lake macquarie waterway flood study · this lake macquarie waterway flood study constitutes a...

LAKE MACQUARIE

WATERWAY

FLOOD STUDY

LAKE MACQUARIE CITY COUNCIL

JUNE 2012

Level 2, 160 Clarence Street Sydney, NSW, 2000 Tel: 9299 2855 Fax: 9262 6208 Email: [email protected] Web: www.wmawater.com.au

LAKE MACQUARIE WATERWAY FLOOD STUDY

JUNE, 2012

Project

Lake Macquarie Waterway Flood Study

Project Number 29076

Client Lake Macquarie City Council

Client’s Representative Greg D Jones Greg Giles

Author P Wongpaibool R W Dewar

Prepared by

Date 26 June 2012

Verified by

Revision Description Date

1 1st Draft Report 24th November 2010

2 2nd

Draft Report 3rd

August 2011

3 3rd

Draft Report 8th September 2011

4 Public Exhibition 14th September 2011

5 Final Draft 24th January 2012

6 FINAL 26 June 2012

LAKE MACQUARIE WATERWAY FLOOD STUDY

TABLE OF CONTENTS PAGE

FOREWORD ............................................................................................................................... i

EXECUTIVE SUMMARY ............................................................................................................ ii

1. INTRODUCTION ........................................................................................................ 1

1.1. Background ................................................................................................ 1

1.2. Objectives ................................................................................................... 2

1.3. The Flood Problem ..................................................................................... 3

1.4. Causes of Flooding ..................................................................................... 4

1.5. Previous Studies ......................................................................................... 5

1.5.1. Lake Macquarie Flood Study - 1998 ........................................................... 5

1.5.2. Tidal Prism Modelling of Lake Macquarie - 2010 ........................................ 7

1.5.3. Lake Macquarie Adaptive Response of Estuarine Shores to Sea Level Rise

– 2010 ..................................................................................................... 8

1.6. Land Use .................................................................................................... 8

1.7. The Entrance Channel ................................................................................ 9

2. AVAILABLE DATA .................................................................................................. 11

2.1. Flood Levels ............................................................................................. 11

2.1.1. Water Level Recorders ............................................................................. 11

2.1.2. Flood Levels from Debris or Other Marks ................................................. 11

2.2. Rainfall Stations ........................................................................................ 12

2.3. Flow Measurements ................................................................................. 13

2.4. Survey ...................................................................................................... 15

2.5. Flood Photographs ................................................................................... 16

2.6. Ocean Levels ............................................................................................ 16

3. APPROACH ............................................................................................................. 17

3.1. Hydrologic Model ...................................................................................... 18

3.2. Hydraulic Model ........................................................................................ 18

3.3. Calibration Events..................................................................................... 19

3.4. Design Flood Modelling ............................................................................ 19

3.5. Climate Change ........................................................................................ 19

3.6. Wind Wave Assessment ........................................................................... 19

4. OCEAN WATER ASSESSMENT ............................................................................. 22

4.1. Approach Adopted in the 1998 Lake Macquarie Flood Study -Reference 1

................................................................................................................. 22

4.2. Adopted Approach .................................................................................... 23

4.2.1. Available Tidal Data .................................................................................. 23

4.2.2. Astronomic Tides ...................................................................................... 24

4.2.3. Ocean Tidal Anomaly ............................................................................... 24

4.2.4. Wave Setup .............................................................................................. 25

4.2.5. Tidal Anomaly Analysis ............................................................................. 27

4.2.6. Summary .................................................................................................. 28

5. HYDROLOGIC MODELLING ................................................................................... 30

5.1. Watershed Bounded Network Model (WBNM) .......................................... 30

5.2. Calibration ................................................................................................ 30

6. HYDRAULIC MODELLING ...................................................................................... 32

6.1. TUFLOW .................................................................................................. 32

6.2. Calibration ................................................................................................ 32

6.3. Design ...................................................................................................... 33

6.3.1. Critical Duration Analysis .......................................................................... 33

6.3.2. Approach for Coincidence of Rainfall and Ocean Levels .......................... 34

6.3.3. Sensitivity Analysis - Varying Ocean Levels.............................................. 35

6.3.4. Variation in Starting Water Level for Design Analysis ............................... 36

6.3.5. Probable Maximum Flood ......................................................................... 37

7. CLIMATE CHANGE ASSESSMENT ........................................................................ 38

7.1. Background .............................................................................................. 38

7.2. Rainfall and Ocean Dominated Flooding ................................................... 39

7.3. Increase in Average Lake Water Level ..................................................... 40

7.4. Flood Extent Mapping ............................................................................... 41

8. REVIEW OF STORM SURGE, WAVE SETUP AND WAVE RUNUP ....................... 42

8.1. Effect of Climate Change on Storm Surge ................................................ 42

8.1.1. Ocean Storm Surge .................................................................................. 42

8.1.2. Lake Storm Surge ..................................................................................... 43

8.2. Local Wind Wave Runup .......................................................................... 44

8.2.1. Design Wind Speeds ................................................................................ 44

8.2.2. Design Wave Climate ............................................................................... 46

8.2.3. Foreshore Profiles and Wave Runup ........................................................ 46

8.2.4. Design Wave Runup and Water Level ...................................................... 47

8.2.5. Results from the 1998 Lake Macquarie Flood Study - Reference 2 .......... 47

9. ACKNOWLEDGEMENTS ...................................................................................... 49

10. REFERENCES ...................................................................................................... 50

LIST OF APPENDICES APPENDIX A Glossary of Terms APPENDIX B Flood Extent Mapping

LIST OF FIGURES Figure 1: Study Area Figure 2: Water Levels Marmong Point Figure 3: Water Levels Belmont Figure 4: Water Levels Swansea Figure 5: Jigadee Creek Gauge Figure 6: Water Levels February 1990 Figure 7: Water Levels June 2007 Figure 8: Rainfall Data 2-4 February 1990 Figure 9: Rainfall Data 7-9 June 2007 Figure 10: Cumulative Rainfall 2-4 February 1990 Figure 11: Cumulative Rainfall 7-9 June 2007 Figure 12: Flood Photographs Figure 13a: Port Stephens Tidal Analysis Figure 13b: Port Stephens Tidal Anomalies Figure 14: Tides and Critical Duration Analysis Figure 15: Model Calibration February 1990 Figure 16: Model Calibration June 2007 Figure 17: Sensitivity Analysis Tides Figure 18: Summary of Design Lake Levels Figure 19a: Climate Change: Hydrographs Figure 19b: Climate Change: Estuary Profiles Figure 20: Assessment of Climate Change: Increased Rainfall Figure 21: Assessment of Climate Change: Sea Level Rise Figure 22: Assessment of Climate Change: Combination of Sea Level Rise and Rainfall

Increase Figure 23: Assessment of Climate Change: Ocean Dominated with Sea Level Rise Figure 24: Wave Runup Levels 100 year ARI Figure 25: Wave Runup Levels- Lake Macquarie Flood Study - Part 2

LIST OF ACRONYMS

AEP Annual Exceedance Probability AHD Australian Height Datum ARI Average Recurrence Interval ALS Airborne Laser Scanning BOM Bureau of Meteorology CSIRO Commonwealth Scientific and Industrial Research Organisation HW Hunter Water Corporation IFD Intensity, Frequency and Duration of rainfall IPCC Inter-governmental Panel on Climate Change LIDAR Light Detecting and Ranging (ALS and LIDAR refer to exactly the same process of

obtaining survey) m metre m3/s cubic metres per second PMF Probable Maximum Flood TUFLOW one-dimensional (1D) and two-dimensional (2D) flood and tide simulation software

program (hydraulic computer model) WBNM Watershed Bounded Network Model (hydrologic computer model) WTP Water Treatment Plant 1D One Dimensional hydraulic computer model 2D Two Dimensional hydraulic computer model

LIST OF TABLES

Table i: Design Event Scenarios (year 2011 conditions) iii Table ii: Summary of Still Water Flood Levels iv Table iii: Summary of Design Flood Levels in Lake Macquarie vi Table 1: Lake Macquarie Waterway: Main Features.......................................................... 1 Table 2: Flood Events ...................................................................................................... 3 Table 3: Factors Affecting the Peak Lake Level ................................................................ 4 Table 4: Peak Design Levels from the 1998 Lake Macquarie Flood Study (Reference 1) ... 6 Table 5: History of the Swansea Channel ......................................................................... 9 Table 6: Water Level Recorders in the Lake Macquarie waterway ................................... 11 Table 7: Availability of Rainfall Data for each Flood Event ............................................... 12 Table 8: Continuously Read (Pluviometer) Rainfall Stations ............................................ 12 Table 9: BOM Daily Read Rainfall Stations ..................................................................... 13 Table 10: Jigadee Creek Gauge – Peak Annual Peak Water Levels and Flows ................. 14 Table 11: Factors Influencing Wave Runup Effects ........................................................... 20 Table 12: Wave Runup Effects – 100 year ARI Flood and 1 year ARI Event ...................... 21 Table 13: Peak Design Ocean Levels (1998 Lake Macquarie Flood Study - Reference 1) . 22 Table 14: Tidal Anomalies and Wave Setup (m) during March 2005 Large Wave Energy

Event ............................................................................................................... 27 Table 15: Estimated Design Ocean Peak Levels .............................................................. 29 Table 16: Adopted Manning’s “n” Values – TUFLOW model ............................................. 33 Table 17: 48 Hour Design Rainfall Intensities (mm/h)........................................................ 34 Table 18: 100 year ARI Wind Data (m/s) .......................................................................... 45


WMAwater J:\Jobs\29076\Admin\Report\LakeMacFloodStudy.docx:26 June 2012

i

FOREWORD

The NSW State Government’s Flood Policy provides a framework to ensure the sustainable use

of floodplain environments. The Policy is specifically structured to provide solutions to existing

flooding problems in rural and urban areas. In addition, the Policy provides a means of ensuring

that any new development is compatible with the flood hazard and does not create additional

flooding problems in other areas.

Under the Policy, the management of flood liable land remains the responsibility of local

government. The State Government subsidises flood mitigation works to alleviate existing

problems and provides specialist technical advice to assist Councils in the discharge of their

floodplain management responsibilities.

The Policy provides for technical and financial support by the Government through four

sequential stages:

1. Flood Study

Determine the nature and extent of the flood problem.

2. Floodplain Risk Management Study

Evaluates management options for the floodplain in respect of both existing and

proposed development.

3. Floodplain Risk Management Plan

Involves formal adoption by Council of a plan of management for the floodplain.

4. Implementation of the Plan

Use of Local Environmental Plans and Development Control Plans to ensure new

development is compatible with the flood hazard. Construction of foreshore

protection works and other measures to protect existing developments.

This Lake Macquarie Waterway Flood Study constitutes a review of the first stage of the

management process, namely to update the 1998 Lake Macquarie Flood Study Part 1 and Part

2. This current study has been prepared by WMAwater for Lake Macquarie City Council and

was undertaken to include the June 2007 long weekend storm/flood event and to incorporate the

implications of predicted climate change.

The results provide the basis for the future management of flood liable lands adjacent to the

foreshores of Lake Macquarie. The study concentrates on those areas of the lake foreshore

within the boundaries of Lake Macquarie City local government area, with little emphasis on land

within the Wyong local government area.



ii

EXECUTIVE SUMMARY

The catchment area of the Lake Macquarie waterway to the Pacific Ocean is approximately 700

square kilometres, of which approximately 95% is within the City of Lake Macquarie. Of this

approximately 110 square kilometres (16%) is the area of the lake. The lake is approximately

22 kilometres in length and up to 8 kilometres wide, with a perimeter of approximately

170 kilometres.

The lake is surrounded by extensive rural and residential developments that value its scenic

quality as well as its commercial and recreational value. The entrance to the Pacific Ocean is by

the narrow and shallow 4 kilometre long Swansea Channel. The lake level is normally at 0.1

mAHD and average tidal fluctuations are ± 0.05m. Elevated ocean levels (high tides and storm

surge) as well as intense rainfall over the catchment cause the lake level to rise. The highest

recorded level is 1.25 mAHD in 1949, with 1.05 mAHD reached in the June 2007 long weekend

storm/flood event, and 1.00 mAHD recorded in February 1990. The June 2007 long weekend

storm/flood event and the February 1990 event were of the order of a 30 year ARI design event.

Flooding causes significant hardship (tangible and intangible damages) to the community and

for this reason Lake Macquarie City Council has undertaken a program of studies to manage

flood risks.

The present study was initiated by Lake Macquarie City Council to research and to update the

1998 Flood Study, to incorporate predicted impacts of climate change and catchment

modifications. The primary objective of the Study is to assess scenarios that would arise due to

climate change, such as an increase in rainfall and sea level rise. The study builds on the 1998

Lake Macquarie Flood Study - Parts 1 and 2, which defined design flood levels for the foreshore

area. In addition, this present study incorporates modelling of the June 2007 long weekend

storm/flood event which occurred subsequent to publication of the 1998 Flood Study.

The outcomes of this Study provide an indication of the impacts likely to occur on lake levels

due to flooding and climate change.

This report does not consider the effects of flooding due to a tsunami.

Reasons for Updating the Hydraulic Modelling Approach

The main reasons for updating the hydraulic modelling approach are as follows:

availability of a two dimensional (2D) hydraulic model,

availability of detailed bathymetric data to better describe the bed of the Swansea

channel rather than the use of cross sections used previously,

availability of Airborne Laser Scanning (ALS) or LiDAR (Light Detecting and

Ranging) survey data that provides a very accurate definition of the topography of

the floodplain,

a more detailed appraisal of design ocean level conditions,



iii

incorporation of data for the June 2007 long weekend storm/flood event in the

calibration process,

incorporation of predicted sea level rises in ocean boundary conditions and lake

still water conditions, and

incorporation of an approach based on the maximum of an ocean dominated

event and a runoff dominated event.

Adopted Hydraulic Modelling Approach

The adopted approach was to establish a TUFLOW hydraulic model based on the available

bathymetric and ALS survey with inflows from a WBNM hydrologic model. A

calibration/verification was undertaken to the February 1990 and the June 2007 long weekend

storm/flood events.

The model was then used for design flood estimation with sensitivity analysis undertaken to

determine the impacts of various model parameters.

Coincidence of Ocean Levels and Runoff

Flood levels on the Lake Macquarie foreshore are affected by runoff from the surrounding

catchments into the lake as well as inflows from the Pacific Ocean, via Swansea Channel during

elevated ocean levels. Elevated ocean levels occur due to a combination of tides (the ocean’s

high tide varies from approximately 0.5 m to 1.1 mAHD during the year) and what are known as

ocean anomalies. The main components of ocean anomalies (difference between the predicted

or astronomical tide and the recorded tide) are storm surge and wave setup. Together these

components can raise ocean levels by up to 1 metre.

As part of the Study, ocean anomalies were investigated and two runoff/ocean design scenarios

were adopted. A design ocean event in conjunction with a similar or smaller magnitude rainfall

event (termed an ocean-dominated event) and a design rainfall event in conjunction with a

similar or smaller magnitude ocean event (termed a rainfall dominated event). A summary of

the design scenarios is provided in Table i).

Table i: Design Event Scenarios (Year 2011 conditions)

OCEAN DOMINATED DESIGN

EVENT

(ARI)

RAINFALL DOMINATED

Peak Design Ocean

Level + Wave Setup

(mAHD)

Co incident Design

Rainfall Event

(ARI)

Co incident Design

Ocean Event

(ARI)

Co incident Design

Ocean Level + Wave

Setup (mAHD)

2.18 100 year PMF 100 year 1.70

1.80 100 year 500 year 100 year 1.70

1.75 100 year 200 year 100 year 1.70

1.70 20 year 100 year 20 year 1.63

1.67 20 year 50 year 20 year 1.63

1.63 20 year 20 year 20 year 1.63

1.41 10 year 10 year 10 year 1.41

1.38 5 year 5 year 5 year 1.38

1.30 2 year 2 year 2 year 1.30



iv

The following conditions were adopted for the design flood analysis:

0.1 mAHD initial water level in the Lake Macquarie waterway, which is the average Lake

water level,

48 hour critical rainfall storm duration inflows (for all design events except the probable

maximum flood {PMF}) in conjunction with the respective ocean tides as shown in Table

i),

design ocean levels based on the design levels in Fort Denison/Sydney Harbour plus a

wave setup component (0.2 m assumed for the 100 year ARI event),

all design tides assume the “shape” of the tidal hydrograph of the May 21st to 27th 1974

East Coast Low event (approximately 160 hours with the peak at 110 hours) as

recorded at Fort Denison in Sydney Harbour. This tidal hydrograph approximates the

100 year ARI design ocean event,

the wave setup component was assumed as 0 m at time zero and was increased

linearly to peak at the same time as the ocean peak (time 110 hours). Thereafter it

decreased linearly to 0 m at time 160 hours,

the peak ocean level was coincided with the peak rainfall burst in the 48 hour duration

event.

Design Flood Approach

A approach was adopted which assumed the maximum of an ocean dominated event and a

rainfall dominated event. The results indicated that, downstream of the Swansea Bridge the

ocean dominated event produces the higher water level but, upstream the runoff dominated

event produces the higher water level. The adopted design flood levels for the lake are provided

in Table ii below, together with a comparison with those adopted previously.

