Minnegang Creek Flood Study Report - Wollongong City Council

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Illawarra Flood Study investigated flooding of Lake Illawarra to evaluate peak floodlevels for a range of design rainfall events under existing catchment and lakeconditions. The results for flooding within Lake Illawarra at Griffin Bay have beenused to derive the tailwater levels for the Minnegang Creek catchment.Three surveys of the catchment were carried out as part of the flood study. G.A.Goodman Surveys Pty Ltd completed the first two of these surveys. The first survey,completed in July 2000, comprised three parts:• a detailed survey of the open channel sections of Minnegang Creek, with surveyedcross sections perpendicular to the direction of flow;• a detailed survey of Barina Park; and• identification of pit/culvert locations within the catchment.The second survey, completed in January 2001, provided more details of the pipeddrainage system, including conduit type and size, invert levels and surface levels ofthe manholes and pipe connectivity. Cross sections and weir profiles along theoverland flow paths were also taken to allow the hydraulic modelling of the catchmentto be completed in more detail.The final survey was completed by Council’s surveyors. It provided further groundlevel information for the area of the catchment downstream of Northcliffe Drive. Italso provided definition of the bridge over Minnegang Creek between Denise St andJane Ave. Finally, the survey provided spot ground levels at the locations of recordedhistorical flood levels.Appendix C contains information on the rain gauge locations near the MinnegangCreek catchment, as well as historical rainfall and flood height data that was used inthe preparation of the flood study.SV8507-D0-001 Rev 2 3-818/10/02
4 Hydrologic Modelling4.1 INTRODUCTIONThe first phase of modelling necessary for the determination of flooding behaviourwithin a catchment is the hydrologic component, which defines the relationshipbetween rainfall and runoff. Hydrologic modelling of the Minnegang Creekcatchment was undertaken to develop runoff hydrographs from design rainfall data forinput into the hydraulic model.This section discusses the methodology associated with the hydrologic modelling ofthe Minnegang Creek catchment using the computer-based RAFTS (Runoff Analysisand Flow Training Simulation) v5.1 hydrologic model (WP Software, 1996). Furtherdetails of the RAFTS model developed for the catchment are provided in Appendix D.4.2 MODEL SELECTIONRAFTS is a widely accepted hydrologic model for urban and rural catchments. Itcalculates design discharges for specified AEP events and storm durations byproducing and combining runoff hydrographs from each specified sub-catchment.RAFTS uses empirical rainfall runoff relationships to produce the runoff hydrographs.It is possible to extract local or total flow hydrographs for each sub-catchment. Theexported RAFTS hydrographs were routed through the catchment using the MIKE 11hydraulic model of the catchment, as discussed in Section 5.For most sub-catchments in the model, a local hydrograph was exported for eachevent. However, due to the layout of the hydraulic model, some sub-catchments withinRAFTS are not located on the branches of the hydraulic model. The local hydrographfor these sub-catchments could therefore not be entered directly into the hydraulicmodel. Hence for these sub-catchments the flows were routed in RAFTS using link lagtimes to a nearby sub-catchment, and a total hydrograph exported to MIKE 11.4.3 MODEL CONSTRUCTION4.3.1 Catchment and sub-catchment boundariesThe overall catchment boundary was determined from the map of the MinnegangCreek catchment provid ed by Council. With consideration of the requirements of thehydraulic model, sub-catchment boundaries were determined from the 1:2000 contourand cadastral maps and the 1982 and 1993 aerial photographs of the catchment. Theadopted catchment boundary and RAFTS sub-catchments are shown in Figure 4-1.SV8507-D0-001 Rev 2 4-918/10/02
Page 1 and 2: Minnegang CreekFlood Study
Page 3: © Kellogg Brown & Root Pty Ltd, 20
Page 6 and 7: SummaryThis report presents the det
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 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
Page 58 and 59: 1% AEP velocities and flow distribu
Page 60 and 61: 2524.52423.5RL (m)231% AEP velociti
Page 62 and 63: RL (m)3433.53332.53231.53130.53029.
Page 64 and 65: Figure 5-6.Table 6-2 Piped drainage
Page 66 and 67: that handrails above these structur
Page 68 and 69: 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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