Elsewhere on the Sound (e.g., Columbia), however, wetlands continue to expand, under the 

 low scenario, through the end of the simulation period. In contrast, at Plymouth on the west end 

 of the Sound, wetlands are rapidly replaced by sheltered water over the period 2075 to 2100. The 

 difference in behavior at the two sites is related to the presence of adjacent lowland. At Plymouth 

 most wetlands are located adjacent to uplands (higher terraces), whereas further east at 

 Columbia, wetlands are located adjacent to undeveloped lowland (low terrace) which can be 

 readily converted to wetlands as mean sea level rises, thus compensating for some wetland loss to 

 sheltered water. 



Even under the high scenario, migration of wetlands onto adjacent undeveloped lowland 

 continues as late as 2075 at such sites as Columbia where abundant lowland is available (Figure 

 4-15). However, after that period, under the assumption that accretion rates declined to 5 mm/yr 

 between 2075 and 2100, wetland area is significantly reduced as rising seas flood out most 

 lowland sites throughout the area. Elsewhere, where less lowland is available, the wetlands 

 maintain themselves at about the same level as under the low scenario until about 2050. In the 

 second half of the century, however, major losses occur as favorable landward sites for marsh 

 migration become rare. For example, all of the wetlands on Manteo Island are lost to rising seas 

 because no adjacent lowland remains, thus cutting off possible wetland migration. 



Wetland behavior at the Charleston, South Carolina, site may not be well simulated by 

 SLAMM because of fine-scale natural and disturbed landscape features that could not be 

 depicted at the scale employed in this study. Charleston harbor is unusual in that the Santee 

 River was diverted into it, causing high sedimentation. In order to maintain this naval port, large 

 amounts of sediments are dredged annually and dumped on the adjacent lowlands. Examination 

 of large-scale maps shows that levees, sea walls, dredge spoil islands, and other alterations of the 

 natural landscape would significantly limit marsh migration. However, because these features are 

 under-represented at the 1 km 2 cell scale, our simulations depict higher marsh migration rates 

 than those estimated by fine-scale studies (Kana, Baca, and Williams 1986 and Chapter 2, this 

 report). Under the high scenario, 75 percent of the existing marshes are lost, but 38 percent of 

 the lowland is converted to marsh. Thus, because the model was developed as a regional-scale 

 model, it is of limited use in simulating small-scale patterns. 



Marsh behavior in the Georgia environment resembles that of North Carolina. High 

 accretion rates (10 mm/yr) enable extensive marshlands to maintain themselves against the rising 

 sea level. The protected marshes on the lee side of undeveloped lowlands can expand onto these 

 lowlands in a seaward movement as well as spread landward onto lowlands further west within the 

 sample area. Elsewhere, however, lowlands replace salt marsh so that the net change is quite 

 small under the low scenario. 



Similarly, under the high scenario, because of available lowland and an accretion rate which 

 equals or exceeds the sea level rise rate the first 50 years of the simulation, salt marshes could 

 expand modestly in area. At lower accretion rates, losses of salt marsh would occur relatively 

 quickly under the high scenario. Given that the rate of sea level rise by 2100 exceeds even the 

 high accretion rate by over three times, running the scenario into later years would result in a 

 substantial net loss of salt marsh. 



Florida Atlantic and Gulf Coasts 



Although the northeastern part of this region is considered part of the South Atlantic 

 region, it is included here because mangroves could become important if the climate warms. 



In north Florida (Matanzas), wetlands are lost by the year 2100 under either low or high sea 

 level rise scenarios, probably reflecting the 5 mm/yr accretion rates that are reasonable for this 

 area. Most of the more extensive freshwater marshes here would be lost, but some protected by 

 upland areas would be preserved. 



112 



