Morphodynamic modelling of entrance breakout for a coastal lake

Entrance breakout is an important process influencing sediment dynamics and hydrodynamic conditions in Intermittently Closed and Open Lake or Lagoons (ICOLLs). Breakout events may occur naturally in response to catchment rainfall or mechanically by excavation of a channel through the beach berm. In New South Wales, ICOLLs with untrained entrances are frequently opened in accordance with entrance opening strategies following increased lake water levels after rainfall. After an ocean entrance closes, coastal processes build the barrier, catchment runoff is trapped and water levels behind the barrier rise. This paper describes the development and validation of a coastal model at Lake Cathie to a measured breach event. The Lake Cathie / Lake Innes Estuary is located on the NSW east coast approximately 20 km south of Port Macquarie. Numerical modelling was adopted to investigate and assess existing environmental conditions and to provide further insight on the impacts of a variety of management strategies. A system based on a hydrodynamic model (TUFLOW – FV), spectral wave model (SWAN) and a sediment transport and morphology model (TUFLOW – MORPH) was used to simulate entrance breakout dynamics. This modelling system is presented. Using historical water level records, two entrance breakout events occurring in May 1996 and April 2002 were identified as suitable for validation purposes. Simulated and measured water levels were compared and excellent validation was achieved. The validation demonstrates the model is capable of reproducing measured behaviour and is suitable for testing the impact of various management options