The Woy Woy Peninsula urban area is bounded by Brisbane Water to the north and east, Broken Bay to the south, and Brisbane Water National Park to the West. The study area is approximately 13km2 as shown in Figure 1. Much of the area is prone to nuisance flooding, especially from long duration rainfall events. Flooding occurs in road reserves and in private property, where it remains until it infiltrates or evaporates. Generally this nuisance flooding may remain for a couple of days.

However, during very wet periods the groundwater table can rise such that flooding remains for several weeks.

This flood study was undertaken to determine the existing flood behaviour of flood prone areas for a range of flood risk levels from the 50% Annual Exceedance Probability (AEP) event through to the Probable Maximum Flood (PMF). Flood behaviour was determined for flood prone areas using mathematical modelling tools developed specifically for the study. Catchment groundwater behaviour, runoff generation, overland flow, channel flow and pipe flow were calculated using the MIKE SHE modelling software. The model allows a distributed, physically based approach to rainfall runoff, with rainfall time series applied directly to a two dimensional grid representation of the catchment surface.

The model was calibrated to the 1988 storm event using flood depths obtained from community consultation and council maps indicating areas historically prone to flooding.

Design rainfall intensities and temporal patterns for the required range of flood risk events were obtained and applied to the model. The flood model predictions indicate that in many areas of the catchment the groundwater table rises to the ground surface, preventing infiltration of rainfall and creating significant areas of ponded water. The existing flow channels and stormwater drainage conduits can be effective at removing this water if the ponded areas are connected to the drainage system and the drainage system is operating effectively.

Plans showing flood depth, flood hazard classification and other results from the design flood events are presented in Figure 13 to Figure 55.

The developed flood modelling tools and reported flood behaviour for existing catchment conditions can now be used as the basis for developing a Floodplain Risk Management Plan for flood prone land in the catchment. The hydraulic model developed for the flood study may generally be used to assess the hydraulic impact of any proposed structural flood mitigation works on flood behaviour.