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POWER AND WATER AUTHORiTY WATER RESOURCES DIVISION FINNIS RIVER FLOODPLAIN MAPPING REPORT 21 I 93 JEROME PAIVA DARWiN FEBRUARY 1993

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Page 1: FINNIS RIVER FLOODPLAIN MAPPING - territorystories.nt.gov.au · power and water authority water resources division finnis river floodplain mapping report 21 i 93 jerome paiva darwin

POWER AND WATER AUTHORiTY WATER RESOURCES DIVISION

FINNIS RIVER FLOODPLAIN MAPPING

REPORT 21 I 93

JEROME PAIVA DARWiN FEBRUARY 1993

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ABSTRACT

This report describes the determination of the boundary of the land liable to flooding (on a 1 : 25,000 topographical map) by a 1 in 100 Annual Exceedence Probability (AEP) flood, on the Finnis River floodplain downstream of the Mt. Bennet dam site.

The 1 in 100 AEP flood peak discharge of the Finnis River at Gitchams was obtained by applying the 100 year Average Recurrence Interval (ARI) design rainfall hyetograph to the RORB model. The RORB model was calibrated using recorded catchment rainfalls and discharge records at Gitchams, for the flood events of February 1975, March 1981 and January 1981. The level reached by the 1 in 100 AEP flood at the upstream end of the floodplain (at the Mt. Bennet dam site) was estimated.

Using the estimated flood level (at the upstream end of the floodplain) and an assumed design storm tide level (at the Finnis coastal area), the boundary of the area liable to flooding by a 1 in 100 AEP flood was determined.

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CONTENTS

ABSTRACT

CONTENTS

LIST OF FIGURES

LIST OF TABLES

1 iNTRODUCTION 1

2 STUDYAREA 1

2.1 General 1 2.2 Climate 3

3 FINN1S RIVER FLOOD ANALYSiS - EARLIER STUDIES 4

4 BASIC DATA 5

4.1 Topographic Maps 5 4.2 Streamflow Data 5 4.3 Flood Data and Pluviometer Records 6

5 COMPUTATION OF DESIGN FLOOD 9

5.1 General 9 5.2 Flood Frequency AnaLysis 10 5.3 Rainfall Runoff Routing Model 10

5.3.1 The RORB Model 10 5.3.2 Calibration of the RORB Model 12 5.3.3 The 1 in 100 AEP Design Flood

from the RORB Model i

5.4 The 1 in 100 AEP Design Flood Estimate 16

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6 FLOODPLAIN MAPPING 17

6.1 General i 6.2 Flood Mapping i 8 6.3 Flood Map i

7 REFERENCES 21

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LIST OF FIGURES

FiGURE 1 LOCATION MAP

FIGURE 2 FIN NIS RIVER CATCHMENT AT GITCHAMS

FIGURE 3 FINNIS RIVER AT GITCHAMS GS 8150180 -

FLOOD FREQUENCY ANALYSIS

FIGURE 4 FINNIS RIVER AT GITCHAMS RORB MODEL CALIBRATION - Kc VERSUS m

FIGURE 5 FINNIS RIVER FLOOD MAP

LIST OF TABLES

TABLE 1

FINNIS RIVER ANNUAL PEAK DISCHARGES AT GITCHAMS GS 8150180

- TABLE 2 RORB MODELLING OF 1 IN 100 AEP FINNIS RIVER PEAK FLOW AT GITCHAMS 1 5

2

7

11

13

23

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1 INTRODUCTION

The Floodplain Management Policy of the Northern Territory Government (Northen Territory Department of Lands 1981) provides for the identification and mapping of land liable to flooding (including coastal surge zones) and sets out guidelines and requirements to allow development to proceed on such land. In accordance with the policy and at the direction of the Northern Territory Floodplain Management Committee, the Power and Water Authority (PAWA) has been carrying out flood estimation and flood mapping of several towns and areas in the Northern Territory.

This report describes the analysis of the extent of flooding by a 1 in 100 Annual Exceedence Probability (AEP) flood and flood mapping of the area within the floodplain of the Finnis River, identified for development by the Ndrthern Territory Department of Lands and Housing.

2 STUDY AREA

2.1 General

The study area is the floodplain of the Finnis River downstream of the Mt. Bennet dam site (Figure 1). It consists of gently undulating lowlands which rise gradually from the plains in the West to the hills of the Finnis ranges to the east. Vegetation in the area is mainly shrubland. The Eastern part of the area consists of tall woodland and the coastal area of mangrove.

