Sugarloaf Creek Flood Study

The study objective was to define flood behaviour on Sugarloaf Creek in terms of water levels,
flows and velocities for design floods ranging between 5 and 100 year ARI, as well as for the
Probable Maximum Flood (PMF). Figure 1.1 shows the Sugarloaf Creek catchment and its
stormwater system. The flood study investigation involved the following activities:
? The collection of flood related data. A Community Newsletter/Questionnaire introducing
the study objectives and seeking information on historic flood patterns was forwarded to
residents in the floodplain. Rainfall data at several daily gauges were also collected. A
previous flood study of the Sugarloaf Creek catchment by Lyall and Macoun Consulting
Engineers (LMCE, 1988) also provided information on historic flooding. Three significant
storms which had occurred over the past 25 years were identified (5 August 1986, 30
April 1988 and 10 April 1998) and their rainfalls used to test the flood models developed
for the study.

? The hydrologic modelling of the catchment of Sugarloaf Creek to determine discharge
hydrographs. All reaches of the piped drainage system of diameter 450 mm or larger were
modelled.

? Application of the discharge hydrographs to a hydraulic model of the main arm of the
creek and its overland flow paths. The model extended from the headwaters of the
catchment (to the west of Bales Park) to its outfall to Sugarloaf Bay downstream of the
Eastern Valley Way.

? Presentation of study results as water surface profiles, as well as diagrams showing
indicative extents of inundation, provisional flood hazard and the hydraulic categorisation
of the floodplain into floodway and flood fringe areas.

? Sensitivity studies to assess the effects on model results resulting from uncertainties in
model parameters such as hydraulic roughness of the floodplain, the effects of partial
blockage of the piped drainage system and the effects on flooding patterns resulting from
future climate change.

The hydrologic modelling approach was based on the DRAINS rainfall-runoff software. DRAINS
derived discharge hydrographs resulting from historic storms for each model sub-catchment area,
which were then applied to the hydraulic model to demonstrate that the models reproduced
observed flood behaviour. The TUFLOW two-dimensional modelling system was adopted for the
hydraulic analysis. Appendix A describes the results of testing the models.
Both the DRAINS and TUFLOW models included the piped drainage system and routed the flows
to the catchment outlet. Hence both models were able to provide independent estimates of the
relative magnitudes of piped and overland flows. However, TUFLOW being primarily a hydraulic
model (as opposed to DRAINS which is hydrologically based) was used to route flows over the
land surface and determine peak flood levels and flow velocities, as well as indicative extents and
depths of inundation.

After testing the models for the historic floods, design storm rainfalls ranging between 5 and 100
year ARI were derived using procedures set out in Australian Rainfall and Runoff (ARR, 2001)
and applied to the DRAINS model to determine discharge hydrographs. The PMF was also
modelled. Flooding patterns derived by TUFLOW for the design flood events are described in
Chapter 6 of the report, with exhibits presented in <strong>Volume 2</strong>.

Tags: Castlecrag, Sugarloaf Creek, Willoughby, hydraulic model, hydrologic model
Organization: Willoughby City Council