Mullet and Brooks Creeks Flood Study Report Part - Wollongong ...

CALIBRATION AND VERIFICATION 5-62 

5.4.3 Topography and Structures 

Topography and structures from the current (2003) DEM and structure survey is used as supplied. 

That is, no changes to sizes of structures or levels are known to have occurred between 1999 and 

2005 that will impact upon the model’s ability to predict the 1999 flood behaviour. 

5.4.4 Culvert and Bridge Blockage Assumptions 

Blockage assumptions were not applied for the 1999 flood event. 

5.4.5 Model Parameters 

Hydrologic model parameters adopted for the 1999 event simulation are shown in Table 5-14. 

Manning’s n values adopted are shown in Table 5-2. 

5.4.6 Downstream Boundary Levels 

G:\ADMIN\B14142.G.CLB_MULLETBROOKSFS\R.B14142.001.05.FINAL.DOC 

Table 5-14 WBNM Model Parameters – 1999 Calibration 

Parameter 1999 

% Impervious See Appendix B 

Lag Parameter 1.0 

Initial Loss - Pervious 60 mm 

Continuing Loss Rate – Pervious 2 mm/h 

Lake Illawarra level data at Koonawarra was available for the 1999 flood event from a continuous 

stage recorder maintained by MHL. Lake levels throughout the flood event are presented in Figure 

3-3. 

5.4.7 1999 Predicted Flows 

Table 5-15 provides predicted flows from the hydraulic model at the listed locations. 

Table 5-15 1999 Predicted Flows at Various Locations 

Location Flow (m 3 /s) 

Freeway F6 Bridge on Mullet Creek 239 

Princes Highway Bridge on Mullet Creek 278 

Cleveland Road, Dapto 88 

Bong Bong Road, Dapto 95 

Ena Avenue, Dapto 81 

Sheaffes Creek (downstream of West Dapto Road) 30 

Robins Creek (Ritchie Crescent) 111 

Reed Park Creek (Fairwater Drive) 16 

Forest Creek (upstream West Dapto Road) 52


5.4.8 Replication of Recorded Levels 

There were no recorded peak flood levels for the 1999 flood event for either Mullet Creek or Brooks 

Creek. Continuous gauge recordings are available in both Mullet Creek and Lake Illawarra. No 

gauge recordings are available in Brooks Creek. 

Figure 5-61 compares the recorded level hydrograph at Mullet Creek with the modelled flood level 

hydrograph. It is apparent from Figure 5-61 that the model reproduces the peak level well with the 

peak modelled level of 4.20 mAHD being within 0.03m of the peak recorded level of 4.17 mAHD. 

The timing of the peak is similar, with the recorded peak occurring at 12:15pm and the modelled peak 

occurring at 12:30pm. In general, the rising limb of the hydrograph is also similar. The falling limb of 

the recorded hydrograph is steeper than that of the modelled hydrograph. Matching the slope of the 

falling limb was impossible without altering the timing of the rising limb and the magnitude of the 

peak. It was decided that priority be given to matching of the peak and the rising limb. 

Some differences in recorded and modelled flood levels are also evident at the start and end of the 

flood. These differences are believed to be related to the weir downstream. A sensitivity analysis on 

the downstream weir level shows that for the current surveyed weir level of 0.8 mAHD, the base 

water level is approximately 0.9 mAHD at the gauge. If the weir is at 1.1mAHD the base water level 

at the gauge is approximately 1.2 mAHD, the base water level recorded by the gauge during the 1999 

flood. This could suggest there may have been a blockage downstream of the gauge during the 

1999 flood event. The 1999 flood extent for Mullet Creek is shown in Figure 5-62. 

There were no recorded flood levels in the Brooks Creek Catchment for the 1999 flood event. The 

flood extent is presented in Figure 5-63. 

5.4.9 Conclusions on 1999 Event Calibration 

Figure 5-61 demonstrates that the model adequately represents the peak of the flood and the timing 

of the peak. 

G:\ADMIN\B14142.G.CLB_MULLETBROOKSFS\R.B14142.001.05.FINAL.DOC


Level (mAHD) 

4.5 

4 

3.5 

3 

2.5 

2 

1.5 

1 

0.5 

0 

23/10/199 

9 21:36 

23/10/199 

9 23:36 

24/10/199 

9 1:36 

k:\B14142.k.clb\1999_gauge_comparison_054.xls 

24/10/199 

9 3:36 

24/10/199 

9 5:36 

24/10/199 

9 7:36 


24/10/199 

9 9:36 

24/10/199 

9 11:36 

24/10/199 

9 13:36 

Time (hrs) 

24/10/199 

9 15:36 

24/10/199 

9 17:36 

24/10/199 

9 19:36 

24/10/199 

9 21:36 

24/10/199 

9 23:36 

Recorded Gauge Level 

Modelled Flood Level 

Figure 5-61 Comparison between Recorded and Modelled Levels at the Mullet Creek Gauge 

25/10/199 

9 1:36 

25/10/199 

9 3:36 

25/10/199 

9 5:36

DESIGN EVENT MODELLING – EXISTING CONDITIONS 6-1 

6 DESIGN EVENT MODELLING – EXISTING CONDITIONS 

6.1 Introduction 

The previous chapter of this report details the hydrologic and hydraulic modelling of three calibration 

events in the Mullet Creek catchment. This chapter describes the design event modelling for the 

20%, 10%, 5%, 2% and 1% AEP flood events and the Probable Maximum Flood (PMF). 

Design floods are hypothetical floods used for planning purposes and floodplain management 

investigations. They are based on having a probability of occurrence specified either as: 

• Annual Exceedance Probability (AEP) expressed as a percentage; or 

• An Average Recurrence Interval (ARI) expressed in years. 

This report uses the AEP terminology as this is standard terminology preferred by WCC. Table 6-1 

provides a description of the different design floods considered as part of this study. 

AEP 1 ARI 2 Comments 


Table 6-1 Design Flood Terminology 

1% 100 years A hypothetical flood or combination of floods which 

represent the worst case scenario likely to have a 1% 

chance of occurring in any one year, or in other words, 

is likely to occur once every 100 years on average. 

















Extreme Flood / PMF 3 A hypothetical flood or combination of floods which 

represent an extreme scenario. It is only used for 

special purposes (eg. design of a dam spillway) where a 

high factor of safety is recommended such as planning 

to prevent possible loss of life, or in consideration of 

floodplain planning (eg evacuation and isolation of 

communities). 

1 Annual Exceedance Probability (%) 

2 Average Recurrence Interval (years) 

3 Probable Maximum Flood


6.2 Design Flood Hydrology 

The WBNM hydrologic models, developed and calibrated for this study, are used to provide inflows to 

the hydraulic model for the Mullet and Brooks Creeks design events. Details on WBNM model 

development are provided in Section 4.2 of this report, and the calibration of the hydrologic/hydraulic 

model is discussed in Section 5.1. 

6.2.1 WBNM Model Parameters 

WBNM Lag Parameter (‘C’) 

During calibration of the models, a lag factor (C) of 1.0 was found to provide the best calibration. This 

value has been maintained for the design event modelling. As discussed in Section 5.1.1, this value 

falls outside the recommended values of 1.3 to 1.8. However, due to the steep escarpments within 

this catchment, the ‘peaky’ nature of the flow and the general ‘down catchment’ movement of storms, 

the lower lag factor is physically applicable to the Mullet/Brooks catchments. Detailed discussion on 

the C value is provided in Section 4.2.3 and Section 5.1.1. 