The area was originally pastoral land but is now no longer used for its original pastoral purpose. However, cattle belonging to the AboriginaL owners of Wagait reserve (Figure 1) are still grazed on the plains to the South - West. The main activity in the area now is fishing, both recreational and commercial, mainly ir i the mangrove areas. Recreational fishing is also popular in the upstream areas of the Finnis River.

Proposals have been formulated for the development of the Northern floodplain of the Finnis River. The proposed land uses are detailed in the Finnis Land Use Structure Plan 1990 (Northern Territory Department of

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2

I 12 15

- -. 12

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SEA DARWIN

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COX

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Brown Point -T Ir

FOG I

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DARWIN

/ ftJTCHAMS 'MT. BURTO sa15,p O DAM

fl TE

- --. BATHELOR (E.b,ancM DAM SITE

cJ

ç Batchelor

Wangin

km20 0 20 40km

LEGEND

CATCHMENT BOUNDARY

GS8150180 GAUGLNG STATiON

TRACK

STUDY AREA

WAGALT ABORIGINAL LAND

LOCATION MAP

Fig. 1

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Lands and Housing 1990). The Finnis Land Use Plan has been prepared within the broader objectives identified by the Darwin Regional Land Use Structure Plan 1990. Within the Darwin Region, the Finnis Land Use Structure Plan identifies the most appropriate role for the Finnis area as a recreational and rural living resource for the regional population. The Firinis area is a largely undeveloped area of high scenic attraction. it is within easy reach of Darwin, and would provide the metropolitan centre with a range of public and private recreational facilities in an open and natural setting. Development would be mainly rural residential, conservational, recreational, tourism, and related service facilities (Northern Territory Department of Lands and Housing 1990).

The purpose of this study is to define the land liable to flooding 1 within the area identified for development on the Northern floodplain of the Firinis River and, if possible, to produce flood maps showing the severity of flooding. This would greatly assist in the development of the area identified

2.2 Climate

The Finnis River catchment falls within the climate zone classified as 'Summer Rainfall - Tropical' (Bureau of Meteorology 1986). This zone is characterized by heavy periodic rains (heavier in coastal areas), generally hot and humid in coastal areas during the summer (November - March). The winter months (May - September) are generally rainless, mild to warm and dry. The months of April and October represent transition periods of change from one season to the other.

The rainfall is seasonal and 80 - 90 % of the annual rainfall occurs in the four months from December to March. Although a substantial portion of the rainfall is caused by thunderstorm activity, flood producing rainfalls in the catchment mainly result from monsoonal depressions or cyclonic activity.

1 Land liable to flooding is defined as land that would be inundated as a result of a flood that has a - statistical chance of one percent of being equalled or exceeded during any one year.

3

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3 FINNIS RIVER FLOOD ANALYSIS EARLIER STUDIES

- Estimates were made of dam site floods for the proposed Mt. Bennet, Batchelor and Mt. Burton dams (Figure 1) in 1979 (Snowy Mountains Engineering Corporation 1979). Frequency analysis of stream discharge

records (1961 - 1978) of Finnis River at Gitchams and catchment area ratios were used. The design flood estimates were as foows.

Flood Discharge (m 3/s)

Damsite ARI

100 1,000 10,000

Batchelor 1,200 1,800 2,300

Mt. Burton 1,300 1,900 2,500

Mt. Bennet 2,700 3,900 5,100

The estimates are preliminary. Only 17 years of record were used and there is uncertainity in extrapolation to infrequent return periods.

A probable maximum flood of 3,900 m 3/s at the Batchelor damsite was obtained in 1981 (Gibb and Partners 1981). The RWSM rainfall runoff model (Goyen and Aitken 1976) was used. This model is a modified version of the RORB runoff routing method (Laurenson and Mein 1988) and was used to generate a catchment runoff hydrograph from a given storm pattern. The rainfall loss employed was a continuing loss of 1.2 mm/hr. However, no mention was made of model calibration or values of model parameters used.