Table ii: Summary of Still Water Flood Levels

Event

(ARI) Still Water Level on the Lake

Macquarie foreshore

(excludes wave runup in the lake)

Still Water Level downstream of

Swansea Bridge

Year 2011 OLD

(mAHD)

Year 2011 NEW

(mAHD)

Year 2011 Difference

(m)

Year 2011 OLD

(mAHD)

Year 2011 NEW

(mAHD)

Year 2011 Difference

(m)

PMF/extreme 2.63 2.45 -0.18 2.01 2.06 +0.05

500 year n/c 1.87 n/c 1.69

200 year n/c 1.69 n/c 1.64

100 year 1.38 1.50 +0.12 1.67 1.57 -0.10

50 year 1.24 1.38 +0.14 1.64 1.54 -0.10

20 year 0.97 1.23 +0.26 1.49 1.50 +0.01

10 year n/c 0.94 n/c 1.27

5 year n/c 0.82 n/c 1.24

2 year n/c 0.65 n/c 1.15

Notes: n/c = not calculated previously

The previous maximum estimated flood level was termed “extreme” rather than a PMF as a rigorous PMF analysis was not undertaken.



v

The main reason that the levels have changed is because of different assumptions regarding the

peak ocean levels and the joint co-incidence of ocean and rainfall events. Changes to the 100

year, 50 year and 20 year ARI levels range from increases of 0.12 m to 0.26 m but in the PMF

there is a reduction of 0.18 m. Re-modelling of design events will always produce minor

changes to flood levels due to the different approaches and models employed.

In addition to inundation due to runoff/ocean levels there is another form of inundation resulting

from waves running up the foreshore (wave runup). This is where waves (caused by wind

acting on the water surface of the lake) break and “runup” the foreshore. Part 2 of the 1998

Lake Macquarie Flood Study investigated the effects of wave runup at 48 locations. The results

indicate that wave runup may increase the still water design lake levels by up to 1 m (average of

0.3 m for the 100 year ARI event). Wave runup heights are highly site specific as they are

affected by local wind speed and fetch (the length of lake across which the wind is blowing) and

local bathymetry and topography.

Climate Change

A worldwide anthropomorphic climate change is projected to raise sea levels and increase

rainfall intensities. The NSW Government has introduced a set of guidelines for the assessment

of raised sea levels and increases in rainfall intensities. As a result, the following climate

change scenarios were analysed for the 5 year, 20 year and 100 year ARI events (results can

be interpolated for intermediate events).

Rainfall Dominated flooding: increase in design rainfall intensities of 10%, 20% and

30%,

Rainfall Dominated flooding: increase in design sea levels of 0.4 m and 0.9 m. Sea

level rise scenarios assume that the initial water level in the lake rises by a similar amount

to the sea level rise, thus for a 0.4 m sea level rise the initial water level in the lake

increases from 0.1 m to 0.5 mAHD,

Rainfall Dominated flooding: combination of increase in design rainfall (10%, 20% and

30%) and increase in design sea levels (0.4 m and 0.9 m),

Ocean Dominated flooding: increase in design sea levels of 0.4 m and 0.9 m.



vi

Table iii) provides a summary of the design flood levels due to projected sea level and rainfall

increases.

Table iii: Summary of Design Flood Levels in Lake Macquarie

Peak Lake Level (mAHD)

Sea Level Rise Rainfall Increase

Event (ARI)

Existing + 0.4m + 0.9m 10% 20% 30%

2 year 0.65 1.04 1.54 0.71 0.77 0.83

5 year 0.82 1.21 1.71 0.88 0.94 1.00

10 year 0.94 1.32 1.81 1.03 1.11 1.19

20 year 1.23 1.61 2.10 1.32 1.40 1.49

50 year 1.38 1.74 2.20 1.50 1.61 1.72

100 year 1.50 1.86 2.32 1.62 1.73 1.84

200 year 1.69 2.05 2.51 1.81 1.92 2.03

500 year 1.87 2.23 2.69 1.99 2.10 2.21

PMF 2.45 2.81 3.27 2.57 2.68 2.79

Note: Underlined levels have been derived by interpolation from model results rather than actual modelling

A summary of the results are:

The effect of rainfall increase varies depending upon the size of the event. At the 5 year

ARI level a 10% rainfall increase approximates a 0.06 m increase in the peak lake water

level while at the 100 year ARI level the increase in rainfall intensity approximates a 0.12

m increase in the peak lake water level.

The effect of a sea level rise varies depending upon the size of the event. At the 5 year

ARI level a 0.4 m sea level rise approximates a 0.39 m increase in the peak water level

while at the 100 year ARI level the increase approximates a 0.36 m increase.

Results for a combined sea level and rainfall increase for the rainfall dominated scenario

generally reflects the addition of the rainfall and sea level increases.



1

1. INTRODUCTION

1.1. Background

The Lake Macquarie waterway is a saline tidal lake, with a permanently open entrance, located

in the Hunter Region of New South Wales, 95 kilometres north of Sydney and 20 kilometres

south of Newcastle (Figure 1). The main features of the lake are provided in Table 1.

Table 1: Lake Macquarie Waterway: Main Features

Total Catchment Area 700 km2

Area of Lake 110 km2 (16% of the total catchment area)

Length of Lake 22 km in a north-south direction

Width of Lake varying from 2 km to 8 km in an east-west direction

Perimeter Length 170 kilometres

Average Water Depth 8 to 9 metres

Maximum Water Depth 11 metres (near Pulbah Island)

Contributing Catchments Subcatchments (refer Figure 1) Area (km2)

Dora Creek 19, 20, 21, 22, 23,24, 25, 26, 27, 28,

29, 30, 31,32 231.2

Stony Creek 9, 10 35.7

L T Creek 11 16.4

Cockle Creek 18, 17, 15, 14, 16, 13 111.1

Mangrove Gully Creek 2 19.5

Pourmalong Creek 6 33.7

Wyee Creek 4 28.6

Lake itself 1 110.8

Subcatchment 3 3 21.1





The Lake Macquarie waterway is the largest coastal lake in eastern Australia and is surrounded

by extensive residential, commercial and industrial developments. The lake is a valuable natural

resource for the region providing commercial and recreational usage as well as being of high

scenic value. The outlet of the Lake Macquarie waterway to the Pacific Ocean is by the narrow

and shallow entrance channel at Swansea (Swansea Channel). Today it has a permanently

open entrance which has been extensively modified by man made structures (filling of the

northern embankment, dredging, sea walls).

The water level in the lake is typically at 0.1 mAHD but can rise to 0.4 mAHD following a period

of high ocean levels. Australian Height Datum (AHD) is the common national geodetic plane

approximating to mean sea level. Under average circumstances, the ocean tide (±0.5 m) has

little impact on the water level (±0.05 m) in the lake. Intense rainfall over the catchment



2

combined with elevated ocean levels can raise the water level in less than 24 hours causing

significant flooding of the foreshore areas and hardship to the community.

1.2. Objectives

The key objective of this Flood Study is to develop a suitable hydrologic/hydraulic model that

can project flood and permanent inundation water levels in Lake Macquarie from rainfall, sea

level rise and storm surge. These results will be used by Lake Macquarie City Council, in

consultation with the community of Lake Macquarie City, to manage flood and permanent

inundation risks to low lying land around the Lake Macquarie waterway.

The key stages in the process are:

Undertaken a comprehensive review of the 1998 Lake Macquarie Flood Study (Part 1 –

Reference 1) and develop suitable hydrologic/hydraulic models to define flood behaviour

over the full range of design events for existing catchment conditions,

Use the hydrologic/hydraulic models to assess various climate change scenarios,

including application of the NSW Government’s sea level rise benchmarks,

Assess the potential increase in storm surge as a result of climate change and its impact

on elevated ocean levels,

Review the potential impact of climate change on the local wind/wave climate as this

affects the extent of wave runup on the foreshore,

Assess the hydraulic and hazard categories for existing and climate change conditions.

This report details the results and findings of the above investigations. The key elements

include:

a summary of available historical flood related data,

establishment of the hydrologic and hydraulic models,

calibration of the hydrologic and hydraulic models,

definition of the design flood behaviour for existing catchment conditions,

sensitivity analysis of the design flood behaviour,

assessment of the impacts of climate change on the still water and wave runup water

levels

re-definition of the flood extent and hydraulic and hazard categories mapping for existing

and climate change conditions.

A Flood Study is a technical document and not easily understood by the general public. A

glossary of flood related terms is provided in Appendix A to assist. If more explanation of terms

or a better understanding of the approach is required, type “NSW Government Floodplain

Development Manual” into an internet search engine and you will be directed to the NSW

Government web site which provides a copy of this manual and further explanation.

Flood levels given in this report relate only to the water level with the lake itself. Design water

levels in the creek systems entering the lake (Cockle Creek, Dora Creek etc.) will be higher than

those shown for the lake.



3

1.3. The Flood Problem

Historical records (started in 1927) show that periodically the level of the lake has risen in

response to heavy rainfall over the catchment and/or elevated ocean levels. This has resulted

in inundation of land and occasionally of building floors. The records show that the highest

recorded level was 1.25 mAHD in 1949 (observed at Marks Point) with the most recent major

events occurring over the June 2007 long weekend (1.05 mAHD) and in February 1990 (1.00

mAHD). The June 2007 long weekend and the February 1990 events were of the order of a 30

year ARI design event according to the design levels in the 1998 Lake Macquarie Flood Study

(Reference 1). Accurate recording of lake levels has only been available since installation of the

NSW state government operated gauges at Marmong Point and Belmont in 1986 (Figures 2 and

3). A further water level recorder was installed at Swansea in 1995 (Figure 4) and due to the

closeness to the ocean, this gauge shows considerable tidal fluctuations in the water level.

The dates and approximate peak lake levels of all known significant floods are shown in Table 2.

The February 1990 and June 2007 events were both greater than a 20 year ARI event (Table 4)

and smaller than a 50 year ARI event (thus both approximately a 30 year ARI event).

Table 2: Flood Events

Date (in order of severity)

Approximate Peak Lake Level (mAHD)

18 June 1949 1.25

Easter 1946 1.20

11 June 1930 1.10

9 June 2007 1.05

2 May 1964 1.00

4 February 1990 1.00

1953 0.90

1926/27 0.80

25 February 1981 0.80

May 1974 0.80

4 March 1977 0.70

Notes: Data obtained from the 1998 Lake Macquarie Flood Study - Reference 1.

Levels are an average of several recorded heights. It is likely that several floods prior to 1970 may not have been recorded.

Water levels are also available on Jigadee Creek, a tributary to Dora Creek (Figure 5). Water

levels for the available recorders in Lake Macquarie and the tide gauge at Port Stephens for the

February 1990 and the June 2007 long weekend storm/flood events are shown on Figures 6 and

7.



4

1.4. Causes of Flooding

Flooding on the Lake Macquarie foreshore may occur as a result of a combination of factors

(Table 3) including:

an elevated ocean level due to an ocean storm surge, wave setup at the entrance

and/or a high astronomic tide,

rainfall over the lake and the tributaries entering the Lake Macquarie waterway,

wind wave action causing wind setup and wave runup on the foreshore within the

lake,

a permanent rise in ocean and lake levels due to climate change.

Table 3: Factors Affecting the Peak Lake Level

Major Factors Comment

Volume of Rainfall It is the volume of runoff entering the lake and not the peak flows from the various tributaries that cause the lake level to rise. Generally this significant volume of rainfall can only be obtained from rainfall over a period of 3 to 7 days. However in the June 2007 long weekend storm/flood event the rainfall was for a period of less than 12 hours.

Size of the Entrance Channel at Swansea

The size (width and depth) of the channel controls how much water is released from the lake, as well as how much enters from the ocean.

Ocean Water Level / Sea level rise

An elevated ocean level can result from a high tide, a storm surge and an ocean wave setup, or a combination thereof. It can also alter as a result of a climate change induced sea level rise.

Local Wave Runup caused by Wind Waves on the Lake

The flood level may be raised in a local area as a result of wave runup. The amount of runup depends upon the local wind/wave climate and the foreshore profile. Little is known about this effect. The main factors affecting the wave climate are the intensity of the wind and the fetch (horizontal distance in the direction of wind over which wind waves are generated). This issue is further discussed in Section 8.

Minor Factors Comment

Initial Water Level There is little variation in the normal water level (Figure 3) except in Swansea Channel and areas close to its entry into the lake.

Antecedent Catchment Moisture Conditions

The “wetness” of the catchment prior to the rainfall event determines the volume of runoff. Generally if the catchment is “very dry” prior to the event it will “soak” up a lot of the rainfall and produce less runoff than from a “wet” catchment.

Volume of Temporary Floodplain Storage (includes the area of the lake)

As the surface area of the lake is very large (110 km2), a minor reduction

in the volume of temporary storage (filling of the floodplain) will have no significant impact upon the peak lake level.

Intensity/duration of Rainfall It is the volume of rainfall rather than the peak intensity of rainfall which is more important. Thus a longer duration of rainfall (say > 12 hours) is more likely to produce flooding rather than an intense burst for say 4 hours.

Level of Catchment Development Sealing of pervious areas (houses, roads, factories, etc.) will increase the volume of runoff. However it is considered that the present extent of development has had only a minor impact, as it represents only a small percentage of the total catchment area.

Catchment Deforestation or Other Agricultural Changes

These activities will tend to increase the volume of runoff. It is considered that these changes have had only a minor impact upon runoff volumes during floods.

Evapo-transpiration Any change in the amount of evapo-transpiration will produce only a minor change in the total runoff volume.

Wind Setup within the Lake The 1998 Lake Macquarie Flood Study (Reference 1) concluded that under average conditions a maximum increase in level of only 0.04 m would occur. This issue is further discussed in Section 8.



5

One of the key considerations in modelling coastal systems is the probability of occurrence of a

combined ocean and rainfall event and the relative magnitude of both. It is considered to be

overly conservative to assume a 100 year ARI ocean event will occur concurrently with a 100

year ARI rainfall event, however there are no data available to accurately define a suitable

approach.

For this reason two scenarios were analysed: a Rainfall Dominated scenario which assumes

the design rainfall over the catchment in conjunction with a design ocean event of equal or

smaller magnitude and an Ocean Dominated scenario which assumes the design ocean event

in conjunction with the design rainfall of equal or smaller magnitude. More details on this

approach are discussed in Section 4.2.

1.5. Previous Studies

1.5.1. Lake Macquarie Flood Study - 1998

The Flood Study completed in January 1998 (References 1 and 2) by Manly Hydraulic

Laboratory was undertaken to determine flood behaviour for the 100 year, 50 year and 20 year

ARI design floods and an extreme flood event. This study determined design flood levels using

two approaches:

Still Water Design Lake Levels (Reference 1): These were obtained using a combination of

hydrologic and hydraulic computer models. The hydrologic model converts rainfall over the

catchment into stream flows. These are input into the hydraulic model which determines the

design still water lake level. The hydraulic model takes account of:

the bathymetry of the lake,

the dimensions of the Swansea entrance channel,

the complex interaction between ocean levels and outflow from the lake,

wind setup across the lake.

The models were calibrated to historical data (November 1983 gauging and May 1974, February

1990 and March 1990 floods) and the critical design storm duration was found to be 6 days (144

hours).

The adopted design levels are shown in Table 4.



6

Table 4: Peak Design Levels from the 1998 Lake Macquarie Flood Study (Reference 1)

Event

(ARI)

Still Water Level

in the Lake

(excludes wave

runup effects)

(mAHD)

Still Water Level in the Swansea

Channel (approximately midway

between the bridge and the ocean)

(excludes wave runup effects)

(mAHD)

Ocean Still Water

Level (includes

storm surge only) at

the Entrance

(mAHD)

Ocean Level

(includes storm

surge and ocean

wave setup)

(mAHD)

Extreme 2.63 2.12 1.78 2.18

500 year 1.75 * 1.80 * n/c n/c

200 year 1.55 * 1.75 * n/c n/c

100 year 1.38 1.70 1.50 1.80

50 year 1.24 1.67 1.47 1.77

20 year 0.97 1.52 1.43 1.63

10 year 0.80 * 1.45 * n/c n/c

5 year 0.65 * 1.4 * 1.38 n/c

2 year 0.45 * 1.3 * n/c n/c

NOTES: * Estimated as part of this present study.

n/c not calculated. The peak levels at each location are not coincident.

These levels are referred to as still water levels in the lake as they exclude the effect of wind

and wave set up in the lake itself. However, an ocean wave set up component is included in

determination of the design ocean level. The impact of lake wind and wave set up is discussed

below.

The “1% AEP” or “100 year ARI" flood has a 1 in 100 chance of being equalled or exceeded in

any year. On a long-term average it will happen once every 100 years, but it is wrong to think it

can only happen once in a century. Because floods are random events, there is still a 1 in 100

chance of the flood occurring next year no matter what happens this year. One of the key

features of the above results is that for all design events, except the extreme flood, the peak

ocean level (i.e. includes ocean wave setup) is higher than the peak still water level in the lake.

The difference is 0.42 m in a 100 year ARI event and 0.66 m in a 20 year ARI event.

Wave Runup: The flood level at a particular location depends upon a combination of the still

water design lake levels and the effects of local wind/wave action (wave runup). The 1998 Lake

Macquarie Flood Study included a separate study (Reference 2) to examine the effects of wave

runup at 48 locations around the lake. The results indicate that wave runup may increase the

local still water design lake levels by up to 1 m (average of 0.3 m for the 100 year ARI event).

