Physical Protection Techniques

Grade Stabilization

A dam, wall, or drop is used to reestablish or maintain streambed elevation (fig. 10). Maintenance of streambed elevation is necessary in stabilizing streambanks. Environmental considerations in designing grade stabilization measures include choice of construction materials, fish passage, plunge pools, number of structures used and their locations, and incorporation of vegetation.

Figure 10.
Grade stabilization structures used to either maintain existing grade or raise streambottom by trapping sediment.

Rock Riprap

Natural rock of appropriate size, density, and gradation is placed on shaped streambanks and in open-toe trenches. A gravel filter is placed under the rock to prevent erosion of soil material from beneath (fig. 11). Extensively eroded outside curves are generally realigned with enough "bend" left for necessary energy dissipation. Environmental considerations include impacts at sites where rock is obtained; preservation of as much existing streamside vegetation as possible during construction; construction during low-flow or dry stream conditions; preservation or enhancement of fish and wildlife habitat; fencing; and seep trenches. Stabilization of streambed grade may be an integral part of this measure.

Concrete grout may be incorporated where channel alignment problems and/or flow velocities require greater protection from the influence of scouring or high-energy flows. Environmental considerations include the use of vegetation,

Figure 11.
A typical streambank in cross section protected with natural rock over gravel bedding and willow cuttings.
The toe trench is used to trap bank seepage and encourage better growth of cuttings and natural vegetation.

visual impacts, aquatic life, and use of colored grout.

Post-and-Wire Revetment

Wire mesh strung on four-inch steel diameter or wooden posts, braced with steel cables along the top and bottom edges of the wire mesh, are used where flow, abrasion, and instream debris conditions will allow. Post-and-wire revetments are commonly used where rock is unavailable or not cost-effective in comparison. Riparian plantings immediately behind wire revetments are an integral part of revetment establishment and maintenance.

Figure 12.
A typical rail-and-cable revetment located
on Rancharia Creek in Mendocino county.

Rail-and-Cable Revetment

Rail-and-cable revetments (fig. 12) are similar in general appearance and function to post-and-wire revetments but are usually taller and made of heavier material to withstand greater water velocity, more abrasion, and debris problems. The revetment is covered with wire mesh with cables strung along the bottom, middle and top of the revetment. Riparian plantings are important components of rail-and-cable revetments.

Rock-and-Wire Revetment

Rock-and-wire revetments are essentially a well-braced double post-and-wire revetment with rock or very coarse gravel placed between the revetment walls. Riparian plantings are an important component of this type of revetment. Although they are expensive, properly located and constructed rock-and-wire revetments are quite stable as long as the wire withstands abrasion.

Pilot Channels

Pilot channels are dug in streambed material and used to "train" water away from newly treated streambanks, keep water out of construction areas, and increase chances of establishing newly planted vegetation.

Gabion Baskets

Gabion baskets are rectangular, heavy-mesh wire baskets filled with coarse gravel, which are placed and then wired together in series. Gravel size must be large enough to prevent leakage through basket mesh. Abrasion may break the wire mesh, requiring periodic maintenance. Gabions are used where large rock is unavailable or not

cost-effective. Plant material, generally in the form of willow cuttings, should be placed under, behind, and within baskets where moisture is adequate for growth.

Jacks and Cabled Trees

Past experience has shown these two methods to be less effective than many other methods. Cabled trees may be considered where ample trees are available and cost of other methods is absolutely prohibitive.

Vegetative Protection Techniques

Woody Cuttings

Cuttings are taken from locally adapted and locally growing plants when possible. Species which have been used include willow (Salix spp.), athel (Tamarixaphylla ), and tamarisk or salt cedar (Tamarixgallica ). Methods and procedures for taking and planting cuttings are provided in construction plans and specifications written for each project. In general, cuttings are laid horizontally in open-toe trenches and on shaped banks, as shown in figure 13, or planted vertically. If natural moisture is inadequate for growth, irrigation must be provided to insure establishment. Vertical plantings should be placed at a minimum depth of 1 m. (3 ft.) with 0.3 m. (1 ft.) of the cutting extending above the ground. The growing tip must extend aboveground. The best results have been obtained by augering holes for cuttings and watering cuttings in by washing soil material in around them. Watering in insures that cuttings are completely surrounded by soil which is devoid of air spaces. Immediate and generous irrigation should be provided at the time of planting. Water jetting can be used to both dig holes and water in plants where adequate water supply and pressure are available and soil texture allows.

Figure 13.
Placement of willow cuttings on bottom of toe trench and on moist areas (wick zone) of sloped banks.

Rooted Woody Plants

Rooted woody plantings are made on the bank side of revetments or on levees or streambanks above bank protection materials. Rooted plants can also be placed in planters set in concrete or gunnited rock. Unless natural seepage is adequate, plantings must be irrigated for at least two or three years for establishment. Plant species selection is influenced primarily by site conditions, soils, climate, plant availability, price, and wildlife habitat needs. Mulching is strongly recommended around and between plants. Herbaceous plants can be used in combination with rooted woody plants but should not create competition for the woody plants.

Herbaceous Plants

Perennial and annual grasses and forbs are seeded on sloping banks where mineral soil is exposed and on any disturbed areas on top of banks. Species selection, site preparation, timing, and seeding are all critical elements. If planted in combination with rooted woody plants, herbaceous seedings should be kept several feet away from establishing woody plants to avoid competition for space, moisture, and nutrients.

Mulching

Mulching around woody plants and over seedlings with straw and erosion control blankets

made of jute, fiberglass, wood fiber, and other materials is recommended on steep and erosive streambanks and levees. Straw should not be chopped in lengths shorter than 15 cm. (6 in.) and should be anchored either mechanically or with fiber mulch and a tackifier. Fiber mulch should be wood cellulose fiber containing no germination- or growth-inhibiting properties. Fiber mulch should be hydromulched as a slurry containing tackifier at a rate of 1,500 pounds per acre. Seed can be either mixed with the slurry or the fiber slurry applied over the seeded area.