2018-05-14 GC Agenda Attachment 1 to Item 6 Lake MacDonald Dr Community Reference Group Investigations Report

7. Flood Mitigation Scenarios for Lake
   MacDonald Drive
Throughout the CRG consultation a selection of flood mitigation options were suggested by CRG
stakeholders with the view of reducing the frequency and severity of flooding in the vicinity of
Highland Drive.

In general terms these options included works to reduce the occurrence of flood waters entering
the overland flow path which leads to Highland Drive and also works to improve the passage of
floodwaters through the main channel of Six Mile Creek.

WMA were engaged to utilise the upgraded flood model to test the effectiveness of each flood
mitigation suggestion (including combinations of these).

For clarity, these options have been analysed only for the purposes of assessing their potential
effectiveness on reducing flood levels and frequency. These preliminary assessments have been
undertaken for the purposes of responding to CRG stakeholder suggestions and should not be
interpreted as a commitment or willingness from Council to undertake any of the works described
in these options without further detailed investigations and appropriate environmental approvals.

The mitigation options modelled and an assessment of the general effectiveness of each option is
described in the following sub-sections. Each option is assessed for its benefit to the Highland
Drive community and impact on other areas. Each option was modelled for a 5% AEP (moderate)
flood event and 1% AEP (major) flood event.


7.1      Mitigation Options

Option 1: Low level earth bund from Liane Drive
The flood plain which extends to the west from the end of Liane Drive is crossed by a number of
shallow depressions which convey flood waters to Highland Drive via the overland flow path as
water levels rise upstream. Refer to Image 5-1. These depressions are expected to be engaged
relatively frequently. The objective of this option is to construct an earth bund to fill across the entry
to these low level depressions and reduce the frequency of minor flood flows entering the overland
flow path. Major flows would overtop the low level bund.

The flood model results demonstrate reductions in flood levels in the vicinity of Highland Drive of
between 100 to 250mm for a moderate flood event; however, no reduction for a major flood event.

Localised upstream increases in flood levels of between 500mm and 1m were evident immediately
upstream of the bund. Downstream along the main branch of Six Mile Creek flood levels increased
by approximately 100 to 250mm for the moderate flood event with no change for the major flood
event.

Refer to Figure 11 and Figure 12.




Lake McDonald Flooding Community Reference Group Report                                           Page 20