USC FloodStudyUpdate Report - Flipbook - Page 4
EXECUTIVE SUMMARY
This flood study provides information about the existing flood risk in the upper South Creek
catchment. The study involved development of computer models of the floodplain. The models
were used to estimate flood levels and map flood extents for a range of flood sizes, from smaller
relatively frequent floods to extreme but very rare floods. The flood modelling tools and spatial
mapping from this study can be used by Council for decision-making about land-use planning,
determining flood impacts of proposed developments, assessing risk for future development due
to flooding and to assess the effectiveness of potential measures to reduce flood risk in the
future.
This study supersedes a previous flood study of the catchment published by WMAwater in 2012.
Due to the rapid development of the catchment, there is a need to update the previous flood
study. Land use in the catchment is in a state of transition from the historic rural use to urban
development. Urban development is generally undertaken on a precinct scale, with a number of
precincts already developed or currently being constructed in the upper catchment, including
Oran Park, Gregory Hills, Gledswood Hills and Emerald Hills. There is planned development for
much of the remaining area of the catchment. There is a need to develop a catchment-wide
flood model that includes the recent precinct developments, and provide an updated regional
modelling approach to assess and control cumulative impacts of multiple precinct developments.
In addition, the previous study utilised Australian Rainfall and Runoff (ARR) 1987 guidelines.
Recently, these guidelines were updated to ARR 2016, and finalised in 2019. With an additional
30 years of data and improvements in computing technology, ARR 2019 presents a significant
update to how floods are estimated.
This study developed an XP-RAFTS hydrologic model which simulates the runoff for individual
sub-catchments for a given rainfall event. These flow hydrographs were then used as inputs into
a detailed TUFLOW hydraulic model which was also developed for this study. The hydraulic
model simulates the flood behaviour. The models represent catchments conditions as at
November 2018.
The upper South Creek study area has been subject to flooding in the past, with notable events
occurring in April 1988, June 1991, February 1992, June 2016 and February 2020. The June
2016 event was used for calibration and the February 2020 event, which occurred during the
course of this study, was used as a verification event. The calibration to the June 2016 event
was considered to be good, while the verification to the February 2020 event was considered to
be reasonable.
Design flood events were simulated using ARR 2019 guidelines. Flood events from the 20%
Annual Exceedance Probability (AEP) through to the Probable Maximum Flood (PMF) event
were simulated. A range of flood outputs were produced, including peak flood depth and levels,
peak flood velocities, hydraulic hazard, hydraulic categories, flood emergency response
classifications, flood risk precincts and the flood planning area. A sensitivity analysis was also
conducted on the adopted model parameters, as well as comparisons to the previous study
results and previous ARR 1987 guidelines.
In the 50% AEP event, flow is generally shallow (