In 1986 (Douglas 1986) the rating curve of the Finrtis River at Gitchamswas extended by PAWA in a study to assist the N.T. Department of Transport and Works in the design of a road crossing on the Finnis River. The extension was based on Manning's equation. The 1 in 50, 1 in 20 and 1 in 10 AEP floods

4

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were estimated to be 1235 m 3/s, 1050 m3/s and 890 m3/s, respectively by frequency analysis of 24 years (1960 - 1983) of Finnis River discharges at Gitchams.

PAWA obtained a preliminary estimate of 11,700 m 3 Is for the probable maximum flood at the Mt. Bennet dam site (Abbey 1987). The ROAB runoff routing model (Laurenson and Mein 1988) was used. The model was uncalibrated and the main RORB parameter Kc was obtained from the regional relationship (IE Aust. 1987). The rainfall loss employed was a continuing loss of 1.4 mm/hr.

4 BASIC DATA

4.1 Topographic Mapping

The study area (Figure 1) is covered by 1:100,000 scale maps with 20 rn contours. The area (except for the Wagait Aboriginal Land downstream of the Mt. Bennet dam site) is also covered by 1 : 25000 scale maps with 5 m contours. A smaLl portion of the study area is covered by 1:10,000 mapping with 2 m contours.

4.2 Streamflow Data

Near the study area, the Finnis River is gauged at the Gitchams gauging station (GS 8150180) (Figure 1). The catchment area at the gauging station is 1016 square kilometres. The gauging station was opened in 1960 and continuous stage recording is being carried out.

Current meter measurements are also carried out at GS 81 501 80 and since 1960, 220 discharge measurements have been made. The current meter measurements are sufficient to derive a stage discharge relationship up to a discharge of 380 m 3/s (highest discharge measurement) at a gauge height of 7.45 m. The gauge height of the highest stage recorded, so far, is 9.84 n (in February 1966). To determine the recorded streamf lows the rating curve thus needed to be extended.

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The cross section at GS 8150180 changes quite markedly at gauge datums of approximately 7.00 m and 10.0 m (Douglas 1986). - A simple log/log extrapolation of the rating curve, up to gauge datum 10.0 m, was therefore not possible.

Discharges were computed using Manning's equation from surveyed cross sections at GS 8150180 for stage heights 9.00 m, 10.0 m, 11.5 m and 12.3 m (Douglas 1986). The discharges obtained were 687 m 3 Is, 960 m 3 /s, 1480m 3/s and 2000m 3/s, respectively

The rating curve at GS 8150180 (up to stage height 7.45 m) obtained from the available discharge measurements was then extended to a stage height of 12.3 m using the computed discharges.

The available stage records were processed using the extended rating curve, and discharges derived for the period 1960/61-1990191. The annual peak discharges at Gitchams for this period are shown in Table 1.

4.3 Pluviometer Records and Flood Data

Within and near the catchment (Figure 2) pluviometers have been operated for different periods as follows.

STATiON NO. STATION NAME LAT. LONG. PERIOD OF RECORD

R 8150180 Finnis River 1258 13045 10.1974 - 06.1977

R 8150181 ps 1307 13043 11.1974 — 07.1977

R 8150010 Upper Finnis 1302 13057 10.1974 - present

R 8150205 East Finnis 1259 13100 08.1984 - present

R 8150202 TaIings Creek 1259 13100 01.1982 - 07.1988

A 8150011 Darwin River 1250 13058 12.1962 - present

6

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URsl500ll

lop

t .

S.-

S.

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, GITCHAMS R;

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/ I

...R815 0181

'S 5'

.5

S. S.

1cm 10 0 10 20 km —i

LEGEND

- CATCHMENT BOUNDARY

GS8150180- RIVER GAUGING STATION

R8150180 0 PLUVIOMETER

FINNISS RIVER CATCHMENT AT GITCHAMS

\L I:I rachL -'

(

R8150202 / 81502O5,.-

I

S. S.

A 815001

0

I I

/

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/ I

- - - \ -

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'S.- -- '

Fig. 2

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TABLE 1

ANNUAL PEAK DISCHARGES AT

GITCHAMS GS 8150180

Water Year Discharge Water Year Discharge (m 3 ls) (m 3 /s)

1960/61 32 1975/76 347

1961/62 336 1976/77 429

1962/63 132 1977/78 373

1963/64 385 1978/79 176

1964/65 356 1979/80 392

1965166 912 1980181 551

1966/67 167 1981/82 362

1967/68 726 1982/83 394

1968/69 812 1983/84 439

1969170 155 1984/85 269

1970/71 311 1985/86 273

1971/72 548 1986187 208

1972/73 569 1987/88 102

1973/74 563 1988/89 360

1974/75 893 1989/90 27

1990/91 565

8

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For the period 1960/61 - 1990/91, major flood events (flood events with peaks above 500. m 3/s) for which pluviometer records are available, are as follows.

