Diversions: Routing Water into Wetlands

ON AN AVERAGE DAY, the Mississippi River propels over half a million metric tons of nutrients and sediment into the Gulf of Mexico, while the wetlands just beyond its banks starve for lack of these very elements. To deliver the river's marsh-building ingredients where needed, four major kinds of diversions penetrate the levees and direct river water into the wetlands.

Crevasses, Old and Simple

The simplest type of diversion is a crevasse, a fissure in the natural levee where the river spills through. The earliest crevasses occurred at weak points in the embankment, but agencies managing the coastal region today are cutting new ones to assist in wetlands restoration. For example, the Delta-wide Crevasses Project in lower Plaquemines Parish proposes to make and maintain up to 30 breaches over 20 years' time, producing a net increase in the area and quality of fresh and intermediate marsh.

Siphons' Scale Adaptable to Location

"Siphons are like huge straws arching over the riverbank, sucking river water in at one end and spewing it out into the wetlands at the other," says Chuck Villarrubia, coastal scientist for the Louisiana Department of Natural Resources. "To establish suction you have to prime siphons, and ship traffic or low river levels can break the suction, making these diversions difficult to operate." They are relatively inexpensive to construct, however, and numerous small siphons can be used up and down the river to pinpoint delivery of fresh water.

Gated Structures Control Outflow

Larger-capacity gated structures employ box-like culverts that tunnel through the levee, diverting water into a receiving area or canal before it enters the estuary. Operators can open or shut gates fitted into the mouths of the culverts to control the timing, volume and velocity of the freshwater flow into the outflow area, and to seasonally moderate the salinity level.

Results from the project at Caernarvon in Breton Sound demonstrate the advantages of a gated structure. In its 12 years of operation, Caernarvon has increased over seven-fold the size of freshwater plant communities, reduced the area of saltwater vegetation by over 50 percent, rejuvenated fish and wildlife populations, and stabilized the marshes. Built with funds from the Water Resources Development Act, Caernarvon is a model for other gated projects, guiding their response to changing weather conditions, growth cycles of shellfish, natural threats such as marshgrass dieback, or requirements of commercial and recreational users of the wetlands.

Experiments indicate that pulsing - releasing a large quantity of water over a relatively short period of time - may best simulate natural floods. According to John Day, professor of oceanography and coastal sciences at Louisiana State University, pulsing pushes fresh water and nutrients over a wide expanse without overfreshening the outfall area. As demonstrated when Caernarvon operated for brief periods at its full capacity, pulsing may also prove useful for land-building purposes.

Sediment Diversions Build Land

Larger particles of sediment are carried deep in the river's channel. To claim them for landbuilding purposes, sediment diversions would make large cuts down into the river bank to allow tens of thousands of cubic feet of sediment-laden water to pass into the marsh every second. Slated for construction in 2003, the first diversion of this kind, West Bay, is expected to create almost 10,000 acres of marsh. The U.S. Army Corps of Engineers suggests that more such diversions, placed downriver from populated areas, can effectively combat the problem of coastal land loss.

Coastal restoration requires a variety of techniques, including the use of diversions of various scales, to achieve the age-old accomplishments of nature. Diversions combine science and engineering to benefit the natural world and the human population that so depends on it.