August 15, 2013
Total time pumped directly into:
- Cell 5 (0.307 acres) = 72.8 hours
- Cell 4 (0.352 acres) = 72.2 hours
- Cell 3 (0.366 acres) = 85.0 hours
Total Time in Study Cells = 231.5 hours
- Estimated cost per acre = $6,000 (excluding labor)
This report can be downloaded in PDF file format by clicking the Dredge and Rainey Report, One Acre Update hyperlink.
Summary of the Small Dredge Project:
In 2009, we established a Coastal Use Permit to fill up to 16 acres of shallow open water that was created by hurricane Ike (2008), using a small dredge to move material that had accumulated on the bottom of an adjacent access canal. A test area was created to learn how to use the small dredge that had been constructed and donated to Audubon Louisiana. An experimental area was identified and pre-surveyed in cooperation with a research group from the Biogeochemistry Institute at Louisiana State University in Baton Rouge, and divided into cells for research replication. The dredge and fill operation began in the experimental cells on June 9, 2011, and was considered complete on July 9, 2013 when the third cell was determined to be full.
In summary, it took two people 231.5 hours of pumping with the mini-dredge to fill 1.021 acres. An additional two cells received a good bit of fill as overflow, but this could not be separated from the total. The work was spread over a period of 2 years as we had time, as containment was constructed, as we made repairs and performed maintenance, and the conditions were right. Had we more people to run the dredge continuously (with no equipment failures) when conditions were right, we could have completed the job in ten 24-hour days or twenty-nine 8-hr days.
The estimated cost of operation to fill an acre with this mini-dredge was roughly $4,300 (Table 1). Operating expenses included diesel costs to run the dredge, boat fuel to transport the operators, and general supplies that were consumed during the general day-to day operations. There were site specific and one time costs like the boardwalk and containment that would vary by site, or safety equipment and upgrades we found necessary. These totaled about $4,500 for us. Maintenance costs for oil changes, and replacement equipment as we wore it out totaled about $2,100. The total expenditure in this project excluding labor was $11,031.
Labor is the biggest issue. If the landowner operates the equipment himself or has people already in his employ that can spend some of their time dredging, this becomes a more affordable endeavor. I used $25/hour as a general rate for an operator, and if they were only spending time dredging, an acre would then cost about $11,550 to fill.
Using these numbers, an acre for $22,581 still sounds like a deal when compared to large-scale operations where 200 acres costs multiple millions of dollars – especially if you only need a few acres restored. Just the cost of mobilizing a large dredge covers the cost of several acres, if not the purchase of the dredge itself!
I can’t begin to calculate our own labor costs. Since this was a prototype, we had to spend time adjusting things for safety, comfort, efficient operation, and to reduce wear and corrosion. We had to learn how to use it, and to explore its limitations and capabilities, as well as our own.
Site preparation for us was the clearing of the levee for a staging area, construction of boardwalks to prevent constant traffic from destroying the marsh root platform, construction of containment, and getting the hose into position. There were only the two of us to perform all needed work.
A lot of time was spent waiting on weather or appropriate water levels. Too little water and anything we pumped would scour under containment, too much water and we would lose material over containment or through the marsh. We took breaks to allow the mud to settle and de-water some. Because we had leaky containment, extra time was needed to replace mud that escaped. The lengthy operation meant repairs due to weathering and salt corrosion, upgrades, and maintenance.
We also had to work around our other job duties and personnel schedules. Had we a dedicated crew, or multiple crews, we could have worked around the clock when the conditions were ideal to fill the cells much more efficiently.
The two remaining cells that are marked out and contained would add another 0.655 acres, and we may yet resume dredging after a break to catch up on other Sanctuary activities. The fill that overflowed from the other cells brought the bottom up to an average of -7” ML, so those areas are well-primed. Additionally, filling the next cell would likely cause back-flow into the LSU research area, and we don’t want to compromise the results of their research.
