Bioengineering to control stream bank erosion

Stream bank erosion is a natural process that can be beneficial to fish since bank failures can deliver needed boulders, gravel, and large woody debris into streams. However, human disturbance can cause erosion that exceeds natural levels, destroying fish habitat and compromising the quality of riparian zones.

Each site is unique and must be evaluated individually. If it is determined that erosion is a problem, it may be appropriate to take some action. Traditionally, this has involved the placement of structures like rip-rap, gabions, or sheet piles, techniques that may be expensive or unsightly.

Bioengineering is an another approach, utilizing live plants alone or in combination with dead or inorganic materials to produce living, functioning systems that not only prevent erosion but can also control sediment and provide habitat. Bioengineering is creative and multi-disciplinary, requiring knowledge of engineering, botany, hydrology, soil science, construction and more. It is a rapidly growing field, subject to innovations and changing design specifications.

Advantages and Limitations
Bioengineering solutions can be adopted in many soil stabilization and erosion control situations.

Advantages include:

• low cost and lower long-term maintenance costs than traditional methods
• low maintenance of live plants after they are established
• environmental benefits of wildlife habitat, water quality improvement and aesthetics
• improved strength over time as root systems develop and increase structural stability
• compatibility with environmentally sensitive sites or sites with limited access.

Limitations include:

• the installation season is often limited to plant dormant seasons, when site access may be limited
• the availability of locally adapted plants may be limited
• labor needs are intensive; skilled, experienced labor may not be available
• training may be required for installers not familiar with bioengineering principles and designs
• traditional practices are often more widely accepted by society and contractors.

Vegetation Types
Selection, procurement and installation of the proper plant material is essential for a successful design. In the case of streambank protection, both herbaceous and woody plants are needed. Herbaceous plants that can grow with their roots underwater are needed at and near the water¹s edge. This root growth adds considerable strength to the soil. Woody plants should be used on the upper slope and upland areas where their roots can grow in soil above the water table.

Proper plant selection is important; appropriate species are usually those found growing nearby. Cuttings can be taken from plants such as willow, cottonwood, thimbleberry, coyote bush, or other species that can root from cuttings. Container grown or bareroot stock such as alder, tan oak, Ceanothus, Douglas-fir, redwood and grand fir is also good. Correct placement and proper planting techniques are critical (see video, page 10).

Protect Plantings
Protect live plantings from animals and humans. Signs may keep people away, but fencing may be needed if animals are a problem. Also, protection from flooding or excess water flowing across the planting is important to establish all bioengineering plantings. Be sure surface drainage and water flow is directed away from the new plantings or protected slope.

Bioengineering Techniques
The following are some of the most common bioengineering practices, used alone or in combinations. Be aware that new methods and materials are constantly being developed.

Contour Wattling. This method is used to control surface erosion by breaking long slopes into shorter ones. Bundles of branches, called wattles or fascines, are placed in shallow trenches along the slope or streambank contour.

Brush Layering. This is used to restore slopes by constructing a fill-slope consisting of alternating layers of live branches and soil, creating a series of reinforced benches. Large quantities of dormant willow branches often used.

Brush Mattress. A mattress-like layer of branches is placed over the streambank to protect soil and slow water velocity. The mat is composed of interwoven, usually dead, branches secured to the soil by live stakes, wire, twine or live branches. Live stakes are often cut from dormant willow. Brush matting helps collect sediment and enables establishment of vegetation.

Coir Fascines. Coir fascines are wattles made from the fibrous outer husk of coconuts. Coir is denser than water so it won't float and is very slow to decay. Coir fascines are readily available and are popular for streambank and wetland restoration. Live plants can be placed into coir fascines to create a natural look.

Prevegetated Mats. Prevegetated mats are live plants grown on a movable mat of organic material. They come in many sizes and materials and are moved and installed in one piece.

Willow Siltation Baffles. These are inexpensive structures that can be used for bank protection and energy dissipation. Flow passes through the structure, where bedload is sorted, energy dissipated, and fines trapped.

Interplanting Rip Rap. Rip rap is composed of various size large stones placed on the soil surface where the water contacts the soil. Live cuttings can be interplanted in rip rap to provide additional slope stability. Root growth below the rip rap will improve soil strength and live vegetation will hide the rocks, presenting a more natural look.

Stakings. Stakes can be live or dead. Live staking is often done with willows to stabilize soil or to stake other materials in place. Manufactured timber stakes, 2 to 3 feet long, are used to secure wattles and coir fascines.

Improving Bioengineering Success

Bioengineering can be effective in many streambank and hillslope erosion situations, but it will not solve all soil erosion or slope failure problems. The success of a project hinges on many factors including proper design, plant selection, proper installation, weather conditions, and outside factors like animal damage. Site evaluation is important to determine whether there is adequate sunlight, soil type, and water quality to support plant growth. Do not expect bioengineering solutions to stop slope failure caused by high water tables or landslides. Nor are they ideal for high stress areas with severe wave action, rapid or long-term water level fluctuations or fast water flows.

The following list includes tips that may help ensure a successful bioengineering project.

1. Do not attempt bioengineering solutions in situations where:
   1. there is severe soil or water contamination;
   2. the stream bottom is degrading;
   3. you can not control human or animal traffic at the site; or
   4. there is too much shade for selected plant species to thrive.

2. Check with your local Natural Resource Conservation Service (NRCS) office for help. Some have staff that can provide technical assistance.
3. Check with local, state and federal regulatory agencies before beginning the project. Get any necessary permits.
4. Water elevation is the most critical element in a successful installation. Be sure you know the normal, high and low water elevations for the site. Know the seasonal changes in water elevation and how rapidly these occur.
5. Be sure to fence out animals and people, if needed. If damage occurs, supplemental planting may be necessary.
6. Be aware of flood or drought conditions that could impact your installation. Severe weather will reduce seedling survival. Supplemental planting may be needed.
7. Provide regular monitoring and maintenance, especially in the first year, to assure adequate plant survival.
8. Plan ahead. Involve the proper design professionals and experts to provide information on hydrology, plantings and structural design. A multi-disciplinary approach will assure success.

Information for this article came from the California Salmonid Stream Habitat Restoration Manual and from a NebGuide: Bioengineering for Hillslope, Streambank, and Lakeshore Erosion Control.