GOLD COAST, QUEENSLAND, AUSTRALIA

COASTAL EROSION AND RELATED PROBLEMS

VOLUME II

PART 1:

INVESTIGATIONS (TEXT AND TABLES)

DELFT HYDRAULICS LABORATORY

THE NETHERLANDS R 257

CP 34

GOLD COAST Q,UEENSLAND AUSTRALIA

REPORT: COASTAL EOOSIQN AND BElA TED PRO)3IEM3

ERRATA LIS T - VOLUME II Part 1

P~ e Location

- List of Tables 35

5

15

16

21

28

30

40

41

51

54

List of Tables 36

10th line from bottom

Table 4. column for 14 knot s velocity

line 1 of Section 3.3.3

line 6 of Section 3.3.3

Table 6, Note.

Table 14. at end

Table 16. at end

bottom line

line 21

9th line from bottom

Table 37, No te F

As Printed Oorrection

U,S, navy U,S. Navy

U,S. navy U.S, Navy

Snapper Rock Snapper Rocks

have has

G. J.

4eavy dividing dividing line

Add the follow-. ing Note:

previal.

en

of

see.

The number in brackets gives the number of times T is z recorded at 0900 hI'S in the month wi thin the appropria te rmge.

Add the follow­ing Note: The nUlllber in brackets gives the numb er of times Tz is

recorded at 0900 hI'S in the month within the appropria te range.

prevail

an

or

. I

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Errata Lis t - Volume II Part 1

Page Lo.cation As Printed Correction

55 Table 37, Site 6 7.5 17.5 2nd last line of data

58 Table 38, 2nd line 0 10 a

of data, column angle to Coa s t

61 line 10 radings readings

70 3rd line from diameter diameters bottom

73 Formula for K k s. s

86 line 22 are area

89 3rd line of 3.5.1 3.5 Section 4.1

89 bottom line mall smalL

93 last line of 3.6 3.5 Section 4.4

94 line 9 even ever

Gold Coast, Queensland Australia

- Coastal Erosion and Related Problems -

Volume [I

(Part 1)

Investigations

(Text and tables)

R 257

1970

Delft Hydraulics Laboratory

Delft The Netherlands

1.

2.

3.

CONTENTS

Volume II I nvestigat ions

Pa r t Text and Tables

I ntroduct ion I •••• •••• , • I •••••••••••••••••••••••••••••••••••

General Description of the Area 2.1. Geography, Population and Development. 2.2. Physiography and Geomorphology •.••••••

. . . . ......... • • I ••••••

2.3. Leve I Datum. I , ••••••••••• , •••••• •• I •••••••••••••••••

The Coast,. "." 3.1. Historical Background ............. f" ., •• '" •• "" I. I ••

3.2. Description of Particular Beaches ......... If ••••••••••• I

3.3. Meteorological Data ......•......... . . . .

3.4. 3.5.

3.3. 1. Genera I.. I........ . ... . · . . . 3.3,2. Winds ................. . · . . . 3.3.3. 3.3.4. 3.3.5. 3.3.6.

Cyclones ..•.. I • •••••• I ••• I •••••••••••• I ••••••

Temperatures •••••..• Atmospheric Pressure. Stream Discharge ••••

. . . . .. . . . .. .

.. ..

.. · ... · . , . . . . .

Water Characteristics. I., •...........•..•• I ••••••••••• Tides •• 3.5.1. 3.5.2.

• •••••••••••••• I •• I •••••••••••••••••••••••••••

Genera I ••••••••• , .•••••..••••••.•..•••.•..••• Elaboration of Data ........ ' ...... .. ...

3.6. Waves ................... I ........ . . . . . . . .. . .. 3.6.1. 3.6.2. 3.6.3. 3.6.4. 3.6.5.

Genera I .................. , .................... . Data and Elaborations ••••••••••• • ••••• I •••••

Comparison and Discussion ............. "' ... . Waverider Information - Detailed Analyses •.••.•. Refraction Diagrams ........................... .

3.7. Currents ... , .... , . . .. . ..... . . .. . . . ..

3.8.

3.9.

3.7.1. General. . . . . . . . . . . . . . . . . ... 3.7-.2. Data and "Elaborations .... to ••••••••••••••••••••

3.7.3. Discussion and Conclusions ................. ' ... . Bottom Composition .. , ....................... . 3.B.1. General ..•...................... f 3. B. 2. Mineral Consistence ........... t • I ••••••••••••••

3.8.3. 3.8.4.

Grain Diameters •••• • • • • • • • • • • • • • • • • • • • • • • • • • I •

Samples from Stream Beds ................ I ••••••

Sed i ment Transport ............................ I •••••••

3.9.1. General ......•... I ••• ,'.1 ••••••• , ., ••••••••••

3.9.2. Computation of Littoral Transport .•••••••••.•••.. 3.9.3. Results ...................................... .

page 1

2 3 3

4 5

12 12 13 16 17 17 18 19 20 20 20 23 23 24 36 38 52 52 52 53 64 68 68 68 69 70 71 71 71 74

CONTENTS (conti nued)

page

3.10. Morphology of Bottom and Shore...................... 76 3.10.1. General Methods of Approach................ 76 3.10.2. Erosion/Accretion Quantities from Shoreline

Changes..................................... 78 3.10.3. Erosion/Accretion Quantities from Soundings.... 80

3.11. Comparison and Summary of Erosion/Accretion Quantities 86

4. River and Creek Mouths 4.1. '.General.i .. "01 ••• '. It •••••• If It ••• ,. ,.1 ••••• f ••• II •• II 89 4.2. Currumbin and Tallebudgera Creeks..................... 89 4.3. Tweed River.......................................... 90

4; 3. 1. Introduction and Statement of the Problem........ 90 4.3.2. Sand Accumulation ................... I ••••• I •• ' 91 4.3.3. Influence on Gold Coast Beaches................ 91

4.4. Nerang River ............. ·............................ 93 4.4.1. Movement of the Entrance .. t •• I., I ••••••••••• I • 93 4.4.2. Shociliogof the Broadwater...................... 94 4.4.3. Tidal Volumes ....................... I ••••••• I' 96

TABLES

page

1. Directional Frequency of Wind Occurrence.................... 13 2. Distribution of Velocities of Winds from NE................... .14 3. Distribution of Velocities of Winds from E..................... 14 4. Distribution of Velocities of Winds from SE.................... 15 5. Monthly Discharge Analysis - Nerang River (1918 - 1968)...... 18 6. Distribution of High - and Low - Water Levels.............. .• 21 7. Distribution of Tidal Ranges - Snapper Rocks and Broadwater.... 22 8. Tidal Characteristics.......................................... 23 9. Cape Moreton - State of Sea Percentage Exceedance (1957 -

1968) ...•......••...•••...•.•.............................. 25 10. HjH at Locations 2 and 3 of Waverider...................... 25 11. HjH~ at Location 1 of Waverider............................. 26 12. Percentage Occurrence of Swell 'Heights and Periods - Cape

Moreton 09.00 hrs .. ........... of • • • • • • • • • • • • • • • • • .. • • • • • • • • • • 26 13. Boundaries between Short and Medium: Medium and Long Periods 27 14. Mean Values within Period Ranges. 0.00. 0 •••• 0. ............ ••••• 28 15. Boundaries between Low and Moderate: Moderate and Heavy

Swell~ ....... oo ............... o .... o................... ..... 29 16. Mean Values within Height Ranges............................ 30 17. Directions, Heights, Periods and Frequencies of Occurrence of

Swell (according to Cape Moreton observations; elaborated with some Waveri der resu Its) • •••••••• 0 • .. • • • .. .. • • • • • • • • • • • • • .. • • • • • • • 31

18. Swell Occurrence according to K. N. M.I. Observations......... 32 19. Sea Occurrence according to K.N.M.1. Observations.......... 33 20. Wave Information from American Sea and Swell Charts (June -

February) ............ ........................................ . 21. Cape Byron - State of Sea Percentage Exceedance (1960 - 1968) 22. Comparison of Swell Observations ............................ . 23. Mean Direction of Predominant Swell. .•••••••••..•.••.•••.•.• ' 24. Comparison of Sea Observations ...•... 8 ••••••••••••••••••••••

25. Avai lable Waverider Records ..•.•.. 8 •••••••••••••••••••••••••

26. Monthly Wave Characteristics from Waverider •...•••••••.....•• 27. Hs - T z Scatter Diagram: Waverider Buoy No.1 ••.•.•.•••..••. 28. Hs - T z Scatter Diagram: Waverider Buoy No.2 .••.••••.••.•.• 29. Hs - Tz Scatter Diagram: Waverider Buoy No. 3 .••••••••••.••• _ 30. Hs - T z Scatter Diagram: All Waverider Buoys .••.•.••••••.•••. 31. Monthly Distribution of Hmax from Waverider Buoy No.1 ...... 32. Monthly Distribution of Hmax from Waverider Buoy No.2 •••.•• 33. Monthly Distribution of Hmax from Waverider Buoy No.3 .••••. 34. Distribution of Hmax from all Waverider Buoys .•.••••••••.••••• 35. Surface Currents from U.S. navy H.O. Pub. No. 568 ........ .. 36. Surface Currents from U.S. navy H.O. Pub. No. 107 ......... . 37. Results of Drift Card Measurements ..•.•....................... 38. Resu Its of Sub-surface Current Measurements •••••••.•...•..•••.

34 35 37 37 38 39 42 43 44 45 46 47 48 49 50 53 53

54-56 57-59

TABLES (continued)

39. Summary of Surf-zone Current Measurements •.•••••..•••...••• 40. Spacing of Rip Currents ................................... . 41. Average Sub-surface Current Velocities ••••••.••...•.••.•.•.• 42. Surf-zone Currents on Same Day at Different Sites ..•••.•..••. 43. Littoral Transports ........................... I ••••••• f •• " ••

44. Littoral Transports Through Profile 9 - Variable Bed Roughness •• 45. Shoreline Changes 1932 - 1962 ............................ . 46. Transverse Transport Rates ...•.•.............................

page

62 63 65 67 75 76 79

84-85

FIGURES

Figure no. Title

1 Locality Plan. 2 Geomorphology of the Area. 3 Depth Contours and Measurement Stations, 4 Location of Beaches and Sounding Lines. 5 Typical Section of 1968 Loose Boulder Wall. 6 Probabi lity of Occurrence of Cyclone affecting Gold Coast. 7 Number of Cyclones per year in Gold Coast Region. 8 Probability of a Certain Number of Cyclones Affecting the Gold

Coast in Any Year. 9 Yearly Discharge of Gold Coast Streams •

. 9a Tide Gauge Records. 10 Percentage of Time that High-Water (Low-Water) Levels are Exceeded

(not Exceeded). 11 Probability of Exceedance of Tidal Range. 12 Wind Fetches. 13 Sea Wave Height Distribution - All Directions. 14 Wave Information August. 15 Wave Information September. 16 Wave . Information October. 17 Wave Information November. 18 Wave Information December. 19 Wave Information January. 20 Wave Information February. 21 Wave Information March. 22 Wave Information April. 23 Wave Information May. 24 Wave I nformation June. 25 Wave Information July. 26 Cape Moreton Swell Observations - Monthly averages 1957 - 1968

at 0900 hr. 27 Cape Moreton Swell Observations - Monthly averages 1957 - 1968

at 1500 hr. 28 Cape Moreton Swell Observations at 0900 and 1500 hr -Average for

years 1957 - 1968. 29 Monthly Variation in Direction of Net Swell - Cape Moreton and

Cape Byron. 30 Wave Period Exceedance August - December 1968. 31 Wave Period Exceedance January - May 1969. 32 Wave Period Exceedance June - November 1969. 33 Wave Information from U.S. Sea and Swell Charts (March - May). 34 Cape Byron Swell Observations - Monthly Averages 1960 - 1968

at 0900 hr.

Figure no.

35

36

37

38 39

40

41

42

43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

67

68

FIGURES (confinued)

Title

Cap~ Byron Swell Observations - Monthly Averages 1960 - 1968 at" 1500 hr. Cape Byron Swell Observations at 0900 and 1500 hr - Average for years 1960 - 1968. Monthly Percentage of Swell from Various Directions at Cape Moreton and Cape Byron. Comparison of Hs according to Draper and Hs calculated from E. Waverider Measurements - Comparison of E Values at Locations I, 2_ and 3. Waverider Measurements - Comparison of Hs Values at Locations I, 2 and 3. Waverider Measurements - Comparison of T z Values at Locations I, 2 and 3. Waverider Measurements - Comparison of T c Values at Locations I, 2 and 3. Percentage of Time that Hmax or ~~ is ~ Value Shown. Probability that Hmax or Hs is ~ Value Shown. Wave Persistance Probabi !ity. Refraction of NE Waves, Period 6 secs. Refraction of NE Waves, Period 8 secs. Refraction of NE Waves, Period 10 secs. Refraction of E "" Wavf3s, Period 6 secs. Refraction of E Waves~ .. Per.iod" 8 secs". Refraction of E Waves, Period 10 secs. Refraction of E Waves, Period 12 secs. Refraction of SE Waves, Period 6 secs. Refraction of SE Waves, Period 8 secs. Refraction of SE Waves, Period 10 secs. Refraction of SE Waves, Period 12 secs. Surface Current Velocities. Probability of Surface Current Velocity~ Value Shown. Direction of Surface Currents versus Wind Direction"," on Day of Release. Average Longshore Current versus Date of Measurement. Average Longshore Current versus Time of year. Average Longshore Current versus Direction of Wind. Average Longshore Current versus Direction of Swell. Probability of Absolute Sub-surface Current Velocity ~ Value Shown. Probability of Directional Sub-surface Current Velocity ~ Value Shown. Planeta Current Measurements (Vertical Current Profiles) Site 6, 30 Aug. 1967. Planeta Current Measurements (Vertical Current Profiles) Site 5, 31 Oct. 1967. Planeta Current Measurements (Vertical Current Profiles) Site 2, 23 Apr. 1968.

FIGURES (continued)

Figure no. Title

69 Sea-bed and Bed-rock Profiles (Omega 5). 70 Sea-bed and Bed-rock Profiles (Omega 16). 71 Sea-bed and Bed-rock Profiles (Omega 22). 72 Sea-bed and Bed-rock Profiles (Omega 41). 73 Sand Samples Omega Lines. 74 Sand Samples Omega Lines. 75 Sand Samples Alpha and Beta Lines. 76 Sand Samples Gamma and Delta Lines. 77 Sand Samples Epsilon and Zeta Lines. 78 Variation in D50 for Alpha Lines (1967 - 1968). 79 Samples from Stream Beds - Tallebudgera Creek. 1969. 80 Samples from Stream Beds - Nerang River 1969. 81 Samples from Stream Beds - Curruinbin Creek 1969. 82 Samples from Stream Beds - Tweed River 1969. 83 Relation Between Littoral Transport and Wave Height. 84,.· Theoretical and Experimental Wave Height Distributions. 85 Profiles for Littoral Transport Calculations. 86 Erosion/Accretion Quantities 1932 - 1962. 87 General Profiles (to 8 miles offshore). 88 General Profiles (to 1 mile offshore). 89 Profiles at Alpha and Beta Lines. 90 Profi les at Gamma and Delta Lines, 91 Profi les at Epsilon and Zeta Lines. 92 Scheme of Area Calculation from Profiles. 93 Profile Areas above Specific Levels. 94 Change in Area above Specific Levels with Time. 95 Cumulative Volume Change 1966 - 1967, 1967 - 1968 and

1966 - 1968, 96 Rate of Volume Change 1966 - 1968. 97 Cumulative Volume Change Between Contours. 98 Li ttora I Transport. 99 Nearshore Profiles South from Tweed River Mouth.

100 Sand Transport Past the Head of Tweed River Walls as a Function of TimE 101 Nearshore Profiles in front of and north from Tweed River Mouth. 102 The Broadwater Area. 103 Migration of Nerang River Mouth. 104 Rate of Linear Growth of The Spit. 105 Volumetric Growth of The Spit. 106 Tidal Discharges through Nerang River Mouth,

PHOTO GRAPHS

1. Greenmount - Coolangatta, 2. Burleigh Beach, 3. Nobby Beach, 4. Broadbeach, 5. Surfers Paradise, 6. Narrow Neck, 7. Main Beach, 8. Currumbin Creek Entrance, 9. Currumbin Creek Entrance,

10. Currumbin Creek Entrance, 11. Tweed River Entrance, 12. Tweed River Entrance, 13. Tweed River Entrance, 14. Nerang River Entrance, 15. Nerang River Entrance,

highwater 27 June 1967. highwater 27 June 1967. highwater 27 June 1967. highwater 27 June 1967. highwater 27 June 1967. highwater 27 June 1967. highwater 27 June 1967. 17 June 1967. 18 January 1969. 6 November 1962. 1 September 1961.

25 November 1963. 18 January 1969.

1 August 1955. 16 April 1969.

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1. INTRODUCTION

In 1964 the Co-ordinator-General's Department in Queensland requested the Delft Hydraul ics Laboratory to examine the beach erosion problems of the City of Gold Coast in Southern Queensland, and to recommend a programme of oceanographic and beach movement investigations necessary for a comprehen­sive study of the erosion phenomena and the design of such protective works as may be warranted. A report on this subject was prepared by the Laboratory in 1965 [1 J. Many results of the recommended investigations became available in the following years.

In 1968 the Laboratory was requested to assist in the processing and evaluation of the measured data and to recommend possible methods for beach improvement and stabilization of the river entrances and related works.

The study was executed in 1969 and 1970, the present report being completed in June 1970. This volume of the report (Volume II) presents the evaluation of the measurements and a study of the erosion phenomena. The recommendations are given in Volume I.

Many persons and organizations gave a valuable contribution to the result of this study. Their names are mentioned in Volume I.

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2. GENERAL DESCRIPTION OF THE AREA

2.1. Geography, Population and Development

The area under consideration forms part of the City of Gold Coast, extending along the Queensland East Coast from the New South Wales border about 20 miles northward to ha If-way South Stradbroke Island (figure 1).

Most of the area serves as a valuable tourist centre for the people from Queensland as well as from the other States of Australia and from over­seas. The main touristical value of Gold Coast City must be found in the beaches. These beaches, composed of fine sand, are gently sloping with excellent possibi lities for surfing, fishing, cruising and other sporting activi­ties. Westward from the coast there are beautiful mountain areas. Establishment of industry, other than tourism, in the area is gaining momentum.

The sub-tropical climate makes the area also in winter very attractive because of mild temperatures and decreased cloudiness, whi Ie during summer the temperature is more moderate than in the tropical part of Queensland. Still during about 90 days a year temperatures are higher than 850 F, with approximately 1 day a year higher than .1000 F.

Average annual rainfall is about 43 inches (Brisbane), varying from about 6 inches a month in summer to 2 inches a month in winter. The rainfall is very erratic, however, not only from year to year, but also from place to place, due to the occurrence of cyclones and thunder-storms.

The permanent population of Gold Coast City has increased from 20,000 persons in 1954 to about 53,000 in 1967; including the holiday population this number mounts up to about 75,000 and is increasing from year to year. Besides the holiday visitors, many people from Brisbane, the capital of the State, frequently make day or weekend trips to the Gold Coast beaches.

The increasing popularity of the area leads to an intensive development of transport services, major' highway improvements and extension of accomodation facilities. In the last tgln years capital investment on building has totalled $ 128.6 million. Many kinds of establishments, caravan parks and camping grounds provide accomodation for more than 115,000 visitors at the one time by now.

Many buildings and correspondih~ roads are situated very close to the beach, so that any retreat of the beaches may· cause severe damage to public and private property, concern over which fact gave rise to the present study of erosion problems.

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2.2. Physiography and Geomorphology

A lot of information about the sub-bottom structure can be derived from a geophysical survey of the Gold Coast in 1967 by the Bureau of Mineral Resources, Geology and Geophysics [2:1.

Figure 2 shows a geomorphological map of the area. Most of the area can be described as coastal plains, one-half to three miles wide, consisting of sand, mud, peat, old dune ridges and estuarine alluvium, atthe western side bounded by mountains. Parallel to the coast several dune ridges are present, covering an area 300 to 1200 ft wide.

The coastal plains are built up on a west to east sloping base of pre-pleisto­cene bedrock. At several locations the bedrock protrudes into the ocean as head­lands (at Point Danger, Coolangatta, Currumbin, Burleigh and South Nobby).

From mid-Pleistocene to recent times the sea level has varied from 100 ft above, to 250 ft below the present level. That means that the coasta I plains have been both exposed and submerged, so that erosion and deposition in a wide range at both sides of the present shore cou Id take place.

The area now occupied by the plain has been built progressively seaward by river and marine deposition. The coastal plains are cut by several rivers and creeks. From south to north there are Tweed River (NSW), Currumbin Creek, Tallebudgera Creek, Nerang River and several other rivers and creeks discharging in the Broadwater. The streams approach the coast almost perpendi cu larly and turn sharply northward near the shore, under the influence of longshore currents. The presence of the large coastal plain shows that during thousands of years depo­sition of sediments has taken place. As presently the general trend at the coast is erosion, some change in the supply of sediments must have occurred.

In figure 3 the undersea depth contours a long the coast are shown. These are quite regular. Only large areas of submarine reefs disturb the regular pattern, especially in the northern part of the area. These reefs however protrude only a short distance above the bed.

2.3. Level Datum

It should be noted that Reduced Level (R.L.), used throughout the investi­gations and in this report, signifies the height of a point above or below Queens­land State Datum. The Zero of State Datum was set as equivalent to Mean Sea Level as determined many years ago from the Port of Brisbane tide gauge at the Pile Light.

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3. THE COAST

3.1. Historical Background

It is not the purpose of this section to give a detailed history of the development of the area. The researching of such information is beyond the scope of this report, and unnecessary as to the conclusions reached. A brief, genera I resume, however, is of value in putting the problem into perspective.

Nothing at a II is known of the area prior to 1800; it is probable that no white man saw it before this time, and the original inhabitants of the area, the Aborigines, kept no written record.

In the latter half of the nineteenth century many sailing ships are reported to have crossed the Nerang River entrance on their voyages between Brisbane and the south, sailing inside Stradbroke Island. At least one such ship, the Scottish Prince, was lost while attempting the crossing. Evidence of chroni­clers of the time suggests that an amount of timber was transported to Brisbane from northern New South Wales via the Tweed River, Pacific Ocean and thence across the bar at Nerang River. Prior to the break-through at Jumpinpin it is probable that the greater flows through the entrance and behind the southern portion of Stradbroke Island were sufficient to maintain greater depths than at present.

Very few maps are available prior to 1900, but some settlement had al­ready commenced by this time at. Southport and at Coolangatta near the mouth of the Tweed River.

A map of the northern portion dated 1917 indicates several allotments in . Southport, but only large parcels of land (farms?) from Main Beach to Broad­beach. It is interesting that at this time a large township had already been surveyed on the southern end of South Stradbroke Island. This town, Moondarawa, has since disappeared as the Nerang River entrance migrated north.

In the first decades of this century, development continued mainly in the Southport and Coolangatta areas, they being serviced by the railway. The construction of the Jubi lee Bridge at Southport and the road southwards to Burleigh Heads in the mid-1920's enabled the northern surfing beaches to be offered for close settlement. Kindler and O'Connor [3J mention that in 1923 there were only two houses between Currumbin Creek and Burleigh Headland.

Development sti II progressed slowly, generally from the two ends towards the centre. For· example, a photo of 1936 [3J shows only about 30 houses between K irra Hill and Coolangatta Creek.

The end of the Second World War marked the beginning of a big expansion in bui Idings accompanied by the inevitable boom in land prices, which has continued, with a few normal recessions of brief duration, to the present time. In the early years of the 1960 decade a significant change occurred in the type of building when multi-storeyed units began. to appear, and these are increasing progressively.

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Originally the area was administered by separate town councils, which later coalesced to form the Town of South Coast Council. On achieving city size, the area took the name City of Gold Coast. The present population growth rate is reportedly the second largest, after Canberra, of any city in Australia.

The whole strip is now fairly densely settled, and in most cases to the top of the frontal dunes. The most concentrated development is in the Surfers Paradise area, while multi-storeyed units are replacing older houses at Coolangatta, Burleigh and Broadbeach, and this trend will probably continue until the whole of the coast is fronted by high-rise buildings.

Erosion problems have recurred at irregular intervals over the past 50 years, when periods of heavy seas, usually associated with the passage of cyclones, caused loss of beach or facilities generally in one or two particular areas each time.

Following such a period late in 1950, the report of Kindler ahd 0' Connor [3J was prepared in 1951. Considering the amount of technical information then available, the conclusions reached are indicative of an accurate appreciation of the prevai ling circumstances. Apparently, however, no action based on this report eventuated, for which omission the price is now to be paid.

In 1964 this Laboratory became involved in the problem, and in 1965 presented a report [1] which contained seven major recommendations. It is pleasing to note that all recommendations have been acted upon. The present report is based largely on action since the 1965 report, and provides a basis for the future deve­lopment of the beach areas.

3.2. Description of Particular Beaches

Some valuable references to beach conditions in earlier times are to be found in reference [3J. Drawing on that information and other sources, this section will point out developments at various beaches which are considered to be significant in terms of past happenings and likely future occurrences. Reference should be made to figure 4, which shows the location of the various beaches within the area, together with the location of sounding I ines for the detailed investigation.

Rainbow Bay

Th is is a' sma II pocket beach located at the southern extremity of the area, and situated between the headlands Snapper Rock and Greenmourit Hill, approximate­ly 900 ft apart. The beach line in plan is usually quite curved, but on occasions a sand spit forms between the two headlands, and the bay is almost completely filled with sand.

A former camping area and now park land is situated between the esplanade and beach, and is almost certainly within the limit of normal migration of the beach line. Erosion of this area occurred early in 1967, but subsequently the area recovered rapidly.

No dune system exists, but the park affords a valuable buffer area, and the area could not be considered as one presenting urgent problems.

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Greenmount-Coolangatta Beach

This sharply curved beach lies between Greenmount and Kirra Hills which are 2,600 ft apart. A park exists between the road and beach, in which park are situated a kiosk and Surf Lifesaving Clubhouse (S.L.S.C.) at Greenmount, and an S.L.S.C. at the Coolangatta end.

Prior to 1967, a small dune existed over some of the length, due in the main to efforts made some 20 to 30 years previously to overcome a problem of wind-blown sand by establishment of vegetation. This dune was destroyed in 1967, and uncovered was a series of earlier protective measures, including a grouted rock wall and a small timber-piled wall. The time of the installation of these measures is not known, but a photograph of the area in an eroded condition in 1935 [3J shows no sign of them, and it seems likely they were constructed after the cyclone of 1936. Photograph 1 shows the area late in June 1967, and the earlier protective measures are clearly visible.

The above-mentioned buildings, particularly the one at Coolangatta beach, are located within the area which is required for the normal variation of the beach line. In order to limit the future landward migration of the beach line, a substantial loose-boulder wall was constructed over the whole length in 1968 (figure 5). Recovery of the beach in the first years after 1967 has been poor, and the wall is subject to much wave action.

The width of available beach has shown great variation over the years, particularly at the Greenmount end, sometimes reaching almost to the tip of the headland.

Kirra Beach

The Kirra beach area lies generally east-west between Kirra Hill and Coolangatta Creek. Immediately west of the headland, a kiosk and bath ing pavilion were constructed in 1936, and suffered wave attack even before the official opening. A timber-piled wall with boulder protection was erected a­bout that time, and beach conditions since have varied from 100 yards of sand to nothing in front of this wall. Further west, the area between the road and beach is occupied by a camping ground. No dune exists.

Extensive erosion occurred during 1967, especially in the easternmost corner, and recovery since that time in this area has been poor.

It is quite obvious that the bothing pavilion and camping area are loca­ted on what should be the beach at times under normal circumstances. A photo­graph taken in 1936 and given in reference [3 J confirms this.

Such beaches are particularly sensitive to changes in wave direction, and the >easternmost corner is almost entirely dependent on north-west to north-east winds for accretion. The camping area contains a few permanent structures in the way of toi lets etc.; wind-blown sand is a problem.

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Coolangatta Creek probably originally discharged adjacent to Kirra Hill, its present course being man-made. Future plans for the creek include the con­struction of a substantial outfall structure, which will handle the storm-water drainage from the areas of K irra, Bilinga and Tugun.

Coolangatta Creek - Flat Rock Creek

The beaches in this area, North Kirra, Bilinga and Tugun, are among the most stable of the area. Even so, fluctuations of beach line have occurred in the past, as witnessed by the boulders, now largely sand covered, which exist at the front of private property.

South-eastward from sounding line 'Gamma 1, the esplanade is fronted by a low we II-vegetated dune system, but north-westward from here to Flat Rock Creek previous erosion has resulted in the loss of the originqlly surveyed espla­nade, and the placement of the above-mentioned boulders.

The esplanade (Pacific Parade) is not required for access to the properties south-east from Flat Rock Creek, but the present protection afforded to these properties is slim. Eventual re-establishment of an esplanade and dune system in this area must be considered desirable.

At Flat Rock Creek, there is a small outcrop of rock on the beach, which provides a control for the beach line south from there. The Creek itself has recently been piped across the beach to discharge adjacent to the rock.

The replacement of individual houses in the area by multi-storeyed units is accelerating.

Flat Rock Creek-Currumbin Creek

The road in this area with its retaining wall of boulders on the beach is obviously built on what should be the beach almost all the time under natural conditions. A short groyne at the northern end, constructed many years ago, appears to have no appreciable effect on beach levels.

The area consists of two small bays - Flat Rock to Elephant Rock and Elephant Rock to Currumbin Creek.

C!,Jrrurnbin Rock is usually separated from the mainland by a channel, but sometimes a tombolo forms, checking transport through this passage. It is reported that this tombolo carried a dune system ~uring the 1920's.

It is apparent that no appreciable gain in beach levels and lessening of wave attack on the road can be hoped for under the existing prevailing conditions.

Currumbin Creek is dealt with in section 4.2. It does offer an area of water in the lee of Cllrrumbin Hill which appears attractive for development for small-boat purposes, the major problem being connected with the unstable shallow entrance.

