Beneficial Use Analysis

Beneficial Use Analysis

The dredged material from the flexible-discharge dustpan dredge demon-

stration was used beneficially for wetlands restoration. It was pumped upstream,

over the dike, adjacent pasture, and existing marshland, and placed in an area

designated by MVN (Figure 2). The right-of-way across the dike, adjacent

pasture, and existing marshland consisted of a 100-ft-wide corridor (Photo 15).

As the placed dredged material built up above the surface of the water in the

marsh area, additional shore line was added to extend the placement further into

the marsh (see Photo 14). Two hydraulic backhoes (swamp buggies) mounted on

swamp tracks were used to move the pipe and build temporary dikes to direct

discharge flow (see Photo 15). Other than the temporary dikes, no other con-

tainment structures were used in the placement process.

Photographs of the BD and AD placement site are shown in Photos 29 and

30, respectively. BD and AD placement site surveys were conducted by the

contractor's survey crew and inspected by MVN, using airboats, mobile DGPS

for horizontal positioning, and spirit leveling for vertical control. The results

from these surveys are plotted on Figures 11 and 12 (BD and AD surveys,

respectively). The difference plot between the BD and AD surveys is shown in

Figure 13. The dredged material deposit's footprint covers an area of approxi-

mately 20 acres. From the surveys, the contractor calculated a deposition volume

of 177,700 cu yd. Assuming a 20 percent loss in fines in runoff, this volume was

adjusted up to a gross volume of 222,000 cu yd. Contract specifications required

that no dredged material exceed a vertical placement height of +3.5 ft MLG, but

Figure 12 (referenced to the MLG datum) indicates elevations that exceed

+3.5 ft. This resulted from the impact of an inspection conducted 13 June of the

placement area, when USACE and OAS personnel discovered several least tern

and American avocet nests containing eggs (Photo 31). The nests had been

constructed some distance from the active placement point and were not being

disturbed, but MVN decided to terminate the grading operations, along with the

dredging operations, at 2100 hr on 13 June to preclude any damage to the nests.

This rapid colonization by the least terns and American avocets, as well as other

species, is an aspect that should be considered for future dredging projects of a

similar nature.

Chapter 5

Beneficial Use Analysis