Notes from Little Lake and Chaland Headland
Field Lessons in Marsh Creation

In theory, creating marsh is simple: make a bowl with earthen sides and fill it with sediment. But constructing a living ecosystem in an ever-changing, dynamic environment presents both scientific and engineering challenges.

A look at two CWPPRA projects exposes some of the complexities and demonstrates CWPPRA’s continual improvement in developing effective, efficient coastal restoration techniques.

Goals Drive Decisions

“Establishing the purpose of a project answers a lot of initial questions,” says Rachel Sweeney, ecologist and project manager for the National Marine Fisheries Service. “Why are we building marsh here rather than there? How much marsh do we need? Are we creating a certain kind of habitat? Is there infrastructure that we want our marsh to protect?”

The goals of the Little Lake Shoreline Protection/Dedicated Dredging Near Round Lake (BA-37) project were to reduce erosion by erecting a rock dike along the Little Lake and Round Lake shorelines and to create or nourish nearly 1,000 acres in an area of open water and broken marsh south of Round Lake. BA-37 is among the largest of CWPPRA projects, but as Daniel Dearmond, civil engineer with the Louisiana Department of Natural Resources (DNR) and construction engineer for Little Lake, says, “From a construction point of view, there are no physical limits to the size of marsh we can build. All we need is a sediment source, a place to put the sediment, and money.”

“A single 1,000-acre project is more economical than three 300-acre projects,” says Cheryl Brodnax, marine habitat specialist with the National Oceanic and Atmospheric Administration. “Size doesn’t significantly increase the expenses of planning and design, securing land rights and mobilizing the dredge operation. But availability of marsh-building material can be a limitation.”

Sediment for Little Lake’s Marsh

Miles from the Mississippi River and dozens of miles from borrow sites in the Gulf of Mexico, the Little Lake project used sediment pumped from the lake bottom to build and nourish the marsh.

“We try to use material from sources that can be replenished with new sediment entering the wetland system, such as the Mississippi or Atchafalaya rivers,” says Dearmond. “That’s preferable to simply moving sediment around within the system, robbing a borrow area in a static, inland lake where — if it fills in at all — it fills in with material from a neighboring sloughing area. But sometimes, with our limited funding, an internal borrow area is the only available source.”

Once the slurry started entering the Little Lake fill site, differences between plan and execution surfaced. “About 80 percent of the project site was open area,” says Dearmond. “We thought there would be enough friction in the material for it to stack up and slope into the existing marsh. But instead there were areas where the material slid underneath the marsh. Patches of marsh lifted up and rode on top of the slurry out of the project area. To get the acreage we wanted we had to adjust our plans and build additional containment dikes.”

“A lesson we learned was to determine if existing marsh by itself provides adequate containment for pumped sediment,” says Brodnax. “Building fewer containment structures saves money and lets the marsh develop its own natural hydrology. When we use dikes to hold newly built marsh, we often have to return and cut gaps in them to allow water to flow in and out. That water exchange is essential for the marsh to become useful wildlife habitat.”

Peak Time for Habitat

Establishing desired habitat depends on building the marsh to the correct elevation, as elevation affects soil hydrology and determines the kind of vegetation that will grow. But, says Dearmond, “Mud is not an engineered material. It’s highly variable and consolidates a lot, which makes determining how much you’ll need to reach a target elevation something of a game.” Settlement tables and a growing body of data on sediment characteristics give scientists clues for solving the puzzle.

Because settling occurs over time, project design specifies not only what the target elevation will be, but also when to achieve it. “Some want to see ideal marsh elevation very quickly,” says Dearmond, “but to allow for consolidation, settlement and subsidence, I would shoot to reach target elevation in five to 10 years.”

“If we build marsh on the high side,” says Darin Lee, senior coastal resources scientist at DNR, “we end up with some kind of vegetation. But if we build marsh too low, we end up with the same open water, even though it’s shallower than before.”