Sensitivity analyses were undertaken to determine the impacts of the following parameters on

lake water levels:

volume of rainfall runoff,

ocean level,

coincidence of rainfall and ocean high tide,

bed friction in the Swansea Channel,

bathymetry of the Swansea Channel,

tide sequence,

wind setup.



7

The results indicated that the critical factor affecting the peak lake water level is the peak ocean

level and altering this level by (say) ±0.2 m results in a similar order of change in the peak lake

water level.

1.5.2. Tidal Prism Modelling of Lake Macquarie - 2010

This study (Reference 3) modelled tidal behaviour of the Lake Macquarie waterway and

examined predicted impacts due to climate change. The study focussed on the Swansea

Channel and the entrance area. The study assessed impacts likely to occur from rising sea

levels, storm surge and changes to channel morphology. A two dimensional MIKE-21

hydrodynamic model was established based on detailed current hydrographic surveys.

The model describes tidal water levels, tidal prism and the flow patterns associated with the

coast and estuary.

Morphological modelling was also included in this study. The aim of the morphological

modelling was to determine the new channel configuration that would be likely due to rising sea

levels. It is believed that sea level rise is likely to increase the rate of scouring within the

channel as a result of increased tidal velocities. The results from the study indicate that the

morphological response is tending toward erosion and this would result in a deeper channel.

The study also looked at storm surge which is predicted to increase due to climate change. The

study used existing extreme water level information to simulate storm surge events. These

synthetic storm surge levels were combined with ocean wave processes and sea level rise

levels to capture the effects of wave set up on the entrance.

Key results from the study are:

Tidal analysis indicates an increase in tidal range over the last 22 years of records,

The entire channel has experienced a net loss of sediment and that scoured sediments

have likely been deposited at either end of the channel,

Channel scour may potentially be slightly accelerated with sea level rise,

With a sea level rise of 0.91 m (year 2100 conditions) the spring tidal range is expected

to more than double with half due to sea level rise and half due to ongoing channel

scour and flushing times will be reduced from 270+ to about 170 days,

the wind setup effects in the lake were estimated as only ±0.05 m.

The study also analysed the impact of sea level rise on the 100 year ARI design ocean event.

The 100 year ARI design event assumed a peak ocean level of 1.5 mAHD based on the May

1974 event (the same as in the present study – refer Section 4) and used the hydraulic model to

estimate wave setup at the entrance.

The results produced a peak level in the lake of 1.25 mAHD but this assumed no catchment

inflows and thus is not comparable to the results in this present study. The study also analysed

the effects of climate change by the year 2100.



8

These effects were assumed as:

increase in sea level of 0.91 m,

an 8% increase in storm surge (the storm surge component was increased from 0.63

m to 0.66 m),

the maximum significant wave height was increased by 15% from 9.3 m to 10.7 m,

the wind speed component was increased by 5%.

The results indicated that the 100 year ARI design ocean storm in the year 2100 (assuming the

above effects) would produce a water level of 2.35 mAHD in the lake (an increase of 1.1 m).

Again this result is not comparable to those provided in this present study as the latter has not

quantified the effects of climate change on storm surge, wave height or wind speed.

1.5.3. Lake Macquarie Adaptive Response of Estuarine Shores to Sea

Level Rise – 2010

The objective of this report (Reference 4) was to gain an appreciation of how the foreshores of

the Lake Macquarie waterway might respond to rising sea levels Ten case study locations were

examined in terms of sediment/rock material, vegetation, back beach form and profile. The

study also examined wind and wave set up. The outcome was to develop a methodology to

investigate foreshore changes to sea level rise that can be re-applied at other sites.

The study established a hydrodynamic model to investigate shoreline erosion and recession.

The model simulated the effect of larger storms on the foreshore profile and was able to look at

seabed forces generated by storm waves at each location. The model results indicate the

factors affecting the shoreline response around the Lake Macquarie waterway are:

Wave climate and near shore depth,

Vegetation,

Sediment type, and

Sediment sources and sinks.

The study concluded that it was likely that the existing Lake Macquarie shoreline would shift in

as a result of sea level rise and shoreline erosion. The extent of the inundation is dependant on

the topography, while the extent of erosion is dependant largely on the sediment type and wave

energy. Mapping potential risk from erosion is site specific and as such was not incorporated

into this plan.

1.6. Land Use

The majority of the lake perimeter is within the Lake Macquarie local government area, with

approximately 15% within Wyong Shire. Wyong Shire includes land to the south around Point

Wollstonecraft.



9

The land use (within the Lake Macquarie local government area) surrounding the lake (assumed

as land below 4 mAHD) comprises the full range of planning zones provided in Local

Environmental Plan 2004 namely:

• Rural (1),

• Residential (2.1 and 2.2),

• Business (3),

• Industrial (4),

• Infrastructure (5),

• Open Space (6),

• Environmental Protection (7),

• National Park (8),

• Natural Resources (9),

• Investigation (10).

1.7. The Entrance Channel

Water levels in the Lake Macquarie waterway are dependent on ocean levels but are controlled

by the entrance channel (Swansea Channel) which connects the Lake Macquarie waterway to

the ocean. The channel is approximately 4 kilometres long and is characterised by numerous

shoals and scoured deeper areas. The entrance at Blacksmiths Point is approximately 350 m

wide (between the breakwaters). As the volume of the Lake Macquarie waterway is so large,

less than one percent is exchanged in each tidal cycle.

A brief summary of the history of the channel is provided in Table 5.

Table 5: History of the Swansea Channel

Year Event

1878 Construction of the Swansea breakwaters commence

1884 Construction of the 1st Swansea Bridge

1887 Construction of the Swansea breakwater is completed

1939 to 1996 Dredging works begin in 1939 and continue in order to improve the

navigability of the channel

1980 to 2001 Salts Bay foreshore is stabilised after a long period of recession

1996 to 2008 Dredging removes 210,000m3 from the upper reaches of the

Swansea Channel

The channel has been extensively altered by human activities notably:

ocean entrance training works (late 1800's) which removed the shoals at the

entrance, producing an increased tidal range in the lake,

construction of the 1st Swansea bridge in 1884 and reclamation of the northern

approach (late 1800's) producing a significant hydraulic restriction at this point,

construction of the 2nd Swansea bridge in 1909,

in the 1950's various dredging and reclamation activities were undertaken in the

vicinity of Elizabeth and Pelican Islands near Marks Point,



10

construction of the 3rd Swansea bridge in 1955,

construction of the 4th Swansea bridge in 1980 (a duplicate bridge structure was

constructed),

more recently in 2006, stabilisation works involving placing ballast around the piers

was undertaken,

dredging of the ocean entrance channel around 1981 and more recently.

The channel has responded to natural and man-made effects through changes in the pattern of

erosion and sedimentation. These are natural phenomena which will always occur, but the

pattern and rate and change is affected by human modifications such as breakwalls, dredging,

and seawalls. Changes to the entrances to coastal lakes such as Lake Macquarie can disrupt

the natural estuarine processes and consequently cause ecological changes in the lake.

Solving one problem with man-made works tends to impact upon other areas. Management of

the estuary and lake environs must therefore consider the broader implications of any works and

their inter-relationships.

Sedimentation in the channel can potentially restrict access for deeper draft vessels. Typically

the depth at low water in the channel is around 2.5 m (requirement for larger vessels) but in

places due to shoaling it may reduce to 1.5 m restricting access. Many of the vessels that are

moored within the Lake Macquarie waterway are capable of ocean sailing as opposed to lake

sailing. According to the Roads and Traffic Authority’s web site the bridge opens about

2000 times each year, and around five to six times per day, allowing up to 4500 boat

movements annually.



11

2. AVAILABLE DATA

2.1. Flood Levels

2.1.1. Water Level Recorders

The main sources of flood level data relevant to this study are the water levels recorders (Figure

1) at Marmong Point, Belmont, Swansea Channel and Jigadee Creek at Avondale, as indicated

in Table 6. The complete historical records for these gauges are shown on Figures 2 to 5 and

the record for the February 1990 and the June 2007 long weekend storm/flood events shown on

Figures 6 and 7.

The Kalang Road gauge on Dora Creek and the Stockton Creek gauge at Morisset are water

level only gauges and have not been used in this study as the hydraulic model of the Lake

Macquarie waterway does not include estimation of the water levels in the various tributary

creeks upstream of the Lake Macquarie waterway itself.

The Jigadee Creek gauge is also excluded from the hydraulic model extent of this study but is of

value as it has been “gauged” and thus a rating curve (relationship between water level and

flow) is available. Design flood levels in these tributary creeks are provided in separate studies

(such as the 1986 Dora Creek Flood Study - Reference 5).

Table 6: Water Level Recorders in the Lake Macquarie waterway

Data Available

NAME Opened February

1990 June 2007

Belmont 1986 Y Y

Marmong Point 1986 Y Y

Swansea 1996 N Y

Jigadee Creek at Avondale 1986 Y Y

Kalang Road at Dora Creek 1993 N Y

Stockton Creek at Morisset unknown N Y

Notes: Opening dates are approximate and records may be available outside those periods

2.1.2. Flood Levels from Debris or Other Marks

Apart from the water level recorders the other source of flood peaks are the surveying of flood

marks recorded during/after the flood. The accuracy of these levels will vary depending upon

the nature of the mark. A “tide mark” on a building wall or fence is probably accurate to within a

few centimetres but surveying of a “vegetative debris” mark is probably only accurate to ±0.3 m

depending upon the exact nature of the mark. These levels have not been considered in this

study due to the availability of the high quality water level gauges for both February 1990 and

the June 2007 long weekend storm/flood events.



12

2.2. Rainfall Stations

Rainfall data from past flood events is required for the calibration of hydrologic models. For this

reason rainfall data has been collected from the relevant rainfall stations (Figure 1) within or

near to the catchment of the Lake Macquarie waterway for the two largest flood events (greater

than 1 mAHD) in the last 30+ years, namely:

4th February 1990,

10th June 2007.

Table 7 indicates the total number of rainfall stations (where data are available) for each flood

event and Table 8 lists the continuously read (pluviometer) stations that have data available.

The pluviometers are operated by either the Bureau of Meteorology (BOM) or Hunter Water

(HW - gauges not operating in 1990).

Table 7: Availability of Rainfall Data for each Flood Event

Type Total February 1990 June 2007

Daily 17 10 7

Continuous 16 2 14

Table 8: Continuously Read (Pluviometer) Rainfall Stations

Station Operator Station Name Data Available

February 1990

June 2007

Bureau of Meteorology Barnsley Y Y

Bureau of Meteorology Martinsville N Y

Bureau of Meteorology Mandalong N Y

Bureau of Meteorology Whitemans Ridge Y Y

Hunter Water R11-Swansea N Y

Hunter Water R14-Wallsend N Y

Hunter Water R32-Morriset/Dora Creek N Y

Hunter Water R33-Wangi Wangi N Y

Hunter Water R38-Hamilton N Y

Hunter Water R39-Kotara N Y

Hunter Water TR100-Eleebana N Y

Hunter Water TR101-Valentine N Y

Hunter Water TR102-Belmont N Y

Hunter Water TR103-Belmont N Y

Hunter Water TR104-Redhead N Y

Hunter Water TR105-Tooltaba N Y

Hunter Water TR106-Dudley N Y

Hunter Water TR107-Swansea N Y

N = data NOT available, Y = data available



13

Table 9 lists the BOM daily read stations that have data available for at least one flood event.

Table 9: BOM Daily Read Rainfall Stations

Number Station Name Opened Closed

61299 Belmont WTP 1990 -

61133 Bolton Point (The Ridge Way) 1962 -

61011 Cockle Ck (Pasminco Metals) 1900 2003

61393 Edgeworth WTP 1990 -

61359 Mt Hutton (Auklet Road) 1986 2005

61377 Swansea (Catherine Street) 1987 -

61322 Toronto WTP 1972 -

61357 Mandalong 1986 -

61323 Dora Creek 1972 1993

61012 Cooranbong 1903 -

61041 Balcolyn (Bay Street) 1999 -

61406 Blacksmiths 2003 -

61376 Eraring (Payten Street) 1993 -

61382 Wyong 1993 -

The location of available rainfall gauges and cumulative rainfall totals for the February 1990 and

the June 2007 long weekend storm/flood events are shown on Figures 8 to 11.

2.3. Flow Measurements

An automatic water level recorder is located on Jigadee Creek (Gauge No: 211008) immediately

downstream of the Newport Road bridge (Figure 1). The gauge was installed in 1969 and is

operated by the Office of Environment and Heritage.

The recorded water levels for the period December 1973 to February 2010 were obtained from

Pinneena (NSW Government’s database of surface water records) and are shown on Figure 5.

The overall quality of the data is reported as being good but the data set is not complete as

some readings are missing for various reasons. The maximum annual water levels were

extracted and are provided in Table 10.



14

Table 10: Jigadee Creek Gauge – Peak Annual Peak Water Levels and Flows

Year Peak Recorded Water

Level (mAHD)(1)

Peak Flow based on the Pinneena Rating Curve

(m3/s)

1974 5.40* 121

1975 5.37 118

1976 5.14 97

1977 5.49* 130

1978 5.56 138

1979 4.16 35

1980 2.92 4

1981 5.8 166

1982 4.75 67

1983 4.39 46

1984 5 86

1985 4.78 69

1986 4.24 38

1987 4.52 53

1988 4.94* 81

1989 5.03 88

1990 5.11 95

1991 3.11 19

1992 4.62* 59

1993 3.24* 8

1994 3.13 6

1995 3.19* 7

1996 3.08* 6

1997 3.59* 20

1998 4.03 27

1999 3.84 25

2000 4.03 27

2001 4.4 50

2002 4.86 73

2003 3.16 10

2004 3.88 26

2005 3.89 27

2006 5.04 84

2007 5.59* 144

2008 4.65* 60

2009 2.72* 5

Notes: Gauge zero at Jigadee Creek gauge is at 2.03 mAHD * gauge records missing in the year so possibly the annual peak was never recorded



15

Figure 3 of the 1986 Dora Creek Flood Study (Reference 5) includes peak water levels for the

1977 and 1981 floods (it should be noted that the level of 6.5 mAHD quoted thereon for the

1981 event at the Jigadee Creek gauge appears to be a typographic error because Table 6 of

Reference 5 lists the peak water level as 5.81 mAHD). The available gauge records (refer to

Table 10) indicate the highest water level occurred in 1981 (February) with five other annual

peaks within 0.4 m of the 1981 peak. The June 2007 long weekend storm/flood event was the

2nd highest on record.

For calibration of a hydrologic model and to a lesser extent a hydraulic model, a recorded flow

(in m3/s) in the tributary river/creek is required. The estimated flow at a given water level is

obtained from a rating curve which provides a relationship between the known water level and

estimated flow. This relationship is derived from velocity readings (obtained from a current

meter) at a known water level and cross sectional water area (obtained by survey). Many of

these velocity readings are taken over a period of years at different water levels (termed

gaugings) and in this way, a rating curve is developed as a “line of best fit” between the

gaugings.

It is relatively easy to obtain “low flow” gaugings as small rises in water level occur frequently

and the gauging party has therefore ample opportunity to undertake them. It is much harder to

obtain “high flow” gaugings as they can only be obtained during large floods (which occur

infrequently) and it may be that the gauging party cannot get access to the site or are otherwise

engaged. Thus all rating curves have few “high flow” gaugings and there is therefore

considerable uncertainty about the flow estimates at high water levels. A graph of the gaugings

indicates how many “high flow” gaugings were undertaken and the height at which they were

taken, from this an estimate of the accuracy of the high flows can be made. Generally there are

no gaugings taken at the peak of a flood and thus the highest gaugings may be several metres

below the peak and the rating curve must be extrapolated.

Gaugings are usually taken from a bridge over the river with several velocity measurements at

various depths and distances across the river. These velocity measurements are averaged and

the flow calculated (flow {m3/s} = mean velocity {m/s}*waterway area {m2}).

The Pinneena rating curves (flow versus height relationship) including the actual gaugings for

two periods are provided as Figure 5. The differences between the two curves should not be

interpreted as there being a difference in the channel morphology at the changeover date.

Rather, rating curves are derived based on the gaugings available at the time, thus the later

rating curve is based on a different dataset to the former or different interpretation.

2.4. Survey

Airborne Laser Scanning (ALS) or LiDAR of the surrounding topography was provided by Lake

Macquarie City Council as part of this study. The ALS did not pick up “below water levels” and

the bathymetry survey was obtained from the 2010 Tidal Prism Modelling Study of Lake

Macquarie (Reference 3). These two datasets were merged together to obtain a grid of the

below water and above water topography.



16

2.5. Flood Photographs

A number of flood photographs taken in the June 1949, February 1990 and June 2007 long

weekend storm/flood events are available and selected photographs are provided on Figure 12.

In the absence of the automatic water level recorders in the lake, these photographs would be a

valuable source of flood height data.

2.6. Ocean Levels

Ocean water level data are available from the Port Stephens gauge. Port Stephens is the

nearest “ocean” gauge and historical records for this gauge were obtained and used to

represent ocean conditions at the entrance to the Lake Macquarie waterway for the historical

events. The highest level recorded since 1986 at the Port Stephens gauge is 1.34 mAHD in

June 1999. The design ocean levels at Fort Denison in Sydney Harbour based on 80+ years of

record (these values are the same as used in the 1998 Lake Macquarie Flood Study -

Reference 1) are:

100 year ARI 1.50 mAHD,





1 year ARI 1.26 mAHD.