Year Flood Peak (m 3 /s)

1974/75 893

1990/91 565

1980/81 551

There is a 36 hour gap in the record at R 815180B during the 1974/75 flood event, from 1200 hours on 14/02/75 to 2400 hours on 15/02/75.

5 COMPUTATION OF DESIGN FLOOD

5.1 General

Design floods for input to a floodplain model can be derived from flood frequency analysis of discharge records or from a rainfall runoff model. Where flood frequency analysis is used, the required peak flow is found from a frequency distribution of recorded annual flood maxima. Rainfall runoff based methods usually use either a rainfall runoff routing model or a unit hydrograph. Rainfall runoff based methods give a complete hydrograph of the design flood.

The selection of the method of flood derivation would depend on the length and variability of the available river records and whether adequate data is available to develop a rainfall runoff model. Chapter 12 of Australian Rainfall and Runoff (IE Aust. 1987), provides quantitative assistance in the choice between flood frequency estimates and rainfall based methods.

9

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5.2 Flood Frequency Analysis

A Log Pearson Type 3 distribution was fitted to the 31 year series (1 960 - 1990) of annual peak discharges (Table 1) of the Finnis River at Gitchams. The annual peak discharges for the years 1960/61 and 1989/90 were detected to be low outliers (using equation 10.31 of ARR), and were therefore excluded from the frequency analysis. A 1 in 100 Annual

a

Exceedence Probability (AEP) flood peak discharge of 1120 m 3ls (with 5% and 95% confidence limits of 1620 m 3 /s and 770 m 3 /s, respectively) was obtained (Figure 3).

The recorded annual floods (1960 - 1991) represent a relatively small sample of possible events from the unknown population of floods for the catchment. The recorded sample would therefore give a biased estimate of the true annual exceedence probability. An adjustment is thus required for expected probability due to the bias introduced by the sample. Section 10.6 in Australian Rainfall and Runoff (ARR) (IE Aust. 1987) gives methods of adjustment for expected probability, based on the statistics of the Log Pearson Type 3 distribution obtained.

The 1 in 100 AEP flood peak was adjusted for expected probability (using TabLe 10.5 of ARR). The adjusted 1 in 100 AEP flood discharge obtained was 1 220 m 3 /s (with 5% and 95% confidence limits of 1760 m 3/s and 840 m3/s, respectively).

5.3 Rainfall Runoff Routing Model

5.3.1 The RORB Model

The runoff routing model RORB version 4 developed by Laurenson and Mein (1988) was used. The model computes the flood hydrograph from rainfall and other channel inputs by subtracting the losses from rainfall and routing the rainfall excess through each subcatchment storage. The model is non linear and accounts for temporal and areal variation of rainfall and losses.

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p

10

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I Oii CO 0

UJ

----------.---------------

DC

DC---

DC-- xe

D

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99,9 99 95 90 80 50 20 10 5 2 1 0.5 0.1

FLOOD FREQUENCY CURVE

ANNUAL EXCEEDENCE PROBABLTY (%)

- - - -. 5% AND 95% CONFIDENCE LNTERVAL

X RECORDED DATA 1960-1990

FINNISS RIVER AT GITCHAMS GS 815 018 0

co FLOOD FREQUENCY CURVE C)

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The key parameters in the model are the dimensionless exponent m, a measure of thenonunearity of the catchment (usually, 0.6 :5; m :5 1.0), and the empirical coefficient K, a dimensionless measure of the storage delay time of the whole catchment.

Subcatchment storages are assumed to be governed by a storage - discharge equation of the form

S = 3600Kc KrQm

where S = storage (m 3 )

0 = outflow discharge (m 3/s)

m = dimensionless exponent

Kc empirical coefficient applicable to the whole catchment and stream network

Kr = dimensionless ratio called the relative delay time applicable to individual reach storages.

