A tree revetment, made by anchoring trees along a stream bank, is an inexpensive, effective way of stopping stream bank erosion. The trees greatly slow the current along the eroding bank; this decreases erosion and allows silt and sand to be deposited along the bank and within the tree branches. The deposited material forms a good seed bed in which the seeds of river trees such as cottonwood and sycamore can sprout and grow. The resulting trees spread roots throughout the revetment and stream bank. By the time the revetment trees have decayed, the bank should be stabilized by the roots of the living trees. As an added benefit, tree revetments provide excellent fish and wildlife cover.
On some streams, permits must be obtained from the U.S. Army Corps of Engineers before tree revetments can be built. Contact the Corps of Engineers or the Missouri Department of Conservation to find out if you need a permit before doing any work on a stream.
If the trees are cut near where they are to be installed, they can be dragged to he revetment site. A four-wheel-drive pickup or tractor can move even large trees in this way.
Once the trees have been moved to the top of the eroding bank, they are ready to be placed against the bank. This can be done with a long cable attached to a truck or tractor on the opposite bank. Trees can also be pushed over the bank edge with a front-end loader mounted on a tractor. Caution: When this method is used, it is important to avoid coming too close to the bank edge because it could collapse. Workers in the stream should stay well clear of cables, chains and trees as they are pulled over the edge.
Construction of the revetment begins at the downstream end of the eroding stream bank. The first tree is moved into place on the eroded surface, with the butt end pointed upstream. The tree is placed tightly against the banks and anchored at both ends. (See the next section for anchoring procedures.) Another tree is then moved into place with it's top overlapping the butt of the first tree, so that no gap between the two exists. The cable used to anchor the butt of the first tree is then secured to the top of the second tree (see above illustration) and a new anchor is put in at the butt of the second tree. This process is continued upstream until the entire bank is covered with trees.
Overlapping the trees ensures that no gaps are left in which erosion can occur. It also reduces anchoring costs by allowing each anchor to hold both the butt of one tree and the top of the next.
It is usually necessary to pull trees tightly against the bank before anchoring them in place. This can be done by attaching a chain or cable to the trees and pulling them against the bank with a truck or tractor.
It is very important that trees in a revetment be well anchored. Many methods, varying in cost, effectiveness and ease of installation, can be used. When choosing an anchoring method, remember that a certain anchors should be used only in specific types of bank soil. Since everything from loose sand to clay to bedrock can be encountered in Missouri stream banks, it is important to determine the composition of the bank before deciding on an anchoring method.
When stream bank soils are rocky, two types of anchors can be used: steel "T-posts" and driven earth anchors. T-posts, which are commonly used for barbed wire fences, can be driven completely into the bank. A cable (3/16-inch aircraft cable or larger) is then attached to each post, wrapped around the tree, and secured with a common cable clamp. Driven earth anchors are more expensive and more difficult to obtain than T-posts. They have excellent holding ability, however, and can be installed in the rockiest of stream banks. They are also attached to the trees with steel aircraft cable. Screw-in anchors, available at most farm supply stores, can be used when stream banks are not rocky.
Regardless of how the trees are anchored, it is very important that they be held tightly against the bank. If cables are loose or too much cable is stretched between the anchors and the trees, flood waters will cause the trees to move violently and they may break free or allow the banks behind them to erode.
Examine the stream bank soil carefully before deciding how to anchor your revetment and match the anchoring method to the soil type. It is important to check for the presence of bedrock at or near the soil surface, because anchors cannot be driven into solid rock. Use a four foot length of steel rod to prove at intervals along the bank to make sure you will be able to drive anchors to at least this depth before hitting bedrock. Contact the Missouri Department of Conservation for more information on anchoring tree revetments.
As floods deposit silt and sand in a new tree revetment, a moist, fertile seedbed is formed. Usually the seedlings of river trees like cottonwood, sycamore and willow will appear in a tree revetment within a year. You can accelerate this process by planting seedlings in the revetment after some silt deposition has taken place. Technical advice on planting streamside trees is available from the Missouri Department of Conservation. Remember that it is vital that trees get started within the tree revetment. Its primary purpose is to protect the bank from flood currents until trees can grow on the bank to permanently stabilize it.
Tree revetments will generally work on medium to small stream bends that are unstable because the original cover of trees has been removed. If a stream bank is unstable because of drastic changes in its channel or watershed, a tree revetment may not work. A good example of such a change is stream straightening (or channelization). Remember that the reason for installing a tree revetment is to provide bank protection until trees have a chance to get started on a bank. If a bank is well covered with trees already and is still eroding, then the stream is probably unstable because of watershed or channel alteration and a tree revetment may not stabilize it.
Stream and bank size should also be considered. If the eroding stream bank is much more than12 feet high, a tree revetment may not be enough to stabilize it. Every stream problem is different, however, and there are a few "rules of thumb" regarding bank stabilization. If you are considering a tree revetment, it is a good idea to get advice from the Missouri Department of Conservation.
Questions? For more information on tree revetments or other methods of stream bank stabilization, contact the Missouri Department of Conservation, Division of Fisheries, P.O. Box 180, Jefferson City, MO 65102. Phone 573-751-4115.
For more information, write:
Missouri Department of Conservation
Jefferson City, MO 65102
The Missouri Department of conservation uses Federal financial assistance in Sport Fish and/ or wildlife Restoration. Because the state utilizes these federal funds, it must comply with federal anti-discrimination law. Under Title VI of the 1964 Civil Rights Act and Section 504 of the Rehabilitation Act of 1973, the federal government prohibits discrimination on the basis of race, color, national origin, disability, age or sex. If you believe that you have been discriminated against in any program, activity or facility as described above, or if you desire further information please write to:
The Office for Human Resources
U.S. Fish and Wildlife Service
Department of the Interior
Washington, DC 20240
Missouri Department of Conservation
P.O. box 180
Jefferson City, MO 65102