Gosford Coastal Lagoons Flood Studies Comparison of Opening Procedure Avoca and Cockrone Lagoons

Storage Volumes: The modelling showed that the approach developed for Terrigal and Wamberal Lagoons could be satisfactorily applied to Avoca Lagoon for the opening of 14 September 1993. In discussions with Paterson Consultants it became apparent that the storage volume of the Lagoon was critical for establishing the peak outflow from the Lagoon. A considerable effort was undertaken in this regard which revealed that a table of height/area of Avoca Lagoon was in error at a level of I.0 m. In addition it was determined that the Mike-II and RUBICON models determine different storage capacities of the lagoon. There was insufficient time to completely resolve this issue, however preliminary work suggested that the Mike- 11 model was over-calculating storage. The differences in storage make no significant difference to the peak flood level. Comparison of Results: For Wamberal, Avoca and Cockrone Lagoons the peak flood levels from both approaches should be very similar. This is because the berm height is the major control and the peak flood level is only just (0.3m) above the berm level. For Terrigal Lagoon the differences in the two approaches are likely to be more significant as the beach berm is much lower (2.5m AHD) and the peak flood level approximately 0.5m higher than the berm. Additional Work: It is recommended that data be collected for future openings. If the data are suitable they should be used in further verification of the hydraulic models.