SPATIAL AND TEMPORAL COMPLEXITIES IN COASTAL SYSTEMS: LESSONS LEARNED FROM LONG TERM BEACH MONITORING AND INTEGRATED GEOLOGIC FRAMEWORK STUDIES IN SOUTH CAROLINA

Orrin Pilkey has brought conceptual simplifications of beach behavior used to model/manage beach change to national attention and debate. A regional program of beach profiling, shoreline mapping and geologic framework studies provide the perspective of beach behavior along the northern coast of South Carolina necessary to assess the assertions of Pilkey. In this sediment-starved system, geologic framework exerts strong influence on beach profile geometry complicating efforts to characterize broad areas by simple equilibrium profiles. A wide range of natural and anthropogenic structures interrupts beach profiles causing deviations from simple concave-up geometries described by processes and sediment grain size. These include coastwise variations in: induration of outcropping strata, sediment cover, proximity to inlets, and influence of engineering structures. Recent high-resolution geophysical surveys and coring transects show remarkably little modern sediment within the active shoreface along broad stretches of the coast. In many areas, profiles are anchored on resistant Tertiary and Cretaceous age strata. Subtle breaks in profile slope also occur at contacts with unconsolidated older deposits and result in alongshore gradients in slope and relief within the beachface.

Framework influences can also result in progressive change in profile geometry even under the influence of constant ambient processes. At one area of historically (>100 y) high erosion rates, the “shoreline” is now defined as “stable” according to commonly used definitions due to the presence of a seawall. Below the surfzone, however, a decade of long-term acquisition of profiles has documented the continued landward migration of the lower shoreface at historical rates resulting in a change in slope and width of the beach system. Such modification over time exemplifies temporal changes and challenges to managing and modeling beach systems complicated by framework variability.

A long-time series of beach profiles at recent nourishment projects near Myrtle Beach show relatively limited alongshore end loss of beach fills. Regional sediment budget calculations also suggest the importance, if not dominance, of cross-shore transport in this system questioning basic assumptions about sediment dispersal in the nearshore of the area.