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

Flood Mitigation Investigation: Ashgrove Drive, Cooroy


4 FLOOD MITIGATION ANALYSIS

4.1   Methodology
Flood mitigation analysis has been undertaking utilising the hydraulic and hydrologic models setup
by GHD for modelling of the Bruce Highway Upgrade. These models have been provided for the
existing case and the highway upgrade case.

The flood mitigation options have been assessed by incorporating them into the highway upgrade
case flood model. This enables flood impact mapping to be produced showing the overall
combined impact of the Bruce highway upgrade in conjunction with the flood mitigation works. This
approach has been chosen based upon the assumption that the mitigation works will be
constructed at the same time as the highway upgrade.

Analysis of various flood mitigation options has been carried out and from these, a preferred option
has been selected in consultation with Noosa Council.

4.2   Base Case
The base case models used for the current project are a copy of existing case models supplied by
GHD. Models has been re-established and run the 10%, 1% and 0.05% AEP design events as
detailed in Section 3.

The hydrologic model has been run for the full range of standard design storm event durations and
the critical event duration has been used for the hydraulic analysis. The critical event durations for
the 10%, 1% and 0.05% AEP are 24hr, 24hr and 2hr respectively.

The base case model results are provided in Appendix A.

4.3   Options Analysis
The following flood mitigation options have been assessed for this project:


       Option 1 – Diversion Channel Parallel to Bruce Highway (Preferred Option)

This is the preferred flood mitigation option coming out of this project. A concept design drawing
is provided in Appendix B. The objective of this option is to collect floodwaters and convey in north-
west direction, parallel to the Bruce Highway towards Cooroy Creek.

In the low lying ground situated in the eastern area of these works, the diversion channel has been
formed with an earth embankment running along the eastern side of the channel. In the western
region of these works the channel can be constructed in cut and it is likely that a bund will not be
required to contain flows (subject to survey of ground and final design of channel).

The basic line-work applied in the TUFLOW flood modelling is shown on Image 4-1 below along
with the mitigated case 1% AEP flooding characteristics. This image shows how the diversion
channel effectively captures flood waters, diverts them away from Ashgrove Avenue and directs
flow to Cooroy Creek

Peak flood depth and flow velocity vector mapping is provided in Appendix C for this preferred
mitigation option. Peak flood level impacts maps are provide in Appendix D. These reflect the
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overall combined impact of the mitigation works in conjunction with the Bruce Highway Upgrade.
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