2018-05-08 Services & Organisation Committee Agenda

SERVICES & ORGANISATION COMMITTEE MEETING AGENDA                                      8 MAY 2018


As part of this investigation, an analysis of historical flood records was undertaken to establish
the number of events in a given period that was likely to cause flooding. The results showed that
between 2000 and 2009 there was a strong pattern of drought conditions with few rainfall events
likely to result in flooding. However, the period 2010 to 2017 showed a strong pattern of high
rainfall, with 4 times as many rainfall events likely to cause flooding observed. This second time
period included the February 2012 flood that recorded an estimated recurrence interval in the
magnitude of 1 in 500 years.

This report was peer reviewed at every stage of the project by HydarLogic and presented to the
community during a Community Reference group meeting. The community were also provided
material, available on Council’s website, to assist in the community member’s preparedness in
the event of disasters.

3.   New Flood Study Outcomes

The new flood study for Six Mile Creek has integrated the various hinterland sub-catchments of
Cooroy, Cooroora and Six Mile Creek with inputs from Lake Macdonald and various smaller
tributaries into a consistent and technologically superior model. The new model has been
developed using TUFLOW modelling software which is consistent with the approach taken for the
Noosa River flood model and uses a 5m resolution within urban areas and a 10m resolution in
the Six Mile Creek main branch.

The new model is an improvement over the existing modelling for the hinterland catchments in
the following areas:
 Integration: by combining all of the existing catchments in the hinterland into a single model
    we have removed the uncertainty of application, particularly at intersections of the models.
 New data: the new model uses an updated 2016 LiDar terrain model for its base terrain
    elevations which has significant improvements in resolution and accuracy over existing terrain
    models.
 Updated terrain: the new model incorporates the changes in terrain that has occurred
    between modelling projects including the Bruce Highway upgrade and upgraded drainage
    features in Garnet Street and Ashgrove Drive.
 Latest modelling software: improved computing power and modelling software has allowed
    for more accurate flood routing and higher resolution modelling to be completed.

The new model provides Council with an improved tool to:
 Understand the impacts of flood design events ranging from a 50% AEP event (1 in 2 years)
   up to a probable maximum flood (PMF) event, including a design event that incorporates
   climate change effects.
 Undertake flooding investigations such as the investigation of the Lake Macdonald
   community (below) or the upgrading of the Lake Macdonald Dam.
 Have confidence that it is uniformly calibrated and applied across the hinterland.
 Apply with the same tools and confidence as the Noosa River Flood model.

4.   Application of Flood Study

Within the localised area of Lake Macdonald, although no house has been inundated from the
previous flood events, the investigation looked at various flood mitigation options proposed by the
community; including a low and high levee bank, straightening of one and two sections of the
preceding water course and a combination of these. The results showed that the straightening
option provided no benefit and the levee banks provided some benefit that was considerably out-
weighed by worsening effects in surrounding parts of the catchment. As a result the report could
not recommend any mitigation options for this community.


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