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Mud Cat's Technical Papers

USING GEOTUBES® FOR ENGINEERING
AND ENVIRONMENTAL PROJECTS
by
Robert N. Blama
Project Manager/Ecologist
Operations Division
U.S. Army Engineer District, Baltimore
Baltimore, MD

The U.S. Army Engineer District, Baltimore (NAB), in conjunction with the National Marine Fisheries Service, conducted a series of demonstration projects using dredged material. One such beneficial use project was located at Twitch Cove, Smith Island.

Smith Island, Maryland and Virginia, is one of few remaining inhabited islands in the Chesapeake Bay. It has historically been surrounded by a myriad of submerged aquatic vegetation (SAV). In recent years, SAV has declined throughout the bay and attempts have been made to reestablish the plants. In addition to the loss of plants, the island itself has experienced losses as a result of erosion.

During the period of June and July 1987, a Federal navigation channel at Smith Island was dredged and the materials, largely silts, were placed along the Eastern shore of the island at Twitch Cove. About 24,000 cu yd of dredged material was placed in order to shoal the bottom depth and provide suitable substrate for establishment of a seagrass bed. The bed would in turn provide a nursery habitat for juvenile fishes and a shedding area for blue crabs. The Twitch Cove site was selected because of its proximity to the navigation channel to be dredged and the absence of SAV. It was hypothesized that the absence of seagrass at the site was due to deeper than optimal water depths, which resulted in insufficient available light under prevailing turbid water conditions. By raising the bottom with dredged material, conditions were intended to be brought within appropriate light levels for seagrass establishment. The area was to be planted with eelgrass (Zostera marina), a native plant found in the Smith Island area.

To help protect the plants from damaging wave energies and retain the dredged material from dispersing, three Longard tubes were placed along the site in a configuration of the letter "L." A Longard tube is a double-lined polyethylene impermeable inner liner and geotextile outer liner, about 110 cm in diameter and 100 m in length. Two holes about 0.5 m in diameter are cut into the fabric and a flange is attached. The tube is then stretched to its entire length and filled hydraulically with sand. The system used was an independent hopper with a 6-in. line. As the tube is filled, it settles to the bottom and once filled, the two holes are capped. The tube is sturdy enough to be walked upon and provides a surface for epibenthic organisms. The weight of the tube prevents it from rolling and also acts to dissipate wave energy to produce a quiescent environment behind the tube.

The dredged material was placed behind the tubes and allowed to settle for about a month, and a 3-acre site was planted. Planting was conducted in September 1987 using a transplant spacing of 2 ft on center. This spacing required the planting of 32,670 units of eelgrass. The plants were obtained from nearby native stands of vegetation.

The substrate elevations throughout the planting area ranged from -2.7 to -4.3 ft mean low water. The tidal range in the area is 2.0 ft and, therefore, covered the tubes with 1-2 ft of water at high tide, which may not have provided the wave dampening effect anticipated.

To obtain a quantitative measure of seagrass survival at the site, the National Marine Fisheries Service conducted surveys in July 1988 and June of 1989 and 1990. A grid of 150 points was laid across the planted area. In July 1988, the survey yielded 33 eelgrass contacts at the 150 grid points. Extrapolation of the 150-m2 sampled area to the 12,541.5 m2 total transplant plot gives an estimate of 2,822 m2 (22 percent) site coverage after 1 year. This represents about 0.69 acre of seagrass habitat. The survey indicates that a substantial portion of the initial transplants had been lost within the first year, but also that the planting units in the central portion of the plot had undergone some detectible growth.

In June 1989, the site was resurveyed in a manner identical to that of the previous year. A total of 31 eelgrass contacts were obtained, and in 1990, 37 sea grass contacts were encountered.

Survival of eelgrass in the central portion of the plot for 2 years after transplanting indicates the success of the Longard tubes to prevent eradication of the plants by storm events or the transporting of dredged material away from the site. Over time, the dredged material can be expected to consolidate and further stabilize. The fact that survival was largely limited to the center of the plot may reflect a response of the planting units to adequate lift of the preexisting bottom only in that area. The volume of dredged material placed at the site may not have been sufficient to optimally raise the bottom. The distribution of surviving seagrass may also be indicative of the amount of protection provided by the Longard tubes. An additional benefit derived from the Twitch Cove project is from the natural recruitment of widgeon grass (Ruppia maritima), which is found in the area.

Another use of geotextile tubes (Nicolon) in the District was to serve as offshore segmented breakwaters for erosion control. The U.S. Fish and Wildlife Service constructed segmented rock breakwaters adjacent to their Eastern Neck Wildlife Refuge. The NAB extended the breakwater design by using two geotextile tubes the same length as the existing breakwaters (about 75 ft), placed dredged material behind the structures, and planted with cordgrass (Spartina alterniflora and S. patents).

One tube was placed parallel to the breakwaters but was at a depth that it is covered by mean high water (mhw). The other tube was placed closer to shore so it would extend above the surface at mhw. The tubes were filled directly from the dredge by using a 12-in. discharge line inserted into the tube. The use of a "Y" valve diverted some of the material as beach nourishment and also controlled the flow of material entering the tube. The tubes were semipermeable, which allowed the water to pass through but retained the material. Since the material was dredged from the channel, it contained all grain sizes along with shell and other material.

After placement of the material, the dynamics of the wave action redistributed the sand to form the expected tombolos. It appears that the Geotubes® are working in the same manner as the breakwaters since the depths behind the tubes are becoming shallower. The nearshore tube has captured enough sand that the beach has extended to the tube. The tubes are also acting as substrate for benthics and algae and many fish have been seined along the tubes.

Discussion

BG Locurcio asked if there were toxins in any of that fill. Mr. Blama responded that the fill was tested and there were no toxins. It was clean fine sand.

(Geotube® is a registered trademark of Ten Cate Nicolon)