To investigate some of these issues, Kana et al. (Chapters 2 and 3) estimate the impact of 

 accelerated sea level rise on wetlands in the areas of Charleston, South Carolina, and Long 

 Beach Island, New Jersey. Charleston has a tidal range of almost two meters, while the New Jersey 

 area has tidal ranges between sixty and one hundred centimeters. In each area, they surveyed a 

 dozen marsh profiles to develop a "composite transect," an average cross section of the marsh. 

 Based on previous studies, they assume that the marshes in both areas could grow upward at a 

 rate of five millimeters per year. 



Figure 1-6 illustrates the composite transect of the Charleston marshes. The low marsh, 

 whose elevation is between 45 and 90 centimeters (1.5 to 3.0 feet) is 550 meters (1800 feet) wide. 

 The high marsh, with elevation between 90 and 120 centimeters (3.0 to 4.0 feet), is about 210 

 meters (700 feet) wide; the transition wetlands, with elevation between 120 and 195 centimeters 

 (4.0 to 6.5 feet), are generally about 150 meters (500 feet) wide. Thus, the average slopes found in 

 the low, high, and transition marsh areas are 0.08, 0.14, and 0.50 percent, respectively, 

 confirming that the slope of the profile increases as one moves inland from the marsh. (The slope 

 immediately above the marsh is approximately 0.55 percent.) 



FIGURE 1-6 



COMPOSITE TRANSECT-CHARLESTON, S.C. 



Highland 47°-; 



z 

 O 



F 



Water 27° 



- 10-YR STORM 



- PEAK YEARLY TIDE 



- SPRING HIGH WATER 



- MEAN HIGH WATER 



- NEAP HIGH WATER 



- MEAN SEA LEVEL 



- MEAN LOW WATER 

 j- SPRING LOW WATER 



Spanina Alternlflora (2.4) 



»10 

 .8 

 -.6 



.4 



♦ 2 

 



-2 



-4 



-6 



1000 



2000 3000 



TYPICAL DISTANCE (FT ) 



4000 



5000 



Composite wetlands transect for Charleston illustrating the approximate percent occurrence and 

 modal elevation for key indicator species or habitats based on results of 12 surveyed transects. 

 Minor species have been omitted. Elevations are with respect to 1929 NGVD, which is about 15 

 cm lower than current sea level. Current tidal ranges are shown at right. 



Source: Kana et al. (Chapter 2) 



A word on what we mean by elevation is in order. Old maps often have contours 

 representing, for example, five feet above sea level. However, because sea level has been rising, a 

 contour that was five feet above sea level fifty years ago may only be four and one-half feet above 

 sea level today. To avoid potential confusion, most maps today express elevations with respect to 

 the "National Geodetic Vertical Datum" (NGVD) reference plane, which is a fixed reference that 

 is unaffected by changes in sea level. 



NGVD was developed in 1929 by estimating mean sea level at twenty-six sites along the 

 North American coast for the preceeding couple of decades. For these sites, zero elevation 

 (NGVD) is the same as mean sea level over that period. For other sites, however, the zero 

 elevation is not necessarily mean sea level for that period. NGVD was developed by a surveying 



13 