.3.2 Calibration of the RORB Model

The RORB model was calibrated for the flood events of 1974/75, 1990/91 and 1980/81. Base flow contributions to the peak flows were insignificant (about 2% of the peak flows). Week's procedure (Laurenson and Mein 1988) was used to obtain an optimum combination of m and K0. For each of the flood events, a range of m values was used. For each m value, K0 was varied until the best fit with that m value was obtained. Graphs of K0 versus m for the flood events were then plotted (Figure 4). The graphs indicated the optimal combination of m and Kc to be 1.0 and 17.0 (average), respectively.

12

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975 FLOOD EVENT

981

990

0

13

0.5 0.6 0.7 0.8 0.9 tO 1.1 1.2

m

FINNISS RIVER AT GITCHAMS RORB MODEL CALIBRATION

Kc VERSUS m

Fig. 4

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The results of the calibrations were as follows.

Flood Peak Iriitiar Continuing Kc Event Loss Loss

(m 3 /s) mm mm/hr

m

0.6 0.7 0.8 0.9 1.0

1974/75 893 0.0 4.42 113.3 63.4 35.2 19.6 10.8

1990/91 565 0.0 0.12 207.0 124.1 73.5 44.0 24.5

1980/81 551 0.0 11.41 117.3 69.1 41.3 24.5 14.4

The RORB model parameter K0 (for m = 0.8) was also obtained from the IRORB regional relationship 1 for the humid region of the Northern Territory (IE Aust. 1987). K0 was computed to be 87.0.

5.3.3 The 1 in 100 AEP Design Flood from the RORB Model

To obtain the 1 in 100 AEP design flood, the 100 year Average Recurrence Interval (ARI) design rainfall from ARR (IE Aust. 1987) together with the 100 year ARI rainfall temporal patterns (JE Aust. 1987) were applied to the RORB model. The rainfall hyetograph for input to the model was derived by first applying an areal reduction factor (IE Aust. 1987) to the 1 in 100 year AEP point rainfall. Storm durations of 12, 24, 36 and 48 hours were used. The results of the modelling are shown in Table 2.

1 For the humid region of the Northern Territory

Kc = 1.8 (A / S05 ) 055

where A is the catchment area in km2

S is the equal area srope of the catchmertt in mlkm.

14

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Peak Flow

(m 3/s)

Kc

17.0 87.0

m=1.0 m=0.8 ii =0mm ii =0mm ci = 1.4 mm/hr ci = 1.40 mm/hr

2140 1590

2180 1820

2240 1980

2030 1810

Table 2

RORB MODELLING OF

I IN 100 AEP FINN1S R1VER PEAK FLOW AT GITCHAMS

Storm Areal Total Rainfall Duration Reduction

Factor

(Hours) (mm)

Point Calchment

12 0.88 246 216

24 0.91 348 317

36 0.91 441 401

48 0.91 480 437

Note : Ko, m, - RORB parameters, ii - initial loss, cI - continuing loss

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5.4 The 1 in 100 AEP Design Flood Estimate

For comparison purposes the 1 in 100 AEP flood peaks at Gitchams derived by frequency analysis and RORB modelling are shown below.

Flood Frequency 1220 m3/s - Analysis

RORB model with regional parameters 1980 m3 /s (Kc = 87.0, m = 0.8)

Calibrated RORB model (Kc = 17.0, m = 1.0) 2240 m3 /s

The extent to which a flood frequency analysis can be extrapolated before an alternative rainfall based method becomes preferable depends on the accuracy of the latter method and on the characteristics of the design rainfall data (IE Aust. 1987). In addition, it depends on the length of the flood record (N) in years, the standard deviation (S) and coefficient of skew (g) of the logarithms of the flood values (IE Aust. 1987).

Based on the above considerations, Australian Rainfall and Runoff (ARR) (IE Aust. 1987) has set out three empirical equations for finding Y (for a flood of AEP of I in Y), one for catchments greater than 150 km 2 where Meteorological Bureau design rainfalls (in ARR) are based on daily data, and the other two for catchments smaller than 100 km 2 where the design rainfalls were based on pluviometer records.

The annual exceedence probability (AEP) at which the rainfall based method becomes more accurate than flood frequency analysis (for a catchment area greater than 150 km2 ), is denoted by 1 in Yj 1 for a rainfall - runoff method calibrated "on site" (and I in 3Y1 for a rainfall runoff method using regional parameters).

