During a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. However, the mechanics of berm morphodynamics have not been fully described. In this study, 26 trials were conducted in a large wave flume to explore the response of a near‐prototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore, inhibiting inundation of the backshore. Net offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion.
- Award ID(s):
- 1541089
- NSF-PAR ID:
- 10302289
- Date Published:
- Journal Name:
- Shore & Beach
- Volume:
- 87
- Issue:
- 4
- ISSN:
- 0037-4237
- Page Range / eLocation ID:
- 3 to 14
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Hurricane Irma (September 2017) was one of the most devastating hurricanes in recent times. In January 2018, a post‐hurricane field survey was conducted on Anegada (British Virgin Islands) to report on the erosional and depositional evidence caused by Hurricane Irma's storm surge and waves. We document the type and extent of hurricane‐induced geomorphological changes, allowing for an improved risk assessment of hurricane‐related inundation on low‐lying islands and carbonate platforms.
Anegada's north shore was most impacted by Hurricane Irma. The surge reached about 3.8 m above sea level and onshore flow depths ranged between 1.2 to 1.6 m. Storm wave action created 1 to 1.5 m high erosional scarps along the beaches, and the coastline locally retreated by 6 to 8 m.
Onshore sand sheets reached up to 40 m inland, overlie a sharp erosive contact and have thicknesses of 7 to 35 cm along the north shore. In contrast, lobate overwash fans in the south are 2 to 10 cm thick and reach 10 to 30 m inland.
Moreover, the hurricane reworked a pre‐existing coast‐parallel coral rubble ridge on the central north shore. The crest of the coral rubble ridge shifted up to 10 m inland due to the landward transport of cobbles and boulders (maximum size 0.5 m3) that were part of the pre‐hurricane ridge.
A re‐survey, 18 months after the event, assessed the degree of the natural coastal recovery. The sand along the northern shoreline of Anegada that was eroded during the hurricane and stored in the shallow water, acted as a nearshore source for beach reconstruction which set in only days after the event. Beach recovery peaked in February 2018, when beaches accreted within hours during a nor'easter‐like storm that transported large volumes of nearshore sand back onto the beach.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.34237/1008741
