technique, known as "leveling," between the twenty-six sites; mean sea level, on the other hand, 

 may be higher or lower at a particular location depending on such factors as rainfall, winds, 

 currents, and atmospheric pressure. This distinction is usually unimportant; even USGS 

 topographic maps printed before 1973 refer to elevations above "mean sea level" when they 

 really mean NGVD. For most practical purposes, the reader of this report can assume that zero 

 elevation at a particular site refers to the level of the sea between 1910 and 1929. All elevations in 

 this report are with respect to NGVD unless otherwise stated. 



The other type of elevational reference is the "tidal datum." Depending upon context, terms 

 such as "mean sea level" can refer to a theoretical concept or a legal definition. The legal 

 definition of mean sea level (MSL) is the average water level observed at a location over the 

 period 1960-78; mean high water (MHW) and mean low water (MLW) are the averages of all high 

 and low tides, respectively, over that period; mean tidal range is the difference between mean 

 high water and mean low water. However, wetlands respond to actual conditions, the average 

 water level of today. Thus, unless otherwise stated, the term mean sea level in this report refers 

 to the average water levels of today, not the legal tidal datum. 



Figure 1-7 illustrates the impact on the composite marsh profile of the low scenario for the 

 period 1980-2075, which implies an 87-centimeter (2.9-foot) rise in relative sea level for the 

 Charleston area. Because Kana et al. assume that sedimentation would enable the surface to rise 

 48 centimeters, the net rise in sea level is equivalent to an instantaneous rise of 39 centimeters 

 (15 inches). As the figure shows, the area of low and high marsh would each decline by about 50 

 percent as they shifted upward and inland. For the high scenario rise of 159 centimeters (5.2 

 feet), the loss would be approximately 80 percent. 



FIGURE 1-7 



SHIFT IN WETLANDS ZONATION ALONG A SHORELINE PROFILE 



< 



u 



Highland 

 2075 



46% 



Water 

 2075 

 33% 



2075 MSL 



LOW SCENARIO 



Conceptual model of the shift in wetlands zonation along a shoreline profile if sea level rise 

 exceeds sedimentation by 40cm. In general, the response will be a landward shift and altered 

 area! distribution of each habitat because of variable slopes at each elevation interval. 



Source: Kana et al. (Chapter 2) 



Although Kana et al. considered alternative scenarios of sea level rise, they did not investi- 

 gate alternative rates of wetland accretion. However, using the data presented in Figure 1-6, one 

 can derive Figure 1-8, which shows marsh loss for various combinations of vertical accretion and 

 sea level rise. For example, an 80 percent loss could occur (1) if the marsh grows upward at 1 

 centimeter per year and sea level rises 1.9 meters by 2100 or (2) if sea level rises 80 centimeters 



14 



