Waves and water level setup during storms can create overwashing flows across barrier islands. Overwashing flows can cause erosion and barrier island breaching, but its sediments can also be deposited as washover fans. These widely different outcomes remain difficult to predict. Here we suggest that breaches develop when the sediment transported by overwashing flows exceed the barrier subaerial volume. We form a simple analytical theory that estimates overwashing flows from storm characteristics, barrier morphology, and dune vegetation, and can be used to assess washover deposition and breaching likelihood. Barrier width and storm surge height appear as two important controls on barrier breaching. We test our theory with the hydrodynamic and morphodynamic model Delft3D as well as with field observations of 21 washover fans and 6 breaches that formed during hurricane Sandy. There is reasonable correspondence for natural but not for developed barrier coasts. Our analytical formulations for breach formation and overwash deposition can be used to improve long-term barrier island models