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Currumbin Creek - Burleigh Headland

The whole area is commonly known as Palm Beach, though there are in fact three official bathing areas - Palm Beach, Pacific and Tallebudgera.

In this section, previous erosion has resulted in the complete loss of the original esplanade area over the southern portion, and partial loss over the northern portion. Towards the southern end, high-water mark is now situated within the limits of private property. Some houses and out-buildings have been damaged or lost in past years, and further damage was suffered in 1967. These happenings have led owners in'the area to attempt their own forms of protection, and today very few gaps exist in tin ill-formed alignment of randomly tipped boulders, the protective value of which is highly doubtful in the present form.

The area of high land (i.e. sand dunes~), originally occupied in this area, was extremely narrow and backed by swamps. There is no doubt that, whatever the reason, the original development layout was situated far too sea­ward, within the limits of migration of the beach line even under normal cir­cumstances. The occupation of this area has worsened the situation by damage to the dunes and prevention of natural recovery of dunes during quieter periods.

At the southern end of this beach, Currumbin Creek causes problems of erosion as its mouth is forced north-westward during its migratory cycle. The position of the mouth of Tallebudgera Creek is kept sensibly stable by Burleigh Headland.

The building development along the front of the area at present consists mainly of private, single-storeyed residences.

Burleigh Headland - South Nobby

The bathing areas of Burleigh and North Burleigh are located on a beach some 6,700 ft long between Burleigh Headland and South Nobby. Due to the fact that a heavy-mineral sand mining lease was current over the frontal dunes until relatively recent times, there now exists a park of about 200 ft width between the formed esplanade and the beach. Landward of the esplanade are to be found multi-storeyed units in numbers which are increasing.

Located within the park area are the S.L.S.C. of North Burleigh, and at the southern end a kiosk with S. L. S. C. and parki ng area. The latter is situated well for.ward of the normal landward limit of beach line migration, and has been somewhat protected from erosion by a boulder wall constructed severa I years ago. A large drainage complex serving the whole area discharges immedia­tely to the north of the parking area, and at present a pile-supported outfall structure is being constructed for this drain. Photograph 2 shows the southern end of this beach late in June 1967.

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The sma II dune which had established itself in front of the park was destroyed in 1967, leaving a raw scarp. Subsequent pedestrian traffic across the scarp made regeneration impossible. In 1969 a system of sand fencing and vegetation was installed along the whole length, and early indications are of a quite successful start to dune re-growth.

The presence of the park as a buffer area, together with the dune works already carried out, make this area one of less urgent problems.

Miami, Nobby and Mermaid Beaches

South Nobby is the last natural controlling headland unti I Point Lookout, some 45 miles to the north, is reached" This major arc is relieved only by Nerang River entrance and Jumpinpin. .

Possibly due to sand mining title also, the area of Miami Beach comprised a park some 100 ft wide seaward of the esplanade prior to 1967, with most of the buildings landward of the esplanade being private houses. The frontal level was particularly low, being only RL 11 to 12 ft towards the north of the area. No dune existed. During 1967, severe erosion of the park occurred, and the low areas were overtopped. Damage to houses was avoided, but some storm-water drains discharging on the beach were destroyed.

In order to prevent further westward migration of the beach and thus to protect the esplanade, a boulder wall (figure 5) was constructed during 1968, extendi ng from South Nobby northward for 3,600 ft.

In the Nobby and Mermaid Beach region, erosion had over the years destroyed the explanade area, and the highwater mark is now within private property over most of the area. In past times a few private owners had attemp­ted to protect their houses by dumping boulders and other means, and this work accelerated after 1967. The protection such as existed prior to 1967 was of little value, but the work since then has been to a better standard. Many houses in the area were damaged during 1967 - see photograph 3.

The westward limit of the beach reached during 1967 was almost certainly further than known before in man's occupany of the area.

Photographs given in the report [3], taken at Mermaid Beach in 1951, show that the regression in 1967 was worse than the 1936 and 1950 erosion. In those photos an even earlier, more severe erosion scarp, is evident and this a Iso was exceeded. These photos are instructive in another way, as they indicate that a considerable amount of building has taken place in that area since that time.

As will be indicated by other considerations later in this report, the now landward maximum position of the shoreline reached in 1967 could be symptomatic of a general continuing erosion in the area, and the fault here may not lie com­pletely in the occupying of the area within the normal migratory cycle of the beach. In any case it is obvious that .the re-establishment of the esplanade area and a wider dune-beach area is of importance for the future safe occupancy of the area.

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Kurrewa Beach

Again because of sand mining, which was completed only a few years ago, there exists here a wide park area ("Pratten Park") in which the only building is a surf I ifesaving clubhouse.

Some regression occurred during 1967, and in 1969 a system of sand fencing and vegetation was installed which is causing a dune to establish rapidly. This area is one with few problems at present.

Broadbeach - Northcliffe Beach

In this area, settlement is becoming more dense, and multi-storeyed units on the front are beginning to become more frequent.

At the southern end, the small park and dune seaward of the esplanade eroded almost completely during 1967, and in 1968 a 2,000 ft length of boulder wall (figure 5) was constructed to protect the remaining esplanade.

North from here, the original esplanade area had been lost prior to 1967, and high-water mark is in many cases within private property. Private efforts at protection followed a similar pattern to those in the Nobby-Mermaid Beach area -see photograph 4.

In 1967 erosion in this area surpassed previously known limits, and again a continual general erosion might be suspected in addition to the harmful occupancy and unrestricted usage of the frontal dune area.

Surfers Paradise

This is the area which has attracted the greatest development in high-rise buildings, motels and so on. It .is difficult to attribute the popularity of this area compared with other areas of the coast to any specific factors.

The southernmost 2,500 to 3,000 ft are similar to the foregoing section (Broadbeach-Northcliffe), and similar remarks apply.

The next section extends approximately 5,000 ft to Narrow Neck, over ·which length exists an esplanade which was eroded, and in some places lost entirely, during 1967, and subsequently a boulder wall (figure 5) was constructed. Photograph 5 shows the area in June 1967 .•

Because of the large and increasing capital development in the area, provision of more adequate beaches and dunes or alternative protection against further erosion is of major importance for this area.

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Narrow Neck

Protection of this area, occupied only by the road (formerly the Pacific Highway, but now the connecting road to Main Beach) commenced almost as soon as the road was constructed in the mid-1920's. Kindler and 0 'Connor [3] give a good resume of the work carried out prior to 1950. The likelihood of a breakthrough from the Nerang River side has been eliminated by the works carried out in connection with the construction of the new bridge and highway.

As a result of the protective works, the area now resembles a "mini­headland" and very seldom is fronted by a beach. Photograph 6 illustrates the position late in June 1967, and the boulder wall constructed at Surfers Paradise in 1968 connects to the Narrow Neck protection works. Under present conditions, continuing maintenance at regular intervals will be required in this area.

Main Beach

This area may be considered in two sections. (i) The southern section stretches 2,200 ft approximately from Narrow Neck north. In this section the original dunes and surveyed esplanade were lost prior to 1950, so that houses in the area were threatened. In the early 1950's a substantial boulder wall was constructed, and this has withstood subsequent wave attack, including the 1967 erosion, reasonably well. Prior to the wall's construction many homes in the area were moved landward within the properties. (ii) The northern section is about 1,500 ft long, and comprises park land, parking area, esplanade and an S.L.S.C. The southern portion of esplanade and parking area was lost during or prior to 1950, and the remainder during 1967. Subsequent­ly, a boulder wall (figure 5) was constructed along this section, joining up with the southern wall. Photograph 7 illustrates the situation in late June 1967 at the junction of these two sections.

The wall construction is intended to limit further westward migration of the beach line, but as a consequence the usable beach in the area will gradually diminish. The area is possibly subject toa continuous slow regression, and must be considered as one requiring action to establish more adequate beaches.

The Spit

A detailed description of the history of The Spit is to be found in section 4.4.1. In brief, it comprises an area some 14,000 ft long, average width 1,310 ft, and average elevation R.L. 8 ft approximately and is growing at approximately 85 ft length per year. The southern quarter is occupied by a variety of enter­prises, and the remainder consists of sparsely vegetated wind-formed dune ridges, with swampy areas between.

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The frontal dune system has a maximum height of R.L. 20 ft, decreasing in the northerly direction, but is interrupted at frequent intervals by low saddles through wh ich water passes during storm periods threatening to cut a new channel. Indeed in 1936 and again in 1951 a channel formed linking the Broadwater and ocean just north of Main Beach. The latter channel was filled in by bulldozer - the mode of closure of the former is unknown, In 1967, wave uprush overtopped the saddles in at least seven places, but no complete channels were cut. These saddles have been made worse by the passage of pedestrians and vehicles through them. This type of action has recently been controlled under the Beach Protection Act.

Some sand mining has been carried out over part of the area in former years, but some heavy-minerals still remain to be won apparently.

In its present condition, the Spit could never be considered as a stable, permanent area, primarily because of the poor system of frontal dunes, and also due to the continuing movement of the northern end.

South Stradbroke Island

There is no settlement on the eastern side of this island, which is characterised by transgressive dunes of heights approximately 30 ft. Sand mining is being carried out on the island.

Wi th the fronta I dunes and Nerang River entrance stabi Iized, the island would offer an attractive area for planned development in the future. If the frontal dunes are not stabilized, the island will in the long term probably vanish because of wind erosion.

3.3. Meteorological Data

3.3. 1. Ger\era I

The most important meteorological information as far as coastal processes are concerned is usually that related to winds. Local winds generate sea, but distant winds and the fetches over which they blow are often more important, being' the source of swell. Methods are available for calculating sea and swell from wind observations, or from winds determined from synoptic charts. The accu­racies of the various methods depend basically on the reliability of the observa­tions or synoptic charts used. However, such methods are no substitute for actua I measurements of waves at the subject locality, and since wave measurements in various forms are available (see section 3.6.), emphasis has been placed on these rather than on theories of wave generation from known or derived winds.

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Temperaturesr and particularly air-sea temperature differences r are now known to be a factor in the mechanism of wave generation r but assume limited importance in the case of the Gold Coast when actua I wave measurements are available.

Storm surgesr or the set-up of water above the normal tidal level r is determined not only by local wind velocitYr fetch and duration r but also by barometric pressure. At the Gold Coast r the presence of low-pressure weather disturbances in the vicinitYr particularly when associated with high-pressure systems in the Tasman Sear may cause significant surge due purely to atmos­pheric pressure differences.

Rainfa Ilr or rather the consequent runoff in rivers and creeks debouching in the arear is a determining factor in the morphology of the adjacent coast and stream entrances.

3.3.2. Winds

The results of wind observations by Dutch ocean-going vessels r collected by the Royal Dutch Meteorological Institute (K.N.M.I.)r are given in iables 1 - 4. Table 1 gives the frequency of occurrence of winds from north-easterlYr easterly and south-easterly directions. Tables 2r 3 and 4 give respectively the frequency of occurrence of wind velocities from the above-mentioned directions. The observations have been made from 1961 ti II 1967 in an area in front of the Queens land coast between 250 and 300 S; 1550 ~ and the coast.

Tables 2r 3 and 4 indicate that the median and mode velocities from the various directions are: north-easterly and easterly directions r 9 knots; south-easter­ly directionr 13 knots.

TABLE 1 Directional Frequency of Wind Occurrence

Month NE E SE Oiher directions Number of + calm observations + confused

j 30 48 70. 102 250 f 31 52 61 81 225 m 35 54 105 94 288 a 28 56 73 94 251 m 19 49 39 129 236 i 5 22 76 122 225 j 9 6 25 142 182 a 19 22 14 178 233 s 37 28 27 113 205 o . 41 49 38 87 215 n 38 46 40 74 198 d 39 34 28 64 165

Numberof observations 331 466 596 1280 2673

percentages 12.4 17.4 22 .. 3 47.9 100

TABLE 2 Distribution of Velocities of Winds from NE

~ Knots month. 1 2 3 4 5 6 7 8 9 10 11 12

i 1 3 2 3 2 5 4 1 f 3 1 1 5 2 1 4 2 6 m 2 1 3 6 1 2 6 4 4 1 0 4 1 1 6 4 1 6 1 1 1 m 3 1 5 3 4 1 i 2 2 1 i 2 1 1 2 1 0 2 2 3 2 6 2 1 s 3 2 5 1 4 3 3 3 2 0 2 6 1 1 3 10 3 3 2 n 1 2 6 7 4 4 1 d 2 5 1 4 3 6 5 1 1

number of obs. 1 20 4 15 46 25 21 21 55 26 13 12

0/0 0.3 6.0 1.21 4.5 13.1 7.6 6.3 6.3 16.6 7.9 3.9 3.6 ~.- L- l....- L..-- _-- -

TABLE 3 Distribution of Velocities of Winds from E

~ Knots month. 1 2 3 4 5 6 7 a 9 10 11 12

i 3 2 14 2 1 1 4 5 1 1 f 2 8 5 7 1 15 3 3 3 m 3 1 3 8 1 5 2 11 2 1 4 a 4 2 2 7 1 1 3 6 5 1 m 4 2 3 2 1 3 6 6 2 1 i 2 2 1 2 4 5 1 1 i 1 1 1 0 2 1 1 2 2 2 2 3 1 2 s 3 1 3 2 1 6 4 1 1 0 1 1 8 3 2 1 16 6 3 n 1 2 2 5 6 1 4 8 6 2 d 1 1 2 2 5 4 2 5 3

number of obs. 22 10 17 61 29 26 22 81 48 13 19

0/0 4.7 2.1 3.6 13.1 6.2 5.6 4.7 17.4 10.3 2.8 4.8

13 14 15 16 17 18 19

3 1 1 2 2 2 2

1 1 2 1 1 1

2

1 1 1

5 2 1 2 1 2 2 1 4 7 1 2 2 4 2 1 3

25 5 8 6 8 11 3

7.6 1.5 2.4 1.8 2.4 3.3 0.9

13 14 15 16 17 18 19

3 2 2 2 1 2 1 4 1 1 2 3 1 7 3 2 4 2 4 8 1 1 3 3 1

1 1 1 1

1 1 1 1 2 3 1

2 1 2 3 2 1 4 1 5 1 2 1

33 19 14 15 7 17 6

7.1 2.4 3.0 3.2 1.5 3.6 1.3 ,

20 21 22 23 24 25

1 1

1 1

1

4 1 1

1.2 0.3 0.3

20 21 22 23 24 25

1 1 2 1

1 1 .1 . 1

1

3 2 1 1 2 1

0.6 0.4 0.2 0.2 0.4 0.2 . . ,

26 27 28 29

26 27 .28 34

1

1

1 1 1

1 2 1 1

0.2 0.4 0.2 0.2 .

number of observations

30 31 35 28 19 5 9

I 19 37 41 !

38 39

331

100

number of 1

observations

48 52 54 56 49 22 6

22 28 49 46 34

466

100

~

.l>­I

TABLE 4 Distribution of Velocities of Winds from SE

~ Knots month. 1 2 3 4 5 6 7 8 9 10 11 12 13

i 1 1 2 2 2 5 5 2 3 8 1 1 1 7 2 2 2 6 4 1 5 m 3 4 2 2 4 10 8 2 2 11 a 1 5 5 2 3 6 5 1 2 11 m 3 1 7 5 1 3 5 2 1 3 i 1 1 6 2 2 3 6 1 4 5 i 1 6 2 3 3 3 a 1 3 2 1 1 2 s 1 2 2 1 2 4 2 1 3 0 3 1 1 3 1 1 2 4 n 3 2 2 4 3 3 3 4 d 2 1 1 2 5 1 1 1

number of obs. 11 5 11 47 18 14 14 48 49 15 21 60 ~o 1.8 0.8 1.8 7.9 3.0 2.4 2.4 8.1 8.2 2.5 3.5 10.1

~ Knots month. 30 31 32 33 34 35 36 37 38 39 40

i 1 1 2 m 4 2 a 1 1 1 m

i 5 1 2 1 2 2 1 i 1 1 0 s 0 1 n d

number 01 obs. 11 1 3 1 4 3 5 1

~o 1.8 0.2 0.5 0.2 0.7 0.5 0.8 0.2

14 15 16 17 18 19 20

1 10 7 1 6 4 4 4 2 5 2 6 2 2 2 7 8 2 11 1 6 3 5 5 3 7

1 1 3 2 1 1 1 1 1 6

3 1 3 1 1 1 1 1 4

3 5 1 2 1 3 1 5 3 3 1 1 2 1 5 1 2·

16 45 30 14 46 12 27

12.7 7.6 5.0 2.4 7.7 2.0 4.5

number of 41 42 43 44 45 observations

70 61

1 105 73 39 76 25 14 27 38 40 28

1 596

0.2 100

21 22 23 24

1 1 1 1

1 2 2 2 2 1 3

1 2 1 8

1 1 2 1 1 1

1

6 7 4 20

1.0 1.2 0.7 3.4

I

I I I

I

I

J I

25 26 27

1 1 4 1

2 1 3 1

1 1 1

9 3 5

1.5 0.5 0.8

28

1 1 1

2

5

0.8

29

2

1

1

4

0.7

I

I

I

1 I

01 I

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Kindler and O'Connor [3J list average ground-wind speeds at the Gold Coast as measured during cyclones or cyclonic depressions between 1921 and 1951. The highest average velocity thus measured was approximately 50 knots, while a figure of 35 knots was exceeded 8 times in the 30 year period. These figures do not include strong winds which may occur from regions of high pressure to the south of the area. .

McGrath [4] I ists maximum wind velocities associated with the cyclones of 1967.

It is concluded that the K. N. M.1. figures are representative of general conditions, and that average velocities above 35 knots are very rare, being generally associated with cyclones or similar disturbances, and usually of a few hours'duration only.

Intermittent records from a recording anemometer at Surfers Paradise, as well as cup-anemometer readings from Southport, Burleigh Head and Coolan­gatta, are avai lable since 1955. These gauges have been operated by the Gold Coast City Counci I. Other wind information from Coolangatta airport is available from the Commonwea Ith Bureau of Meteorology. However, for reasons mentioned in section 3.3. I., it was considered unnecessary to utilize this information.

Continuous wind direction and velocity records have been taken at South Nobby since early in 1969. These have not been required for the present study, but will be useful in the future when and if the need to fully investigate wave spectra in the area becomes evident, as well as for information they will yield on cyclonic winds etc.

3.3.3. Cyclones

Information on cyclones have come from:

a. Charts of cyclone tracks in the Queensland region 1924 - 1960. b. "Austra I ian Hurricanes and Related Storms" S. S. Visher and D. Hodge, of the

Commonwealth Bureau of Meteorology,covering the period 1867 - ~24 [5]. c. "The Occurrence of Tropical Cyclones in Australian Region" by A. T. Brunt and

G. Hogan, covering the period 1907 - 1956 [6]. d. Annual Reports for 1963/64, 64/65, 65/66, 66/67 on tropical cyclones in the

Australian region.

At present, Australian meteorologists restrict the term "tropical cyclone" to low-pressure tropical disturbances occurring between December and April, and divide them into classes I, 2 and 3 depending on wind speed and area of influence. Low-pressure Glisturbances with similar behaviour do occur outside this period, but these are regarded by the meteorologists as originating from other causes, and hence not true tropical cyclones.'

However, the effects on the coast from these intense low-pressure disturbances is similar no matter when they occur, so for the purpose of this report all low­pressure disturbances with maximum wind velocities of the order of 35 knots

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or greater, will be grouped under the collective title "cyclone" . . In the case of Queensland, the point of origin, path followed,and

speed of travel of cyclones are all extremely variable. Assuming 10 mph as a representative value for speed. of travel, a major cyclone may cause on a section of the coast very high wind velocities to be maintained for a period of the order of 24 hours, and moderate swell and heavy seas for up to a few days. Wind gusts may reach 150 knots, but maximum wind speeds of the order of 35 to 70 knots would be usual. Central pressures may fall to around 950 mb.

Figure 6 shows the probability of a cyclone which effects the Gold Coast occurring in any month, the information being obtained from the sources listed above. February and March are the most likely months. Figure 7a shows the distribution of occurrence of cyclones affecting the Gold Coast over the last 100 years, and figure 7b the running-average over successive ten year periods.

In figure 8 is shown the probability of any number of cyclones occurring per year. Since the probability of no cyclones is 40 % approximately, the pro­bability of one or more cyclones occurring per year is 60 '1'0. The probability of 7 cyclones occurring in any year (the 1967 case) is approximately 0.5 '10.

The low central pressures of cyclones can contribute to the storm surge. It has been estimated [4] that pressure differentials could contribute 1 ft of surge. It is not possible to calculate the total surge that might be realized during the worst cyclones, but an amount of 3 ft was measured during one cyclone in 1967 [4].

3.3.4. Temperatures

General information on seasonal air temperatures is given in section 2.1. Wet and· dry bulb temperatures have been recorded twice daily at South Nobby since early 1969, but such detailed information is not pertinent to this exercise.

3.3.5. Atmospheric Pressure

Barometer readings corrected to mean sea level have been taken twice daily at South Nobby since early 1969, but are of no interest to this study. Pressure changes during cyclone passages may be important, and have been dis­cussed in section 3.3.3.

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3.3.6. Stream Discharge

Information about stream discharge in the area has come from:

a. Queensland Irrigation and Water Supply Commission{ in the case of Nerang River{ Tallebudgera and Currumbin Creeks{ and

b. New South Wales Water Conservation and Irrigation Commission{ in 'the case of Tweed River.

The total annua I discharge is extremely variable{ as shown on figure 9. In the case of the Nerang River { the variation is between 25 x ]03 and 400 x 103 acre feet{ while Tallebudgera Creek varies between 5 x ]03 and 100 x 103 and Currumbin Creek between 5 x ]03 and 60 x 103 acre feet per year over the approximately 50 years for which records are available. The available records for the Tweed River cover on Iy a few years{ but exhibit a similar large variation.

The distribution of high annual discharges is also extremely erratic. Figure 9 shows for example that between 1919 and 1967 annual totals greater than or equal to 200 x 103 acre feet in the Nerang River have occurred in 1927{ 1928{ 1931{ 1947{ 1950{ 1954{ 1955{ 1956{ 1959{ 1963{ 1967.

The monthly variation of peak discharges is also extremely variable. Table 5 gives the distribution of total monthly discharges for the Nerang River.

TABLE 5 Monthly Discharge Analysis - Nerang River 1918 - 1968

Number of times discharge occurs in month of

Discharge Oct Nov Dec Jan Feb Mar Apr May June July Aug Sep

o - 9.99 45 43 35 29 30 23 30 40 37 43 47 49 10 - 19.99 2 3 7 5 4 5 11 2 5 1 2 1 20 - 29.99 1 0 5 6 4 4 21 1 1 3 1 -30 - 39.99 1 1 1 1 1 4 2 1 3 0 - -40 - 49.99 - 1 1 2 2 3 3 3 1 0 - -50 - 59.99 - - - 3 3 4 0 3 0 0 - -60 - 69.99 - - - 1 0 2 1 - 1 1 - -70 - 79.99 - - - 1 1 2 1 - 0 1 - -80 - 89.99 - - - 0 0 1 - - 1 0 - -90 - 99.99 - - - 0 3 1 - - 0 1 - -

100 -109.99 - - - 0 0 0 - - 1 - - -110 -119.99 - - - 2 0 1 - - - - - -120 -129.99 - - - - 0 - - - - - - -130 -139.99 - - - - 1 - - - - - - -140 -149.99 - - - - 0 - - - - - - -150. -159.99 - - - - 1 - - - - - - -

Discharge in thousands of acre feet (month Iy tota I)

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It shows that the heaviest discharges are likely in the months of January to March and June and July. A detailed analysis of the highest discharges in each month showed that these are usually associated with the presence of a cyclone in the adlacent area.

Daily discharge totals were available for the Nerang River between 1/10/53 and 30/9/54 and from 1/10/65 till 30/9/68. They show that spates are usually of 2 to 5 days duration, with really heavy discharge persisting for 1 day or less.

All the discharge figures quoted above are as measured at gauging stations located some miles upstream from the estuary mouth. The catchment area further downstream will contribute to the discharge, but the effect of high discharges on flushing action in the estuaries is mitigated by the planar, somewhat swampy nature of the terrain traversed by these streams immediately upstream of their estuaries, enabl ing retention of water and attenuation of flood waves to be effected.

3.4. Water Characteristics

The ocean water in the area is generally clear and free of pollution, with a temperature "quite suitable for bathing all year. Following periods of heavy rain discolouration due to sediment carried directly from the land and via the rivers may be observed for a few days, while after periods of heavy seas, seaweed in the surf and on the beaches causes a minor nuisance. In spring of most years algal slicks (tricodesmium) originating on the "Barrier Reef are seen close inshore [7].

Detailed measurements of many water characteristics including chlorinity, density, turbidity, temperature, dissolved oxygen, phosphate and nitrate content etc. were made. These are of primary importance to the proposed Outfa II Sewers, and are dealt with" in a recent Report [8]. "

The area covered by the Omega lines (figure 4) was investigated for near­surface temperatures in January - February 1967, and vertical profiles of tempera­ture variation Were made as well. The results are given in the report of the Bureau of Mineral Resources, Geology and Geophysics [2]. These measurements indicated the presence of slightly cooler (24.5 - 25.0 oc) river water flowing eastward and south-eastward from the Nerang River mouth and south-eastward from the Tweed River mouth. Near the coast, immediately south from Nerang River entrance and north-west from PI. Danger, cooler water masses (23.0 -24.5 oc) were located. The mean near-surface temperature during the measure­ments was 25 oC, and winter" temperatures would be 2 - 3 oC lower.

The temperature variations as observed from the profiles are a maximum of a few degrees Centigrade, and show no evidence of important stratification, the temperature decreasing slowly from surface to bed. The chlorinity and den­sity measurements also did not show any stratification of importance. Only near the mouth of the Nerang River was some larger density difference found, about 0.5 kg/m3, which was restricted"to the upper layer of about 0.6 m.

The conclusion is that there are no density currents which could influence the transport of sediment a long the coast.

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3.5. Tides

3.5.1. General

The tidal cycle obtaining in the area is semi-diurnal with strong inequality. During the course of the investigation leading to this report, tide gauges have been operated at Snapper Rocks, Currumbin Rock, Caloundra and Noosa (Neyrpic bubbler gauges), Tweed River mouth and Broadwater - all the aforementioned being permanent installations - and at Currumbin Creek, Cobaki, Paradise Point, Swan Bay and Dunwich - short period installations (Munro and Ott float gauges). The indications are that the (ocean) tide approaches the coast more or less perpendi­cularly, and thus causes I ittle water movement of importance a long the coast (see figure 9a and section 3.7.2.).

3.5.2. Elaboration of Data

Recorded h igh- and low-water leve Is have been extracted from the records for Snapper Rocks, Tweed River mouth and Broadwater over a ten month period from 24 Nov. 1968 to 28 Sep. 1969. An ana lysis of these levels is shown in table 6 The diurnal inequality is evident with most frequent high waters having levels of + 1. 4 ft and + 2.4 ft approximately.

These levels contain residuals, the most important of which are due to storm surge. Although it has not been possible in the present exercise to perform any study on the residuals themselves, the probability lines of figure 10 contain "ordinary" residuals and thus may be considered as probability of a certain water level being exceeded (or not exceeded in the case of the low water line) rather than probability of a certain tide level being exceeded. Thus figure 10 may be used for design purposes, provided that an allowance is made for "extraordinary" storm surge such as may arise from the passage of a severe cyclone (see section 3.3.3.).

Although the gauges at Snapper Rocks and Tweed River mouth are physica lIy very close, the Tweed River Bar has an effect on the latter, and wave action at the former makes recording of accurate mean level variation with time difficult. Nevertheless the agreement between the gauges as shown in figure 10 is quite reasonable. The bar effects on high-water levels are slight, but the effect on low­water levels is quite marked, especially in the case of Broadwater.

Probabilities of certain tidal ranges for rising and falling tides at Snapper Rocks and Broadwater are I isted in table 7 and are plotted on figure 11. There is some difference in the probabilities for rising tides as compared with falling tides, particularly in the case of the Broadwater tides. Nevertheless the differences are sufficiently small to be neglected and a single line (for each gauge) used.

TABLE 6 Distribution of High- and Low-Water Levels

Snapper Rocks Tweed River Broadwater 0

Cum. 0/0 Cum. '10 Cum. /0 NO Level (ft) N° Cum. Exceedance NO Cum. Exceedance Cum. Exceedance

-

4.51 to 4.75 0 0 0 4.26 to 4.50 4 4 0.76 4.01 to 4.25 1 5 0.95 0 0 0 3.76 to 4.00 4 9 1. 71 0 0 0 2 2 0.35 3.51 to 3.75 9 18 3.41 3 3 0.60 4 6 1.04 3.26 to 3.50 34 52 9.86 23 26 5.16 19 25 4.32 3.01 to 3.25 33 85 16.13 24 50 9.93 24 49 8.48 2.76 to 3.00 41 126 23.90 32 82 16.29 53 102 17.62 2.51 to 2.75 43 169 32.05 23 105 20.83 38 140 24.2 2.26 to 2.50 46 215 40.75 76 181 36.95 74 214 37.0 2.01 to 2.25 35 250 47.40 44 225 44.70 41 255 45.8 1. 76 to 2.00 54 304 57.6 46 271 53.8 54 30~ 53.5

I 1.51 to 1.75 33 337 64.0 35 306 60.3 34 343 59.4 I

'" 1. 26 to 1.50 60 397 75.3 76 382 75.9 85 r--r- 428 577 14.0 100 I ~

I 1. 01 to 1.25 33 430 81.6 43 425 84.5 46 1 474 576 82.0 99.83 0.76 to 1:00 5L 487 92.5 407 465 510 92.4 100 59 2 533 575 92.25 99.65 0.51 to 0.75 22 1 509 524 96.59 100 21 2 486 509 96.50 99.80 21 3 554 573 95.85 99.31 0.26 to 0.50 13 5 522 523 99.05 99.81 12 7 498 507 99.06 99.41 17 16 571 570 98.89 98.88 0 to +0.25 4 4 526 518 99.81 98.86 4 3 502 500 99.80 98.04 5 35 576 554 99.65 96.0 0 to -0.25 ~21 I 527 514 100 98.09 o 16 502 497 99.80 97.50 2 60 578 519 100 90.0 -0.26 to -0.50 34 493 94.09 o 61 502 481 99.80 94.40 153 459 79.5 -0.51 to -0.75 32 459 87.5 ~44 I 503 420 100 82.4 126 306 53.0 -0.76 to -1. 00 61 427 81. 5 91 376 73.7 125 180 31.2 -1.01 to -l.25 49 366 69.8 59 285 55.9 43 55 9.53 -l.26 to -l.50 86 317 60.5 112 22p 44.3 12 12 2.08 -1.51 to -1.75 61 231 42.1 49 114 22.35 0 0 0 -1.76 to -2.00 94 170 33.4 52 65 12.75 -2.01 to -2,.25 36 76 14.69 6 13 2.55 -2.26 to -2.50 30 40 7.63 6 7 1. 37 -2.51 to -2.75 5 10 1. 91 1 1 0.20 -2.76 to -3.00 4 5 0.95 0 0 0 -3.01 to -3.25 1 1 0.19 Note~ Readings below heavy -3.26 to -3.50 0 0 0 dividing are low water levels,

those above high water leve Is.

