2015-03-31 I&S Committee - Item 4 - Attachment 1 - Flood Mitigation Investigation Ashgrove Dr Cooroy

Flood Mitigation Investigation: Ashgrove Drive, Cooroy


5 DISCUSSION ON THE PREFERRED OPTION

5.1   Peak Flood Level Impacts
The flood mapping figures provided in the appendices of this report demonstrate that the diversion
channel is effective at diverting floodwaters away from Ashgrove Drive and is effective for events
with magnitudes of up to the 0.05% AEP event (1 in 2000 chance every year).

The peak flood level impact figures provided in Appendix D represent the overall combined impact
of the mitigation works in conjunction with the Bruce Highway Upgrade.

The only adverse offsite flood impact which is predicted to be caused by the diversion channel is
a 25mm increase in the peak 1% AEP flood level over Cooroy Creek, west of Ashgrove Drive. This
impact occurs over the Greenwood Grove development site however it is in an area that is
designated for open space / drainage reserve so it is likely that agreement could be reached with
the developer (PEET) for these impacts.



5.2   GHD and DTMR Comments

DTMR and their engineers have reviewed the results and the modelling of the preferred mitigation
option and provided a number of comments. These are listed here below along with a HWMC
response:


Table 5-1 GHD Comments and HWMC Response
  DTMR / GHD Comment                                       HWMC Response

  The surface water levels are reduced in Ashgrove
                                                           Agreed
  Drive


                                                           Increased water levels in Cooroy Creek do not
                                                           adversely impact any existing homes or
                                                           infrastructure. The impacts are over land owned by
  The surface water levels are increased in Cooroy
                                                           a developer in areas that are to be designated as
  Creek downstream of the Bruce Highway (afflux
                                                           open space / drainage reserve however Council will
  increases from 10 mm to 25 mm)
                                                           need to seek permission for these impacts from the
                                                           developer (PEET) and ensure flood freeboard to
                                                           future lots will be achieved.


  Considerable amount of vegetation needs to be
  cleared to construct drain. Environmental impacts        Council to action.
  will need to be considered


                                                           There is potential to construct this drain entirely in
                                                           cut however this will mean an earthworks balance is
                                                           not achievable. Furthermore, it would no longer be
                                                           possible to direct environmental low flows along the
  Drain consists of a bund on the eastern side (i.e.
                                                           existing Ashgrove Drive tributary. Due to water
  drain is not all cut down below existing ground level)
                                                                                                           T


                                                           levels in Cooroy Creek it is expected that
                                                           construction of a bund will allow a better level of
                                                                                                 AF




                                                           flood immunity to Ashgrove Drive however a design
                                                           without a bund may still be able to provide a
                                                           significant improvement.
                                                                                            R
                                                                                      D




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