U.S. Geological Survey Open-File Report 2008-1206
Coastal Change Along the Shore of Northeastern South Carolina: The South Carolina Coastal Erosion Study
Sandy beaches represent a public resource that is vitally important to the economic and environmental health of coastal communities. In northeastern South Carolina, however, the natural coastal system has been widely disturbed by human activities. The dynamic and mobile beach is progressively colliding with static infrastructure, which prevents it from migrating landward in an orderly fashion. Direct and indirect effects of shoreline migration on development in this complex system likely will increase due to anticipated global warming and accelerated rates of sea-level rise. Beach nourishment is the preferred method of erosion control, but experience shows that nearly all nourished beaches have a limited lifespan, so repeated infusions of new sediment are required (NRC, 1995; Trembanis and others, 1999). The long-term need for beach fill in the Grand Strand region will require improved projections of coastal-erosion rates and identification of new sediment sources.
The SCCES has developed a comprehensive strategy to determine the factors and processes that control sediment movement in the coastal zone. The strategy consists of three distinct parts. The first part is to map the geologic framework of Grand Strand beaches and adjacent areas of shallow seafloor. The second part is to document fluctuations in shoreline position over time, including both long-term and short-term changes. The third part is to develop a conceptual sediment budget for the coastal system, conduct sediment-transport measurements at selected sites, and ultimately construct numerical models of sediment flux in the nearshore area.
Part 1 - Geologic Mapping. The geologic framework of the Grand Strand exerts strong control on the production, movement, and ultimate deposition of sediment. The degree to which geology controls sediment movement along the coast was, until recently, difficult to measure because collecting data in energetic areas adjacent to the beach was not feasible. However, improvements in technology now provide efficient methods to explore and map the shallow seafloor. High-resolution maps of seafloor topography and sediment texture support conceptual models of sediment generally moving from northeast to southwest along the coast. The Pee Dee River has migrated southward for millions of years, too, as shown by a series of buried river channels mapped beneath the seafloor. This migration has shifted the major supply of fluvial sediment away from the center of the Grand Strand and thereby produced a generally sediment-limited inner shelf dominated by rocky outcrops. Geologic mapping of the inner shelf has identified large sandy deposits that represent potential sources for beach-nourishment sediment in the future.
Part 2 - Shoreline Change. Aerial photography and beach-profile measurements show that the active beach system has been migrating landward, but at different rates at different sites along the coast. Also, the rate of retreat at any single site is not uniform over time. In Myrtle Beach, for example, substantial variations in beach-fill stability have been observed where a large beach-nourishment project was completed in 1998. Areas of low stability coincide with extensive rocky outcrops mapped in shallow water just seaward of the reconstructed beaches. The outcrops appear to interrupt sediment-dispersal pathways and alter patterns of sediment accumulation along the beach; the result is localized areas of enhanced erosion. The monitoring period includes some of the largest storms to affect the region, such as Hurricane Hugo (1989) and the storm named “The Storm of the Century” (1993). These storms were major erosional events that removed the thin veneer of beach sediment to the greatest extent of any storms over the last twenty years. Beach profiles measured during this deeply eroded condition documented the base of the active sediment layer where it overlays a rocky substrate. In many places, sediment on the modern beach is less than 1 m (3.3 ft) thick, even though nourishment projects have frequently placed large amounts of new sediment on the beach.
Part 3 - Ocean Processes and Sediment Budget. Ongoing efforts aim to quantify the volume of sediment moving through the coastal system and examine the role of coastal oceanographic processes causing erosion. Time-series measurements of ocean processes help us to address questions raised by geologic mapping. For example, what is the precise role of the large sandy shoal offshore of Myrtle Beach in controlling the distribution of wave power along the shoreline? Mining the shoal to rebuild the beach might alter the height and direction of waves approaching the beach and thereby locally increase its vulnerability to erosion. Oceanographic experiments have acquired data to explain the sediment-transport processes responsible for shaping the shoal and adjacent shoreline. Eventually, observational data and numerical models will be used to explain regional circulation dynamics and to predict patterns of sediment flux around the shoal and adjacent surf zone.
The results of this integrated approach to understanding coastal erosion have helped municipalities and government agencies protect public health and stabilize beaches. For example, the City of North Myrtle Beach used geologic maps produced by the SCCES to design stormwater outfalls, which extend offshore so that discharge will not circulate back onto the beach and affect water quality. The results of this study generated substantial cost savings for the construction of 12 outfalls by obviating the need for expensive surveys at many potential sites. Instead, the city accessed the regional database to determine suitable locations. Every major beach-nourishment project in South Carolina over the past 10 years has relied on beach-profile data and mapping products from the SCCES. The U.S. Army Corps of Engineers has reduced expenses up to $50,000 per year per project, depending on the shoreline length, by incorporating the study results as part of its planning process. The City of Myrtle Beach and Horry County (for Arcadia Shores and Surfside/Garden City) similarly use the data to generate annual maintenance reports for nourishment projects. In this sediment-limited region, South Carolina is committed to beach nourishment to mitigate coastal erosion, and sediment is already being mined from borrow areas beyond the 3-mile limit of State waters. The regionally comprehensive framework approach described in this report supports efforts to efficiently manage coastal sediment resources and reliably project their long-term availability and cost.
Mapping products and data generated by the SCCES have challenged some long-held assumptions about how beaches evolve, especially how they respond to storms and rising sea level. Most models of coastal change assume that waves and currents are acting on homogeneous deposits of unconsolidated sediment, and that beaches maintain a constant geometry (smooth and concave up) over time as they migrate landward. The degree to which these assumptions are violated along the Grand Strand was initially identified by the long series of beach profiles collected in this study. The profiles clearly show that older, erosion-resistant deposits underlie the beach and inner shelf. Rock ledges exposed on the shoreface interrupt the standard concave-up profile used in the traditional models. Integration of shoreline-change studies with geologic-framework mapping conclusively demonstrates that, in many places along the Grand Strand, virtually no modern sediment is present on the shallow seafloor adjacent to the beach. This absence highlights the relative importance of the actively eroding inner shelf and shoreface as major components of the regional sediment budget.