-22-

TABLE 7 Distribution of Tidal Ranges

(a) Snapper Rocks

Rising Tide (L.W.-H.W.) Falling Tide (H.W._L.W.)

Range (ft) No Cum 0 ;0 Exceedance No Cum /to Exceedance

6.51 - 7.00 0 0 0 0 0 0 6.01 - 6.50 2 2 0.38 o. 0 0 5.51 - 6.00 14 16 3.06 17 17 3.26 5.01 - 5.50 41 57 10.91 33 50 9.60 4.51 - 5.00 48 105 20.1 47 97 18.6 4.01 - 4.50 55 160 30.6 59 156 29.9 3.51 - 4.00 68 228 43.6 72 228 43.65 3.01 - 3.50 78 306 58.6 79 307 58.9 2.51 - 3.00 76 382 73.1 79 386 74.1 2.01 - 2.50 76 458 89.6 69 455 87.3 1. 51 - 2.00 36 494 94.5 44 499 95.8 1.01 - 1.50 19 513 98.28 18 517 99.23 0.51 - 1.00 8 521 99.81 4 521 100 0 - 0.50 1 522 100 0 521 100

(b) Broadwater

Rising Tide (L.W.-+H.W.) Falling Tide (H.W._L.W.)

Range (ft) No Cum 0 ;0 Exceedance No Cum /to Exceedance

4.51 - 5.00 0 0 0 0 0 0 4.01 - 4.50 28 28 4.86 0 0 0 3.51 - 4.00 63 91 15.81 35 35 6.06 3.01 - 3.50 70 161 28.0 101 136 23.6 2.51 - 3.00 81 242 42.1 118 254 44.0 2.01 - 2.50 101 343 59.6 131 385 66.7 1.51 - 2.00 97 440 76.5 119 504 87.4 1.01 - 1.50 105 545 93.0 60 564 97.75 0.51 - 1.00 29 574 99.83 13 577 100 0 - 0.50 1 575 100 0 100

-23-

From figures 10 and 11 and from other calculations based on the classical method of tidal analysis, table 8 is constructed.

TABLE 8 Tidal Characteristics

Snapper Rocks Tweed River Broadwater Mouth

Median level of all high waters + 2.0 + 1. 9 + 1. 9 Median level of all low waters - 1.6 - 1.0 - 0.5 Median tidal range 3.4 feet - 2.4 feet Mean High Water Spring Tide Level + 2.47 - + 2.25 Mean Low Water Spring Tide Level - 2.21 - - 0.77

Note~ Levels are in feet to State Datum

3.6. Waves

3.6.1. General

Besides human interference in the natural conditions at the Gold Coast, the wave climate is certainly the most important factor determining the shape of the coast and the rate of erosion or accretion. North-easterly to south­easterly winds at sea have practically unlimited fetches to generate waves travelling unobstructedly to the Gold Coast (see figure 12).

Information about waves has been obtained from:

a. Cape Moreton observations from 1957 till 1968. b. Waverider measurements since August 1968. c. Observations of Dutch ocean-going vessels from 1961 till 1967. d. Sea and Swell charts of the Pacific Ocean. e. Cape Byron observations from 1960 till 1968. f. South Nobby observations.

Together this information gives a reasonably good picture of the phenomena of sea and swell along the Gold Coast. The data are sufficient to serve as a base for considerations about the beach erosion problems and some computations of the I ittora I transports of sediments.

-24-

3,6,2. Data and Elaborations

a. Cape Moreton Observations

State of sea: Since 1957 visual observations of the height of sea at Cape Moreton-'Cire-available. Table 9 shows the frequencies of exceedance of a certain sea height per month. Figure 13 indicates the average annual values. The period of the observations is long enough to avoid significant errors, e.g. figures 14 -25 show that there is no basic difference between the averages over the periods 1957 - 65 and 1957 - 68, despite the inclusion in the latter period of 1967, a relatively rough year. It may be assumed that the percentages of exceedance derived from these observations are a good long-term average for sea wave heights.

Swell: Since 1957 swell observations from Cape Moreton are available also. Fi'9L;;:es 26 - 28 give the results of these swell observations. According to these figures the net annual swell is from direction 1060 , and the variation in direction throughout the year is shown in figure 29.

b. Waverider measurements (see also section 3.6.4.)

Since August 1968 Waveriders have been in operation at the locations shown on figure 3. Every day from the 0900 hrs analogue chart record of one buoy, usually the one at location 2, the significant wave height Hs and the wave period T z (zero-up-crossing period) are calculated by hand, using the method of Tucker [9]. These records are usually 20 minutes long.

S:J!,l1'p"a!i~,:r:_~£t~_~<£~_0..?!~~~1 The sea height, visually observed at Cape Moreton, is compared to the measured Hs of the Waverider, which is the resultant significant height of sea and swell. Figures 14 - 25 give the ex­ceedance graphs of 0900 hrs Cape Moreton observat ions and 0900 hrs Waverider measurements from August 1968 ti II November 1969. Comparing these exceedance graphs the impression is that the visually observed sea heights are probably slight­ly underestimated at the lower heights (3 ft) and overestimated by a larger pro­portion at the higher heights (if the Cape Moreton observations are considered to be observed at location 2 of the Waverider). It must be kept in mind that the waves at Cape Moreton are observed from the shore and are influenced by refrac­tion and other shallow water effects. The rates at which the wave heights at the Waverider location 2 are changed by refraction and shallow water are indicated in table 10. In this table the ratios of the wave heights H at locations 2 and 3 and the deep water wave heights Ho are given with respect to wave periods and direction&. The table has been derived from the refraction diagrams which are described in section 3.6.5. of this report. 'Table 11 lists the H/Ho ratios for the other Waverider location.

-25-

TABLE 9 Cape Moreton - State of Sea Percentage Exceedance Years 1957 - 68 . H ~h (ft) Time Jat) Feb Mar Apr May June July Aug Sep Oct Nov Dec

0 0900 100 100 100 100 100 100 100 100 100 100 100 100 1500 100 100 100 100 100 100 100 100 100 100 100 100 Mean 100 100 100 100 100 100 100 100 100 100 100 100

0.5 0900 99.3 100 100 99.7 100 100 100 100 100 100 100 100 1500 100 100 100 100 100 100 100 100 100 100 100 100 Mean 99.6 100 100 99.9 100 100 100 100 100 100 100 100

1.0 0900 97.7 97.4 98.7 99.5 99.0 9.9.7 98.6 96.4 97.4 98.1 98.3 97.6 1500 98.9 100 100 98. 1 97.5 98.2 99.3 96.8 99.0 100 98.9 100 Mean 98.3 98.7 99.3 98.8 98.2 99.0 99.0 96.6 98.2 99.0 98.6 98.2

2.0 0900 70.5 66.7 75.0 76.4 70.0 72.3 64.1 58.0 63.8 69.6 66.9 67.2 1500 76.3 72.3 75.6 73.3 65.6 70.8 60. 1 57.7 62.3 74.9 75.6 70.0 Mean 73.4 69.5 75.3 74.8 67.8 71.5 62.1 57.9 63.0 72.2 71.2 68.6

4.0 0900 28.3 24.2 32.5 31.1 23.2 24.7 19.4 18.8 19. 1 20.4 19.4 19. 1 1500 28.3 20.4 29.7 32.2 23.2 27.8 16.9 16.5 16.7 19.0 18.2 16.2 Mean 28.3 22.3 31.1 31. 7 23.2 26.2 18. 1 17.6 17.9 19.6 18.8 17.6

8.0 0900 3.7 5.9 5.1 7.8 3.0 5.9 0.8 1.1 0.8 2.4 2.2 2.2 1500 6.1 5.0 5.7 8. 1 3.2 8.5 2.2 1.1 1.1 2.2 3.0 2.9 Mean 4.9 5.4 5.4 8.0 3. 1 7.2 1.5 1.1 1.1 2.3 2.6 2.5

13.0 0900 0.3 0 0.5 0.6 0.3 0.6 0.3 0 0 0 0 0 1500 0.4 0 0 1.1 0 1.1 0.4 0 0 0 0 0 Mean 0.3 0 0.3 0.8 0.2 0.8 0.3 0 0 0 0 0

20.0 0900 0 0 0 0 0 0.6 0 0 0 0 0 0 1500 0 0 0 0.4 0 0.7 0 0 0 0 I 0 0 Mean 0 0 0 0.2 0 0.6 0 0 0 0 0 0

TABLE.lO H/HQ at Locations 2 and 3 of Waverider

Period in secs

Direction 6 8 10 12

NE 1 0.98 0.94 -E 1 0.98 0.94 0.92 SE 1 0.90 0.87 0.76 .

-26-

TABLE 11 H/Ho at Location 1 of Waverider

Period in secs

Direction 6 8 10 12

NE 1 0,99 0.95 ' -E 1 0.99 0;95 0.92 SE 1 0.83 0.81 0.63

The swell heights and periods observed at Cape Moreton are indicated as respectively low, moderate or heavy and short, medium or long. Using the avai­lable 0900 hrs Waverider results from August 1968 till November 1969 (figures 14 - 25, showing exceedance graphs of wave heights,and figures 30 - 32, showing exceedance graphs of wave periods Tz) and comparing these results with the results of the observations of swell heights and periods at Cape Moreton (see table 12) covering also the period from August 1968 till November 1969, approximate scales can be determined for the expressions low, moderate, heavy and short, medium, long.

Table 13 gives the compared readings from figures 30 - 32 (waverider periods T z) and table 12 (swell period indications).

TABLE 12 Percentage Occurrence of Swell Heights and Periods - Cape Moreton 0900 hrs

Swell Height Swell Period

Month Low Moderate . Heavy Short Medium Long

Aug '68 77.4 22.6 - 19.3 74.2 6.5 Sept 73.4 26.6 - 23.3 60.0 16.7 Oct 96.7 3.2 - 3.2 74.2 22.6 Nov 56.7 36.7 - 36.7 53.3 3,3 Dec 64.6 12.9 - 12.9 64.6 -Jan '69 80.7 3.2 - - 80.7 3.2 Feb 35.7 60.7 - 42.9 46.3 7.1 Mar 67.7 25.8 - 19.4 71.0 3.2 Apr 66.7 33.3 - 23.4 46.6 30.0 May 38.8 58.1 3.2 38.7 54.9 6.5 June 83.3 16.7 - 16.7 46.7 36.7 July 61.3 38.8 - 35.5 29.0 35.5 Aug 87.1 9.6 - 3.2 58.0 35.5 Sep 90.0 6.6 - 3.3 56.7 36.6 Oct ·67.7 25.9 3.2 25.9 45.2 25.8 Nov '69 60.0 36.7 - 33.4 10.0 53.3

-27-

TABLE 13 Boundaries between Short and Medium: Medium and Long Periods (in seconds)

Month Short-Medium Medium-Long

Aug '68 5.4 11. 9 Sep 5.45 8.4 Oct 3.3 7.55 Nov 6.1 8.75 Dec 6.3 -Jan '69 5.2 8.35 Feb 6.65 8.75 Mar 6.55 8.35 Apr 6.35 7.3 May 6,55

.

8.3 June 6.4 8.5 July 6.95 7.85 Aug 6.5 8.5 Sep. 5.5 8.7 Oct 6.2 7.45 Nov '69 7.3 7.6

Average 6.04 8.43 Standard 0.91 1.04 Deviation

Thus short periods may be considered to be less than 6 seconds, medium periods between 6 and 8.5 seconds and long periods more than 8.5 seconds. In table 14 the mean periods of swell within these boundaries are derived from the Waverider analysis sheets. Between brackets the number of times is given that in the month a T z is recorded at 0900 hrs in the appropriate range, and the agreement in overa II percentage of occurrence is reasonable ..

-28-

TABLE 14 Mean Values within Period Ranges

Month Mean of periods Mean of periods Mean of periods less than 6 secs. from 6 to 8. 5 secs. greater than 8.5 secs.

Aug '68 4.9 (6) 7.2 (11) 10.9 (7) Sep 5.2 (8) 7.4 (18) 10.2 (4) Oct 4.8 (7) 7.1 (17) 9.8 (2) ·Nov 5.1 (10) 6.8 (19) 9.2 (1) Dec 4.8 (5) 7.0 (17) 10.3 (1) Jan '69 5.2 (4) 7.0 (11) -Feb 5.1 (4) 6.9 (22) 9.5 (2) Mar 5.7 (4) 7.0 (27) -Apr 5.5 (4) 7.1 (25) 9.2 (1) May 5.8 (3) 7.2 (26) 9. 1 (1) June 5.7 (2) 7.0 (17) 9.1 (11) July 5.8 (3) 7.3 (33) 9. 1 (5) Aug 5.6 (1) 7.4 (19) 9.1 (11) Sep 5,6 (4) 7.2 (13) 9.3 (13) Oct 5.6 (3) 7.0 (27) 9,8 (1) Nov '69 - 7.2 (20) 9.7 (10)

weighted mean 5.25 7.1 9.55

number occurring 68 312 70

percentage occurring 15. 1 69.4 15.5

'Yo occurring same period at short medium long C. Moreton 21. 1 54.5 20.2

-29-

Table 15 gives the compared readings from figures 14 - 25 (Waverider Hs) and table 12 (swell height indications)

TABLE 15 Boundaries between Low and Moderate; Moderate and Heavy Swell (in ft)

Month Low-Moderate Moderate-Heavy

Aug '68 4.22 -Sep 3.57 -Oct 6.72 -Nov 3.37 -Dec 4.52 -Jan '69 4.04 -Feb 4.36 -Mar 5.01 -Apr 4.16 -May 2.91 8. 13 June 5.10 -July 4.32 -Aug 7.43 -Sep 5.00 -Oct 4.50 6.35 Nov '69 4.86 -Average 4.63 7.24

Standard deviation 1. 10 0.89

Thus low and heavy swell may be considered to have associated significant wave heights of respectively less than 4.6 ft, and more than 7.2 ft, with moderate swell between these boundaries.

In table 16 the average values of the significant wave heights within these ranges are mentioned in a simi lor fashion to that followed for the pe­riods in table 14.

-30-

TABLE 16 Mean Values within Height Ranges (in ft)

Mean of Hs Mean of H between Mean of Hs greater Month less than 4.63 ft 4.63 and 1.24 ft than 7.24 ft

Aug '68 2.99 (21) 5.60 (3) -Sep 2.88 (28) 4.84 (1) -Oct 2.62 (24) 5.66 (2) -Nov 2.98 (28) 5.00 (2) -Dec 2.96 (27) 5.40 (3) -Jon '69 2.74 (31) - -Feb 3.61 (12) 6.09 (10) 8.34 (6) Mar 4.05 (16) 4.96 (13) 8.02 (2) Apr 3.04 (27) 6.08 (2) 8.98 (1) May 2.69 (18) 5.77 (10) 8.36 (3) June 3. 14 (21) 5.20 (17) 7.66 (2) July 3.13 (19) 5.43 (12) -Aug 3.38 (22) 5.87 (5) 9.04 (4) Sep 2.97 (28) 5.67 (2) -Oct 2.99 (24) 5.54 (7) -Nov '69 3.25 (17) 5.56 (13) -weighted mean 3.04 5.51 8.42

number occurring 363 92 18

0/0 occurri(1g 76.9 19.5 3.8

~o occurring low moderate heavy for some period C. Moreton 69.2 26.1 0.4

Now from the overage of the 0900 hrs and the 1500 hrs annual overage values of swell height and period (figure 28) and using the results from tables 13 - 16, table 17 may be constructed.

c. K. N. M.I. Observations

Dutch ocean-going vessels have observed waves in an area in front of the Queensland coast between 25

0 and 300 S and between 1550 E and the coast from

1961 till 1967. The information is collected by the Royal Dutch Meteorological Institute (K. N.M.I.). The tables 18 and 19 give the period, the height and the frequency of occurrence of respectively swell and sea from north-easterly, easterly and: south-easterly directions.

TABLE 17 Directions, Heights, Periods and Frequencies of Occurrence of Swell (according to Cape Moreton observations; elaborated with some Waverider results)

Swell Height in ft Swell Period in seconds Percentage of time occurrina from

Observed Range Hs Average Hs Observed Range Tz Average Tz NE E SE

Low < 4.6 3.0 short <6 5.2 0 0 0 Low < 4.6 3.0 medium 6-8.5 7.1 3.95 21.6 14.15 Low < 4.6 3.0 long >8.5 9.6 0.65 15.75 9.6

Moderate 4.6-7.2 5.5 short <6 5.2 0.7 12.0 12.7 Moderate 4.-6-7.2 5.5 medium 6-8.5 7.1 0.1 1.9 3.4 Moderate 4.6-7.2 5.5 long >8.5 9.6 0 0.55 0.6

Heavy > 7.2 8.4 short <6 5.2 0.05 0.35 0.3 Heavy > 7.2 8.4 medium 6-8.5 7.1 0 0.1 0.05 Heavy > 7.2 8.4 long :;. 8.5 9.6 0 0.05 0.05

total 5.45 52.30 40.85 _._-- --- -_.-

1) Other directions Or calms

1)

1.40

-I

I W

-32-

TABLE 18 Swell Occurrence according to K N M 1 Observations .. Period in Significant Percentages of time occurring from

seconds Height in ft Nt t:. SE

..,;5 0.8 - - 0.05 1.65 0.95 0.80 0.96 3.28 1.28 2.15 2.16 4.90 0.66 1.25 1.25 6.55 0.28 0.18 0.33 8.20 0.18 - 0.09 9.85 - - 0.05

6 and 7 0.8 - 0.05 -1.65 0.18 0.83 0.52· 3.28 2.42 5.45 4.08 4.90 2.37 2.32 4.80 6.55 0.33 1.40 1. 73 8.20 0.04 0.27 0.43 9.85 - - 0.09

13.1 - - 0.05 8 and 9 1.65 0.05 0.33 0.39

3.28 0.66 1. 12 1.20 4.90 0.38 1. 96 2.07 6.55 0.57 1.07 2.22 8.20 0.05 0.23 0.87 9.85 - - 0.24

11.5 - 0.09 0.15 14.8 - - 0.05 24.6 - - 0.05

10 and 11 1.65 0.19 0.05 0.05 ·3.28 0.15 0.27 0.48 4.90 0.09 0.23 0.97 6.55· 0.15 0.51 0.63 8.20 - 0.33 0.81 9.85 - 0.19 0.52

11.5 - - 0.10 13.1 - 0.09 0.15 14.8 - 0.05 -16.4 - - 0.05 24.6 - - 0.05 26.2 - - 0.05

12 and 13 1.65 - - 0.05 3.28 - 0.14 0.05 4.90 - - 0.05 6.55 - 0.05 0.14 8.20 - 0.05 0.14 9.85 - 0.15 0.05

11.5 - 0.05 0.34 13.1 - 0.09 0.14 21.3 - 0.05 -

14 and 15 0.8 - - 0.06 3.28. - 0.05 -9.85 - - 0.05

11.5 - - 0.05 28 - - 0.05

16 and 17 0.8 - - 0.05 16.4 - - 0.10 18 - 0.05 -19.7 - 0.05 -

20 and 21 11.5 - - 0.05

. T ota 1 percentage 11 22 29

Number of observations 233 466 615

Number of observations of other directions and calms: 806 (= 38'}'0)

Totol number of swell observations : 2120 (= 100'}'0)

-33-

TABLE 19 Sea Occurrence according to K.N.M I. Observations

Period in Significant Percentages of time occurring from seconds Height in ft NE E SE

(:5 0.8 2.28 3.45 1.65 1.65 4.85 6.55 4.90 3.28 2.59 4.15 5.14 4.90 0.58 0.68 2.04 6.55 0.19 0.08 0.36 8.20 - . - 0.17

6 and 7 0.8 - - -1.65 0.03 0.08 0.08 3.28 0.16 0.48 1. 16 4.90 0.24 0.92 2.17 6.55 0.03 0.24 0.57 8.20 - 0.16 0.17 9.85 - - 0.16

13.1 - - 0.04 8 and 9 1.65 - - -

3.28 - - 0.05 4.90 - 0.08 0.04 6.55 - 0.08 0.08 8.20 - - 0.21 9.85 - 0.03 0.24

11.5 - - 0.12 13. 1 - - O. 12

10 and 11 6.55 - 0.03 0.16 9.85 - - 0.08

11. 5 - - 0.04 14.8 - - 0.08 29.5 - - 0.04

~12 1.65 0.03 - -Tota I percentage 11 17 20

Number of observations 280 433 510

Number of observations of other directions and calms: 1321 (= 52<J'0

Total number of sea observations : 2544 (= 100<J'0)

d, Sea and Swell charts

Information obtained from the American atlas of Sea and Swell charts [1( about the relevant area near the Gold Coast is given in figure 33, which is representative for the months of March-May, and in table 20 which gives similar information for the other months,

-34-

TABLE 20 Wave Information from American Sea and Swell Charts

June - August Sea results from 180 observations of which

0 calms. 3;1,0 were

Swell results from 170 observations of which 8;0 were calms.

Direction Sea Swell (from)

NW 8/,0 predominantly moderate, -W 10~0 predominantly moderate, 7~0 predominantly moderate. SW 23/,0 of which 29~0 low and 54~0 mod, 19~0 of which 18~0 low and 330/0 mod. S 19<yo of which 31<yo low and 49~0 mod, 32~0 of which 19~0 low and 430/0 mod. SE 23<yo of which 190/0 low and 67~0 mod, 21/,0 of which 17/,0 low and 630/0 mod.

September - November 0

Sea results from 169 observations of which 2Zo were calms. Swell results from 151 observations of which 6/,0 were calms.

Direction Sea Swell (from)

N 11~0 predominantly moderate, - , NW 10<Yo predominantly moderate, 7~0 predominantly moderate. SW - 13~0 predominantly low. S 22~0 of which 19~0 low and 70~0 mod, 28/,0 of which 30~0 low and low 53~0 mod, SE 23/,0 of which 33~0 low and 62~0 mod, 25~0 of which 27cro low and low 59~0 mod, E 10<yo predominantly low, 7/,0 predominantly moderate. NE 10<yo predominantly low, -

December - .February Sea results from 136 observations of which 0

6Zo were calms. Swell results from 129 observations of which 13/,0 were calms.

Direction Sea Swell (from)

N 10/,0 predominantly moderate, -SW - 7;(0 predominantly low. S 11;{,opredominantly moderate, 16Zo of which 45~0 low and 35cro mod. SE 29;{,0 of which 26;{,0 low and 67~0 mod, 266,0 of wh ich 47/,0 low and 47~0 mod. E 16;0 of which 64;0 low and 36<yo mod, 17;0 of which 59<yo low and 23<yo mod. NE 15cro of which 40~0 low and 55~0 mod, 10~0 predominantly low.

-35-

e. Cape Byron Observations

State of Sea: Sea height observation at Cape Byron are available from 1960 until 1961Ci'he-sea height percentage exceedance at 0900 and 1500 hrs are given in table 21, and the 0900 hrs values are plotted on figures 14 - 25. The average annual curve isshown on figure 13.

TABLE 21 Cape Byron - State of Sea Percentage Exceedance Years 1960 - 1968 • . H Month

(ft) Time Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec

0 0900 100 100 100 100 100 100 100 100 100 100 100 100 1500 100 100 100 100 100 100 100 100 100 100 100 100 Mean 100 100 100 100 100 100 100 100 100 100 100 100

0.5 0900 99.7 99.7 99.6 99.6 99.7 99.6 99.6 100 99~6 100 99.6 100 1500 99.7 98.9 98.1 98.1 99.0 98.1 98.2 100 98.9 99.6 99.7 99.6 Mean 99.7 99.3 98.8 98.8 99.4 98.8 98.9 100 99.2 99.8 99.6 99.8

1.0 0900 79.6 75.8 77.4 72.1 71.4 72.9 72.7 65.0 71.5 82~ 7 78.9 83.5 1500 87.5 81.6 82. 1 80.7 73.1 73.7 77.1 69.2 80.8 88.8 88.2 88.3 Mean 83.6 78.7 79.8 76.4 72.2 73.3 74.9 67.1 76.2 84.8 83.6 85.9

2.0 0900 62.8 55.4 59.1 49.1 48.1 52.2 46.2 37.8 43.4 44.4 45.6 50.8 1500 68.1 58.5 64.2 58.5 53.7 57.7 47.7 43.9 55.2 55.2 59.3 64.1 Mean 65.4 57.0 61. 6 53.8 50.9 54.8 47.0 40.8 49.3 49.8 52.4 57.4

4.0 0900 18.0 23.2 22.2 20. 1 15.5 26.3 13.2 6.5 10.1 8.5 10.4 16.9 1500 21.9 29.9 27.3 24.1 15.2 25.2 14.7 11. 6 15.2 16.4 16.0 25.8 Mean 20.0 26.6 24.8 22.1 15.4 25.8 14.0 9.0 12.6 12.4 13.2 21.4

8.0 0900 3.3 6.7 7.9 6.0 4.7 10.4 3.2 1.0 2.0 2.0 1.9 2.4 1500 5.4 8.3 10.5 9.3 3.7 10.8 3.6 1.9 2.2 1 .. 7 3.4 2.8 Mean 4.4 7.5 9.2 7.6 4.2 10.6 3.4 1.4 2.1 1.8 2.6 2.6

13.0 0900 0.8 1.2 2.5 1.5 1.5 2.6 1.4 0.5 0 0 0 0.4 1500 1.1 1.6 3.0 1.9 0.8 3.4 1.4 0.5.

0

0 0 0.4 0.4 Mean 1.0 1.4 2.8 1.7 1.2 3.0 1.4 0.51 0 0 0.2 0.4

20.0 0900 0.4 0.4 0 0 0.8 0 0.7 0 0 0 0 0 1500 0.7 0 0 0 0.8 0.4 0.7 0 0 0 0 0 Mean 0.6 0.2 0 0 0.8 0.1 0.7 0 0 0 0 0

-36-

Assuming the sea height climates to be similar at Cape Moreton and Cape Byron, the impression is gained from figures 14 - 25 that the observa­tions at Cape Byron further underestimate the lower wave heights and over­estimate the higher heights. However, the average annual values of excee­dance (figure 13) compare quite well.

Swell: The average monthly and annual swell height and period distribu­tions are-;hown on figures 34 - 36. They differ from the Cape Moreton distri­butions (figures 26 - 28) chiefly' by exhibiting increased percentage from the easterly direction. Figure 37 illustrates the differences, but shows that the trends are similar. However, when the average net direction per month is com­puted, it is seen that Cape Moreton and Cape Byron give practically identical results (see figure 29), and it is concluded that both sets of observations are "valid", and that the differences in percentages from the same direction are due to refraction and shallow water effects. It may be assumed that the scales determined for the Cape Moretonswell height and period classifications apply also at Cape Byron.

f. South Nobby Observations

Visual observations of sea height and swell have been made daily at South Nobby since February 1969. The sea height information is plotted on figures 14 - 25. However, the available period is too short to enable compa­risons to be made with other data, or conclusions to be drawn.

3.6.3. Comparison and Discussion

In tables22 and 23 swell observations of Cape Moreton, the American Atlas, the K. N. M. I. and Cape Byron are compared. K. N. M.1. and Atlas ob­served wave heights and directions at open sea in deep water. The observed wave heights and directions at Cape Moreton and Cape Byron are influenced by refrac­tion and shallow water. Therefore, it is difficult to compare the Cape Moreton, Cape Byron observations to the K. N. M.1. and Atlas observations. The scales of

. low, moderate, heavy, short, medium and long swell, determined by comparing Waverider measurements and Cape Moreton observations, are applied to Cape Moreton, Cape Byron and Atlas observations.

K. N.M.I. and Atlas give rather great percentages of time that swell origi­nates from other than north-easterly, easterly and sO\lth-easterly directions. Of course these other swell directions do not occur at the shore stations Cape Moreton and Cape Byron. It is believed that these other directions are noticed at the shore as low swell coming from easterly and south-easterly directions, thus increasing the frequencies of occurrence at Cape Moreton and Cape Byron for the lower wave heights especially from the east and south-east.

