Small-scale marsh restoration at the Paul J. Rainey Wildlife Sanctuary was officially started on November 22, 2010 with the first mud pumped from a dead end canal to the target test area by the Audubon owned mini-dredge, the John James. This is part of a project initiated through and partially funded by TogetherGreen.
The dredge was designed as simply as possible. The concept was to develop a user-friendly dredge that would be small enough to be portable, but powerful enough to efficiently move sediment for small-scale marsh restoration work in areas of limited accessibility, and be cost-effective for landowners to use without having to wait for state or federal large-scale projects. We wanted to put marsh restoration capability into the many hands of the people motivated by local concerns.
The “dredge” is an open pontoon barge 9ft wide, 24ft long, and 12ft high; drawing about eighteen inches of water. It is not self-propelled so requires a tender boat to move it into place. The pump is suspended from the overhead beam by a hydraulic winch to move it up and down and a tractor drive to move it forward and back. Spuds on the rear of the barge act as stabilizers and pivoting hold-fasts, and two winch-controlled front cables attached to anchors move the barge side to side.
To dredge, the 15hp Toyo submersible pump is lowered to the substrate where water jets on the bottom ring help break up the accumulated sediment and the recessed impeller pushes it up into the 4-inch diameter transport hose. This is more efficient than a topside pump sucking it up.
We found that in our thick sticky mud, we could not drag the pump across the substrate, but had to raise and lower it like a cookie cutter. Approximately one hour is needed for a 35-40 ft arc of the dredge, dropping the pump about 10-12 times. Each “hole” seems to be about 3-4 ft wide. Since it isn’t an excavator, we don’t have to worry about over dredging – it won’t cut into the original, consolidated bottom of the canal.
At its maximum, this pump is said to be capable of moving up to 70% solids at a rate of 20cy/ hour, although we have not been able to acheive this. Tests of our system showed we produced 18 cy/hour of dense slurry with the potential to produce 25cy/hr through a hose that is 560 feet long. A larger pump could move more sediment, but would require a 10” hose that is much more difficult to handle. We move mud to a distance of 600 feet, and it takes roughly 200 hours to fill an acre to 1ft depth.
Though this may sound like a slow process, the dredge has relatively low fuel requirements (5 gallons of diesel per 6-hr day) and can be handled by two people for low per-acre costs. The effluent can be flowed directly from the pipe into open water or sprayed to distribute the sediment more evenly through a fragmented marsh.
Small dredge technology has great potential as a tool for the coastal landowner, marsh management toolbox, but like any tool, it has specific and limited applications. This dredge was not intended for open water locations where it would be subject to much wave action. Nor is it meant for deep-water work—the spuds are only 20 feet long. Marsh restoration with a small dredge is relatively time consuming, and requires a bit of resourcefulness to overcome inevitable challenges posed by each specific site.
As designed, the best application of the small dredge technology is in shallow, limited access waterways for small projects (<5 acres) within 600 feet of a sediment source.
Some suggested applications include:
- augmenting fragmented marsh,
- enhancing existing marsh with thin sediment layering,
- filling an acre or two of interior marsh,
- Dual purpose dredging such as clearing a boat slip or a park pond and restoring or enhancing nearby degraded marsh
- clear out and reset sediment traps in the Atchafalaya Floodway that decrease sedimentation in the sensitive cypress swamps, and perhaps barge the sediment to an area that needs it.
The coastal marshes are riddled with abandoned canals, some of which act as sediment traps as valuable material is eroded out of the marsh. That sediment is not likely to move back into the adjacent, starving marsh because of spoil banks and direction of flow, nor are the canals likely to fill in completely and revegetate on their own. A squadron of small dredges could visit these canals on a rotating basis to replace the sediment back into the marshes from which it came. Even a small influx of sediment into the marsh system would help sustain and enhance these critical coastal habitats.
The Audubon Louisiana’s small dredge and others like it can’t solve all of Louisiana’s marsh loss problems, but they could help in a local setting to replace a few acres at a time – and if that acre or two was in your backyard or your park, it would make a difference to you and the wildlife in your area, and it would be an acre or two that isn’t lost. And if your neighbor and his neighbor did it, there would be that many more acres not lost.
The next steps in this process are to explore ways of improving efficiency and reliability, compare the cost and capabilities of other small dredges, work with the state to streamline marsh creation permits, work out a business plan for use by landowners, and start making marsh!
NOTE: National Audubon Society is not in the dredging business, although we would be pleased to show you our operation. Please contact Javeler Construction Co, Inc for costs of small dredge construction or rental rates.