Engineers Build Land with Dredged Sediment
Marsh Creation as a Coastal Restoration Strategy
Erosion and subsidence exact a heavy toll on Louisiana’s coastal zone, claiming nearly 2,000 square miles of land in the past 75 years. But scientists and engineers seeking to rebuild barrier islands and restore marshes have a powerful technique at their disposal: marsh creation.
“By replacing sediment compacted by subsidence or eroded away by coastal forces such as tides and waves, marsh creation rebuilds land, restoring marshes and barrier islands to an elevation that can support wetland habitat,� explains Russ Joffrion, a civil engineer with the Louisiana Department of Natural Resources (LDNR). “It’s the only restoration technique that immediately reverses land loss.�
Moving Earth With Pumps and Pipes
Over millennia, sand, silt and clay delivered via the muddy floodwaters of the Mississippi River built the wetlands of Louisiana’s coastal zone. Marsh creation replicates the natural land-building process in a controlled — and much accelerated — fashion.
Workers remove sediment from a “borrow site� using a pipeline dredge, a specialized vessel outfitted with a suction pump and pipe. A cutterhead, a large drill-like assembly, is attached to the end of the suction pipe. As the cutterhead spins, it agitates sediment at the bottom of the borrow site; the pump sucks sediment and water into the pipe, then sends it through a pipeline to the “fill site� — the restoration project area. Once in place, the sediment settles and the water runs off.
Engineers must determine not only how much slurry to pump into a marsh creation project, but also how fast to pump it. “We’re trying to achieve the target elevation as quickly as possible, but we don’t want to breach the containment dikes — doing so wastes sediment, increases cost, slows the project down and potentially infringes on the land rights of neighboring properties,� explains the NRCS’ John Jurgensen.LDNR
Marsh construction and dewatering can take days or weeks, depending on the type of sediment pumped and the project’s parameters. But in all cases, says John Jurgensen, a civil engineer with the Natural Resources Conservation Service, “The minute we finish construction, there’s land out there — a huge mudflat that will re-vegetate within a year. We’ve built land where just days before there was open water.�
Marsh Creation Projects Meet Specific Goals
CWPPRA partners carefully evaluate every proposed project site to determine whether it is a good candidate for marsh creation.
“Marsh creation is feasible in broken marsh areas and in shallow, open-water areas — no more than two feet deep,� Joffrion says. Fishing, hunting and oyster leases must be reviewed and landowners’ permission obtained before any work can begin. “We also need to work with industry to locate all existing oil and gas pipelines in the area. Hitting a pipeline poses risk to both workers and the environment.�
Billy Hicks, USACE
For a marsh creation project at Bayou LaBranche, the U.S. Army Corps of Engineers partnered with LDNR to restore 300 acres of wetland using pumped sediment. The Corps has been creating land with this technique since the 1970s, building some 9,000 acres in marshes and on barrier and bird islands. The knowledge and experience that the Corps, LDNR and other CWPPRA agencies gain from each project are shared and applied to the design and construction of successive projects.
After a project site is selected, LDNR’s Ecological Review Unit scientists determine how high to build the marsh platform. "The elevation of the marsh is critical to the ecosystem’s productivity because it is a determining factor for the establishment and sustainability of the desired plant community," says Susan Hill, the unit's supervisor. Land built too high might be colonized by upland woody vegetation instead of marsh grasses appropriate to the area. "Alternatively, if the marsh is built too low, it could remain inundated, preventing vegetation from becoming established.� Scientists collect data on elevation, vegetation and other characteristics of the created marsh and compile the data into monitoring reports. As engineers design new projects, ecological review scientists consult these reports in an adaptive management process, seeking ways to enhance projects’ performance. “With each project we complete, we learn more about how better to restore our wetlands,� Hill says. “We are constantly striving to improve our marsh creation designs and obtain even better results.�
Elevation Gradient Creates Conditions for Varied Plant Life
Barrier Islands Host Diverse Habitats
Source : Hester, Mark W., Elizabeth A. Spalding, and Carol D. Franze. “Biological Resources of the Louisiana Coast: Part 1.� Journal of Coastal Research Special Issue No. 44, Spring 2005. 134-145. PHOTO CREDITS: widgeon grass: Edward G. Voss @ USDA-NRCS; shoal grass: Ronald C. Phillips @ NOAA; turtle grass: R. A. Howard @ USDA-NRCS; smooth cordgrass: USDA-NRCS; black mangrove: USDA-NRCS; marshhay cordgrass: Larry Allain @ USDA-NRCS; marsh elder: Larry Allain @ USDA-NRCS; wax myrtle: Larry Allain @ USGS-NWRC; bitter panicum: USDA-NRCS; sea oats: R. A. Howard @ USDA-NRCS; beach morning glory: R. A. Howard @ USDA-NRCS; sea rocket: Pedro Acevedo-Rodriguez @ USDA-NRCS.Monitoring Programs Provide Current Data
Marsh Creation by the Numbers
Marsh creation relies on not only pumps, pipes and sediment but also information. To calculate how high to build new marsh, engineers must know the current elevation of a project site.
Spanning the state’s coastal zone, a network of survey monuments provides elevation data for nearly 300 sites in marshes and on barrier islands. Each monument has a unique designation corresponding to a data sheet available from the Louisiana Department of Natural Resources (LDNR) Web site. Data include the monument’s elevation, latitude and longitude, and aerial and ground-level photos of the monument site. “Elevation benchmarks placed on or near the ground’s surface sink quickly as the land beneath them subsides,� says Stephen Melton, an LDNR surveyor. “These monuments consist of steel rods driven 60 to 120 feet into the ground for extra stability. Regularly updating elevation data using GPS survey techniques also helps us ensure accuracy.�
And to glean lessons from CWPPRA’s past projects, scientists must continually monitor habitat health. Through ongoing collection of salinity, elevation, water level and vegetation data at more than 390 wetland locations, LDNR’s Coastwide Reference Monitoring System (CRMS) lets CWPPRA agencies evaluate both individual restoration projects and the health of wetlands across the coast. Using CRMS data, scientists seek lessons to apply to new marsh creation endeavors.