Continuing Loss 

A continuing loss of 2.0mm/hr was found to provide the best calibration for each of the three 

calibration events: 1975, 1984 and 1999. Thus, a continuing loss of 2.0mm/hr has been maintained 

for the design event modelling. 

Initial Loss 

Initial losses for the calibration events were found to range from 0mm to 60mm. However, the 

adopted initial loss for design events should always be 0 mm, regardless of initial losses determined 

in calibration. The reasons for this are: 

• Initial losses are event specific, being a function of pre-event rainfall and not a function of the 

catchment; 

• Design events are storm bursts. That is, they occur as part of a larger rainfall event, some or all 

of the initial loss will be ‘supplied’ by the pre-burst rainfall (Book 6, AR&R, 2001); and 

• To assume an initial loss of 0mm is conservative. 

Thus, an initial loss of 0mm is adopted for the design events. 

6.2.2 Critical Durations 

Assessment of the critical durations for each of the catchments was undertaken using the 

hydrologic/hydraulic model. That is, the critical duration was that duration found to provide the 

highest peak flood levels as predicted by the hydraulic model. Details on this assessment are 

provided in Section 6.3.3. In summary, the critical duration for Mullet Creek was found to be a 

combination of the 2 hour and 6 hour durations while Brooks Creek was found to have a critical 

duration of 2 hours. 



The critical duration for Lake Illawarra is 36 hours (L&T, 2001). This duration was also simulated for 

Mullet and Brooks Creeks to provide the highest peak flood levels in the lower portion of each creek. 

6.2.3 Design Rainfall Estimation – 20%, 10%, 5%, 2% & 1% AEP 

Intensity-Frequency-Duration Parameters 

Design rainfall totals were derived for the catchment from Australian Rainfall and Runoff (AR&R) 

(1987). This is the standard approach for the development of design hydrographs in Australia. 

Design rainfall totals were derived for two locations representing the rainfall intensities across the 

Mullet/Brooks Creek catchments. Figure 6-2 shows the two locations used to derive design event 

rainfall, one in the upper catchment and one at the catchment outlet to Lake Illawarra. Table 6-2 

shows the Intensity-Frequency-Duration (IFD) parameters derived from AR&R (1987) for the two 

locations. 


Table 6-2 Design Rainfall IFD Parameters 

IFD Parameter Lower Catchment 

(Lake Illawarra) 

Location 1 Location 2 

Upper Catchment 

(Forest Creek) 

50% AEP, 1 hour intensity 45 mm/hr 48 mm/hr 


50% AEP, 72 hour intensity 2.75 mm/hr 4 mm/hr 



2% AEP, 72 hour intensity 6.75 mm/hr 9 mm/hr 

Average Regional Skewness 0 0 

Design Rainfall Depths 

Design rainfall depths for the three durations discussed in Section 6.2.2 (2 hour, 6 hour and 36 hour) 

are provided in Table 6-3. 

Table 6-3 Adopted Design Rainfall Depths 

Design Event 

AEP 

2 hour 

Duration 

6 hour 36 hour 

Location 1 Location 2 Location 1 Location 2 Location 1 Location 2 

1% 1570 mm 1995 mm 409 mm 515 mm 217 mm 280 mm 

2% 1407 mm 1759 mm 393 mm 454 mm 191 mm 247 mm 

5% 1193 mm 1456 mm 305 mm 376 mm 157 mm 205 mm 

10% 1031 mm 1231 mm 261 mm 318 mm 132 mm 174 mm 

20% 906 mm 1055 mm 226 mm 273 mm 112 mm 149 mm 

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Temporal Distribution of Design Rainfall 

The temporal distribution of rainfall is derived from the temporal patterns presented in AR&R (1987).


Spatial Distribution of Design Rainfall 

The spatial distribution of design rainfall for the 20%, 10%, 5%, 2% and 1% AEP events was 

determined automatically by the WBNM hydrologic model. The total depth of rainfall in each 

subcatchment for each storm is calculated as the weighted depth from all surrounding WBNM 

‘raingauges’ (in this case, two locations as shown in Figure 6-2). This weighting is calculated from 

the inverse square of the distance between each WBNM ‘raingauge’ and the centre of each 

subcatchment. 

6.2.4 Probable Maximum Flood 

The Probable Maximum Flood (PMF) is the largest flood that could reasonably be expected to occur 

in a catchment and is due to the Probable Maximum Precipitation (PMP). The theoretical definition of 

the PMP is “the greatest depth of precipitation for a given duration meteorologically possible for a 

given size storm area at a particular location at a particular time of year, with no allowance made for 

long-term climatic trends.” (WMO, 1986). 

2 hour and 6 hour Durations 

The 2 hour and 6 hour PMP totals, temporal and spatial patterns were derived using the Bureau of 

Meteorology’s guidelines entitled The Estimation of Probable Maximum Precipitation in Australia: 

Generalised Short-Duration Method (BOM, 2003). The Generalised Short-Duration Method (GSDM) 

is recommended in AR&R (2001) for PMP estimates for durations of 6 hours or less. 

• The 2 hour duration PMP rainfall depth at the centre of the catchment is 510mm. 

• The 6 hour duration PMP rainfall depth at the centre of the catchment is 819mm. 

The spatial distribution of the PMP for the 2 and 6 hour duration events was derived using the GSDM 

(BOM, 2003) and amendments to this method published in December 1996. This approach is 

specific to Australia and assumes a stationary storm by distributing the design rainfall across a series 

of ellipses that can be orientated in a controlled direction across the catchment. The PMP depth is 

allocated in a stepped distribution across the ellipses with all points between consecutive ellipses 

having a constant rainfall depth. Figure 6-3 shows the ellipses used in the spatial distribution of the 

PMP for Mullet and Brooks Creeks catchments and the PMP depths for the 2 and 6 hour durations. 

The temporal distribution of the PMP for the 2 and 6 hour duration events was also derived using the 

GSDM (BOM, 2003). 

The WBNM hydrologic model was run with the estimated PMP depths and spatial and temporal 

patterns derived from BOM (2003) to produce inflow hydrographs for the hydraulic model for the 2 

and 6 hour PMF events. 

36 hour Duration 

The BOM (2003) method of PMP estimation is not suitable for durations longer than 6 hours and an 

alternative method was required to estimate the inflow hydrographs to the hydraulic model for the 

36 hour PMF event. To obtain an accurate estimate of the 36 hour duration PMP, AR&R (2001) 

suggests the use of a longer duration generalised PMP method. Wollongong lies within the 

Generalised Southeast Australia Method (GSAM) zone. The GSAM PMP estimation techniques can 



only be undertaken by the Bureau of Meteorology as a fee-based service. PMP estimates provided 

by the BoM according to the GSAM are necessary in dam design or other critical areas. However, for 

the purposes of this study, it was agreed that a preliminary estimate of the 36 hour PMP depth would 

suffice. This preliminary estimate was based on the ratio of the 1% AEP and PMP depths calculated 

for the 2 hour and 6 hour durations. The process is described as follows. 

For each subcatchment the ratio between the peak flows of the PMF and the 1% AEP flood events 

was calculated for the 2 and 6 hour durations. 