16

= FN05exp(O.02N) whore F depends on the values of S and g and is taken from Table 12.2 in ARR

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On this basis, for the Finnis River at Gitchams, the flood frequency estimate can be used up to an annual exceedence probability (AEP) of 1 in 30 before the calibrated ROAB estimate becomes more accurate (and up to an AEP of 1 in 90 before the regionalised RORB estimate can be used).

The average error of estimate from calibrated rainfafl runoff model is 14% (whilst that from a rainfall runoff model using a regional parameter is 25 - 70%) (IE Aust. 1987).

The 1 in 100 AEP flood peak (2240 m 3/s) from the calibrated RORB model was therefore taken to be the design flood estimate for floodplain mapping.

6 FLOODPLAIN MAPPING

6.1 General

The scale of the topograghical maps available for the study area (except for the Wagait aboriginal land area downstream of Mt. Bennet dam site) was 1 25,000 with 5 'm contours. The accuracy of the topographical maps 1 did not permit the use of either a 1 - D or 2 - D gradually varied flow finite difference model, for simulating the flood flow.

Cross sections of the Finnis River in the study area, at the Gitchams gauging station CS 8150180 and at the Mt. Bennet dam site were available (Douglas 1986 and GHD 1980).

With the available information it was not possible to determine flood contours, seventies of flooding etc. Only the approximate boundary of the area liable to flooding was determined.

As most of the area identified for development in the Finnis Land Use Structure plan has been set aside as public open space, there was no major benefit in obtaining additional information (larger scale topographical maps, additional cross sections etc.) to determine flood contours, seventies of flooding .etc.

17

1 The error in interpolating levels from 5 m contour maps is about ± 1.5 m

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6.2 Flood Mapping

The floodplain of the Finnis River proposed for development, is downstream of the Mt. Bennet dam site (Figure 1).

From the rating curve 1 for Gitchams GS 8150180 (section 4.2), for a flood discharge of 2240 m 3 /s, the flood level was 34.80 m AHD (gauge height 12.54 m). The corresponding flood discharge at the Mt. Bennet dam site was estimated 2 to be 2450 m 3/s.

Cross sections of the Finnis River, at the gauging station GS 8150180 and at the Mt. Bennet dam site were available, Using Mannings equation and assuming uniform flow, the 1 in 100 AEP flood level at the dam site was estimated to be 19.70 m AHD. A value of 0.05 for Manning's "n" was used and was considered reasonable for the terrain.

The coastal areas of the Finnis River catchment would be subjected to tropical cyclones. These large scale storms and their associated severe winds, heavy rains, storm surge and extreme waves can combine to produce very high water levels in excess of the normal astronomical tide. These increased water levels may threaten flooding of low lying coastal areas.

The total water level (or the storm tide level) experienced during the passage of a tropical cyclone is made up of the astronomical tide level, the cyclone storm surge and the effect of wave action on the surge level at the coast. It has been estimated that the storm surge resulting from a 100 year return period storm has a probability of between 0.1 % and 0.01 % per annum (Department of Housing and Construction 1981). However, it was concluded in the Darwin Storm Surge study (Department of Housing and Construction 1981) that the selection of minimum design storm tide level should take account of the probability of exceedence of a high storm tide and the resultant damage to life and property, the cost and consequences of the associated major flooding, and the risks the community and Government are

1 The derived rating curve in section 4.2 is valid up to a gauge height of 12.3 m (at a discharge of 2000 rn 3/s). It was further extended, logarithmically, using the computed discharges at the gauge heights of 10.0 m, 11.5 m and 123 m. The equation obtained for the extension was In(h) = -6.7118 + 2.2439 In (q) -0.1356 In 2 (q) where h is the gauge height in metres and q is the discharge in m 3/s.

was assumed that the ratio of the flood discharges equals the ratio of the catchment areas to the power 0.7

(

18

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prepared to accept (Department of Housing and Construction 1981). Risk levels of the order of 1 % per annum to 0.1 % per annum were considered acceptable in the study, for coastal townships around Darwin (Department of Housing and Construction 1981). The study recommended that storm tide levels which have an estimated probability of exceedence of 0.1 % per annum be adopted for planning new developments.