TABLE 22 Comparison of Swell Observations

Swell Percentage of Time Occurring from

Height (Hs) :eriod (~) NE E in feet In secOn C. NIoreton KNMI Atlas C. Byron C. Moreton KNMI Atlas C. Byron

<: 4.6 <6 0 2.23

~ 2

0 0 2.95 ~ 3.7

0 <:4.6 6-8.5 3.95 2.96 4.50 21.60 7.06 4.50 <4.6 >8 .. 5 0.65 0.70 4.90 15.75 1. 24 14.00

4.6-7.2 <6 0.70 0.94

~ 0

3.65 12.00 1.43 t 4.8 5.65

4.6-7.2 6-8.5 0.10 3.18 1.30 1. 90 5.24 3.15 4.6-7.2 >8.5 0 0.72 0.35 0.55 2.30 1.35

> 7.2 <6 0.05 0.18

~ 0

0.50 0.35 0 ~ 2.1

0.95 >7.2 6-8.5 0 0.06 0.35 0.10 0.43 0.20 >7.2 >8.5 0 0.02 0 0.05 1.31 0.25

Totals 5.45 10.99 2 15.55 52.30 21. 96 10.60 30.05

Other Directions and calm: - Cape Moreton - 1.40~o K.N.M.1. - 38 /,0 Atlas - 57.7 ~o Cape Byron - 4.4 /,0

TABLE 23 Mean Direction of Predominant Swell

Cape 1v\oreton KNMI Atlas Cope Byron

NE: 45 x 5.45 ; 245 45 x 10.99; 495 45 x 2 ; 90 45 x 15.55 ; 700 E· 90 x 52.30 ; 4700 90 x 21.96 ; 1975 90 x 10.6 ; 954 90 x 30.05 ; 2700 SE: 135 x 40.85 ; 5510 135 x 29.08 ; 3930 135 x 29.7 ; 4015 135 x 50.0 ; 6750

98. 60 + 10455 62.03 • 6400 42.30. 5059 95.60 + 10150

1060 1030 1200 1060

-- -----

C. Moreton KNMI

0 3.17 14.15 5.40 9.60 1.54

12.70 1.58 3.40 8.68 0.60 3.94 0.30 0.14 0.05 1.25 0.05 3.38

40.85 29.08

SE Atlas

i 7.6

~ 16.2

~ 5.9

29.70

C. Byron I

0 4.80

21.50 12.25 7.40 2.05 1.15 0.45 0.40

50.00

I W '-l I

-38-

As sea is observed at Cape Moreton and Cape Byron without consideration of directions, these sea observations are compared to the K. N. M. I. observations from all directions on the basis of the frequencies of exceedance in figure 13. In table 24 the K. N. M.1. and Atlas observations are compared directly, using again the determined scales for low, moderate and heavy sea. Only the K. N. M.1. observations provides periods, so as for as.periods are concerned no comparison can be made. '

TABLE 24 Comparison of Sea Observations

Percentages of time occurring from

Height in NE E SE ft KNMr Atlas KNMI· Atlas KNMI Atlas

low < 4.6 9.94 4.0 14.71 6.5 12.98 7.4 moderate 4.6-7.2 1.04 2.1 2. 11 4.3 5.42 17.2 heavy > 7.2 0 0.2 0.19 0.2 1.47 3.2

1-1 - 6.3 17 11.0 20 27.7

K.N.M.I.: other directions and calms '" 520/0. Atlas: other directions. '" 520;0; co I ms '" 30;0.

"

There is a close agreement between the frequencies of occurrence of wind directions (taole 1) and the K. N. M. I. sea directions. Considerable differences occur between K.N.M.I. and Atlas observations. K.N.M.1. observes more frequently low seas. The Atlas gives more frequently moderate and heavy seas.

A calculation of sea height from wind velocities has not been made. As sea originates from local winds and little is known about fetches during periods with local winds, a check of the figures of tabre 24 with the aid of calculations of sea heights derived from wind velocities is thought to be not very meaningful.

3.6.4. Waverider Information - Detailed Analyses

All wave information considered in sections 3.6.1. - 3.6.3. gives some insight into the wave phenomena. The information is useful in considering general problems. For instance the information can serve as a base for littoral transport considerations and computations, but for determining more precise design wave heights and their frequencies of occurrence for structures, more detailed know­ledge is desirable. This kind of wave information must be derived from the results of the Waverider measurements

Waverider buoys were sited at locations I, 2 and 3 (see figure 3) and com­menced operation on 8 August 1968. Failure of mooring lines due to natural causes and vandalism resulted in not full service being obtained from all three positions

-39-

all of the time, and in addition noise proved to be troublesome for the frequency of buoy 3. Nevertheless a considerable number of good records was obtained, as shown in table 25.

TABLE 25 Available Waverider Records

Buoy Period Analysed No of reliable records Max;,/ossib~~ ~o 1 8 Aug. 1968 - 16 Dec. 1969 1605 1980 2 8 Aug. 1968 - 20 Sep. 1969 1253 1632 3 8 Aug. 1968 - 17 Mor. 1969 522 884

*) assuming 4 records at 6 hr intervals per buoy per day

The length of digital record from the Preflok recorder analysed was as follows:

8 Aug. 1968 - 1 Nov. 1968 10 min. approximately 1 Nov. 1968 - 20 May 1969 10 - 15 min. approximately

20 May 1969 - 8 Sep. 1969 20 - 25 min. approximately 8 Sep. 1969 - 16 Dec. 1969 25 min. approximately

The analysis procedure was, in brief:

1. The water heights in each record were treated in blocks of 2 minutes length (i.e. 240 water heights at 0.5 sec intervals). Any such block exhibiting an error count > 2'10 of the block length was rejected, and when for the retai­ned blocks in a whole record the error count was> 4'10, the whole record was rejected. Errors may arise from punching mistakes, noise, or tape feed problems.

2. In each valid record, the mean water level per block was calculated, and subtracted from the digitised water heights to give water height above (or below) mean level (I}')' .

3. The number of zero-&ossings in the upward direction (Nz) was counted. 4. The maximum crest height (HA) and the maximum trough depth (HB) were found

in each record, a parabolic interpolation being made between digitized 'l j values to find the true maxima and minima.

5. The number of crests (Nc) was counted. 6. Zero-up-crossing period (T z) was calculated

T = No of valid observations (seconds) z 2 x N

z

(since observations are spaced at 0.5 second intervals)

-40-

7. Crest period (T c) was calculated.

T - No of va I id observations ( d ) c - 2 x N secon s

c

8. Spectral width parameter (E) was calculated

9. E was calculated from

E '" 2 ~ ~ "I j2 l .~ No of valid observations 5

E is analagous to Energy., and may be shown to be theoretically equal to (H )2/8 (ref. [10]).

10. Dr~s·' or root mean square water displacement from mean level was calculated from Tucker's formula given in [11].

D '" 1/2 (HA

- HB) (2ef 1/2 (l + 0.289 e-1 - O.247e-2f 1

rms

where e '" log N e z

11. Significant wave height H was calculated from s

Hs '" (H A - HB) x factor 1.

where the factor I depends on Nz and is from Draper's Table [11].

12. The probable 6 hr maximum wave height H was calculated from max

H '" D x factor 2 max rms

where the factor is calculated from the figure in Draper's paper [IIJ.

Values of e were found between 0.49 and 0.95, and .. although Eo has latterly been shown to be a rather unreliable indicator of spectral width [12J, these values indicate that "sea" conditions previal rather than "swell".

-41-

In order to check how Hs as calculated by the program (Draper's Method) compares with Hs as calculated from E, figure 38 was prepared for Buoy 1 for October 1969. It shows that the Draper Hs tends to be lower than Hs calculated from E on the average, but only by 0.1 - 0.2 foot. Since for "real" ocean waves both methods must be regarded as approximations, there is little to choose between the two estimates and in any case the agreement is quite good. Further, since Hs from Draper's method was the one calculated by the computer, it was decided to use this estimate of the significant wave height. Svasek [12] introduces a term Hu' the energy height of the equivalent simple sine wave, and thus his Hu is in fact (8E)l/2 i.e. Hs calculated from E. ,

The three buoys do not give precisely the same results, nor should they be expected to. However, their agreement is remarkably good as figures 39 - 42 exem­plify.

For each buoy, values of Hs and Tz in 0.5 ft and 0.5. sec .increments res­pectively were extracted month by month and scatter diagrams prepared. Table 26 lists the most commonly occurring (i.e. modal) Hs, Tz and Hs - Tz combination for each month. In some months, more than one value (or combination) occurred equally frequently. As well a median value of Hs and T z was calculated by linear interpolation from the values bracketing: the 50'10 exceedance value. No seasonal variations of real importance are in evidence, and it was thus decided to treat all available information from each buoy as en entity.

Tables 27 - 29 are scatter diagrams for each buoy, and wave significant steepness lines, where significant steepness is defined by

H s Significant Steepness = ------.,..

5.12 (T )2 z

are ·also shown. Table 26 lists the modal and median values, which are seen to vary but little.

The cumulative exceedance of Hs is plotted for each buoy on figure 43. The consistency of the results is very high. The slightly increased percentages of exceedance of buoy 2 compared with buoy 1 would in fact be expected conside­ring the refraction diagrams (see sections 3.6.5. and 3.6.2b.). The small differen­~e which buoy 3 exhibits from the other two buoys is not significant when the shorter period of operation of buoy 3 is considered.

Having in mind the agreement between the buoys, it was concluded that a single scatter diagram and single cumulative exceedance graph representing all valid records of all buoys would be quite meaningful. Accordingly table 30 and figure 44 were prepared.

A similar procedure to that for Hs was carried out for Hmax, with the excep­tion that Hmax - T z scatter diagrams were not prepared, as they have no real meaning. Tables 31 - 34 and figures 43 and 44 show the results SO obtained.

-42-

TABlE 26 Monthly Wove Characteristics from Woverider • == Hs in feet s - T in seconds - z

Modo! Values Median Values

Buoy No ----P 1 2 3 1 2 3

Month H, Tz Hs·Tz H, Tz Hs-Tz H, Tz Hs-Tz H, Tz H, Tz H, Tz comb. comb. comb.

August 1968 4.75 5.75 4.75' } 2.75 5.25 2 75'1 3.25 3,75 3. 25'} 3.50 5.23 3.80 5.25 3.82 5.13 (from 8th) 2.75 5.755 5.25$ 4.25 3.75$

September 1968 - - - 2.75 4.25 2.75' 2.75 4.25 2.75'] 3.27 4.46 3.40 4.47 4.25$ 3.75, - -

October 1968 2.75 3,75 3.50' } - - - - - - 3.15 4.15 - - - -3.25 4.25 4.00$ 3.75

November 1968 2.75 4.25 3.00' 1 - - - 3,75 5.25 4.25'} 3.37 4.63 - - 3.55 4.75 3.25 3.75$ 4.25 5.25$

ond 4.25' 1 2.25 4.75$ ond

325' } 5.25$

December 1968 3,75 5.25 3.25' I 3.25 4.75 325'\ 3.25 4.25 3.25} 3.62 4.90 3.67 4.87 3.54 4.85 4.75$ 4.25$ 4.25$ ond ond

4.25' I 5.25$

3.25'\ 5.25

5

January 1969 3.25 4.50 3.25' ! 3.25 4.25 3.25' I 3.25 4,75 3. 25'} 3.40 4.57 3.20 4.62 3.25 4.69 4.24, 4.25s 4.75$

Febr"",), 1969 4.75 5.25 4.75' l 3.75 5.75 4.75' I 4,75 5,75 4.75'} 4.88 5.42 5.39 5.50 5.69 5.75 5.75$ 5.75$ 5.75

5

March 1969 4.25 5.25 4.25' l 5.25 5.25 5.25'l - - - 4.55 5.30 4.76 5.40 - -4.75, 5.25,

April 1969 3.25 4,75 3.25' I 3.25 4.75 3.25 } - - - 3.53 4.87 3.34 4.90 - -4.75$ 4.75

NK>y 1969 2.25 5.75 5.25' I 2.75 5.75 6.00'} - - - 4.31 5.37 4.63 5.33 - -5.25$ 5.50$ and

6.25' } 5.75$

June 1969 3.75 6.25 3.75' } 3.75 5.75 3.75' ! - - - 3.87 5.61 4.27 5.75 - -6.25$ 4.25$

Jvly 1969 4.25 6.25 4.50' l 5.25 5.75 5.50' ! - - - 4.35 5.78 4.55 5.60 - -5.505 5.50s

August 1969 3.75 5.75 3.75' I 3.25 5.25 3.25' ! - - - 4.29 5.65 4.28 5.49 - -5.75$ 4.75,

ond 4.25'1 4.75$

September 1969 3.25 4.75 2.75' } - - - - - - 3.29 4.97 - - - -4.75,

October 1969 3.25 4.75 3.25' I - - - - - - 3.65 4.97 - - - -4.75,

November 1969 4.25 5.25 4.25' ! - - - - - - 4.73 5.89 - - - -6.25$

ond 5.25' } 5.25$

December 1969 3.25 4.25 3.25' } - - - - - - 3.43 4.22 - - - -(to 16th) 3.75$

Whole Available 3.25 5.25 3.25' I 3.25 5.25 3.25' I 3.25 5.25 3.25:1

3.78 5.11 3.99 5.22 3.66 4.85 Period 4.75$ 4.75, 4.75,

ond 4.75'} 5.25$

Whole Available 3.25 5.25 3.25' I 3.81 5.12 Period all b!Joys 4.75s combined

-43-

TASLE 27 Hs - T7 Scatter Diagram: - Waverider Buoy No 1

P rlod Aug t 1968 D , , "' - ecem be 1969 , B N 1 UOt 0

~ 2.01 2.51 3.01 3.51 4.01 4.51 5.01 5.51 6.01 6.51 7.01. 7.51 8.01 8.51 9.01 9.51 10.01 10.51 Cum. ,/0

1 1 I I I I I I I I I I I I I I I I

H, 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 Exceedonce

0 - 0.50 0.51 - 1.00 r-... 2' r-I-1.01 - 1.50

2 ~ I-} ~ I No of occurrences 100.0

1.51 - 2.00 I ~~- "'II, 55 1-2-~ I 99.7

2.01 - 2.50 5 '20_ 24 r-22, 14 8 7 -1- 3 96.9

2.51 - 3.00 5 I~' '~~ '-~;, 49 h9_ 15 9 6 i'--4-:---.. 89.0

3.01 - 3.50 I II 45 NL 53 h~, 15 6 5 :---. 67.0

3.51 - 4.00 2 24 "42, 50 37 '~;, 6 4 2 ~ 58.50

4.01 - 4.50 10 24 1'-~7 ,,:;~, 29 16 ,10 4 3 0 "1 -- 43.50

4.51 - 5.00 4 14 ~, 35 N~ 18 10, 9 2 1 0 1 32.35

5.01 - 5.50 I 5 :"~.5 20, ,:5

4 ';, 3 0 0 I ~ 22.00

5.51 - 6.00 \ 2 II 18, 21 II 10 7 ';, I 15.95

6.01 - 6.50 4 13 :,,~8 10, 4 I I 0 I 10.71

6.51 - 7.00

\ I 2 I~, 10 '{, 2 ').... 7.35

7.01 - 7.50 2 9 7 9 3 3

~ 5.30

7.51 - 8.00 I 5 2 I 1 4 I 3.12

8.01 - 8.50 I 4 4, I ~, I 3

~ 2.18

8.51 - 9.00 1 I

'\.~, I

i'k 1.06

9.01 - 9.50 I 0 I I

~ 0.81

9.51 - 10.00 :\ I 0 0 0 iZ 00 0.560

10.01 - 10.50 I I ~g\ I ~

0.37

10.51 - 11.00 I I 0.187

11.01 - 11.50 I

~ 0.062

11.51 - 12.00

\ ~ 12.01 - 12.50 12.51 - 13.00 13.01 - 13.50 13.51 - 14.00 14.01 - 14.50 14.51 - 15.00

~ 15.01 - 15.50 15.51 - 16.00 Wove steepness 16.01 - 17.00 \ 17.01 - 18.00 18.01 - 19.00 '\ 19.01 - 20.00 20.01 - 21.00 21.01 - 22.00 22.01 - 23.00 23.01 - 24.00 24.01 - 25.00

0·8 0« 0 0 ~ ~ ro ~

., M ~ 0 0- '" " ." 0- M 0 0-

j~ 0 '" gO .,; .,; f! .; .,; i ~ .,; N d d d 0

~ 0- 0- ro ~ M

" W

~ • g ." a; !:1

., '" '" <'l '" 0- !::: " ;!: ~ ~ ~

tz~ 0 0 ro ~ ~ '" M

d d M '" ~ i i :5 '" .; ..; ..: d d d

u 0

No of records used: 1605

-44-

TABlE 28 Hs - Tx Scolter Diagram: - Woverider Buoy No 2

Pe 'od Au st 1968 S t be 1969 " , ~v - eE em r 8 N 2 \Jol 0

~ 2.01 2.51 3.01 3.51 4.01 4.51 5.01 5.51 6.01 6.51 7.01 7.51 8.01 8.51 9.01 9.51 10.01 10.51 Cum. ~o

I I I I I I I I I I I I I I I I I I

H, 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 Exceedance

0 - 0.50 :: f=:=: 0.51 - 1.00 1.01 - 1.50 ~ : Ii- - 1--1- No of occurrences 100

1.51 - 2.00 I R;: """8. I 2 t- 2 -~ 99.84

2.01 - 2.50 2 I'-I?, 13 ---19~ II 8 t-I_ t---3~

97.80

2.51 - 3.00 I 8 i'~~, '~!, 33 t--23~ 20 17 4 r-r-- 90.70

3.01 - 3.50 6 29 '~, ~ 45 t--30~ 12 4 3 h-- 77.60

3.51 - 4.00 7 1'-?5 21 1'22, 6 2 I ro-- 61.20

4.01 - 4.50 8 29 i'-~9 28, 27 8 t--.I}. 5 3 0 1--0.

-~ 0 I 48.50

4.51 - 5.00 3 12 I?, 40 I'~ 10 r---~,

0 0 0 0

~ 35.90

5.01 - 5.50 4 18 ~k 2h

~ 7 2 I I I 26.20

5.51 - 6.00 12 15 I 4 i'~, I 0 2 ~ 18.02

6.01 - 6.50 I I 10 N~ N-.. I 0 I 12.43

6.51 - 7.00

\ I 8 ~, 6 I 2

"" 9.01

7.01 - 7.50 I 0 6 9 3 2 2

~ 6.70

7.51 - 8.00 3 9 '~, I ",~, 2 4.550

8.01 - 8.50 3 I 3 0

"" 3.110

8.51 - 9.00 I 0 0 2 2 f'-~, 2 f'!, 2 1.834

9.01 - 9.50

1\ I 0 I 0 ~

1.038

9.51 - 10.00 0

rS I 0.798

10.01 - 10.50 2 ~< ~ 0.559

10.51 - 11.00 I 0.399

11.01 - 11.50 r\ "" 0.239

11.51 - 12.00

1\ 0.080

12.01 - 12.50 I

~ 0.080

12.51 - 13.00

f~ 0

13.01 - 13.50 13.51 - 14.00 14.01 - 14.50 1\ 14.51 - 15.00 15.01 - 15.50

Wave steepness 15.51 - 16.00 \ 16.01 - 17,00 17.01 - 18.00

1'\ 18.01 - 19.00 19.01 - 20.00 20.01 - 21.00 21.01 - 22.00 22.01 - 23.00 23.01 - 24.00 24.01 - 25.00

• f! ~ '" I<: '" 0 ~

0 0 ,~g Ol .., 0 "

0 Ii! ~ ;:: 0 ro ro 0 '" '"' "- '" "!

.., 0 0

• u 0 0: &: sO iii i ~ I:i i 0: .,; N - ci ci ci ci ci 0

!ill ~ '" U x

w .... • ro 0 !;t ro ro 0 ro 0 ;;\ '" :':! ~ ro ;g N ~ ro ro < 0 .., N "< ro 0 "< '"' 0 0

~~ ci ci N ...: M ~ N ~ g ... N - ci ci a a 0 a 0

0 No of records used; 1253

-45-

TABLE 29 H$ - Tz Scatter Diagram: - Waverider Buoy No 3

Pe'ad "",,u t 1968 Mo ch 1969 " , , " r 8 N 3 uol 0

~ 2.01 2.51 3.01 3.51 4.01 4.51 5.01 5.51 6.01 6.51 7 .. 01 7.51 8.01 8.51 9.01 9.51 10.01 10.51 Cum. ~o

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

H, 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 Exceedance

0 " 0;50 • 0;51 " 1;00 -1.01 " 1.50 1't-+ 1 -0· J 1 No of occurrences 100.0

1.51 - 2.00 4 3 2 r-l· t-+ 98.47

2.01 - 2.50: ~:h~,r--t I--~- 6 0 I---r--94.05

2.51 " 3.00 2 11 i'i~, i'!~-- 13 '-14 4 6 86.5

3.01 - 3.50 6 14 I'~O N~- 13 r--..:~ 8 1 1 I---I---70.5

3.51 - 4.00 1 11 14, 16 12 h 3 0 1 :--- 53.1

.4.01 " 4.50 1 4 14 1"1,2 '17, 4 i'{ , 1 :--- 38.5

4.51 " 5.00 2 7 10, 10 9 2 0 2 ;---...... 1 27.60

5.01 - 5.50 6 6 t'--~. '3'

I'i 2 I'-i, 0 1 1 ~ 19.34

5.51 - 6.00 1 3 1 1 0 t"--L t-- 13.40

6.01 - 6.50 2 1 "'!, Ii 0 1 1 1 10.53

6.51 " 7.00 1 1 2 0 1 1'~, 1 7.66

7.01 - 7.50 1\ 1 1 Ii 3 I'Z 0 1 5.56

7.51 " 8.00 1 f'-~, 2 2 3.26

8.01 " 8.50 1 0 K 2.30

8.51 " 9.00 1

~ ~ ~ 1 1. 531

9.01 " 9.50

1\ 2 ·0 1. 150

9.51 " 10.00 ~ 1

I' 0.766

10.01 " 10.50 1 K 0 0 1 0.575

10.51 " 11.00 ~ 0.192

11.01 - 11.50 ~i 0

11.51 " 12.00 \ '\ 12.01 " 12.50 12.51 " 13.00 13.01 " 13.50 Wove steepness

~ 13.51 " 14.00

1\ 14.01 " 14.50 14.51 " 15.00 15.01 " 15.50 15.51 - 16.00 16.01 " 17.00 17.01 " 18.00 18.01 " 19.00 19.01 " 20.00 20.01 " 21.00 21.00 " 22.00 22.01 " 23.00 23. 01 " 24.00 24.01 " 25.00

r~1 ~ '11 '" ~ '" ~ ;i; '" ~ gj 0- '" '" ... '" '" ~ '" ~ '" '"

'1' 0 .; M .,; oi ::i <i :!i 0: .,; .. oi - .,; d .,; 0 E 0 g 0- 0- '" '" ;;0 x g

.Vw ~

L • :0 '" &l 0 0 0 ~ ~ :;\ <\ N ~ ~ '" !!: ~ '" ~~ i:! '" 0 '" N 0- '" '"

. 0 • .,; .,; ~ ~ ~ ~ ~ ..; M .,; .,; .,; .,; d .,;

No of records used: 522

-46-

TABLE 30 Hi ~ Tz. Scatter Diagram: - All Waverider Bools

3.01 3.51 6.51 7.01 7.51 8,01 8.51 9.01 9.51 10.01 JO.51 1 1 1 1 1 1 1 1 1 1 1

3.50 4.00 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11,00

0.51 - 1.00 1.01 - 1.50 No of occurrences 100,0

1.51 - 2,00 2 99.50

2.01 - 2.50 96,95

2.51 - 3.00 89.40

3.01 - 3,50 75.95 3.51 4.00 58.70 IS

4.01- 4.50 44.55 1.65

4.51 - 5.00 1 32.95 9.82

5.01 - 5,50 3 23.12 6.80

5.51 - 6.00 2 16.30 5.00 6,01 - 6,50 2 1 11.32 3.31

6.51 - 7.00 8.02 2.16

7.01 - 7.50 5.96 2.19

7.51 - 8.00 3.67 1. 12

8.01 - 2.54 1. 12

.8.51' - 1.42 0.47 9,01 - 0.266

9.51 - 0.21

10.01 - 3 0.21

10.51 - 11 0.15

11.01 - 11 0.089

11.51 - 0

12.01 - 12.50 0,0296

12.51 - 13,00 0 0

13.01 - 13.50 13.51 - 14.00 14.01 - 14.50 14.51 - 15,00 15,01 - 15.50 15.51 - 16.00 16.01 - 17.00 17,01 - 18.00 18.01 - 19.00 >1 ~o of time 19.01 - 20.00 20,01 - 21.00 21.01 - 22.00 22.01 - 23.00

- 24.00

0 ;}. :2 f! '" "- Sl <l g '" !;/ ~ ~ ~

'" ~ '" 0

.,; " gO ili .0 M i ;i i " .,; .; .,; d d d ,; 0

~ ~ '" "-

'" ;;: 'I 0 f! 'I ~ 0 ~ '" ~ f! S! <I '" '" M ~

0 0 '" 0 '" 0 0 0

.,; .,; M " ci i i 0 " ..; '" ,; ,; d d ,; ,; 0

No of records used: 3380

TABLE 31 Monthly Distribution of HmgX from Waverider Buoy No 1

Period: August 1968 - December 1969 8uoy No I

H""", Aug Sep Oct Nov Dec Jan Feb Mar Apr May June July I Aug I sepl Oct I Nov I Dec Total C Hmox ,/0

o - 0 .. 50 um ... IExceedance 0.51 - 1.00 1.01 - 1.50 1.51 - 2.00 2.01 - 2.50

'2.51 - 3.00 1 3.01 - 3.50 2 3.51 - 4.00 8 4.01 - 4.50 4 4.51 - 5.00 3' 5.01 - 5.50 8 5.51 - 6.00 9 6.01 - 6.50 3 6.51 - 7.00 4 7.01 - 7.50 4 7.51 - 8.00 2 8.01 - 8.50 4 8.51 - 9.00 4 9.01 - 9.50 6 9.51 - 10.00 5

10.01 - 10.50 4 10.51 - 11.00 2 11.01 - 11.50 I 11.51 - 12.00 2 12.01 - 12.50 0 12.51 - 13.00 5 13.01 - 13.50 0 13.51 - 14.00 1 14.01 - 14.50 1 14.51 - 15.00 0 15.01 - 15.50 1 15.51 - 16.00 0 16.01 - 17.00 2 17.01 - 18.00 18.01 - 19.00 19.01 - 20.00 20.01 - 21. 00 21.01 - 22.00 22.01 - 23.00 23.01 - 24.00 24.01 - 25.00

1 2 2

No of records used: 1605

Note~

1 1 3 3 3 4 3 5 6 7 2 6 4 2 1 1 3 o 3 o o o I o 1

1 3 7

10 9

14 14 7 6 9 8 6 7 5 1 o 3 3 o 1

4 7 6 7

12 11 12 14 10 6 8 4 3 4 3 1 2 1 3 o o o o 1

[ill a 'i. from ego 16.01 - 16.50 b i. from ego 16.51 - 17.00

3 5 8 8

15 16 20 11 9 6 4 4 3 2 1 3

1 o 4 4 4 2 2 5 3 6 3 7 2 2 o 4 4 2 o 1 1

*

1 o 3 4 5 3 7 6 5 7 2 6 o 1 3 3 o I o o 1

2 7 3 2 2 4 11' 3 4 8 10 9 7 3

13 3 9 14 7 8 16 4 10 16 5 11 7 3 7 4 5 7 7 4 9 1 2 5 3 6 4 4 3 2 2 2 3 2 7 ,6 1 4 3 o 6 0 o 3 1 o 0 1 o 4 0 o 2 3 1 6 2 2 0 o 2 0

02 j 0 a

3 5

10 3 6 7 5 6 7 7 6 7

10 6 6 2 7 2 2 a o 1

No. of occurrences

2 5

1 8 3 14 5 10

11 11 8 14

10 9 11 10 6 7 4 4 9 9 3 4 6 1 7 1 2 0 2 2 2 2 3 0 2 0 o 2 2 1 o 0 o 0 2 1

2 3 5 8 8

11 12 13 8 7 8 7 5 2 1 4 3 1

1 1 2 4 o 5 2 5 5 6 5 7 4

12 5 3 4 6

1 3 4 3 ? 1 3 5 2 1 o 3 1 0

o o o 2 4 9

13 12 6 1 3 4 3 1 2 2

1 1605 100.0 2 1604 99.94 5 1602 99.81

26 1597 99.45 36 1571 98.0 66 1435 95.60 98 1469 91.50 92 1371 85.50

133 1279 79.6 146 1146 71.4 137 1000 62.3 124 863 53.7 95 739 46.00 83 644 40.15 99 561 35.00 63 462 28.80 77 '399 24.85 44 322 20.05 40 '278 17.31 37 238 14.82 36 201 12.52 28 165 10.28 25 137 8.54 15 112 6.98 19 97 6.04 10 78 4.86 15 68 4.24 13 53 3.30

2

20 15

Q 0.436

1 ~59-~~~.ll!L /1 I 2~Z IU"~9 125 I/o r 0 0.062 0

I

~

-48-

TABLE 32 Monthly Distribution of Hmgx from Woverider Buoy No 2

Period- August 1968 .. September 1969 Buoy No 2

Hmox Aug 5ep Oct Nov Dec Jan Feb Mor Apr Moy June July Aug Sep Total Totol (Cum)

o - 0.50 0.51 - 1.00 1.01 - 1.50 1.51 - 2.00 2.01 - 2.50 2.51 - 3.00 3.01 - 3.50 3.51 - 4.00 4.01 - 4.50 4.51 - 5.00 5.01 - 5.50 5.51 - 6.00 6.01 - 6.50 6.51 - 7.00 7.01 - 7.50 1.51 - 8.00 '8.01 - 8.50 8.51 - 9.00 9.01 - 9.50 9.51 - 10.00

10.01 - 10.50 10.51 - 11.00 11.01 - 11.50 11.51 - 12.00 12.01 - 12.50 12.51 - 13.00 13.01 - 13.50 13.51 - 14.00 14.01 - 14.50 14.51 - 15.00 15.01 - 15.50 15.51 - 16.00 16.01 - 17.00 17.01 - 18.00 18.01 - 19.00 19.01 - 20.00 20.01 - 21.00 21.01 - 22.00 22.01 - 23.00 23.01 - 24.00 24.01 - 25.00 25.01 - 26.00 26.01 - 27.00 27.01 - 28.00