It is interesting to note that there is less than a 0.3 m difference between the 1 year ARI event

and the 100 year ARI event and the highest astronomic tide in a year reaches approximately

1.1 mAHD. No accurate estimates of ocean levels for events greater than the 100 year ARI are

available. However an indicative estimate for an extreme event is of the order of 1.8 m to 1.9

mAHD. These levels are applicable along the NSW coast where there is no wave setup

component.

The May 1974 Fort Denison (Sydney Harbour) tide is the highest on record (1.48 mAHD) and

approximates the 100 year ARI ocean level. This tide encompasses a storm surge component

of 0.5+ m and a high tide of 0.9 mAHD. Due to the gauge location in Sydney Harbour this

record does not include any wave setup component.



17

3. APPROACH

The approach adopted in flood studies to determine design flood levels largely depends upon

the objectives of the study and the quantity and quality of the data (survey, flood, rainfall, flow

etc.). In the absence of an extensive historical flood record, a flood frequency approach cannot

be undertaken for the Lake Macquarie waterway and must rely on the use of design rainfalls and

establishment of a hydrologic/hydraulic modelling system. A diagrammatic representation of the

flood study process is shown below.

CA TCHMENT INFORMA TION

sub-areas

land-use

stream length

observed runoff volumes or rates

RA INFA LL DA TA

historical or design storm events

rainfall depths (Isohyets)

temporal patterns (intensity v

time)

MODEL BOUNDARY CONDITIONS

downstream ocean/tide levels

upstream inflow hydrographs

direct rainfall - lateral inflows

CA LIBRATION/VERIFICATION

Computational Modelling Software

HYDROLOGIC ANALYSIS

QUANTIFY CA TCHMENT RUNOFF

estimated flow hydrographs

HYDRAULIC

CHARACTERISTICS

topographic data

bridge/culvert details

ov erflow weir structures

define flow paths

stream roughness values

OBSERVED FLOOD

BEHAVIOUR

peak heights

stage or flow hydrographs

relative timing of ev ents

velocity estimates

general observations

COMPUTER MODEL PARAMETERS

storage-routing coefficient

rainfall losses

CA LIBRA TION/VERIFICATION

Computational Modelling

Software

QUA NTIFY FLOOD

BEHA VIOUR

flood levels

flows

velocities

HYDRA ULIC ANALYSIS

REVIEW

Diagram 1 - Flood Study Process



18

3.1. Hydrologic Model

Inflow hydrographs to the Lake Macquarie waterway are required as input to the hydraulic

model. Typically in flood studies a rainfall-runoff hydrologic model (converts rainfall to runoff) is

used to provide these inflows. A range of runoff routing hydrologic models is available as

described in the 1987 edition of Australian Rainfall and Runoff (Reference 6). These models

allow the rainfall depth to vary both spatially and temporarily over the catchment and readily lend

themselves to calibration against recorded data. The Watershed Bounded Network Model

(WBNM) was adopted for the reason that it was used in the 1998 Lake Macquarie Flood Study

(Reference 1) and the 2004 Jigadee Creek Flood Study (Reference 7) and this allowed a

comparison between the results from these studies.

For the historical events (used to calibrate the TUFLOW hydraulic model) historical rainfall data

was input to the WBNM model to obtain the inflows and the WBNM model could be calibrated to

the flow data from the Jigadee Creek stream flow gauge.

3.2. Hydraulic Model

Originally it was intended to use the hydraulic model established for the previous 1998 Lake

Macquarie Flood Study (Reference 1) or an updated hydraulic model used for the 2010 Tidal

Prism Modelling Study of Lake Macquarie (Reference 3). However the former model did not

include the current bathymetry and needed to be updated and the latter model could not be

obtained for licence reasons. Thus a new hydraulic model had to be established. The

availability of high quality ALS and aerial photographic data means that the study area is

suitable for two dimensional (2D) hydraulic modelling. Various 2D software packages are

available (SOBEK, TUFLOW) and the TUFLOW package was adopted as it is widely used in

Australia.

In TUFLOW the ground topography is represented as a uniform grid with a ground elevation and

Manning’s “n” roughness value assigned to each grid cell. The size of grid is determined as a

balance between the model result definition required, the dimensions of the river channel (as a

rough guide the channel should have over 4 cells widths in order to accurately define it) and the

computer run time (depends on the number of grid cells).

The adopted approach was to establish a 40m by 40m grid TUFLOW model. The model

extends from the ocean to the upstream limit of Swansea Channel (Figure 1) with the lake itself

represented as a storage node.

By modelling historical flood events and matching the model versus the recorded data the

TUFLOW model can be “calibrated” or tuned to replicate actual flood events. This process is

critical to the success of the approach and comprises the majority of the effort in the study.

Water level recorders (Marmong Point, Belmont and Swansea) provide a continuous record of

the water level for the duration of the historical flood events and these data are used to compare

to the results from the TUFLOW model.



19

3.3. Calibration Events

The choice of calibration events for flood modelling depends on a combination of the magnitude

of the flood event and the quality and quantity of available flood height data. Clearly it is

preferable to use recent events as generally they have a higher quality and quantity of data

(February 1990 and the June 2007 long weekend storm/flood events). Calibration to earlier

events (say 1949) is not possible due to the lack of pluviometer and water level data.

The quality and quantity of available data for each flood event has varied considerably over the

years. For the February 1990 flood event, pluviometers at Barnsley and Whitemans Ridge were

the only gauges in existence. For the June 2007 long weekend storm/flood event, pluviometer

data were available at 18 locations within or around the catchment (Table 7).

For February 1990 water level data was available from the Belmont and Marmong Point gauges,

whilst for the June 2007 long weekend storm/flood event additional data was available from the

later installed Swansea gauge.

3.4. Design Flood Modelling

Following model establishment and calibration the following steps were undertaken:

Design tributary inflows were obtained from the WBNM hydrologic model and included

in the TUFLOW hydraulic model,

Sensitivity analysis was undertaken to assess the effect of changing model

parameters and the assumed ocean boundary conditions.

3.5. Climate Change

The calibrated hydrologic/hydraulic models were used to assess the effects of climate change

induced sea level rise (+0.4 m and +0.9 m) as well as rainfall volume increase (10%, 20% and

30%). The basis for these climate change impacts are provided in Section 7. For the sea level

rise scenarios it was assumed that the design ocean hydrograph was elevated by the magnitude

of the assumed sea level rise (i.e no change to the assumed storm surge or wave setup

component in the derivation of the design ocean hydrograph). For the rainfall increase

scenarios the design rainfalls over the catchment were increased by the nominated percentage

(10%, 20%, 30%) within the hydrologic model (i.e an increase in volume and intensity but no

change to the adopted temporal pattern or loss rates).

3.6. Wind Wave Assessment

The actual flood level at a site depends upon a combination of the still water level (determined

from the hydrologic and hydraulic modelling approach) and the effect of local wind/wave action

(wave runup). The wave runup effect at Lake Macquarie depends upon a number of interrelated

factors summarised in Table 11.



20

Table 11: Factors Influencing Wave Runup Effects

General Factors

Comment

Maximum Fetch across Lake Macquarie

The length of open water used to determine the wind wave

condition (varies from 1.5 km to 9 km). Direction of Maximum Fetch

Design wind data vary depending upon the direction (by up to

20%). Approximate Offshore Water Depth

Can vary from 1 m to 5 m. This influences the breaking of the

waves.

Local Factors

Comment Offshore Beach Profile

The slope of the lake bed can vary significantly.

Foreshore Beach Profile

The slope and vegetation type influence the extent of wave

activity. Embankment or Seawall

Many locations have stone or earthen embankments. The

height, slope and location of these structures relative to the

shoreline and buildings influences the breaking waves. Location of Nearest Building

Some buildings are located on the shoreline whilst others are

over 50 m away.

The 1998 Lake Macquarie Flood Study Part 2 – Foreshore Flooding (Reference 2) uses a

“Guideline” method to combine wind setup and catchment runoff water levels to determine the

100 year ARI (and the 20 year ARI) design runup levels at the 48 locations around the foreshore

of Lake Macquarie.

The guideline method for the 100 year ARI event was to adopt the highest of either:

100 year ARI design lake level (taken as 1.38 mAHD) with an approximate 1 year ARI

wind velocity,

the 100 year ARI wind velocity with an approximate 1 year ARI design lake level of 0.4

mAHD).

The results showed that there were no locations where the second scenario (the higher wave

runup condition) produced the highest runup levels and only one location (Site 4, Bolton Point)

where the difference was less than 0.3 m and at that location there was no development likely to

be affected.

The results from the 1998 Lake Macquarie Flood Study (Reference 2) for the nominated 48 sites

around the lake indicate (for the 100 year ARI event) that the maximum increase as a result of

wave runup is 1.0 m, the minimum is 0.1 m and the average is 0.3 m, as summarised in Table

12. This table also shows the increase in local water level due to wave runup above the 1 year

ARI design lake level scenario.



21

Table 12: Wave Runup Effects – 100 year ARI Flood and 1 year ARI Event

% of Sites with Runup Level below

Runup Level above 100 year ARI

(m)

Runup Level above 1 year ARI

(m)

10% 0.2 0.4

20% 0.2 0.6

30% 0.2 0.6

40% 0.2 0.6

50% 0.3 0.7

60% 0.3 0.7

70% 0.4 0.8

80% 0.5 0.9

90% 0.5 1.2

100% 1.0 1.4

The key points regarding the use of wave runup data are summarised below:

• Wave runup effects produce an increase in the design flood level (Table 12) and also

require that the structural integrity of any proposed structure be more closely examined.

• Council has adopted a 0.5 m freeboard (for setting floor levels of residential buildings)

above the 100 year ARI flood level. A significant component of this freeboard

allowance is to cater for the effects of wave runup.

• 90% of the 48 sites analysed have a wave runup effect of 0.52 m or less for the 100

year ARI scenario.

• Of the remaining 10% for the 100 year ARI scenario, at four out of the five sites the

high level is due to the effect of an existing building or structure on the foreshore. The

remaining site is at Marmong Point where the level is attributable to the particular

beach profile.

• Wave runup effects will generally only occur over a small percentage of the lake

foreshore in a given event (in the prevailing wind direction).

• The effects will vary in time and space as a result of changing foreshore profiles. This

may occur naturally (sedimentation, erosion, vegetation growth) or as a result of human

activities (construction).

• Buildings located close to the foreshore will experience the greatest wave runup impact

(increased design flood level and increased potential for structural damage). Further

away from the foreshore the impacts reduce significantly. The zone of influence of the

wave runup effect varies considerably depending upon the topography of the area. In a

relatively flat area (Swansea) the impact may be up to 50 m whilst in a steeply rising

foreshore area the impact may be 5 m or less.

• Of the factors influencing wave runup (Table 11) only three, foreshore beach profile,

embankment or seawall and location of nearest building, can possibly be modified to

reduce the impact.

Further discussion on wave runup and the likely impacts of climate change on wave runup are

provided in Section 8.



22

4. OCEAN WATER ASSESSMENT

4.1. Approach Adopted in the 1998 Lake Macquarie Flood Study -

Reference 1

The 1998 Lake Macquarie Flood Study (Reference 1) used ocean entrance design hydrographs

as the downstream boundary conditions for the hydraulic model of the lake. The likely maximum

ocean entrance levels during the design flood events were determined by examining the ocean

level component parts, namely:

astronomic tide,

storm surge (barometric and wind stress effects),

wave setup at the entrance.

Table 13 lists the “still water” (tide and storm surge levels but no ocean wave setup component)

ocean levels adopted. The adopted temporal pattern of the ocean levels was the May 1974

event which is generally assumed as approximating a 100 year ARI ocean event in Sydney

Harbour.

The 1998 Lake Macquarie Flood Study (Reference 1) undertook a review of the ocean wave

setup component at the entrance to the Lake Macquarie waterway and concluded that the likely

maximum wave setup is of the order of 0.4 m in an extreme event. The adopted peak ocean

levels for design are shown in Table 13.

Table 13: Peak Design Ocean Levels (1998 Lake Macquarie Flood Study - Reference 1)

Event (ARI)

“Still Water” Tide and

Storm Surge Level

(mAHD)

Wave

Setup

(m)

Peak Ocean

Level

(mAHD)

20 year 1.43 0.2 1.63

50 year 1.47 0.3 1.77

100 year 1.50 0.3 1.80

Extreme 1.78 0.4 2.18

The adopted approach in Reference 1 for determining design flood levels in the lake assumed:

a 6 day (144 hours) critical storm duration,

an ocean hydrograph based on the temporal pattern of the May 1974 event,

the time of the peak ocean level approximates the time of peak rainfall intensity,

joint coincidence of the same design ocean and rainfall event (i.e a 100 year ARI

rainfall and 100 year ARI ocean level are coincident and a 50 year ARI rainfall and 50

year ARI ocean level are coincident etc.).

The study examined the effects of wind setup on the lake and concluded that the maximum was

less than 0.05 m in the lake. The report states “Because wind setup is small during flooding

events, and requires winds from specific directions, the impacts of winds on lake water levels



23

were not included in the modelling of design flood water levels”.

4.2. Adopted Approach

The procedures and assumptions used to determine the maximum design levels in the 1998

Lake Macquarie Flood Study (Reference 1) were “standard” at the time of the assessment.

However, since 1998 further investigations and long term ocean and entrance water level data

collection and analyses have provided a much better understanding of the processes operating

at estuary entrances during storms. This is particularly the case for wave setup and the impacts

on flood levels inside entrances. As a result, the assumptions and procedures used for the 1998

Lake Macquarie Flood Study (Reference 1) are now considered to be conservative.

The following sections examine relevant available data and determine design ocean

hydrographs that better reflect the conditions applying at the entrance to the Lake Macquarie

waterway during design rainfall events.

The basic methodology for this Flood Study Review is similar to that used for the 1998 Lake

Macquarie Flood Study (Reference 1) in that the individual component parts that make up

elevated ocean levels at the Lake Macquarie waterway entrance were examined, and summed

to produce design ocean levels. An allowance for sea level rise due to climate change was

added to produce levels for the years 2050 and 2100.

The significant water level components affecting the entrance to the Lake Macquarie waterway

are:

astronomic tide,

tidal anomaly:

o storm surge (barometric and wind stress effects),

o oceanographic effects (shelf waves, ocean currents, temperature variations),

wave setup.

The latter two components are likely to be affected by predicted future climate change.

4.2.1. Available Tidal Data

The tidal record for Fort Denison in Sydney Harbour is over 125 years long. Since completion of

the 1998 Lake Macquarie Flood Study (Reference 1) the almost continuous record from 1914

has been digitised and analysed to accurately determine its astronomic and anomaly

components (1995 Harmonic Analysis of NSW Gauge Network - Reference 8). Since around

1984 there has also been accurate tidal data recorded at a number of ocean and estuary

entrance locations along the NSW coast. These data sets have been analysed by numerous

studies and provide a much improved understanding of tidal conditions and influences along the

NSW coast and inside estuary entrances (for example 1992 Mid NSW Coastal Region Storm-

Tide Surge Analysis – Reference 9).

The Fort Denison gauge, although within Sydney Harbour, is considered to be a “deep still



24

water” gauge location. This basically means that the gauge records the ocean astronomic tide

plus ocean tidal anomaly components (such as storm surge and oceanographic effects) without

significant interference from non-ocean effects such as breaking or broken waves, catchment

runoff, shallow water effects, local wind shear, etc. Other “deep still water” gauge sites along

the NSW coast include Coffs Harbour, Crowdy Head, Port Stephens (Tomaree), Jervis Bay and

Batemans Bay.

In addition to the “deep still water” sites, there are also gauges located just inside estuary

entrances that respond closely to ocean conditions but are also influenced to some (varying)

extent by non-ocean effects. These gauges record the ocean astronomic tide and the ocean

tidal anomaly components, but also some wave and/or estuary effects. The Lake Macquarie

waterway (Swansea) gauge installed in 1995 is an example, as are the Hastings River (Port

Macquarie), Manning River (Harrington) and Wallis Lake (Forster) gauges.

4.2.2. Astronomic Tides

Astronomic tides are caused by the gravitational and centripedal forces between the earth and

moon, and to a lesser extent the sun and other planets. They can be predicted with accuracy

based on the harmonic movements of these bodies. Along the NSW open coast, astronomic

tides are very similar in terms of their levels and timing. There are two high and two low tides

per day, with a range of up to around 2.0 m during the summer and winter “King” tides.

Analysis of the long term tidal harmonics for Fort Denison shows that the highest astronomical

tide level is approximately 1.1 mAHD, and that a level of 0.6 mAHD is exceeded around 10% of

the time. The 0.6 mAHD level is also approximately the Mean High Water Springs tide level (the

average of two highest new moon and full moon tides).

Harmonic analyses for the other “deep still water” gauge locations along the NSW coast, as well

as many of the entrance gauge locations give very similar harmonic constituents to Fort Denison

(1995 Harmonic Analysis of NSW Gauge Network - Reference 8). This similarity shows that the

maximum astronomic tide level at these locations (including the entrance to the Lake Macquarie

waterway) is also less than 1.1 mAHD and that an astronomic tide level of 0.6 mAHD would be

exceeded around 10% of the time.

