February 29, 2016
For the last 5 years, Audubon has owned and been operating a mini-dredge, the “John James” on its sanctuary property to build marsh and to explore the cost and feasibility of such equipment for use by other landowners. Through a NFWF grant, we were recently presented a unique opportunity to demonstrate the use of another small dredge, the Amphibex 400 owned by Crosby Dredging Co., to create marsh in the same permitted area. Still considered a small dredge, the Amphibex is only 13 feet longer than our mini-dredge, but it can move 10 times more material in the same amount of time as the Audubon mini-dredge.
We were given this opportunity, in large part, because we had an area already permitted for marsh restoration by a small dredge, and could get started immediately. Our small dredge had only filled a little over an acre of a 16 acre permitted area (Figure 2), and we had just renewed the permit to start another project. Bertucci had just acquired the Amphibex dredge and were eager to test its capabilities, so worked with us to make the proposal to NFWF possible. They agreed to give us 30 days of 24 hr/day dredging and a lot of in-kind services to train their crew and see what the Amphibex could do. We wrote the proposal in June, it was accepted in September and we started work in October. The quick turn-around was necessary to get the dredging done before the usual winter weather lowered water levels that would prevent access.
Prior to dredging, we place a number of tools into the pond to keep track of water levels and mud fill. PVC poles with color marks were used as water level poles and were placed on a grid to visually check fill level (Figure 3). The green mark was at the average level of the surrounding marsh (ML) which is approximately 1 foot above Mean Sea Level and was considered the end goal for optimum marsh growth. The center of the color marks was 2 or 4 inches above or below ML. The top of the pole is +12” ML. Because the churned up sediment would dewater and settle lower than the initial placement height, we hoped for anything between 3 and 8 inches above marsh level. If too low, marsh plants could not thrive; if too high, it would more than likely result in bushes instead of marsh.
In addition to the water level poles, 30 settling disks were placed randomly throughout the pond to measure fill and monitor compaction over time (Figure 4). Perforated metal disks were placed with a snow pole through one of the holes so that the metal disk would lay on the bottom. When covered by mud, we could probe through the mud with a measuring stick to the pan to measure height of fill and amount of depression of the bottom by the overlying weight. In addition, the bottom blue portion was at ML and could also be used to watch waterlevel and mud level.
Both of these instruments were used to visually estimate the progress of the fill, and later the progress of settling. Along with a known water level, maps could be created showing the estimated status of the target pond.
Audubon owns a GPS linked fathometer system for measuring shallow water depths. It can be mounted on any boat including our airboat. We used it to obtain data on the depths of the borrow canal and the target pond before dredging, and measured the canal immediately after dredging (figures 5 & 6). Not wanting to disturb the new unconsolidated fill in the pond, we have not measured it yet, but will wait for high water at a later time. The canal will be surveyed every six months to monitor the refill rate.
Our mini-dredge was a great little machine and had created an acre of mudflat in 220 hours of dredging. This was spread over 2 years as Timmy and I had time to operate it, and after 3 years the vegetation had almost completely filled it in (Figure 3). We learned a lot about how mud moves, how high to stack our fine material to make marsh level, and what conditions were good for retention. All of the operation was detailed in weekly or monthly updates on the “Dredge blog.”
On October 9, 2015, the Amphibex 400 began pumping into the permitted pond from the permitted canal, dredging material from the entrance to North Canal on the east and moving backwards toward the west.
Timmy and I discovered that the mud movement was similar to what we knew, but the volume of it coming out of a 10” pipe instead of our 4” pipe made some things very different. For one thing, the material actually stacked higher than we expected — the volume building high before its weight would cause it to slump and spread. Timmy went every day in the airboat to level the mound and spread it into areas it was reluctant to go. The heavier material falls out closest to the discharge, and material won’t fill areas where water has no outlet. We went into the pond twice a day to see how the fill was progressing, and to assess when to move the discharge end of the pipe to spread the heavier material around.
