Flood Investigation & Stormwater Quantity Management Plan

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

                                      Table 3.10: Mitigated Scenario Peak Flows
                                                                Peak Flow (m3/s)
                   ARI                                                     Difference (%) from
                                                    Mitigated WBNM
                                                                            Existing Scenario
                   2                                      0.91                      2
                   5                                      1.45                      2
                   10                                     1.83                      5
                   20                                     2.29                      4
                   50                                     2.59                      -1
                   100                                    2.90                      -6


The results confirm that the proposed detention storages are effective in mitigating the increase in peak flows from the
development. While an increase in peak flow still occurs for the 2 to 20 year ARI events, the increases are negligible
(no greater than 5%) and will not be perceivable. Therefore the negligible increases are acceptable. Table 3.11 below
summarises the peak water surface level (WSL) in the detention storages for each ARI. Refer to Appendix B for a
detailed output summary of the mitigated scenario WBNM.
                                   Table 3.11: Maximum Detention Storage WSL’s.
                                                              Peak WSL (m.AHD)
                   ARI
                                                 Dodonaea Close Basin    Twigrush Bio-Basin
                   2                                    10.81                   8.95
                   5                                    11.32                   9.16
                   10                                   11.59                   9.25
                   20                                   11.90                   9.35
                   50                                   12.04                   9.39
                   100                                  12.11                   9.44

For the Dodonaea Close Basin the road embankment (minimum crest level 11.93m.AHD) is overtopped during the 50
and 100 year ARI peak flows. The depth of overtopping during the 100 year ARI event (180mm) is less than the
maximum 250mm allowed for as specified in Table 7.03.1 of QUDM (2007) to maintain suitable access across the
environmental corridor. The maximum depth of ponding within the Twrigrush Bio-Basin is 1.94m. This is 160mm
below the top of the basin embankment however more then 300mm of freeboard will be provided between the
maximum WSL and proposed lots 136 and 146 adjacent to the basin. Overtopping of the overflow weir has been
limited to events above and including the 50 year ARI, with depth over the weir only 140mm during the 100 year ARI.

3.6      Peak Flows for Hydraulic Analysis
For hydraulic modelling purposes (refer to Section 4) peak flows within the environmental corridor downstream of the
site boundary were determined. The mitigated scenario WBNM was further configured with the areas and fraction
impervious values of the external sub-catchments D1, D2 and D3 and specified in Table 3.3. Peak 100 year ARI flows
determined by the model at each specific location along the environmental corridor are presented below in Table 3.12.
                         Table 3.12: Peak 100 Year ARI Flows within Environmental Corridor
             Location      Peak Flow (m3/s)      HEC RAS Cross Section        Reach Position
             A                   1.07                    348                  Top of environmental corridor
             B                   1.38                    274                  U/S Dodonaea Road
             C                   3.11                    189                  U/S Shire Drive
             D                   5.18                     60                  Adjacent Detention Basin


B09182.W-02A                                                                                                       Page 8