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Geotextile mat helps hold North Dakota river in place

Case Studies, Features | September 1, 2008 | By:

The Grand Forks, North Dakota river jumped its banks; now a geotextile mat helps hold it in place.

On the riverfront in Grand Forks, North Dakota, in a park called the Town Green, is a concrete obelisk that commemorates floods. The tip of that obelisk, 16.5m above the Red River of the North’s normal water elevation, is for 1997. That year, the so-called “Flood of the Millennium” inundated the downtowns of Grand Forks and East Grand Forks, Minnesota, forcing the evacuation of more than 50,000 people, triggering a fire that burned six city blocks, and costing more than $2 billion in damage.

Almost immediately after the floodwaters subsided, the federal government, through the U.S. Army Corps of Engineers, offered $100 million and agency expertise to raise the levees and clear land to ensure that such a disaster would not happen again. The cities said no. Instead they created a master plan for an 880-hectare recreational area that would stretch along 4.8km of both states’ waterfronts and include parks, trails, and sports fields. This was to be flood control not as functional imposition, but as local amenity. The project ultimately received more than $450 million in federal assistance, which was supplemented by local and state funding.

Now, 11 years later, the Red River Greenway is complete, and the obelisk in the Town Green looks out over art and music festivals, the afternoon lunch crowd and a wide, sluggish river fringed with native prairie. Yes, native prairie, not stone armoring. While the large scope and cost of this project were certainly unusual, so, too, is the aesthetic.

“One of the concerns early on,” remembers Tom Whitlock, ASLA, a landscape architect with Minneapolis-based Damon Farber Associates, “was that [residents] wanted the whole shoreline natural. They didn’t want to riprap the edge.” Whitlock worked on the project from its inception in the fall of 1997, and has outlasted Army Corps project managers, city officials and other consultants. He remembers being all for the idea, since this is likely what the Red River Valley would have looked like before European settlement, but saw some immediate concerns, namely the fact that the Red would surely flood again and rip out whatever he planted there.

To satisfy community desires and Army Corps mandates, Whitlock and his team devised a simple detail that relies heavily on geotextile fabric – then put that fabric completely out of sight. The sloping flood levees that rise from the edges of the water were left uncompacted, but were scarified to loosen the soil. Then Enkamat® was rolled out on the slopes and anchored at the bottom with a 1.52m width of riprap. Enkamat is manufactured by Colbond, a Dutch company with production facilities in Europe and the U.S. The mat, a 3D woven pattern is designed for both soil stabilization and ease of seeding. Laid on the earth, it looks a bit like a tangled fishing net. Contractors spread a 50.8mm thick layer of topsoil and studded the slopes with a combination of seeds and small plant plugs, the latter placed into holes cut in the mat allowing them to root through to the native soil below. A layer of biodegradable fiber blanket was placed over the top for additional erosion control.

Two different plant mixes were developed, depending on proximity to the water line. The “Shoreline Mix” leans heavily on Big Bluestem grass and also includes milkweed, aster, several sedges, wild rye, bulrushes, switchgrass, and a locally appropriate variety of other wildflowers. The “Upland Mix” includes mostly Little Bluestem grass, wild rye, and switchgrass, with some Big Bluestem, black-eyed susans, coneflowers, primroses and other prairie wildflowers. The plants have thrived, covering the banks of the river in a native carpet, camouflaging the levees. “The result,” says Whitlock, “is this lush landscape. You can’t even tell the fabric is there.”

But the fabric is working. During construction of the Greenway in the early 2000s, the Red River flooded again. The waters rose up over the restored prairie banks, up over the Town Green, almost submerging the obelisk. When the waters receded, the prairie was largely intact, and the plants rebounded easily throughout the summer. That flood turned out to be the third highest flood on record, but, remembers Whitlock, “It was a non-event.”

After the heart-wrenching flood of 1997, the residents of Grand Forks and East Grand Forks wanted a return to their prairie roots. They wanted to see flowers and waving grasses on the edge of the Red instead of stone-clad slopes hemming in their river with an engineered look. The geotextile fabric Whitlock used is making that happen. Though it is virtually invisible, it is facilitating the restoration of a native landscape in a place where that is hardly ever done. Today, one of the great prairie rivers of North America actually looks like a prairie river.

Adam Regn Arvidson is a landscape architect and freelance writer based in Minneapolis. He is founder of Treeline, a design/writing consultancy that focuses on environmentally sustainable projects.

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