Another thing we discovered was that the Amphibex could move A LOT more material. The 220 hours that marked the end of the mini dredge project that took 2 years, was reached by the Amphibex in 10 days! In 220 hours, where we filled a little over one acre, it had created 9 acres of soft mudflat! Bertucci had promised us at least 8 acres and they still had 20 more days to dredge, so at this point we realized the entire pond would likely be filled (Figure 6).
The second half of the dredging period was plagued with high water, making visual assessments of the fill difficult. At times, I would stick my arm over the side of the airboat to feel for the soft, fluffy sediment layer to help in our determination of when the pond was full. We needed enough fill so that it would dewater and be high enough for plants to grow, but not so high that all we would get is bushes.
By November 5, we decided the pond was full. There were about 3 acres in the west end that completely covered all of the marker poles, so we knew it was at least 12 inches above marsh level in that area. The containment we had installed was rather flimsy, and we were afraid too much mud against it would blow it out. It was functioning very well as a wind and wave buffer and we didn’t want to lose it. Therefore, at 22 days with a total of 338 hours of dredging, the Amphibex 400 had effectively filled the remaining portion of our permitted pond to well above marsh level (figure 7). The post-dredging fathometer survey confirmed Bertucci’s assessment that 24,000 cubic yards of material was moved from the canal to the pond.
As the Amphibex pulled out, our job was just beginning. This marked the beginning of the two-year monitoring stage, and we planned to watch and measure mud settlement, bird use, vegetation growth and refill rate of the borrow area. November 6 was considered “Day 1.”
Marshbird and bird use monitoring is being done roughly every two weeks, along with photographic documentation of the water level and mud exposure. Two trail cams were set up at either end of the pond to monitor wildlife movement. They became even more important for monitoring environmental conditions, showing water level changes as well as storm events, explaining and illustrating what was occurring with the soft mud fill.
During the dredging operation, wading birds were attracted to the new mud, would fly over but could find no solid place to land. Egrets would perch on the marsh edge, the pipe itself, any floating object, and even pick things off the surface in flight. The new, soft mud was rich in food items, forcing fish, shrimp and crabs to the surface and exposing many benthic creatures pumped from the bottom of the canal.
High water, low water, in and out. Tidal movement, with flooding and draining, redistributes the sediment creating waterways to accommodate the flow. In time, the fine grained sediments either flush out or settle and dewater to become more firm. After 2 months, the exposed mudflats started developing a thin layer of bright green algae, proving that at least the sediment surface is stabilized enough for it to grow. Nature is reforming what humans have disturbed.
As time passed, the tide moved in and out, and the mud settled and became more firm, birds could finally walk on parts that were more solid. After a month, medium sized birds such as Black-necked Stilts and Avocets moved in. Later, dowitchers, a little shorter, arrived, then the short-legged peeps. Green-winged Teal discovered the shallow open water when the tide came up. By the end of 3 months, pelicans, cormorants, Roseate Spoonbills, Caspian Terns, and all the others find their own niche in the diversity provided by the settling mud and moving water.
The following two figures illustrate how natural processes have been redistributing the dredge fill into a more natural configuration. The sheer weight of the soft mud caused it to self-level while unconsolidated, with the high areas pushing down and out on the surrounding mud until pressures are equalized. Therefore it isn’t surprising that the high areas settled so quickly to match most of the fill area (Figures 23 & 24). The areas near the outfall locations are composed of larger particles and are more dense than other areas, so should spread and dewater less to stay somewhat more elevated. Tidal flow moves through the area from the west end where the containment is located, and from two major areas through the marsh on the east side, reforming the fill into waterways that can accommodate the flow.
If the west end stabilizes and vegetates quickly, it will act as a buffer for the eastern parts of the pond to stabilize, retaining the desired high diversity of water and mud elevations for a more natural, marsh-ecosystem restoration scenario.
Karen A Westphal and Timmy J Vincent, National Audubon Society Audubon Louisiana and the Paul J Rainey Wildlife Refuge 5615 Corporate Blvd, Suite 200B, Baton Rouge, LA 70808 225-768-0921x202 office, firstname.lastname@example.org