Willow Burritos for Streambank Stabilization
Willow “burritos” consist of willow cuttings, native soil, and straw mulch rolled up with high-strength coir erosion control blanket to provide a method of stabilizing the toe of streambanks or slopes using natural biodegradable materials. Used successfully on a number of projects in the Denver metropolitan area, willow burrito applications provide a rapid and aesthetically pleasing treatment for sensitive stream environments.
At a stream restoration project in Highlands Ranch, CO, willow burritos were installed at the suggestion of Grant Gurnee and Gary Bentrup in 1998. After that project’s success, willow burritos have since been used by Muller Engineering Co. in other projects. One such project is Lilley Gulch, completed in 2003 with Jefferson County Open Space and Urban Drainage and Flood Control District. Territory Unlimited, with landscaping subcontractor CDI, was the contractor awarded the project.
Lilley Gulch, typical of many streams and creeks along the Front Range in Colorado, had been experiencing channel instability because of increased peak flows from urbanization. With a base flow, Lilley Gulch provided an ideal environment for a dense vegetative cover, such as willows, to stabilize the banks. The design of the project included a low-flow channel within a flatter bench. The goal was to define the low-flow channel edge with a softer treatment. Willow burritos were suggested as an alternative to riprap to better serve this purpose. Flow velocities were low enough to be considered stable with a vegetative cover (5 feet per second or less in a 100-year storm event). In addition, if successful, the willow burritos would facilitate more rapid revegetation of the channel edges.
Willow cuttings from nearby sources are harvested during dormancy, ideally just prior to spring budding. In the Denver area, this time frame typically occurs in March. After cutting, all side branches and leaves are trimmed. Ideally, willow cuttings are installed immediately in the willow burrito with a sufficient source of water, to prevent drying. Willow cuttings can, however, be stored with their entire length soaked in water for up to a week or as specified by a qualified ecologist. Soaking their entire length allows for maximum absorption of water. At a minimum, the butt-end of the willow cuttings should be stored in water.
The key to a successful installation is having sufficient water to support willow growth, such as a baseflow in a stream channel. In an ideal environment, adventitious roots will spread from the cuttings into the natural materials of the willow burrito and the surrounding soil. After a few years, the willows ideally will have grown sufficiently so that once the twine and blanket begin to degrade, the roots and structure of the willows hold the toe of the slope in place.
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| Photo 2 |
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| Photo 3 |
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| Photo 4 |
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| Photo 5 |
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To fabricate willow burritos, coir erosion control blanket (13 feet wide works well) is laid out on the ground and cut into a manageable length, such as 20 feet. Woven coir mat has been used because of its longer life (approximately three years) and high-strength characteristics. The blanket should be placed so that one long edge (in this case, 20 feet) is located along the toe of the slope where the final burrito is to be located, and the other long edge is upslope. The toe of the slope should be trenched, so that approximately one-third of the final willow burrito is keyed below the channel bottom and is in contact with moist soil. Willow cuttings (0.25 to 0.75 inch in diameter and 36 inches long) are laid on the blanket side by side over the middle portion of the blanket width (leaving approximately 3 feet clear on each side). A 1-inch layer (minimum) of mulch—wood, straw, willow stake trimmings, or other natural materials—combined with native soil is then placed over the willow cuttings (photo 1).
The blanket is then rolled up (photo 2)starting on the upslope end, resulting in a 12- to 18-inch-diameter roll, and tied at 18-inch intervals with biodegradable twine (photo 3). The ends of the burritos are secured by twisting and tying with twine. Adjacent willow burritos are butted firmly up against each other, forming a continuous line. Once in place, the willow burrito is secured into the ground with 36-inch-long wood stakes at 24-inch intervals through the middle of the burrito, leaving only 1 to 2 inches of stake above the surface. Wood stakes are made up of 2- by 4-inch wood boards that are cut on the diagonal. This wedge shape helps prevent the willow burrito and blanket material from slipping over the top edge of the stake and more firmly secures the burrito in place. Wood stakes are preferred over metal fasteners in sensitive environments.
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A few of the lessons learned include making sure that the burritos, once rolled up, are in the proper location, because once they are fabricated or become wet, they are heavy and difficult to move. Burritos should be rolled up tightly so that they maintain their shape and do not lose much of their height once staked into the ground. It takes some experimentation to determine the right amount of willow cuttings and mulch/soil filling to get the full 12 to 18 inches of diameter once rolled. In addition, it is helpful to have extra wood stakes around, because some will break as they are hammered into the ground. A longer wood stake length (such as 36 inches) provides adequate depth of burial and proper stabilization of the willow burrito.
Lilley Gulch willow burritos have been a success, with the stabilized stream and surrounding open space providing a neighborhood amenity and wildlife habitat. Willows have grown approximately 4 feet tall in just a single growing season (photos 4 and 5). Where design flow velocity is not expected to exceed that which a good vegetation cover can protect the soils, willow burritos are considered a viable and more environmentally sensitive alternative to riprap toe protection. The estimated cost for materials and installation is approximately $16 per linear foot.