2 o 7 4 5 5 4 6 2 3 3 4 5 3 6 4 2 3 1 4 1 1 3 o 1 1 o o

~ o

~

1 3 o 9

13 17 11 8

10 8 8

11 5 4 3 o 2 1 1 1 o o 1 o 1

No of records used: 1253

5 4 6 5 3 4 1 1 2 3 o 1 o 1 o o o o o o o o o

b1

1 1 1 o 1 o o o o o 1 1

1 5 1 8 2

12 13 15 10 9 8 4 8 5 2 5 4 o 2 o 1 o o o

~

1 2 4

13 16 22 16 13 13 4 7 3 2. 1 o 2

No of occurrences

3 1 3 4 0 4 11 3 4 2

2 5 6 1 6 1 5 9 7 1 7 2 11113348 3 1 15 4 8 3 14 4 5 12 3 5 8 9 9784868 4861768 7 17 2 3 7 10 11 4 13 2 4 3 9 7 51452239 2 14 0 6 7 12 7 6 13 3 0 4 5 2 2524243 6615184 4307053 2112342 1107330 4 0 0 2 1 1 1 5202222 4002221

4 01

01~3 30

1 3

~ ~O 03

1-'1 :t/3<'4-h'0"-! 0 2 0 I YO . V-! 0/\ 0 1Al 1/\ ~ 1~ ~ 0/\ 0

r';1) :i:: ~ =H o

o o tyo

2 4 4 4 5 4 4 5 2 7 1 1 4 o 1 1

2 1

16 20 52 68 80 93 95 96 90 69 97 61 61 66 37 37 39 27 19 16 13 16 12 16 10 7/5

-5.17 2A

2 '(1;0 00

1253 1251 1250 1234 1214 1162 1094 1014 921 826 730 640 571 474 413 352 286 249 212 173 146 127 111 98 82 70 'i4

44/37 32/27 20/18 13/11

8/7

/1

'70

Hmax j'o Exceedonee

100 99.84 99.76 98.5 96.9 92.75 87.4 80.95 73.5 65.9 58.2 51.05 45.5 37.8 32.95 28.05 23.85 19.88 16.90 13.80 11.63 10.12 8.85 7.81 6.54 5.58 4:31

•. 01/2.95 2.55/2.15

1.595/1 435 l.u3 :/0.877 O.7Y8/0 71R

O. 638/ O.~~5B 0.558/0 478 0.239/0080 u.0870 08 0.08/0 OR O.OiVO.08

-49-

TABLE 33 Monthly Distribution of Hmax from Waverider Buoy No 3

Period· August 1968 - March 1969 Buoy No 3

H max

Aug Sep Oct Nov Dec Jan Feb Mar Toto I T ota I (Cum)

o - 0.50 0.51 - 1.00 1.01 - 1.50 1.51 - 2.00 2.01 - 2.50 2.51 - 3.00 3.01 - 3.50 7 3.51 - 4.00 2 4.01 - 4.50 3 4.51 - 5.00 2 5.01 - 5.50 3 5.51 - 6.00 7 6.01 - 6.50 2 6.51 - 7.00 5 7.01 - 7.50 8 7.51 - 8.00 5 8.01 - 8.50 1 8.51 - 9.00 4 9.01 - 9.50 1 9.51 - 10.00 1

10.01 - 10.50 4 10.51 - 11.00 5 11.01 - 11.50 4 11.51 - 12.00 5 12.01 - 12.50 2 12.51 - 13.00 1 13.01 - 13.50 2 13.51 - 14.00 2 14.01 - 14.50 0 14.51 - 15.00 1 15.01 - 15.50 1 15.51 - 16.00 2 16.01 - 17.00 ~2 17.01 - 18.00 I ~2 18.01 - 19.00 19.01 - 20.00 20.01 - 21.00 21.01 - 22.00 22.01 - 23.00 23.01 - 24.00 24.01 - 25.00

1 o 5 4 9 6

11 3 5 6 6 8 3 2 1 2 1 1 2 1 1 o 1

No of records used: 522

3 1 3 o 2 2 6 4 1 1 3 1 2 3 1 2 1 1 1

2 6 5 2 4 8 1 7 3 6 3 7 1 1 1 2 1 1 ]

1 2 4 1 5 7 7

11 8 8 7 6 3 5 3 2 1 2 1 1 o o 4 o 2 o o

'70

1 o o 5 8

12 9

20 10 16 4 6 5 5 2 o o 1 o o 1

No. of occurrences

1 o 1 2 3 2 4 1 2 4 5 1 o o 2 2 2 1 3 3 2 3 o

2/1 0/2 o

2/1 0/1 0/1 o/i

3 522 11 519 9 508

18 499 20 481 29 461 43 432 43 389 44 346 41 302

1 46 261 o 26 215 o 31 189 1 27 158 o 20 131 2 16 111 3 14 95

9 81 13 72 8 59 6 51 5 45 5 40 8 35 3 27 6 24 2 18

2A 16/14

o/j 3/3

~ 0

o Hmax /0 Exceedance

100 99.42 97.4 95.5 92.1 88.3 82.7 74.5 66.2 57.8 50.0 41. 15 36.2 30.2 25.1 21.25 18.2 15.5 13.8 11. 3 9.76 8.62 7.66 6.70 5.16 4.60 3 45

3.06/2 68 :l. 1071. 916 1. 15/1 15 1. 1570 765

0.575/0575 0.383/0.383 0.192/0.192

o 0

-50-

TABLE 34 Distribution of Hmox from all Woverider Buoys

H max

o - 0.50 0.51 - 1.00 1.01 - 1.50 1.51 - 2.00

No Total (Cum)

2.01 - 2.50 1 3380 100 2.51 - 3.00 7 3379 99.97 3.01 - 3.50 17 3372 99.8 3.51 - 4.00 51 3355 99.3 4.01 - 4.50 74 3304 97.85 4.51 - 5.00 138 3230 95.6 5.01 - 5.50 195 3092 91.1 5.51 - 6.00 215 2897 85.7 6.01 - 6.50 269 2682 79.4 6.51 - 7.00 285 2413 71.4 7.01 - 7.50 274 2128 63.0 7.51 - 8.00 260 1854 54.9 8.01 - 8.50 190 1594 47.15 8.51 - 9.00 211 1404 41.5 9.01 - 9.50 187 1193 35.35 9.51 - 10.00 144 1006 29.75

10.01 - 10.50 159 862 25.50 10.51 - 11. 00 95 703 20.80 11.01 - 11.50 86 608 17.98 11.51 - 12.00 89 522 15.45 12.01 - 12.50 71 433 12.80 12.51 - 13.00 53 362 10.71 13.01 - 13.50 46 309 9.14 13.51 - 14.00 33 263 7.78 14.01 - 14.50 43 230 6.81 14.51 - 15.00 25 187 5.53 15.01 - 15.50 37 162 4.79 15. 51 - 16. 00 h-i2(d5'r-+--",dc 112¥-c5+-~i<'l3,---7l)<70'---I 16.01 - 17.00 h-,:14(L+'/l164-.:.;:1 0~0/1~8~6+-...;2:-:,' 9~6f-' /-j-2~5~4y 17.01 - 18. 00 N13fL:/f'1 f,4-;7~OA~:7+-~2:;.:' 0~7L' /-,J71~6>1S81>Lj5 18.01 - 19.00 ~/8 4Y38 1.211/11?3 19. 01 - 20. 00 1--,-O/.<-;40'-f--,3~0f-r= /2,4--+-c0" "= 8!:ll:ll. /'---;C'O ,,"",7-,-:1 Oy 20.01 - 21.00 '/2 20/19 0.592/0 562 21.01 - 22.00 2/2 lY15 0.503/0.443 22 • 01 - 23. 00 f--,3,f--7 ~5--1f---;1 V?---i'l O,-+~O,.:..' 3",8",4f. /"-T0-'-" 2'-'.9-':'--16 23. 01 - 24. 00 f-----.::2fL-f /----I1----;5'f-'f-/-+-;:0~.'" 114 ;;;:84' /0~0,,-,8'-L..j9 24.01 - 25. 00 !----,;1f-JJ-~-+----,,2iY--/+U::.;.~r, 0:5",9~L //~ 00"'-13 25.01 - 26.00 hUPf /i0---\-_.-tl/C-j1y -;0,:-' 0,,;3,£-1 /i~0!.L0~3!-1 26.01 - 27.00 h0;LIA:,;0----lI----;ViC+1 -+--;.O~. 0~3/~0~06i3y 27.01 - 28.00 yo Yo 0.03/0

No of records used: 3380

-51-

From tables 30 and 31 it may be concluded that the most commonly occurr-ing wave conditions at the Gold Coast have the following characteristics:

Hs 3.25 ft Tz 4.75 secs Hmax 6.75 ft

In general, the waves are rather steep, having significant steepness approx­imately between 1/100 and 1/20, with the most frequent wave conditions having significant steepnesses in the order of 3/100. These values are far removed from pure swell conditions and are typical of waves within or very near the generating area, confirming the indication gained from the € values. The likelihood is that locally generated waves mask most swell from distant generating areas and a com­plete spectral analysis would be necessary to determine the presence of longer period swell waves.

Considering the values listed in tables 10 and 11 (section 3.6. 2b.) there appears to be no requirement to correct the values obtained. This is one of the advantages of recording in relatively deep water (i.e. for wave-recorders).

Unfortunately (for the sake of this analysis only) no cyclones have occurred during the recording period. This lack of occurrence of cyclones influences the frequencies of exceedance of wave heights in deep water given in figure 44. If cyclones are taken into account the percentages of time, especially of the higher wave heights, will increase on the average by something like one percent (l 0/0). There seems to be no need to calculate theoretically the exact influence of cy­clones on the graph of figure 44 because of the uncertainty of available methods. It might be proposed to have a Waverider in operation during a cyclone in order to get an idea of wave heights during cyclones.

Figure 44 may be used as a design graph for wave heights Ho (deep 'water conditions) with a high degree of confidence.

Since the duration of attack of waves is an important factor in design, figure 45 was prepared from the analysed records. From this figure may be obtained the length of time for which a Hs will be maintained at or above any value with the appropriate probability.

The K. N.M.1. observations are the only other source of wave information that includes rather detailed information about periods. However, a comparison between Waverider and K:N.M.1. results is not very meaningful. In fact the com­parison hardly can be made. The period in which the Waverider operated clid not coincide with the period of the K. N.M.1. observations. The Waverider does not distinguish between wave directions of between sea and swell. K. N. M.1. observations report sea and swell separately. Sometimes sea and swell occur separately, but often they occur at the same time. This makes the separate visual observation of sea and swell very difficult, especially if they originate from close directions. Low swell of long periods hardly can be noticed visually. In section 3.9.2., where the results of the K. N. M.1. observations served as a base for the littoral trans-port computations, the Waverider measurements not being directive and the results of them not being' available at the time of the computations, it is considered for ease of computation that sea and swell occur separately.

-52-

The Waverider does not show periods longer than 11 seconds. As stated before, the most frequent waves attacking the Gold Coast are originated within the area or in near-by areas. These locally generated waves will mask most of the longer swell. To determine the presence of longer swell and to check the K.N.M.1. results on this point a complete spectral analysis of Waverider results would be necessary.

Keeping in mind the above-stated differences between methods of measur­ing or observing wave conditions, the Waverider and K.N.M.I. results - al­though a direct comparison cannot be made - do not seem to be contradictive.

3.6.5. Refraction Diagrams

Waves from a north-easterly to a south-easterly direction can approach the Gold Coast and are the main cause of sediment movements along the coast.

The celerity of waves in shallow water - ciepth less than half the wave length - decreases with the depth. This causes an attenuation of the direction of travel of the waves in such a way that the crests tend to become more paral­lel to the contour lines. The effect of this refraction depends on the period (i.e. the wave length) of the waves. In order to provide data for further cal­culations about the quantities of sediments travelling along the coast, refraction diagrams have been drawn for the followi ng cases:

a. Waves from NE; periods 6, 8 b. Waves from E; periods 6, 8, c. Waves from SE; periods 6, 8,

3.7. Currents

3.7.1. General

and 10 sees (see figures 46 - 48). 10 and 12 secs (see figures 49 - 52). 10 and 12 secs (see figures 53 - 56).

As to their origin, the currents can be divided into three categories:

- ocean currents, related to the East Australian .Coast Current and drift currents caused by local winds.

- tidal currents, oscillatory with a period of 12 hours. - currents originating from the waves.

(i) (ii) (iii)

General information about currents has been obtained from:

The Royal Australian Navy. The United States Navy Hydrographic Office [14, 15]. Generalised surface circulation diagrams such as are given in references [7, 16, 17J.

Tables 35 and 36 summarize the information obtained from source (ii) above.

-53-

TABLE 35 Surface Currents from U. S. Navl TABLE 36 Surface Currents from U. S. H.O. Pub. 568 Navy H. O. Pub. 107

Month Southerly drift in Number of Southerly drift in n. mi les per day observations n. miles per day

i 21.6 3 16.0 f 17.5 4 30.0 m 33.4 9 -a 36.5 3 16.0 m 19.2 1 25.0

i 14.2 3 8.0 i 24.7 5 -a 30.0 1 20.0 s 19.2 1 -0 24.3 7 20.0 n 20.4 6 21.0 d 38.4 3 9.0

Mean: 25.8 n.m;/day 18.3 n.m./day (1 knot) (0.75 knot)

In general the indications are that the surface circulation offshore in the Gold Coast area is southward for most of the year. This East Austral ian Coast Current can reach· velocities up to several knots.

Information related much more to the actual erosion problem has been obtained from the results of the various current measurements:

- Surface current measurements with drift cards. - Sub-surface current measurements with float poles. - Littoral current measurements in the surf-zone.

3.7.2. Data and Elaborations

a. Surface currents -----------------

.-, .

Surface currents were measured at several sites (see figure 3), using drift cards. The cards were released at sea and posted back to the Co-ordinator-General's Department after inscribing of time and place of finding by members of the public. The results of these experiments are given in table 37. In these tables also the results of sub-surface current measurements (see 3.7. 2b.), excuted simultaneously, are listed.

-54-

TABLE 37 Results of Drift Card Measurements

Site: 1 No. of Releases : 9 No. of Results: 9 + IF

Release day Av. Travel Surface Vel. Av. Sub-SurLVel. Note Wind

Date Time Speed Dir" Distance Dirn Mox.1 Mean Vel. Oirn

hrs. m.p.h. (from) miles feet per min. It/min

17- Aug. 66 0915 6-8 S 3.3 3100 46.5 12.0 11.34 3240

17 Aug. 66 1600 6-8 S 1.75 3050

7.2 6.2 11.34 3240

25 Apr. 67 0830 10-15 NW-N' 1.45 15]0 28.4 22.2 14.45 1370

25 Apr. 67 1500 10-15 NW-N 1. 15 1690 67.5 59.5 14.45 1370

4 July 67 0815 0-5 variable 176.0 3550 37.4 28.8 13.5 12010

4- July 67 1130 0-5 variable 176.0 3550 37.4 28.8 13.5 120jO 4 July 67 1445 0-5 variable 176.0 3550 37.4 28.8 13.5 1201,0

27 Mor. 68 0730 6-10 SSE-SE 18.0 3440

19.4 16.8 27.7 314 67~0* 27-Mor. 68 0730 6-10 SSE-SE 5.0 318

0 47.5 30.7 27.7 314

0 33/0 .. 27 Mor. 68 1400 6-10 SSE-SE 16.8 342

0 29.8 18.8 27.7 314

0

* 67<jto of cords found travelled in av. dir" 344°, and 33)'0 in av. dirn 3180

F. This means that on 9 experiments oords were recovered, but 1 exper~ment gave two "separate ll

results-see .

Site: 2 No. of Releases: 3 No. of Results: 3 + 3

Release Dal Av. Travel Surface Vel. Av. Sub-Surf. Vel Note Wind

Date Time Speed Dirn Distance Dir" Max. / Mean Vel. Oil hrs. m.p.h. (from) miles feet per min. ft/min

30 Aug. 66 0756 6-20 E-NNE 0.65 3030 2.1 1.3 9.5 339

0

30 Aug. 66 1402 6-20 E-NNE 0.6 2720

1.1 - 9.5 3390

23 Apr. 68 1200 5-8 SE 1.7 3440

5.65 4.06 8.6 3340

60t,0 23 Apr. 68 1200 5-8 SE 12.4 05

0 11.5 - 8.6 334

0 3;{;0

23 Apr. 68 1200 5-8 SE 16.3 1650

6.5 - 8.6 3340

3;{;0 23 Apr. 68 1200 5-8 SE 0.8 212

0 2.75 1.4 8.6 334

0 35/,0

Site: 3 . No, of Releases : 2 No. of Results : 2

Release Day Av. Travel Surface Vel. Av. Sub-Surf. Vel. Wind

Max. I Mean Date Time Speed Dir" Distance Dir" Vel. Dir" hrs, m.p.h. (from) miles feet per min. ft/min

24 Aug. 66 0820 5-10-5 W-NE-E 2.5 2740

38.6/ 23.4 18.5 2070

24 Aug. 66 1515 5-10-5 W-NE-E 2.5 2050

14.8 6.7 18.5 2070

Site: 4 No. of Releases : 4 No. of Results : 4

Release Day Av. Travel Surface Vel. Av. Sub-Surf. Vel. _'iii d

Date Time Speed Dirn Distance Dirn Max. Mean Vel. Dirn

hrs. m.p.h. (from) miles feet per min. ft/min

25 Aug. 66 0938 0-14 SE 0.85 3150

11.5 11.5 8.2 3220

25 July 67 1045 5-8 E-S 9.7 00 14.2 10.0 2.3 2680

25 July 67 1640 5-8 E-S 13.0 020

8.1 8.1 2.3 2680

2 Sep. 66 0955 5-15 W 16.0 110

4.5 4.0 3.75 980

-55-

TABLE 37 (continued)

Site: 5 No. of Releases: 8 No. of Results : 7

Release Day Av. Travel Surface Vel. Av. Sub-Surf. Vel. -vi nid

Max. I Mean Date Time Speed Dirn Distance Dirn Vel. oil hfS. m.p.h. (from) miles feet per min. ft/min

29 Sep. 66 0804 8 S-ESE 1.95 3120 28.9 28.2 2.0 2830

29 Sep. 66 1418 8 S-ESE 1.95 3120 30.2 23.5 2.0 2830

18 July 67 1030 0-5 NE 0.8 2120 18.6 13.8 8.2 2730

18 July 67 1700 0-5 NE No Cards Found 31 Oct. 67 0745 9 E 1.2 2860 20.1 12.6 23.7 1670

31 Oct. 67 1410 9 E 1.3 2920 4.2 3.9 23.7 1670

1 Nov. 67 0800 0-5 ESE 3.3 3250 44.6 33.5 3.25 2300

1 Nov.67 1400 0-5 ESE 1.6 3030 40.2 37.6 3.25 2300

Site: 6 No. of Releases : 4 No. L of Results: 3 + 4

Reloos. Day Av. Travel Surfoce Vel. Av. Sub-Surf. WI. Wind

Max.1 Mean Date Time Speed Dirn Distance Dirn Vel. Dirn Note hrs. m.p.h. (from) miles feet per min. h/min

29 Aug. 67 1430 6 SS'!k;!;.N 3.0 1520 5.5 - 51.8 1160

27 Mar. 68 0730 6 SW-E 2.2 1720 20.8 - 19.5 1980

27 Mar. 68 1400 6 SW-E 4.0 2900 4.0 3.8 19.5 1980 15lo

27 Mor. 68 1400 6 SW-E 14.0 3270 16.2 14.6 19.5 1980 15,0

27 Mor. 68 1400 6 SW-E 37.0 3500 12.3 9.7 19.5 1980 38ro

27 Mor. 68 1400 6 SW-E 16.0 1600 7.5 9.7 19.5 1980 23;{,0

27 Mor. 68 1400 6 SW-E 38.0 1600 20.6 9.7 19.5 1980

8,0

L: No cords found from one release.

Site: 7 No. of Releases: 31 M No. of Results: 22 + 21

Reloose Day Av. Travel Surface Vel. Av. Sub-Surf. Vel. Wi;d~

Max. I Moon Date TIme Speed Dirn Distance Dirn Vel. Dirn Note hrs. m.p.h. (from) miles feet per min. It/min

6 May 68 0700 10-20 SE 11 000 35.8 29.4 37.3 3520

20 Moy 68 1246 12 SW 6.5 1810 23.6 21. 7 6.75 3540 71~0 20 May 68 1246 12 SW 15.4 1580 19.6 19.4 6.75 3540 29"/0 21 May 68 0655 0-5 W-NE 15.4 1580 28.4 26.3 15.3 2070

25 May 68 0655 8 W 50.0 Ig3g 55.0 27.0 4.5 2090 N 30 May 68 1300 10 W 3.1 3200 5.8 - 53. 1 3500

3 June 68 1255 16-8 SW-W 15.4 1560 51.2 30.8 10.6 3560

4 June 68 0645 6-8 NW-N 13.5 1640 23.3 13.7 24.6 1800

4 June 68 1130 6-8 NW-N 15.4 1550 46.0 33.5 24.6 1800

5 June 68 0650 13 ~NW-N 10 3590 10.8 8.6 16.5 1820

9 Sep. 68 0740 0-20 S-SE 1.4 3220 13.7 - 8.0 3310

9 Sep. 68 1240 0-20 S-SE 1.5 3250 33.0 - 8.0 3310

10 Sep. 68 0840 6-10 SE-E 1.3 2190 14.0 - 6.8 3250

10 Sep. 68 1240 6-10 SE-E 0.9 2470 17.5 - 6.8 3250

11 Sep. 68 0740 0-12 NE 4.3 1870 40.0 21.0 27.8 1730

11 Sep. 68 1150 0-12 NE 2.9 1930 36.5 - 27.8 1730

12 Sep. 68 0730 0-14 NE 2.9 1930 181.0 35.0 25.3 1880

12 Sep. 68 1150 0-14 NE 2.8 1930 82. 1 30.0 25.3 1880

13 Sep. 68 0850 6-16 NW-NE 13.2 1640 37.5 31.4 45.6 1700

13 Sep. 68 1340 6-16 NW-NE 15.6 1570 31.2 25.7 45.6 1700

14 Sep. 68 0700 0-14 NW-NE 3.5 3420 5.5 - 12.1 1910 56lo

14 Sep. 68 0700 0-14 NW-NE 5.1 1840 37.4 11. 1 12. 1 1910 44 yo 15 Sep. 68 0800 0-16 NW-ESE 0.9 2470 13.7 - 4.9 3030

15 Sep. 68 1110 0-16 NW-ESE 1.7 2100 34.7 - 4.9 3030

M: No cards found from 9 releases.

N: To clear .point Danger.

-56-

TABLE 37 (continued)

Site, 7A No. of Releases: 24 p

No. of Results , 14 + 1

Release Day Av. Travel Surface Vel. Av. Sub-Surf. Vel. \\ inct

Date Time Speed Dir" Distance Dirn Max. IMean Vel. Dirn Note hrs. m.p.h. (from) miles feet per min ft/min

6 May 68 1216 10-20 SE 11.5 3570 41.0 - 57.2 01 0

25 May 68 0630 8 W 15.5 1560 41.0 - 27.5 1750

3 June 68 1230 16-8 SW-W 15.4 1580 49.2 20.0 8.1 990

4 June 68 0620 6-8 NW-N 15.4 1580 47.5 30.8 40.4 1850

4 June 68 1105 6-8 NW-N 15.6 1560 27.5 - 40.4 1850

5 June 68 0630 13 NW-N 10.0 3590 11. 3 - 17. 1 1650 27';'0 5 June 68 0630 13 NW-N 1.6 2480 1.7 1.4 17. 1 1650

730/0 16 Sep. 68 0720 6-8 S-SE 3.2 3270 10.4 - 26.0 3350

16 Sep. 68 1220 6-8 S-SE 2.0 2270 6.2 - 26.0 3350

17 Sep. 68 0840 0-16 NE 2.1 2260 20.1 - 12.4 2310

17 Sep. 68 1250 0-16 NE 17.8 1560

20.4 - 12.4 2310

18 Sop. 68 0730 6-3 SE-NE 3.0 2070 27.8 11.3 32.0 1780

18 Sep. 68 1140 6-3 SE-NE 3.1 2050 15.2 9.7 32.0 1780

19 Sep. 68 0740 0-18 SWNNE 3.1 2050 31.0 - 13.0 2120

19 Sep. 68 1150 0-18 SWNNE 4.0 1980 63.0 39.0 13.0 2120

p, No cards found from 10 releases

In figure 57 the mean surface velocities are plotted for selected sites versus the percentage of time that a certain value is exceeded. No account is taken of the direction of the current. The sites excluded from this figure have too few results to give a meaningful plot. In figure 58 the same graph is plotted but no distinction is made between the sites. Figure 59 is a plot re­lating the direction of travel of the drift cards to the average direction towards which the wind blew on the day of release.

b. Sub-surface Currents ---------------------Sub-surface currents were measured at various sites (see figure 3), using

vaned float poles with the measuring area centered opproximately 13.5 ft below the surface. At each site the currents were analysed into two component direc­tions, being upcoast-downcoast and onshore-offshore. The "upcoast" bearing is defined as the average bearing of the adjacent shoreline in a northerly general direction. "Downcoast" is the back bearing of "upcoast", "offshore" and "onshore" are respectively "upcoast" plus and minus 900

•

-57-

Since this analysis revealed the predominance of longshore components over transversal components, further elaboration was concentrated on the former components. For each site, the longshore component was firstly plotted against time and then to a common tide base. However, no useful relationships could be established between any tidal or other phenomena and the variation in longshore component velocity throughout each experiment. In order to further elaborate the data, it was decided that the variation of both longshore and transverse components throughout each experiment was sufficiently small to make a mean value for each component from each experiment meaningful. These values are listed in table 38.

TABLE 38 Results of Sub-surface Currents Measurements .

Average Velocity(ft/min.) Net Current Ratio

Date Longshore On/Offshore Net Angle to True Av. longsh. Vel. x) xx) Coast xxx) Bearing Av. NetVel.

Sites 1 + lA 25 April 1967 -14.4 -0.8 14.5 30 . 1370 0.99 26 April 1967 + 4.5 0 4.5 0 3200 1.00 17 Aug. 1966 +11.3 -0.9 11.4 4 0 32440 0.99 27 Mar, 1968 +27.5 +2.8 27.6 5 0 3141

0 0.99 4 July 1967 -12.7 -4.5 13.4 19l

o 1201

0 0.95

5 July 1967 +17.5 +4.8 18.3 1510 304lo 0.96

Site 2 30 Aug. 1966 + 9.0 +3.0 9.5 180 3390 0,95 23 April 1968 + 7.8 +3.4 8.5 230

3340 0.92

Site 3 24 Aug. 1966 -15.2 +10.5 18.5 350 207

0 0.82

15 Sep. 1964 + 1.0 +16.7 16,8 861.0 265lo 0.06 Site 4 25 Aug, 1966 + 6,5 +5.0 8.2 380 3220 0.79 2 Sep. 1966 - 0.5 -3.7 3:7 820 98

0 0.14

25 July 1967 - O. 1 +2.3 2.3 8710 167lo 0.04

Site 5 29 Sep. 1966 + 1. 2 +1.6 2.0 530 2830 0.60 18 July 1967 + 3.7 +7.3 8.2 63,0 272~0 0.45 19 July 1967 +26.0 +7.4 27.0 16

b 3200 0.96

31 Oct. 1967 -23.1 +4.2 23.5 1010 16610 0.98

1 Nov. 1967 - 0.9 +3.2 3.3 740 2300 0.27 28 Mar. 1968 + 3.0 +1.0 3.2 180 3180 0.94 Site 6 29 Aug. 1967 -51.8 - 3.8 51.8 40 116

0 1.00

30 Aug. 1967 -80.0 +30.6 85.6 21 0 141 0 0.93 27 Mar. 1968 - 4.0 +19.2 19.5 781 0 1981

0 0.21

x) xx) xxx) xxxx) See page 59

Remarks xxxx)

0 0 I I I

I 0 0

I 10+ I I

I I l I 10 40

-58-

TABLE 38 (continued)

Average Velocity (ft/min,) Net Current Ratio

Date Longshore On/Offshore Net Angle to True Av. longsh. Vel. x) xx) Coast xxx) Bearing Av. NetVel.

