The catchment area to Old Pittwater Road is 11.9 hectares (Figure 1). Downstream of Old Pittwater Road runoff flows through the Warringah Mall Shopping Centre and into Brookvale Creek through Warringah Golf Course. Ultimately the runoff enters Manly Lagoon and discharges into the Pacific Ocean. The catchment area to Old Pittwater Road is entirely comprised of residential urban development, there are no non-residential developments. The catchment has two main tributary branches entering from Ryan Place which join upstream of Brookvale Avenue within No. 21 Ryan Place.

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 an Overland Flood Study. The term overland has been used to signify that flooding results from runoff exceeding the capacity of the piped drainage system and/or is of sufficient magnitude to represent a hazard, as opposed to flooding from the overtopping of a natural or artificial defined creek system.

The only area where there is some form of defined creek system is immediately downstream of Ryan Place. The remainder of the natural system has been destroyed over time by urban development. Today overland flow resulting from heavy rainfall does not generally follow a defined path but travels between buildings, under or through fences, along driveways, roads, etc., until it reaches Old Pittwater Road. The route may vary depending upon the size of the rainfall event and may change as residents construct fences or buildings.

The primary objectives of this Overland Flood Study are:

• to define the flood behaviour of the catchment by quantifying flood levels, velocities and flows for a range of design flood events under existing catchment conditions,
• to assess the hydraulic categories and undertake provisional flood hazard mapping in accordance with the NSW Floodplain Management Manual (2001) (Reference 5),
• to formulate a suitable hydraulic model that can be used in a subsequent Floodplain Risk Management Study,
• to provide a preliminary assessment of floodplain management measures.

Flood Problem Areas: The study has indicated that Brookvale Avenue and Old Pittwater Road will be crossed by floodwaters in the 100y ARI event. The yards of many properties along these roads will also be inundated.

Outcomes: The main outcomes of this study are as follows:

• full documentation of the methodology and results,
• preparation of flood contour, hazard and extent maps for the catchment,
• establishment of a MIKE-Storm model which will form the basis for a subsequent Floodplain Risk Management Study and Plan,
• preliminary review of possible floodplain management/mitigation measures.