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Figure 5.6. Measurements of wind speed and direction (top) collected offshore of Myrtle Beach from February 8–29, 2004. Weather maps in bottom panels show three different storm patterns crossing the region. More »

5.5 Numerical Modeling

Numerical models developed as part of the SCCES show that offshore bathymetric features, such as the sandy shoal seaward of Myrtle Beach (Figure 5.5), affect nearshore processes through the modification of waves that pass over them.  The direction and magnitude of sediment transport on the inner shelf are influenced by different patterns of storms that pass through South Carolina: 1) tropical cyclones (that is, tropical storms and hurricanes), 2) cold fronts, and 3) warm fronts (Figure 5.6).  Storms develop because the atmosphere contains regions of high and low pressure that move across North America.  Winds blow counterclockwise around low-pressure systems, bringing cooler air to the south and warmer air to the north.  As these air masses move, they create a cold front to the south of the low pressure and a warm front that extends to the east from the low pressure.

• TROPICAL CYCLONE: Low pressure is east of Long Bay and moving to the north.  As it passes offshore of the coast, winds initially blow out of the northeast (dashed arrow) and shift to the northwest (solid arrow).  Strong winds, waves, and currents are directed generally towards the south throughout the storm.  Net sediment flux along the coast is towards the southwest.

• COLD FRONT: Low pressure is north of Long Bay and moving to the east.  A trailing cold front crosses the region and is accompanied by a change in wind direction.  Wind initially blows out of the southwest (dashed arrow) and shifts to the northeast (solid arrow).  More sediment is transported when the wind blows from the southwest because the larger fetch generates stronger waves.  Net sediment flux along the coast is towards the northeast.

• WARM FRONT: Low pressure is west of Long Bay and moving to the north.  An extending warm front crosses the region and is accompanied by a rapid change in wind direction.  Wind initially blows out of the northeast (dashed arrow) and shifts to the southwest (solid arrow).  Even though the warm front has an opposite wind pattern to that of a cold front, the fetch is larger when the winds are from the southwest.  Net sediment flux along the coast is also towards the northeast.

The duration, magnitude, and frequency of the different storm types dictate the long-term sediment flux in the region.  Cold fronts and warm fronts generally drive sediment transport toward the northeast; tropical storms generally drive it to the southwest (Figure 5.6).  If transport is roughly equal in both directions, it provides a potential mechanism for partially preserving the sandy shoal, which somehow has survived erosion over the last 5000 years or so since it was formed (Barnhardt and others, 2007).  Instead of storms moving sediment in one consistent direction and eventually eroding the shoal, these observations suggest that storms could have shifted sediment back and forth across the shelf.  Additional modeling will help us identify the oceanographic processes responsible for maintaining the large shoal and explore possible sediment pathways connecting the shoal to the beach.

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