MORPHODYNAMICS AND HYDRODYNAMICS OF THE GEORGIA COAST

The Georgia Bight extends for a distance of approximately 1200 km between Cape Hatteras, North Carolina, and Cape Canaveral, Florida. At its apex, the Georgia coast is a classic mesotidal barrier coast that falls within the mixed energy / tide-dominated field (Hayes 1979, 1994; Davis and Hayes, 1984). Spring tidal ranges are 2 to 3 m, the second highest on the US east coast, and are the dominant hydrodynamic forcing agent; mean wave height ranges from only 0.6 to 1.0 m (Hayes and Sexton, 1989). Dominant winds are typically produced by extratropical cyclones while hurricane events are less significant compared to the flanks of the Georgia Bight in Florida and North Carolina.


Map of the Georgia coast with barrier islands (click on thumbprint)

In the early Pleistocene, the Georgia coast was a wave-dominated system similar to that of North Carolina today (Rhea, 1986). Large cuspate deltaic headlands were developed at the paleo-mouths of the Savannah and Altamaha Rivers, while large strandplain systems similar to those at Nayarit (Mexico) and the Doce River (Brazil) characterized the coast. Today, the Georgia coast is tide-dominated and is characterized by eleven relatively short drumstick barrier islands that are separated by large estuaries and backed by expansive salt marshes. Four of the barriers are developed (Tybee, Sea, St. Simons, and Jekyll Islands) and are experiencing growing developmental pressures; the remaining seven islands are in relatively pristine states with minimal development and little coastal infrastructure at risk from erosion. Georgia's barrier islands average about 8 km in length which contrasts markedly with those further to the north and south on the Georgia Bight which attain average lengths of as much as 38 km (Brown, 1977; Hayes, 1994). Large tidal prisms favor relatively stable inlets between barriers, with ebb-dominated flow fields and well-developed ebb tidal deltas. Additionally, extensive marsh development behind the islands and well-developed back-barrier drainage networks tend to enhance inlet stability. At large time and space scales, longshore sediment transport is predominantly southward in response to the highest wave-energy events being associated with northeasterly winds; however, local reversals do occur seasonally (due to different summer and winter wave regimes) and spatially (such as in coastal areas immediately downdrift of ebb tidal deltas).

Click on these two thumbprints to see views of Tybee Island, GA

As part of the long-term Holocene rise in sea level, barrier islands on the Georgia coast are moving landward with sand being supplied from cannibalistic shoreface retreat (Swift et al., 1991) and from fluvial and tidal-channel reworking of Pleistocene deposits on the back sides of the barrier coast. Today, more than 70% of the available Holocene sand in the coastal system is stored in well developed ebb tidal deltas which act as temporary sediment sources and sinks (Hayes, 1994). Little new sediment is being supplied to the coastal system either by Piedmont-draining rivers (such as the Altamaha and Savannah) or Coastal Plain rivers (such as the Satilla and Ogeechee). This is due to anthropogenic effects (such as river damming, dredging, increased stream-bank vegetation) and natural processes (primarily fluvial base level adjustments to the Holocene rise in sea level).

At human time scales, coastal response to the ongoing Holocene transgression is significantly more variable as processes such as tidal-channel switching, swash bar welding, navigation enhancements, coastal engineering, and storm events exert control on the supply and sink of littoral sediments. The interaction of the retreating shoreface with older (antecedent) stratigraphy also influences the distribution of erosional and accretional zones, and the concomitant direction of shoreline change, as new sources of littoral sand are intersected by shoreface hydrodynamic forces. Variation in the along-coast occurrence of paleo-sand deposits (primarily paleo-inlet and paleo-barrier sequences) will affect the direction and rate of shoreline change because of spatial variability in sediment supply to the littoral system. At the same time, however, variations in the rates and directions of shoreline change are also strongly controlled by variations in the wave and tidal energy flux, which in turn influence sediment flux along the coast.

WHY IS COASTAL RESEARCH IMPORTANT?

A recent report by the H. John Heinz III Center for Science, Economics and the Environment states that, for the US coast as a whole, approximately 1500 homes will be lost to erosion each year for the next several decades at an annual cost to coastal landowners of approximately $530 million (Heinz Center, 2000). Several recent National Science Foundation (NSF), US Geological Survey (USGS), and National Research Council (NRC) documents on future research priorities in the arena of coastal geology recognize that there is a critical need for high-resolution integrated research in the coastal zone, specifically on coastal change processes (FUMAGES, 1996; NRC, 1990; NSF, 1999; NSF-EAR, 1999; Fletcher et al., 2000; NSF 2000; USGS, 2000). These documents, developed by leading US and international research geoscientists, highlight the fact that a thorough understanding of sedimentary processes, morphodynamical substrate evolution, and coastal stratigraphic frameworks is required to better understand complex, non-linear, coastal systems and to develop effective process-response models. Once this level of scientific understanding is achieved, coastal zone management (and coastal hazard mitigation specifically) can move from reactive and crisis-management decision making toward a more proactive stance that will reduce the economic and human losses that are associated with increasing development of the dynamic US coastal zone.

Georgia's coast is an ideal "field laboratory" within which to study coastal evolution in a tide-dominated setting, both under natural conditions and under conditions of limited coastal engineering and/or renourishment. Georgia's four developed barrier islands (Tybee, Sea, St. Simons, and Jekyll Islands) are prime candidates for coastal change and shallow stratigraphic investigations of partly engineered/renourished systems. Georgia's seven other islands are either state, federal, or non-profit foundation-owned islands and are not as critically in need of baseline data due to the lack of infrastructure at risk from coastal erosion. However, these undeveloped islands do serve as models for contrasting the coastal response of undeveloped systems with those of developed or partly engineered systems within the same hydrodynamic regime (i.e., on the apex of the Georgia Bight).

