sedimentation rates are high wetlands can maintain the distribution of their habitats only if they 

 shift along the coastal profile— moving landward and upward, to keep pace with rising sea levels. 

 Total marsh acreage can only remain constant if slopes and substrate are uniform above and 

 below the wetlands, and inundation is unimpeded by human activities such as the construction of 

 bulkheads. Titus, Henderson, and Teal (1984), however, point out that there is usually less land 

 immediately above wetland elevation than at wetland elevation (See Figure 1-5). Therefore, 

 significant changes in the habitats and a reduction in the area they cover will generally occur with 

 accelerated sea level rise. Moreover, increasing development along the coast is likely to block 

 much of the natural adjustment in some areas. 



Louisiana is an extreme example. Human interference with natural sediment processes and 

 relative sea level rise are resulting in the drowning of 100 sq km of wetlands every year (Gagliano, 

 Meyer Arendt, and Wicker 1981; Nummedal 1982). There is virtually no ground to which the 

 wetlands can migrate. Thus, wetlands are converting to open water; high-marsh zones are being 

 replaced by low marsh, or tidal flats; and saltwater intrusion is converting freshwater swamps and 

 marsh to brackish marsh and open water. 



COASTAL HABITATS OF THE CHARLESTON STUDY AREA 



As shown in Figure 2-1, the case study area, stretching across 45,500 acres, is separated by 

 the three major tidal rivers that converge at Charleston: the Ashley, Cooper, and Wando Rivers. 

 In addition, the study area covers five land areas: 



■ West Ashley, which is primarily a low-density residential area with expansive 

 boundary marsh; 



■ Charleston Peninsula, which contains the bulkheaded historic district built partly on 

 landfill; 



■ Daniel Island, which is an artificially embanked dredge spoil island; 



■ Mount Pleasant, which derives geologically from ancient barrier island deposits 

 oriented parallel to the coast; and 



■ Sullivans Island, which is an accreting barrier island at the harbor entrance. 



Six discrete habitats are found in the Charleston area, distinguished by their elevation in 

 relation to sea level and, thus, by how often they are flooded (Figure 2-2): 



■ highland - flooded rarely (47 percent of study area) 



■ transition wetlands - flooding may range from biweekly to annually (3 percent) 



■ high marshes - flooding may range from daily to biweekly (5 percent) 



■ low marshes - flooded once or twice daily (12 percent) 



■ tidal flats - flooded about half of the day (6 percent) 



■ open water - (27 percent) 



This flooding, in turn, controls the kinds of plant species that can survive in an area. In 

 Charleston, the present upper limit of salt-tolerant plants is approximately 1.8-2.0 m (6.0-6.5 ft) 

 above mean sea level (Scott, Thebeau, and Kana 1981). This elevation also represents the effec- 

 tive lower limit of human development, except in areas where wetlands have been destroyed. The 

 zone below this elevation (delineated on the basis of vegetation types) is referred to as a critical 

 area under South Carolina Coastal Zone Management laws and is strictly regulated (U.S. 

 Department of Commerce 1979). 



39 



