Barrier Breaching Versus Overwash Deposition: Predicting the Morphologic Impact of Storms on Coastal Barriers

• Published in: Journal of geophysical research. Earth surface Vol. 126; no. 6
• Main Authors: Nienhuis, Jaap H., Heijkers, Leoni G. H., Ruessink, Gerben
• Format: Journal Article
• Language: English
• Published: 01.06.2021
• Subjects: barrier island; breaching; Delft3D; inlet; overwash; storm
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2021JF006066

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. Plain Language Summary Storms can have big impacts on coastlines. Beaches and dunes can be eroded, their sands can be blown inland, coverings houses and streets. Sometimes, there is so much erosion that new channels have formed that breach the island and make a new connection between the ocean and the lagoon. Here, we develop and test a new theory to predict these outcomes ahead of time. We find that high storm surges along narrow dunes are most vulnerable to breaching. Key Points New analytical theory compares overwashing flow against barrier volume to predict breaching and washover deposition We test our theory against Delft3D simulations and Hurricane Sandy observations: vegetation and island elevation help to prevent breaching Developed barrier islands do not follow predicted trends, suggesting alternative controls on overwashing and breaching