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.温伯勒泻湖（Wamberal Lagoon）是坐落于戈斯福德市（Gosford City）与怀永市政局（Wyong Council）地方政府辖区内的小型沿海泻湖（图1）。该泻湖表面积约0.5平方千米，在温伯勒海滩（Wamberal Beach）处注入太平洋。其汇入太平洋的总集水区面积约为6.6平方千米，泻湖自身面积占总集水区的8%。 周边不少物业地势低洼，过往洪水曾造成财产损失与运营中断。近年来尚未出现大规模财产损失的情况。为缓解洪水问题，市政局会在泻湖水位接近临界值时，以机械方式开启泻湖入海口。在洪水期间开启泻湖与海洋连通的作业既存在难度，有时也颇具危险性。因此，适配设计洪水工况的海洋与航道入口条件，是本研究的核心考量因素。 过去30年间，该集水区从以乡村为主的社区转型为更高度城市化的社区。温伯勒泻湖及其部分毗邻滨水区已被划为温伯勒泻湖自然保护区，区域人口有所增长，公众对环境议题的关注度也有所提升。这些变化已对泻湖产生影响，且未来仍存在进一步变化的可能。因此，有必要明确当前的洪水问题，并对集水区及洪泛区内的未来开发进行审慎管理。 作为制定温伯勒泻湖整体洪泛区管理计划的第一阶段，戈斯福德市市政局委托韦伯·麦基翁联合事务所（Webb, McKeown & Associates）开展本次洪水研究。 本次洪水研究的核心目标如下： * 确定现有工况下温伯勒泻湖及其支流的洪水演进特性； * 建立集水区数值模型，以确定设计工况下的洪水流量、流速与水位； * 对模型进行结构化设计，以便可用于研究集水区开发及防洪方案对洪水演进特性的影响。 本次洪水研究未涵盖因城市排水设施不足引发的局部洪水问题，此类问题已纳入市政局开展的其他独立研究范畴。 本报告详述了本次洪水研究的成果与发现，核心内容包括： * 现有可用数据汇总； * 水文与水力学模型的选择依据； * 上述模型的率定过程； * 设计洪水演进特性的确定； * 所采用参数的敏感性分析。 本地此前已开展过多项相关研究，参考文献1汇总了所有相关过往报告，其中部分内容将在2.3节展开详细讨论。本报告中所有水位均采用澳大利亚高程基准面（Australian Height Datum, AHD），该基准为近似平均海平面的全国统一平面高程系统。