Tidal-flat areas were estimated using aerial photos and shaded patterns shown on USGS 

 topographic sheets. The marsh was initially lumped together (high and low marsh) to determine 

 representative areas for each Charleston community. The total number of acres for this zone was 

 divided into high- and low-marsh areas by applying the typical percentage of each along the 

 composite transect (70 percent low marsh and 30 percent high marsh). The transition zone areas 

 were estimated from the digitized computer maps. 



WETLAND SCENARIOS FOR THE CHARLESTON AREA: 



MODELING AND RESULTS 



After establishing the basic relationships among elevation, wetland habitats, and occurrence 

 of species for Charleston, the next steps in our analysis were to develop a conceptual model for 

 changes in saltwater wetlands under an accelerated rise in sea level and to apply the model to the 

 case study area. 



Scenario Modeling 



Based on an earlier EPA study (Barth and Titus 1984), we chose three scenarios of future 

 sea level rise (described in Chapter 1, page 9): baseline (current trends), low, and high. 6 To be 

 consistent with the study, we projected the scenarios to the year 2075—95 years after the 

 baseline date of 1980 used to determine "present" conditions; we also assumed that the current 

 rate of relative sea level rise in Charleston is 2.5 mm/yr, although more recent studies suggest 3.4 

 mm/yr. 



The model for future wetland zonation also accounted for sedimentation and peat 

 formation, which partially offset the impact of sea level rise by raising the land surface. 

 Sedimentation rates are highly variable within East Coast marsh/tidal-flat systems, with published 

 values ranging from 2 to 18 mm (.08 to .71 in) per year (Redfield 1972; Hatton, DeLaune, and 

 Patrick 1983). Ward and Domeracki (1978) established markers in an intertidal marsh 20 km (12 

 mi) south of the Charleston case study area and measured sedimentation rates of 4-6 mm (.16-.24 

 in) per year. Hatton, DeLaune, and Patrick (1983) reported comparable values (3-5 mm, or 

 .12-.20 in, per year) for Georgia marshes. Although the rate of marsh accretion will depend on 

 proximity to tidal channels (sediment sources) and density of plants (baffling effect and detritus), 

 we believe the published rate of 4-6 mm per year is reasonably representative for the case study 

 area (Ward and Domeracki 1978). Thus, for purposes of modeling, we assumed a sedimentation 

 rate of 5 mm per year. Obviously, the actual rate will vary across any wetland transect, so this 

 assumed value represents an average. Lacking sufficient quantitative data and considering the 

 broad application of our model, we found it was more feasible to apply a constant rate for the 

 entire study area. 



As shown in Table 2-3, the combined sea level rise scenarios and sedimentation rates yield a 

 positive change in substrate elevation for the baseline and a negative change for the low and high 

 scenarios. The positive change for baseline conditions follows the recent trend of marsh 

 accretion in Charleston. 



For each of these three scenarios, we considered four alternatives for protecting developed 

 uplands from the rising sea: no protection, complete protection, and two intermediate protection 

 options. Protective options consist of bulkheads, dikes, or seawalls constructed at the lower limit 

 of existing development, which is generally the upper limit of wetlands (S.C. Coastal Council 

 critical area line). Figure 2-5 illustrates the various options. If all property above today's wetlands 

 is protected with a wall, for example, the wetlands will be squeezed between the wall and the sea. 

 Table 24 illustrates the intermediate protection options, whose economic implications were 

 estimated by Gibbs 0984). 



46 