4.2.3. Ocean Tidal Anomaly

As mentioned, the ocean tidal anomaly component recorded at a “deep still water” gauge

location is made up of storm surge and oceanographic effects. This anomaly is recorded as a

variation from the predicted astronomic tide level.

The storm surge component is the increase in ocean water level that occurs during storms as a

result of inverse barometric pressure and wind stress. Barometric pressure causes a localised

rise in ocean water levels of about 0.1 m for each 10hPA drop in pressure and strong onshore

winds produce surface currents that cause a build up of water against the coastline.



25

The oceanographic component of the tidal anomaly covers a range of other factors that can

affect ocean water levels. The most important of these are the shelf waves generated by large

storms remote from the NSW coast. These waves are long and low, with heights of up to 0.2 m

and periods of many days. When these waves reach the eastern continental shelf they are

confined and migrate along the coast producing elevated ocean water levels.

The size and occurrence of oceanographic effects is hard to determine accurately. However, for

the purposes of determining an ocean hydrograph for the Lake Macquarie waterway this is not

necessary, as statistically these effects are accounted for in the overall “deep still water” tidal

anomaly analysis.

An analysis of “deep still water” anomalies along the NSW coast (1992 Mid NSW Coastal

Region Storm-Tide Surge Analysis – Reference 9) found very good correlation between

anomaly levels and occurrence north and south of the Lake Macquarie waterway between

Crowdy Head and Batemans Bay. This correlation reflected the size and similarity of the

weather systems along the coast despite the more localised nature of the effects, and the

remote nature of shelf waves. As a result of the correlation it is reasonable to assume that the

tidal anomaly conditions near the entrance to the Lake Macquarie waterway would be similar to

those at Fort Denison.

Analysis of the tidal anomalies recorded at Fort Denison since 1914 shows that the maximum

“deep still water” increase is around 0.6 m (as occurred in May 1974) and that a 0.2 m level

occurs for around 5% of the time, but a 0.4 m level occurs for less than 0.1% of the time (1992

Mid NSW Coastal Region Storm-Tide Surge Analysis – Reference 9). However, there is a

correlation between a storm event capable of producing major flooding in a large catchment

such as the Lake Macquarie waterway catchment and a storm event likely to produce a large

storm surge tidal anomaly.

A major flood producing storm event is likely to last several days and be associated with very

low barometric pressure and strong onshore winds (as well as very heavy rain). Based on the

above, it is reasonable to assume that the maximum tidal anomaly (storm surge plus

oceanographic effects) would be less than 0.6 m. However, because of the strong correlation

between the flood/rainfall event and the conditions likely to produce a high storm surge, an

anomaly level of greater than 0.4 m could be expected.

4.2.4. Wave Setup

Wave setup occurs in the surf zone where the shoreward kinetic energy of the breaking and

broken waves is converted to gravitational potential energy in the form of increased water levels.

Wave setup is largely confined to the nearshore area and is highly dependent on factors such as

the wave height, wave length, water depth and embayment geometry.

Wave setup along exposed NSW beaches can be of the order of 1.5 m during very large energy

wave climate conditions, but this setup is only maintained if the wave energy remains high for a

sustained period (in excess) of an hour. Wave setup can be relieved by a lull in wave energy,



26

by alongshore rips and currents and at estuary entrances. The extent of the relief is highly

dependent on the specific site conditions and the method used to calculate setup in the 1998

Lake Macquarie Flood Study (Reference 1) is still valid.

“Deep still water” locations not in the breaker zone, such as Fort Denison, Coffs Harbour,

Crowdy Head and Port Stephens (Tomaree) gauge locations have negligible wave setup

because there is no significant capacity for the waves to break and convert shoreward kinetic

energy into increased water levels. This is reflected in the correlation between the astronomic

tide predictions at these sites. However, most estuary entrance locations are exposed to ocean

waves and have shallow foreshore conditions capable of producing breaking waves under some

high energy wave climate conditions. These locations are inside the breaker zone and under

these conditions will be affected by wave setup to some extent.

The degree to which estuary entrance locations are affected by wave setup depends on the

exposure of the site and the capacity of the waves to break and produce setup. It also depends

on how quickly any setup can be relieved by flow into the estuary.

Some locations with relatively high exposure and shallow bed conditions such as the entrance to

the Manning River experience significant wave setup. Other locations with some protection but

with shallow bed conditions such as the entrance to the Lake Macquarie waterway (Swansea) or

the Hastings River (Port Macquarie) have significant setup during larger wave climate

conditions, but negligible setup during low conditions. Other, semi-protected and deep

entrances, such as the entrance to Wallis Lake, have very little wave setup under most

conditions.

Analysis was undertaken of the 22nd and 23rd March 2005 large energy wave event. The low

pressure system causing that event was centred off the coast of NSW between Sydney and

Coffs Harbour moving south to north. The central pressure dropped to 996 hectopascals and

winds were south easterly at around 35 knots. Under these conditions, a storm surge anomaly

of between 0.3 m and 0.4 m could be expected at “deep still water” gauge locations.

Table 14 sets out the tidal anomalies recorded at a number of “deep still water” gauges as well

as the tidal anomaly plus wave setup at a number of estuary entrance gauges during the event.

The table also shows an approximation of the tidal anomaly component at the estuary entrance

locations based on the adjoining “deep still water” locations, and by subtraction the resultant

wave setup component at the estuary entrances.



27

Table 14: Tidal Anomalies and Wave Setup (m) during March 2005 Large Wave Energy Event

Gauge Location Gauge

Type

Maximum

Anomaly +

Setup

High Tide

Anomaly + Setup

Estimated

Storm Surge

Resultant

Wave Setup

Coffs Harbour Deep S W 0.34 0.31 0.35 0.0

Hastings River Estuary 0.65 0.33 0.40 0.25

Manning River Estuary 0.77 0.65 0.40 0.37

Wallis Lake Estuary 0.44 0.22 0.40 0.04

Port Stephens Deep S W 0.45 0.43 0.45 0.0

Hunter River Estuary 0.46 0.36 0.45 0.0

Lake Macquarie Estuary 0.50 0.16 0.40 0.10

Port Jackson Deep S W 0.29 0.28 0.30 0.0

Shoalhaven

River

Estuary 0.26 0.11 0.25 0.0

Batemans Bay Deep S W 0.27 0.12 0.25 0.0

The analysis shows that significant wave setup occurred at the Hastings River and Manning

River entrances of 0.25 m and 0.37 m respectively. Such a response is in keeping with the

wave exposure and shallow nature of the entrances. Similarly, the smaller 0.1 m results for the

Lake Macquarie waterway entrance, which is well sheltered from south easterly waves, and the

even smaller 0.04 m setup for the Wallis Lake entrance, which is both well sheltered and deep,

are as expected.

These wave setup differences were also reflected in the analysis of tidal anomalies for the years

between 1987 and 1991 (1992 Mid NSW Coastal Region Storm-Tide Surge Analysis –

Reference 9). All the estuary entrance sites show good correlation with Port Jackson during low

wave climate conditions, but the Hastings River and Lake Macquarie deviate significantly during

larger wave climate conditions.

Assuming sustained large energy wave breaking occurs across the Lake Macquarie waterway

entrance during a major storm event, there should be some wave setup at the entrance. The

level of setup would initially be partially relieved by flows into the estuary, and later by the bed

scour and the entrance rip formed by catchment outflows. However, provided the wave energy

is sufficiently large and sustained wave setup would occur. Based on the available information,

the maximum wave setup during a major flood event is unlikely to be greater than 0.2 m.

4.2.5. Tidal Anomaly Analysis

A limited tidal anomaly analysis was undertaken to determine the magnitude of the recorded

anomalies at Lake Macquarie and whether this accords with the storm surge component

assumed for the May 1974 event.

Water level data in the Swansea Channel at the mouth of the Lake Macquarie waterway have



28

been recorded continuously since late 1995. It should be noted that this gauge is located at

Swansea near the bridge and thus is influenced by entrance conditions within the Swansea

Channel and is not representative of the ocean level. These data can be compared with the

“predicted” tidal data to estimate the difference in water levels resulting from any tidal anomaly.

To some extent the Swansea gauge will be influenced by elevated water levels in the Lake

Macquarie waterway, resulting from runoff from the catchment (as occurred in February 1990

and the June 2007 long weekend storm/flood events). The value of this record is limited due to

its relatively short period of operation. A much longer record is available at the Port Stephens

gauge (25 years as opposed to 15 years at Swansea).

Manly Hydraulics Laboratory (MHL) undertook a comparison of the recorded versus predicted

water levels at the Port Stephens tidal gauge to obtain the residual or anomaly (Figure 13). The

results indicated that the maximum water level recorded at Port Stephens (datum conversion of

-0.944) is 1.34 mAHD with the maximum predicted level of 1.23 mAHD approximately 0.1 m

lower than the maximum recorded levels. 75 incidences of an anomaly greater than 0.3 m were

recorded with the largest being 0.56 m (May 1997).

In conclusion, based on the limited data and analysis undertaken, the storm surge component

assumed for the May 1974 event (0.5+ m) is comparable with the maximum value recorded at

the Port Stephens gauge.

However, the highest two anomalies at Port Stephens were approximately 0.1 m greater than

the third largest and further investigation of the record for the largest anomaly (May 1997) was

undertaken. The record shows that the anomaly is not a smooth line, rather it consists of peaks

and troughs which can vary by over 0.1 m in an hour. The peak anomaly of 0.56 m is one such

peak and a more representative anomaly value for this period is 0.5 m.

4.2.6. Summary

Based on the above assessment and in conjunction with advice from the NSW Office of

Environment and Heritage the maximum ocean boundary levels as set out in Table 15 have

been determined for the Lake Macquarie waterway entrance for current and year 2050 and 2100

conditions (refer Section 7).

The levels are based on the design ocean levels at Fort Denison (Section 2.6) with the addition

of a wave setup component (the adopted amount is less than assumed in Table 12 of the 1998

Lake Macquarie Flood Study - Reference 1).



29

Table 15: Estimated Design Ocean Peak Levels

Design Event

(ARI)

Ft Denison

Design

Ocean Level

(mAHD)

Wave

Setup

(m)

Peak Ocean Level

(mAHD)

(year 2011)

Peak Ocean Level

(mAHD) with 0.4m

Sea Level Rise

(year 2050)

Peak Ocean Level

(mAHD) with 0.9m

Sea Level Rise

(year 2100)

PMF/Extreme 1.78 0.4 2.18 2.58 3.08

500 year 1.60 0.2 1.80 2.20 2.70

200 year 1.55 0.2 1.75 2.15 2.65

100 year 1.50 0.2 1.70 2.10 2.60

50 year 1.47 0.2 1.67 2.07 2.57

20 year 1.43 0.2 1.63 2.03 2.53

10 year 1.41 0.0 1.41 1.81 2.31

5 year 1.38 0.0 1.38 1.78 2.28

2 year 1.30 0.0 1.30 1.70 2.20

Selected historical and design tides are shown on Figure 14.



30

5. HYDROLOGIC MODELLING

5.1. Watershed Bounded Network Model (WBNM)

The WBNM hydrologic runoff-routing model was used to determine inflows from the local

catchments to the Lake Macquarie waterway. This model is widely used throughout NSW and

the model layout for the Lake Macquarie waterway is shown as Figure 1. The model input

parameters are a storage lag factor (termed C which accounts for the attenuation in the peak

flow as floodwaters travel downstream) and the rainfall initial and continuing loss.

If data are available the model can be “calibrated” to historical flow records by including the

historical rainfall data and adjusting the model parameters until a good match to the recorded

data is achieved. The main issue with this approach is the limited amount of pluviometer

records available. Pluviometer data are required to provide a temporal pattern to be applied to

the daily rainfall records. It is known that the rainfall temporal patterns can vary greatly across

even a small area and thus over these relatively large catchments the availability of only a few

pluviometers means that the resulting “accuracy” of the calibrated model is low.

5.2. Calibration

The only flow gauging records available are for Jigadee Creek (Table 9 and Figures 1 and 5).

The accuracy of the flow gauging has been investigated in the 2004 Jigadee Creek Flood Study

(Reference 7) which concluded that it was likely that the recorded flow gauging was

underestimating the flows at high flood levels. The 2004 Jigadee Creek Flood Study (Reference

7) established a WBNM and a 1D hydraulic model (MIKE-11) and undertook a joint

hydrologic/hydraulic model calibration to the recorded stage hydrographs for the February 1981

(largest on record), June 1989 and February 1990 events. This approach is the most accurate

that is possible as it relies on a hydraulic model, using surveyed cross sections, to match to the

recorded heights through adjustment of the model parameters in WBNM to determine the

necessary inflows. The adopted design model parameters in the 2004 Jigadee Creek Flood

Study (Reference 7) were:

C value = 2.3,

Initial Loss = 20 mm,

Continuing Loss = 2.5 mm/h.

These parameters were identical to those adopted in the 1998 Lake Macquarie Flood Study

(Reference 1) with the exception of an urban initial loss of 10 mm and a rural initial loss of 25

mm. For the present study the following parameters were adopted for the calibration (February

1990 and the June 2007 long weekend storm/flood events) and design events:

C value = 2.4,

Initial Loss = 10 mm,

Continuing Loss = 2.5 mm/h,

On the Lake Macquarie waterway no losses assumed.



31

A comparison between the WBNM flows and those based on the Pinneena record and in the

2004 Jigadee Creek Flood Study (Reference 7) for February 1990 and the June 2007 long

weekend storm/flood event is shown on Figures 15 and 16. As noted above the WBNM flows

are much higher than the Pinneena record. The results provide a reasonable match to the data

using the rating curve obtained in the 2004 Jigadee Creek Flood Study (Reference 7). The

adopted parameters were very similar to Reference 7 and thus the model hydrographs in each

study are very similar. A slight change in the C value was adopted in this study to more closely

align the peaks and a slight reduction in the initial losses was required.

Sensitivity analysis was not undertaken to assess the impacts of varying the above hydrologic

model parameters for the following reasons:

the hydrologic\hydraulic modelling approach was calibrated in tandem to recorded

levels, thus any significant change to any of the parameters would require an

adjustment of other parameters (say Manning’s “n”) to achieve the same calibration,

the effect of varying the hydrologic model parameters (within an acceptable range) is

small given the large surface area of the lake and the significant tidal/oceanic

influence on flood levels.



32

6. HYDRAULIC MODELLING

6.1. TUFLOW

A TUFLOW 2D hydraulic model was established, calibrated to historical events (February 1990

and the June 2007 long weekend storm/flood events) and used for design flood estimation.

The TUFLOW model extended from upstream of the Swansea Channel to the Pacific Ocean

(Figure 1) and was based on the same bathymetry as used in the 2010 Tidal Prism Modelling of

Lake Macquarie (Reference 3). The model covered an area of approximately 21 km2 using a

40m by 40m grid. The lake itself was modelled as a 1D storage node which had an

area/elevation relationship based on the ALS and connected to the 2D domain. Each grid cell is

assigned a ground level and a Manning’s “n” value which reflects the hydraulic roughness of the

topography.

The only hydraulic structure included in the model was the Swansea Bridge. This bridge

represents a significant hydraulic restriction due to the width of the piers, the presence of eddies

around the piers and because there are two sets of piers (see photographs below).

Aerial view of Swansea Bridge looking upstream View between bridge structures indicating

piers and other restrictions

6.2. Calibration

The calibration process was based on matching the TUFLOW results to produce the best fit to

the recorded water level data for the most recent flood events in February 1990 and the June

2007 long weekend.

The inflows from the calibrated WBNM hydrologic model were included into TUFLOW and the

model run for both events. The Manning’s “n” within the Swansea Channel was adjusted so that

the modelled stage hydrograph at the water level gauges matched the recorded hydrograph.

This was an iterative procedure and the adopted Manning’s “n” values are provided in Table 16.

However it should be noted that other combinations of hydrologic and hydraulic parameters

could produce similar results. The February 1990 event, which had two peaks, was sensitive to



33

a change in Manning’s “n” value whereas the June 2007 long weekend storm/flood event which

only had one peak was less sensitive.

It was considered that there was no justification for varying the Manning’s “n” value within the 2D

area except between the channel and the floodplain.

Table 16: Adopted Manning’s “n” Values – TUFLOW model

Description February 1990, June 2007

and Design Events

Swansea Channel 0.025

General floodplain 0.080

The calibration results are provided in Figures 15 and 16.

It should be noted that the emphasis in calibration / verification of the computer models was to

find the optimal balance of model parameters (such as roughness) that gave the overall best

match to observed historic flood behaviour. This set of parameters could then be used to

estimate design flood behaviour.

For this study, there was only a very limited amount of historic flood data but it was of high

quality (from a gauge). It is likely that the channel bed has varied between 1990 and 2007 due

to erosion/sedimentation and/or dredging. Unfortunately there is no detailed information

available that allows for these changes to be included in the calibration process.

The quality of match to the peak was better for the June 2007 long weekend storm/flood event

than February 1990 with the match to the shape of the hydrographs good, particularly for the

June 2007 long weekend storm/flood event. Overall it was considered preferable to determine a

consistent set of modelling parameters and assumptions, rather than modifying the parameters

for each event as this would provide the best estimate of design flood behaviour under present

conditions.

Sensitivity analysis could be undertaken into the effects of changing the Manning’s “n” values (or

any other parameter) but as the hydraulic model is calibrated to recorded data this approach is

of limited value, as any significant change would require a re-calibration.

