The Wollongong City catchment area includes the suburbs of Wollongong (south of Crown Street), Coniston, and northern Port Kembla, and drains towards Wollongong Golf Course and JJ Kelly Park, ultimately discharging into Port Kembla Inner Harbour via the Gurungaty Waterway and Tom Thumb Lagoon. There have been several recorded instances of flood-producing storms in the catchment, including March 1978, March 1983, December 1990, March 1995, March 2011 and February 2012.
The key objective of the Flood Study was to develop a suitable hydraulic model that can be used as the basis for a Floodplain Risk Management Plan for the study area, which will assist Wollongong City Council to undertake flood-related planning decisions for existing and future developments. Previous hydraulic modelling of the study area was limited in extent, and did not estimate flood levels or flows in residential areas of the catchment.
Other goals of the study were:
- to determine the flood behaviour including design flood levels and velocities over the full range of flooding up to and including the PMF from storm runoff in the Wollongong City Catchment and from tidal influences;
- to provide a model that can establish the effects on flood behaviour of future development;
- to assess the sensitivity of flood behaviour to potential climate change effects such as increases in rainfall intensities and sea level rise; and
- to assess the provisional hydraulic categories and undertake mapping of provisional hazard, preliminary emergency response planning classifications, and preliminary flood planning extent areas.
The drainage characteristics of the catchment have been significantly altered as a result of urbanisation, particularly in the last 50 years. Most sections of creek and open channel have been covered in the course of development of the catchment, in most instances replaced with pipes. Isolated sections of open channel remain in the upper catchment, as well as within the golf course and JJ Kelly Park. Urbanisation has resulted in the following impacts:
- an increase in the proportion of paved area and consequent reduction in rainfall infiltration, with increases in runoff (both in terms of peak flows and volumes);
- the introduction of embankments (road and rail) and other impediments to flow (such as fences) along major drainage paths, creating trapped depressions where temporary ponding of flood waters can occur; and
- an increase in development density within lower lying areas of the catchment where flood issues are more problematic.