Minnegang Creek Flood Study Report - Wollongong City Council

More documents

Recommendations

Info

only affects land around specific railway stations and therefore the Minnegang Creekcatchment is not affected by the policy. The effectiveness of any on-site stormwaterdetention (OSD) policies, in relation to future development within the catchment, willbe assessed in the Floodplain Risk Management StudyMost areas within the catchment are fully developed within their respective land zoneentitlements. The large undeveloped area south east of Flagstaff Road and HilltopAvenue is zoned Low Density Residential - Zone 2(a) and hence could be developedin the future. In order to ascertain the impact on existing flooding within thecatchment, the RAFTS model was modified so that the sub-catchments in thiscurrently undeveloped area were considered fully developed. Residentialdevelopment, with an imperviousness of 40%, was assumed for these areas. Existingroads in these sub-catchments were retained with a percentage imperviousness of95%. The resulting modelled flows were then compared to the flows based on existingdevelopment conditions.For storm durations of 90 minutes and 2 hours, the developed conditions led to anincrease of flows in individual sub-catchments of less than 5%, whilst the increase inthe total flow in Minnegang Creek was less than 0.2%. It was therefore assumed thatthe effects of future development on flooding within the catchment are negligible andhence future development conditions were not included in the hydraulic modelling.Hydrographs from RAFTS, at critical locations along Minnegang Creek have beenincluded in Appendix D for both the existing and future development conditions.SV8507-D0-001 Rev 2 4-1418/10/02
5 Hydraulic Modelling5.1 INTRODUCTIONHydraulic modelling of the Minnegang Creek catchment was undertaken to translatethe runoff hydrographs generated by RAFTS into the flood levels, flow distributionsand flow velocities necessary to adequately describe the existing flooding behaviourwithin the catchment.This section discusses the methodology associated with the hydraulic modellingundertaken as part of the study, including model selection, model construction,selection of boundary conditions and verification of the model with historical floodevents.5.2 MODEL SELECTIONThe range of hydraulic models available can be broadly grouped into two categories:steady state and unsteady state. Steady state models, such as HEC-RAS (HydrologicEngineering Centre, 1997), perform hydraulic calculations based on a fixed startingwater level and assume that discharge through the model remains constant at all times.This invariably results in the coincidence of peak flows at a confluence, which islikely to overestimate the true discharge at that point. Unsteady hydraulic models, onthe other hand, allow for the use of time-varying boundary conditions, which forcesthe model to more closely simulate actual conditions in a catchment by accounting forthe timing of peak discharges contained within full runoff hydrographs.For this study, the MIKE 11 v3.20 (Danish Hydraulic Institute, 1998) one dimensionalimplicit finite difference model for unsteady flow computation was selected, primarilyfor its ability to account for the difference in timing of peak discharges in MinnegangCreek and its tributaries. A further advantage of MIKE 11 over a steady state model isits ability to model the effects of dynamic storage, which results in the attenuation of aflood hydrograph as it is routed along a river channel. Dynamic storage refers to thestorage volume inherent along any open channel reach, which is generally small wherechannel cross sections are steep and narrow but can be significant where a moreextensive channel floodplain exists. It is considered that dynamic storage will have asignificant role in the natural attenuation of flows in the lower part of the MinnegangCreek catchment, which further justifies the use of MIKE 11 for this particular study.To complement the hydraulic modelling of the overland flow paths within thecatchment with an assessment of the capacity of the exist ing piped drainage system,RatHGL v4.2 (WP Software, 1997) was used to model the piped drainage system.RatHGL is an urban stormwater piped network analysis and design program thatmodels peak flows, and can be linked with RAFTS for direct import of peak designSV8507-D0-001 Rev 2 5-1518/10/02
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 20 and 21: Illawarra Flood Study investigated
Page 23 and 24: The catchment was divided into 64 s
Page 25: To determine the peak flow from the
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
Page 70 and 71: 35.01% AEP flood level - blocked ca
Page 72 and 73: 20.01% AEP flood level - blocked ca
Page 74 and 75: Appendix AReferencesBureau of Meteo
Page 76 and 77:
Appendix BGLOSSARYSV8507-DO-001 Rev
Page 78 and 79:
hydraulicshydrographhydrologyMIKE 1
Page 80 and 81:
Appendix CHistorical Flood DataRECO
Page 82 and 83:
Time Rain(mm)Time Rain(mm)Time Rain
Page 85 and 86:
Appendix DRAFTS HYDROLOGIC MODELSV8
Page 87 and 88:
MHE .5359 .3573 12.10 12.10 5.000 1
Page 89 and 90:
Mannings n 0.025 0.025 0.025 0.025
Page 91 and 92:
Mannings n 0.025 0.025 0.025 0.025
Page 93 and 94:
MH94 67.500 15.00 1.500 2.500 .0000
Page 95 and 96:
Appendix EMIKE 11 HYDRAULIC MODELSV
Page 97 and 98:
EXIST ; DENISE2 ; 0.005; 0.133; 100
Page 99 and 100:
22.40; 88.40;23.30; 100.00;25.10; 1
Page 101 and 102:
27.26; 128.96;27.43; 137.64;28.08;
Page 103 and 104:
1; 1.250;0;1.800; 1.250; 0.000; 0.0
Page 105 and 106:
PIPE_10; 0.108; EXIST;57.000; 159.0
Page 107 and 108:
Appendix FMIKE11 RESULTS SUMMARYThi
Page 109 and 110:
BRANCH CHAINAGE PMF 1% AEP 2% AEP 5
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
show all

Minnegang Creek Flood Study Report - Wollongong City Council

Create successful ePaper yourself

Delete template?

Save as template?