HYDRODYNAMIC FRAMEWORK OF LONG BAY, SC: MEASUREMENTS OF WAVES & COASTAL CURRENTS

Coastal erosion is the result of spatial variations (i.e., gradients) in sediment transport rates. Sediment particles are lifted from the seabed by the combined action of waves and currents and then are transported predominantly by the mean currents. In addition to influencing local sediment transport, offshore currents can affect waves that also cause surf zone sediment transport and shoreline erosion.

While other studies in the USGS SC/GA Coastal Erosion Study, Phase II, focus on mapping the geological framework of the northern SC coast, we are mapping the forces acting on the seabed. These forces create erosional “hotspots” or depositional features as found on seismic and acoustic imagery of the seabed.

This poster presents the initial stage of a two-part study. Measurements of waves and currents are obtained throughout the northern study area of the USGS SC/GA Coastal Erosion Study. This stage aims at describing the inner shelf flow dynamics in Long Bay, SC for a variety of environmental conditions. The second part of the study will utilize these data to model the hydrodynamics of the area.

The main objectives of the measurement program are to:

1) identify the relative influences of the tides and winds and the control they exert on the sediment carrier (i.e., mean currents); and

2) identify the change in wave propagation (height and direction) as a function of depth, current strength and direction.

In Long Bay the tides can explain only 30 to 40% of the variability of the coastal flow. The tidal currents alone have a strength of approximately 10 cm/s. The tidal ellipse is oriented perpendicular to the coastline offshore and rotates in a direction along the coastline in shallower waters. The majority of the energy (remaining 60 to 70%) is attributed to wind forcing. Wind effects are highly variable and depend on the wind direction relative to the coastline. Wind creates strong longshore currents that superimpose themselves onto tidal currents. Longshore currents can be either to the north or south depending on the wind direction. Wind-induced cross-shore currents can be directed in different directions near the surface and bed respectively.