Waves and water level setup during storms can create overwashing flows
across barrier islands. Overwashing flows can cause erosion, barrier
breaching, and inlet formation, but their sediments can also be
deposited and form washover fans. These widely different outcomes remain
difficult to predict. Here we suggest that a breach develops when the
sediment volume transported by overwashing flows exceeds the barrier
subaerial volume. We form a simple analytical theory that estimates
overwashing flows from storm characteristics, barrier morphology, and
dune vegetation, and which can be used to assess washover deposition and
breaching likelihood. Our theory suggests that barrier width and storm
surge height are 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, where traditional sediment
transport equations do not readily apply. Our analytical formulations
for breach formation and overwash deposition can be used to improve
long-term barrier island models.