Marsh Restoration using Small Dredge Technology at the Paul J. Rainey Wildlife Sanctuary in Southwestern Louisiana
March 26, 2011
The mini-dredge was a concept born of need. The hurricanes of 2005 (Rita) and 2008 (Ike) destroyed hundreds of acres of marsh in the National Audubon Society’s Paul J. Rainey Sanctuary (Rainey) and the surrounding area of Vermilion Parish, Louisiana, leaving behind shallow, open water areas that are not likely to fill in or re-vegetate under current conditions. A way to economically promote regrowth and restoration of the lost marsh was highly desired.
To dredge and pump sediment back into some of these more remote and sensitive areas through traditional methods would cost tens of thousands of dollars even if the work vessels could fit in the smaller waterways. Dr. Irv Mendelssohn’s research group from Louisiana State University Biogeochemical Laboratory conducted an experiment with a small pump to add sediment to small marsh plots, and success led to questions of how to do this on a scale between the two extremes.
Developing the Dredge
Paul Kemp and Melanie Driscoll of National Audubon Society Louisiana Coastal Initiative approached a local construction company, Javeler Construction Co, Inc., out of New Iberia, with an idea for a portable, small-scale dredge that could be used by landowners in limited access areas. Funding through the Together Green organization made the concept tangible and provided $50K for marsh restoration activities at Rainey, but the funding provided could not be used for dredge design and construction. Fortunately, the construction company was so excited by the idea, they decided to donate the finished prototype, estimated to be worth $100K, to Audubon for testing, proof of concept, demonstrations, and educational uses.
The “dredge” is a barge 9ft wide by 24ft long, drawing about a foot of water (Figure 1). It is not self-propelled so requires a tender boat to move it into place. The pump is suspended from a winch on an overhead track so that it can be moved 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. The 15hp submersible pump is lowered to the substrate where it agitates the sediment and pushes it up into the transport hose under pressure. This is more efficient than a topside pump sucking it up, and theoretically will pump up to 70% solids at a rate of 20cy/ hour through a 4” hose, up to 1000 feet away. At this rate, it requires 75 hours to fill an acre to 1ft depth. Though this may sound like a slow process, the dredge has relatively low fuel requirements and can be handled by two people for low per-acre costs. The effluent under pressure can be flowed directly from the pipe into open water or sprayed to distribute the sediment more evenly through a fragmented marsh (Figure 2).
Design and construction took longer than expected, because the construction company tested various alternatives to make it more efficient. Instead of the trailerable version Audubon had envisioned, it became a truckable version that could pump sediment more efficiently. Mobilization costs would still be 10 times less than traditional dredging operations.
On June 22, 2010, the dredge was finally trucked to Intracoastal City, where Audubon took possession and towed it to the Paul J. Rainey Wildlife Sanctuary headquarters. Audubon staff then customized the dredge for safety and comfort including shade and solar lights. A few technical upgrades allow the dredge to be operated by one person once dredge pipe is connected and it has been pushed into place by another boat. The first mud was moved into a test area for marsh creation in November 2011. Timmy Vincent and Karen Westphal of the National Audubon Society Louisiana Coastal Initiative are co-leads for marsh restoration projects using the small dredge.
The Marsh Restoration Plan
The Paul J. Rainey Wildlife Sancturay headquarters is in Vermilion Parish, on the west side of Vermilion Bay in southwest Louisiana (Figure 3). The cumulative impacts of the hurricanes of 2005 (Rita) and 2008 (Ike) ripped out marsh and opened up new shallow ponds. The numerous stretches of open water were not likely to fill in or re-vegetate naturally, and now interior marsh is in danger of erosion from wind driven waves. Using funds from a grant from the Walton Family Foundation, Audubon contracted with LSU to help us learn how to effectively make use of this new technology. The original LSU study site was at the end of a small oil & gas canal not far from the Rainey headquarters. A 16-acre pond near the LSU site and close to a canal was chosen to test the small dredge (Figure 4).
The current funding supports the LSU study plan to fill a test cell and 5 study cells, each measuring about 1/3 of an acre, for a total of 2 acres to be filled of the 16 acres permitted (Figure 5). With their help, we will learn the best method to fill areas with the material available, such as what kind of containment is necessary and how high to stack it so that when it de-waters it will be at an appropriate marsh level. Cost analysis will done for cost/acre assessment. We plan to share our findings with other landowners and the public.
Current Marsh Creation Status
Small-scale marsh restoration at the Paul J. Rainey Wildlife Sanctuary officially started on November 22, 2010 with the first mud pumped from the adjacent canal to the target test area. Timmy Vincent and Karen Westphal of the National Audubon Society handled the dredge and the Louisiana State University group consisting of Dr. Irv Mendelssohn, Sean Graham, John Cross and Joe Baustian handled the pond preparations. Settling disks for measuring the depth of fill were placed in the pond connected to cane poles that were marked with the target fill-elevation. Eddie Weeks from LSU brought a remote controlled plane mounted with a remotely operated camera to provide an aerial overview of the operation (Figure 6).
