A Case Study: Atchafalaya Sediment Delivery Project

Although the Atchafalaya River is one of coastal Louisiana’s few delta-building rivers, some of its sediment is either dredged to maintain navigation or carried through the channel and deposited farther out in the bay. The result is a loss of crucial land-building material in the lower Atchafalaya River delta.

The Atchafalaya Sediment Delivery project (ASD) is part of Louisiana’s effort to address this problem. Located within Atchafalaya Bay, about 18 miles southwest of Morgan City, the project was completed in March of 1998. Its purpose is to increase the amount of fresh water and associated sediments delivered from the Atchafalaya River eastward to a 2,000-acre section of the Atchafalaya Delta Wildlife Management Area.

ASD consists of two channels constructed from East Pass, through Natal Channel and Castille Pass, extending out into the wildlife management area. The combined length of the channels is about two miles. According to Dr. Erik Zobrist, CWPPRA program officer for the National Marine Fisheries Service and project leader for ASD, the project was crucial because the freshwater flow from the Atchafalaya was being choked off. “The sediment carrying capacity of Natal Channel and Castille Pass was significantly less than it could have been,” said Zobrist.

Part of the ASD project called for using dredged materials taken from Natal Channel and Castille Pass to create delta lobe islands and marsh habitat within the project area. Using natural deltas for a blueprint, 800,000 cubic yards of sediment were used to create land with elevations that would encourage the growth of marsh vegetation. This is an important component of the project since vegetation traps additional sediments during flooding and reduces both water velocity and erosion. These dredged sediments were deposited to build sites with higher elevations on the upper ends, to resemble a naturally formed delta. The project did not include vegetative plantings, with the hope that dredged sediments would carry their own seed source, which they did.

Soon after the sediment was placed, vegetation began to grow on the newly created wetlands. “Plant species that had not been seen in this area for years began to appear, but eventually the vegetation that you’d expect to dominate such as upland willows, shrublands and submerged aquatic vegetation really took hold,” said Zobrist.

Project Area Habitat Class	1994	1997	1998
Fresh Open Water	1,251.9	849.7	660.2
Submerged Aquatics	0.0	643.3	864.8
Fresh Marsh	142.1	216.8	230.9
Beach/Bar/Flat	750.9	429.7	302.2
Wetland Forested	0.0	28.1	78.2
Upland Barren	0.0	0.0	14.2
Total	2,181.6	2,181.6	2181.4

Table 1 (Source: Atchafalaya Sediment Delivery Progress Report Number 1)

Not all project results were as immediate or noticeable as the islands’ vegetative growth, but early monitoring results are promising. Table 1 provides a breakdown of habitat classes for the pre-construction years 1994 and 1997 and the first post-construction year, 1998. Based on November of 1998 photoimagery from the National Wetlands Inventory, 78.4 wetland acres were created by ASD, including 14.1 acres of fresh marsh, 14.2 acres of upland barren habitat, and 50.1 acres of forested wetland.

A second analysis of project results was also completed in late 1998 by Brown, Cunningham & Gannuch Inc. and reported in the engineering firm’s “Engineering Closure Report.” In this case, the survey showed that a total of 281 acres was created on the five sediment deposition areas in ASD’s project area. Based on the elevations of these created lands, the breakdown was 211 acres of subaqueous marsh, 49 acres of emergent marsh and 21 acres of lobe island habitat.

Since project dollars are limited, the evaluation phase of the ASD has been reduced to basic monitoring, and the extent of delta-building improvement can now best be judged by how closely the results compare with the natural system. Zobrist is optimistic. “When you can build a project that works with the forces of nature rather than against them, you tend to get a bigger bang for your buck, you spend taxpayers’ dollars more wisely, and you get better results in the long run.