Slip, sliding away: WNC creek banks imperiled by erosion

As Western North Carolina’s population expands, our landscape’s capacity to absorb rainfall is increasingly pushed to — and beyond — its limit. As trees are cut to make way for development, and hard surfaces such as concrete cover more and more ground area, the amount of stormwater that runs into streams and rivers is growing. And that runoff carries with it debris, sediment, pollutants and heat, all of which affect the ecology of local waterways.

The North Carolina Cooperative Extension Service has published a detailed guide to help property owners address and mitigate stream bank erosion. The publication, Small-scale solutions to stream bank erosion, is available for free online (http://avl.mx/2uu) and explains why stream banks often need attention and how to improve them. Some tips and advice from the publication are outlined below.

Crumbling banks

Even a moderate amount of rain can quickly discharge a large volume of water into streams and creeks. One inch of rain falling on a 1-acre parking lot, for example, can drain 27,000 gallons of water into waterways. When that much water roars through, stream beds can shift course and can also become more deeply incised into the surface of the surrounding land. Fast-moving water doesn’t allow much sediment to be deposited and, without replenishment, the stream bank may begin to erode.

Area streams also face another threat: People like to look at them. In order to clear the view, property owners often mow right up to the edge of the stream bank. Though understandable, this practice also contributes to erosion, since relatively shallow-rooted turf grass washes away more readily than a mix of plantings.

Erosion degrades wildlife habitat by adding sediment to the water and by altering the physical shape of the stream. Erosion also results in the loss of riparian, or river bank-related, land, which is itself a rich and unique habitat for wildlife and plant species. Shifting creek banks can also damage bridges, fences and roads, not to mention houses and other buildings.

First steps

So what is a responsible steward to do? The answer is not always simple. According to Gary Higgins, a representative of the Buncombe County Soil and Water Conservation District, there are a few things landowners need to know when considering a stream bank repair.

The first step, Higgins says, is to figure out whether your property and stream lie within the area designated as the 100-year floodplain. A fact sheet from the U.S. Geological Survey explains, “The term ‘100-year flood’ is really a statistical designation, and there is a 1-in-100 chance that a flood [of a particular] size will happen during any year.” Thus, the 100-year floodplain is the area that will be inundated when a flood of that magnitude occurs. Most cities and towns have flood zone maps available for the public to view, or you can visit the website www.freeflood.net to search for your property address. The website is free to use, though it does require that you create an account with a valid email address.

If the property does fall within the 100-year floodplain, you will need to obtain permits from the Army Corps of Engineers, the North Carolina Department of Environmental Quality, the county where the land is located and possibly other entities. If it does not, you may be able to do some repairs and maintenance without permits.

Next, examine the stream in detail. Higgins points out that “Every situation is different. You have to know what the stream is trying to do, what is natural.” Some things to note include the size and direction of the flow; the stream’s depth and width; how steep the stream bank is; and what sort of plants grow there, including what kinds of root systems are in place.

Erosion-control alternatives

Once you have a good understanding of the conditions on your property, there are several possible solutions if you do have an eroded bank. The first is also the simplest: Allow nature to reclaim the bank. Instead of mowing to the edge, leave a buffer of vegetation up to 15 feet from the bank. This method is cost-free and requires no work, but it can leave the area looking weedy, and some undesirable invasive plants could take over. The bank may still experience some erosion until the plant buffer matures.

Or you could choose to halt the progress of the erosion by planting appropriate native vegetation without moving any earth. Trees and shrubs work to hold the soil in place and to shade the stream, allowing the water to maintain the cooler temperatures hospitable to native stream dwellers like fish, crayfish, salamanders and dragonflies. Experts advise planting native trees 5 feet from the bank’s edge and small shrubs 3 feet apart at the water’s edge, planting in rows wherever the soil remains saturated.

According to the pamphlet Is Your Backyard Washing Away? published by North Carolina State University in collaboration with various agencies, some good choices for native plantings in our area include beautyberry, sweet pepperbush, cardinal flower, redbud, sweetspire, fringe tree and soft rush. The planting work is best done while plants are dormant, between October and March. This erosion control method is relatively inexpensive, but it isn’t as permanent as regrading.

The longest-lasting and most visually appealing results require grading the bank to a 3:1 (about 18 percent grade) slope and planting with native plants. While the use of grading machines may compact the soil, the benefits of a safer, more stable bank can be worth it. After grading, the area will require trenching to control water flow, as well as placement of a tough but biodegradable matting material like coir, held in place with 2-foot wooden stakes driven into the ground. Straw and temporary seeding can also help to retain the top layer of soil until permanent plantings become established.

One final aspect of stream bank repair to consider is cost.  Variables that affect cost include accessibility to the site and types of plants used. Intensive grading to achieve a desired slope or the presence of a lot of in-stream rock structures can drive the price up dramatically. In the case of small projects, the permits and engineering can cost as much as the restoration itself, especially for properties in the 100-year floodplain.

The Buncombe County Soil and Water Conservation District works with the United States Department of Agriculture program, Environmental Stream Initiative, on stream bank repairs of 1,500 lineal feet or more. Sometimes landowners will band together to use this program collectively. You can learn more by contacting the Buncombe County Center of the Cooperative Extension Service at (828) 255-5522 or the Buncombe County Soil and Water Conservation District at (828) 250-4786.