The Narrabeen Lagoon Flood Study has been prepared for Warringah and Pittwater Councils (The Councils) to define the existing flood behaviour in the Narrabeen Lagoon catchment and establish the basis for subsequent floodplain management activities.

This study updates previous studies on the Lagoon including the Narrabeen Lagoon Flood Study (PWD, 1990) and studies of the individual tributary streams, providing a holistic assessment of flooding within the catchment. The current Flood Study considers land use changes subsequent to previous modelling investigations, the influence of the Narrabeen Lagoon entrance on flood behaviour and the influence of potential climate change.

The primary objective of this Flood Study is to define the flood behaviour under historical, existing and future conditions (incorporating potential impacts of climate change) in the Narrabeen Lagoon catchment for a full range of design flood events. The study provides information on flood levels and depths, velocities, flows, hydraulic categories and provisional hazard categories. The Flood Study has also identified the impact on flood behaviour as a result of future climate change and potential changes in the catchment and lagoon entrance. Specifically, the study incorporates:

• Compilation and review of existing information pertinent to the study and acquisition of additional data including survey as required;
• Undertaking of a community consultation and participation program to identify local flooding concerns, collect information on historical flood behaviour and engage the community in the on-going floodplain management process;
• Development and calibration of appropriate hydrological and hydraulic models;
• Determination of design flood conditions for a range of design events including the Probable Maximum Flood (PMF), 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, 10%, 20% and 50% AEP events for catchment derived flooding and the 0.5%, 1%, 2%, 5%, 10% and 20% AEP events for ocean derived flooding; and
• Assessment of potential impact of climate change using the latest guidelines.