For the last 5 years, Audubon has owned and been operating a mini-dredge on its sanctuary property to build marsh under Coastal Use Permit #P20090649 (CUP). Through a NFWF grant, we have recently been presented with a unique opportunity to demonstrate the use of another small dredge, the Amphibex 400 (Figure 1) owned by Crosby Dredging Co., to create marsh in the same permitted area. The Amphibex is only 13 feet longer than our mini-dredge but can move 10 times more material in the same amount of time as the Audubon mini-dredge. A new Audubon-owned, GPS-linked fathometer system will allow us to monitor and document the refill rate of the borrow area.
In 2009, Audubon commissioned a dredge from a local construction company that could be used by landowners, putting cost-effective restoration capabilities into their hands. Unlike larger dredge designs, it was constructed as a simple tool and has limited utility in terms of creating substantial acreage. This was our mini-dredge, the “John James.”
Between 2010 and 2013, we managed to fill one acre of open water (see before/after photos at end). This left 15 acres of calculated fill in the existing CUP, and a permit extension to 2017 was obtained for further work in the same dredge and fill footprints.
With the greater capability of the Amphibex, instead of the marsh terraces proposed in the permit extension documents, 5-10 acres will be filled at the west end of the permitted pond to increase the marsh integrity of the area (Figure 1). Reduction of wind fetch and slower water flows, reduction of edge erosion, and promotion of vegetative growth (both marsh and submerged aquatics) is expected in the remaining pond. The new marsh will also benefit various species of marsh birds of conservation concern, as well as other wildlife and estuarine species. Changes in water clarity, vegetation species diversity, expansion in the remaining pond, persistence of the created marsh and refill rate of the borrow area will be monitored.
Comparing data between fill areas in the same pond by two different dredges will allow us to provide guidelines for use of this novel restoration technique. A cost and logistic comparison to Audubon’s mini-dredge will be included in Audubon’s Business Plan for Landowners on the Use of Small Dredges in Louisiana (2016). This project will further demonstrate capabilities of small dredge technology.
The project location is within the Audubon Paul J. Rainey Wildlife Sanctuary in Vermilion Parish, southwest Louisiana (Figure 2). A 16-acre pond that was created when Hurricane Ike removed marsh next to a dead-end oil & gas location canal was selected and permitted for dredge and fill with a small dredge.
The average water depth of the pond to marsh level is approximately 10-12 inches. Material will be pumped to a target of 0 to +3 inches above surrounding marsh substrate and will be contained using bagasse/hay bags, Roseau cane bundles, or untreated plywood and 2x4s that will naturally degrade and decompose over time; and connected to marsh at one or both ends to allow for overflow filtering of the effluent and as a source of vegetative colonization (Figure 3). Overflow of effluent into the surrounding marsh is expected, and acts as a fertilizing agent enhancing the health of that marsh. The Amphibex will work for 30 days, and the acreage created will depend upon the character of the fill.
Because of the fine-grained nature of the fill material, stacking above water level is not likely, self-levelling occurs, and natural drainage is expected to develop augmented by the movement of resident alligators as documented in the nearby mini-dredge marsh creation site. Therefore, no reworking of material will be needed.
The pond bathymetry and topography will be surveyed before fill is initiated, after dredging is completed, and then periodically for at least 3 years. To document plant succession, we will catalog plant species composition and measure vegetative growth. Three areas separate from contiguous marsh will be manually planted by collecting nearby plants, to compare rates with natural colonization.
Initial measures of success will be the number of acres of subaerial mudflat successfully created. Long-term measures of success will include increase in marsh area, improvement in water quality that promotes submerged aquatics, and persistence of the created marsh for a number of years.