Decadal Evolution of Shoreface Geometry in South Carolina, USA

A long-term statewide beach surveying program in South Carolina has compiled a substantial record of beach and shoreface geometry across a region of diverse hydrodynamic and geologic framework. Initiated to support statewide beachfront regulatory obligations, the accumulated time-series has also afforded long-term monitoring of several beach nourishment projects and record of evolution of profile geometry in areas progressively interfacing with coastal development as well as relatively undisturbed systems on a decadal scale. Work to date has focused on the influence of underlying geologic framework on profile geometry and variability. In sediment starved areas of South Carolina's Grand Strand, profile geometry is often better defined as a series of linear segments rather than a smooth concave up form. The breaks in slope frequently reflect stratigraphic changes observed in nearshore geophysical records and cores collected in the lower beachface. The time series documents 13+ years of behavior, revealing decadal-scale changes across the full active shoreface. These changes are relevant to consideration of long term coastal evolution. Estimates of shoreline change have traditionally been defined by change in subaerial features or datums such as the wet-dry line defined on aerial photos or Mean High Water (MHW) contour. Across the state, and much of the nation's shoreline, these features are progressively encroaching on development and are being influenced by hard and soft erosion protection strategies. In practice, some sections of the South Carolina coast have been defined as stable (zero long term erosion rates) as the subaerial shoreline features have been artificially stabilized. These same areas continue to require beach nourishment and other engineering efforts to maintain beach volume and function. Profiles across the shoreface over the past 13 years in several such locations show evidence of continued migration of the lower shoreface. The resulting steepening of the beachface may be expected to modify prevailing hydrodynamics and sediment dispersal and demonstrate change in profile geometry on annual-decadal scales. Efforts to predict long term coastal behavior are traditionally been based on some variation of the Bruun Rule, which presumes migration of persistent profile geometry in the face of rising sea level. Any approximation of persistent profile geometry requires the subaerial and shoreface systems to migrate uniformly at all depths. The decoupling of the upper beach system by modifications such as groins, seawalls, and beach nourishment from the mid-to lower-shoreface suggests that profile geometry for a site may evolve not only on annual time scales, but on decadal time scales as well. Additionally, at one location experiencing substantial expansion of coastal dune fields over the past 20 years, the shoreface has continued migrating landward also resulting in a net steepening of the nearshore. Within the time series of shoreface profiles are glimpses of the incremental development of the marine unconformity surface characteristic of transgressing shorelines. The composite envelop of profile minima is locally expressed as a series of scarps into older underlying substrate with limited expression on the fair-weather sea floor. In some areas relief on the order of 0.5 meters exists which may become emergent during storm events and also influences processes and sediment dispersal. Recent efforts towards modeling morphological behavior in predicting longer term coastal evolution have shown substrate slope influences transgression rates (Stolper et al, 2005). The growing database of actual behavior across the shoreface on decadal scales may provide important verification to developing models and investigating coastal behavior on time periods longer than single events but shorter than geologic time scales evidence on the inner shelf.