Review of Lagoon Entrance Modelling for Flood Studies

Separate Flood studies have been undertaken for four coastal lagoons in the Gosford area, that is, Avoca, Cockrone, Terrigal and Wamberal Lagoons.\r\n\r\nDifferent analytical approaches have been used for Avoca-Cockrone and Terrigal-Wamberal Lagoons. The physical processes involved in the break-out of the lagoons through the beach are complex. Thus, there is a potential for the different analytical processes to produce significantly different results.\r\n\r\nFor this study, separate MIKE-11 models were established for Terrigal-Wamberal Lagoons. These models were calibrated against the documented openings of August and September 1993.\r\n\r\nMIKE-11 models have been previously established for Avoca and Cockrone Lagoons as part of the Flood Studies.\r\n\r\nThe Avoca Lagoon hydraulic model established for the flood study was modified to remove the source of numerical noise which resulted in the over-estimation of the volume of water discharged to the ocean during the opening calibration event. The re-calibration did not alter the parameters for initial breakout of the lagoon.\r\n\r\nThe amendment of channel lengths in the Avoca model was also undertaken to remove a minor over-estimation of storage. The revised hydraulic model produced an estimated one percent AEP design flood level 20mm lower than that obtained in the flood study. This variation in estimated flood levels is not considered significant.\r\n\r\nIt is noted that the critical duration storm (that is, the design storm that produces the greatest water level in the lagoon) was the 9 hour storm for each lagoon.\r\n\r\nThe design flood level estimates for the one percent AEP flood in Terrigal Lagoon and Wamberal Lagoon determined by the review hydraulic model were within 30mm of the flood study estimates. This can be considered to provide independent verification of the flood study estimates.

针对戈斯福德（Gosford）区域的四座滨海潟湖——阿沃卡（Avoca）潟湖、科克龙（Cockrone）潟湖、特瑞格尔（Terrigal）潟湖与万伯勒尔（Wamberal）潟湖，已分别开展洪水研究。
针对阿沃卡-科克龙潟湖与特瑞格尔-万伯勒尔潟湖，本次研究采用了不同的分析方法。潟湖经海滩溃决的物理过程极为复杂，因此不同分析方法可能会得出差异显著的结果。
本研究针对特瑞格尔-万伯勒尔潟湖构建了专属的MIKE-11模型，并以1993年8月及9月的有记载潟湖开口事件对模型进行率定。
此前作为洪水研究的一部分，已针对阿沃卡潟湖与科克龙潟湖构建了MIKE-11模型。
本次洪水研究构建的阿沃卡潟湖水力学模型经修改，以消除数值噪声来源——该噪声曾导致率定溃决事件中排海水量被高估。本次重新率定并未改变潟湖初始溃决的相关参数。
同时还对阿沃卡模型的河道长度进行了修正，以消除小幅高估蓄水量的问题。经修正后的水力学模型得出的1%年超越概率（Annual Exceedance Probability）设计洪水位较原洪水研究结果低20毫米，该洪水位估算值的差异被认为不具有显著性。
需说明的是，各潟湖的临界历时暴雨（即能使潟湖水位达到峰值的设计暴雨）均为9小时历时暴雨。
经复核的水力学模型得出的特瑞格尔潟湖与万伯勒尔潟湖1%年超越概率设计洪水位估算值，与原洪水研究结果的偏差均在30毫米以内，该结果可视为对原洪水研究估算值的独立验证。