SHOREFACE GEOLOGICAL DATA FROM NATURAL AND DEVELOPED BARRIER ISLANDS PROVIDE INSIGHTS INTO COASTAL VULNERABILITY

Recent field and reduced-complexity morphodynamic modeling efforts have shown that sediment availability, or the balance between coastal sediment fluxes, and sea-level rise drive barrier island evolution. Interestingly, modeled responses are particularly sensitive to fluxes from the shoreface. Also, humans are increasingly recognized for their role in modifying coastal sediment availability by reducing/eliminating overwash, installing coastal engineering structures that fix coastal features in place, and/or adding sediment via beach nourishment. However, little is known about shoreface response to natural versus human-induced variations in sediment availability and possible impacts to future coastal resilience. To explore this, we compare three US Atlantic barrier island systems with varying degrees of human intervention. Rockaway Beach, NY is the developed endmember, has been fixed in place with seawalls and groins, and is regularly nourished. The undeveloped endmember is Cedar Island, VA, which has migrated landward approximately 15-30 m/y since 1984 without any human alterations. The intermediate example is Fire Island, NY, which is sparsely populated and regularly nourished but has few coastal engineering structures to impede natural sediment exchanges. Geophysical data reveal variations in shoreface geomorphology between the study areas. For heavily-developed Rockaway, the shoreface is steep and the geologically-defined shoreface toe is close to shore, likely the result of being fixed in place for decades. At Cedar, the shoreface is almost entirely devoid of modern sediment and the slope is gradual, both responses to its recent, rapid transgression. At Fire Island, shoreface geomorphology reflects past periods of seaward progradation and alongshore extension resulting in a relatively sediment-rich shoreface with the shoreface toe frequently extending past the 10-m depth contour. The lack of shoreface sediment at Rockaway and Cedar suggests those barriers will be less resilient than Fire Island to increases in sea level, since landward sediment fluxes cannot be replenished with fluxes from the shoreface. Future work will explore the feedbacks between shoreface sediment availability, morphodynamic equilibrium, and human modifications.