Minnegang Creek Flood Study Report - Wollongong City Council

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SummaryThis report presents the details and findings of the Flood Study carried out for theMinnegang Creek catchment. The Floodplain Risk Management Study and Plan willbe undertaken following the completion of this Flood Study.Existing flood behaviour within the Minnegang Creek catchment has been determinedbased on hydrologic and hydraulic modelling for the 20%, 5%, 2% and 1% AnnualExceedence Probability (AEP) events, as well as the Probable Maximum Flood (PMF)event. Flood behaviour has been defined in terms of design flows, flood surfaceprofiles, flow velocities, flood extents and flood contours.Hydrologic modelling of the catchment was undertaken using RAFTS v5.1(WP Software, 1996). The hydraulic modelling component of the study wasundertaken using MIKE 11 v3.20 (Danish Hydraulic Institute, 1998), an unsteady statehydraulic model. RatHGL v4.2 (WP Software, 1997) was used to complement theMIKE 11 model in assessing the capacity of the existing piped drainage system in thecatchment.HYDROLOGYThe 90 ha Minnegang Creek catchment was divided into 64 sub-catchments. RAFTSwas used to determine a runoff hydrograph from each of these sub-catchments foreach design storm event. Design storm events were defined using Intensity FrequencyDuration (IFD) data for the Wollongong area. The Probable Maximum Precipitation(PMP) was derived in accordance with Generalised Short - Duration Method asdescribed in Bulletin 53 (Bureau of Meteorology, 1994).Due to the absence of historical streamflow data within the Minnegang Creekcatchment, calibration of the RAFTS model was not possible. Instead, the flows fromthe RAFTS model were compared to estimates made using the Probabilistic RationalMethod (PRM). Design flow rates derived by each method were similar. Anassessment of the sensitivity of the RAFTS model to the adopted parameters foundthat reasonable variations to the adopted parameters would not significantly alter theresults.To assess the effect that future development may have on flooding within thecatchment, land zoning presented in Council’s Local Environmental Plan (LEP)(1990) was considered as part of the study. An increase in developed conditions to themaximum allowable under the provisions of the LEP led to an increase in the totalflow in Minnegang Creek of less than 0.2%. Council’s Urban Consolidation Policydoes not apply to the Minnegang Creek catchment. It was therefore assumed that theeffects of future development on flooding within the catchment are negligible andhence future development conditions were not modelled in the hydraulic modelling ofthe catchment.SV8507-DO-001 Rev 218/10/02ii
HYDRAULICSThe MIKE 11 model included Minnegang Creek, the tributary from Gordon Crescent,the tributary from Melinda Grove, the three minor branches draining the area betweenRanchby Avenue and Hilltop Avenue and the minor branch draining the area betweenCanberra Road and Minnegang Creek. Boundary conditions identified in the modelwere discharges and water levels. The runoff hydrographs from RAFTS were enteredas discharges into the MIKE 11 hydraulic model. The tailwater levels at thedownstream end of the model were derived from the Lake Illawarra Flood Study(Lawson and Treloar, 2000).VERIFICATIONVerification of the combined RAFTS and MIKE 11 model results was undertaken forthe historical event of 17 August 1998. Rainfall data was obtained from the SydneyWater owned gauge located at the Berkeley Sports and Social Club. Runoffhydrographs were generated in the RAFTS model using the historical rainfall datarecorded at five minute increments. These hydrographs were routed through the MIKE11 hydraulic model and the resulting peak water levels were compared to the recordedflood levels for the event.The results of this verification indicated that the MIKE 11 model is accuratelyreproducing the response of the catchment to 17 August 1998 event. However, itshould be noted that this event is considered to have an Average Recurrence Interval(ARI) of approximately 2 years. During the August 1998 flood event, the higherrainfalls were recorded further north of the Minnegang Creek catchment. Ideally,calibration of the hydraulic model would be carried out using a historical event ofsimilar magnitude to the design events (such as the 1% AEP event) to be modelled.This was not possible due to the lack of historical data corresponding to large rainfallevents in the catchment.SENSITIVITYAn assessment of the sensitivity of the MIKE 11 model found that the results for the1% AEP design event did