Summary

The model update of Mullet Creek has been prepared for Wollongong City Council (Council) to define the existing flood behaviour in the Mullet Creek. The model update was required due to a number of recent changes within the catchment and to take advantage of innovations and improvements in computer flood modelling. The model update provides a holistic assessment of historic, current and future flood risk and establishes the basis for the subsequent Floodplain Risk Management Study and Plan.

Flood risk has been assessed through the establishment of appropriate numerical models. The study has produced information on flood flows, velocities, levels and extents for a range of flood event magnitudes under existing catchment and floodplain conditions and considering the influence of potential climate change on future flood behaviour. Specifically, the study incorporates:

• Compilation and review of existing information pertinent to the study;

• Restructuring of the hydrologic and hydraulic flood models developed for the 2010 flood study and 2011 model extension, and subsequent calibration;

• Development of a baseline flood model incorporating recent changes to the Mullet Creek catchment;

• Determination of design flood conditions for a range of design events - including the 20% Annual Exceedance Probability (AEP), 10% AEP, 5% AEP, 2% AEP, 1% AEP, 0.2% AEP and Probable Maximum Flood (PMF);

• Examination of potential impact of climate change for the 1% AEP design event (20% increase in rainfall); and

• Presentation of study methodology, results and findings in a concise report and flood mapping compendium.

The Mullet Creek study area is located to the west of Lake Illawarra. Mullet creek contains a number of ephemeral tributaries including; Dapto Creek, Reed Creek and Robins Creek. The creeks drain a combined catchment area of approximately 75 km2 and extend from the Illawarra Escarpment in the west to Mullet Creek adjacent to the Princes Highway, where they discharge into Lake Illawarra.

The headwaters of Mullet creek originate at the Illawarra escarpment, at an elevation of approximately 600m AHD, before flowing a distance of some 18 kilometres to its outlet at Lake Illawarra. The headwaters are very steep – the elevation falls from a maximum of 600m AHD to 50m AHD in the foothills only 2.5 kilometres further east. These steep catchment conditions provide for dynamic flooding and geomorphic conditions in the catchment. The floodplain near Lake Illawarra is broad and flat, extending up to 2.8 kilometres in width.