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

7.3      Flood storage & blockages
Within the Six Mile Creek catchment there are a number of hydraulic structures that affect the
performance of the flood plain and consequently the flooding extent experienced during flood
events. Hydraulic structures within the catchment can affect the flood waters by either presenting a
blockage or they can provide storage of flood waters such that they do not contribute to flooding at
its peak.

Several questions have been raised both within the CRG and subsequent to it about specific
structures, such as the SEQ water pipes that cross in the vicinity of the lake McDonald community,
and more generally about the consequence that these hydraulic structures have on the
performance of the flood plain.

Firstly, storage devices such as dams or retention ponds that are on private land either for water
storage or flood mitigation are assumed to be full when modelling catchment wide response to
flooding. Given that the volume currently within these devices for any given event is unknowable, a
conservative approach is taken by assuming these storage devices provide no attenuation to flood
waters.

This is in line with assuming that the ground soil condition is in a fully saturated state before any
design flood event takes place. By making these two assumptions, the flood model is assuming a
‘worst-case’ scenario for water storage when flooding events do occur.

While this assumption removes the benefit of flood storage within the model, new developments
within any catchment are required within the planning scheme to present non-worsening of water
run-off into the catchment. Typically, this is done by designing water retention basins that
effectively offsets natural water retention in the soil with man-made retention in basins.

Secondly, structures that are within the flow path of flood waters can block flood waters, which can
alter the peak flood levels both upstream and downstream of the structure. Six Mile Creek varies
between urbanised through Cooroy, Pomona and Cooran to moderately wooded in other areas. As
a consequence of this, vegetation and build up including fallen trees and sedimentation may block
structures within the flood water flow path.

The design blockage is the blockage condition that is most likely to occur during a given design
storm and needs to be an “average” of all potential blockage conditions to ensure that the
calculated design flood levels reflect the defined probability. This blockage condition is well defined
within Australian Runoff and Rainfall and the adopted blockage for this flood model are provided in
table 2 below.

Table 2: Hydraulic Blockage Factors

      Location         Width/Diameter      50% to 5% AEP       2% and 1% AEP            < 1% AEP
Cooroy Township              < 3m                25%                  50%                 100%
Cooroy Township              > 3m                0%                   10%                  20%
   External to               < 3m                50%                 100%                 100%
Cooroy Township
   External to               > 3m                10%                  20%                  20%
Cooroy Township




Lake McDonald Flooding Community Reference Group Report                                        Page 32