Tidal flats, marshes, mangrove swamps, sheltered beaches, high dunes, and sheltered water 

 can become exposed beaches by the process of "washover." If an adjacent exposed beach in the 

 direction of the dominant waves is converted to water or tidal flat, the cell in question becomes 

 beach, with an average elevation slightly above sea level to insure that the beach is not immedi- 

 ately inundated and eroded. This mimics the in-place "drowning" of barrier beaches (Leather- 

 man 1983) and their eventual stepwise retreat over back-barrier marshes and lagoons once they 

 are low enough to be subject to washover (Sanders and Kumar 1975, Rampino and Sanders 

 1980, Buttner 1981). Washover leads to a migrating beach in seven out of eight cycles; inundation 

 during the other cycle results in a breach in the barrier island. 



Each cell category is represented by a pattern and a color, so that the primary output from 

 the model is colored maps for user-specified intervals of years for a given area and rate of rise in 

 sea level. Summary statistics for all categories are provided for 25-year intervals and for wetlands 

 for 5-year intervals so that the progressive impact on coastal wetlands can be assessed. 



Assumptions and Simplifications. Because the model is intended to be used for regional 

 analysis of long-term trends, several simplifying assumptions have been made that may not be 

 appropriate for detailed analysis of local and short-term conditions: 



■ Each square-kilometer cell is represented by only one (dominant) category and by 

 average elevation; this results in pocket beaches and marshes and narrow barrier 

 beaches being under-represented; furthermore, gradual changes seem to occur 

 instantaneously when the threshold average elevation of the cell is reached; 



■ Continued residential and commercial development of coastal zones is ignored; only 

 those areas developed when the maps were published are subject to protection; given 

 current trends and policies, this may not be a reasonable assumption; 



■ Freshwater discharge is ignored in distinguishing freshwater from saltwater wetlands; 

 this is most noticeable in the Florida Everglades, which are modeled as mangrove 

 swamp due to their elevation near sea level; 



■ Sedimentation and accretion rates are related to the extent of existing wetlands; in 

 most areas this results in a decrease in sedimentation as marshes disappear, 

 coinciding with the decrease brought about by sediments "hanging up" further inland 

 in the deepening estuaries; however, in areas where extensive lowlands are inundated 

 and converted into wetlands, this algorithm will predict increased 

 sedimentation— perhaps more than is reasonable; 



■ No distinction is made among East Coast, West Coast, and Gulf Coast marshes; the 

 same algorithms are used for accretion, erosion, and position within the tidal range 

 for all three regions; SLAMM also does not distinguish between mature and new 

 marshes; 



■ No provision is made for changing vegetation due to global warming trends; in 

 particular, mangroves will not be simulated in more northerly areas where they do not 

 already occur; 



■ Cliff retreat is not modeled, nor is the increased supply of sediment to the coastal 

 regime due to cliff erosion; this could affect areas such as Cape Cod, Massachusetts, 

 and Oxnard, California; 



■ Actual bathymetry is not considered nor is the effect of changing bathymetry on wave 

 energy; beach migration is permitted in sheltered water but not in open sea; this 

 seems to be a reasonable simplification for essentially all areas; 



■ The change in erosion by tidal currents with changing morphometry and bathymetry 

 is not modeled; 



■ Changes in storm tracks and in the erosive energy of storms concomitant with 

 climatic change are not modeled. 



105 



