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. Different 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. For 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. MIKE-11 models have been previously established for Avoca and Cockrone Lagoons as part of the Flood Studies. The 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. The 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. It 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. The 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.