6.3. Design

6.3.1. Critical Duration Analysis

The Lake Macquarie waterway design inflows to the TUFLOW model were determined using the

calibrated WBNM model. Four evenly spaced design Intensity, Frequency, Duration (IFD)

locations were chosen and the closest sub catchments to these locations were assigned to that

IFD. WBNM automatically assumes an areal reduction factor based on the catchment size

(approximately 0.95).



34

A range of storm durations for the 100 year ARI event were input into TUFLOW and the duration

which produced the highest lake level determined as the critical duration (a static ocean level of

0.1 mAHD was taken to eliminate issues with the coincidence of the peak ocean level and peak

flow). The results are shown on Figure 14 which indicates that the critical storm duration is the

48 hour event (design rainfalls shown on Table 17). The 36 hour event produces a similar but

slightly lower peak lake level, the 48 hour duration was adopted as it has a greater volume. This

duration is significantly shorter than the assumed 144 hour (6 day) duration adopted in the 1998

Lake Macquarie Flood Study - Reference 1 (Section 4.1). A 48 hour duration appears more

reasonable and also is compatible with the relatively rapid response (24 hours) in the June 2007

long weekend event.

Table 17: 48 Hour Design Rainfall Intensities (mm/h)

IFD Locations 2 year

ARI

5 year

ARI

10 year

ARI

20 year

ARI

50 year

ARI

100 year

ARI

1 (N) 3.24 4.25 4.86 5.64 6.70 7.51

2 (SE) 3.24 4.34 5.01 5.86 7.02 7.90

3 (S) 3.43 4.56 5.24 6.12 7.30 8.21

4 (SW) 3.50 4.78 5.58 6.61 8.00 9.09

Note: Locations represent the north, south east, south and south west parts of the catchment

The 100 year ARI 48 hour design inflow hydrographs are provided in Figure 14. These show

the peak inflows occur approximately 20 hours after the commencement of the design rainfall

event.

6.3.2. Approach for Coincidence of Rainfall and Ocean Levels

Peak water levels in the Lake Macquarie waterway result from a combination of rainfall over the

catchment and elevated ocean levels. Thus the assumed design ocean level in conjunction with

the design rainfall event over the catchment will affect the resulting design flood level in the Lake

Macquarie waterway. The approach adopted in the 1998 Lake Macquarie Flood Study

(Reference 1) was to combine the design rainfall event with the same design ocean level as

shown in Table 4 (i.e. a 100 year ARI rainfall event occurs in conjunction with a 100 year ARI

ocean event, a 50 year ARI rainfall event occurs in conjunction with a 50 year ARI ocean event

etc.).

There is no definitive combination of rainfall and ocean levels that has been universally adopted

in NSW. The former NSW Department of Environment, Climate Change and Water (now Office

of Environment and Heritage) produced the Flood Risk Management Guide, incorporating sea

level rise benchmarks in flood risk assessments in August 2010 (Reference 10) which

supersedes the approach adopted in the 1998 Lake Macquarie Flood Study (Reference 1).

According to the 2010 Flood Risk Management Guide (Reference 10) the entrance to Lake

Macquarie is a Class 2 entrance (catchments that drain directly to the ocean via trained or

otherwise stable entrances). The “default” ocean condition for this type of entrance is to assume

a 100 year design ARI ocean hydrograph with a peak of 2.6 mAHD (Figure 7.1 of Reference



35

10). However, this is superseded if a site specific analysis of elevated water levels at the ocean

boundary is undertaken. The latter approach has been undertaken as part of this present study,

as reported in Section 4.

The 2010 Flood Risk Management Guide (Reference 10) recommends an approach that

combines a design ocean event with a design rainfall event and suggests the following

scenarios:

100 year ARI ocean flooding with 20 year ARI catchment flooding with coincident peaks,

20 year ARI ocean flooding with 100 year ARI catchment flooding with coincident peaks,

neap tide cycle with 100 year ARI catchment flooding with coincident peaks.

In conjunction with advice from the NSW Government Office of Environment and Heritage, the

combination of ocean and rainfall for design flood analysis as shown in Table i) in the summary

was developed. The design scenarios are defined as either Rainfall Dominated (design inflow

event) or Ocean Dominated (design ocean event) mechanisms.

The following conditions were adopted for the year 2011 design flood analysis:

0.1 mAHD initial water level in the Lake Macquarie waterway,

48 hour critical rainfall storm duration inflows (for all design events except the PMF)

in conjunction with the respective ocean tides as shown in the table above,

design ocean levels based on the design levels in Fort Denison/Sydney Harbour

plus a wave setup component,

all design tides assume the “shape” of the tidal hydrograph of the May 21st to 27th

1974 event (approximately 160 hours with the peak at 110 hours) as recorded at

Fort Denison in Sydney Harbour. This tidal hydrograph approximates the 100 year

ARI design ocean event,

the wave setup component was assumed as 0 m at time zero and was increased

linearly to peak at the same time as the ocean peak (time 110 hours). Thereafter it

decreased linearly to 0 m at time 160 hours,

the peak ocean level was coincided with the peak rainfall burst in the 48 hour

duration event.

6.3.3. Sensitivity Analysis - Varying Ocean Levels

Figure 17A indicates the effect on lake water levels of different ocean levels in combination with

the 100 year ARI 48 hour design rainfall event:

In combination with the 20 year ARI ocean event produces an increase in the peak lake

water level of approximately 0.55 m compared to with a 0.1 m static ocean water level

(this represents the “normal” water level in the lake and by assuming a static water

level excludes the influence of ocean tides),

In combination with the 20 year ARI ocean event produces an increase in the peak

water level of approximately 0.35 m compared to with the June 2007 tide,

Varying the coincidence of the peak outflow and the peak ocean level for the 100 year



36

ARI 48 hour event and the June 2007 tide makes less than 0.05 m difference in the

peak water level in the lake,

The peak of the June 2007 long weekend storm/flood event was only 0.1 m lower than

the 100 year ARI 48 hour event combined with the June 2007 tide. This indicates that

the June 2007 long weekend rainfall was approaching the 100 year ARI rainfall

intensity, though a direct comparison is not possible due different storm durations and

areal distributions.

Varying the timing of the peak rainfall and ocean hydrograph can change the peak level

in the lake by approximately ± 0.05 m.

Figure 17B indicates that:

The 100 year ARI 48 hour event and the 20 year ARI ocean event produces a peak

level in the lake of 1.5 mAHD. This scenario is in accordance with the 2010 Flood Risk

Management Guide (Reference 10) and constitutes the Rainfall Dominated flood

scenario,

The 20 year ARI 48 hour event and the 100 year ARI ocean event produces a peak

level in the lake of 1.3 mAHD. This scenario is in accordance with the 2010 Flood Risk

Management Guide (Reference 10) and constitutes the Ocean Dominated flood

scenario,

Comparison of the above two flooding scenarios indicates that the Rainfall Dominated

flood scenario produces the greater flood level in the lake (1.5 mAHD) and should

therefore be adopted as the 100 year ARI design flood scenario for the lake.

Downstream of the bridge the Ocean Dominated flood scenario produces the higher

peak levels. This scenario is not considered in detail in this study but is evaluated for

climate change (Section 7),

The Rainfall Dominated peak lake level of 1.5 mAHD is 0.12 m higher than that

derived in the 1998 Flood Study (refer Table 4 - derived using the design rainfall in

combination with the 100 year ARI ocean event),

Various runs were undertaken to assess the variation in lake level for different design

rainfall and ocean event scenarios (Figure 17). The 100 year ARI 48 hour rainfall event

in combination with a 100 year, rather than a 20 year ocean event increases the lake

level by 0.05 m (approximately the difference in ocean level peak). The 100 year ARI

48 hour rainfall event in combination with the May 1974 tide results in a lake level of

1.4 mAHD. Increasing the design ocean event from the 20 year ARI to the 100 year

ARI in combination with the 20 year ARI design rainfall increases the lake level by

approximately 0.05 m.

The resulting design flood levels in the Lake Macquarie waterway, including flood levels that

incorporate sea level rises by 2050 and 2100, are shown in Table ii) and Table iii) in the

summary and in Figure 18.

6.3.4. Variation in Starting Water Level for Design Analysis

The 2011 initial water level in the lake for all design runs was taken as 0.1 mAHD and this level



37

was derived from the long term average water level as shown on Figures 2 and 3. If the design

rainfall event is preceded by more rain, or a period of elevated ocean levels, the initial water

level in the lake could be greater than 0.1 mAHD. Changing the initial water level in the lake for

all the design events makes no difference to the peak level as the design events are run for over

100 hours prior to the peak rainfall burst, in this time the effect of the initial water level

dissipates. Sensitivity analysis was undertaken into the effect of varying the initial water level for

the 100 year ARI 48 hour rainfall event in combination with a much shorter duration tidal

hydrograph (the initial 2 day period prior to the start of the 48 hour design rainfall was omitted, it

is during this period that the elevated ocean levels “pump up” the lake levels). The results vary

depending upon the run duration but for a 40 hour run the results are shown below:

Peak with 0.1 mAHD initial water level = 1.30 mAHD,

Peak with 0.5 mAHD initial water level = 1.38 mAHD,

Peak with 0.7 mAHD initial water level = 1.42 mAHD.

In conclusion varying the initial water level in the Lake Macquarie waterway for the design flood

analysis makes only a slight difference to the resulting peak water level.

6.3.5. Probable Maximum Flood

The Probable Maximum Flood (PMF) was determined using the methodology provided in the

Bureau of Meteorology’s 2003 Estimation of Probable Maximum Precipitation in Australia –

(Reference 11) which indicated a critical storm duration of 6 hours. The peak outflows from the

WBNM hydrologic model (i.e not routed through the lake) are 3,220 m3/s in the 100 year ARI (48

hour duration) event and 10,250 m3/s in the PMF (6 hour) event.

The results shown in Table ii) in the Executive Summary for the Rainfall Dominated flood

scenario indicate a lower PMF level than that given in the 1998 Lake Macquarie Flood Study

(Reference 1) by approximately 0.18 m. The key reasons for this difference are due to different

ocean level assumptions, the use of a 2D hydraulic model that encompasses the entire

floodplain across the Pacific Highway and approaches for estimation of the PMF inflows (the

1998 Lake Macquarie Flood Study - Reference 1 adopted a runoff hydrograph three times the

100 year ARI event whereas this present study adopted Reference 11).



38

7. CLIMATE CHANGE ASSESSMENT

7.1. Background

The 2005 NSW Government Floodplain Development Manual (Reference 12) requires that

Flood Studies and Floodplain Risk Management Studies consider the impacts of climate change

on flood behaviour.

Since completion of the 1998 Lake Macquarie Flood Study (Reference 1), current best practice

for considering the impacts of climate change (sea level rise and rainfall increase) have been

evolving rapidly. Key developments in the last four years have included:

release of the Fourth Assessment Report by the Inter-governmental Panel on Climate

Change (IPCC) in February 2007 (Reference 13), which updated the Third IPCC

Assessment Report of 2001 (Reference 14);

preparation of Climate Change Adaptation Actions for Local Government by SMEC

Australia for the Australian Greenhouse Office in mid 2007 (Reference 15);

preparation of Climate Change in Australia by CSIRO in late 2007 (Reference 16), which

provides an Australian focus on Reference 13;

release of the Floodplain Risk Management Guideline Practical Consideration of Climate

Change by the NSW Department of Environment and Climate Change in October 2007

(Reference 17 - referred to as the DECC Guideline 2007);

adoption by Lake Macquarie City Council of the Lake Macquarie Sea Level Rise

Preparedness and Adaptation Policy in August 2008 and the preparation of interim

development assessment procedures in areas vulnerable to sea level rise;

Hunter, Central and Lower North Coast Regional Climate Change Project — Report 3:

Climate Change Impact for the Hunter, Lower North Coast and Central Coast Region of

NSW (Hunter and Central Coast Regional Environmental Strategy, 2009 (Reference 18).

In October 2009 the NSW Government issued its Policy Statement on Sea Level Rise

(Reference 19) which states: “Over the period 1870-2001, global sea levels rose by 20 cm, with

a current global average rate of increase approximately twice the historical average. Sea levels

are expected to continue rising throughout the twenty-first century and there is no scientific

evidence to suggest that sea levels will stop rising beyond 2100 or that the current trends will be

reversed.

Sea level rise is an incremental process and will have medium to long-term impacts. The best

national and international projections of sea level rise along the NSW coast are for a rise relative

to 1990 mean sea levels of 40 cm by 2050 and 90 cm by 2100. However, the 4th

Intergovernmental Panel on Climate Change in 2007 also acknowledged that higher rates of sea

level rise are possible”;

In August 2010, the former NSW Department of Environment, Climate Change and Water

issued the following:



39

Flood Risk Management Guide (Reference 10): Incorporating sea level rise benchmarks

in flood risk assessments,

Coastal Risk Management Guide (Reference 20): Incorporating sea level rise

benchmarks in coastal risk assessments.

In addition an accompanying document Derivation of the NSW Government’s sea level rise

planning benchmarks – October 2009 (Reference 21) provided technical details on how the sea

level rise assessment was undertaken, using peer reviewed scientific research from the IPCC,

CSIRO, BOM and other scientific agencies.

As a result of the information provided in the above and other documents, and to keep up-to-

date with current best practice, this study incorporates an assessment of climate change.

Although there are some minor variations in the sea levels predicted in these studies, policies,

and guides, they all agree on an ocean level rise on the NSW coast of around 0.9 m by the year

2100 relative to 1990 levels.

The most recent guideline, the NSW Sea Level Rise Policy Statement (2009) (Reference 19)

and associated guides, indicates a 0.9 m sea level rise by the year 2100 and a 0.4 m rise by the

year 2050. It should be noted that climate change and the associated rise in sea levels will

continue beyond 2100.

The climate change scenarios in the earlier DECC Guideline 2007 (Reference 17) suggested for

undertaking rainfall sensitivity analysis in flood studies are indicated below.

increase in peak rainfall and storm volume:

low level rainfall increase = 10%,

medium level rainfall increase = 20%,

high level rainfall increase = 30%.

A high level rainfall increase of up to 30% is recommended for consideration in the DECC

Guideline 2007 (Reference 17) due to the uncertainties associated with this aspect of climate

change and to apply the “precautionary principle”. A 30% rainfall increase is probably overly

conservative. The Hunter & Central Coast Regional Environmental Management Strategy 2009

(Reference 18) climate change study of the Hunter, for example, predicted an increase of spring

rainfall of about 15% by 2080, and a drop in the other three seasons, although this does not

predict the intensity of individual design events. A timeframe for the provision of definitive

predictions of the actual increase is unknown. The DECC Guideline 2007 (Reference 17) is

currently the only NSW reference providing guidelines for rainfall increases for design flood

analysis due to climate change.

7.2. Rainfall and Ocean Dominated Flooding

The following scenarios were modelled for the 5 year, 20 year and 100 year ARI events (results

have been interpolated for intermediate events and are provided in Table iii) in the Executive

Summary). It should be noted that the same 48 hour critical duration rainfall event adopted for



40

the scenarios described in Section 6 has been adopted for all the climate change scenarios

(except for the PMF) as the next longer duration (72 hours) design event produces much lower

results for these scenario (Figure 14).

Rainfall Dominated flooding: increase in design rainfall volume of 10%, 20% and 30%,

Rainfall Dominated flooding: increase in sea level of 0.4 m and 0.9 m for the design

ocean event. All sea level rise scenarios assume that the initial water level in the lake

rises by a similar amount to the sea level rise, thus for a 0.4 m sea level rise the initial

water level increases from 0.1 m to 0.5 mAHD,

Rainfall Dominated flooding: combination of increase in design rainfall volume (10%,

20% and 30%) and increase in sea level (0.4 m and 0.9 m) for the design ocean event,

Ocean Dominated flooding: increase in the design ocean event of 0.4 m and 0.9 m.

A summary of the results shown on Figures 18 to 23 are as follows.

Figures 18 and 19: These graphs summarises the results for the 100 year ARI event

including the increase in design rainfall of 10%, 20% and 30% and the increase in

design ocean level of 0.4 m and 0.9 m.

Figure 20: This graph shows the results for the 5 year, 20 year and 100 year ARI events

with a 10%, 20% and 30% increase in design rainfall. The effect of rainfall increase

varies depending upon the size of the event. At the 5 year ARI level a 10% rainfall

increase approximates to a 0.06 m increase in peak water level while at the 100 year

ARI level the increase approximates to a 0.12 m increase.


with an increase in sea level of 0.4 m and 0.9 m (Rainfall dominated). The effect of a

sea level rise varies depending upon the size of the event. At the 5 year ARI level a 0.4

m sea level rise approximates a 0.4 m increase in peak water level while at the 100 year

ARI level the increase approximates a 0.35 m increase.


for a combined sea level and rainfall increase for the rainfall dominated scenario. In

summary the results reflect the addition of the rainfall and sea level increases.

Figure 23: This graph (as well as Figure 19) shows the results for the 5 year, 20 year

and 100 year ARI events for the ocean dominated scenario. It can be seen that this

scenario produces greater flood levels downstream of the bridge than in the lake

(because the elevated ocean level is not at its peak for long enough to elevate the entire

lake. This statement does not agree with the outcomes of the 2010 Tidal Prism

Modelling of Lake Macquarie - Reference 3 – refer Section 1.5.2). The results indicate

that flood levels downstream of the bridge are increased by a similar magnitude to the

sea level increase.

