5.6 Summary

In most places along the Grand Strand, the modern beach is a thin veneer of sand.  As a result, it is particularly sensitive to relatively small changes in the annual balance between sediment gains and losses.  A sediment budget was constructed to determine the directions and annual rates of sediment movement within the coastal system.  South of Murrells Inlet, there is clear evidence of long-term net longshore transport towards the south.  North of Murrells Inlet net transport appears to be more variable.  Large losses occur where sediment moving alongshore is trapped by tidal inlets (Murrells and Little River), accumulates in a large spit complex (North Island), or exits the coastal compartment at the southern boundary (Winyah Bay).

Two important assumptions underlie our sediment-budget calculations.  First, the budget is balanced so that the total volume of sediment in the Long Bay system remains constant.  Losses of sediment must be offset by gains of new sediment.  Second, the coastal system is closed and receives no sediment from adjacent coastal compartments to the north and south.  Sediment is not transferred around the shoals at Cape Fear and Cape Romain.  Lacking significant fluvial or alongshore input, the only potential sources of new sediment are internal to the system, primarily erosion of older deposits within Long Bay.  These potential sources include beach and shoreface deposits, which have been closely monitored by beach profiling, and inner-shelf deposits, which lie offshore in deeper, less accessible areas.  Estimates of sediment derived from beach and shoreface erosion (Table 5.1) are maximum values that are based on the assumed steady landward migration of sediment indicated by an idealized beach profile (Figure 5.2).  If erosion across the idealized beach profile is uneven (Figure 4.3), however, the volume of new sediment might be significantly lower and more sediment would be assumed to originate from inner-shelf erosion to balance the budget.  In either case, the scale of total sediment losses greatly exceeds the estimates of new sediment derived from beach and shoreface erosion.  Although the absolute amounts of sediment moving out of the beach system are arguable, erosion of the inner shelf must be an important contribution to the regional sediment budget.

Experiments using moored instruments have shown that shelf sediment responds to storms in a variety of ways.  Depending on the type and path of given storm, waves and currents of differing intensity and direction can be expected.  Some events drive sediment towards the northern boundary of the system, and some drive sediment towards the southern boundary.  The net result over a given year or decade will depend on the relative frequency and intensity of the different types of storms.  Continued improvement of models of sediment transport within the Bay will aid us in refining the regional sediment budget and prediction of future coastal response.  The potential for shifts in storm frequency, intensity, and tracks that may accompany climate change can be expected to strongly affect sediment movement in the area.

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