“With each project we’re learning just how high to pile containment, how to calculate optimal elevation, how soil quality determines project quality,” says Brodnax. “We’re also learning how to build projects more economically, how to leverage the advantages of scale, and how to improve the language of construction contracts so that we get what we want from each dollar we spend.”

Marsh on Barrier Islands

Engineers have learned from past projects that marshes are an essential structural component of barrier island restoration. “Behind the sandy dunes, these areas provide a platform for windand wave-driven sand to roll onto, saving it from falling into water and supporting the incremental migration of the barrier island system,” says Patty Taylor, an environmental engineer and project manager for the Environmental Protection Agency. “The marshes help to sustain the island’s width, reducing the likelihood tidal inlets will cut new passes from the Gulf of Mexico to interior estuaries.”

Creating marsh on barrier islands is similar to creating interior marsh, but Taylor says island conditions present special problems. In determining dune height and platform width, design engineers must include wind, wave energy and tidal range in their calculations. A location typically 10 to 15 miles offshore complicates construction logistics. Construction schedules must take seasonal weather patterns, shorebird nesting and bird migrations into consideration. Storms can alter the landscape, splitting one island into two. Storms may also cause an unexpected escalation in fuel costs, labor shortages and damage to natural features already incorporated into a project’s design.

Lessons in Land-building at Chaland Headland

The goals of the Chaland Headland portion of project BA-38 were to prevent breaching of the barrier shoreline between Pass La Mer and Chaland Pass and to protect and create over 400 acres of dune, swale and intertidal marsh habitat. Enhancing these landscape features would shield wetlands and infrastructure to the north of the headland from storms, wind and waves originating in the Gulf of Mexico

“Katrina hit right as construction of the Chaland Headland section of project BA-38 was starting,” says Brodnax. “The storm reshaped the project’s footprint. Before we could continue, we had to update our survey and recalculate the amount of material we needed.”

The project’s design specified making tidal creeks and access channels for water to flow in and out. During construction it became evident the creeks would form on their own. “Before, during and after construction we collect data to verify engineering assumptions,” says Sweeney. “We have to stay flexible while implementing a project, adjusting to lessons we learn along the way.”

Lessons from Chaland Headland included realizing the benefits of using a small dredge and more flexible pipe. “A smaller dredge delivering less volume with less force reduced the danger of blowing out containment dikes,” says Sweeney. “And it slowed the pace of production so we didn’t have to shut down to dewater — we could leave one section to settle and move the pipe to another area. Because the pipe was lightweight plastic, we could move it easily and direct sediment into all the nooks and crannies of the project area.”

Applying lessons from previous projects, the Chaland Headland project erected sand fencing within a week of finishing a section of rebuilt dune. “Wind was blowing our sand away,” says Sweeney. “We wanted to trap that sand and get a grass cover growing as soon as possible.”

As the project ages, rates of settlement, the contour of land, hydrologic measures and vegetative surveys will be monitored and the collected data will inform the design of future projects. “We’ll never get every single detail correct,” says Dearmond, “but we have to go out and build marsh anyway. We design to the best of our ability and handle the variables as we encounter them in the field.”

Building the Future

Even the largest marsh creation projects built to date are dwarfed by Louisiana’s annual 24-square-mile loss. “Each project is small compared to the entire coastline of Louisiana,” says Dearmond. “But to the local community, each project is very significant. Our current condition in Louisiana didn’t develop overnight; it’s taken 40 or 50 years. The first pipeline canal cut through the marsh didn’t have a significant impact on Louisiana’s coastline, but over time, each canal has become very significant. It’s the same situation with restoration — we’re fixing one piece at a time. For future generations, I think each project will prove very significant.”

“The long-term vision is to build islands and marshes with sediment from the Mississippi River,” says Brodnax. “CWPPRA’s smaller projects lay the groundwork for the next level. They will segue into the large-scale picture of restoring coastal Louisiana using renewable resources.”

“If we have time, money and sediment to build with, marsh creation can combat Louisiana’s problem very effectively,” says Lee. “In a geologic timescale, sustainability will come from river diversions. Marsh creation works in societal time. It’s a very important first step.”