Objectives

Sutherland Shire Council engaged Webb, McKeown & Associates to prepare the Kurnell Township Flood Study, utilising current technology and data. The information and results obtained from the study will be used in the preparation of the Floodplain Risk Management Study and Plan. The specific objectives of the Kurnell Township Flood Study are to:

• Develop suitable hydrologic and hydraulic models to define flood behaviour, and that can be used in the subsequent Floodplain Risk Management Study and Plan.
• Use these models to quantify flood levels, velocities, flows and flood extents for the 20%, 5% and 1% Annual Exceedence Probability (AEP) design storm events and the Probable Maximum Flood (PMF). The impact of both surface runoff and tidal inundation will be considered.
• Assess the provisional hydraulic categories and undertake provisional flood hazard mapping in accordance with the NSW Floodplain Development Manual 2005 (Reference 2).

Outline

This report details the methodology, results and findings of the Kurnell Township Flood Study, the key elements of which include:

• a background to the study area (Chapter 2),
• a summary of available flood related data (Chapter 3),
• the development and validation of the hydrologic and hydraulic models (Chapters 4 to 7),
• the definition of the design flood behaviour for existing conditions through the analysis and interpretation of model results (Chapters 8 and 9), and
• a sensitivity analysis of key model parameters and an analysis of the potential impacts of climate change (Chapters 10 and 11).

A glossary of flood related terms is provided in Appendix A.

CONCLUSIONS

Detailed numerical models to quantify the hydrology and hydraulics of the Kurnell catchment have been established, making best use of the data currently available. These models have been used to define the design flood behaviour for existing conditions.

Results indicate that flooding in Kurnell is currently dominated by relatively shallow, slow moving water throughout the majority of the study area. However, the extent of flooding, the ponding times, and the lack of clear evacuation routes have the potential to increase the flood risk. Whilst rainfall is currently the primary cause of flooding for most areas within Kurnell, the impacts of climate change has the potential to increase the dominance of tidal flooding. Hence future management strategies should consider the effects of both rainfall and tidal flooding.

The current models are significantly more detailed and refined compared with any previous studies or investigations. Given the level of detail used in the present study and the ability of the current models to better represent dynamic flow and the complex overland flow paths through much of the floodplain, means that the results can be interpreted with a greater level of confidence compared with any previous estimates of flood behaviour.

Importantly, the models developed for the current study are suitable for use in a subsequent Floodplain Risk Management Study and/or other assessments of redevelopment options within the catchment. However, it should be noted that any specific site investigations should undertake more detailed flood analyses to gain sufficient information specific to the area. The current results are intended for use at a catchment scale rather than an individual site basis.