Evergreen Revetments Guide No

Evergreen Revetments Guide No. 12 EVERGREEN REVETMENTS An evergreen revetment is a protective “wall” made of freshly cut cedars, pines, firs or recycled Christmas trees which are anchored into an eroded streambank. It absorbs stream energy and traps sediments. This buffer sys- tem, shown in Figure 1, slows or halts active erosion, creates a place for sediments to deposit and allows vegetation to re-establish on the streambank. An evergreen revetment alone may not be sufficient to hold soils in place after the evergreens decay. For long-term stability, dormant cuttings of willow or other rapidly-rooting species should be planted within the revetment and above it on the bank face to accelerate the re-establishment of root structure (see Guide No. 07 Restoring Streambanks with Vegetation). A buffer strip of na- tive hardwoods should also be planted, if absent, along the top of the bank (see Guides No. 08 Trees for Ditches and No. Figure 1. Evergreen Revetment 13 Forested Buffer Strips). An evergreen revetment is one where high velocity flows can be design, installation and maintenance of several biotechnical practices de- expected. In other situations, a steps for evergreen revetments. scribed in the Ohio Stream Manage- combination of structural and bio- The guidance provided is based on ment Guides. These practices use technical practices may provide both guidelines from Missouri and on field vegetative or other natural materials strength and habitat. No project experience here in Ohio. As with to achieve stream management should be undertaken without an any construction project in a stream, objectives, usually erosion control. understanding of the functions of the Ohio Department of Natural Re- One of the chief advantages of bio- stream energy and the source of sources recommends you consult technical practices is that they help any problems to be corrected. Guide with the applicable local, state and restore natural stream features, like No. 03, Stream Management and the federal authorities listed in Guide 06, in-stream habitat and streambank Stream’s Natural Processes will pro- Permit Checklist for Stream Modifi- vegetation. Guide No. 10, Biotech- vide an overview of stream dynamics cation Projects, prior to construction. nical Projects in Ohio, provides an and the impacts land and channel The extent of permit requirements overview of biotechnical practices. management practices have on will depend on the location and de- It also maps over 50 project sites streams. Technical assistance sign of your project. and list contacts who can arrange about stream dynamics can also be for site visits. obtained at your local Soil and Water WHERE TO USE Some site conditions and/or Conservation District. Its number is EVERGREEN REVETMENTS project objectives (such as pro- listed under county government in Evergreen revetments work tecting existing structures on the the local phone directory. best on medium to small streams streambank) will require use of more The purpose of this Ohio Stream which have erosion problems due to traditional, structurally-engineered Management Guide is to describe inadequate tree cover on the stream- solutions. This is particularly true the generally suitable site conditions, banks. This lack of tree cover usually means there is adequate sunlight to Since the revetment itself does not and put the culled trees to good supplement the revetment by planting use live vegetation, construction can use. Christmas trees gathered for dormant cuttings and hardwood trees. take place in any season. Construc- recycling in January have also been Always look for the cause of erosion tion in late winter or early spring will successfully used. But trees which when considering how to solve it. If allow the revetment to catch silt from have been dead for some time are the stream is just naturally meander- spring floods while it still has most of more brittle and lose needles and ing, protective measures should not its needles and fine branches in tact. branches during construction. be installed unless really necessary. In addition, February and early March The more limbs and fine branches A bank that is well covered with trees are good months for planting dormant a tree has the more it will continue to but still eroding indicates that the cuttings to supplement the revetment slow the stream’s current after the channel is undergoing adjustments with new root structure in the bank. needles are gone, allowing sediments due to changes in land use or channel to accumulate within the anchored configuration in the upper watershed, Regrading the slope — Ev- trees. Both time and money can be and an evergreen revetment will ergreen revetments are generally saved by using the largest trees avail- probably not stabilize it. Biotechnical used as an alternative to expensive able as long as they are compatible practices are not likely to stabilize an regrading, so they are most often with the stream’s size (see worksheet eroded area if streambank erosion is used in locations were regrading is calculations). Cut the trunk near the very prevalent in your stream, an in- not necessary. Providing safe work- lowest limbs; anything below that is dication that the stream is undergoing ing conditions is a key factor in deter- excess weight to carry. Trees cut a systematic change (e.g. deepening mining if a revetment should be built prior to construction and stored on or widening). at all or if the bank needs regraded site should be placed above the flood- If the water depth at the toe of before construction. Regrading loos- plain. This will prevent them from the slope is less than three feet, a ens soil and increases the chance of floating downstream should a storm revetment should stabilize the bank. revetment blow-out, so regrade only occur before construction begins. Channel depth at the site needs to be if absolutely necessary. Some of those with field experi- shallow enough to safely stand and Some eroded banks have a ence prefer to drill holes in the butt work during low flows. Generally, if slumped area at the base and a more end of each tree trunk (Figure 2). the target streambank is over 12 feet vertical section toward the top. If the During construction, cable is threaded (ft.) high, an evergreen revetment soils in the slump are stable and there through the hole first, then wrapped may not be adequate to stabilize it. is enough area to safely work, regrad- around the trunk above the bottom However, revetments have been suc- ing the base area may not be neces- limbs. When using this method be cessfully built on higher banks. If the sary, but the top should be regraded sure to size the hole appropriately for water depth at the toe of the slope is after construction. If there is an the cable you are using (see Equip- more than three feet, the revetment overhang, for safety’s sake it should ment, below). Others prefer wrapping alone may not be able to protect the be taken off first, before construction cable twice around the trunks above toe and stabilize the bank. The toe takes place lower on the bank. the lower limbs and have found this of the slope is where the bank slope method adequate for resisting stream turns and becomes the channel bot- Tree Selection — Freshly cut flow velocities, especially in lower tom. Consider using a tree kicker cedars, firs or pines work best. Ce- gradient streams. upstream to deflect flows if they are dars resist decay the best, but are undercutting the bank (see Guide No. not available in all parts of Ohio. It is Anchoring Methods — Anchor- 11, Tree Kickers). often possible to work with Christmas ing the trees into the streambank in tree farms to help cull their stock a manner that will resist the force DESIGN AND of water is imperative. There are CONSTRUCTION several different types of anchors GUIDELINES available, and they vary in availability, Enclosed is a reproducible Ever- cost, ease of use and effectiveness green Revetment Worksheet which in different soils. takes you through the calculations • Disk anchors (Figure 3-a) are needed for designing a revetment available at most hardware or and estimating the amount of materi- farm supply stores. They are als needed for construction. Many often used to tie down mobile hands make constructing a revetment homes. Use a model with at least “light” work, but a minimum of 3 to a four-foot arm. Since they are 5 people in good physical condition screwed into the bank they are should be on site to build even a Figure 2 . Wrapping and used in soils, preferably clays, one-row revetment on a small site. Clamping Cable to Tree Trunks where there are no rocks. At- 3a 3c 3b Figure 3. (a, b &c) Types of Anchors tach the wrapping cable with a The strength of the materials used This usually means the entire length saddle clamp after screwing in should be based on expected stream of the outside bend must be covered the anchor. flows and project size. Choose from by the revetment. The first row of • T-posts (Figure 3-b) can also be cable with saddle clamps, high tensile trees needs to be placed so that the purchased at farm supply stores. wire with crimping sleeves (attached tree crown (the widest part of the tree, These fenceposts are best used with crimping pliers), or use num- i.e., the bottom of the foliage area for on small streams. They can be ber nine fencing wire which can be evergreens) rests on the toe of the driven all the way into even rocky twisted tight with pliers. These ma- bank slope as shown in Figure 1.

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