MCU14/0056 Stormwater Quantity Management Plan Att D Information response

5.0      HYDRAULIC CALCULATIONS
 5.1     Existing Scenario Model Setup
       To assess the existing inundation levels through the site, a HECRAS hydraulic model was
       set up using ALS survey data obtained from the Department of Natural Resources and
       Mines (DNRM). This ALS data matched the survey data presented in Figure 4, Appendix A.
       The model consists of a single branch through the waterway on the eastern side of the site.
       The locations of the cross sections used in the HECRAS model for the existing and
       developed scenarios are presented in Figure 4, Appendix A. Two 3.2x2.45m box culverts
       which convey flows under Myall Street were included in the model. The flow through the
       waterway is sub-critical and is therefore governed by the downstream boundary condition. A
       ground slope of 1 in 5000 was used to represent the downstream boundary condition.

       The inflow for the model was determined using the URBS model. Section 4.0 demonstrates
       that a peak discharge of 44.04 m3/s flows through the waterway at the site during a Q100
       event. This peak discharge was input at XS 70 in the HECRAS model.

       As a conservative modelling approach, flow from the external catchment was only permitted
       to flow in the western channel of the waterway through the site.

       The following table presents the model parameters used in the existing HECRAS model:

       Table 5.1 – HECRAS Manning’s Roughness Coefficients
                    Land use                             Mannings’s n value
                   Dense grass                                   0.04
                      Roads                                      0.02
         Waterway and highly vegetated
                                                                 0.08
                    areas

       An inundation plan for the existing site conditions is presented in Figure 7, Appendix A.




                                       J4279 Report 1.0.doc
                                         14 January 2015                                    Page 10