However, we have determined that filling open water is probably not the best use of a dredge this size, and would like to pursue another project to fill broken marsh and monitor how that affects the surrounding degraded marsh.
Summaries by Cell
The cell arrangement and numbering were determined by the LSU research group, and we filled them in reverse order.
At 0.307 acres, Cell 5 was the smallest of the 5 cells planned and marked. Dredging began on June 9, 2011 and was completed by November 15, 2011 in 72.8 hours over a 5 month period. For a month of that time in August, the hose was disconnected and allowed to flow into Cell 4 while the water level was too low to add fill to the now shallow Cell 5. A good bit of time in July was spent installing containment along the outer perimeter of the experimental area, to try to contain as much overflow material as possible. A pontoon was also constructed to help move the outfall around.
Three small fingers of water that were connected to the Cell 5 were incompletely filled. The hose was not long enough to reach into them, and with no outlet, the current carrying mud would not flow into them. Mud overflowed the reed containment into cells 4 and 3. Vegetation was observed spreading into the new fill as early as August — 2 months after starting.
In September 2011, Tropical Storm Lee sat over the site for 24 hours. The storm surge came up 4 feet above normal, then dropped to 3 feet below normal, overtopping levees and inundating marsh areas with 11 ppt salt water. The winds shredded the blue tarp over the dredge and tangled the dredge hose under it, displaced the canoe and pirogue, and rearranged the hose in the pond. The storm also prompted a delay in dredging in order to start construction of the two buildings that were presently in the form of stacked materials, and that was feared might float away in a bigger storm if not anchored to the ground. Recovery efforts included a custom-made, heavy-duty canopy for the dredge.
Preliminary dredging into Cell 4 (0.352 acres) occurred from August 4, 2011 through September 3, 2011, and material was allowed to overflow into 4 as Cell 5 was completed, so that Cell 4 was almost a third full by that time. The hose was removed from Cell 5 November 16, 2011 and dredging was dedicated to Cell 4 until it was considered full on May 10, 2012. For 2 days, while we adjusted hoses as the dredge was moved along the source canal, the hose was separated and the outfall was directed into Cell 1. The total dredge time to fill Cell 4 was roughly 72.2 hours over a 6 month period.
Cell 4 bordered the outer pond and the containment 4c between the two was the deepest depth we had to span. Water pressure frequently scoured under the reed or plywood so that we lost a good bit of material to the outer pond. A 4-foot deep piece of plywood finally served to block the flow, although it was barely above marsh level and subsequently acted as a weir, allowing high water to cross. A plywood wall was constructed for the longer length of containment 4b between cells 4 and 2. Material was also lost through the reed containment 4a between cells 4 and 2 and created a rather deep and wide delta in Cell 2. In spite of these losses, Cell 4 fill was fairly consistent at marsh level when considered full.
At 0.366 acres, Cell 3 was the largest of the 3 cells that have been filled, and was also the deepest, with a natural channel that crossed from the marsh to the outer pond. Dredging began June 5, 2012 and was considered full on July 9, 2013 after 85 hours of effort over an 11 month period of time. An unusually extended period of low, low water prevented continuous work which was sporadic after December with only a couple of days of work in each the months of Feb, April, June and July.
The substrate had plenty of time to consolidate between dredging episodes and was much harder to fill consistently. The natural channel appeared to suck a lot of material away into the marsh, and at first we were losing a good bit into Cell 2 through the marsh or under the containment. Vegetation was starting to spread at the part of the cell that we filled first way before we managed to fill the north, deeper end. The outfall was moved every hour during the last few days of dredging to attempt to spread the fill, but even this allowed high spots to form as the low spots refused to stay filled with tidal flow and higher water.
Karen A Westphal and Timmy J Vincent, National Audubon Society Audubon Louisiana and the Paul J Rainey Wildlife Refuge 6160 Perkins Road, Suite 215, Baton Rouge, LA 70808 225-768-0921x202 office, firstname.lastname@example.org