LAND LOSS PATTERNS AND PROPOSED SEDIMENT DISTRIBUTION

Land loss is ubiquitous, occurring both in interior areas and on the edges of water bodies, including the Gulf of Mexico. Any solution approaching a large-scale or optimal restoration will encounter conflicts with navigation, flood control, oil and gas, and other land uses on the one hand and the need for large-scale redistribution of Mississippi River freshwater and sediment on the other. One of the most dramatic and long-term examples of this conflict is the dam placed across the Bayou Lafourche distributary in 1904 as a flood protection measure for Donaldsonville, Louisiana. While fulfilling its authorized purpose to prevent flooding in the city, it also reduced the natural flows of freshwater and sediments to the Barataria and Terrebonne Basins. Prior to dam construction, flows amounting to approximately 15 percent of the Mississippi River flows (Kesel, 2003) had nourished the wetlands and maintained elevations relative to sea level rise (U.S. Army Corps of Engineers, 2004a). According to the report of the Governor’s Office of Coastal Activities Science Advisory Panel Workshop (Gagliano, 1994), Terrebonne and Barataria Basins each lost almost 30 square kilometers (km2) per yr (11.6 square miles [mi2] per yr) between 1978 and 1990.

Thus, flood control contributes to land loss, and reversing this land loss will require reflooding the area in order to preserve human habitation and agricultural productivity. Because of the extent of observed land loss across the entire Louisiana coastal area, it is clear that the constraints of existing development and the need for a minimum amount of water in the Mississippi River will limit the amount and location of any restoration. An accepted constraint of the LCA Study is that the Mississippi River switching, as would occur under natural conditions, will not be allowed. Also, the minimum Mississippi River flows must be sufficiently large so that the industrial and municipal water supply for New Orleans can be maintained (U.S. Army Corps of Engineers, 2004a).

An average subsidence of 0.25 centimeters (cm) per yr (0.1 inches [in] per yr) results in an annual volumetric deficit of 75 million cubic meters (m3) (98 million cubic yards [yd3]). For stability to be maintained, this volume must be replaced by a combination of siliciclastic and organic matter. Considering two ratios of siliciclastic to total matter (1:10 and 1:4), the two associated annual volumes of mineral sediment required are 75 million m3 (98 million yd3) and 18.8 million m3 (24.6 million yd3). Annual delivery costs, if distributed over the entire coastal area, would be $450 million and $1.13 billion, respectively, based on an average slurry pump distance of 120 kilometers (km) (74.6 miles [mi]) and a cost of $0.50 per m3/km ($1.05 per yd3/mi). The volumes of siliciclastic sediment range



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