Between 2009 and 2014, Audubon owned and operated a mini-dredge on its sanctuary property to build marsh under Coastal Use Permit #P20090649 (CUP). Through a NFWF grant in 2015, we were presented with a unique opportunity to demonstrate the use of another small dredge, the Amphibex 400 (Figure 1) owned by Bertucci Construction Co., to create marsh in the same permitted area. The Amphibex was only 13 feet longer than our mini-dredge but moved 10 times more material in the same amount of time as the Audubon mini-dredge. A new Audubon-owned, GPS-linked fathometer system allows us to monitor and document the refill rate of the borrow area.
The project location was 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 adjacent to a dead-end oil & gas location canal had been permitted for dredge and fill with a small dredge.
Between 2010 and 2013, the mini-dredge was used to fill one acre of open water. This left 15 acres to fill in the existing CUP, and a permit extension to 2017 was obtained for further work in the same dredge-and-fill footprints. However, instead of utilizing the mini-dredge, we were able to use the Amphibex for this work.
Amphibex 400 proposed to work for 30 days to fill at least 5-10 acres of the permitted pond with slurry, allowing overflow to increase the marsh integrity of the area. The actual result was 15 acres filled with a thick slurry in 23 days, working sometimes 24 hours/day. High water at the time of material transport allowed higher resultant stacking of fine material, as well as loss or settlement through the surrounding marsh (Figure 3).
Reduction of wind fetch and slower water flows, reduction of edge erosion, and promotion of vegetative growth (both marsh and submerged aquatics) was expected, and observed, in the remaining pond. The new marsh benefits a variety 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 is being monitored.
The average water depth of the original pond to marsh level was approximately 10-12 inches. Material was pumped to a target of +3 to +12 inches above surrounding marsh substrate and was partially contained using existing marsh landscape and bagasse/hay bags pinned between 2x4s that naturally degraded and decomposed over time. Overflow of effluent into the surrounding marsh was expected, and filled depressions and sub-aqueous channels, acting as a fertilizing agent to enhance the health of that marsh.
Because of the fine-grained nature of the fill material, stacking above water level was temporary as it slowly spread by its own weight to self-level. An airboat was used during the pumping process to help spread the more cohesive material away from the outfall and into areas it would not naturally flow. Natural drainage developed and was augmented by the movement of resident alligators. Therefore, no reworking of material was needed as the site matured and it was allowed to naturalize.
Vegetative plantings had been planned, but the natural colonization seemed to be more than adequate and it was not worth disturbing the soft, shallow substrate to use mechanical means to plant more vegetation.
The pond bathymetry and topography was surveyed before fill was initiated. Elevation after fill was estimated by water level and marker poles on the ground until a drone was acquired in 2016. Plant species composition and vegetative growth was documented by aerial photography collected by drone or Google imagery every three months.
The canal bathymetry is being monitored to establish a refill rate for possible reuse in the future. 24,300 cubic yards of material were removed and transferred to the fill area, and as of July 2018, 4,559 cubic yards have accumulated. If this rate is linear, it suggests the same canal could be used again for borrow in 10 years.
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 Guidance for Utilizing Small Dredge Technology for Restoration of Marsh on Private Properties (2018). This project further demonstrates the capabilities of small dredge technology.