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

Flood Mitigation Investigation: Ashgrove Drive, Cooroy


Channel Geometry:
For the purpose of this investigation, the cross section of the diversion channel has been assumed
to be a trapezoid with a top width of 13m and a base width of 7m. The depth of cut required to
form the channel varies along the length of the channel and ranges between approximately 0.5m
and 2m.

The design bed level at the upstream extent of the channel is 111.5 m AHD. At the downstream
extent of the channel the bed level reduces to 110.5 m AHD.

The channel has a total length of nearly 200m. A long section of the channel concept design is
provided in Image 6-1 below. It also shows the natural surface profile (based on aerial LiDAR) and
the predicted peak flood surface profiles.




Image 6-1 Longitudinal Channel Profile


Bund Requirements:
The concept design allows for an earth bund along the northern side of the channel. It is
recommended that the height of this bund should be at least 300mm above the predicted 1% AEP
flood level, the profile of which is shown on Image 6-1.

If the existing Ashgrove Drive tributary holds environmental value downstream of the diversion,
then it may be worth incorporating a low flow diversion culvert that would allow environmental flows
to continue along the existing tributary. This will be difficult to achieve if the channel is not formed
using a bund.

There is potential to negate the need for this bund by forming the channel entirely in cut by lowering
the bed level of the channel however the effectiveness of the channel without a bund will need to
be assessed.
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J00158D1V1                                                                                           12