2003 - Innovative Alternative for Wetlands Restoration

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Transport and distribution of Dredged material by large Hovercraft. This has pertained to a couple of areas in Alaska and occasionally in the continental US.

INNOVATIVE ALTERNATIVE FOR WETLANDS RESTORATION: TRANSPORT AND DISTRIBUTION OF DREDGED MATERIAL BY LARGE HOVERCRAFT

Trudy J. Olin, Michael R. Palermo, Anthony C. Gibson
Environmental Laboratory
US Army Engineer Waterways Experiment Station
Vicksburg, MS 39180

Abstract

Thin layer placement of dredged material in wetlands areas has been proposed as a viable disposal alternative that could address the need for sediment and nutrient resources for deteriorating wetlands. The focus of this study addresses the technical and economic feasibility of a conceptual "hoverbarge" as a transport mechanism for dredged material nourishment of wetlands.

Transport and distribution of dredged material in coastal wetlands is motivated by the need to address severe and ongoing losses due to subsidence and impacts of anthropogenic activities. Hovercraft have been proposed as an environmentally sensitive transport alternative. The scope of the study was constrained by limited available data pertaining to environmental effects of hovercraft traffic on wetlands and lack of actual performance and cost data.

Environmental Effects

Studies of the impact of hovercraft on wetland soils and vegetation are limited but generally seem to indicate that effects are minimal and temporary, except under certain circumstances (Planning Systems Inc. 1984). However, the effects of continuous operation over a restricted area has not been evaluated.

Technical and Economic Feasibility

Technical Feasibility. At the time of this study, a hoverbarge prototype had not been developed. The technical feasibility of hovercraft transport of dredged material into wetlands was evaluated based on identifiable technical issues, status of the current technology, necessary physical and operational modifications, and potential interfaces with dredging operations. The principal technical issues include: load capacity, stability, loading/offloading operations, performance, and environmental considerations.

The load capacity of hovercraft is limited; currently 75 tons maximum domestically. The maximum feasible payload size is projected to be 300 tons, according to industry experts. A 300 ton payload corresponds to a volume of approximately 247 cuyd, depending upon the material and solids concentration, as compared to the 1500 to 4000 cuyd capacities of conventional barges. Interim storage of sediments would be required because typical dredged material production far exceeds foreseeable utilization rates. The stability of hovercraft in transporting bulk cargo is unknown. Trim and balance problems have been reported under certain conditions. Modifications of the craft to address this concern, as well as to facilitate loading, offloading and distribution of materials, would be required. Operating performance in a low speed, high capacity application has not been demonstrated.

Economic Feasibility. The economic analysis of the hoverbarge concept was based on actual capital and operating costs of military hovercraft, the LCAC and the LACV30, and industry estimates. Because significant differences exist between the proposed hoverbarge prototype existing hovercraft, the cost analysis cannot be considered to be definitive. However, an estimated cost range was established which was compared to pipeline transport, the most economical conventional transport alternative for the volumes and distances projected (Souder, Paul S., Tobias, Leo, Imperial, J.F., Mushal, Frances C. 1978).

Conclusions

From a technical perspective, the hoverbarge concept appears to be viable but the load capacity of hovercraft is limited and the volume of sediments that can be transported is small relative to conventional means of transport. Estimates indicate that due to high capital and operating costs, the use of hovercraft to transport and distribute slurried sediments will be significantly more expensive than other methods available. Environmental justification as a compensating factor has not been adequately demonstrated at this time.

Literature Cited - Planning Systems Incorporated 1984. "Environmental Assessment (EA) for Landing Craft Air Cushion (LCAC) Program at NCSC, Panama City, Florida", PSI Job No. 1293, NCSC Contract No. N61331-84-M- 0127, Naval Coastal Systems Center, Panama City, Florida.
Souder, Paul S., Tobias, Leo, Imperial, J.F., Mushal, Frances C. 1978. "Dredged Material Transport Systems for Inland Disposal and/or Productive Use Concepts", Technical Report D-78-28, General Research Corporation, McLean, Virginia.