This report has been prepared for Warringah Council to define the nature and extent of flood in the Dee Why South Catchment. Flood modelling was completed to define flood behaviour for a range of storm events from 1 year Average Recurrence Interval (ARI) to Probable Maximum Flood (PMF). The modelling shows that significant flows are conveyed in the piped drainage network and overland through roads and properties. Generally the main overland flowpath starts from several branches at Alfred Street to Beverley Job Park. Flows in the open channel at Victor Road and Redman Road combine with overland flows from Mooramba Road, Fisher Road, and Pittwater Road at the intersection of Redman Road and Pittwater Road. Overland flows are then conveyed along several roads and properties to Dee Why Lagoon as well as in the open channels between Pacific Parade / Oaks Avenue and downstream of Dee Why Parade. In a 1% AEP event, the results show that ponding of runoff occurs at several locations with restricted outlet capacity.

High pedestrian areas in Dee Why CBD also experience overland flow inundation, particularly along Redman Road, Pittwater Road, Oaks Avenue and Howard Avenue. In a 1% AEP event floodway and high provisional hazard flow conditions along the open channels, on roads, and some properties including in the Dee Why CBD.

A series of climate change scenarios were also modelled to evaluate potential impacts from elevated sea levels and increased rainfall intensity. Results showed that the modelled increases to Lagoon level have an impact to flood inundation of low-lying land near the Lagoon. Most properties within the Dee Why South catchment do not show a significant change in inundation extent for the modelled scenarios. Modelled increases in rainfall intensity showed a rise in peak water levels across the catchment, particularly in trapped low points such as on Sturdee Parade and Alfred Street.

Council applies land use planning and development controls to manage development within flood prone areas. The flood result and mapping provided in the document will assist Council with future land use planning, development controls and floodplain risk management.

The next stage of the floodplain risk management process following the adoption of the Flood Study is the Floodplain Risk Management Study and Plan. This next stage will investigate various floodplain risk management measures and prioritise these measures for implementation.