• The average ratio between the 2 hour PMF and the 2 hour 1% AEP peak flows for all 

subcatchments is 1.62. 

• The average ratio between the 6 hour PMF and the 6 hour 1% AEP peak flows for all 

subcatchments is 1.58. 

A factor of 1.60 was therefore applied to the 36 hour 1% AEP inflow hydrographs to estimate the 

36 hour PMF hydrographs for input to the hydraulic model. The spatial and temporal distribution of 

rainfall in this event is therefore the same as for the 1% AEP design event as described above. 

6.3 Design Flood Hydraulics 

6.3.1 Model Geometry 

As agreed with Council, the TUFLOW model geometry used for the 1999 calibration event was 

adopted for use in the design event modelling with no changes to the model geometry. 

6.3.2 Manning’s n 

The Manning’s n values used for the design event modelling are unchanged from those used for the 

calibration events. These values are used in both the 1D and 2D areas of the model. The values 

used are shown in Table 6-4. Land use category locations are as shown in Figure 6-4 (a 

reproduction of Figure 5-2). 




Table 6-4 Manning’s ‘n’ for design Event Modelling 

Material No. Description 

Manning's ‘n’ 

Mullet Ck Catchment Brooks Ck Catchment 

1 Sand bed/bank 0.030 0.030 

2 Grassy Channels 0.070 0.070 

3 Grass (maintained) 0.035 0.035 

4 Pasture 0.050 0.050 

5 Roads 0.025 0.025 

6 Concrete storage areas 0.050 0.050 

7 Dense riparian vegetation 0.150 0.150 

8 Dense land vegetation 0.150 0.150 

9 Creek bed (middle of creek) 0.040 0.040 

10 Building 3.000 3.000 

11 Urban block 1.000 1.000 

12 Concrete in channel 0.050 0.050 

13 Dams, open water, bare soil etc 0.060 0.060 

14 Lightly vegetated creek 0.080 0.080 

15 Dense creek vegetation 0.150 0.150 

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6.3.3 Critical Duration 

A summary of the critical duration assessments is provided previously in Sections 6.2.2. This section 

provides further detail. 

To determine the critical duration for the Mullet and Brooks Creeks catchments the 

hydrologic/hydraulic design models were run for the 1% AEP event for a range of storm durations 

between 0.5 and 12 hours. The peak water levels for these durations were investigated in order to 

determine which duration(s) resulted in the highest peak water levels across the study area. 

For Brooks Creek, the 2 hour storm duration was found to produce the highest flood levels across the 

study area. For Mullet Creek, a combination of the 2, 6 and 9 hour durations was found to produce 

the highest flood levels. The 2 hour storm was critical in the upper parts of the catchment, the 6 hour 

storm in the mid-reaches and the 9 hour storm in the lower reaches on the downstream side of the 

Princes Highway. As peak flood levels in the lower reaches are governed by Lake Illawarra levels it 

was decided in agreement with Bewsher Consulting and Council that the 2 and 6 hour storm 

durations would be used to determine the flood envelope for Mullet Creek. 

In addition, a 36 hour storm duration simulation was also undertaken. In the Lake Illawarra Flood 

Study (L&T, 2001) this was found to be the critical duration for the Lake Illawarra catchment (see 

Section 6.3.4). The inclusion of the 36 hour creek design event, in conjunction with the 36 hour Lake 

Illawarra event, is expected to provide the highest peak flood levels in the lower portions of each 

creek. 

6.3.4 Downstream Boundary Conditions 

Both Mullet and Brooks Creeks flow into Lake Illawarra, in the northwest corner of the lake. The 

downstream boundary conditions for the hydraulic models of the Mullet and Brooks Creeks 

catchments will therefore be water levels in Lake Illawarra.


Lawson and Treloar Pty Ltd completed the Lake Illawarra Flood Study for Wollongong City Council, 

Shellharbour City Council and the Lake Illawarra Authority in July 2001 (L&T, 2001). This study 

involved an investigation of flooding of Lake Illawarra to evaluate the current peak flood level 

estimates for the 50%, 20%, 10%, 5%, 2% and 1% AEP design events and the PMF. The 36 hour 

storm was found to be the critical duration storm that produced peak lake levels. 

For the purposes of this study, the downstream boundary levels of Mullet and Brooks Creeks for the 

36 hour duration design events have been sourced directly from L&T (2001) model results. However, 

the 2 hour and 6 hour durations were not simulated by L&T (2001) and were required to be simulated 

in the model for the purposes of this study. At Wollongong City Council’s request, Cardno Lawson 

Treloar PTY Ltd (CLT) undertook these simulations for the 20%, 10%, 5%, 2% and 1% AEP design 

events and the PMF. 

CLT provided a spreadsheet containing stage time hydrographs for the 20%, 10%, 5%, 2% and 1% 

AEP events and the PMF for the 2, 6 and 36 hour durations at three catchment outlet locations on 

Lake Illawarra. Council provided this data to BMT WBM via email on 10 January 2006. 

The three locations on Lake Illawarra are Macquarie Rivulet, Duck Creek and Mullet Creek. No direct 

time series was available at the exact location of the Brooks Creek outlet. A comparison of the stagetime 

hydrographs at Macquarie Rivulet (south), Duck Creek (middle) and Mullet Creek (north) 

indicated that there was little variation across that length of Lake Illawarra and that therefore it would 

be appropriate to use the stage-time hydrograph at the Mullet Creek outlet for the Brooks Creek 

outlet as well. Figure 6-1 shows the stage-time hydrograph on Lake Illawarra used at the Mullet and 

Brooks Creeks outlets for the 1% AEP events for the 2, 6 and 36 hour durations. Table 6-5 shows 

the peak Lake Illawarra levels at the Mullet and Brooks Creeks outlets for all event scenarios 

simulated in this flood study. 

Level (m AHD) 

2.5 

2 

1.5 

1 

0.5 

0 

0 6 12 18 24 30 36 

Time (hours) 

42 48 54 60 66 72 

K:\B14142.k.clb\Tuflow\Boundaries\[Lake Illawarra Flood Study Stage Hydrographs 10 Jan 2006.xls] 

Figure 6-1 Stage Time Hydrograph at Mullet and Brooks Creeks Outlets 


100y 36hr 

100y 6hr 

100y 2hr


Table 6-5 Peak Levels in Lake Illawarra at Mullet and Brooks Creeks Outlets 

Peak level in Lake Illawarra * (m AHD) 

Event (AEP) 2 hour 6 hour 36 hour 

PMF 1.89 2.63 3.26 

1% 1.18 1.61 2.31 

2% 1.09 1.46 2.04 

5% 0.95 1.28 1.85 

10% 0.86 1.11 1.61 

20% 0.79 0.97 1.44 

* From Cardno Lawson & Treloar, January 2006. 


6.3.5 Design Event Combinations 

In conjunction with the Technical Working Group (TWG), it was decided to use equivalent creek and 

downstream boundary events to produce the design event combinations. 

For each AEP design event three durations were simulated: the 2 hour, 6 hour and 36 hour. The 

downstream boundary conditions applied at Lake Illawarra were for the same event on the Lake 

Illawarra catchment as the Mullet and Brooks Creeks catchments. That is, for the 1% AEP 6 hour 

design event, the 1% AEP 6 hour storm on the Mullet Creek catchment was combined with the stagetime 

hydrograph for the 1% AEP 6 hour storm on the Lake Illawarra catchment. 

