Further information re Request to Change Approval MCU 132003.221132.8

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Basin BlA and B1C alone do not have adequate capacity to replace the Dodonaea basin. The reason

behind OPTION 2 failing to meet requirements is that Basins B1A and B1C (as stated previously) only

mitigate their relevant catchments. The Dodonaea basin previously mitigated stormwater not only
from   urban   development catchments of               B1A    and   B1C, but   also    catchments      B1B     (upper
environmental corridor catchment) and catchment A (external catchment upstream).


OPTION 3: Strategy incorporates Basins 81A and B1e, Original Twigrush Basin and an optimized

Dodonaea Basin.


OPTION 3 offers a strategy with the Twigrush basin at its original size as well as Dodonaea, B1A and

BlC basins.


The configuration of Dodonaea Basin outlet also updated to achieve non-worsening objectives. The

configuration of the outlet for the Dodonaea Basin to achieve this objective is presented in Table 3
below. The Twigrush outlet structure has been kept to its original configuration and that for this

option runoff from Sub Catchment 2B2 bypasses the Twigrush Basin.

                                  Table 3: Options 3 Dodonaea Basin Configuration


       OPTION 3                                Dodonaea Basin                       Twigrush Basin
       Upstream Catchment                           A,B1B                             C1, C2 & C3
       Basin Invert RL (m.AH D)                       9                                   7.5

       Basin Top RL (m.AHD)                         12.25                                 9.6

                                                                                1 x 525mm dia RCP
       Outlet Structure                    1 x 450mm dia RCP
                                                                                2 x 225mm dia RCP




The results of the analysis indicate that the outlet structure configuration as shown above (Table 3)

achieves the non-worsening objective. Results are shown in Table 4 below.


                                               Table 4: Options 3 Results


                                  Existing WBNM                 Developed WBNM           Ch;mge (%)from Existing
ARI
                              (Peak Flow m3Is)                  (Peak Flow m3/s)                    Scenario

2                                      0.89                             0.83                         -6.7%

5                                      1.42                             1.39                         -2.2%

10                                      1.75                            1.75                           -




20                                     2.20                             2.19                         -0.3%

50                                      2.61                            2.53                         -3.2%

100                                     3.07                            2.89                         -5.9%




Results show that there are insignificant changes in             peak water surface levels (WSL’s) within the
Dodonaea basin as previously specified in the previous report B09182.W-02A. Results are presented

in the Table 5 below. Changes in Volume in the Dodonaea basin are presented in Table 6 below.




B09182.W _Options_2_3_40NLY_130314.docx                                                                         Page 2