Erosion mitigation on stream bank decreases stormwater impact on larger river

LINKEDINCOMMENTMORE

A stretch of creek that runs through the Black Mountain Golf Course has taken on a dramatically different look in recent months.

Erosion caused the banks of Tomahawk Branch, which feeds into Lake Tomahawk, to recede, leaving a jagged, cliff-like wall where there was once a bank with thriving vegetation. However, the recent restoration of the embankment impacts much more than the aesthetics of the small body of water.

In 2006 the Swannanoa River was placed on the state’s 303(d) impaired waters list, due to an insufficient population of benthic macroinvertebrates (also known as benthos), aquatic plants and animals that live in and around streams, rivers and lakes. The 22-mile Swannanoa River feeds into the French Broad River at the confluence of the bodies of water on the grounds of the Biltmore Estate.

Black Mountain adopted an Upper Swannanoa River Watershed Plan in January 2016 to address the impacts of stormwater runoff in the seven-mile stretch of the river and its tributaries that pass through the town.

“The decrease of that life in the river is attributed directly to stormwater runoff,” said Josh Harrold, the director of planning and development for Black Mountain. “There are increased volumes of water (in the river), the temperature of the stormwater is warmer since much of it flows across pavement. The turbidity of the water, since it picks up soil and other matter before going into the river, are all factors that impact the life in and around it.”

Although heavy industrial and agricultural operations are no longer prevalent in the watershed, the urbanization of the area has led to an increase in stormwater runoff, making projects like the Tomahawk Branch restoration vital, according to Harrold.

“Tomahawk Branch ultimately feeds into the Swannanoa River,” Harrold said. “It does go through Lake Tomahawk before getting there. There is more development upstream from Tomahawk Branch which contributes to the stormwater that comes through that stream.”

The erosion impacting the stream was largely due to a combination of that increase in stormwater and the practice of cutting the vegetation along the banks of the 1,200-foot stretch visible from Tomahawk Avenue, between Ninth Street and Laurel Circle Drive.

“The golf course was clearing that bank for years,” Harrold said. “Those banks then degraded and eventually fell into Tomahawk Branch, which led to it carrying that muddy water into the lake and ultimately the Swannanoa River.”

The town received a grant for $73,500 from the N.C. Division of Water Resources last November to cover half of the anticipated cost of repairing the embankment. The town was responsible for paying the other half of the project. The restoration was completed for around $90,000, according to Harrold.

Wildlands Engineering was responsible for designing the project while South Core Environmental handled the construction involved with rebuilding the banks.

“They didn’t restore every inch of bank in the 1,200 linear foot section,” Harrold said. “What they did was restore specific sections of that stream that needed it the most. Those portions were the ones with the heaviest and most turbulent water.”

The banks of Tomahawk Branch were excavated, according to Harrold, which widened the floodplain area within the stream itself.

“That way the water can move more easily within the creek bed without eroding the bank,” he said.

The banks were then stabilized with the addition of crushed rock and dirt, according to Harrold.

"Then they put matting over that, which was seeded," he said. "Then they planted live stakes, which are basically limbs of specific trees that thrive in that particular environment."

The reinforcing of the bank prevents loose dirt from falling into rushing stormwater when the stream rises.

The Tomahawk Branch restoration project is just “one step” in the restoration of the Upper Swannanoa River Watershed, according to Harrold.

“We got funding with the help of the Land of Sky Regional Council,” he said. “We received a 319(h) grant, which is through the Environmental Protection Agency, and it’s specifically for stormwater projects.”

Funds from that grant will go toward three projects that will help ease stormwater runoff in the watershed in different ways. The first is for a bioretention cell near the parking lot at the golf course. A bioretention cell captures rainwater and removes sedimentation and other contaminants before releasing it into the soil.

“Another project will be done at the (Black Mountain) library,” Harrold said. “It’s more of an educational piece than an actual stormwater treatment one. It will help show the public how stormwater runoff impacts the entire watershed.”

A third project will take place on Church Street in the coming months as well.

“What we’re hoping to do there is called a green street project,” Harrold said. “And that involved putting a few bioretention cells in a series along the street and as that stormwater comes down it can be treated before making its way to the storm drains.”

For more information on the Upper Swannanoa River Watershed Plan visit townofblackmountain.org.

LINKEDINCOMMENTMORE
Read or Share this story: https://www.blackmountainnews.com/story/news/2017/05/31/tomahawk-branch-restoration-addresses-erosion/102185194/