In a storm surge study for the Greater Darwin Area (Blain Bremner and Williams 1983) the 1 % per annum probability of exceedence storm tide level estimates, at locations nearest to the Finnis coastal area at Brown Point, Bynoe Harbour, East Point and Charles Point were 5.7 m AHD, 5.6 m AHD, 5.3 m AHD and 5.0 m AHD. The respective 0.1 per annum probability of exceedence levels were 6.1 m AHD, 6.2 m AHD, 6.5 m AHD and 6.2 m AHD (Blain Bremner and Williams 1983). These storm tide levels were obtained by considering the combined probabilities of tide, surge height and wave setup. A simulation approach was employed using a mathematical model.

Based on the above considerations, a design storm tide level of 5.8 m AHD was considered acceptable for the Finnis coastal area.

From the 1 in 100 AEP flood hydrograph it was found that the flood discharge would exceed 90 % of its peak (2240 m 3 /s) for a period of 18 hours. It is very likely therefore that a high tide would obtain at the Finnis River outfall during this period. Head losses from the dam site to the floodplain were found to be negligible and therefore its effect on the flood slope would be insignificant. The boundary of the land liable to flooding for the study area (except for the coastal area) was therefore obtained by assuming a uniform slope, from the 1 in 100 AEP flood level at the Mt Bennet dam site (19.70 m AHD) to the high tide level (about 4.0 m AHD) at the Finnis river outfall. This slope was computed to be 0.00033. Near the coastal area this boundary would be enveloped by the 5.8 m AHO contour which forms the coastal area design flood boundary.

6.3 Flood Map

The flood inundation map of the study area for a 1 in 100 AEP flood event is shown on the 1 : 25,000 scale topographical maps (with 5. m contours) (Figure 5).

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Only the extent of flooding in a 1 in 100 AEP flood event has been shown. Due to the error (± 1.5 m) in interpolating levels from 5 m contour maps it was not possible to show the flood contours and the hazardous floodway 1 areas on the topographical map.

The flood map produced (Figure 5) is only indicative and shows the approximate extent of flooding.

I Hazardous floodway is defined as an area where the depth of flooding is greater than 2 m, or where the product of depth in metres and velocity in rn/s exceeds one.

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7 REFERENCES

ABBEY P. (1988) - Mt Rennet Dam Spillway Desn Flood Studies Internal File Records - Water Resources Division, Power and Water Authority, 1988.

BLAIN BREMNER AND Greater Darwin Storm Surge Study WILLIAMS PTY. LTD. Volume 3 (1983) Northern Territory Department of Lands,

Darwin, 1983.

BUREAU OF District Rainfall Deciles - Australia METEOROLOGY(1986) Bureau of Meteorology, Melbourne, 1986.

DEPARTMENT OF Storm Surge at Darwin HOUSING AND Northern Territory Department of Lands, CONSTRUCTION (1981) Darwin, 1981.

DOUGLAS, S.J. (1986) Finnis River Flood Hydrology Study Department of Mines and Energy, Water Resources Division Report No.220/1986, 1986.

GOYEN, A.G.,. A Regional Stomiwater Drainage Model AITKEN, A.P. (1976) Proceedings of Hydrology Symposium,

Institution of Engineers, Australia 40 - 44, 1976.

GUTTERIDGE HASKINS Estimated Costs of Potential Dams & DAVY (1980) in the Darwin Region"

Power and Water Authority, 1988.

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22

IE AUST. (1987) Australian Rainfall and Runoff The Institution of Engineers, Australia,

- 1987.

LAURENSON, E,M., RORB - Version 4 Runoff Routing Program AND MEIN, R.G. (1988) User Manual

Department of Civil Engineering, Monash University, Victoria, 1988.

NORTHERN TERRITORY Land Use in Floodplains DEPARTMENTOF LANDS Northern Territory Department of Lands, (1981) Darwin, 1981.

NORThERN TERRITORY Einnis Land Use Structure PIan 1990 DEPARTMENT OF LANDS Northern Territory Department of Lands AND HOUSING (1990) and Housing, Darwin, 1990.

SNOWY MOUNTAINS Drwin Water Supply Future Source ENGINEERING -Appraisal Study CORPORATION (1979) Northern Territory of Australia

Department of Transport and Works, 1979.

SIR ALEXANDER GIBB Finnis River (Batchelor) Dam Studies AND PARTNERS (1981) tVolume 1

Northern Territory of Australia Department of Transport and Works, 1981.

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Figure 5

FINNIS RIVER FLOOD MAP

23

Scale 1 : 259000

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