Site 7 6 May 1968 +37.1 + 5.2 37.7 80 3520 0.98

20 May 1968 +,6.7 + 0.7 6.7 0 359,0 1.00

21 May 1968 -13;6 + 7.0 15.5 270 20l~ 0.88 22 May 1968 -37.9 - 6.0 38.3 90 171

0 0.99 23 May 1968 -27.7 - 5.0 28.0 100 1700 0.99 24 May 1968 -17.3 -18.3 25.3 4640 133!0 0.68 25 May 1968 - 3.9 + 2.2 4.5 29!0 209!0 0.87 30 May 1968 +52.1 + 8.8 53.0 9,0 350!0 0.98 31 May 1968 -10.7 + 1.5 10.8 gl:> 1880 0.99 3 June 1968 +10.5 + 0.8 10.5 40 3560 1.00 4 June 1968 -24.6 + 0.1 24.6 0 1800 1.00 5 June 1968 -16.4 + 0.6 16.4 20 1820 1.00 9 Sep. 1968 + 6.9 + 4.0 8.0 300 3300 0.86

10 Sep. 1968 + 5.5 + 3.9 6.7 35,0 32410 0.82 11 Sep. 1968 -27.3 - 3.4 27.6 7° 1730 0.99 12 Sep. 1968 -25.0 + 3.6 25.4 8

0 1880 0.98 13 Sep. 1968 -44.7 - 7.7 45.2 940 170io 0.99 14 Sep. 1968 -11. 9 + 2.3 12.1 11 !o 19H

o 0.98

~5 Sep. 1~ +2.7 + 4.1 ~.9_ 56,0 3031 0 0.55 Site 7A - - r- - ,---' -- ---6 May 1968 +57.2 - 0.8 57.3 10 01 0 1.00

20 May 1968 - 2.4 - 4. 1 4.8 5910 12010 0.50 21 May 1968 -34.9 + 3.0 35.1 50 185t> 0.99 22 May 1968 -48.0 - 7. 1 48.6 810 17]10 0.99 23 May 1968 -46.6 - 4.9 46.9 66 174t> 0.99 24 May 1968 -23.0 -15.6 27.8 34

0 1460 0.83 25 May 1968 -27.3 - 2.3 27.5 50 1750 0.99 3 June 1968 - 1.3 - 8.0 8.1 810 990 0.16 4 June 1968 -39.9 + 3.7 40.0 51

0 18s! 0 1.00 5 June 1968 -16.4 - 4.7 17.2 15io 164!0 0.95

16'Sep. 1968 +23.7 +10.8 26.0 24!0 335!0 0.91 17 Sep. 1968 - 7.6 + 9.5 12.2 51 !o 23Ho 0.62 18 Sep. 1968 -31.7 - 1.2 31. 8 2!0 177t 1.00 19 Sep. 1968 -10.8 + 7.0 12.8 330 213 0.84 20 Sep. 1968 -25.3 - 5.4 26.0 120 1680 0.97

x) xx) xxx) See page 59

-59-

TABLE 38 (continued)

Average Velocity (ft/min.) Net Current Ratio

Date Longshore On/Offshore Net Angle to True Av.longsh. Vel. Remarks x) xx) Coast xxx) Bearing Av. NetVel. xxxx)

Site 8 15 July 1969 -47.1 +15.4 49.6 18° 138° 0.95 16 July 1969 -39.5 +12.3 41.3 ln° 137!0 0.96 22 July 1969 -41.7 +17.1 45.0 22~0 142Ao 0.93 23 July 1969 -31. 8 +20.1 37.6 32t 152~0 0.85 2 Sep. 1969 -72.2 + 6.0 72.5 5 125° 1.00

~~1~ -99.0_ +22.3 101.4 12° 132 10 0.98 Site 9

- f-"

21 April 1969 -16.3 - 5.0 17.0 17° 148° 0.96 22 April 1969 + 2.7 -2.4 3.6 41° 26° 0.75 16 May 1969 - 2.9 . - 1. 7 3.4 30° 135° 0.85 21 May 1969 + 1. 9 - 1. 7 2.6 42° 27° 0.73 22 May 1969 +37.0 +22.1 43.0 31° 314° 0.86 23 May 1969 +22.1 + 5.5 22.7 14° 331° 0.97 2J~ly 1969 +14.1 + 5.1 15.0 15° 330° 0.94 3 July 1969 +21.8 +13.7 25.5 32° 313° 0.85

10 July 1969 +36.6 - 9.7 37.7 15° 00° 0.97 21 July 1969 -44.2 - 4.7 44.4 6° 159° 1.00 23 July 1969 - 4.0 + 1.2 4.2 17° 182° 0.95 2 Sep. 1969 -58.0 -15.0 60.0 15° 150° 0.97

_4 Sep. 1969 -±2'~1- +13.8 15.8_ 61° 284° I--- O~ - r- --Site 10

f-- - --16 July 1969 -20.0 +10.8 22.7 28io 208!0 0.88 0 21 July 1969 -36.3 - 7.9 37.0 12~0 16n° 0.98 22 July 1969 -30.0 - 1.2 30.0 2° 178° 1.00 I

3 Sep. 1969 -38.0 -10.0 39.2 15° 165° 0.97 I 4 Sep. 1969 - 0.4 - 9.0 9.0 8n° 92!0 0.04 I

x) + is upcoost direction. - is downcoast direction.

xx) + is onshore direction. - is offshore direction.

xxx) "Angle to Coast" given is the Angle < 90° from upcoost or downcoost direction as appropriate.

xxxx) o is outflow tide. I is inflow tide.

-60-

Two types of float poles were used, Type I with the ratio

proiecte~ ar.e: of vanes = 23, prior to May 1968, prolecte Win age area

and type II with the similar ratio of 73 thereafter. The substitution was designed to minimise wind effects on the results, but figure 60 shows no significant difference between the results obtained with either type.

In figure 61 is plotted the average longshore current versus the time of year to investigate possible seasonal effects on current velocities. In figure 62 the average longshore current is plotted versus the average wind direction during the experiment. Figure 63 is a plot of the average longshore velocity component versus average swell direction during the course of the experiment. Figures 64 and 65 indicate the frequencies of the daily average longshore velocity component. In figure 64 three lines are drawn - for upcoast, down coast and combined (absolute) values. In figure 65 upcoast currents are considered positive and downcoast currents negative, thus taking into account the current directions.

Some measurements of current profiles in a vertica I direction were made using a "Planeta" pendulum current meter, and the results are shown in figures 66 - 68. Also on these figures are plotted the velocities and directions of the current as determined from the movement of the float poles.

There appears to be no general shape for the vertical velocity profile, and a reasonable approximation to the measured profiles is a constant velocity from just below the surface to just above the bed. There appears to be a reasona­bleness about the directional change with depth, but the overall change is rather small. The slow change observed in direction with depth is probably due to the diminishing effect with depth of the orbital currents which are superimposed on a general current fairly steady in direction and fairly constant with depth.

The agreement between the mean current velocity over the whole depth as calculated from the vertical profiles, and the velocity as indicated by the float poles is quite good, and the directional agreement is also of a high order. In all cases however the float poles tend to give a velocity slightly less than the mean determined from the pendulum current meter, the discrepancy being largest at low velocities as would be expected.

Both methods are subject to the measuring errors inherent in such work, The agreement in the results is a good indication that the velocities and directions determined from the float pole measurements are adequately representative of the current over the whole depth profile.

c. Surf-zone (Littoral) Currents -.--------------------------Surf-zone currents were measured at three locations, namely at The Spit

(Zeta lines), at Palm Beach (Delta lines) and at Tugun (Gamma lines),(figure 4). A small quantity of'tracer dye w(ls dropped in the surf-zone. The progrEissof the dye

-61-

patch was followed and positioned as well as the travel I ing time from one position to another position. The drop positions were varied laterally along the beach during each experiment as necessary to avoid the dye going sea­ward immediately in a rip current. The results of the measurements are sum­marized in table 39, together with general information about waves and wind. The velocity is presented as the daily average of the body average velocity, the "body" being the central position of the dye patch, determined visually. The average velocity was calculated by dividing the total distance travelled by the total time. This was done for both the body and the head of the dye. The radings of the body average velocities were not always obtainable. From the results of the experiments a reasonably good mean value for the ratio

Head average velocity Body average velocity

could be computed, being 1.25. In the cases that the body average velocity was not obtainable, it was thus computed from the head average velocity .. From the tests performed at each site on a certain day the daily average of the body average velocity was determined and this value is presented in table 39.

During many of the tests, the dye moved some distance from the drop point inside the breaker-zone, and then went seaward in one or more places, due to rip currents. In a few cases the dye later returned through the breakers towards the shore. Table 40 lists the distance from drop positions to rips, and thus gives an idea of the spacing between rip currents.

TABLE 39 S, of Surf· C< Meo' "'""' .... '1: ... , .>vn-,<.<.>na ..... urrent .. ",,,,~,,,,,,,"~,,~

~ 1969 0900 hr 0900 hr (Visual)

""'t. riment Dote At No. of Hs from T,_ Weather from S. Nobbv N° The Spit Palm Beh Tugun Tom Waverider Woverider Sea Swell Swell (ft) (sees) (ft) Oirn

1 21(4 8 3.53 7.8 H I"", E· low

2 21(4 8 3.53 7 .• H long

E low 3 22/. 6° 1.69 8.4 0

mod. NE low

• 22/4 5° 1.69 8.' 0 mod •

NE low 5 6/5 7 5.21 7.2 1-2 mod.

E low 6 6/5 7 5.21 1-2

mod. E 7.2 low

7 7/5 5 7." 1-2 mod.

E 6.7 low 8 7/5 6° 7." 6.7 1-2

mod. E low

9 21/5 7° 2.02 6.2 1-2 mod.

NE low 10 21/5 7 2.02 6.2 1-2

mod. NE low

11 21/5 6 2.02 6.2 1-2 mod. NE low

12 22/5 !-1 mod. 6 1.61 5.8 low NE

mod. 13 22/5 6 1.61 5.8 H low NE

" 22/5 6 1.61 5;8 H mod.

NE low 15 23/5 •• 0 0

mod. E 7 2.74

low 16 2:V5 7 2.74 8.0 0

mod. E low

17 25/6 8 3.24 9.9 1-1 long

E mod. 18 25/6 7 3.24 9.9 !-1

Ion. E mod.

19 26/6 7 3.48 •• 6 1-2 10"9

E mod.

20 26/6 7 3.48 '.6 1-2 long

E mod.

2-4 mod.

E 21 27/6 6 4.08 6.5 mod. 22 mod. 27/6 6 4.08 6.5 2-4 mod. E

3.03 6.0 1-2 mod. E 23 2f7 7 law·

24 2/7 7 3.03 6.0 1-2 mod. E low

25 3/7 7 2.79 6.8 1-2 . short

E mod.

26 3/7 6 2.79 6 •• 1-2 "crt E mod.

4/7 7 5.20 •• 2 1-2 mod. E 27 mod.

7 . 5.20 8.2 1-2 mod. E 23 4/7 mod.

a - one test gives no usable results.

+ I.IpCOCISt direction of SUMone current. - downcoost diredion of turf-zone current.

All velocities in ft/m;n (oxeopt wind).

Weather ot Site DoilyAv. of Durtltion Soc Swell Swe~l SodyAv. Vel. of Expt.

Difl (h~)

_th 3'-4' choppy - + 82.5 6.5

"""oth choppy - - + 37.5 7

smooth 2' - +38 5.5

,",ooth 1'-2' - + 10.5 • slight 4'-5'-4' E +100 6.5

slight 4'-5'-4' E +100 6.5 ,,"oath slight 4'-6' E +119 5.5 slight mod. 4'-5' SE-E +112 5.5

slight 0 - +11 6.5

,,"ooth 2' E - 24 6.5

slight _II NE - 21 5.5 slight choppy

0 - + 26 5.5

slight 2' E +33 5.5 ,,"ooth choppy 0 - + 31 5.5

- - - + 29 6.5

""OOth 2'-3'· E +64 6.5

0-1' -. SE + 69.5 7.5

0-1' mod. E + 31 6.5 low

".,..th 10"9 SE + 49 6.5 -. low

0 mod. low E + 47.5 6.5

1'-2' 10"9 SE low + 52.5 5.5

mod. SE 0'-1'

~~. + 45.5 5

2'-8' SE _46 6.5 low

1'-4' 1o"" SE + 31.5 6.5 low ,

2'-4' mod. SE + 18.5 6.5 low

1'-2' mod. SE +23 5.5 low

1'-2' 10", SE +98 6.5 low

4'-8' ,hort

E +108 6 mod.

local Wind Av.Vel. Av.

(mph) Dirn

10 NNW

5 NE

0 -0 -

10 SE

12 SSE

10 SSE-E

10 S

4 NW

5 NW

7 NNE

15 SE

12 SSE

15 SSE-S

- -0 -

va,. var. 4-10 SE-E-NW

3 ENE-E

va,. var. 5-20 NW-SE va,. va,. 0-. E-SE-SW

5 WSW

10 S

18 SE

10 SSW-S#

6 SE

10 ESE-SE

0 -6 NW

Site

9

9

9

9

----9

9

9

9

9

9

9

9·

------9

9

9

9

--

Offsho.", Currents Av. Vel. Mean Djr

17.0 , .1480

, 148° 17.0

3.6 26°

3.6 26°

- -- -- -- -2.6 2,0

2.6 2,0

2.6 2,0

43.0 3140

43.0 3140

43.0 3140

22.7 331°

22.7 331 0

- -- -- -- -- -- -15.0 330

0

15.0 330

0

25.0 3130

25.5 : 3130

- -- -

I

'" "" I

-63-

TABLE 40 Spacing of Rip Currents

Distance Drop to Distance Drop to Expt. No. Test No. Rip(s) in ft. Expt. No. Test No. Rip(s) in ft.

1 2 > 1300 16 5 800 3 1450 6 800 4 1450 7 800 5 1450 and 1200 17 1 130 6 1200 and 400 5 800 7 1450, 1200 and 400 18 1 530 8 1050 and 400 3 530

2 6 650 4 400 7 650 20 3 920

3 2 800 22 6 530 3 920 24 6 650 4 400 and 800 7 650 5 400 and 800 25 7 130 6 130 26 3 650

5 1 400 4 650 2 400 5 650 3 250 28 5 650 6 400 and in at 800

7 2 450 10 3 530

4 530 5 530 6 530 and in at 800 7 400

13 5 650 6 650

15 3 250 16 1 530

2 800 3 650 4 650

-64-

3.7.3. Discussion and Conclusions

There appears to be little point in attempting to correlate surface currents with wind directions as the drift cards were often at sea for some days and detailed wind records are not available. Nevertheless some important con­clusions can be drawn from figure 59:

(i) There are two significant surface currerit directions, namely about 3300

and 150

0, being the average upcoast-downcoast bearings.

(ii) Wind direction, in an increasing rate with increasing wind speeds, is certainly one factor determining the surface current direction, but not the only one.

Especially for sites 6, 7 and 7A the plots on figure 59 show a considerable scatter. On these sites other factors, such as wave directions or in- and outflow of river entrances, prevail.

In table 37 the results of the surface current measurements are listed. In the same table also the results of the sub-surface float pole measurements, executed on the same day that the drift cards were released, are mentioned. In general there exists a reasonable directional agreement between float poles and cards. The agreement between mean drift card velocity and average float pole velocity is less. .

. From the results of the sub-surface float pole measurements the following general conclusions are drawn:

(i) The longshore component is .generally much greater than the on-, offshore component.

(ii) Both components can change considerably in magnitude and even in direction during the course of a test. This was particularly so at sites 3, 6 and 8, and for some experiments at sites 1 and 7.

(iii) There appears to be no correlation between either component and any tidal phenomena.

Table 38 gives the daily average longshore and on-, offshore velocities. From the components, the net mean velocity was obtained for each experiment in both magnitude, direction relative to the adjacent coast and absolute direction. In addition for each experiment the ratio

average longshore velocity average net velocity

was calculated. The value of this ratio indicates that at sites 1, lA, 2, 6, 7, 7A, 8, 9 and 10 the net velocity is predominantly parallel to the adjacent coastline. At sites 3, 4 and 5 the on-, offshore components are large relative to the longshore component. This could be due to tidal influences of the adjacent Nerang River (sites 3 and 4) and Tallebudgera Creek (site 5). It is concluded that the net velocity

-65-

is predominantly parallel to the coast, except in the vicinity of the mouths of rivers and creeks where the in- and outflow wi II have a ma jor effect on net current velocities and directions at times when the general current parallel to the shore is weak.

TABLE 41 Average Sub-surface Current Velocities

Site Approx. distance to Av. of Upcoast Av. of Downcoast Average of all Long-shore in miles Current in Currents in shore Currents

ft/min. ft/min. ft/min.

B I,B - 53.5 } 9 2,0 IB.O 25.1 30.4 10 2,0 - 24.9

7A 1,5 24.3 40.4

1 IA 1,2 17.5' 12.7 lB.5 3 1,2 1.0 15.2

1 0,6 14.4 14.4 ) 7 0,6 17.4 21. 7 5 0,6 8.5 12.0 14.9 4 0,5 6.5 0.3

) I I

6 0,5 - 45.3 2 0,5 8.9 -

Table 41 lists the average values of upcoast and downcoast longshore velocity components at each site. Averages are taken over the three groups

in

of sites at approximately! mi Ie, I mile and 2 miles from the shore. The fairly definite indication is that the magnitude of the velocity increases as the distance from the coast increases. This is confirmed by consideration of the results of mea­surements at sites 7 and 7Ai however with the modification that when the upcoast component is weak at site 7 the current at 7A may well be down coast . When the upcoast velocity is strong at 7, it is upcoast and greater in magnitude at 7A.

In figure 61 is plotted the average longshore current versus the time of year, and the trend suggested is that down coast currents tend to increase during the year reaching maximum values in spring (August, September) with a corres­ponding decrease in upcoast currents.

In figure 62 the average current direction is plotted versus the wind direction during the experiment. The resulting distribution is fairly random, although there is the suggestion from the plot that down coast velocities are more likely to occur with winds from W over N to NE and upcoast velocities with winds from other directions.

-66-

Figure 63 is a plot of average longshore velocity component versus average swell direction, with on Iy those points for which the swell direction during the course of the experiment was fairly constant being plotted. From the plot it appears that a trend may be evident as follows: If the average longshore velocity component is directed downcoast, then its magnitude is independent of the swell direction. However if the average velocity component is directed upcoast, then its magnitude increases as the swell direction goes from N over E to SE.

The facts found and stated above may well fit the hypothesis that the cur­rents are spawned by the East Australian Coast Current, being in fact part of that general current but diminishing in value as the coast is approached in the case of dawn coast currents; I n the case when upcoast currents are observed, these may be due to eddies from the 'East.Australian.CoasLCurrent.

The variation in direction, which often occurs from day to day, and the variation in velocity is sugge~tive of eddies, somewhat unstable.

As far as the littoral (surf-zone) current is concerned it is evident that over the tests conducted, the predominant movement was upcoast. From the weather information shown in table 39, downcoast movements might be expected in experi­ments 3, 4, 9, 10, 11, 12, 13 and 14 only (the experiments with reported NE swell) •. Downcoast currents occured in experiments 9, 10, 11 and 5, the latter being a purely temporary movement occasioned by a rip current.

It is concluded ·that the direction of longitudal surf-zone current is upcoast when the waves originate from E and SE directions and downcoast when the waves come from a nor.th-easterly direction. The fact that some upcoast movements were observed under swell conditions from NE does not conflict with this general state­ment, as even in these cases the vyaves at the site were small and the directions of the waves and resulting littoral currents were modified possibly by rips and/or the offshore bar.

As indicated in table 39, severa I surf-zone experiments were conducted simultaneously at two or more sites, and table 42 gives the results, of them.

At Palm Beach and Tugun the results are very nearly equal on anyone day. At The Spit .quite <;>ther .values of the velocities were measured, sometimes even with an opposite direction to that at Palm Beach and Tugun.

The littoral current velocities vary considerably from day to day. The littoral current velocity measured is a function of:

-. breaker wave height - angle of wave incidence - wave period - the profile within the surf-zone

local winds - . genera I current

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TABLE 42 Surf-zone Currents on Same Day at Different Sites

Experiment No. Velocity in ft/min. at

The Spit Palm Beach Tugun

1 and 2 82.5 37.5 -3 and 4 38.0 10.5 -5 and 6 - 100 100 7 and 8 - 119 112

9,10 and 11 11 -24* -21* 12,13 and 14 26 33 31

15 and 16 29 64 -17 and 18 69.5 - 31 19 and 20 49 47.5 -21 and 22 - 52.5 46.5 23 and 24 46 ., 31.5 25 and 26 18.5 28 -27 and 28 - 98 108

* velacities directed down coast

Most of fhese' fac.tors have been taken into account in Eagleson's [ 18J formula:

a

= the value of the longshore current velocity = the breaker height = the breaker depth = the ratio of, group velocity cg to wave celerity

1 [ 41lhb/Lb J = "2 1 + sinh 41lhb/Lb

= beach slope !'pb = angle between breaker crests and coastline 2 f = Darcy-Weisbach resistance coefficient = 8g/C g = earth acceleration C = Chezy resistance coefficient Lb = wave length (at breaking) h = water depth.

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Calculating with this formula, using visually observed wave heights and wave directions mentioned in table 39 and the results of the refraction diagrams, the longshore velocities at the three sites and comparing the results with the results of the measurements, there seems to be a reasonable agreement between the experimental and the computed values for the sites Palm Beach ond Tugun especially at times of high wave heights. At The Spit the measured velocities are considerably' lower than the computed velocities.

3.8. Bottom Composition

3.8. I. General

The nearshore bottom consists of gently sloping bed-rock, covered by a layer of sand. By echo soundings and seismic measurements [2] the thickness of the sand layer has been determined. Some typical cross-sections are shown in figures 69 - 72. The sand layer has an average thickness of 20 - 30 ft at distances from 0.5 to 7 miles from the shoreline. Nearer to the beach the thickness of the sand layer is increasing. At several places reefs are penetrating the sand layer (see figures 7J and 72).

Information about grain diameters and mineral consistence has been made available by sampling. Between 1966 and 1968, 2400 samples (approximately) have been taken along the various sounding lines. Also in 1968 some sub-bottom jet-pipe boring and sample recovery, which yielded valuable information, was carried out off the Gold Coast by a heavy-mineral sand mining company.

3.8.2. Mineral Consistence

The samples generally have a quartz content of 90 - 95 910. Also broken shell has been found in most samples; in the nearshore samples 2 - 5 910, increasing to 10 - 20 % at three miles offshore, with local areas up to 80 - 90 910. Other groins, mostly rock fragments, may occupy up to 5 910 of the sample.

Along the whole coast heavy minerals are found, important for their high proportion of rutile and zircon. The content of heavy minerals in the sand samples is averaging 0.6 910. Large concentrations are mainly found at present or post shorelines and dunes. Most of the high-grade deposits in the area have been exploited already by intensive miningj other deposits are unavailable because of surface deve­lopments. The largest known reserves of heavy mineral sand in the State are found on North Stradbroke Island. Detai led information about the exploitation of mineral sands is given in the Connah's Report [19] • The influence of sand mining activities on the cOdstal erosion is discussed in section 3.10.1.

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3,8.3. Grain Diameters

The bottom samples contain fine to moderate sized grains. On the beaches and the bar in front of the beaches the material is slightly coarser than in between and further seaward. I n areas deeper than 50 ft the materia I is generally coarser aga in. D50 values are consistently between 0,2 mm and 0.3 mm, with some local offshore areas having higher values up to 0.6 mm or even 2 mm near Stradbroke Island.

Figures 73 and 74 show an overall plan of the area with median grain dia­meters of the samples taken along the Omega lines (1966). The samples at the Omega 14 line have a slightly smaller median diameter than at the other lines. It can not be concluded from this, however, that the coarser material does not pass the Omega 14 line, because of the possible spread in diameters, The results of sam­pling in this area in 1970 show already coarser material at the Omega 14 line (see figure 73), In the northward direction there is a trend that the diameters become somewhat smaller, which is normal considering the shape of the coastline between Point Danger and North Stradbroke Island.

On various locations (Alpha to Zeta lines), detailed sampling has been carried out in 1967 and 1968, The most important results are shown in figures 75 - 77, which give a picture of the values of Dl0, D50 and D90 in the various groups of sounding lines for one sampling time. The distribution of grain sizes along the Alpha, Beta and Zeta lines appears to be rather similar. The D50 has a fairly constant value of about 0,2 mm up to the measured distance of 5500 ft offshore, Only within the first 2000 ft the D50 is a little higher.

Also the Gamma, Delta and Epsilon lines show much similarity, Here the finest material is found between 2000 ft and 4000 ft offshore (D50~ 0,15 - 0,20 mm). Near the beach the 'sand is slightly coarser. Further offshore than 4000 ft the diameters are gradually increasing again. Between 5000 ft and 6000 ft, D50 is 0.25 mm to 0.30 mm. The spread in the diameters along these lines is also increasing in the offshore direction, mainly as far as the presence of coarser material in concerned (Dl0~ 0,25 mm - 0,30 mm at about 3000 ft, and 0.4 mm - 0,5 mm at 5000 ft). In 1967 samples are taken at the ends of several Omega-lines (approximately 8 miles offshore), The grain diameters are generally of the same size as at the offshore ends of the Alpha, Beta etc. lines.

In figure 78 the variation in D50 at the various dates of sampl ing is given for the Alpha lines, It is remarkable that in October 1968 the sand is definitely finer than in 1967, This change extended even to depths of 70 ft and more, which means that transport at these depths should be taken into accounL

As far as sufficient samples from different dates are avai lable from the other lines, no significant variations in time can be recognized.

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The skewness

and sorting

coefficients of all samples were determined. Little variation from the normal distribution was evidenced by the skewness values (0.9 - 1.1), while sorting values tend to remain fairly constant between 1.2 and 1.5, increasing in the offshore direction.

3.8.4. Samples from Stream Beds

During 1969, 280 samples were taken from the beds of Nerang and Tweed Rivers< Tallebudgera and Currumbin Creeks at locations shown on figures 79 - 82, and on which are shown also the results of gradings of typical samples.

In the Nerang River, the gradings up to the junction with Little Tallebudgera Creek, that is in the Broadwater and the portion of Nerang River running northward parallel to the coast, are simi lar to offshore gradings, or slightly coarser. Beyond this point, clay and mud begin to predominate, while beyond Nerang Town the bed is gravel. Thus bed material in the Broadwater and up to six miles upstream would be suitable for beach nourishment usage.

In Tallebudgera Creek the bed material retains a remarkably consistent grading up to 4 miles from the mouth, such grading ~eing almost the same as the offshore samples. Beyond this point the bed material becomes markedly more coarse.

The gradings in Currumbin Creek exhibit high similarity to approximately 1. 5 miles upstream, and then become coarser.

The bed material in the mentioned lower reaches of both of these Creeks would be quite suitable for beach nourishment purposes.

In the lower reaches of the Tweed River the median grain diameter tend to be larger, i. e. slightly coarser sand, than in the other streams sampled and on the beach and offshore areas.

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3.9. Sediment Transport

3.9. 1. General

In general and certainly on the Gold Coast, ocean and tidal currents are relatively weak and are ,not able to transport bottom materials on their own. The oscillatory wave motions, however, can be so intense that the waves bring sediments into suspension and then the currents can carry the sediments away.

In shallow water the front of a wave crest becomes steeper than the back. The corresponding forward (shoreward) flow is stronger and lasts for a shorter time than the return current. As the transport capacity of a flow is approximately pro­portional to some power of the velocity (in general a fourth to a seventh power), it depends on the steepness of the waves whether there is a net seaward or net shoreward material transport.

Near the shore, the waves break with high water velocities into a turbulent mass of water which contains suspended sediment picked up from the bottom. The profile of the beach tends to be in a state of equilibrium determined by the charac­ter of the attacking waves. This state of equilibrium continuously changes with variations in the direction, the height and the period of the waves that approach the coast.

If the wave crests arrive at an angle with the shoreline, the breakers will cause a longshore current. The magnitude and the direction of the subsequent trans­port of sediments along the coast depends primarily on the angle of the wave inci­dence.

If the overall picture of changing wave directions and wave heights in front of a coast results in this way in a net sediment transport along the coast, a state of equilibrium is only possible when this net sediment transport has the same magni­tude along the whole coast".

3.9.2. Computation of Littoral Transport

Through a number of profiles along the Gold Coast the longshore transport capacity is computed, using the method described in publication No. 58 of the Delft Hydraulics Laboratory [20]. These computations give an insight into the di­rection and magnitude of the sediment transport. Together with the information elaborated from the sounding surveys, the accretion in front of the Tweed River Walls, 'the growth of the Spit etc, the computations yield data required to solve th!'} general erosion problem and to indicate the sequence of works to be executed.

The profiles (5, 7, 8 and 9) through which the sediment transports are cal­culated, are'situated perpendicular to the shoreline and are indicated on figures 46 - 56. The tota I length of these profiles is about 600 metres. The profiles are aivided in belts of equal lengths of about 20 to 30 metres. Hardly any major movement of sediments occurs further seaward than 600 m, as is concluded from the results of these computations. This is also confirmed by the results of the succes­sive sounding surveys, which show that below a depth of about 30 ft the profiles

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have not changed significantly throughout the years of survey. Starting with a certain wave direction, wave height and wave period in deep water, the wave heights and wave lengths within each belt of the profi Ie are computed, taking into account the direct influence of shallow water on heights and lengths and the influence of the refraction of waves on the wave heights. At a depth of 1.3 times the wave height the waves will break. Within the breaker-zone it is as­sumed that the wave height diminishes· linearly to zero from the breaker point till the beach line. Within the breaker-zone the littoral current velocity is computed from the formula of Eagleson [18]. This computed velocity agrees rather well with the results of the surf-zone current measurements. The current is assumed to be

. invariable with the distance from the coast (within the breaker-zone). The direction of this current is taken as down coast when north-easterly waves occur and upcoast when easterly and south-easterly waves occur (see section 3.7.3.).

Outside the breaker-zone the longshore current velocity is taken as 20 ft/min. (0. I rrVs), which seems to be a rather good average value according to the results of the longshore current measurements (section 3.7. 2b.). As for the dIrection of the current outside the breaker-zone, in general the same criteria are used as those applied to the current within the breaker-zone, i. e. downcoast for north-easterly waves and upcoast for easterly and south-easterly waves.

for the actual computation of the bed-load transport (Sb) in each belt a normal bed-load transport formula is used,. in which however the bed shear is increased as a result of the wave motion.

in which: ~

C a U

a

Sb = b • D • v

C-

= 0.0575 Co h

= 18 log (12 k) _ 1T H _--,1....,.-,­- T . h 21Th

Sin -C L

= (...5:.)3/2 C

1

o

C1

= 18 log (12 Dh ) 90

ADC2

_ 0.27 ___ --:-_0 __ ...-.-_

fI V2 f I + ~ (t; ~0)2 } V9. e

notations: Sb = bed-load transport in m3/s per unit of width

b = factor (= 5) D = 50 /,0 grain diameter in m D. = i /,0 grain diameter in m

I

-73-

Y '" current velocity in m/s Co '" bottom roughness in ml/2,Cs g '" earth acceleration in m/s2

A '" relative density = (Ps - Pw)/p '" ripple coefficient '" (CoIC1)3~ J.l

C1 U hO

k H T L

= bottom roughness due to groins alone '" maximum orbital velocity above bottom = water depth in m '" factor for ripples'" 1/2 (ripple height) = wove height in m '" wove period in seconds '" wove length in m

in m/s

in m

For the calculation of the suspended-load transport (5s) Bijker appl ies the procedure of Einstein [21J. The concentration of suspended load immediately above the bed is derived from the bedload transport, assuming that this. bed-load will be transported in a layer immediately above the bed with a thickness equal to that of the fictitious bed roughness

in which:

notations: 5 s w

K Y-J

'1 and '2

1

.( h

(1 - y)z In y dy y .

