Harris Creek Flood Study Report

Background

A detailed flood study has been prepared for the Harris and Williams Creeks catchments to help Council update their existing flood information and help plan for, predict, and manage flood risk within the catchment.

The flood study covers parts of the suburbs of Holsworthy, Hammondville, Voyager Point and Pleasure Point and focused on local overland (stormwater) flooding as well as flooding from overtopping of Harris and Williams Creeks.

The study focused on the lower parts of the catchment which are urbanised and holds major thoroughfares such as Heathcote Road and the East Hills railway line. It documents flood behaviour across the catchment for a range of historic and ‘design’ floods. This includes information on flood discharges, levels, depths, and flow velocities.

Study Approach

A hydrologic computer model was developed to simulate the transformation of rainfall into runoff, as well as to predict flood characteristics such as water depth and velocity. The reliability of the results produced by the computer models were verified against historic floods in June 2016, February 2020, and July 2022.

The model verification involved comparing historic flood information provided by Council and the community (including flood photographs) to ensure the models were able to reliably represent past floods

The verified models were then used to define flood behaviour for a range of ‘design’ flood sizes, such as the 1% AEP (1 in 100 chance per year flood), which is used to inform planning and risk management decisions to protect the community.

Study Outcomes

The model development, verification and results from the study have been documented in a draft flood study report. The report includes detailed mapping for a range of possible flood events (including the Probable Maximum Flood, which is the largest flood that could occur). It also presents the Flood Risk Precincts and Flood Planning Area which are used to inform planning and development decisions.

The published result sets include a representation of the Heathcote Road upgrade. Other key outcomes of the study include:

• In frequent flood events up to the 100-year ARI (1%AEP), mainstream flooding is contained to the main creeks and does not impact any properties in the study area.
• Flood levels north of Heathcote Road are dominated by Georges River flooding
• Properties in urban areas are generally predicated to be free from overland flooding in events up to and including the 100-year ARI (1%AEP) flood.
• Heathcote Road is predicated to be free from inundation in events up to and including the 100-year ARI (1%AEP) once the current road upgrade work is completed.