NATIONWIDE LOSS OF WETLANDS: 

 A FIRST APPROXIMATION 



Methods 



The case studies of South Carolina and New Jersey illustrate the hypothesis that a rapid rise 

 in sea level would drown more wetlands than it would create. Nevertheless, to demonstrate the 

 general applicability of this hypothesis requires more than two case studies. Although this project 

 did not have the resources necessary to conduct additional field surveys, we wanted to develop at 

 least a rough estimate of the likely nationwide loss of coastal wedands. 



Armentano et al. (Chapter 4) use topographical maps, information on tidal ranges, and 

 a computer model to estimate the impacts of sea level rise on 57 sites comprising 4800 

 square kilometers (1,200,000 acres) of wedands, over 17 percent of all U.S. coastal wetiands. 

 For each square kilometer they assigned a single elevation. If the map has ten-foot contours, 

 and most of a square is between five and fifteen feet above sea level, they assigned the entire 

 square an elevation of ten feet. If the map shows that a particular area is marsh, they gave it 

 the marsh designation and an elevation based on a linear interpolation between the 

 shoreline and the first contour, generally at elevation 10 feet. Their data base also 

 considered whether a particular area is developed or undeveloped, and whether there is an 

 existing flood-protection wall or bulkhead. 



Although their data base was much more coarse, Armentano et al. use a more 

 sophisticated model for projecting the impact of sea level rise than Kana et al. The latter 

 simply subtracted estimated vertical accretion from relative sea level rise for the year 2075, 

 to yield an estimate of net substrate change for the entire period. Armentano et al. also 

 subtract vertical accretion from relative sea level rise, but in five-year increments. Once an 

 area is below spring high tide, it is assumed to be marsh; once it is below mean low water, it 

 converts from marsh to open water. This procedure makes it possible to display results of 

 wetland loss for particular years, and to consider changes in marsh accretion rates during 

 the forecast period. Armentano et al. also account for changes in exposure to waves due to 

 destruction of barrier islands and spits. 



Because elevations are estimated crudely, one should be suspicious of individual results. 

 Although marsh is generally found at elevations ranging from mean sea level to spring tide, 

 Armentano et al. assign it all to a single elevation for a particular cell based on contours that 

 generally describe elevation of adjacent dry land, not the elevation of the marsh, rounded to 

 the nearest half meter. If the change in water depth (relative sea level rise minus accretion) is 

 small, the model assumes no loss of marsh; whereas some marsh would actually be lost. 

 Conversely, for a water depth greater than the estimated elevation above mean low water, all 

 the marsh is assumed lost; whereas the marsh between that elevation and spring high tide 

 would actually remain marsh. Similarly, the model may tend to underestimate marsh 

 creation for small rises in sea level while overestimating creation for larger rises. 



The estimates by Armentano et al. were based on a number of conservative assumptions that 

 may tend to understate wetland loss. They assumed that the New England, Florida, and Texas 

 marshes are not subsiding, whereas tide gauges indicate that these areas are subsiding between 

 one and two millimeters per year (Hicks et al. 1983). Moreover, they assumed that sea level rise 

 would not convert marsh until mean low water had risen above the marsh; by contrast, marsh is 

 often not found below mean sea level, and in the case of Charleston, Kana et al. found that it is 

 generally at least 30 centimeters above today's mean sea level (NGVD elevation 45 centimeters). 

 Finally, the linearity assumption tends to understate marsh loss in areas where the profile is 

 concave, as in Figures 1-5 and 1-6 and most coastal areas. 



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