Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
Proposed Residential Development at Doonella - Stage C

Appendix C for extract). At the upstream extents of the model the boundary conditions were entered as ‘Normal
Depth’ with a slope of 6.0%.
4.2.4    Hydraulic Structures
The outlet pipe and road crown levels determined from modelling the Dodonaea Close detention basin have been
adopted for modelling the crossing within HEC RAS. Refer to Table 3.9 for details of the configuration adopted.
The crossing over the environmental corridor and culverts under Shire Drive were entered into the model using
information adopted from survey.
Refer to Drawing No. B09182.W-SK02 in Appendix D for further details of the hydraulic structures.
4.2.5    Peak Flows
The peak flows for the 100 year ARI event from Table 3.12 were entered into the model.
4.2.6    Results
The peak 100 year ARI water surface levels and comparison to proposed minimum lot levels are presented below in
Table 4.2. HEC-RAS output results are presented in Appendix C. The water surface levels along the environmental
corridor were used to generate the flood extent lines shown on Drawing No. B09182.W-SK02 in Appendix D.
                                                Table 4.2: Peak 100 Year ARI WSL
          HEC-RAS                   100 Year ARI            Proposed Minimum             Location
          Cross Section             WSL (m.AHD)             Lot Level (m.AHD)
          348                          14.29                      17.13                  U/S end of corridor
          308                          12.32                      16.87
          274                          12.03                      15.08
          252                          12.03                       NA                    Upper Twigrush Court intersection
          234                          12.03                      13.68                  U/S of Dodonaea Close crossing
          213                           8.92                      12.20                  D/S of Dodonaea Close crossing
          198                           8.66                      11.85
          166                           7.81                      11.10
          141                           7.77                      10.40
          121                           7.75                      10.25
          101                           7.73                       9.95
          90                            7.73                       9.95                  U/S of Shire Drive crossing
          60                             7.6                      10.42                  D/S end of model
                   Please note adjoining subdivision have less freeboard (existing development) but freeboard still >300mm.

As the above results show the proposed lot levels adjacent to the environmental corridor have more than adequate
freeboard (minimum of 1.64m adjacent to Lots 155 and 156) in accordance with QUDM (2007). Flood extents for the
peak 100 year ARI flows are presented on Drawing No. B09182.W-SK02 in Appendix D.
As determined from the hydrological analysis, overtopping of Dodonaea Drive in the 100 year ARI event occurs. The
HEC RAS model results indicate that the level of ponding over the road crown as 100mm (WSL 12.03m.AHD). With
the velocity of overtopping flows determined to be less than 0.4m/s, the Depth-Velocity Product (D.V) is negligible.
Therefore the modelling results indicate Dodonaea Close will be trafficable during the 100 year ARI.
4.2.7    Sensitivity Check
A sensitivity check was undertaken for the channel using a Manning’s value 0.15. Results of this analysis indicate that
flood levels are only slightly higher (maximum difference 120mm at Cross Section166) than those modelled using the
Manning’s values adopted in Table 4.1. Therefore the proposed lot levels provided are adequate. Refer to
Appendix C for HEC RAS output results for the sensitivity check



B09182.W-02A                                                                                                                  Page 10