WHAT IS THE CURRENT STATE OF KNOWLEDGE FOR THE GEORGIA COAST?

The Georgia coast is the last great unmapped area on the Atlantic coast in terms of the acquisition of shoreline-change data for coastal management and hazard mitigation purposes. Shallow stratigraphic framework studies of the Georgia barrier system were at a peak in the 1970s and early 1980s but significantly less work has been conducted during the late 1980s and 1990s (Taylor et al., 1995). While Georgia only recently (1998) adopted its Coastal Management Program under NOAA's National Coastal Zone Management Program (OCRM, 1999), as yet there is neither an effort in place to obtain baseline data on the state of the shoreline nor an effort to regularly map the shoreline. Nationally, obtaining this type of fundamental scientific information is very important for future coastal management and hazard mitigation planning (NRC, 1990). This information is all the more important for Georgia considering that the South Atlantic states collectively could lose as much as 1000 square miles of coastal lands over the next 100 years (NOAA, 1999). Unlike most other states participating in the NOAA coastal management program, Georgia does not have good up-to-date information on average annual erosion rates which are an important input for determining setback lines for (sustainable) coastal development.

The most up-to-date published information on coastal change in Georgia is contained in Nash (1977) and Griffin and Henry (1982). While highly accurate given the methods and equipment used at the time, by today's standards both studies have limitations in terms of being able to quantify coastal change to the degree of accuracy that is possible and generally required since the advent of modern GIS and shoreline-mapping systems (Anders and Byrnes, 1991; Crowell et al., 1991; Danforth and Thieler, 1992; Thieler and Danforth, 1994a,b). Other works on the Georgia coast are generally focused on individual developed islands and inlets, generally in connection with beach nourishment, coastal engineering, or navigation-channel dredging projects (Pilkey and Richter, 1965; Oertel, 1975; Howard and Frey 1980; Henry et al., 1987; Frey and Howard, 1988). Most recently, FEMA-sponsored coastal hazard assessments have been conducted in Glynn County on the central Georgia coast (Heinz Center, 2000).

WHO BENEFITS FROM COASTAL CHANGE RESEARCH?

COASTAL RESEARCHERS
An understanding of sedimentary processes, morphodynamical substrate evolution, and coastal stratigraphic frameworks is required by coastal scientists to better understand complex, non-linear, coastal systems and to develop effective process-response models. Once this level of scientific understanding and predictive capability is achieved, predictive models of future coastal change can ultimately be used to help reduce the economic and human losses that accompany development of the dynamic US coastal zone.

Manipulating shoreline data to obtain statistically valid information on the directions and rates of long- and short-term shoreline change, for example, would be a major step in filling an existing knowledge void (in terms of availability, completeness, and currency) for the Georgia coast. Similarly, determining upper shoreface and barrier system stratigraphic frameworks would reveal the role exerted by shallow stratigraphic and surficial sedimentary frameworks on the erosion, dispersal, and deposition of sediment on the upper shoreface and barrier system, as well as on variability in shoreline change vectors. Future studies would then be able to build upon this morphodynamic / stratigraphic framework by focusing on hydrodynamic processes to produce a cross-disciplinary and integrated understanding of the process-response characteristics of the coastal system. This latter work would require meteorologic and physical oceanographic research components in order to quantify the wind, wave, and tidal forces that cause sediment movement on the upper shoreface. An added benefit for researchers is that an understanding of the processes controlling coastal change in Georgia would be somewhat transferable to similar mesotidal systems both nationally and internationally. Coastal geologic research forms a valuable component in the wealth of cross-disciplinary knowledge that coastal scientists are currently assembling to understand coastal change and its influence on living systems.

COASTAL MANAGERS
Good coastal-change information generally paves the way for coastal hazard management strategies to evolve from reactive and crisis-management decision making toward proactive decision making. The Georgia Coastal Management Program would, for example, stand to gain significantly from coastal research because a centralized source of coastal geoscience data is not currently available to Georgia's coastal managers. Since Georgia has only recently spun up its own NOAA-sponsored coastal management program, this type of information is doubly important in helping the state get up to speed in developing databases comparable to other coastal states. Dissemination of research-derived information in a format and content usable to coastal managers would provide the Georgia Program with a sound knowledge of the rates, magnitudes, directions, and factors controlling coastal change in Georgia. Information on Georgia's coastal vulnerability, similar to that generated by the USGS as part of a national assessment of coastal vulnerability (Thieler and Hammar-Klose, 1999), would also be very useful to Georgia. Similarly, results from shoreface and shallow framework studies would provide relevant information on, for example, the fate of beach nourishment. The fundamental scientific knowledge generated through coastal geoscience research forms an integral input to the decision making process in coastal zone management and planning as the Georgia coast continues to experience developmental pressures in the early 21st century.

THE GENERAL PUBLIC, EDUCATORS, AND STUDENTS
It has been recommended at the national level (NRC, 1999), and is now being adopted as policy (USGS, 2000), that geoscience research by organizations such as the USGS be conducted within the framework of societal relevance (applied geologic research) and not just as research for the sake of research. Results should ideally be available at a level of understanding that is geared to the different audiences that need, or stand to gain the most from, that information. The general public are the ultimate end users and beneficiaries of coastal geoscience research being recommended, formulated, and conducted at the national level within the framework of earth systems science. This is because coast-related policies that incorporate the research results become implemented at the state and community level where the public are directly affected and can have input. Educators play a role in the process by passing new (geo)scientific knowledge on to students who ultimately become members of the general public. A general public better informed on coastal-related issues is a general public better able to make informed decisions on promulgating sustainable economic development of the dynamic coastal zone.

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