Z"'~ K~

f U} 1/2 11 + 1/2 (~ yO)2

= suspended-load transport in m3/s per unit of width '" fall velocity of 50 <yo grain diameter in m/s '" kappa, factor of Yon Karman = 0.4 = shear velocity in respect to waves and current in m/s '" integrals (see Einstein).

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According to the results of the bottom samples analyses (section 3.8.3.) the 50 '10 grain diameter and the 90 ~o grain diameter are respectively determined to be 0.000225 and 0.000350 m (average). The k-value, factor for ripples being half the ripple height, is chosen to be O. 17 m. In a number of cases the computations are also executed with k-values of 0.10 and 0.25 m, thus considering the influence of the roughness on the magnitude of the transports. The value Sb + Ss for all belts gives the total longshore transport capacity through the profile caused by the wave characteristics (direction, period and height) for which the computation is set up.

The above-mentioned computations are done for four profi les and for the three wave directions NE, E and SEt Each direction is combined with wave periods of 6, 8, 10 and 12 seconds. For each combination the computation is executed with diffe­rent wave heights. Thus the relationship between the transport through a profi Ie by NE, E Or SE waves of 6, 8, 10 or 12 seconds periods could be established.

It is concluded that the transport may be considered to be proportional to the square of the wave height (e.g. see figure 83). From this conclusion and the assumption of a normal wave height distribution in front of the Gold Coast, it can be derived that the wave height which has to be applied in the computations equals 0.7 of the signi­ficant wave height (see figure 84).

For the calculation of the littoral transport per year through the four profiles along the Gold Coast (see figure 85) the wave heights, periods, directions and the frequencies of occurrence of these phenomena, elaborated from the K. N. M.l. obser­vations (section 3.6. 2c), have been used. It is assumed that the visually observed wave height equals the significant wave height. This value is multiplied by 0.7 and thereafter used in the computations. The K. N. M.1. observations were the most detai led ones - apart from the Waverider measurements, which were not all available at the moment of carrying out these computations. Especially should figure 84 be compared with actual data from the Waverider when such become available.

3.9.3. Results

Table 43 gives the results of the littoral-transport capacity computations. The transports caused by sea and swell from a north-easterly, easterly and south-easterly direction, inside and outside the breaker-zone, are given j,1 cubic yards per year. The net transport through a profi Ie is computed as the sum of the transports caused by sea and swell inside and outside the breaker-zone. The transports are calculated using a value of 0.17 m (= 0.56 ft) for the fictitious bed roughness. Since the fictitious roughness is equal to half the ripple height, the equivalent ripple height is about I ft. This is rather well in accordance with reported ripple heights at Palm Beach and Coolangatta. These ripple heights are observed up to about a depth of 5 ft. Sometimes a definite pattern, consisting of ridges and troughs parallel to the coast, was observed, but mostly no set pattern could be distinguished. In order to obtain an idea about the influence of the bed roughness on the magnitude of the transports, for profile 9 (south of Tweed River - Alpha lines) the transport computations are repeated for bed rough­nesses of 0.10 and 0.25 m (= 0.33 ft and 0.82 ft). The results are given in table 44. It is evident that the bottom roughness influences the magnitude of the littoral transport, but within the range of practical values not to a major extent.

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TABLE 43 Littoral Transports

Profile 7. The Spi t (Zeta lines 1966) Profile 5. Tallebudgera Creek (South 1966)

A B A B

Swell: NE - 110,000 - 35,000 - ~O,OOO - 70,000 E - - 140,000 + .90,000 + 115,000

SE + 480,000 + 170,000 + 90,000 + 75,000 Sea: NE - 55,000 - 25,000 - 5,000 - 50,000

E - + 70,000 + 40,000 + 45,000 SE + 225,000 + 55,000 + 20,000 + 10,000

+ 540,000 + 99,000 (15/,0) + 215,000 + 125,000 (37/,0)

+ I,OO~OOO - 365,000 = 635,000 . + 485,000 - 145,000 = + 340,000 (+ 158 ;0 - 58/,0 = 100 <yo = net transport) (+ 143 <yo - 43 <yo =100 <yo = net transport)

Profile 8. Tugun (Gamma lines 1966) Profile 9. South of Tweed River (Alpha 1962)

A B A B

Swell: NE - 5,000 - 55,000 - 50,000 - 70,000 E + 55,000 + 115,000 + 60,000 + 155,000

SE + 50,000 + 35,000 + 175,000 + 160,000 Sea: NE - - 25,000 -. 15,000 - 55,000

E + 5,000 + 40,000 + 20,000 + 80,000 SE + 10,000 + 5,000 + 75,000 + 90,000

+ 115,000 + 115,000(50 <yo) + 265,000 + 360,000 (58 /'0)

+ 315,000 - 85,000 = + 230,000 (+ 137 <yo - 37 <yo = 100 <yo = net transport)

+ 815,000 - 190,000 = + 625,000 (+ 130 <yo - 30 <yo = 100 /,0 = net transport)

Transport in· cubic yards per year. + = upcoast (northerly direction). - = downcoast (southerly direction). A: transport inside breaker-zone. B: transport outside breaker-zone. bed roughness: 0.17 m (= 0.56 ft). underlined figures: net transgort through profile. figures (<yo) are ratio t t t <yo. ne ranspor

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TABLE 44 Littoral Transport Through Profile 9 - Variable Bed Roughness·

Bed roughness: O. 1 m (= 0.33 ft) Bed roughness: 0.25 m (= 0.82 ft)

A B A B

Swell: NE - 90,000 - 95,000 - 45,000 - 55,000 E + 60,000 + 195,000 + 35,000 + 130,000

SE + 270,000 + 230,000 + 155,000 + 145,000 Sea: NE - 30,000 - 80,000 - 5,000 - 50,000

E + 10,000 + 90,000 + 5,000 + 70,000 SE + 90,000 + 110,000 + 60,000 + 75,000

+ 310,000 + 450,000 (59 /,0) + 205,000 + 315,000 (61 /'0) + 1,055,000 - 295,000 = + 760,000 (+ 139 '10 - 39 '10 = 100 <yo - net transport)

+ 675,000 - 155,000 = + 520,000 (+ 130 <yo - 30 <yo = 100 '10 = net transport)

Transport in cubic yards per year. +=upcoast (northerly direction). - =downcoast (southerly direction). A: transport inside breaker-zone. B: transport outs ide breaker_zone. underl ined figures: net transport through profile.

figures (/,0) are ratio B 0 net /'0. transport

3.10. Morphology of Bottom and Shore

3. 10. 1. Genera I Methods of Approach

If a solution has to be given for an erosion problem, it is important to under­stand the phenomena which cause the erosion and to have information about the quantities involved. Dependant on the available data, different approaches are possible. For the Gold Coast the following methods can ·be useful:

1. Theoretical computation of littoral transport. For this purpose the coast has been divided into a number of sections. For each section the littoral transport can be calculated, considering the wave conditions of the specific area. From the deficit or surplus between various sections the erosion or accretion as a function of time can be derived. In section 3.9. these computations were dealt with.

2. Estimation of erosion or accretion from regression or ·transgression of the shoreline. As these data are available over a 30-year period, they give information about the longterm tendencies.

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3. Study of detai led soundings in various profiles along the coast. The~e soundings have been started in 1966, so that they give information concerning the last few years' only. The results are interesting, however, because several cyclones occurred in this period. As the surveys are rather detailed and frequent they give an interesting picture of the behaviour of the coast after a cyclonic period.

Besides the influence of waves and currents, also other factors are affecting the condition of the beaches, and their effects will be included in the figures found from the above-mentioned approaches 2 and 3. These factors are mostly man­induced, for example sand removals for building activities or mineral sand recovery, deterioration of dunes,' etc.

The cjuantities involved with mineral sand mining can be estimated, using the table of Saleable Beach Sand Production from Connah's paper [19). In the years 1941 - 1962 approximately 280,000 tons of heavy minerals were produced from the Gold Coast beaches and N. Stradbroke'lsland. Before that period no mineral in any important quantity was j1roduced. Assuming a relative density of 3.5 as qn average, this yields 106,000 cu. yd from beaches and dune areas over this period, which is about 5,000 cu.yd year. So considering the quantities, the mineral sand mining does not give, serious losses for the coast as a total. It should be carefully watche,d, however, that the exploitation 'does not give rise to deteriorations of beaches and dunes at certain locations.

The quantities of sand u~ed for other purposes are more difficult to estimate. ,It may be assumed that these total qU~lntities are also less important than the local deteriorations which may be caused. Besides, at a coast where sand supply is a problem, it is not acceptable that sand from beaches and dunes is removed.

The dunes and the upper parts of the beaches serve as storage areas for sand, ,which can be used in times of severe wave attack on the beaches. Disturbances of these areas by man can undermine this function. Large-scale flattening of dunes for house'building purposes which has happened in the past must never be allowed again in any circumstances, in this or any other area of the State. Even any local sand removal which leads to disturbance of natural vegetation will increase the possibilities for the wind to carry the sand away. The efforts being made to catch

'the sand by fences, etc. show a rapid build-up of the dunes, which means that the quantities transported by wind are not negl igible.

Several parts of beaches have been protected by sea walls. These walls give protection for the hinterland, but downdrift from the walls in the long term and in front of the walls during stdrm periods, increased erosion takes place (see section 3.10.2., Narrow Neck). The same effect is noticed on a smaller scale where private properties have been protected. In that case the adjoining areas suffer from increased losses. This illustrates that protective works cannot be carried out separately, without consideration of possible negative effects elsewhere.

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3.10.2. Erosion/Accretion Quantities from Shoreline Changes

Plans prepared by the Gold Coast City Council in 1962 show changes in the position of the shorelines during the period 1932 - 1962.

An attempt is made to estimate the eroded or accreted quantities, using the depth of no-change found from recent soundings. Beyond the inaccuracies in this calculation method, the shorelines on the G.C.C.C. plans have to be considered with certain restrictions due to the following:

~ The shoreline on a certain map is a rather arbitrary one for the considered period, because the real shorelines show daily and seasonal variations. Gene­rally the H. W. M. on the day of survey is used. Also the influence of protective structures built during the surveyed period has to be taken into account.

Nevertheless the figures derived from the maps give interesting infprmation, from which tendencies in behaviour of various beaches can be stated. T;!,e figures are given in table 45, together with remarks about the presence of protective structures. The results are illustrated in figure 86, where the eroded or accreted quantities are mentioned at the various locations.

Study of this map leads to the following remarks. North of Narrow Neck the erosion rate is great. Since about 1920 protective works have been carried out at Narrow Neck, so that this can be considered as a fixed point, with a high erosion rate downdrift and no erosion immediately updrift.

For the stretch Narrow Neck to South Nobby the average erosion rate per year is 11 - 15 yd3/yd. The spread in this figure is due to the uncertainty of the rates between First Avenue and Peerless Avenue (Broadbeach). For Palm Beach 10 yd3/yd per year has been found and for the beaches between Currumbin and Snapper Rock this figure is 5.8 yd3jyd. , Based ?n ,these figures, the average loss for the s'tretch b~tween .Poiht ~a~ger a~d, Narrow ~.ec,k ,~s, l.o.7yd3/yd per year (total' 255, 000 yd3/yr), ,'with, an 'increasing erosion rate i.n the northward direction.

If erosion quantities are calculated from regression of the shoreline, the ca Iculated losses have to be reduced because part of the regression is caused by rise of the sea level. Mean sea level rise is generally determined to be 1. 12 mm/yr. Assuming that beyond the 50 ft depth contour, located at an average distance of 3400 ft from the dune crests, no transport occurs, the reduction of the above erosion rates will be about 1. 4 d3 d er ear, which yields an average erosion of 9.3 yd3jyd per year 15,000 yd yyear.

The average horizontal regression of the shoreline is in the order of 1 ft/year. Although this average regression is not strong, figure 86 shows that at various locations the erosion is much more serious, so that these locations require special attention.

TABLE 45 Shorelines Changes 1932 - 1962

General kea Period Location Average Rate Section Type Depth of "No Height of Dune Quantity Ac.:cumu lated Romack> -I Change (I'/ye.,) Length' Section Change" (lee,) Quantiry (Ieet) (miles) (lee') yd3/yd.'I' yd3/yr

Main Beach 1932-62 (a) S.L.S.C.-San Souci -251 ~.4 . 0.30 Zeta 50' 20' -65.3 - 34,500 No Protective Works till Boulder WaH, 1968 ,

(b) San Souci-<:able St. N -198 -S.6 0.37 Zeta 50' 20' -67.0 - 43,500 BoulderWatl Constr'd 1950's. SayJJnrestricted to 1955, (c) Coble St.-Higman St. ( N:k) - SS -2.9 0.50 Zeta 50' 20' -22.6 - 19,800 Protection commenced 1920's. Boulders and tim-

berpi!es now nave fixed this point effectively Surfers-Broad- 1932-62 (a) Higman St. -View Av. - 18.5 -".6 0.48 Epsilon 50' 20' - 4.8 - 4,000 Small Dune remained until June 1967. Boulder beach Wall constr'd 1968

(b) View Av.-Clifford St. 0 0 0.57 0 0 BoulderWali Constr'd 1968 (c) Clifford St.-Frederick St. -40 -1.3 0.42 Epsilon SO' 20' -10.1 - 7,500 Someprivateboulderwallsconstr'din 1950's & 60's (d) Frederick St.-First Av. - 75 -2.5 0.45 Epsilon SO' 20' _19.4 - 15,400 Someprivateboulderwallsconstr'din1950's & 60's

Broadbeach 1953-62 First Av.-Peerless Av. + 59 +6.6 1.29 Epsilon SO' 20' +51.2 +116,000 Not a reliable or representative period Mermaid Beach 1932-62 Peerles Av.-Pacific (Heron) Av. -92 -3.1 1.07 Epsilon 50' 20' _24.2 - 45,400 Several privatewalls-mostconstr'd 1960's

I

::(l I

Nobby-Miami 1932-62 (a) Pacific St.-Beach Av. - 58 -1.9 0.58 Epsilon 50' 20' _14.S - 15,100 Someminor(private} boulder walls (b) 8eoch Av.-Hythe S,. +23 +O.S 0.63 Ep$ilon 50' 20' + 6.2 + 6,900 Boulder Wall, 1968

Tallebudgera- 1932-62 S.L.S.C. Tolle-Palm Beach Av. - 50 -1.7 1.66 Delta 40' 12.5 _10.0 - 29,000 Manyprivateboulderwallsin 1950's & 1960's Palm Bch Av Currumbin 1932-62 (a) Curro Rk:-Eleph. Rk.-Flat Rk. -255 -8.5 0.69 Gamma 35 15 -47.2 - 57,200 OldWa\1 exists inarea

(b) Flat Rk.-Tooloona St. -106 -3.5 0.5 Gamma 35 20 _21.3 - 18,800 Old (Private'?) BoulderWa 1.lsexist Tugun 1932-62 (a) Tooloona St.-Shell St. 0 0 0.22 0 0

(b) Shell St.-John St. -48 -1.6 0.61 Gamma 35 20 _ 9.8 - 10,000 Old (Private?) BoulderWalls, largely covered

Tugun-Bilinga 1932-62 (a) John St.-Mills St. -123 -4.1 0.5 Gamma 35 20 _25.0 - 22,000 Old (Private?) BoulderWalls, largely covered

Nth Kirra (b) Mills St.-Coolangotto Ck. +100 +3.3 0.89 Gamma 35 20 +20. 1 + 31,500 Sand Ounesstill exist and growing

Coolongatto 1932-62 (0) Kirra Seoch +141 +4.7 0.65 Beta 21 50 12.5 +32.6 + 37,300 Dunes destroyed by Pedestrian T raffi c

(b) Greenmount - 2S -0.9 0.5 Beta 11 50 20 _ 7.0 - 6,200 Earlysolidwallsconstr'd 1920's & 1930's

(c) Rainbow Bay - 39 -1.3 0.2 Beta 1 40 12.5 _ 7.6 - 2,700 NewBoulderWall1968

Note: - sign refers to regression/erosion of shoreline. + sign refers to transgression/accretion of shoreline.

- .. - - ... - .. -- ... ~

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3.10.3, Erosion/Accretion Quantities from Soundings

From 1966 - 1968 soundings were taken at the various sounding lines shown in figure 4 • The Omega lines were surveyed once during that period, the Eta lines annually and the other lines bi-annually, so that very detailed information is available.

To determine the average slopes of the ocean bottom,the Omega lines are most suitable. Figure 87 shows the general shapes of the profiles along several Omega lines. The slope of the first mile of the ocean bottom appears to be rather similar for the various lines, having an average value of 1.4 0/0. At a depth of approximately 80 ft the slope becomes flatter (0.4 'Yo), changing to 0.1 0/0 at depths between 150 and 160 ft. Only directly west from Point Danger the slopes are different from the average along the Gold Coast (see Omega 12). Here the foreshore is flatter (0.7 0/0), while between 1.5 and 4 miles from H. W.M. the depths are greater than in the other profiles.

Figure 88 gives a more detailed picture of the mean profiles during the survey period. Again the different shape of the profi Ie in front of Greenmount Beach can be recognized (Beta lines).

The high frequency of surveying enables an insight to be obtained into the consequences of rough-weather periods. Early in 1967 several heavy cyclones occurred at the Gold Coast. Soundings were taken before, between and after the cyclonic periods (resp. June-Aug '66, May-July '67, Sept-Oct '67), while the soundings in 1968 (May and October) give an idea of the behaviour of the beaches after such periods.

" From the soundings, profiles have been drawn. For some specific profiles or mean profi les the shapes at the various dates of survey are compared in figures 89 - 91. A study of these profiles yields the following information.

Due to the cyclones mainly the upper part of the profile has been eroded. Much sand was moved from the beach in the transverse direction to depths of about 20 ft. Changes in profile are noticeable unto depths of about 35 ft. Probably also in deeper areas some deposition will take place. In this respect also a study of grain diameters leads to the conclusion that at greater depths slight but definite changes take place, which are unlikely to be caused by inaccuracies of measurement because of "the great regularity of the changes (see figures 75 - 78).

After the cyclone periods the sand starts moving again towards the beach under influence of more moderate waves. The soundings from October 1968 show for most beaches a profile which has been restored to an important degree. Unfavou­rably the period of surveys is not yet long enough to draw definite conclusions about possible complete restoration. During the preparation of this report some figures from soundings in 1969 in the area between Point Danger and Tallebudgera Creek became available. It appears that for this area the beach has accreted in 1969. It might be expected, however, that some material will be lost, because of deposition in deep water (deeper than 35 ft) during the rough weather, while under normal conditions the waves do not have the capacity to carry it back.

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An attempt has been made to determine the magnitude of volume changes along the coast, using the following procedure.

From 30 selected lines along the coast, areas have been calculated above specific levels at, in general, 3 times in the period 1966 - 1968. As a good approximation the surveys. can be centralised at the following periods:

mid-August 1966 early-July 1967 mid-July 1968,

indicating approximately one year between each survey. On each line, the position of the R.L. + 5 ft line (2a ft approximately·

above M. H. W. $.) at the time of the first survey was determined, and this line used as the landward boundary of areas calculated. Areas were calculated above the following levels by computer:

+ 5, 0, -5, -10, -20, -30, -50 (feet).

Figure 92 indicates how the areas are calculated from each profile, and it is evident that changes above R.L. + 5 ft, on the high part of the beach and/or dunes,may contribute some material to the profile between R.L. + 5 ft and - 50 ft in times of erosion, and the reverse in times of accretion, but this area is not considered in this exercise.

The contours - 10, -20, -30 and -50 ft were selected as being those likely to confine the zones of most importance.

The data are plotted in various ways. Figures 930 and 93b indicate the areas above the selected levels at each profi Ie for each of the three surveys. The area above R.L. -10 ft (i.e. between R.L. -10 and R.L. + 5) decreases very slowly in a northward direction from Eta 4 (Fingal) to Eta 66 (Narrow Neck), and increases slowly thereafter to the Nerang River. The same comments would apply to the area above R.L. - 20 ft (i.e. between R.L. - 20 and R.L, + 5). For the areas above R. L. - 30 and R. L. - 50 ft, there is a great difference between the calculated values around Greenmount and Kirra Beach and at the Nerang River Mouth, compared with the other areas.

The conclusion may be ·drawn, that the coastline between + 5 and - 50 ft, becomes gradually steeper between Tugun and Main Beach, and between +5 and - 20 ·ft gradually steepens from Letitia Spit to Main Beach. Between - 20 and - 50 ft, wide and flatter shelfs exist between Point Danger and K irra and in the vicinity of Nerang River Mouth. Similar conclusions can be derived from figures 3 and 88.

In figure 94 the respective changes in area above the levels in the periods 1966 - 1967, 1967 - 1968 and 1966 - 1968 are shown. The period 1966 - 1967 (figure 94) was one of severe erosion of the usable beach and dunes. Along practi­cally the whole coast the area above R.L. - 10 has decreased. The areas above levels of greater depths generally show fewer losses or even gains,· which illustrates again the transversal transport in periods of rough weather. It is interesting that the high gains to the profiles above R.L. - 30 and R.L. - 50 ft especially have occurred

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just south of the mouths of all four streams debouching in the area. This may point to a kind of interrupting effect of the longitudinal transport by discharges of the streams, which increases the transversal movements. In the case of Tallebudgera and Currumbin Creeks it would appear that the major gain has been made between R.L. - 10 and R.L. - 20 ft, but in the other two cases it is between R.L. - 20 and R.L. - 30 ft. The total gain between + 5 and - 50 ft includes of course material

'derived from erosion of the beach and dunes above R. L. + 5 ft. The period 1967 - 1968 (figure.· 94b) shows to some extent a reverse picture

of the period before. South from Tugun there are continuing losses at all levels, but north from there the materia I moves back to the sha II ower areas near the shore (above - 20 ft).

The overall picture of the period 1966 - 1968 (figure 94c) is one of loss above all levels from Point Danger to Tugun and gain in the area of The Spit, with the intermediate area being variable.

Integration of the curves in figure 94 represents the volume change in the chosen periods. This volume change is plotted in a cumulative way in figure 95. To a great extent figures 95a, 95b and 95c give an affirmation. of the.above remarks w.ith respect to figure 94.· The slopes of the lines show the loss rates. It appears that in the period 1966 - 1967 the loss rate increases in general in northern direction (figure

. 95a, R.L. - 10 and - 20 ft). Figure 95b indicates clearly the complete lack of gain above R. L. - 10 from Currumbin Creek southward, while north from there the material move~ back to the shore from the deeper water. From additional soundings in 1969, which were incomplete during the preparation of this report, it appears however that there is a definite accretion after 1968 in the whole area between Point Danger and Currumbin Creek.

Figure 95c gives the volume loss over the two years. From this figure the rates of volume changes in yd3/yd can be derived, which are plotted in figure 96. There appears to be a clearly defined general trend, viz.

Fingal to Rainbow Bay - accretion Rainbow Bay to North Kirra - erosion at an increasing rate going north North K irra to Bilinga - erosion at a decreasing rate going norih Bilinga to Flat Rock Creek - accretion at an increasing rate going north Flat Rock Creek - Tallebudgera Ck. - accretion at d decreasing rate going north Burleigh Head - South Nobby - erosion at an increasing rate going north South Nobby - Broadbeach - erosion at a decreasing rate going north Broadbeach - South Stradbroke Is. - accretion at an increasing rate going north

If these trends are compared with the erosion/accretion figures from figure 86 no similarity can be recognized. Several areas which show generally erosion have accreted in the period 1966 - 1968. It may be concluded that the long-term erosion has to be distinguished definitely from the short-term erosion during heavy weather, the first being mainly caused.by clifferenriCd' littoral transport and the latter by trans­verse transport.

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Another view of eroded and accreted areas is given in figure 97 in which are plotted volume changes between contours as derived from the same data.

In order to obtain further figures of transverse transport rates, changes in areas above specific contours for the profiles at the groups of lines (figures 89 - 91) were calculated. In this case the limit chosen for the calculation of area was such that landward of the limit no change was evident over the period covered by the pro­files, and so these figures are more indicative of the complete profile than the former, which did not consider the area above R. L. + 5 ft. The resu Its are given in table 46 in the form of volumetric gain or loss rates between two contour levels in the peri od between two surveys.

The total change in any profile between two successive surveys does not equal zero because

(i) errors in sounding, interpretation of mean profile and area determination are inherent,

(ii) small changes may occur beyond R. L. - 50 ft contour (iii) the measured changes are the net effect evidenced by the profile of transverse

and longshore transports.

Of these, (iii) is the most important. Nevertheless the figures obtained are good indicators of net profi Ie changes over relatively short periods, and may be conve­niently considered as transverse transport changes. From the changes can be seen from which parts of the profile material is removed, and on which parts it is de­posited, and the (average) rates at which these changes take place.

Over the whole period, apart from Letitia Spit (Alpha I ines) the beach above R. L. 00 has suffered a net erosion arising from a severe erosion early in the period and later recovery, and as mentioned earlier this recovery continued in 1969 in so far as figures are avai table.

Plans were also prepared showing differences in bed levels between consecutive soundings of the areas covered by the Alpha, Beta etc. lines, but these gave no additional information of consequence anclwere not as convenient as the previously mentioned procedures.

Plots were made for several profiles of distances measured from a fixed point to various offshore contours. Because of the short duration of the surveys and the fact that the slow rates of overall erosion are masked by the greater transient cyclone­produced phenomena in the short term, no definite trends were evident. The continu­ation of such plots for a much longer time can however yield valuable information.

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TABLE 46 Transverse Transport Rates

Group of Area between Erosion/accretion in yi/yd. yr Overall Difference lines levels at first in Area (ft2)

Survey (ft2)

~ Alpha I-II Level 10/7/66 10/7/66- 30/5/67- 6/9/67- 2/5/68-(0) 1 (ft) 30/5/67 6/9/67 3/5/68 17/10/68 10/7/66 - 17/10/68

(334 days) (99 days) (239 days) (169 days)

+ 1510 5 1210 . - 15.0 - 4.0 + 38.5 + 43.5 + 281 + 5to 0 2006 - 47.0 - 50.0 + 42.5 + 200.5 + 582

o to-10 7057 + 43.0 + 106.0 - 98.0 + 180.0 + 770 -10to-20 12205 - 30.0 + 184.5 + 38.5 - 344.0 - 990 -20to-30 18216 + 220.0 - 186.0 - 148.0 - 45.5 + 241 -30to-40 23668 + 40.5 - 115.5 - 7.5 + 75.5 + 309 -40 to- 50 29540 + 3.0 - 169.0 + 9.5 + 147.0 + 276

Totals + 214.5 - 234.0 - 124.5 + 257.5 +1469

~ Beta II Leve 21/6/66 21/6/66- 25/5/67- 12/9/67- 29/5/68-(+25ft)1 (ft) 25/5/67 12/9/67 29/5/68 24/10/68 21/6/66 - 24/10/68

(338 days) (1 10 days) (260 days) (148days)

+17.5t,+ 5 988 - 88.5 - 28.5 + 7.5 + 2.5f - 756F

+ 5 to 0 1077 - 78.0 - 58.0 + 35.0 + 34.0F - 45l 0 to-10 4021 - 108.0 - 193.5 + 101. 0 + 246.0 + 116

-10 to-20 13980 + 59.5 - 700.0 - 12.0 + 84.0 -1175 -20 to-30 28428 - 100.0 + 374.5 + 84.0 - 18.0 + 657 -30 to-40 35698 - 70.5 + 194.5 + 46.5 + 70.0 + 493 -40 to-50 40711 + 9.5 - 74.5 + 47.5 + 79.5 + 474

Totals - 376.0 - 485.5 + 309.5 + 498.0 - 648

~ Gamma Level 23/6/66 23/6/66- 22/5/67- 20/9/67- 24/6/68-(+37.5 ft) 1 (ft) 22/5/67 20/9/67 24/6/68 11/11/68 23/6/66 - 11/11/68

(333 days) (121 days) (278 days) (140 days) - it .

