Wamberal Lagoon is a small coastal lagoon within the Gosford City and Wyong Council Local Government areas (Figure 1). The lagoon has a surface area of approximately 0.5 square kilometres, and discharges to the Pacific Ocean at Wamberal Beach. The total catchment area to the Pacific Ocean is approximately 6.6 square kilometres and thus the area of the lagoon represents 8% of the total catchment.

A number of properties surrounding the lagoon are very low lying, and flooding in the past has caused damage and disruption. There has not been significant loss of property on any occasion in recent years. In an attempt to reduce the flood problems, Council mechanically opens the entrance when the lagoon approaches a critical level. This task of opening the lagoon to the ocean during floods can be both difficult and at times dangerous. The ocean and channel entrance conditions appropriate to design flood conditions were therefore major considerations in the study.

In the last 30 years the catchment has changed from a predominantly rural community to a more urbanised community. The lagoon and part of the adjoining foreshore have been dedicated as the Wamberal Lagoon Nature Reserve and there has been an increase in population and a heightened awareness of environmental issues. These changes have affected the lagoon and there is the potential for further changes. There is a need therefore to define the existing flood problem and carefully manage future development within the catchment and upon the floodplain.

As the first stage in the development of an overall Floodplain Management Plan for Wamberal Lagoon, Gosford City Council engaged Webb, McKeown & Associates to prepare a Flood Study.

The primary objectives of this Flood Study are to:

• determine the flood behaviour of Wamberal Lagoon and its tributaries under existing conditions,
• set up a numerical model of the catchment to determine flood flows velocities and levels for design events,
• formulate the model such that the effects on flood behaviour of catchment development and flood mitigation options can be investigated.

The Flood Study does not consider local flooding which may result from inadequate urban drainage provisions. These issues are being examined in other separate studies undertaken by Council.

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

• a summary of available data,
• reasons for the choice of hydrologic and hydraulic models,
• calibration of these models,
• establishment of design flood behaviour,
• sensitivity analyses of the adopted parameters.

A number of previous studies have been undertaken in the local area. A compilation of all relevant previous reports is contained in Reference 1 and several of these are discussed in detail in Section 2.3. All levels in this Report are to Australian Height Datum (AHD). AHD is the common national plane approximating mean sea level.