not vary significantly as a result of changes to the modellingparameters. The adopted tailwater level did not significantly affect flood levelsupstream of Northcliffe Drive.CRITICAL STORM DURATIONThe critical storm duration for the catchment, based on the peak flow rates generatedby the RAFTS modelling, was found to be the 2 hour storm for each AEP eventmodelled. This duration was verified by modelling the 1% AEP event for 30, 60 and90 minute durations, as well as the 2, 3 and 6 hour durations in MIKE 11. Thisshowed that the 2 hour storm was leading to the critical flood levels in the catchment.DESIGN EVENTS MODELLEDThe hydraulic modelling of design events was subsequently undertaken for the 20%,5%, 2% and 1% AEP events and the PMF event to determine the flood behaviour forexisting conditions in the catchment. The MIKE 11 modelling has enabled theexisting flood behaviour in the catchment to be described in terms of flood profiles,flood extents and flood surface contours, peak discharges and depth-averagedvelocities.SV8507-DO-001 Rev 218/10/02iii
Page 1 and 2: Minnegang CreekFlood Study
Page 3: © Kellogg Brown & Root Pty Ltd, 20
Page 8 and 9: The RatHGL modelling indicated that
Page 10 and 11: FIGURESSectionPageFigure 2-1 Catchm
Page 12 and 13: ABBREVIATIONSAEPAHDARIAR&RDCPIFDLEP
Page 14 and 15: 2 Catchment DescriptionThe Minnegan
Page 16: Figure 2.1CATCHMENT AND STUDY AREAM
Page 20 and 21: Illawarra Flood Study investigated
Page 23 and 24: The catchment was divided into 64 s
Page 25 and 26: To determine the peak flow from the
Page 27 and 28: 5 Hydraulic Modelling5.1 INTRODUCTI
Page 29: Table 5-1 MIKE 11 river branchesBra
Page 32 and 33: elationship that is determined from
Page 34 and 35: interaction between the numerical m
Page 36 and 37: significantly affected since the ex
Page 38 and 39: Since Points A and B correspond to
Page 41 and 42: 5.9 MODEL SENSITIVITY ANALYSESSensi
Page 43 and 44: 555053.09RANCHBY AVEElevation (m AH
Page 45 and 46: Creek and pass directly over Northc
Page 47: 5.12.4 Overland flow pathsAs discus
Page 50 and 51: Table 6-1 Peak design dischargesBra
Page 52 and 53: 55.050.0PMF Flood Level1% AEP Flood
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1% AEP velocities and flow distribu
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2524.52423.5RL (m)231% AEP velociti
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RL (m)3433.53332.53231.53130.53029.
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Figure 5-6.Table 6-2 Piped drainage
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that handrails above these structur
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flooding within the catchment are n
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35.01% AEP flood level - blocked ca
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20.01% AEP flood level - blocked ca
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Appendix AReferencesBureau of Meteo
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Appendix BGLOSSARYSV8507-DO-001 Rev
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hydraulicshydrographhydrologyMIKE 1
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Appendix CHistorical Flood DataRECO
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Time Rain(mm)Time Rain(mm)Time Rain
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Appendix DRAFTS HYDROLOGIC MODELSV8
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MHE .5359 .3573 12.10 12.10 5.000 1
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Mannings n 0.025 0.025 0.025 0.025
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Mannings n 0.025 0.025 0.025 0.025
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MH94 67.500 15.00 1.500 2.500 .0000
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Appendix EMIKE 11 HYDRAULIC MODELSV
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EXIST ; DENISE2 ; 0.005; 0.133; 100
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22.40; 88.40;23.30; 100.00;25.10; 1
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27.26; 128.96;27.43; 137.64;28.08;
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1; 1.250;0;1.800; 1.250; 0.000; 0.0
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PIPE_10; 0.108; EXIST;57.000; 159.0
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Appendix FMIKE11 RESULTS SUMMARYThi
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BRANCH CHAINAGE PMF 1% AEP 2% AEP 5
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Minnegang Creek Flood Study Report - Wollongong City Council

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