6.4 Culvert and Bridge Blockage Assumptions 

6.4.1 WCC Blockage Policy 

Wollongong City Council developed a policy for assumptions regarding blockage of culverts and 

bridges. This policy is presented below: 

The following blockage factors are to be applied to structures across all watercourses when 

calculating design flood level: 

(i) 100% blockage for structures with a major diagonal opening width of less than 6m; 

(ii) 25% bottom up blockage for structures with a major diagonal opening width of greater 

than 6m. For bridge structures involving piers or bracing, the major diagonal length is 

defined as the clear diagonal opening between piers/bracing, not the width of the 

channel at the cross-section; 

(iii) 100% blockage for handrails over structures covered in (i) and for structures covered 

in (ii) when overtopping occurs. 

This policy is open to interpretation in relation to the treatment of blocking bridge openings described 

in (ii) above. If the major diagonal length is less than 6m in a single span, the question remains if that 

requires a 25% blockage of that particular span or the entire bridge. It was decided in conjunction 

with Council that if a single span is greater than 6m then a 25% blockage would be applied to all 

spans of the bridge. 



It was decided in conjunction with Council that three blockage scenarios would be undertaken for 

Mullet and Brooks Creeks. These scenarios were considered to cover the worst possible case for 

each AEP design event. The cases are discussed below and shown in Figure 6-5. 

• Case 0: All culverts and bridges fully open. 

• Case 1: All culverts and bridges blocked either 100% or 25% blocked depending on diagonal 

opening span according to Council policy. 

• Case 2: Partially blocked case. This case was developed following a review of the Case 0 and 

Case 1 results in order to develop a scenario that might result in higher peak flood levels in some 

locations by keeping some culverts and bridges blocked whilst opening others. The structures to 

be opened were chosen by assessing the impacts of fully blocking all culverts compared to the 

unblocked cases. The culverts chosen to unblock for Case 2 were those for which blocking 

resulted in the more significant impacts on peak water levels compared to the unblocked case. 

In particular, it was expected that blocking the F6 Freeway bridge but fully opening the Princes 

Highway bridge would result in higher peak flood levels on the floodplain upstream of the 

Freeway. In this scenario, all bridges and culverts are considered to be blocked according the 

blockage policy with the exception of the following, which are considered to be open: 

Culvert MC14_0050_R which runs underneath Avondale Colliery Road in the south of the 

catchment; 

Culverts MC30_0120_C and MC30_0100_C under Homestead Drive and Horsley Drive in 

Horsley; 

Culverts RC10_0150_C under Shone Avenue in Horsley; 

Culverts RC22_0020_C, RC21_0020_R and bridge RC21_0030_B at West Dapto road in 

Wongawilli; 

Bridges MC10_0200_B and MC10_0180_B where Mullet Creek flows underneath the 

Princes Highway; 

Culverts RW1_0510_C, RW10_0410_R, RW10_0310_C, GC10_0060_C, GC10_0040_Cd 

and GC10_0030_R under the railway line and Princes Highway at the northern extent of the 

study area; and 

Culverts BC10_0610_R, BC10_0410_R and BC10_0310_R on Brooks Creek underneath 

Emerson Road, Fowlers Road and Byamee Street respectively. 

The location of the open bridges and culverts is shown in Figure 6-5. 

In all three design cases (Case 0, Case 1, Case 2) the handrails were assumed to be fully blocked. 


DESIGN EVENT RESULTS – EXISTING CONDITIONS 7-1 

7 DESIGN EVENT RESULTS – EXISTING CONDITIONS 

7.1 Interpretation of Results 

7.1.1 General Issues 

The interpretation of the maps and other presentations in this report should be done so with an 

appreciation of any limitations in their accuracy. The accuracy of the predicted flood flows, levels and 

extents in this flood study is dependent on the accuracy of the data used to develop the hydrologic 

and hydraulic models and the ability of the models to represent the behaviour of the catchment during 

a storm event. While the points below highlight these limitations, it is important to note that the 

results presented provide an up-to-date, reliable and accurate prediction of design flood behaviour. 

Points to remember are: 

• Recognition that no two actual floods behave in exactly the same manner; 

• Design floods are a best estimate of an ‘average’ flood for their probability of occurrence; 

• The DEM has been generated from photogrammetry supplied to BMT WBM in August 2003. As 

flood depths and flood extents are determined using the DEM, their accuracy should be 

interpreted accordingly; and. 

• Approximations are used to represent the PMF event (refer to Section 6.2.4). 

7.1.2 Uncertainty in Design Flood Results 

All design floods are based on statistical analyses of recorded rainfall data. The longer the period of 

recordings, the greater the certainty. For example, derivation of the 1% AEP event rainfall from 5 

years of recordings would have a much greater error margin than from 100 years of recordings. 

Similarly, the accuracy of the hydrologic and hydraulic computer models is dependent on the amount 

and range of reliable rainfall and flood level recordings for model calibration. The results of an uncalibrated 

model have a greater error margin than a calibrated model. 

The error margin in this study is regarded as moderate due to: 

• A reasonable amount of rainfall and flood level data; 

• Calibration and verification of the flood models to three historical events; 

• Uncertainties associated with data used to develop the DEM (see below); and 

• Approximations used for the PMF event. 

In this hydrologic/hydraulic model the main source of inaccuracy is within the topographic data. The 

topographic data used in this model was collected by photogrammetry and ground survey (see 

Section 3.1). The DEM was checked against eight ground survey sections. The channel crosssections 

within the 1D areas of the model are derived from cross-sections extracted from the DEM 

(see Section 4.3.2). The accuracy of the model predictions in this flood study would be improved if 

surveyed channel sections were available, particularly along the many reaches where the channel is 



overgrown with vegetation and photogrammetry methods may not have been able to capture the true 

ground surface. 

One area of particular concern is in the most upstream reaches of the Mullet and Brooks Creeks 

catchments where the flow appears to be mostly contained within the creek channel. Here, the inbank 

areas of the creek cross-sections are important in determining flows/levels and, as these crosssections 

have been extracted from the DEM, the accuracy of these in-bank areas may not be high. 

It should be recognised that the model has been developed with the best data available at the time of 

development. 

7.1.3 Definition of Peak 

Unless otherwise stated, presentations in this report are based on peak values of flood level and 

flows. The peak flood level does not occur everywhere at the same time and, therefore, the values 

presented are based on taking the maximum which occurred at each computational point in the 

model during the entire flood. Hence, a presentation of peak levels does not represent an 

instantaneous point in time, but rather an envelope of the maximum values that occurred over the 

duration of the flood event. 

7.1.4 Terminology 

The following terms are used to describe the flood model results. 

Velocity 

Velocity is defined as the speed at which the floodwaters are moving. Typically, modelled velocities 

in a creek or river are quoted as the depth and width averaged velocity, i.e. the average velocity 

across the whole river or creek section if one-dimensional model is used; and depth average if a twodimensional 

model is used. It is worth noting that velocities can be significantly higher than the 

figures quoted at some locations due to localised effects (eg in channel areas or outside bends). 

Water Level/Flood Level 

The flood level or water level is the height or elevation of floodwaters relative to a datum (typically the 

Australian Height Datum). 

Depth 

The water depth (m) is the height or elevation of floodwaters above ground level. This should not be 

confused with water level which is the height of the water relative to a datum (not ground level). 