May 2007
Willow Burritos for Streambank Stabilization
Willow “burritos” consist of willow cuttings, native soil, and straw mulch rolled up with high-strength coir erosion control blanket to provide a method of stabilizing the toe of streambanks or slopes using natural biodegradable materials. Used successfully on a number of projects in the Denver metropolitan area, willow burrito applications provide a rapid and aesthetically pleasing treatment for sensitive stream environments.
At a stream restoration project in Highlands Ranch, CO, willow burritos were installed at the suggestion of Grant Gurnee and Gary Bentrup in 1998. After that project’s success, willow burritos have since been used by Muller Engineering Co. in other projects. One such project is Lilley Gulch, completed in 2003 with Jefferson County Open Space and Urban Drainage and Flood Control District. Territory Unlimited, with landscaping subcontractor CDI, was the contractor awarded the project.
Lilley Gulch, typical of many streams and creeks along the Front Range in Colorado, had been experiencing channel instability because of increased peak flows from urbanization. With a base flow, Lilley Gulch provided an ideal environment for a dense vegetative cover, such as willows, to stabilize the banks. The design of the project included a low-flow channel within a flatter bench. The goal was to define the low-flow channel edge with a softer treatment. Willow burritos were suggested as an alternative to riprap to better serve this purpose. Flow velocities were low enough to be considered stable with a vegetative cover (5 feet per second or less in a 100-year storm event). In addition, if successful, the willow burritos would facilitate more rapid revegetation of the channel edges.
Willow cuttings from nearby sources are harvested during dormancy, ideally just prior to spring budding. In the Denver area, this time frame typically occurs in March. After cutting, all side branches and leaves are trimmed. Ideally, willow cuttings are installed immediately in the willow burrito with a sufficient source of water, to prevent drying. Willow cuttings can, however, be stored with their entire length soaked in water for up to a week or as specified by a qualified ecologist. Soaking their entire length allows for maximum absorption of water. At a minimum, the butt-end of the willow cuttings should be stored in water.
The key to a successful installation is having sufficient water to support willow growth, such as a baseflow in a stream channel. In an ideal environment, adventitious roots will spread from the cuttings into the natural materials of the willow burrito and the surrounding soil. After a few years, the willows ideally will have grown sufficiently so that once the twine and blanket begin to degrade, the roots and structure of the willows hold the toe of the slope in place.
| Photo 1 |
|
| Photo 2 |
|
| Photo 3 |
|
| Photo 4 |
|
| Photo 5 |
|
To fabricate willow burritos, coir erosion control blanket (13 feet wide works well) is laid out on the ground and cut into a manageable length, such as 20 feet. Woven coir mat has been used because of its longer life (approximately three years) and high-strength characteristics. The blanket should be placed so that one long edge (in this case, 20 feet) is located along the toe of the slope where the final burrito is to be located, and the other long edge is upslope. The toe of the slope should be trenched, so that approximately one-third of the final willow burrito is keyed below the channel bottom and is in contact with moist soil. Willow cuttings (0.25 to 0.75 inch in diameter and 36 inches long) are laid on the blanket side by side over the middle portion of the blanket width (leaving approximately 3 feet clear on each side). A 1-inch layer (minimum) of mulch—wood, straw, willow stake trimmings, or other natural materials—combined with native soil is then placed over the willow cuttings (photo 1).
The blanket is then rolled up (photo 2)starting on the upslope end, resulting in a 12- to 18-inch-diameter roll, and tied at 18-inch intervals with biodegradable twine (photo 3). The ends of the burritos are secured by twisting and tying with twine. Adjacent willow burritos are butted firmly up against each other, forming a continuous line. Once in place, the willow burrito is secured into the ground with 36-inch-long wood stakes at 24-inch intervals through the middle of the burrito, leaving only 1 to 2 inches of stake above the surface. Wood stakes are made up of 2- by 4-inch wood boards that are cut on the diagonal. This wedge shape helps prevent the willow burrito and blanket material from slipping over the top edge of the stake and more firmly secures the burrito in place. Wood stakes are preferred over metal fasteners in sensitive environments.
A few of the lessons learned include making sure that the burritos, once rolled up, are in the proper location, because once they are fabricated or become wet, they are heavy and difficult to move. Burritos should be rolled up tightly so that they maintain their shape and do not lose much of their height once staked into the ground. It takes some experimentation to determine the right amount of willow cuttings and mulch/soil filling to get the full 12 to 18 inches of diameter once rolled. In addition, it is helpful to have extra wood stakes around, because some will break as they are hammered into the ground. A longer wood stake length (such as 36 inches) provides adequate depth of burial and proper stabilization of the willow burrito.
Lilley Gulch willow burritos have been a success, with the stabilized stream and surrounding open space providing a neighborhood amenity and wildlife habitat. Willows have grown approximately 4 feet tall in just a single growing season (photos 4 and 5). Where design flow velocity is not expected to exceed that which a good vegetation cover can protect the soils, willow burritos are considered a viable and more environmentally sensitive alternative to riprap toe protection. The estimated cost for materials and installation is approximately $16 per linear foot.