Wamberal Lagoon Flood Study

Wamberal Lagoon is a small coastal lagoon within the Gosford City and Wyong Council Local\r\nGovernment areas (Figure 1). The lagoon has a surface area of approximately 0.5 square\r\nkilometres, and discharges to the Pacific Ocean at Wamberal Beach. The total catchment area\r\nto the Pacific Ocean is approximately 6.6 square kilometres and thus the area of the lagoon\r\nrepresents 8% of the total catchment.\r\n\r\nA number of properties surrounding the lagoon are very low lying, and flooding in the past has\r\ncaused damage and disruption. There has not been significant loss of property on any occasion\r\nin recent years. In an attempt to reduce the flood problems, Council mechanically opens the\r\nentrance when the lagoon approaches a critical level. This task of opening the lagoon to the\r\nocean during floods can be both difficult and at times dangerous. The ocean and channel\r\nentrance conditions appropriate to design flood conditions were therefore major considerations\r\nin the study.\r\n\r\nIn the last 30 years the catchment has changed from a predominantly rural community to a more\r\nurbanised community. The lagoon and part of the adjoining foreshore have been dedicated as\r\nthe Wamberal Lagoon Nature Reserve and there has been an increase in population and a\r\nheightened awareness of environmental issues. These changes have affected the lagoon and\r\nthere is the potential for further changes. There is a need therefore to define the existing flood\r\nproblem and carefully manage future development within the catchment and upon the\r\nfloodplain.\r\n\r\nAs the first stage in the development of an overall Floodplain Management Plan for Wamberal\r\nLagoon, Gosford City Council engaged Webb, McKeown & Associates to prepare a Flood\r\nStudy.\r\n\r\nThe primary objectives of this Flood Study are to:\r\n\r\n*\tdetermine the flood behaviour of Wamberal Lagoon and its tributaries under existing\r\nconditions,\r\n*\tset up a numerical model of the catchment to determine flood flows velocities and\r\nlevels for design events,\r\n* formulate the model such that the effects on flood behaviour of catchment\r\ndevelopment and flood mitigation options can be investigated.\r\n\r\nThe Flood Study does not consider local flooding which may result from inadequate urban\r\ndrainage provisions. These issues are being examined in other separate studies undertaken\r\nby Council.\r\n\r\nThis report details the results and findings of the Flood Study. The key elements are:\r\n\r\n* a summary of available data,\r\n* reasons for the choice of hydrologic and hydraulic models,\r\n* calibration of these models,\r\n* establishment of design flood behaviour,\r\n* sensitivity analyses of the adopted parameters.\r\n\r\nA number of previous studies have been undertaken in the local area. A compilation of all\r\nrelevant previous reports is contained in Reference 1 and several of these are discussed in\r\ndetail in Section 2.3. All levels in this Report are to Australian Height Datum (AHD). AHD is the\r\ncommon national plane approximating mean sea level.