In the Charleston area, development is prohibited in the transition wetlands, which extend 

 75 centimeters (2.5 feet) above the high marsh. Thus, Kana, Baca, and Williams (Chapter 2) 

 estimate that in the low scenario, protecting development will not increase the loss of marsh 

 through 2075, although it would increase the loss of transition wetlands. For the high scenario, 

 however, protecting development would result in a 100 percent loss of high marsh (compared with 

 a 71 percent loss), and would increase the loss of low marsh slightly (from 84 to 86 percent) by 

 2075. As Figure 1-8 shows, a two-meter rise by 2100 could result in a 100 percent loss of all 

 marsh if development is protected. 



Kana et al. do not explore the implications of protecting development in the New Jersey 

 study. About one half of the marsh in that study falls within Brigantine National Wildlife Refuge, 

 and hence is off-limits to development. New development in the other part of the study area must 

 be set back 50 to 300 feet from the marsh. 10 Although the buffer zone would offer some 

 protection, eventually the marshes here would also be squeezed out. 



The development of coastal areas may have one positive impact on the ability of marshes to 

 adapt to a rising sea. The development of barrier islands virtually guarantees that substantial 

 efforts will be undertaken to ensure that developed islands do not break up or become 

 submerged as the sea rises. Thus, these coastal barriers will continue to protect wetiands from 

 the larger ocean and gulf waves for at least the next several decades and, in some cases, much 

 longer. 11 



This positive contribution may be offset to some extent by human interference with the 

 natural overwash process of barrier islands. Under natural conditions, storms would supply 

 marshes on the bay sides of barrier islands with additional sediment, to enable them to keep pace 

 with sea level rise. On developed barrier islands, however, public officials generally push the 

 overwashed sand back to the oceanside beach, which could inhibit the ability of these barrier 

 marshes to keep pace with sea level rise. In many instances, however, these marshes have already 

 been filled for building lots. 



Louisiana and Other River Deltas 



Although natural processes would permit a large fraction of most river deltas to keep pace 

 with sea level, human activities may thwart these processes. Throughout the world, people have 

 dammed, leveed, and channelized major rivers, curtailing the amount of sediment that reaches 

 the deltas. Even at today's rate of sea level rise, substantial amounts of land are converting to 

 open water in Egypt and Mexico (Milliman and Meade 1983). 



In the United States, Louisiana is losing over 100 square kilometers (about 50 square miles) 

 per year of wetlands (Boesch 1982). Until about one hundred years ago, the Mississippi Delta 

 gradually expanded into the Gulf of Mexico. Although the deltaic sediments tend to settle and 

 subside about one centimeter per year, the annual flooding permitted the river to overflow its 

 banks, providing enough sediment to the wetlands to enable them to keep pace with relative sea 

 level rise, as well as expand farther into the Gulf of Mexico. 



In the middle of the 19th century, however, the Corps of Engineers learned of a new way to 

 reduce dredging costs at the mouth of the Mississippi River. Two large jetties were built to confine 

 the river flow, preventing the sediment from settling out and creating shoals and marsh in and 

 around the shipping lanes. Instead, the sediment is carried out into the deep waters of the Gulf of 

 Mexico. The "self-scouring" capability of the channels has been gradually increased over the 

 years. The banks of the lower part of the river are maintained to prevent the formation of minor 

 channels that might carry sediment and water to the marsh, and thereby slow the current. The 

 system works so well that dredging operations in the lower part of the river often involve 

 deliberately resuspending the dredged materials in the middle of the river and allowing it to wash 

 into the Gulf of Mexico, rather than disposing of the dredged spoils nearby. Although the 

 channelization of the river has enabled cost-effective improvements in navigation, it prevents 

 sediment, fresh water, and nutrients from reaching the wetlands near the mouth of the river. 



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