Coir Rolls With Brushlayering for Successful Streambank and Shoreline Stabilization
Coir rolls combined with brushlayering provide immediate and long-term erosion control with habitat benefits, making this an ideal application for shoreline and streambank stabilization projects. The tough, long-lasting, and biodegradable coir logs provide the initial toe protection against wave action or stream flow. By the time the coir logs begin to biodegrade after several years, the live brushlayer cuttings have established an extensive root system, and the vegetation buffer offers further protection against breaking waves and high flows. The coir roll is used as a base for the cuttings to be laid upon at a 10-20° angle, which stimulates rooting and growth.
Begin by installing the coir rolls according to manufacturer specifications, with 2 in. of the roll above the mean water level. Adjoining rolls should be tightly laced together end to end. It is important to secure the logs firmly into the substrate with wooden stakes and strong rope. Attempts to save staking time by simply cabling the logs down have proven unsuccessful. After installation, prepare the soil bed behind the logs for brushlayer cuttings. Determine the soil level by laying a straight cutting on the coir roll with approximately 20% of the cutting extending past the roll, with the basal end angled downward into the soil. The soil bed should be at a higher elevation adjacent to the coir roll (angling downward toward the bank) and should be firmly compacted. Once the soil bed is prepared, lay the cuttings in a slightly crisscrossed fashion, as this provides additional tensile strength. The cuttings should be properly harvested and handled and should be of an appropriate species that forms adventitious roots, such as willow (Salix sp.) or cottonwood (Populus sp.).
Next, backfill over the cuttings with soil, covering the lower 80% of the branches (leave 20% of the cutting exposed to sunlight). Use water to wash soil between the branch layers. The backfill should be compacted slightly, graded to an appropriate angle, and seeded and mulched with native grass species. If additional height or soil stability is needed, another coir roll or a soil wrap can be added above in a tilelike fashion until the desired height is achieved.
This technique was used in the Oak Bottom Shoreline Erosion Control Project in northern California, shown here. Serious shoreline erosion in the Whiskeytown National Recreation Area, caused by visitor access to Whiskeytown Reservoir and wave action from boats, was reducing the campground area. In addition to coir rolls and brushlayering, soil wraps made from coir mats were used to build up the banks and actually recapture some of the lost shoreline. Brushlayers were integrated into the soil wrap designs for further stabilization. A few experimental areas used brushlayering with rock toe protection and live siltation. Some shoreline areas were protected with coir logs and soil wraps but were left unvegetated to encourage visitors to use established trails for access to the lake. These user-friendly “soft” engineering techniques were more desirable than gabions or riprap, as the area was heavily used in the summer by boaters and swimmers.
User fees from the recreation area paid for the project. California Conservation Corps and seasonal employees were trained in these biotechnical techniques and did the construction, stabilizing about 1,000 lin. ft. of shoreline with biotechnical treatments. The total cost for the project was approximately $60,000 ($25,000 for materials and $35,000 for consulting and labor). This amount included erosion control work within the campground area to correct drainage and some applications of cellular confinement systems for trail stabilization.
A few mistakes were made in the installation of the coir rolls. For example, in some areas the coir rolls were installed slightly below the mean water level (the reservoir is low in the winter and full in the summer). Note that the project work occurred in the early spring, and the live cuttings had to be watered during dry spells until the reservoir was full. Experimental cables to secure the coir rolls and save staking time did not work well, causing a few of the coir rolls to loosen and separate slightly from the bank.
Overall, the results have been excellent, and one important aspect of the project was that visitors could see and appreciate what their user fees are for. Visitors have been very pleased with the project, the aesthetics of the campground shoreline have been greatly improved, and the variety of willows is growing vigorously. The coir rolls, coir mats, and brushlayers have taken a beating from moored boats and bare feet and have withstood the heavy use with amazing durability. Some of the willow cuttings have grown more than 10 ft. tall after only one season. Young fish also utilize the new habitat.
* This Biotech Tip and drawing are adapted from the CD “Bio Draw 1.0: Compendium of Biotechnical Soil Stabilization Solutions,” by John McCullah, Salix Applied Earthcare.
