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.
