A document by Christopher Sherwood. Click on the document to view its contents.

A document by Christopher Sherwood. Click on the document to view its contents.

There is significant delay in revegetation of storm-induced washout areas compared to washover areas on barrier islands. North Core Banks, North Carolina, was affected by repeat hurricanes in different ways: Hurricane Florence (2018) deposited large washover fans 0.5-1 m thick, and Hurricane Dorian (2019) removed 1-3 m of sediment from washout channels. Aerial surveys captured vegetation recolonization on the Florence washover fans within a year but, after Dorian, surveys showed that the washout channels quickly filled with marine sand and remained mostly unvegetated for three years. We identified comparable outcomes on the coasts of Texas and New York. These observations reveal an interruption in the normal cycle of vegetation and dune growth that is key to rebuilding barrier islands after storms. The lack of vegetation in the former washout channels prolongs vulnerability to overwash, further delaying recovery. Our findings have implications for best-management practices and modeling of coastal geomorphic evolution.

Barrier islands are especially vulnerable to hurricanes and other large storms, owing to their mobile composition, low elevations, and detachment from the mainland. Conceptual models of barrier-island evolution emphasize ocean-side processes that drive landward migration through overwash, inlet migration, and aeolian transport. In contrast, we found that the impact of Hurricane Dorian (2019) on North Core Banks, a 36-km barrier island on the Outer Banks of North Carolina, was primarily driven by inundation of the island from Pamlico Sound, as evidenced by storm-surge model results and observations of high-water marks and wrack lines. Analysis of photogrammetry products from aerial imagery collected before and after the storm indicate the loss of about 18% of the subaerial volume of the island through the formation of over 80 erosional washout channels extending from the marsh and washover platform, through gaps in the foredunes, to the shoreline. The washout channels were largely co-located with washover fans deposited by earlier events. Net seaward export of sediment resulted in the formation of deltaic bars offshore of the channels, which became part of the post-storm berm recovery by onshore bar migration and partial filling of the washouts with washover deposits within two months. The partially filled features have created new ponds and lowland habitats that will likely persist for years. We conclude that this event represents a setback in the overwash/rollover behavior required for barrier transgression.

The lower shoreface, a transitional subaqueous region extending from the seaward limit of the surf zone to beyond the closure depth, often serves as a sediment sink or source in sandy beach environments over annual to millennial time scales. Despite its important role in shoreline dynamics, however, the morphodynamics of the lower shoreface remain poorly understood. This knowledge deficit is partly due to the absence of sediment compositional data across the seabed and to the challenges inherent in measuring subtle bed changes (mm-cm/yr) over historical time scales. It is also unclear how diverse lithologies and long-term changes in wave climate influence shoreface morphodynamics as previous work often considers these steady-state systems in equilibrium. To better understand the controls on shoreface dynamics, we extend an existing energetics-based framework to model sediment transport across theoretical shoreface equilibrium profiles under various physical and geologic disequilibrium conditions. We further incorporate varying shoreline input flux scenarios (i.e., accretion, erosion) to investigate potential coastline inheritance controls on shoreface evolution. Equilibrium profile shapes and disequilibrium sediment transport rates are more sensitive to changes in sediment settling velocity than wave period, indicating that grain size provides a strong geologic control on shoreface morphodynamics. We find that at depths greater than 20 meters, shallow water wave assumptions predict larger sediment transport rates (~1-8 orders of magnitude) than linearly shoaled waves. Furthermore, for linear wave theory, we find an abrupt, discontinuous offshore transition where the bed response to changing wave climates becomes exceptionally slow. Our results provide insight into the sediment dynamics that drive the spatiotemporal evolution of the shoreface, improving our understanding of the interactions between onshore and nearshore processes and geological inheritance.