The SLAMM Model 



Description. Because no previous researchers had developed a satisfactory model, it was 

 necessary for us to develop a simulation model suitable for analyzing the impact of sea level rise 

 on coastal wetlands. The model, called SLAMM (Sea Level Affecting Marshes Model), simulates 

 the long-term change in coastal areas due to rising sea level. The model employs a reasonably 

 straightforward but complex set of decision rules to predict the transfer of map cells from one 

 category to another (Figure 4-8). These rules embody assumptions of linear, average responses. 

 They may not apply in detail for any particular area; however, they are suitable for policy 

 development on a regional basis, providing an estimate of the magnitude of the problems and 

 suggesting the nature of the regional policies needed to mitigate those problems. 



Figure 4-8 summarizes the model. The average elevation for a cell is determined by subtract- 

 ing the sea level rise for a five-year time step from the previous average elevation for that cell. 

 When the average elevation drops below 3.5 m above mean sea level, undeveloped and devel- 

 oped upland are transferred to undeveloped and developed lowland, respectively. Developed low- 

 land is considered to be "protected lowland" if it incorporates a protective dike or levee (a 

 characteristic noted in the input data) or if the user has chosen the option of having all developed 

 areas protected automatically. Protected lowlands are not permitted to change by the year 2100, 

 even under the scenario of the highest projected sea level rise. 



Undeveloped lowlands and developed but unprotected lowlands are subject to inundation 

 when the average elevation is less than the mean high water (MHW) during spring tides (MHWS), 

 which is approximated as half-again as high as MHW. An inundated cell becomes "tidal flat" 

 (actually rocky intertidal, but the two are combined) if the coast is rocky. If the cell is adjacent to 

 open water it becomes exposed beach; otherwise, it can become one of three categories: tidal flat 

 if erosion is greater than low (as determined by the average fetch of the adjacent sheltered water); 

 mangrove swamp if the region is tropical (as indicated by the presence of mangroves in the map 

 area); or salt marsh. High and low salt marsh are not distinguished nor are differences in levee 

 versus back-marsh accretion rates where the latter two have been differentiated in the literature; 

 accretion rates from back marsh areas have been employed because levee marshes occupy 

 relatively small areas. 



The average elevation of wetlands is a function of relative sea level and accretion due to 

 sedimentation and accumulation of organic material. As a simplification, accretion is considered 

 to be an approximate function of the areal extent of existing wetlands; extensive wetlands are 

 considered to indicate high sedimentation and accretion rates (Table 4-5). The influence of this 

 assumption has been tested for several locations and is described in the Results section. When 

 the average elevation of a marsh is less than the level of the embayed MHWS tide plus 0.25 m, 

 the wetland is considered to be saltwater; otherwise it is considered freshwater. (The embayed 

 tide is taken from the source map or, if unavailable, is estimated to be two-thirds the oceanic tidal 

 range; it is assumed that tidal ranges that are amplified by embayments will be noted on the 

 map.) Because freshwater and salt marshes cannot be distinguished using topographic maps, this 

 algorithm is applied to the input data as well as being used during the simulation. However, if the 

 cell is initially freshwater marsh and is protected by a dike or levee, the cell remains freshwater 

 marsh regardless of its elevation. In some areas (especially southern Florida and Louisiana), the 

 extent of freshwater marshes may be underestimated significantly because the influence of 

 freshwater discharge and a coastal freshwater lens is not considered. If the area is tropical, the 

 saltwater wetland is considered to be mangrove swamp; otherwise, it is considered to be salt 

 marsh. Table 4-6 illustrates accretion and subsidence rates for the study areas. 



If a salt marsh is adjacent to open ocean or if erosion is heavy (as indicated by the average 

 fetch) or if the average elevation is below mean sea level, the cell is converted to tidal flat. If the 

 average elevation is less than embayed mean low water (MLW) and the marsh is adjacent to water, 

 or if the average elevation is below MLW (which is assumed to be lower than embayed MLW) 



101 