7.2 Design Flood Behaviour – Summary 

7.2.1 Flood Mapping of Design Flood Behaviour 

The flood mapping presented in this report is a maximum flood envelope. For each AEP design 

event on Mullet Creek, the flood envelope is the maximum flood level from nine simulations (three 

durations: 2 hour, 6 hour and 36 hour and three blockage cases: Case 0, Case 1 and Case 2). For 



Brooks Creek the flood envelope is the maximum of six simulations (two durations: 2 hour and 

36 hour and three blockage cases: Case 0, Case 1 and Case 2). 

Figure 7-2 shows the predicted flood envelope for the 1% AEP design event. The figure shows 

coloured depth contours and labelled peak flood level contours. Note that in the Mullet Creek 

catchment, there is a low-lying area in the Reed Creek floodplain with complex floodplain interactions 

between two waterways (marked with blue-hatching in Sheet 2 of Figure 7-2 to 7-7). These 

waterways have been approximated using a 1D modelling approach. Flood levels and depths should 

be considered indicative only in this area. 

It should be noted that the actual exceedance probability of the modelled flood shown in Figure 7-2 is 

less than 1% as it depends on a particular combination of 1% AEP design storms on the Mullet 

Creek, Brooks Creek and Lake Illawarra catchments together with a particular blockage scenario. 

Flood envelopes for the 20%, 10%, 5% and 2% AEP and PMF flood events are shown in Figure 7-3 

to Figure 7-7. Similarly, the actual recurrence intervals for these floods as they appear in the figures 

are greater than that with which they are labelled. 

7.2.2 Peak Flood Levels at Selected Locations 

Table 7-1 shows the modelled peak flood levels for the 1% AEP design event at selected locations for 

the unblocked case for the 2, 6 and 36 hour durations. The duration that produces the highest 

predicted flood levels for each location is highlighted in bold. The flood behaviour at these nine 

locations will be discussed in more detail in Section 7.3.2. 

Table 7-1 Peak Levels for the 1% AEP Design Event at Selected Locations (Case 0) 

Location 

Peak 1% AEP Flood Level (m AHD) 

2 hour 6 hour 36 hour 

Freeway F6 Bridge on Mullet Creek 3.58 3.90 3.96 

Princes Highway Bridge on Mullet Creek 5.32 5.55 5.44 

Cleveland Road, Dapto 15.56 15.57 15.43 

Bong Bong Road, Dapto 10.86 10.89 10.76 

Ena Avenue, Dapto 21.14 21.04 20.55 

Sheaffes Creek (d/s of West Dapto Road) 8.13 7.93 7.64 

Robins Creek (Ritchie Crescent) 12.54 12.46 12.02 

Reed Park Creek (Fairwater Drive) 14.75 14.81 14.79 

Forest Creek (u/s of West Dapto Road) 14.00 13.96 10.62 


Predicted peak flood levels for the 1% AEP design event at all 1D nodes are shown in Appendix F. 

Table 7-2 shows the modelled peak flood levels at the same locations for the 1%, 2%, 5%, 10% and 

20% AEP and the PMF design flood events. The modelled flood levels shown are the maximum 

flood level from the three durations for each design event in the unblocked case. 



Location 

Table 7-2 Peak Flood Levels for a Range of Design Events at Selected Locations (Case 0) 


Peak Flood Level (m AHD) for Design Events (AEP) 

PMF 1% 2% 5% 10% 20% 

Freeway F6 Bridge on Mullet Creek 5.95 3.96 3.78 3.61 3.40 3.22 

Princes Highway Bridge on Mullet Creek 7.22 5.55 5.33 5.07 4.84 4.65 

Cleveland Road, Dapto 16.19 15.57 15.51 15.42 15.33 15.25 

Bong Bong Road, Dapto 11.59 10.89 10.82 10.68 10.51 10.40 

Ena Avenue, Dapto 22.17 21.14 20.88 20.59 20.31 20.07 

Sheaffes Creek (d/s of West Dapto Road) 8.77 8.13 7.98 7.84 7.73 7.60 

Robins Creek (Ritchie Crescent) 14.53 12.54 12.29 11.99 11.75 11.52 

Reed Park Creek (Fairwater Drive) 15.71 14.81 14.74 14.70 14.65 14.60 

Forest Creek (u/s of West Dapto Road) 15.54 14.00 13.87 13.67 13.50 13.36 


7.2.3 Peak Flows at Selected Locations 

Table 7-3 shows the predicted 1% AEP flows at selected locations for the unblocked case for the 2, 6 

and 36 hour durations. The duration with the highest flows for each location is highlighted in bold. 

The flood behaviour at these nine locations will be discussed in more detail in Section 7.3.2. 

Table 7-3 Peak Flows for the 1% AEP Design Event at Selected Locations (Case 0) 

Peak 1% AEP Flow (m 3 Location 

/s) 

2 hour 6 hour 36 hour 

Freeway F6 Bridge on Mullet Creek 585 769 806* 

Princes Highway Bridge on Mullet Creek 640 733 664 

Cleveland Road, Dapto 348 348 252 

Bong Bong Road, Dapto 396 409 317 

Ena Avenue, Dapto 356 341 243 

Sheaffes Creek (d/s of West Dapto Road) 117 89 55 

Robins Creek (Ritchie Crescent) 479 456 338 

Reed Park Crescent (Fairwater Drive) 41 46 46 

Forest Creek (u/s of West Dapto Road) 183 169 122 

Note: All flows are the total 1D and 2D flow at each location 

* There is a significant head drop across the F6 Freeway in the 36 hour event with high predicted water levels in the storage area 

between the Princes Highway and the F6 Freeway. 


Predicted peak flows for the 1% AEP design event at all 1D channels are presented in Appendix F. It 

should be noted that in some places the flows presented in Appendix F appear to decrease moving 

downstream through the catchment, this is where flow has broken out of the 1D channel onto the 2D 

floodplain. 

Table 7-4 below shows the predicted peak flows at the same locations for the 1%, 2%, 5%, 10% and 

20% AEP design flood events. The flows shown are the maximum flow from the three durations for 

each AEP in the unblocked case.


Location 

Table 7-4 Peak Flows for a Range of Design Events at Selected Locations (Case 0) 


Peak Flows (m 3 /s) for Design Events (AEP) 

PMF 1% 2% 5% 10% 20% 

Freeway F6 Bridge on Mullet Creek 2057 806 691 600 496 414 

Princes Highway Bridge on Mullet Creek 1474 733 638 535 458 392 

Cleveland Road, Dapto 972 348 299 248 197 163 

Bong Bong Road, Dapto 1218 409 348 280 218 179 

Ena Avenue, Dapto 912 356 309 252 201 166 

Sheaffes Creek (d/s of West Dapto Road) 224 117 99 79 64 52 

Robins Creek (Ritchie Crescent) 1139 479 410 333 275 228 

Reed Park Crescent (Fairwater Drive) 214 46 40 34 28 23 

Forest Creek (u/s of West Dapto Road) 376 183 157 129 107 89 

Note: All flows are the total 1D and 2D flow at each location 


7.2.4 Peak Flood Level Profiles 

Longitudinal flood level profiles for Mullet and Brooks Creeks for the 20%, 10%, 5%, 2% and 1% AEP 

and PMF design flood events are shown in Figure 7-9 and Figure 7-10. Profile locations are shown in 

Figure 7-8. The predicted peak flood level profiles shown are the maximum flood level for each AEP. 

