A field data set of the artificial breaching of a coastal lagoon berm is presented, and includes a detailed analysis of the breach evolution in plan and elevation, together with water levels and flow velocities. A semi-coupled two-dimensional (depth averaged) numerical model describing both the shallow water hydrodynamics and morphodynamics is developed and tested against the field data. Key hydrodynamic and morphodynamic processes are discussed, and strategies to model these processes are presented and evaluated, such as accounting for modified roughness under transcritical flows and testing an improved algorithm for widening of the breach channel through side wall erosion. While further research is warranted, the processes relating to the erosion of the breach side walls and sediment transport under transcritical flow regimes were found to be essential to developing a realistic model of the overall breach process. A new channel bank erosion model is developed and implemented, which shows improved performance.