This report details the investigations carried out to determine design flood levels for Cockrone Lagoon and the floodplain adjacent to the lagoon within the study area. Cockrone Lagoon is a small, shallow coastal lagoon located behind MacMasters Beach. The lagoon entrance is normally closed due to the action of coastal dynamic processes which result in the formation of a beach berm across the entrance. The lagoon is perched some 2.5 metres above mean sea level. Gosford Council has adopted a maximum water level for the lagoon, termed "let-out-level" of RL 2.53 m AHD. The lagoon entrance is normally opened when the water level in the lagoon rises above the let-outlevel. The surface area and storage volume of Cockrone Lagoon at the "let-out-level" are 43.8 hectares and 630 megalitres respectively. The catchment area of the lagoon is 7.1 square kilometres. The lagoon may breakout naturally when runoff from the catchment causes the water level in the lagoon to rise above the level of the beach berm. This has happened on seven occasions since records commenced in the late 1970's. The beach berm may be overtopped by wave action during ocean storms and this may cause the water level in the lagoon to rise. Two notable ocean storms occurred in May 1974 and June 1978. These events caused little damage to properties located near the lagoon entrance.

The floods suitable for calibration occurred in April 1988, February 1990 and February 1992. The rainfall recorded for these events had estimated average recurrence intervals of 10 to 20 years. Analysis of the lagoon openings shows Cockrone Lagoon is opened approximately twice per year with the most openings occurring in June and July. The seasonal variation of opening occurs at the end of the wetter half of the year when the lagoon is likely to be higher from rainfall and lower evaporation rates prevail. The entrance to the lagoon is closed by coastal processes after breakout. The time taken to close is variable with durations of between two and 48 days being recorded. Analysis of the beach berm records suggests an average beach level of RL 3.3 m AHD over the period of Council "managed" openings. The maximum level reached was RL 4.0 m AHD.

An hydrologic model of the catchment was established using the package RORB while the lagoon hydraulics were modelled using MIKE-11. The break-out process through the beach berm was simulated using the DAMBREAK module of MIKE-11.