For Mullet Creek, this is the maximum flood level from nine simulations (three durations and three 

blockage cases). For Brooks Creek, the profile is the maximum flood level from six simulations (two 

durations and three blockage cases). 

7.2.5 Provisional Hazard Categories 

Provisional hazards are calculated according to the criteria provided in the NSW Floodplain 

Development Manual (NSWG, 2005) Figure L2, reproduced here as Figure 7-1. 

NSW Floodplain Development Manual (NSWG, 2005) Figure L2 

Figure 7-1 Provisional Hazard Categories


High hazard is defined as representing: 

• possible danger to personal safety, 

• evacuation by trucks difficult, 

• able-bodied adults would have difficulty wading to safety, and 

• potential for significant structural damage to buildings. 

Low hazard is defined as representing: 

• trucks can evacuate people and possessions if necessary, and 

• able-bodied adults would have little difficulty wading to safety. 

Hazards are mapped according to these criteria for Mullet and Brooks Creeks in Figure 7-11. For 

areas that are represented in 1D, the hazard is taken as the maximum hazard along each 1D crosssection 

to provide a conservative approach. 

7.3 Design Flood Behaviour – Mullet Creek 

7.3.1 General Discussion 

General Behaviour 

Model results indicate that, in general, the flood behaviour of Mullet Creek is not complex. In the 

upper reaches of the catchment model results show that flow is relatively contained within the 

channel and a narrow floodplain in the 1% AEP design event. 

In the middle and lower reaches of the Mullet Creek catchment, where the channels become more 

overgrown and meandering, model results indicate that out of bank flow becomes more significant. 

Upstream of the Princes Highway, where flood water backs up behind the crossing, peak flood 

depths greater than 1.5m are predicted in the 1% AEP design event. The predicted velocities of this 

floodplain flow are generally between 1.0 and 1.5m/s during the 1% AEP design event but there are 

locations where out of bank velocities are predicted to be greater than 2.0m/s. 

There is an extensive floodplain area between the Princes Highway and the F6 Freeway that is 

predicted to cover both the golf course and racecourse. In this floodplain area model results indicate 

inundation depths greater than 1.5m in the 1% AEP design event. 

Downstream of the F6 Freeway there is a northern and a southern outlet to Lake Illawarra. Model 

results indicate that the principal flow route to the southern outlet is via the main channel of Mullet 

Creek. Flood water is predicted to reach the northern outlet via the Tank Trap Channel, Hooka Creek 

and overland through the park. A greater proportion of flow leaves the catchment through this 

northern outlet than via the main Mullet Creek channel to the southern outlet. Inundation depths 

downstream of the F6 Freeway are predicted to be greater than 1.5m in parts of the park and 

memorial gardens but are generally shallower with predicted depths from 0 to 1m in the 1% AEP 

design event. Predicted overland velocities in this area are less than 1.0m/s. 



Throughout the Mullet Creek catchment there are many small bridge and culvert structures. The 

major constrictions on flow within this study area are the crossings of the Princes Highway and F6 

Freeway. Flood water is predicted to back up behind these constrictions in all design events 

modelled as part of this study. Model results indicate that these major crossings will not be 

overtopped in a 1% AEP design event. In the longer duration events there is significant storage of 

flood water in the area between the Princes Highway and the F6 Freeway. 

Darkes Road, West Dapto Road and Bong Bong Road are all predicted to be overtopped in sections 

during the 1% AEP design event. Some sections of these roads are also overtopped in the more 

frequent design flood events. Consequently, there is a risk of West Dapto becoming inaccessible if 

significant overtopping of these roads occurred as predicted during a major flood event. This risk is 

increased by the potential blockage of the structures under these roads. 

1% AEP Event 

In the steep upper parts of the catchment, the predicted peak flood levels are due to the 2 hour 

duration events in the 1% AEP design event. The peak levels are derived from a combination of the 

Case 0, Case 1 and Case 2 blockage scenarios. In the mid reaches of the catchment, between 

Horsley and the Princes Highway, the 1% AEP peak flood levels are predicted to be due to the 6 hour 

duration event with Case 1 or Case 2 blockages. Downstream of the F6 Freeway the longer duration 

36 hour event (with Case 1 or Case 2 blockages) results in the highest predicted peak flood levels 

because of the dominating influence of the downstream boundary at Lake Illawarra. In the high 

volume event (36 hour duration) the area between the Princes Highway and F6 Freeway acts as a 

large storage area with high predicted peak flood levels. 

The predicted flood extent for the 1% AEP design event shown in Figure 7-6 has been compared to 

the actual flood extent for the 1984 calibration event (provided by Council) and the modelled flood 

extent for the 1984 calibration event presented in this study (Figure 5-53). The modelled flood extent 

for the 1% AEP design event is predicted to be smaller than that of the actual and modelled February 

1984 calibration event. The February 1984 flood event was estimated to have an average recurrence 

interval greater than 500 years (WM, 1997). 

The predicted flood extent for the 1% AEP design event was also compared to the modelled flood 

extents for the March 1975 and October 1999 calibration events (Figure 5-26 and Figure 5-62). The 

modelled extent for the 1% AEP design event is more extensive than the modelled extent of both 

these calibration events. The 1% AEP design event flows in Table 7-3 can be compared to the 

calibration flows in Table 5-6, Table 5-10 and Table 5-15. This shows that the flows from both the 

1975 and 1999 calibration events are lower than the 1% AEP design event flows. The 1984 

calibration event flows are higher than the predicted 1% AEP design flows in Table 7-3. 

The modelled extent for the 1% AEP design event shown in Figure 7-6 has also been compared to 

the results of previous studies (WM, 1997). See Section 7.3.3 for further discussion. 

Other Events 

In the more frequent flood events (5%, 10% and 20% AEP design events) the influence of levels in 

Lake Illawarra is predicted to extend further upstream with peak levels in the floodplain on the 

upstream side of the Princes Highway resulting from the 36 hour duration scenarios. 



In the PMF flood event model results indicate less influence of Lake Illawarra levels on peak flood 

levels in the Mullet Creek catchment. In the PMF flood event the predicted peak flood levels in the 

upper and middle parts of the catchment, as far downstream as the Princes Highway, are due to the 

2 hour duration events and a combination of the Case 0, Case 1 and Case 2 blockage scenarios. 

Between the Princes Highway and the F6 Freeway the 6 hour duration event with Case 2 blockages 

is predicted to result in the highest peak flood levels. Downstream of the F6 Freeway model results 

show a combination of the 6 hour duration events with Case 0 and Case 2 blockage causing the 

highest peak flood levels, with the 36 hour duration event (with Case 2 blockages) causing the 

highest predicted peak flood levels in a small area around the shore of Lake Illawarra. 

7.3.2 Detailed Discussion 

Several areas of the Mullet Creek catchment have been highlighted by Council as being of particular 

significance in this flood study. The modelled peak flood levels and flows at these locations are 

included in Table 7-1 to Table 7-4. The predicted flood behaviour at these specific areas of interest 

will be discussed in this section, primarily focussing on the 1% AEP design event. It should be noted 

that throughout this discussion the left and right creek banks are defined as though looking 

downstream and the discussion progresses from the upper reaches of the catchment downstream to 

Lake Illawarra. 

