Caernarvon Freshwater Diversion Project

 wetland, erosion

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The below was copied from www.mvn.usace.army.mil/prj/caernarvon/caernarvon.htm

AUTHORITY:
The project was authorized by the Flood Control Act of 1965, (PL 89-298), the Water Resources Development Act (WRDA) 1974 (PL 93-251), and WRDA 1986 (PL 99-622).

PROJECT LOCATION:
The project is located on the east bank of the Mississippi River near the St. Bernard, Plaquemines Parish line.

PROJECT PURPOSE:
The project diverts fresh water and its accompanying nutrients and sediments from the Mississippi River to coastal bays and marshes in Breton Sound for fish and wildlife enhancement. Benefits include restoration of former ecological conditions by controlling salinity and supplementing nutrients and sediments. The bays are important to oyster production and as breeding areas for shrimp and food fishes, while the marsh areas produce food for fur-bearing animals, alligators, and migratory waterfowl. A total of 16,000 acres of marshland will be preserved and 77,000 acres of marshes and bays will be benefited by the project. The diversion takes place under regulated conditions developed from monitoring the impact on the environment and the fish and wildlife. Deterioration of the marshes below New Orleans has long been recognized. This deterioration stems from factors such as subsidence, erosion, and saltwater intrusion. The introduction of fresh water and alluviums from the Mississippi River, via the control structure, will serve to reduce this degenerating trend in the local area. The project will benefit existing commercial fisheries by enhancing marsh conditions, thereby improving the fish and wildlife resources of the area. The total average annual benefits include $8,706,000 for fish and wildlife and $449,000 for recreation, totaling $9,155,000.

PROJECT RESULTS:
The project has been operated since 1991. Prior to the diversion, the area was losing about 1,000 acres annually. The project aims to enhance emergent marsh vegetation, reduce marsh loss, and increase significant commercial and recreational fisheries and wildlife productivity in the Breton Sound estuary. Rigorous monitoring of the project by the Louisiana Department of Natural Resources Coastal Restoration Division since 1988 has shown that the project has accomplished these goals thus far. Monitoring data show that freshwater marsh plants increased over 7 times while brackish marsh plants increased by almost half since operation of the structure began, and the amount of salt marsh vegetation has decreased by more than half. Caernarvon has helped to reduce marsh loss as well. Results show a net increase in marshland of 406 acres within the sampled area, which originally contained 2,289 acres of marsh. This comparison represents a three-year period of operation from 1992-1994, which translates to an increase of 5.9% per year. Commercial and recreational fisheries production has increased as well. Since operation of the project, oyster industry productivity on the public seed grounds, a key link in the viability of the State’s oyster industry, has increased over three orders of magnitude, and the average number of large mouth bass caught has almost doubled. Wildlife has benefited as well. The number of waterfowl using the marsh has increased dramatically since Caernarvon operations began. Similar increases are also noted in the number of alligator and muskrat nests. Water quality monitoring by the U.S. Army Corps of Engineers has shown no significant water quality degradation, fish kills, or algal blooms. Freshwater diversion from Caernarvon may have been responsible for keeping the red tide out of the Breton Basin in 1996. Analysis of monitoring data by Louisiana State University scientists has indicated that nutrients, especially nitrogen, are removed by the marsh prior to reaching the Gulf of Mexico. Additionally, the high population of oysters generated since operation began facilitates removal of nutrients and algae and promotes water clarity.

PROJECT FEATURES:
The project consists of a diversion structure containing five 15-foot square gated culverts and inflow and outflow channels. The design discharge is 8,000 cubic feet per second; however, the actual amount of diverted flow depends on a detailed operational plan. The Corps of Engineers constructed the project and the Louisiana Department of Natural Resources is responsible for its operation. The Caernarvon Interagency Advisory Group consisting of 14 representatives of federal and state agencies, fisheries, and landowners provides overall operational oversight. Construction was completed in February 1991 at a cost of $26.1 million. The federal share was 75% of the costs and the State of Louisiana’s share was 25%.