368 +11.5to+ 5 - 10.5 - 38.5 + 26.5 - 39.0 - 155 + 5 to 0 842 - 17.5 - 8.0 + 15.0 + 11.0 - 26

0 to-W 4160 - 27.0 + 139.0 - 29.0 + 146.5 + 503 -10 to-20 10305 - 24.5 + 243.0 0 - 155.5 - 12 -20 to-30 17028 + 88.5 - 231.0 - 84.0 - 4.0 - 548 -30 to-40 23045 + 16.5 - 119.5 + 42.5 - 83.5 - 215 -40 to-50 29591 + 14.0 - 184.5 + 40.5 - 9.0 - 190

Totals + 39.5 - 199.5 +11.5 - 133.5 - 643

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Table 46 (continued)

~ Delta 11-21 Level 6/7/66 6/7/67- 28/5/67- 18/10/67-(+37.5ft)1 (ft) 18/10/67 18/10/67 29/7/68

(326 days} (143 days) (285 days)

+ 8.51/'0+ 5 194 - 12.0 - 25.0 - 1.5 + 5 to 0 647 - 10.0 - 106.0 + 2.5

0 to-1O 3589 + 3.5 - 294.0 + 88.0 -10 to-20 8735 + 49.0 + 215.0 - 199.0 -20 to-30 16092 + 135.5 + 124.0 - 153.5 -30 to-40 23392 + 55.0 - 4.0 - 51.0 -40 to-50 29644 + 57.0 - 31.5 - 24.0

Totals + 278.0 - 121.5 - 338.0 ._-

~ Epsilon Leve 5/7/66 5/7/66- 1/6/67- 1/10/67-{+62. 5 ft} 1 eft} 1/6/67 1/10/67 19/8/68

(331 days) (122 days) (323 days)

+20to+ 5 892 - 43.0 - 80.5 + 35.0F + 5to 0 1074 - 65.0 + 6.5 + 21. OF

Oto-l0 4671 - 130.5 - 137.5 + 85.5 -10to-20 9333 - 72.5 - 328.0 + 395.5 -2010-30 16939 + 166.0 + 24.0 - 76.0 -30to-40 21890 + 65.0 - 35.0 + 2.5 -40 to-50 26756 + 27.0 - 95.0 - 2.0

Totals - 53.0 - 645.5 + 460.5

~ Zeta Leve 15/8/66 15/8/66- 13/7/67- 4/10/67-{-62.5ft)1 (ft) 13/7/67 4/10/67 2/9/68

(332 days} {83days} (334 days)

+11'0+ 5 1860 - 97.0 - 161.5 + 52.0 + 5to 0 2028 - 103.5 + 188.5 + 15.5

Oto-l0 6699 - 134.0 + 34.0 + 83.0 -10to-20 11848 - 129.5 + 377.0 + 91.0 -20to-30 18361 + 189.5 + 313.0 - 108.0 -30to-40 24614 + 122.0 + 70.5 - 86.0 -40 to-50 32965 + 59.5 + 214.0 - 103.0

Totals - 93.0 +1035.5 - 55.5

Distance from zero chainage taken as limit for area calculation * Approx. level on profile at the limit line

29/7/68-13/11/68 6/7/66 - 13/11/68 (107 days)

+ 36.0 - 97 + 134.0 - 83 + 331. 0 + 489 - 247.5 - 897 + 13.5 + 481 + 37.5 + 168 - 11.5 + 149

+ 293.0 + 210 - --

19/8/68-4/12/68 5/7/66 - 4/12/68

(107 days)

+ 111. OF - 19r + 41.0F _ 239F

+ 228.0 - 198 - 515.0 + 216 - 182.5 + 339 - 121.0 + 121 - ·16.5 - 126

- 455.0 + 94

2/9/68-26/11/68 15/8/66 - 26/11/68 (85 days)

+ 185.5 - 310 + 110.0 - 103 + 168.5 + 4 - 235.5 - 36 - 58.5 +1182 + 82.5 + 609 + 51.5 + 188

+ 304.0 +1534

F The construction in 1968 of a boulder wall in these areas has a slight influence on these figures

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3. II. Comparison and Summary of Erosion/Accretion Quantities

In section 3.9. the theoretical littoral transport at various profiles along the coast has been calculated. From the littoral transports, erosion/accretion figures can be derived. Further information about erosion and accretion comes from maps giving shoreline positions during the period 1932 - 1962 (section 3.10.2.) •

In this section the results of the two methods will be compared. Also an attempt wi II be made to check the calculated transport figures by comparing these with data obtained by other methods.

Li tlora I transport

In a situation which is more or less in equilibrium, considering the whole Australian East Coast, littoral transport passing Point Danger should originate in the sediment contribution of rivers south from this point. It is difficult to determine which rivers may contribute to the sand supply for the Gold Coast. A certain area along the New South Wales Coast has to be taken into account. Here the assumption is made that the rivers south from Wooli Head are of minor importance for the sand transport at Point Danger, because of the distance and the many training works at the river mouths.

Between Wooli Head and Point Danger the major rivers are Clarence River, Richmond River and Tweed River.;:The catchment areas of the smaller . rivers and creeks between can be added to those of the three rivers.

Holeman [24] states that the sediment yield per year of major rivers at the Australian East Coast is about 85 tons per square mile of catchment are. With a solid rock weight of 2650 kg/m3 this leadLto an erosion of 0.01 mm/year. Assuming a specific weight of the loose sediments of 1650 kg/m3, the following figures are found.

River Catchment Area Vol. of Sediments' per year

Clarence R. 8,500 sq. miles 520,000 cu.yd Richmond R. 2,300 140,000 Tweed R. 950 60,000

Total II ,750 sq. miles 720,000 cu.yd

This figure is of the same order as the theoretically calculated littoral transport near Tweed River (625,000 cu. yd/year). Also these figures correspond rather well with the accretion figures in front of the Tweed River walls (470,000 cu.yd/year, see section 4.3.2.), regarding the fact that not all transport has been blocked during construction of the walls.

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The computations show that the littoral transport along the Gold Coast increases going north. The calculated figures are 230,000 cu. yd/year at Tugun, 340,000 cu.yd/year at Tallebudgera Creek and 635,000 cy.yd/year at The Spit (see table 43). At Tugun approximately 135 <yo of the net transport is directed northward and approximately 35 ~o southward. These percentages also increase in the north-ward direction. The same statement can be made in general about the magnitude of the transverse transports (see section 3.10.).

Furthermore it is concluded from the results of the computations that practi­cally all littoral transport occurs within the first 2000 ft from the beach. At the southern end of the coast about half of the total transport occurs within the breaker­zone, increasing to 85 <yo within the breaker-zone at the northern end, the breaker­zone in general being within 300 ft from mean sea level position on the shore.

Erosion/Accretion figures

In section 3.10.3. it is shown that short-period fluctuations of the coastline are due mainly to transverse transports. The period 1966 - 1969' shows that material moved away from the beach by transverse transports during heavy weather, is gene­rally carried back to the beach again under normal wave conditions. The long-term erosion, however, will be caused mainly by differences in littoral transports along various sections of the coast. .

From the results of the littoral transport computations, the yearly regression of the coastline can be calculated. Other factors, which also influence the beha­viour of the coastline, such as net transverse transports, rise of mean sea level, wind erosion, mining activities and accumulation of sand in the river entrances, are not taken into account.

Between The Spit and Tallebudgera Creek the yearly regression of the shore­line is calculated to be about 5 ft. From Tallebudgera Creek to Tugun a regression of about 4 ft is found. This is more than, but still of the same order of magnitude as the regression of the coast I ine derived from maps of the last thirty years (figure 86). The connection between erosion/accretion and differences in littoral transports can also be illustrated by accumulating the eroded or accreted quantities of figure 86 from south to north. The results eire given in figure 98. Because the amount of sand passing Point Danger and remaining close to the shore is not known (to be discussed hereafter), the figures are based on the calculated transport of Tugun at 230,000 cu. yd/year. Adding the erosion or accretion northward from Tugun would yield a littoral transport of about 530,000 cu.yd/year at Narrow Neck (figure 98). At approximately 2 miles north from Narrow Neck the calculated littoral transport is 635,000 cu. yd/year, and figure 98 shows a rather good correspondence between the two methods of approach.

The calculated regression south from Tugun is dependant of the amount of sand which passes Point Danger and which is not lost in deep water in front of the steep coast at Point Danger. The computations show an excess of sand between Tweed River and Tugun of approximately 400,000 cu.yd/year. If all the sand passing Point Danger remains close to the shore, thus serving as a supply for the Gold Coast

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beaches, this excess of sand should lead to an accretion of 15 ft per year for the beaches between Point Danger and Tugun. In fact the beaches between Kirra Hill and Bil inga show an accretion of about 4 ft per year over the last thirty years (figure 86). At the other beaches between Tugun and Point Danger, regression of the shoreline has been noticed on the average.

Naturally, part of the transport south from Tweed River walls will be blocked by the walls (see section 4.3.) or get lost in deep water. Although the beaches directly west from Point Danger are regressing, in figure 88 it can be seen that a wide and flat shelf of sand exists in front of these beaches at distances between 1000 and 6000 ft from the shore-line. To build up this shelf, deposition of large amounts of sand during a long time has been necessary, so that the presence of this shelf is likely to be the explanation for the seeming lack in sand balance between Tweed River and Tugun.

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4. RIVER and CREEK MOUTHS

4. I. General

At several locations rivers and creeks stream out into the ocean. Details of annual discharge, tides, and bed composition are given in sections 3.3.6, 3.5. ),ond 3.8.4. respectively.

The average sediment load of these local streams is quite small. Based on the figures of Holeman [24] the following is found:

Stream Catchment Area Vol. of Sediments per year

Nerang River 190 sq. miles 12,000 cu.yd Tallebudgera Ck. 38 2,400 Currumbin Creek 20.5 1,300

Total 15,700 cu. yd

The discharge distribution (figure 9) indicates that these sediments would be delivered to the coast at erratic intervals.

When the beach is unprotected the sediments of the river, together with the northward transported beach sand, develop a sand spit on the southern side of the mouth. This sand spit grows in a northern direction, so that the real river mouth is also moving northward. This must have been the case already ages ago, as the last few mi les of most rivers are bending to the north (e. g. Tweed River, Tallebudgera Creek and Nerang River. When such a spit reaches a head­land, or when the mouth is stabi lized by training walls, the movement stops (Tallebudgera Creek, Tweed River). Still the possibility exists that the sediments form a bar just in front of the mouth, which might block the opening entirely.

4.2. Currumbin and Tallebudgera Creeks

When the mouth is situated just north of a headland, the spit cannot develop to any great extent because of the erosion which occurs from time to time after storms. An example of this kind is Currumbin Creek. Here the situation is complicated by the presence of a rock at a distance of 1000 ft in front of the headland. There are times that the space between the rock and the coast is filled up with sand, partly checking the littoral transport, while now and then the sea breaks through so that the norma I northward transport re-occurs. Photographs 8 ,.. 10 show Currumbin Creek Entrance in various situations, with the mouth in a southerly, intermediate and northerly position respectively.

During times of low fresh-water discharge in the Creek, (the majority of the time) the littoral supply forces the mouth northward and westward (photograph 10). The tidal compartment·· is so mall that the mouth is often sealed off completely,

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especially at low tide. The north-west migration of the creek mouth causes erosion of the southern part of Pa 1m Beach and endangers private property from time to time. Eventually a high discharge in the Creek cuts a new mouth straight out (photograph 8), the old channels and mouth sand up (photograph 9), and the cycle commences again.

Other photographs available since 1930 confirm this cycle, the length of which is dependent on littoral supply of sand and intervals between high discharges. From figure 9 it is estimated that the cycle would average four to five years.

The entrance of Tallebudgera Creek is of minor interest for the present study, it being not favoured for (smail-boat) navigation due to the proximity of the rocky Burleigh Headland. The bar here also almost completely closes the creek at times. Still the possibility exists that in the future some navigational usage of Tallebudgera Creek entrance might be desirable, considering the increasing development of canal estates connected to this creek.

4.3. Tweed River

4.3.1. Introduction and Statement of the Problem

Tweed River is a tidal river, emerging to the sea one-third of a mile south from the border between New South Wales and Queensland. Near the ocean the river is bending northward, due to the formation of Letitia Spit, caused by the littoral transport of sand. The river has hardly any significant fresh water discharge, so that only the tidal volumes are passing through the mouth in general. No measurements of tidal volumes are available, but information on tides is given in section 3.5. Sand is deposited in the mouth during flood tide and some is washed out again during ebb. Due to the sand movement the mouth was not stable, so that around 1880 training walls were constructed. These walls' caused an accretion of the beach southward from the mouth. Also a bar was formed in front of the entrance.

In the years prior to 1960 the accretion had progressed to such an extent that the walls lost their function (see photograph 11). Also the bar became inconvenient for navi'gation. For that reason during the years 1962 - 1964 new training walls were constructed in front of the old walls (see photographs 12 and 13).

The new wa lis caught a considerable amount of sand, even during construction, thus influencing the general picture of littoral transport. Before that time the sand passing Tweed River mouth was transported around Point Danger, so that material was supplied for the Gold Coast Beaches. As a consequence of the construction of the new Tweed River Training Walls by the N.S.W. Government some questions arose:

1. What will be short and long term effects of the walls, considering the sand supply for the Gold Coast beaches.

2. If there is an unfavourable influence, how can this be overcome.

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4.3.2. Sand Accumulation

. To gather information about the immediate influence of the walls many soundings have been made. Soundings were made by the New South Wales Public Works Department in the period 20 November 1962 to 21 October 1964 during construction of the walls. From the soundings made it appears that in this period an amount of about 900,000 cu.yd has been trapped by the walls. This gives an average of 39,000 cu.yd/month or 470,000 cu.yd/year, which compares rather' well with the theoretical transport in the area (see section 3.9.3.).

From soundings between July 1966 and October 1968 profi les have been drawn to get a picture of the accretion several years after completion of the walls. Some of those profiles are given in figure 99. It appears that during the cyclonic weather of early 1967 much materia I has been transported from the beach seaward to depths from 20 to 30 ft (see profi les of May/June 1967). In the following period the beach is accreting again. Beyond the depth of 35 ft hardly any change in bottom profile can be recognized, although in the long term sediment deposition in much deeper areas may take place (see section 3.8.3.).

If a dam is blJilt perpendicular to a straight coastline of infinite length the amount of sand passing around the head of the dam can be calculated as a function of time, according to Pelnard-Considere's theory [25]. The result of such a computation is given in figure 100. The theory has been developed for gravel beaches, but from model fests and prototype measurements it is kno'M1, that. the 'dotted line on the figure is found for sand beaches.

In this case the beach has a length of about 2 miles (from Fingal Head to Tweed River Entrance). This means that the re-establishment of the original transport will be reached much sooner than would be the case for an infinitely long beach assumed by the theory. Between Fingal Head and the south wall a wedge of material will be accumulated. A rough estimation of the total amount of sand in this wedge can be made. With a length of the wall of 400 m (1300 ft) and the boundary of accretion at the 35 ft depth contour the total content of the wedge will be 1/2 x 400 x 13.5 x 3000 x 1/2 =4 x 106 m3 (5.5 x 106 cu.yd),assuming that the build­up of the beach will be to a level of + 10 ft (see figures 99 and 101). Unlike the asymptotic decrease of the amount of sand trapped every year at a coastline of infinite length, in this case the accumulated amount might be estimated to decrease almost linearly (------ line). Acc'ording to this assumption the original transport will be re-established approximately after 20 - 25 years.

4.3,3, Influence on Gold Coast Beaches

The considerable amount of sand trapped by the Tweed Walls (estimated to be approximately 5.5 x 106 cu, yd) is naturally affecting the Gold Coast beaches, especially the beaches just north of Point Danger. Beside Tweed River, other river mouths along the New South Wales Coast have been trained to some degree [26]. The majority of these works were carried out in the period 1880 to 1910, In 1950

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a major training scheme was started on the Clarence River. Most of these constructions will interrupt the sand transport during a considerable time.

Looking at the consequences of Tweed Walls it appears that after some 20 years the original sand transport will be reached again. Still to be considered is whether there will be remaining effects. The sand has to pass the Tweed River mouth in deeper water now. The bar is situated farther away from the beach so that it would not be impossible that large quantities of sand wi II be lost in deep water.

From the profiles northward from the walls and right in front of them (figure 101) it appears, however, that the bar is still at such a depth that the material can be influenced by the waves and transported shoreward again. Secondly, it can be ob­served that the beaches immediately northward from the walls are accreting again (see figure 10IB).

Photograph 11 gives a clear picture of the sand movement in the situation before extension of the walls. Sand is eroded from the bar and moved in a more or less straight line towards Point Danger. There seems to be no reason why this would not occur in the future.

Although in the future no considerable remammg losses should be expected, in the next 20 years the natural sand supply for the Gold Coast beaches will be less than before construction of the Tweed Walls, even leaving recent training works at other New South Wales rivers out of question.

The results of the littoral transport computations showed an excess of sand supply passing Point Danger (see table 43). In section 3. 11. is demonstrated that the excess of sand did not result in comparable accretion of the beaches directly west from Point Danger, this sand being deposited further offshore in the area of the Beta-lines. As it is reasonable to assume that the division of sand quantities moving nearshore and offshore will not change by construction of the Tweed Walls, the conclusion can be drawn that the sand supply for the beaches wi II decrease to the same percentage as the decrease in total sand transport at Point Danger.

According to figure 98 the near-shore transport directly west from Point Danger can be estimated at 270,000 cu.yd/yr. Neglecting any shoaling of Tweed River Mouth, the sand supply at Point Danger is calculated to be approximately 625,000 cu. yd/yr, so that the deposition in the area of the Beta-lines and losses in deep water wi II be about 350,000 cu. yd/yr. Apparent Iy around 40 'Yo of the sand supply at Point Danger travels directly along the beaches. The total loss for the beaches will then be 40 'Yo of the amount trapped at the Tweed Walls, i. e. 40 'Yo of 5.5 x 106 cu.yd = 2.2 x 106 cu. yd.

This loss will be noticed in particular at Greenmount and Kirra Beaches. Duranbah Beach, immediately to the north of the north training wall should not suffer erosion, and indeed has accreted since the walls were built because it is now completely protected from littoral currents, and can be affected only by wave conditions causing transverse sediment movement. Its erosion or accretion depends on the predominating steepness of the impinging waves, and in common with all such pocket beaches would be expected to show general accretion.

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Since the littoral transport capacity increases northward from Tugun, it is probable that the short term effects of the Tweed River walls (1962 - 1964) will have no major impact north from Tugun.

Thus it may be concluded, that in the absence of further disturbing influences, the conditions prior to the construction of the 1962 - 64 walls will be restored about 1985. In the meantime, lack of littorally supplied sediment will cause erosion of the beaches, but only south from Tugun. This effect will be most marked in the earlier years from the present to 1985, and ways of overcoming these effects are given in Volume I of this Report.

4.4. Nerang River

Nerang River, with a catchment area of about 190 sq. miles the most important river in the Gold Coast area, discharges into a lagoon, the Broadwater, near South­port (see figure 102). The Broadwater is separated from the ocean by South Strad­broke Island and by a narrow land tongue, called The Spit. The opening between South Stradbroke Island and The Spit is generally referred to as Nerang River En­trance. Information on tides, bed composition and discharges is given in sections 3.6., 3.8.4., and 3.3.6. respectively. Photographs 14 and 15 show the entrance.

4.4. I. Movement of the Entrance

The position of Nerang River Entrance is heavily influenced by the littoral transport, as it shows a continuous inigration. Investigations on this subject have been done by Connah and Brooks [27 and 28]. Some of the results of those inves­tigations are stated below.

The various data showing the position of the Entrance and the origin of these data are given in a graph in figure 103 •

. The oldest data came from persons who knew the locality at the end of the 19th century. It is fairly sure that about 1900 The Spit was only reaching a few chains north of the present Jubilee Bridge. There 'are various opinions about the situation before that time. Local residents recall a stable position between 1885 and 1900. A plan prepared by the Harbours and Marine Department in 1958/62, however, shows some older data from which a southward migration of about 1 mi Ie in the years 1865 - 1900 can be derived. These data have to be interpreted carefully, since no reliable measurements are available.

From about 1900 ti II now a northward migration of the mouth has been measured. The general public opinion is that the changed conditions are a conse­quence of the diminishing of discharges through Nerang River Entrance, caused by the breakthrough at Jumpinpin in 1896. Certainly the Jumpinpin breakthrough will have had an effect on the depth of the entrance and bar, but the effect on the mi­gration of the entrance is very doubtful. Indeed in times past, the Nerang certainly .discharged at Broadbeach, and the river area including Chevron and Macintosh Island etc. behind Surfers Paradise is a "Broadwater" of some few hundred years a99.' The Geophysical Report (2J indicates that the river has never discharged in

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the same place for long, so that the coastal area from Main Beach to Broadbeach has been formed by the same processes now moulding The Spit.

It is not impossible that the whole process of migration has been one of a stop-start nature connected with variations in littoral supply of sand and possible break-throughs of sth Stradbroke Is. in earlier times and which have subsequently healed. The lack of long-term records, and the extreme complexity of the hydrau­lic regime of southern Moreton Bay make the actual sequence of events in the cen­turies preceding the present a matter of conjecture, and there appears little possi­bi Iity of even determining the precise time-history of entrance movements or their causes.

In the years 1901 - 1968 the northern point of The Spit has moved north­ward over a distance of approximately 210 chains (average linear growth of 200 ft/yr). Especially in the years between 1920 and 1940 the movement has been very strong. Figure 9 indicates that discharges during this period have been less than subsequently, which may be a possible reason. In the last few years the rate of growth has slowed down a little. Between 1944 and 1968 a fairly constant rate of about 85 ftjyr has been measured (see also figure 104).

The southern point of South Stradbroke Island shows a similar movement. The point of The Spit has the tendency to move seaward from the point of South Strad­broke Island, forming a bar in front of the mouth. The following erosion of South Stradbroke Island and breakoffs of the bar line them up again.

Based on several reasonable assumptions, such as the shape of the cross-section of The Spit, the depth of the channel, etc. the volumetric growth of The Spit has been calculafed. The calculated growth as a function of time is plotted in figure 105. The growth rate has a rather constant value of 156,000 cu.yd/yr. From recent­ly made surveys of five profiles across The Spit, it appeared that the shape assumed for calculations was quite good. The error in the volumes can be in the order of 5 - 10 /0.

4.4.2. Shoaling of the Broadwater

Together with the northward migration of Nerang River Entrance sand is depo­sited in the Broadwater, especially in the vicinity of the Entrance. Also a conside­rable silting of Nerang River itself has taken place in the last fifty years. Only very small ships can pass above Jubilee Bridge now, and even in the Broadwater, dredging of navigation channels is a constant problem.

Calculations with relation to the Broadwater have been done for two purposes:

1. To obtain an estimate of the rate of shoaling of the Broadwater, which gives further information about the sand balance a long the coast.

2. To estimate the available volumes of sand above certain levels, which might be used for sand supply at certain beach areas.

Only one suitable survey can be used, carried out from November 1967 to February 1968. The survey area extended from Jubilee Bridge to Crab Island. Because only one survey is available some assumptions have to be made. For this purpose the

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Broadwater is divided in three sections, bounded by E-W lines (see figure 102): Section I covers the Broadwater from Jubilee Bridge to approximately 2000 ft south of the Entrance. Because the Entrance is moving north and the shoaling is taking place in the vicinity of the Entrance mainly, section I is considered to be the area where shoaling has almost been completed. Section II is extending from the northern end of section I to approximately 3000 ft north of the centre of the Entrance, which is the area in which shoaling would be presently taking place. Section III covers the remaining part of the Broadwater up to Crab Island, where hardly any shoaling has been noticed.

The following average bottom levels can be calculated from the survey:

Section Section II Section III

- 3.42 ft - 5.59 ft - 9.66 ft

Regarding the history of the development of Nerang River Entrance it might be assumed that sections I and II originally had the same average depth as section III, viz. - 9.66 ft. Then the average shoaling of section I is 9.66 - 3.42'" 6.24 ft.

From the calculation on the growth of The Spit it appeared that the average growth rate between 1901 and 1968 was 200 ft/year. The average width of section

being 4070 ft, for the shoaling of the Broadwater can be found

200 x 6.24 x 4070 '" 188 000 dV 27 ,cu. y year.

Over the last 20 - 25 years the linear growth of The Spit has been about 85 ft/year, which yields to a shoaling rate of the Broadwater of 80,000 cu. yd/year.

In the above figures several influences have been neglected, such as

- Sand removals for reclamation. - Sand deposits due to the breakthrough in South Stradbroke Island on 2 Nov.

1938, forming Tragedy Island. - Sand deposits from erosion of South Stradbroke Island (partly shoaling the Broad­

water, partly contributing to the total transport along the coast).

The net effect of these factors will not result in serious errors. More im­portant is the contribution of Nerang River to the siltation, estimated to be 12,000 cu. yd/year. Measurements of sediment transport in the river have not been done. However from analysis of water samples hardly any suspended load has been found, but it is likely that as a result of the climatic circumstances the sediment discharges may occur in short peri ods.

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Besides the growth of The Spit and the siltation of the Broadwater quantities of sand are used for the bui Id-up of the bar in front of the Entrance. A rough es­timate can be made of these quantities. The area of the fully developed bar is a­bout 20,000 sq. yd, with a height of about 2 yd. From time to time the bar breaks off, after which the formation of a new bar starts again. If it is assumed that break­offs occur on an average twice per year, a quantity of 80,000 cu.yd/year is found for the build-up of the bar. This sand is also contributing to the northward transport along South Stradbroke Island.

Compiling this section the following figures are found.

Linear growth of The Spit Average 1901 - 1968: Average over last few years:

200 ft/year 85 ft/year

Volumetric growth of The Spit, Average 1901 - 1968: 156,000 cu.yd/year

Shoaling of Broadwater Average 1901 - 1968: Average over last few years:

188,000 cu.yd/year 80,000 cu.yd/year

Sediment contribution of Nerang River: 12,000 cu.yd/year

Sediment avai lable in Broadwater

Volume (yd3) above

Section RL - 10 RL - 20 Surface area

I 12.5 x 106 31.2 x 106 1170 acres II 3.7 x 106 12.2 x 106 525 acres III 0.2 x 106 6 x 106 390 acres

Total 16.4 x 106 49.4 x 106 2085 acre~ ..

4.4.3. Tidal Volumes

Measurements of velocities at various depths at several points on surveyed cross-sections were made on six occasions during 1969, at times when the fresh water discharge of the river was effectively zero. The first two experiments (6 and 19 June) were conducted simultaneously on cross-sections near the Jubilee Bridge and Anglers Paradise (see figure 102). The flow at the entrance was obtained by addition of the retained volume between the sections. These experiments indicated that the inflow and outflow across each section were nearly equal. For example, the experiment of 19 June yields the following information:

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Volume in Acrefeet Cross-section Area Max. flow rate . Max. velocities sq. feet in cu.secs measured in

Section Inflow Outflow Inflow Outflow fps

Ang lers Paradise 7000 7060 20,140 24,000 21,600 0.8 - 0.9 Bridge 1470 1620 37,400 5,800 3,800 1.0 - 1.2

The remaining eXP:iriments were conducted at the entrance itself, on a cross­section of area 20,075 ft • Maximum velocities of 6 - 7 fps were measured during the experiments of 23 and 26 September (inflow and outflow tide) and 2 - 2.5 fps on 10 July. Figure 106 shows the results obtained. The measurements for inflow and out-flow I ie on the same curve, indicating that there is no residual circulation in the basin involving Jumpinpin.

REFERENCES

Delft Hydraulics Laboratory. "Queensland Coastal Erosion; Recommendations for a Comprehensive Coastal Investigation';" Report R 257 of 1965.

2 Cifali G., Hart G., Mann P.E., Polak E.J. and Wiebenga W.A., "Coastal Erosion, Geophysical Survey of the Gold Coast, Queensland 1967:" Record No. 1968/18, Bureau of Mineral. Resources, Geology and Geophysics.

3, Kindler J. E., and O'Connor C. "Report on Erosion of Beaches in the Town of South Coast;~' Co-ordinator - General's Department, 1951.

4 McGrath B..L. "Erosion of Gold Coast Beaches 1967;~' Journal I.E. Aust., Vol 40, no 7-8, 1968.

5 Visher S. S. and Hodge D., "Austra I ian Hurri canes and Re lated Storms;" Commonwealth of Australia Bureau of Meteorology, 1925.

6 Brunt A. T. and Hogan J. "The Occurrence of Tropical Cyclones in Australian Region.~' Proc. Trop. Cyclone Symp. Brisbane, 1956.

7 Maxwell W.G.H. "Atlas of The Great Barrier ReeL~' Elsevier Pub\. Co. 1968.

8 Delft Hydraulics Laboratory. "Gold Coast Queensland, Ocean Outfall Sewers," Report R 398 of 1970.

9 Tucker M.J. "Analysis of Records of Sea Waves;~' Proc. Inst. Civ. Engrs. Vol 26, 1963.

10 Longuet - Higgins M. S. "On the Statistical Distribution of the Heights of Sea Waves." Jour. Marine Research, Vol XI, No 3, 1952.

11 Draper L. "The Analysis and Presentation of Wave Data - Plea for Uniformity." Proc. Tenth Conf. on Coastal Eng,. Tokyo, 1966.

12 Svaiek J.N. "Statistical Evaluation of Wave Conditions in a Deltaic Area.~' Pl'oc. Symp. on Wave Research, Delft, 1969.

13 U.S. Navy Hydrographic Office. "Atlas of Sea and Swell Charts for North-West and South-West Pacific Ocean." Pub. No. 799 CE, 1943.

14 U. S. Navy Hydrographic Office. "Atlas of Surface Currents for South-Western Pacific Ocean." Pub. No. 568, 1959.

15 U.S. Navy Hydrographic Office. "Atlas of Pilot Charts for South Pacific and Indian Oceans." Pub. No. 107.

REFERENCES (continued)

16 Wiegel R.L. "Oceanographical Engineering" Prentice - Hall 1965.

17 Wyrtki K. "The Surface Circulation in the Coral and Tasman Seas." C.S.i.R.O. Divn. of Fisheries and Oceanography Tech. Paper No.8, 1960.

18 Eagleson P. "Theoretical Study of Longshore Currents on a Plane Beach." M.I.T. Dept. of Civil Eng. Hydr. Lab. Report No. 82 of 1965.

19 Connah T.H. "Beach Sand Heavy Mineral Deposits of QueeAsland," Queensland Department of Mines, Pub. No. 302, 1961.

20 Bijker E.W. "Littoral Drift as a Function of Waves and Current." Delft Hydraulics Laboratory Pub. No. 58, 1968.

21 Einstein H.A. "The Bed Load Function for Sediment Transportation in Open Channel Flow.JI U.S. Dept. of Agr., Tech. Bull. No. 1026, 1950.

22 Groen P. and Dorrestein R. "Zeegolven," K. N. M.I., De Bilt 1958.

23 Delft Hydraulics Laboratory "A Study for the Sekondi Naval Harbour." Report R 248, M 852, M 853; 1966.

24 Holeman J. N. "The Sediment Yield of Major Rivers of the World." Water Resources Res. Vol. 4, No.4, 1968.

25 Pel nard - Considllre R. "Essai de Theorie de I'evolution des formes de rivage en plages de sable et de galets." Les energies de la Mer, Ive Journees de L'Hydraulique de la Societe Hydrotechnique de France, Tome 1, 1954.

26 Floyd C.D. "River Mouth Training in New South Wales." Proc. Eleventh Conf. on Coastal Eng'i' London 1968.

27 Connah T.H. "Stradbroke Is. Erosion and Broadwater Silting" Queensland Government. Mining Journal, Dec. 1946.

28 Brooks J. H. "Stradbroke Is. Erosion and Broadwater Silting" Queensland Government Mining Journal, Aug. 1953.