Near Ena Avenue, overland flow is predicted to occur on both sides of Mullet Creek during the 1% 

AEP design flood event. A significant proportion of the total flow at this point is predicted to be on the 

floodplain with velocities up to 2m/s and some flow into the back gardens of properties along this 

road. The peak flows and levels in the 1% AEP design flood at Ena Avenue occur during the 2 hour 

event with no blockages. It is important to note that a separate, more detailed, study has been 

undertaken for the Ena Avenue area (Forbes Rigby, 2003). More reliable flood extents for Ena 

Avenue itself are contained in that study. 

Cleveland Road is predicted to be overtopped in each of the modelled design flood events. There is 

significant out-of-bank flow predicted on the left bank of the creek at this location during the 1% AEP 

design event and also a small amount of flow through the grounds of Dapto High School. 

Across Bong Bong Road, some flow is predicted to bypass the main channel on the right bank side 

and flow along the railway line and Burringbar Street with modelled velocities up to 1.5m/s and 

depths of greater than 1.5m. Flow is also predicted to break out of the left bank of Mullet Creek 

upstream of Bong Bong Road and to bypass the bend in the creek. Some of this flow is predicted to 

flow north towards the small tributary emerging from Horsley. Downstream of this location, model 

results show a significant flow path parallel to Hamilton Street that bypasses the creek meanders with 

modelled velocities between 1 and 2m/s and depths greater than 1.5m. 

On the northern side of Horsley, near Huxley Drive, the model predicts out-of-bank flow from the right 

bank of Robins Creek with modelled velocities greater than 2m/s and depths greater than 1.5m. 

Downstream of this location, near Sunnybank Crescent, there is a significant high velocity (3 m/s) 

break-out from the right bank which flows onto the floodplain between Mullet Creek and Robins 

Creek. Floodplain depths are predicted to be greater than 1.5m with velocities of approximately 

1.2m/s. Some of this flow is predicted to re-enter Mullet Creek upstream of Darkes Road, which is 

overtopped in the 1% AEP design event. 



Some inundation of the railway line is predicted between Darkes Road and Bong Bong Road during 

the 20% AEP design event. The inundation of the railway becomes significant during flood events 

with an average recurrence interval of 5%s or greater. 

Between the Princes Highway and the F6 Freeway, the major flowpaths are along Mullet Creek and 

Gibsons Creek. There is also a significant volume of water predicted to flow across the floodplain at 

low velocities (0.1 to 0.3 m/s). Much of the area between these two roads, including the golf course 

and parts of the racecourse, is predicted to be inundated to depths of greater than 1.5m in the 1% 

AEP design event. The modelled extent of inundation does reach some properties on Windsor 

Crescent in Brownsville. The F6 Freeway crossing forms the lowest major constriction on the Mullet 

Creek catchment and water flows under this bridge with predicted velocities between 1.5 and 2m/s 

the 1% AEP design event. Model results indicate that neither the Princes Highway nor F6 Freeway 

crossings of Mullet Creek are overtopped in the 1% AEP design event but both roads may be subject 

to inundation during the PMF event. 

7.3.3 Comparison with the Previous Study 

The 1% AEP design event flood extent and peak flood levels have been compared to the results from 

the 1997 Flood Study Review by Webb McKeown (WM, 1997). The areas covered by the two 

studies are similar but the hydraulic modelling undertaken as part of BMT WBM’s current flood study 

extend further upstream on several tributaries. WM (1997) did not undertake 2D modelling, nor did it 

include Brooks Creek. 

Figure 7-10 presents a comparison of the modelled 1% AEP flood extent of WM (1997) and BMT 

WBM’s current flood study. In general, the predicted flood extents by BMT WBM’s current flood study 

match reasonably well to the flood extents from WM (1997) in the upper and lower parts of the 

catchment. In the middle reaches of the catchment, there are some discrepancies between the 

results of BMT WBM’s current flood study and the results of WM (1997). 

Table 7-5 compares the peak flood levels at selected locations on the main channel of Mullet Creek 

from WM (1997) and the current flood study for the 1% AEP design flood event. The peak flood 

levels shown from BMT WBM’s current flood study are the maximum levels from the nine 1% AEP 

scenarios (three durations and three blockage cases). The peak flood levels from WM (1997) are the 

1% AEP design flood levels from the 6 hour duration event with no blockages. 



Location 

Table 7-5 Comparison of Peak Flood Levels with Previous Studies 


2006 Current Flood 

Study (BMT WBM)m AHD 

1997 Flood Study 

Review (WM*)m AHD 

Mullet Creek at Gibsons Creek confluence 4.2 3.9 

Mullet Creek d/s of Princes Highway 5.4 5.1 

Mullet Creek u/s of Darkes Road 6.4 7.1 

Mullet Creek at Wongawilli Spur Line 6.7 7.3 

Mullet Creek u/s of Bong Bong Road 11.2 11.1 

Mullet Creek u/s end of Burringbar Street 12 12 

Mullet Creek u/s of Cleveland Road 16.1 16 

Mullet Creek at Ena Avenue 21 20.9 

Robins Creek u/s of Darkes Road 6.7 7.1 

Robins Creek u/s of Wongawilli Road 8.6 9.2 

Robins Creek d/s of Forest Creek confluence 12.7 11.9 

Sheaffes Creek d/s of West Dapto Road 9.8 9.8 

Dapto Creek d/s of West Dapto Road 13 12.9 

Note: The peak levels shown from the BMT WBM flood study are the maximum flood level for each design event derived from the three durations and three 

blockage cases. The peak levels from WM (1997) do not include any blockage scenarios. 

* Webb McKeown (1997) 


The levels presented in Table 7-5 are generally well matched between the two flood studies. 

However, there are four locations with differences between the two sets of results of greater than 

0.5m. It is likely that differences in peak flood levels on the floodplains may be substantially different 

between the two studies. 

There are several factors that may explain the differences between the peak levels of the current 

flood study and the WM (1997) flood study. The reason for the differences is likely to be a 

combination of the factors listed below: 

• WM (1997) results were determined using a quasi-2D hydraulic model (RUBICON) of the lower 

catchment and a steady state hydraulic model (HEC2) of the steeper upper reaches. The 

current study has used a fully dynamic linked 1D/2D model (TUFLOW) which is better suited to 

modelling the 2D nature of flow in this catchment. 

• As the current study used the 2D/1D TUFLOW model, calibration to a substantial number of 

recorded peak flood levels (1984 event) on the floodplain was possible. WM (1997) were only 

able to undertake a calibration to those recorded flood levels in the main channel. 

• The topographic data used in the current flood study is photogrammetry data collected by 

HATCH on behalf of Wollongong City Council. Channel cross-sections in the upper reaches of 

the model were derived from the DEM created from this photogrammetry. WM (1997) relied on 

cross sections from ground survey collected between 1980 and 1985. 

• The WBNM Hydrologic Model used in the current flood study was developed specifically for this 

study, however it is based on the previous WBNM model (WM, 1997). Changes to the model 

have been made to facilitate TUFLOW boundaries and to improve the representation of the 

catchment where appropriate, as digital data available for this study allowed more accurate 

definition of the sub-catchments. The same lag factor (C = 1.0) has been used in both WBNM


models. The initial loss of 0mm is the same in both WBNM models but the continuing loss 

values are higher in the WM (1997) model than in the current flood study, 2.5mm/hr compared to 

2.0mm/hr. 