The 4-inch diameter dredge pipe used to convey sediment from the dredge in the canal to the pond was assembled on top of the previously constructed walkway to have a limited impact on existing marsh. The test area is being used to explore various dredge and fill techniques as we learn how to operate the small dredge most efficiently. LSU has selected the eastern end of the 16-acre pond for their study area, and dredging to fill each of the 1/3-acre cells will be started this spring.
The first trial was with the open end of the pipe curved around under the walkway and into a small cove on the east side of our test area. We had access to the pipe end this way to watch closely the result of what was done at the dredge. The aerial pictures were taken during this time. The next trial was with a “spreader” attached to the end of the hose that sprayed the mud from 6 feet high over a larger area (Figure 7). Both resulted in a build-up of material that was readily apparent. The heavier parts of the material settle relatively quickly in the quiet waters of the shallow embayment, with the plume of finer material spreading very slowly into the larger pond. By the next day, the water was clear enough to see the bottom and observe the results of our time and effort.
We tested a no-containment scenario first to see if the sediment would “stack” or build up on top of itself and stay in place. The sediment did settle out of the water rather quickly, but just as quickly, the flowing mud made streams that quickly conducted the mud out to the greater pond. The extreme low water conditions that persisted throughout the severe winter would not allow a buildup of sediment, and the wind-driven waves tended to re-distribute the new sediment away from our test area. It became evident that containment, a barrier of some kind, was necessary.
What to use for containment was discussed in great detail. Originally, the Natural Resource Conservation Service (NRCS) wanted to test a silt boom with our project, but decided our 1-3 ft depth was too shallow for an appropriate test. Readily available silt fencing was determined to be not sturdy enough. Plywood and 2x4s could make a wall, but seemed extreme and would require a lot of labor. Bales of hay or rice straw were considered, but there was no way to be sure of what pesticides, other chemicals, or weed seeds would come along with them, and transportation would be an issue. We also have an unused length of oil boom in storage since the BP event that might be useful. We needed something that would be bio-degradable and wouldn’t have to be removed later, would not bring in weed seeds or toxic chemicals, would let the water out and keep the sediment in, keep waves from moving mud around, and could be easily transported and handled by the staff on hand.
Then, as we were clearing the path for the next walkway, it became obvious that suitable material might be all around us. In the Rainey area, any land elevated above marsh is covered by tall cane (Roseaucane or Phragmites). Tall cane can be cut and tied into bundles that are easy to carry, stack, transport and tie down, and it is a renewable and almost unlimited nearby resource. It’s a local and native organic addition to the marsh mud. We cut cane (Figure 8).
Small tallow tress (Sapium sebiferum), which are a common invasive species, were cut to use as poles. We used the boardwalk for convenience as we tested the concept. The bundled cane was laid within the poles in an overlapping fashion, stepped on to mash them into the mud and each other, and then tied within the poles so they would not float.
The next dredging session was exciting and encouraging as the containment did everything we had hoped. It diffused the flow so that there were no large exit streams, it raised the level of the water by an inch or two, and it stopped wind agitation so that the majority of the mud stayed in the enclosed area (Figure 9). We are aware that the containment will need to be monitored and augmented as the sediment builds up, but the first tests have been extremely positive. In addition, within the 15-30 minutes it took to shut the dredge down and get to the pond to view our progress, black-necked stilts and sandpipers were already on site to catch the minnows that were pushed to the top half inch of clear water (figure 10).
Opportunities provided by Additional Funding
Audubon has permitted the entire 16-acre pond, and would like to maximize marsh-building potential of the mini-dredge not only at Rainey (Figure 8), but in other areas as well. Additional funding would make this possible.
“Pennies for the Planet” is providing funding to do marsh grass planting to help stabilize the filled areas, and shorten the time to create marsh habitat. We are hoping to get local school children to grow the plants for this purpose, to encourage local participation.
We don’t intend to go into the dredging business. We will be restoring marsh on our own land or as demonstration projects for other landowners, and hope to use it as a catalyst to work out a permitting method with the state that is more streamlined and suitable for restoration of smaller areas. We plan to hold a few workshops for the public when we have more experience with all of the issues involved, and do a cost/time per acre analysis.
Additional funding would help with any of these later initiatives.Karen A Westphal, National Audubon Society Louisiana Coastal Initiative, 6160 Perkins Road, Suite 215, Baton Rouge, LA 70808, 225-768-0921 office, email@example.com