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

5. Six Mile Creek Flood Study
A flood study is an investigation into the flooding characteristics of a location. A modern flood study
will typically utilise a flood model which is a computer simulation of rainfall and runoff in a
catchment and its waterways. Flood models require information to represent the physical
characteristics of the contributing catchments and the waterways which convey the runoff.

Most flood modelling is a two-step process. The first step is to develop a model which simulates
the runoff generated from rainfall over a catchment area. This step is referred to as hydrology. The
second step is then to take this runoff and to simulate how it might travel down the main
waterways. This second step is referred to as hydraulics. Both hydrology and hydraulics relate to
the study of water.

Flood studies can be used for land use planning; infrastructure design, development assessment
and disaster management.


5.1      Previous studies
Over time several flood studies had previously been undertaken to inform planning and design
decisions for the floodplains of Cooroy Creek, Six Mile Creek and Cooroora Creek (Pomona).
Whilst each of these locations are connected within the catchment the models were prepared at
different times using different software and inconsistent parameters.

In terms of available flood studies, Highland Drive is awkwardly positioned between the lower
extent of previous flood models prepared for Cooroy Creek and the upper extent of flood models
prepared for Six Mile Creek. This created problems for confidently interpreting flooding conditions
at the overlap of these two areas as flood models are constrained by the nearby boundaries
conditions applied at the top and bottom of the models.


5.2      Six Mile Creek Flood Study Upgrade
The Six Mile Creek flood study upgrade has improved the continuity of modelling between the
Cooroy Creek system and main branch of Six Mile Creek. The upgrade process has delivered a
contemporary suite of connected flood models for the entire catchment (to downstream of Cooran).
The modelling platform is a two dimensional flood model (TUFLOW) and utilises detailed ground
terrain information obtained from aerial laser scanning (sometimes referred to a LiDAR).

The upgraded flood model has been calibrated to the February 2012 event. The calibration
process has referred to rainfall and peak water records and the observations and experiences
provided by CRG stakeholders.

To ensure planning and infrastructure designs are consistent throughout regions standardised
design rainfall patterns are applied to flood models to generate design flood results.


5.3      Design Flood Events
Design rainfall events are not real rainfall events. Design rainfall events can vary in magnitude to
reflect different sized flood events which may need to be considered for different forms of planning
and infrastructure design.

Design flood events are often described with reference to the statistical frequency that they are
expected to occur. This is sometimes referred to as the Average Recurrence Interval (ARI). An
example application of this is the minimum floor levels of residential homes are required to be


Lake McDonald Flooding Community Reference Group Report                                         Page 9