The net change in areas under the various scenarios listed in Table 2-6 indicates that all 

 habitats would undergo significant alteration. Even under the baseline scenario, which assumes 

 historical rates of sea level rise, 20-35 percent losses of representative marsh areas are expected 

 by 2075. Protection under the low scenario (as outlined by Gibbs 1984) would have virtually no 

 effect on high or low marsh coverage; but it would cause a substantially increased loss of 

 transition wetlands. Under the high scenario with protection, highland would be saved at the 

 expense of all transition and high marsh areas and almost 90 percent of the low marsh. Even 

 under the low scenario, sea level rise would become the dominant cause of wetland loss in the 

 Charleston area. 



RECOMMENDATIONS FOR FURTHER STUDY 



This study is a first attempt at determining the potential impact of accelerated sea level rise 

 on wetlands; there remains a need for case studies of other estuaries. Louisiana provides a 

 present-day analog for the effect of rapid sea level rise on wetlands because of high subsidence 

 rates along the Mississippi Delta (see Gagliano 1984). Additional studies in that part of the coast 

 should attempt to document the temporal rate of transformation from marsh to submerged 

 wetlands. 



Accurate wetland transects with controlled elevations are required to determine the 

 preferred substrate elevations for predominant wetland species. With better criteria for elevation 

 and vegetation, we can use remote-sensing techniques and aerial photography to delineate 

 wetland contours on the basis of vegetation. Scenario modeling can then proceed using 

 computer-enhanced images of wetlands and surrounding areas, for more accurate delineation of 

 marsh habitats. Using historical aerial photos, it may also be possible to infer sedimentation rates 

 by changes in plant coverage or species type, which could be related to elevation using some of 

 the criteria provided in this report. 



Another problem that remains with this type of study is the frame of reference for mean sea 

 level. For practical reasons, mean sea level for a standard period (18.6 years generally) cannot be 

 computed until after the period ends. Therefore, fixed references, such as the NGVD of 1929, are 

 used. But sea level in Charleston has an elevation of about 15 cm (NGVD). If everyone uses the 

 same reference plane for present and future conditions, the problem may be minor. But it does 

 not allow us to determine modal elevations with respect to today's sea level. The transects 

 surveyed for the present study suggest that S. altemiflora (low marsh) grows optimally at an 

 elevation of 75 cm (2.45 ft) above mean sea level, close to mean high water (U.S. Department of 

 Commerce 1981). Compared with today's mean sea level in Charleston, S. altemiflora probably 

 tends to grow as much as 15 cm below actual mean high water, which may confuse the reader 

 who forgets that the NGVD is 15 cm below today's sea level. 



The basic criteria for delineating elevations of various wetland habitats in this study can be 

 easily tested in other areas. By applying normalized flood probabilities (similar to those depicted 

 in Figure 2-7), it will be possible to measure marsh transects in other tide-range areas and relate 

 them to the results for Charleston. 



Normalized Elevations 



The absolute modal elevation for each species is site-specific for Charleston. Presuming that 

 the zonation is controlled primarily by tidal inundation, it is possible to normalize the data for 

 other tide ranges based on frequency curves for each water level. Figure 2-7 contains two such 

 "tide probability" curves, based on detailed statistics of Atlantic Coast water levels given in 

 Ebersole (1982) and summarized in Appendix 2-A. The graph of Figure 2-7A gives the 

 probability of various water levels for Charleston. In Figure 2-7B, the data have been normalized 



51 



