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

10. Conclusions
The CRG consultation process and associated investigations informed by the upgrade of the Six
Mile Creek flood model support the following conclusions.

The February 2012 flood event was a statistically exceptional event based upon the intensity and
duration of the rainfall which led to the event. The magnitude of this event is significantly greater
than the standard (1% AEP) design flood event typically adopted for land use planning and
infrastructure design.

Without the benefit of long-term water level records in the vicinity of Highland Drive or in the
upstream catchment it is not possible to explore changes in the frequency of runoff from rainfall
events in the catchment over periods of time. Long-term rainfall records do show seasonal and
decadal trends in rainfall volumes which may account for observations of increased runoff/flood
frequency over recent periods compared to periods of lower average rainfall volumes. Runoff from
the contributing catchment is generated from relatively short duration (i.e. hours) rainfall events.
The frequencies of these events are not readily discernible from the long-term data for Eumundi.

The potential effects on flooding conditions of increased development in the contributing catchment
has been investigated and determined that planned increases in urbanisation within the Cooroy
township urban footprint will not increase peak flood levels in the vicinity of Highland Drive, Lake
MacDonald.

A range of flood mitigation options have been tested to evaluate the potential effectiveness of each
option to reduce the frequency or severity of flooding in the vicinity of Highland Drive. Whilst some
options demonstrated modest reductions in localised peak flood levels no option was found to be
viable due to associated impacts on adjacent properties, cost benefit indicators and environmental
constraints.

An investigation found when outflow from Lake MacDonald Dam coincides with flood flows from
the main branch of Six Mile Creek a backwater effect can result which may elevate peak flood
levels and extend the duration of inundation in the northern end of Highland Drive.
Three new flood warning stations have recently been installed upstream of Highland Drive. Real-
time flood reading from these gauges can be accessed via BoM website and may provide warning
of impending flooding conditions. Records from these stations will also assist with improving
Council’s understanding of the rainfall and runoff relationship for this catchment over time.




Lake McDonald Flooding Community Reference Group Report                                        Page 39