U.S. Geological Survey Open-File Report 2008-1206
Coastal Change Along the Shore of Northeastern South Carolina: The South Carolina Coastal Erosion Study
The constant shifting of sediment along shorelines presents a fundamental challenge to the prediction of beach behavior. A valuable approach to managing coastal resources is to consider the sediment that moves in and out of or is stored within a beach system in terms of an annual sediment budget (Komar, 1996). A balanced sediment budget means that, over time, equal amounts of sediment are transported into and out of a coastal compartment. Ideally, annual losses are offset by annual gains, and the beach remains relatively stable; however, an imbalance in the amount of sediment gained or lost during the year destabilizes the beach and causes changes in its shape and/or position.
A good way to think of a sediment budget is in terms of a bank account: when more sediment is deposited than removed, the sediment budget has a surplus and the beach grows seaward. Conversely, when the sediment removed exceeds deposits, the sediment budget is in deficit and the beach retreats landward. Coastal erosion occurs when waves and currents remove sediment faster than it can be replaced. If a beach has a large positive balance of sediment, a small deficit will have a minor impact on its overall condition. However, if the initial balance is small (that is, the beach system stores little sediment), relatively minor losses can cause significant changes in the shape and/or position of the beach.
The major components of a sediment budget are 1) sources that provide new sediment, 2) sinks where sediment is lost to the active beach, and 3) transport pathways along which sediment is exchanged between different parts of the coastal system (Figure 5.1). Sediment budgets typically are based on limited data that contain inherent uncertainties. Accurate estimates of sediment gains and losses are difficult to determine, particularly along coasts with numerous sources and sinks. The Grand Strand, however, is a sediment-limited system that is relatively free of complications associated with inlets and estuaries. It provides an unusual opportunity to determine a first-order approximation of sediment losses and gains (Table 5.1) and to identify transport pathways within a relatively simple coastal compartment. This section incorporates some text previously published by Gayes and others (2003) with new data and figures to provide an overview of sediment movement in the region.