Further information re Request to Change Approval MCU 132003.221132.8

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       New Peak Flow Management Strategy (Option 3)
       The Option 3 strategy to mitigated peak flow increase from the
                                                                              development involves incorporating
       changes     to the   configuration of the previously proposed Twigrush and Dodonaea Basins, with two new
       Detention Basins designated as Basins B1A and B1C.


       Changes to the Twigrush Basin and provision of two new basins allows for the reconfiguration of the
       Dodonaea Basin outlet to incorporate a fauna crossing, with the objective to also reduce
                                                                                                peak flood
       levels and times of inundation within this basin.


       The configuration of all the detention basins within tIle
                                                                 strategy to achieve the objectives is presented
       in Table 1 below. Note that for Option 3 Sub Catchment 2B2 now bypasses the                  Basin. Refer
                                                                          Twigrush
       to   Drawing No. B09182.W-SK01B in Attachment 2 for the updated Stormwater Management
       Catchment Plan.


                                                    Table 1: Option 3 Basin Configurations




                                                       1,080                             1,100                                        1,210

                                                        1.51                             1.81                                         2.11


                                                       9.31                              131                                          7.61                                9

            Basin Top RL (m.AHD)                       10.8                              14.8                                         9.7                               12.25

            Oudet Structure                      1 x 225 dia Rep                  1 x 225 dia Rep                 1 x 375mm dia Rep                           1 x 450mm dia Rep
                                   1.   A depth of 300mm is provided for                             treatment below the detention bottom.




       The previous Watershed Bounded Network Model (WBNM) for the site has been updated to

       incorporate the Option 3 strategy. Assumptions and methodology are consistent with previous
       investigations and analysis for the site. Details of the updated developed sub-catchment areas and
       calculated fraction impervious values are presented in Table 2 below. Refer to Drawing
       No. B09182.W-SKOl IssueB for the location and delineation of each sub catchment area.


                                            Table 2: Developed Scenario Sub-Catchment Details




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