Toward Quantifying Shoreface Contributions to Littoral Sediment Budgets

Variability in sediment volume plays a critical role in coastal evolution. Over centuries to millennia, it controls barrier island formation and, over decades to centuries, it influences coastal change signals. However, the importance of sediment volume variability to coastal response over smaller (event to interannual) time scales remains largely unexplored. Nearshore geology, specifically spatial changes in the shoreface ravinement surface, could be used to better quantify nearshore sediment availability, which may improve understanding of coastal response and recovery over short time scales. Furthermore, quantifying relationships between shoreface morphologic change and sediment thickness could provide insights on sediment flux from below the shoreface ravinement, which is a poorly constrained component of littoral sediment budgets.

We utilize nearshore geophysical data to examine alongshore variability in littoral sediment volumes as defined by the shoreface ravinement surface and identify regions of the shoreface where sediment may be liberated from below that surface. Geophysical data from Fire Island, NY show that the primary ravinement surface on sediment-starved shorefaces is a continuation of the transgressive ravinement surface identified on the shelf. In contrast, multiple shoreface reflection surfaces are observed within sediment-rich shorefaces. This suggests that perceived excesses in nearshore sediment may be limited at smaller scales by changes in sedimentology within the shoreface wedge, which has implications for post-storm beach recovery. In North Carolina, where nearshore geology is similar to that of sediment-starved locations at Fire Island, the relationship between bathymetric variability and sediment thickness differs with regional sediment availability and nearshore sandbar morphology. Cross-shore locations at which bathymetric variability is greater than sediment thickness may indicate where relict sediments are actively being ravined. These data provide unique insights into cross-shore and alongshore variability in the flux of sediment from the shoreface that may contribute to littoral sediment budgets.