Appendix P Drainage, Flooding, Stormwater Management Part 5

Flooding, Drainage and Stormwater Management
Sanctuary Villages
4. Hydraulic modelling
The HEC-RAS (Version 3.1.2, April 2004) computer model, developed by the Hydrologic
Engineering Centre, U.S. Army Corps of Engineers was used to determine the 100 year
water profile within Congewai and Quorrobolong Creeks south of the development precincts.
The programme uses the standard step method to determine water surface profiles for
gradually varied flow.
4.1 Modelling parameters
The hydraulic analysis has been undertaken as a steady state simulation. A brief discussion
on model parameters is provided below. Storage within Ellalong Lagoon has been
accounted for in the hydrologic model described in Section 3.
4.1.1 Flow regime
A mixed flow regime analysis was undertaken for the analysed portion of Congewai and
Quorrobolong Creeks situated within the study area.
4.1.2 Boundary conditions
For mixed flow, both upstream and downstream boundary conditions are required for model
simulation. The upstream boundary conditions assumed normal flow with a creek slope of
0.003. The sensitivity of the model to this slope has not been examined but is likely to be
negligible given that flow is subcritical.
The downstream boundary condition comprised a fixed water level that was obtained from
the Wollombi Valley Flood Study prepared by Patterson Britton and Partners. A level of
110.1m AHD, which is situated approximately 150m upstream of Millfield Bridge, was
adopted. The sensitivity of the model to this starting level has not been examined.
4.1.3 Peak flows
A summary of 100 year ARI peak flow rates used within the hydraulic model are provided in
Table 4-1. These flows were taken from the XP-RAFTS model for the 2160 minute critical
storm duration. Note that the existing and developed peak flow rate estimates for the critical
duration storm event are identical.
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