7.3. Increase in Average Lake Water Level

Sea level rise will increase the “average” water level in Lake Macquarie by a similar amount to

the sea level increase. The 2010 Tidal Prism Modelling of Lake Macquarie (Reference 3)

indicates that the tidal range may also change. The “average” water level in Lake Macquarie is



41

0.1 mAHD and this will rise to 0.5 mAHD (+0.4 m) in the year 2050 and to 1.0 mAHD (+0.9 m) in

the year 2100. The available survey data is not accurate enough to map the current average

water level of 0.1 mAHD as this relies on an accurate definition of the ground/water interface

which cannot be accurately defined by ALS. Appendix B provides maps showing the extent of

inundation to the 1.0 mAHD contour (year 2100 average water level).

7.4. Flood Extent Mapping

Appendix B provides maps showing the extent of inundation in the following events:

Year 2011: existing 100 year ARI water level of 1.5 mAHD,

Year 2050: 100 year ARI water level of 1.86 mAHD (assumes sea level rise of 0.4 m),

Year 2100: 100 year ARI water level of 2.32 mAHD (assumes sea level rise of 0.9 m),

Year 2100: the “average” lake water level of 1.0 mAHD (assumes sea level rise of 0.9 m).



42

8. REVIEW OF STORM SURGE, WAVE SETUP AND WAVE RUNUP

8.1. Effect of Climate Change on Storm Surge

Storm surge is the increase in ocean water level that occurs during storms as a result of the

inverse barometric pressure effect and wind stress. Together with wave setup at the mouth of

the estuary these effects cause a local raising in the ocean level. These effects have been

investigated as part of this study (Section 4.2.3) however they may be impacted by climate

change as implied in the IPCC 2007 report (Reference 13).

Storm surge affects estuaries in two ways, as an elevated ocean surge that is translated into the

estuary through the entrance and as an internally generated estuary wind setup and barometric

pressure effect.

8.1.1. Ocean Storm Surge

Ocean storm surge impacts on design water levels in Lake Macquarie were also considered as

part of the 2010 Tidal Prism Modelling Study of Lake Macquarie (Reference 3). Both this

reference and the present study adopted a year 2011 design ocean storm surge component of

0.63 m for the 100 year ARI storm event (the same as the highest recorded May 1974 storm

residual peak). Based on recommendations made by the CSIRO in their 2007 Projected

Changes in Climatological Forcing for Coastal Erosion in NSW (Reference 22), the 2010 Tidal

Prism Modelling Study of Lake Macquarie (Reference 3) increased this component by 8% to

produce a year 2100 climate change affected ocean storm surge component of 0.68 m and also

increased the assumed wave height and local wind speed (wave setup effects).

To determine the year 2100 design Lake Macquarie water level (100 year ARI level due to

ocean influence in the absence of catchment runoff effects), the 2010 Tidal Prism Modelling

Study of Lake Macquarie (Reference 3) added the 0.68 m ocean storm surge to a climate

change sea level rise of 0.91 m (a total increase of 0.96 m) and translated it into the lake using

their hydraulic model, producing a design lake level of 2.35 mAHD. This level is 1.1 m higher

than their modelled year 2011 design lake level of 1.25 mAHD. The 0.14 m additional difference

between the year 2100 level and the year 2011 level is presumed to be due to increased depth

and hence conveyance along the entrance channel resulting from climate change effects over

time in the estuary.

The present study (Table iii in the Summary) indicated that under year 2011 conditions the 100

year ARI rainfall plus 20 year ARI ocean levels produced a lake level of 1.5 mAHD, but when the

year 2100 climate change sea level rise of 0.9 m was included, the lake level reached only 2.32

mAHD. The -0.08 m difference is explained by increased conveyance in the channel but this

time the greater out flows have reduced the peak lake level.

Based on the above, it is concluded that climate change induced increases in storm intensity

could increase ocean storm surge levels by around 0.05 m. This increase in ocean level would

then increase the entrance conveyance producing a reduction in the height of rainfall dominated



43

flooding in the lake and/or increase the height of ocean storm surge flooding in the lake by up to

0.15 m.

8.1.2. Lake Storm Surge

Although theoretically storm surge is caused by wind setup and inverse barometric pressure

effects, estuaries along the NSW coast are much smaller than the weather systems and so

barometric pressure gradients across an estuary are small. As a result, pressure effects cannot

develop and barometric pressure can be disregarded as part of an internal NSW estuary storm

surge assessment. However, wind setup can develop locally within the lake and needs to be

considered.

Wind setup is caused by wind drag on the water surface (wind stress) creating surface currents

that convert to water level increases against a land mass (1984 Shore Protection Manual -

Reference 23). Wind stress is proportional to the square of wind speed, thus any significant

increase in wind speed has the potential to increase wind setup. However, conversion to setup

or an increase in the local lake level requires quite restricted conditions related to shallow depths

and topography and will not develop where relieving flow paths (back flows) can form.

An investigation into design water levels and wave climate as part of the 1997 Port Stephens

Flood Study – Stage 2 (Reference 24) found that internal wind setup in that estuary was up to

around 0.5 m at some restricted bay heads but generally, wind setup levels were less than 0.05

m. The 2010 Tidal Prism Modelling Study of Lake Macquarie (Reference 3) also found that wind

setup was less than 0.05 m. It should be noted that the 1997 Port Stephens Flood Study

(Reference 24) included winds from all directions while the 2010 Tidal Prism Modelling Study of

Lake Macquarie (Reference 3) considered only wind from the southeast and associated with two

major coastal storm events, the May 1974 “Sygna” storm and the June 2007 “Pasha Bulker”

storm.

An investigation into wind setup within Lake Macquarie was also undertaken as part of the 1998

Lake Macquarie Flood Study Part 1 – Design Lake Water Levels and Wave Climate (Reference

2). However, this investigation used hourly exceedance wind speeds with occurrences

measured in days or months. As a result, the wind speeds (refer Table 18) and hence the

assessed wind setup from the 1998 Lake Macquarie Flood Study (Reference 2) is much smaller

than would be determined for say a 20 or 100 year ARI design storm event. As a result the

outcomes from the 1998 Lake Macquarie Flood Study (Reference 2) were disregarded.

The implication from the above is that although wind setup in estuaries can be substantial, the

conditions required are not usual and that this is particularly the case for Lake Macquarie where

for southeast storm conditions the foreshore morphology and water depths are generally quite

conducive to the formation of relieving flow paths and hence setup levels are low (<0.05 m).

Further, in relation to the impacts of climate change induced ocean (and lake) water level

increases, although the 1997 Port Stephens Flood Study (Reference 24) did not examine the

effects of a climate change sea level rise, it did model wind setup at different water levels. The



44

modelling showed that the predominant effect of increasing water levels was to reduce wind

setup. This finding is consistent with the fact that significant local setup is difficult to generate

and that relieving flow paths are usually facilitated by increasing foreshore water depths.

In shallow restricted bays exposed to winds from the south to east (i.e southern secondary lows

and east coast lows) there theoretically could be potential for climate change increased wind

speeds to increase wind setup by the difference in the squares of the velocities. Therefore, an

increase in wind speed of say 20% could potentially increase wind setup by 44%. At a restricted

bay head this could increase setup from say 0.5 m to 0.7 m. However, generally where setup is

less than 0.05 m any increase would be minimal and even in a restricted bay any general

climate change sea level rise would increase the relieving flow path and hence reduce any

possible increase.

The critical direction to achieve maximum wind setup is likely to be from the south resulting in

increased water levels in the north and north east (Speers Point and Warners Bay). At Speers

Point the foreshore land is largely parks and at Warners Bay (North Creek catchment) the

buildings are on the landward side of The Esplanade and thus largely protected. Investigation of

the water level gauges for the June 2007 long weekend event (Figure 7) indicates that there

may have been some wind setup (as the Belmont record is lower than the Marmong Point

record).

In conclusion, wind setup may be higher than estimated in References 2 and 3 and although

climate change induced storm intensity could increase wind speeds and hence the potential for

increased wind setup in the lake it is likely that these would be small (<0.05 m) and would be

compensated for by increased flow relief. Any potential change within shallow restricted bays

with exposure to winds from the south to east may need to be assessed on a site by site basis.

8.2. Local Wind Wave Runup

The heights and periods (wave climate) of local wind waves are largely related to wind speed,

duration, fetch and water depth (1984 Shore Protection Manual - Reference 23). When waves

reach a shoaling foreshore they break and runup, potentially increasing the inundation level.

Increasing the height and/or period of a wind wave could therefore increase the runup and

inundation level. Increases in storm intensity (and hence wind speed) due to climate change

and/or sea level rise therefore have the potential to increase wave climate and inundation levels.

8.2.1. Design Wind Speeds

Table 18 provides three sources of wind data that could be used to determine wave runup levels

on the foreshore of Lake Macquarie, namely:

The 1998 Lake Macquarie Flood Study Part 1 (Reference 1) used AS 1170.2, 1989

Loading Code Part 2 Wind Loads to source 20, 50 and 100 year ARI 3 second gust wind

speeds and the 1984 Shore Protection Manual (Reference 23) to modify this data to 10

minute average gusts at 10 m height to estimate the resultant wave climate at 48

locations around the foreshore of Lake Macquarie.



45

For comparison purposes the same data as used in the 1998 Lake Macquarie Flood

Study Part 1 (Reference 1) was further modified as part of this present Flood Study and

30 minute 100 year ARI wind gusts were determined.

As part of the 1997 Port Stephens Flood Study – Stage 2 (Reference 24) design water

levels and wave climate were estimated using 38 years of wind data from Williamtown

Airport (20 km north of Newcastle) and the 30 minute average maximum gust at 10 m

height for a 100 year ARI event is provided.

The final column (Lake Macquarie – Munmorah) shows the 1 hour, 1% exceedance wind speed

(this value indicates the wind speed that is exceeded 1% of the time during the year – i.e 3-4

days a year) used to determine wave setup in the 1998 Lake Macquarie Flood Study (Reference

2).

Table 18: 100 year ARI Wind Data (m/s)

Lake Macquarie Port Stephens Lake Macquarie

Direction (Ref 1)*

AS 1170.2,

10 min, 10m

(Present Study)**

AS 1170.2,

30 min, 10m

(Ref 25)

Williamtown,

30 min, 10m

Munmorah

1 hour, 1%

Exceedance

N 24 23 16 9

NE 25 23 14 10

E 24 23 16.5 11

SE 29 27 16.5 10

S 28 26 21 12

SW 28 27 25 12

W 30 29 37 10

NW 28 27 36 11

Note: * using AS 1170.2, 1989

** using AS 1170.2, 2002

Comparison of the wind data shows that there are substantial differences between the AS

1170.2 data and the Williamtown data. Reconciliation of these differences is beyond the scope

of this study, but it is relevant to note that the AS 1170.2 data used for Lake Macquarie is

generally higher and therefore more conservative than the Williamtown data used for the 1997

Port Stephens Flood Study (Reference 24). It is also relevant to note that it is only winds from

the west and northwest that are not more conservative and these are the least relevant in terms

of assessing wave runup levels as these directions have a relatively short fetch.

Based on the AS 1170.2, 1989 wind speeds adopted in the 1998 Lake Macquarie Flood Study

(Reference 1) suggest a reasonable but potentially conservative wind speeds were applied.

Analysis of Mascot (Sydney Airport) or the now longer term Williamtown wind data would

provide a more rigorous outcome.



46

8.2.2. Design Wave Climate

The 1998 Lake Macquarie Flood Study Part 1 – (Reference 1) used the “Simplified Wave

Prediction Model” as set out in the 1984 Shore Protection Manual (Reference 23) to estimate the

resultant wave heights and periods at the 48 sites around the foreshore of Lake Macquarie.

However, since 1984 this approach has been significantly modified and a revised method of

“Wave Hindcasting and Forecasting” was developed in the 2002 Coastal Engineering Manual

(Reference 25). As a result of the changes (and even when using the 1984 Shore Protection

Manual - Reference 23) WMAwater were not able to fully reproduce the wave climate data

presented in the 1998 Lake Macquarie Flood Study (Reference 1). As a result, five sites

covering a range of fetch lengths and directions were examined to test the sensitivity of the

system to climate change increases in ocean/lake levels and storm intensity/wind speeds.

One of the main differences between the approaches in the 1984 Shore Protection Manual

(Reference 23) and the 2002 Coastal Engineering Manual (Reference 25) was that the influence

of shallow water effects on wind wave development was found to be far less than initially

assumed. As a result, the 2002 Coastal Engineering Manual (Reference 25) recommended that

all wind wave estimations be made on the basis of deep water wave growth.

This change in approach has significant ramifications for the wave climate calculations in Lake

Macquarie for two reasons:

it increases the likely wave heights and periods, and

it makes consideration of fetch depths irrelevant.

In relation to the first point, as mentioned above, this present study was not able to fully match

the wave climate analysis undertaken for the 1998 Lake Macquarie Flood Study (Reference 1).

The reason for this is unclear but could be because Reference 1 used shallow water

calculations, did not adjust wind speed for fetch related wind duration, contains

typographical/translational errors, etc. These have been assumed in the present analysis,

irrespectively the size of the “differences” was small (<0.2 m and <0.3 seconds).

In relation to the second point, the fact that water depth is no longer considered significant when

estimating the wave climate means that any climate change increase in ocean/lake levels would

not affect the wave heights or periods used to calculate wave runup. Note however, this does

not mean that an increase in lake level would not affect the foreshore profile and hence runup.

This is examined in the following Section.

8.2.3. Foreshore Profiles and Wave Runup

The 1998 Lake Macquarie Flood Study Part 2 – Foreshore Flooding (Reference 2) used seven

different “typical” foreshore profiles to calculate possible wave runup. Increasing the lake still

water level in response to sea level rise has the potential to change the assessment

requirements for some cross sections. Based on the available information, it was determined

that there would be no change in assessment methods for the year 2050 increase (sea level rise

of 0.4 m) but that a year 2100 increase (sea level rise of 0.9 m) would change Type 3 foreshores



47

to Type 4 and Type 2 foreshores could change to Type 3, depending on the height of the

foreshore wall.

The implications of the above changes depends on the specific site conditions, but generally a

Type 3 to 4 change would marginally increase runup, while a Type 2 to a Type 3 change should

reduce runup. There are only two locations identified with Type 3 foreshores occurring at the

100 year ARI lake level and none at the 1 year ARI level. As a result it is unlikely that changes

in the foreshore type would significantly affect the wave runup levels.

8.2.4. Design Wave Runup and Water Level

As part of this present 2011 Flood Study, the height of the 100 year ARI design lake level has

increased from 1.38 mAHD to 1.50 mAHD, further reinforcing the first scenario as the design

condition. As a result, irrespective of the effect climate change increased storm intensity/wind

speeds may have on the lake wind wave climate, the only runup level that needs to be re-

examined would be the first scenario (1 year ARI runup level).

Examining the results from the 1998 Lake Macquarie Flood Study (Reference 2) for the 1 year

ARI wind wave assessment showed that the maximum significant wave height at any location

was 0.5 m and that the maximum runup was 0.5 m except at one location (Site 3, Marmong

Point) where it was 0.8 m. Four other sites indicated maximum runup levels greater than 0.5 m

but these were only where the waves would break against a building, without the building the

runup levels were within 0.5 m. As part of the present study, the wind speeds were increased by

10% and 20% to determine the sensitivity of the calculated wind wave heights and periods and

to indicate the implications of a possible climate change induced increase in wind speed. The

likely maximum affect on design runup levels for a 20% increase to the 1 year ARI 10 m/s wind

speed, resulted in an increase in the estimated significant wave height of less than 0.1 m and

the change in runup of less than 0.1 m. The only exception was at Site 3 where it was

marginally greater.

Further, as an initial check a 20% increase was applied to the 100 year ARI design wind speed

at some of the sites covering a range of site conditions. The resultant increase in wave height

was from around 15% or 0.1 m for fetch lengths around 2 km, up to around 25% or 0.3 m for

fetch lengths around 6 km. The resultant increase in period was from around 15% for fetch

lengths around 2 km and 20% for fetch lengths around 6 km. These changes would not be

sufficient for the lake plus runup levels calculated by second scenario (high wind speed) to

exceed the first scenario (high lake level).

8.2.5. Results from the 1998 Lake Macquarie Flood Study - Reference 2

The estimated increase in the 100 year ARI water level due to wave runup taken from the 1998

Lake Macquarie Flood Study (Reference 2) is shown on Figure 24. Figure 25 graphs the wave

runup levels for the two scenarios of:

the 100 year ARI design lake level with an approximate 1 year ARI runup level,

the 100 year ARI runup level with an approximate 1 year ARI design lake level.



48

The results on Figure 25 indicate that the 100 year ARI runup levels are significantly higher than

those assumed for the 1 year ARI. Of issue therefore is the joint probability of the lake water

level and the wave runup level. It is generally accepted that a 100 year ARI water level in

combination with the 100 year wave runup level would imply an event of great ARI than the 100

year ARI. However there is no data available to suggest what the joint probability of these two

conditions should be for the range of design flood events.

