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Coastal Change Processes

South Carolina Study Site

image of south carolina location of deployment

Location of deployment sites of oceanographic equipment offshore of Myrtle Beach, SC, as part of the South Carolina Coastal Erosion Study. The equipment were deployed to investigate a large sand deposit located on the inner shelf. (Denny and others, 2013).

image of south carolina deployment period storm analysis

Deployment period storm analysis. (A) significant wave height (red line) and wind speed and direction (arrows). Shading represents storm type (blue = cold front, red = warm front, yellow = low-pressure). (B) Vertical profiles of suspended-sediment concentration. (C) Cumulative along-shore and cross-shore sediment flux. Net sediment flux for this study period was onshore and towards the southwest. (Warner and others, 2012)

image of visualization model output of current bottom stress

Visualization of model output of current bottom stress (N m-2) distribution during a storm event in Long Bay, SC. The numerical models are being used to understand the process that control oceanographic circulation and sediment transport in the region.

image of a rip current experiment

Example of a rip current experiment. Colors show bathymetry and arrows show velocity vectors after 1 h of model simulation. Similar to results in Kumar and others, 2012.

Overview

Understanding the processes that control local sediment fluxes is critical in evaluating regional vulnerability to coastal erosion. This project task involves the analysis of observational data collected as part of the South Carolina Coastal Erosion Study (SCCES), and additional coastal process modeling for the Grand Strand region. Modeling based on the physical oceanographic observations will increase our understanding of the hydrodynamic and sediment transport processes responsible for shaping the inner shelf and adjacent shoreline. Both the observational data and numerical models will be used to understand local circulation dynamics and to predict inner shelf and surf zone sediment transport patterns

Geologic Framework

The geologic mapping component of the SCCES describes the geologic framework of the Grand Strand inner shelf including its bathymetry, the texture and distribution of seafloor sediments, and the structure and composition of sedimentary units that underlie the seafloor and adjacent upland. The inner shelf is sediment limited, with modern sediments forming a patchy and discontinuous veneer that is generally less than 0.5 m thick or absent across much of the region. Modern sediment abundance generally increases from north to south, and the thickest accumulations form low-relief ridges and shoals that lie seaward of tidal inlets and estuaries on the adjacent shore. Inner shelf sediment textural trends and seafloor morphology indicate that long-term net sediment transport is directed to the southwest across the region.

An elongate sediment shoal aligned obliquely to the coast offshore of Singleton Swash in North Myrtle Beach, South Carolina (see top right) was identified as an opportune site to collect oceanographic observations for investigating regional hydrodynamics and sediment transport and evaluating the trends inferred from the geologic mapping data. The shoal is a Holocene marine deposit that consists of fine to medium, clean sand and represents a potential source for aggregate material needed for ongoing beach nourishment activities in the Myrtle Beach area. The influences that this feature may have on the along-shore distribution of wave power and local sediment transport are not clear due to a lack of observation data and numerical modeling of the region. Mining the shoal could disrupt important mechanisms controlling beach vulnerability to coastal erosion.

Oceanographic Observations

Oceanographic instrumentation were deployed at eight sites on the inner-shelf offshore of Myrtle Beach, SC  during the period October 2003–April 2004 to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. (Sullivan and others, 2006). http://woodshole.er.usgs.gov/pubs/of2005-1429/

Our measurements show that sediment mobility is caused by unique combinations of waves and wind driven currents that occur during three predominant types of storm patterns: cold fronts; warm fronts; and low-pressure storms. The passages of these types of storms create changing wind patterns resulting in different sediment mobility and net sediment transport directions (see middle right). A cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north

Observations identify that during cold fronts more sediment is transported when winds are northeastward and directed onshore creating a net sediment flux to the north–east (Warner and others, 2012). Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment  the culmination of these events (figure below) yielded a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf (Denny and others, 2013).

Numerical Modeling

Regional numerical models are being used to identify the significant processes that occur during storm events and relate sediment transport during these events to coastal changes. More detailed modeling along the coastline has increased our knowledge of nearshore circulation and wave driven flows. The modeling system has been advanced to include wetting and drying formulations to allow inundation on beaches and shallow estuarine systems (Warner et al.,2013) and we have included many relevant physical processes for surf zone flows such as the vortex force formulation (Kumar and others, 2012). There have also been applications of the modeling system to increase coastal safety by furthering our understanding of rip currents as demonstrated in Kumar and others, 2011 and Voulgaris and others, 2011.


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