Spatial variability in shoreline change along the Atlantic coast of Delaware: Influence of the geologic framework

The oceanographic and sediment-transport processes governing shoreline change are rarely resolvable from the spatially and temporally limited datasets available. Until more comprehensive data become available, the complex interactions of the myriad processes controlling long-term shoreline change are assumed to be reflected in the migration of the high-water line or comparable shoreline feature. Given the limitations of this approach, other data that can provide context for interpreting historical changes or constraining erosion forecasts become critical. The antecedent geologic framework, including the land surface that is being inundated and eroded during transgression, can provide such a context. The geomorphology and spatial and temporal patterns of shoreline change are, to varying degrees, dependent upon antecedent topography and sediment variations across that surface.; Comprehensive geophysical field-data collection and analysis were conducted along the Delaware Atlantic Coast to refine the geologic framework and explore the ways in which the framework influences modern geomorphology and the long-term retreat of the beach system. Ground-penetrating radar profiles collected on the uplands northwest of Bethany Beach revealed the internal structure of and the spatial relationships among the early Stage 5 paleoshorelines preserved on the emergent Coastal Plain. As documented in high-resolution, Chirp (2–10 kHz) seismic-reflection profiles collected in the nearshore zone, the Holocene sand sheet is generally only a 1–2 meter-thick veneer overlying Pleistocene units, which are the submerged, eastern portions of the Rehoboth and Bethany headlands. The locations of Pleistocene and Holocene fluvially and tidally incised valleys, some of which flank the headlands, were mapped in the shoreface and across the inner shelf.; Spatial and temporal anomalies in historical shoreline-change data were observed coincident with the major sedimentologic and age discontinuities documented in the geologic framework. Geostatistical analyses of shoreline-change data revealed spatial dependence up to an alongshore range of 8 to 11 km, which is equivalent to the large geologic transitions in the shoreface (e.g., Pleistocene versus Holocene, lagoonal sediments versus relict shorelines). Thus, the results presented here can be used by coastal managers, scientists, and engineers to provide a scientific basis for incorporating geologic data into the identification of future erosion hazard areas and in other hazard mitigation activities.