There is obviously some joint co-incidence of these conditions, as indicated in the June 2007

long weekend event where the “Pasha Bulker” tanker was swept onto Nobby’s Beach at

Newcastle by strong winds and there was intense rainfall causing flooding. However at the time

of the peak water level in the Lake Macquarie (approximately 6am on 9th June 2007) the rainfall

and high winds had largely ceased (refer Figures 7 and 11).

The joint co-incidence of these two conditions can only be clarified as further data becomes

available. As there has only been two events for which data have been available (February

1990 and June 2007) this issue may take a considerable time to resolve and it may be prudent

to consider comparable data for say Wallis Lake and Tuggerah Lakes.



49

9. ACKNOWLEDGEMENTS

This study was carried out by WMAwater and funded by Lake Macquarie City Council and the

NSW State Government. The assistance of the following in providing data and guidance to the

study is gratefully acknowledged:

Lake Macquarie City Council,

NSW Office of Environment and Heritage,

Ministry for Police and Emergency Services Department of Attorney General and

Justice,

Council’s Floodplain Management Committee,

Residents surrounding the foreshores of the Lake Macquarie waterway.



50

10. REFERENCES

1. Lake Macquarie City Council

Lake Macquarie Flood Study Part 1 – Design Lake Water Levels and Wave

Climate Report

Manly Hydraulics Laboratory, Report MHL 682, January 1998

2. Lake Macquarie City Council

Lake Macquarie Flood Study Part 2 – Foreshore Flooding

Manly Hydraulics Laboratory, Report MHL 715, April 1998

3 Lake Macquarie City Council

Tidal Prism Modelling of Lake Macquarie, Volumes 1 and 2

Worley Parsons Report No. 301020- 02167- 01, September 2010

4 Lake Macquarie City Council

Lake Macquarie Adaptive Response of Estuarine Shores to Sea Level Rise

Cardno Lawson Treloar, LJ2857/R2629, June 2010

5 Public Works Department

Dora Creek Flood Study

Report No. 85019, May 1986

6 Pilgrim H (Editor in Chief)

Australian Rainfall and Runoff – A Guide to Flood Estimation

Institution of Engineers, Australia, 1987

7 Chase, Burke & Harvey

Jigadee Creek Flood Study for Proposed Development at Lot 2 DP778019

& Lot 15 DP129150

Webb McKeown & Associates Pty Ltd, July 2004

8 Department of Commerce

Harmonic Analysis of NSW Gauge Network

Manly Hydraulics Laboratory, MHL604, 1995

9 Department of Commerce

Mid NSW Coastal Region Storm-Tide Surge Analysis

Manly Hydraulics Laboratory, MHL621, 1992

10 Department of Environment, Climate Change and Water

Flood Risk Management Guide

August 2010



51

11 Bureau of Meteorology

The Estimation of Probable Maximum Precipitation in Australia:

Generalised Short-Duration Method

Australian Government, 2003

12 NSW Government

Floodplain Development Manual

April 2005

13 Fourth Assessment Report “Climate Change 2007” - Synthesis Report

Intergovernmental Panel on Climate Change, 2007

14 Third Assessment Report “Climate Change 2001” - Synthesis Report

Intergovernmental Panel on Climate Change, 2001

15 Australian Greenhouse Office – Department of the Environment and Water Resources

Climate Change Adaptation Actions for Local Government

SMEC, 2007

16 Climate Change in Australia – Technical Report 2007

CSIRO, 2007

17 Floodplain Risk Management Guideline - Practical Consideration of Climate

Change

NSW Department of Environment and Climate Change (DECC), October 2007

18 Hunter, Central and Lower North Coast Regional Climate Change Project 2009 –

Report 3, Climatic Change Impact for the Hunter, Lower North Coast and Central

Coast Region of NSW

Hunter & Central Coast Regional Environmental Management Strategy, 2009

19 NSW Sea Level Rise Policy Statement

New South Wales Government, October 2009

20 Coastal Risk Management Guide

Department of Environment, Climate Change and Water NSW, August 2010

21 Department of Environment, Climate Change and Water

Derivation of the NSW Government’s Sea Level Rise Planning Benchmarks

October 2009

22 McInnes K L, Abbs D J, O’Farrell S P, Macadam I, O’Grady J, & Ranasinghe R

Projected Changes in Climatological Forcing for Coastal Erosion in NSW

DECC NSW, CSIRO Marine & Atmospheric Research, 2007



52

23 Coastal Engineering Research Center

Shore Protection Manual

US Department of Army, 1984

24 Manly Hydraulics Laboratory

Port Stephens Flood Study – Stage 2

Port Stephens and Great Lakes Councils, 1997

25 US Army Corp of Engineers

Coastal Engineering Manual

US Dept of Army, 2002


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J:\Jobs\29076\Admin\Report\LakeMacFloodStudy.docx:26 June 2012 A1

APPENDIX A: GLOSSARY of TERMS

Taken from the Floodplain Development Manual (April 2005 edition)

acid sulfate soils Are sediments which contain sulfidic mineral pyrite which may become extremely

acid following disturbance or drainage as sulfur compounds react when exposed

to oxygen to form sulfuric acid. More detailed explanation and definition can be

found in the NSW Government Acid Sulfate Soil Manual published by Acid Sulfate

Soil Management Advisory Committee.

Annual Exceedance

Probability (AEP)

The chance of a flood of a given or larger size occurring in any one year, usually

expressed as a percentage. For example, if a peak flood discharge of 500 m3/s

has an AEP of 5%, it means that there is a 5% chance (that is one-in-20 chance)

of a 500 m3/s or larger event occurring in any one year (see ARI).

Australian Height Datum

(AHD)

A common national surface level datum approximately corresponding to mean sea

level.

Average Annual Damage

(AAD)

Depending on its size (or severity), each flood will cause a different amount of

flood damage to a flood prone area. AAD is the average damage per year that

would occur in a nominated development situation from flooding over a very long

period of time.

Average Recurrence

Interval (ARI)

The long term average number of years between the occurrence of a flood as big

as, or larger than, the selected event. For example, floods with a discharge as

great as, or greater than, the 20 year ARI flood event will occur on average once

every 20 years. ARI is another way of expressing the likelihood of occurrence of a

flood event.

caravan and moveable

home parks

Caravans and moveable dwellings are being increasingly used for long-term and

permanent accommodation purposes. Standards relating to their siting, design,

construction and management can be found in the Regulations under the LG Act.

catchment The land area draining through the main stream, as well as tributary streams, to a

particular site. It always relates to an area above a specific location.

consent authority The Council, Government agency or person having the function to determine a

development application for land use under the EP&A Act. The consent authority

is most often the Council, however legislation or an EPI may specify a Minister or

public authority (other than a Council), or the Director General of DIPNR, as

having the function to determine an application.

development Is defined in Part 4 of the Environmental Planning and Assessment Act (EP&A

Act).

infill development: refers to the development of vacant blocks of land that are

generally surrounded by developed properties and is permissible under the

current zoning of the land. Conditions such as minimum floor levels may be

imposed on infill development.

new development: refers to development of a completely different nature to that

associated with the former land use. For example, the urban subdivision of an

area previously used for rural purposes. New developments involve rezoning and

typically require major extensions of existing urban services, such as roads, water

supply, sewerage and electric power.

redevelopment: refers to rebuilding in an area. For example, as urban areas

age, it may become necessary to demolish and reconstruct buildings on a

relatively large scale. Redevelopment generally does not require either rezoning

or major extensions to urban services.

disaster plan (DISPLAN) A step by step sequence of previously agreed roles, responsibilities, functions,


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actions and management arrangements for the conduct of a single or series of

connected emergency operations, with the object of ensuring the coordinated

response by all agencies having responsibilities and functions in emergencies.

discharge The rate of flow of water measured in terms of volume per unit time, for example,

cubic metres per second (m3/s). Discharge is different from the speed or velocity

of flow, which is a measure of how fast the water is moving for example, metres

per second (m/s).

ecologically sustainable

development (ESD)

Using, conserving and enhancing natural resources so that ecological processes,

on which life depends, are maintained, and the total quality of life, now and in the

future, can be maintained or increased. A more detailed definition is included in

the Local Government Act 1993. The use of sustainability and sustainable in this

manual relate to ESD.

effective warning time The time available after receiving advice of an impending flood and before the

floodwaters prevent appropriate flood response actions being undertaken. The

effective warning time is typically used to move farm equipment, move stock, raise

furniture, evacuate people and transport their possessions.

emergency management A range of measures to manage risks to communities and the environment. In the

flood context it may include measures to prevent, prepare for, respond to and

recover from flooding.

flash flooding Flooding which is sudden and unexpected. It is often caused by sudden local or

nearby heavy rainfall. Often defined as flooding which peaks within six hours of

the causative rain.

flood Relatively high stream flow which overtops the natural or artificial banks in any

part of a stream, river, estuary, lake or dam, and/or local overland flooding

associated with major drainage before entering a watercourse, and/or coastal

inundation resulting from super-elevated sea levels and/or waves overtopping

coastline defences excluding tsunami.

flood awareness Flood awareness is an appreciation of the likely effects of flooding and a

knowledge of the relevant flood warning, response and evacuation procedures.

flood education Flood education seeks to provide information to raise awareness of the flood

problem so as to enable individuals to understand how to manage themselves an

their property in response to flood warnings and in a flood event. It invokes a

state of flood readiness.

flood fringe areas The remaining area of flood prone land after floodway and flood storage areas

have been defined.

flood liable land Is synonymous with flood prone land (i.e. land susceptible to flooding by the

probable maximum flood (PMF) event). Note that the term flood liable land covers

the whole of the floodplain, not just that part below the flood planning level (see

flood planning area).

flood mitigation standard The average recurrence interval of the flood, selected as part of the floodplain risk

management process that forms the basis for physical works to modify the

impacts of flooding.

floodplain Area of land which is subject to inundation by floods up to and including the

probable maximum flood event, that is, flood prone land.

floodplain risk management

options

The measures that might be feasible for the management of a particular area of

the floodplain. Preparation of a floodplain risk management plan requires a

detailed evaluation of floodplain risk management options.

floodplain risk management

plan

A management plan developed in accordance with the principles and guidelines in

this manual. Usually includes both written and diagrammatic information

describing how particular areas of flood prone land are to be used and managed

to achieve defined objectives.


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flood plan (local) A sub-plan of a disaster plan that deals specifically with flooding. They can exist

at State, Division and local levels. Local flood plans are prepared under the

leadership of the State Emergency Service.

flood planning area The area of land below the flood planning level and thus subject to flood related

development controls. The concept of flood planning area generally supersedes

the Aflood liable land@ concept in the 1986 Manual.

Flood Planning Levels

(FPLs)

FPL=s are the combinations of flood levels (derived from significant historical flood

events or floods of specific AEPs) and freeboards selected for floodplain risk

management purposes, as determined in management studies and incorporated

in management plans. FPLs supersede the Astandard flood event@ in the 1986

manual.

flood proofing A combination of measures incorporated in the design, construction and alteration

of individual buildings or structures subject to flooding, to reduce or eliminate flood

damages.

flood prone land Is land susceptible to flooding by the Probable Maximum Flood (PMF) event.

Flood prone land is synonymous with flood liable land.

flood readiness Flood readiness is an ability to react within the effective warning time.

flood risk Potential danger to personal safety and potential damage to property resulting

from flooding. The degree of risk varies with circumstances across the full range

of floods. Flood risk in this manual is divided into 3 types, existing, future and

continuing risks. They are described below.

existing flood risk: the risk a community is exposed to as a result of its location

on the floodplain.

future flood risk: the risk a community may be exposed to as a result of new

development on the floodplain.

continuing flood risk: the risk a community is exposed to after floodplain risk

management measures have been implemented. For a town protected by levees,

the continuing flood risk is the consequences of the levees being overtopped. For

an area without any floodplain risk management measures, the continuing flood

risk is simply the existence of its flood exposure.

flood storage areas Those parts of the floodplain that are important for the temporary storage of

floodwaters during the passage of a flood. The extent and behaviour of flood

storage areas may change with flood severity, and loss of flood storage can

increase the severity of flood impacts by reducing natural flood attenuation.

Hence, it is necessary to investigate a range of flood sizes before defining flood

storage areas.

floodway areas Those areas of the floodplain where a significant discharge of water occurs during

floods. They are often aligned with naturally defined channels. Floodways are

areas that, even if only partially blocked, would cause a significant redistribution of

flood flows, or a significant increase in flood levels.

freeboard Freeboard provides reasonable certainty that the risk exposure selected in

deciding on a particular flood chosen as the basis for the FPL is actually provided.

It is a factor of safety typically used in relation to the setting of floor levels, levee

crest levels, etc. Freeboard is included in the flood planning level.

habitable room in a residential situation: a living or working area, such as a lounge room, dining

room, rumpus room, kitchen, bedroom or workroom.

in an industrial or commercial situation: an area used for offices or to store

valuable possessions susceptible to flood damage in the event of a flood.

hazard A source of potential harm or a situation with a potential to cause loss. In relation

to this manual the hazard is flooding which has the potential to cause damage to

the community. Definitions of high and low hazard categories are provided in the


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Manual.

hydraulics Term given to the study of water flow in waterways; in particular, the evaluation of

flow parameters such as water level and velocity.

hydrograph A graph which shows how the discharge or stage/flood level at any particular

location varies with time during a flood.

hydrology Term given to the study of the rainfall and runoff process; in particular, the

evaluation of peak flows, flow volumes and the derivation of hydrographs for a

range of floods.

local overland flooding Inundation by local runoff rather than overbank discharge from a stream, river,

estuary, lake or dam.

local drainage Are smaller scale problems in urban areas. They are outside the definition of

major drainage in this glossary.

mainstream flooding Inundation of normally dry land occurring when water overflows the natural or

artificial banks of a stream, river, estuary, lake or dam.

major drainage Councils have discretion in determining whether urban drainage problems are

associated with major or local drainage. For the purpose of this manual major

drainage involves:

$ the floodplains of original watercourses (which may now be piped, channelised

or diverted), or sloping areas where overland flows develop along alternative

paths once system capacity is exceeded; and/or

$ water depths generally in excess of 0.3 m (in the major system design storm

as defined in the current version of Australian Rainfall and Runoff). These

conditions may result in danger to personal safety and property damage to

both premises and vehicles; and/or

$ major overland flow paths through developed areas outside of defined

drainage reserves; and/or

$ the potential to affect a number of buildings along the major flow path.

mathematical/computer

models

The mathematical representation of the physical processes involved in runoff

generation and stream flow. These models are often run on computers due to the

complexity of the mathematical relationships between runoff, stream flow and the

distribution of flows across the floodplain.

merit approach The merit approach weighs social, economic, ecological and cultural impacts of

land use options for different flood prone areas together with flood damage,

hazard and behaviour implications, and environmental protection and well being of

the State=s rivers and floodplains.

The merit approach operates at two levels. At the strategic level it allows for the

consideration of social, economic, ecological, cultural and flooding issues to

determine strategies for the management of future flood risk which are formulated

into Council plans, policy and EPIs. At a site specific level, it involves

consideration of the best way of conditioning development allowable under the

floodplain risk management plan, local floodplain risk management policy and

EPIs.

minor, moderate and major

flooding

Both the State Emergency Service and the Bureau of Meteorology use the

following definitions in flood warnings to give a general indication of the types of

problems expected with a flood:

minor flooding: causes inconvenience such as closing of minor roads and the

submergence of low level bridges. The lower limit of this class of flooding on the

reference gauge is the initial flood level at which landholders and townspeople

begin to be flooded.

moderate flooding: low-lying areas are inundated requiring removal of stock


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and/or evacuation of some houses. Main traffic routes may be covered.

major flooding: appreciable urban areas are flooded and/or extensive rural areas

are flooded. Properties, villages and towns can be isolated.

modification measures Measures that modify either the flood, the property or the response to flooding.

Examples are indicated in Table 2.1 with further discussion in the Manual.

peak discharge The maximum discharge occurring during a flood event.

Probable Maximum Flood

(PMF)

The PMF is the largest flood that could conceivably occur at a particular location,

usually estimated from probable maximum precipitation, and where applicable,

snow melt, coupled with the worst flood producing catchment conditions.

Generally, it is not physically or economically possible to provide complete

protection against this event. The PMF defines the extent of flood prone land, that

is, the floodplain. The extent, nature and potential consequences of flooding

associated with a range of events rarer than the flood used for designing

mitigation works and controlling development, up to and including the PMF event

should be addressed in a floodplain risk management study.

Probable Maximum

Precipitation (PMP)

The PMP is the greatest depth of precipitation for a given duration

meteorologically possible over a given size storm area at a particular location at a

particular time of the year, with no allowance made for long-term climatic trends

(World Meteorological Organisation, 1986). It is the primary input to PMF

estimation.

probability A statistical measure of the expected chance of flooding (see AEP).

risk Chance of something happening that will have an impact. It is measured in terms

of consequences and likelihood. In the context of the manual it is the likelihood of

consequences arising from the interaction of floods, communities and the

environment.

runoff The amount of rainfall which actually ends up as streamflow, also known as

rainfall excess.

stage Equivalent to Awater level@. Both are measured with reference to a specified

datum.

stage hydrograph A graph that shows how the water level at a particular location changes with time

during a flood. It must be referenced to a particular datum.

survey plan A plan prepared by a registered surveyor.

water surface profile A graph showing the flood stage at any given location along a watercourse at a

particular time.

wind fetch The horizontal distance in the direction of wind over which wind waves are

generated.

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