South Creek is a 7.3 km2 catchment which drains to the southern shore of Narrabeen Lagoon through Cromer Golf Course (Figures 1, 2 and 3). The catchment area is predominantly occupied by urban development (59%) including both residential and commercial/light industrial development, with the remainder comprised of open space and forested areas (41%). The catchment has two main tributary branches. South Creek is the major watercourse and Wheeler Creek is the main tributary draining a significant proportion of the western catchment.

In view of the increasing urban development and the need to accurately define the nature and extent of the flood problem, Warringah Council engaged Webb, McKeown & Associates to undertake a Flood Study.

The primary objectives of this Flood Study are:

• to define the flood behaviour of the South Creek catchment by quantifying flood levels, velocities and flows for a range of design flood events under existing catchment and floodplain conditions,
• to assess the hydraulic categories and undertake provisional flood hazard mapping (in accordance with the NSW Floodplain Management Manual (2001)),
• to formulate a suitable hydraulic model that can be used in a subsequent Floodplain Risk Management Study to assess various floodplain management measures, including the effects of future development.

As directed by Council, the scope of this study is such that:

• the extent of the hydrologic model covers the entire South Creek catchment draining to Narrabeen Lagoon,
• the hydraulic model incorporates South Creek from the Lagoon up to Brooker Avenue and Wheeler Creek up to 1.65 km upstream from the confluence with South Creek.

In general, this Flood Study does not consider flooding from local drainage which may result from inadequate urban drainage provisions.

This report details the results and findings of the Flood Study investigations. The key elements include:

• a summary of available flood related data,
• calibration of the hydrologic and hydraulic models,
• definition of the design flood behaviour for existing conditions through the analysis and interpretation of model results.