• WM (1997) used an unsteady downstream boundary at Lake Illawarra with a peak water level of 

1.53m AHD. The current flood study has used different downstream boundary conditions for 

each AEP and storm duration derived from the model used in the Lake Illawarra Flood Study 

(L&T, 2001). The peak water levels at the outlet of Mullet Creek for the 1% AEP design event 

are 1.18m AHD, 1.61m AHD and 2.31m AHD for the 2 hour, 6 hour and 36 hour durations 

respectively. The peak water levels at this location for all design events are shown in Table 6-5. 

• WM (1997) do not account for the blockage of any structures in the Mullet Creek catchment 

during a flood event. The current flood study has included three blockage scenarios for each 

design event: an unblocked case; a fully blocked case; and a partially blocked case. 

7.4 Design Flood Behaviour – Brooks Creek 

7.4.1 General Discussion 

General Behaviour 

In the most upstream reaches of Brooks Creek (500m from the upstream extent of the model) model 

results for the 1% AEP design event indicate that the flow is contained within the creek channel. 

Model results show that there is out-of-bank flow on one or both sides of the channel for the entire 

length of the creek downstream of this reach with significant volumes of overland flow predicted in the 

lower reaches of the catchment in the 1% AEP design event. There is also predicted overland flow in 

the lower reaches in more frequent design events. 

The floodplain in the Brooks Creek catchment is significantly narrower than in the Mullet Creek 

catchment as shown in Figure 7-1 to Figure 7-6. The catchment of Brooks Creek is predominantly 

residential and there are several streets where model results indicate properties may be affected by 

flooding. 

1% AEP Event 

In the 1% AEP design event the predicted peak flood levels in the upper parts of the catchment are 

due to the 2 hour duration events with Case 1 and 2 blockages. In the mid reaches of the catchment 

between Emerson Road and Webb Park, the peak water levels are from the 2 hour duration events 

with Case 0, 1 and 2 blockages. At the downstream extent of the model, near the outlet of Brooks 

Creek into Lake Illawarra, the 36 hour duration event is predicted to cause the highest water levels in 

the 1% AEP flood. The influence of flood levels in Lake Illawarra is predicted to extend upstream 

from the outlet of Brooks Creek to Eleebana Crescent. 

Other Events 

In the more frequent design events (20%, 10% and 5% AEP) peak flood levels in the upper and 

middle reaches of the Brooks Creek catchment are predicted to be due to the 36 hour duration event. 

This occurs on the upstream side of Emerson Road and upstream and downstream of Byamee 

Street. At all other locations the peak flood levels are predicted to be due to the 2 hour duration 



event. In these events the predicted influence of flood levels in Lake Illawarra is less extensive in the 

lower reaches of the catchment. 

In the 2% AEP design event the source of the predicted peak flood levels is similar to that for the 1% 

AEP design event with the 2 hour durations events producing the highest flood levels throughout the 

upper and middle parts of the catchment. The model results indicate that the source of peak flood 

levels in the PMF event is again similar to that described for the 1% AEP design event. 

7.4.2 Detailed Discussion 

The predicted flood behaviour through the Brooks Creek catchment will be discussed in more detail in 

this section, focussing primarily on the 1% AEP design event. It should be noted that throughout this 

discussion the left and right creek banks are defined as though looking downstream and the 

discussion progresses from the upper reaches of the catchment downstream to Lake Illawarra. 

Upstream of Emerson Road, where Brooks Creek flows parallel to the F6 Freeway, there is some 

out-of-bank flooding predicted on both sides of the creek during the 1% AEP design flood. Despite a 

relatively narrow flood extent predicted at this location, properties on Beltana Avenue, Brown Avenue 

and St James Avenue are still predicted to experience inundation depths greater than 1.5m. 

Emerson Road is predicted to be partially inundated in the 1% AEP design event but not completely 

overtopped. 

Downstream of Emerson Road, the model results show some out-of-bank flow within a fairly narrow 

floodplain until the crossing at Lakelands Drive, which is predicted to be overtopped in the 1% AEP 

design event. Downstream of Lakelands Drive some overland flow is predicted to occur through the 

park on the right bank and also some flow along Cambridge Road on the left bank with predicted 

depths up to 0.5m. Some of the overland flow along Cambridge Road is predicted to flow onto 

Fowlers Road and the crossing at Fowlers Road is predicted to be overtopped in the 1% AEP design 

flood. 

Between Fowlers Road and Byamee Street, out-of-bank flow is predicted on both sides of the 

channel during the 1% AEP design flood with inundation affecting the back of some properties on 

Toronto Avenue and Byamee Street. The crossing at Byamee Street is predicted to be overtopped in 

the 1% AEP design flood. Out-of-bank flooding on both sides of the channel downstream of this 

location is predicted to affect properties on Bambil Crescent and Culgoa Crescent with depths up to 

0.5m. A significant volume of flow is predicted to bypass the bend in the channel on entering Webb 

Park, with predicted inundation depths on this corner of Culgoa Crescent up to 1.0m in the 1% AEP 

design flood. 

In the lower reaches of Brooks Creek there are three flow paths towards the outlet on Lake Illawarra. 

Model results indicate that the main flow path into Lake Illawarra in the 1% AEP design event is along 

the main creek channel and underneath Lakeside Drive. The bridge at Lakeside Drive is not 

predicted to be overtopped in the 1% AEP design event but it may be overtopped in the PMF event. 

Floodwater also flows down Edgeworth Avenue and is predicted to overtop Lakeside Drive at this 

location to reach the lake. There is a significant volume of flow in the 1% AEP design event that is 

predicted to bypass the meanders in the creek channel and to flow in a southwest direction across 

the park. Model results show this overland flow rejoining the main creek to flow under Lakeside Drive 

and into Lake Illawarra in the 1% AEP design event. 



7.4.3 Comparison with Previous Study 

As discussed in Section 7.3.3, Brooks Creek was not included in the 1997 Flood Study Review by 

Webb McKeown (WM, 1997). There are no other previous studies of Brooks Creek with which to 

compare the results of the current flood study. 

7.5 Conclusions 

The calibrated hydrologic/hydraulic models were used to simulate the 20%, 10%, 5%, 2% and 1% 

AEP design flood events and the PMF. The flood levels and envelopes presented in Figure 7-1 to 

Figure 7-6 are the maximums from three design event durations and three blockage scenarios for 

Mullet Creek and two design event durations and three blockage scenarios for Brooks Creek. 

The 2 hour duration event is critical in the upper parts of the Mullet Creek catchment, the 6 hour 

duration in the middle reaches and the 36 hour event in the lower reaches where the flooding is 

dominated by levels in Lake Illawarra. The results show significant out-of-bank flooding on the middle 

and lower reaches of Mullet Creek and the overtopping of several roads in the catchment. The outof-bank 

flow tends to occur predominantly through meandering and overgrown sections of the creek. 

On Brooks Creek, the dominant critical duration event is the 2 hour flood event. On the lowest parts 

of the creek the 36 hour duration event is critical as peak flood levels are dominated by the water 

levels in Lake Illawarra. The flow is generally confined to a narrow floodplain but there are areas of 

overland flow affecting properties in the middle and lower parts of the catchment.

Mullet and Brooks Creeks Flood Study Report Part - Wollongong ...

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