VEGETATION ALONE

1. CATEGORY

2.0 – Bank Armor and Protection

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Planting, landscaping or vegetative stabilization. Sometimes known by the type of vegetation establishment technique employed, such as seeding, sodding, transplanting, or hydraulic mulching. Establishment of vegetation by planting or inserting live cuttings is addressed in Live Staking.

4. DESCRIPTION

Ground cover materials used for erosion control can be classified into inorganic materials (cements and rocky materials) versus organic materials (inert materials, living plants, and petro-chemicals) as shown schematically in Figure 1. Vegetation can be viewed as a living, organic ground cover consisting of grasses, legumes, forbs, and/or woody plants.

Vegetation can be used alone under special circumstances, but it also lends itself well to conjunctive use with other erosion control techniques in a mutually beneficial manner. In fact, living plants can be used in conjunction with nearly every other type of groundcover shown in Figure 1.

5. PURPOSE

Vegetation is established on bare soils in order to help prevent surficial erosion, minimize subsurface soil movement, provide habitat, and enhance aesthetics or visual appearance. Vegetation is useful for mechanically stabilizing soil to the depth of the root zone. Root depth varies greatly with the type and age of the vegetation, but most roots occur within the top 50 cm (20 in) of soil. Some hydrologic influences of vegetation, e.g., soil moisture removal by transpiration, extend below the root zone.

Vegetation stabilizes soil against both surficial erosion and shallow mass movement in a variety of ways (see Gray and Sotir, 1996, for a detailed discussion of this topic). In general, vegetation increases the resistance to surficial erosion in the following ways:

• Roots reduce soil compaction by increasing porosity, increasing infiltration, and slowing runoff. Root fibers reinforce soil and increase shear strength by adding tensile resistance to the soil mass.

• Fine roots help to bind soil particles together and make them less susceptible to lift-off and plucking, or removal by tractive stresses exerted by flowing water and wind.

• The above ground parts of a plant shield soil particles against displacement caused by raindrops or wind. Vegetative litter in the form of stems, twigs, and leaves, increases friction in channels and decreases velocities adjacent to a vegetated bank.


Figure 1. Classification of different groundcover materials (Gray & Sotir, 1996).

6. PLANNING

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Benefits are closely tied to increasing habitat quality through increased shade, shelter, forage, and nesting sites for a broad range of species. Vegetation also tends to improve visual appearance, and is particularly true when used in combination with hard or structural armor.

8. HYDRAULIC LOADING

In general, vegetation cannot withstand the high velocities and shear stresses that hard armor systems can endure. In addition, allowable velocities for vegetation tend to be affected more by duration of flow than those for hard structures. Combining grass with soft armor cover such as turf reinforcement mats and erosion control blankets improves resistance to higher flow velocities. These general trends are illustrated in Figure 5. Erosion resistance of mature, herbaceous ground cover is based primarily on the results of tests on grass lined channels in flumes or dam spillways (Chen and Cotton, 1988). Depending on a number of plant-soil variables, the erosional resistance of mature stands of tested grasses ranges from 20 to 160 N/m² (0.4 to 3.3 lbs/ft²). Performance and allowable flow velocity are affected by plant species, density (% cover) and condition of the vegetative cover, and type of bank soil, as shown in Table 2.

Bank Material/Protection	Shear stress, N/m2	Velocity, m/s	Condition	Source
Bermuda grass, erosion resistant soils, 0-5% slope		2.4	design	USDA, 1947
Bermuda grass, erosion resistant soils, 5-10% slope		2.1	design	USDA, 1947
Bermuda grass, erosion resistant soils, over 10% slope		1.8	design	USDA, 1947
Bermuda grass, easily eroded soils, 0-5% slope		1.8	design	USDA, 1947
Bermuda grass, easily eroded soils, 5-10% slope		1.5	design	USDA, 1947
Bermuda grass, easily eroded soils, over 10% slope		1.2	design	USDA, 1947
Grass Mixture, erosion resistant soils, 0-5% slopes		1.5	design	USDA, 1947
Grass Mixture, erosion resistant soils, 5-10% slopes		1.2	design	USDA, 1947
Grass mixture, easily eroded soils, 0-5%slopes		1.2	design	USDA, 1947
Grass mixture, easily eroded soils, 5-10% slopes		0.9	design	USDA, 1947
Grasses: Lespedeza sericea, Weeping lovegrass, Yellow bluestem, Kudzu, Alfalfa, Crabgrass, Common lespedeza; erosion resistant soil, 0-% slope unless on side slopes		1.1		USDA, 1954
Grasses: Sericia lespedeza, Weeping lovegrass, Yellow bluestem, Kudzu, Alfalfa, Crabgrass, Common lespedeza; easily erodible soil, 0-% slope unless on side slopes		0.8		USDA, 1954
Dense sod, fair condition (class D/E) moderately cohesive soil	17		limit	Austin and Theisen, 1994
Bermuda Grass, fair stand <12 cm tall, dormant	44		limit	Parsons, 1963
Bermuda Grass, good stand, <12 cm dormant	54		limit	Parsons, 1963
Bermuda Grass, excellent stand 20 cm tall, dormant	132		limit	Parsons, 1963
Bermuda Grass, excellent stand, 20 cm green	137		limit	Parsons, 1963
Bermuda Grass, excellent stand, >20 cm tall, green	156		limit	Parsons, 1963
12.5 cm of excellent growth of grass/woody vegetation on outside bend	49		limit	Parsons, 1963
Sod revetment, short period of attack	20		design	Schoklitsch, 1937
Turf (immediately after construction)	10		limit	Schiechtl and Stern, 1994
Turf (after 3-4 seasons)	100		limit	Schiechtl and Stern, 1994
Reed Plantings (immediately after construction)	5		limit	Schiechtl and Stern, 1994
Reed Plantings (after 3-4 seasons)	30		limit	Schiechtl and Stern, 1994
Deciduous tree plantings (immediately after construction)	20		limit	Schiechtl and Stern, 1994
Herbaceous and woody		2.4	design	USACE TR EL 97-8

9. COMBINATION OPPORTUNITIES

Vegetation can be combined with most other river training and bank armor systems. Erosion Control Blankets (ECBs), Turf Reinforcement Mats (TRMs), Geocellular Containment Systems (GCSs), and Articulated Concrete Blocks (ACBs) are designed for combined use with vegetation. Plantings can be employed for upper and mid-slope protection on banks protected by structural toe protection like Longitudinal Stone Toe and Longitudinal Stone Toe with Spurs, or softer forms of toe protection, such as Coconut Fiber Rolls. Vegetation can also be used in combination with other bank protection or stabilization techniques, such as Slope Flattening and drainage measures, such as Chimney Drains or Trench Drains. In addition, transplanting and seeding can be employed in conjunction with soil bioengineering techniques, such as Live Fascines and Live Staking.

10. ADVANTAGES

Vegetation improves resistance to both surficial erosion and subsurface soil movement in ways that have been described previously. In addition, vegetation provides important habitat and aesthetic values. Vegetation is relatively inexpensive compared to other methods and does not require heavy lifting or other specialized equipment. Vegetation normally becomes stronger and more effective with time, thus providing increasing protection to streambanks as the vegetation becomes well established. In contrast, inert ground covers tend to degrade or become weaker over time.

11. LIMITATIONS

Vegetation has strength limitations, particularly in its early stages, when the plants are not yet well-established. Additionally, vegetation is not as strong when used alone as it is when combined with other techniques or materials, such as erosion control fabrics and more structural procedures, such as biotechnical erosion control techniques. Plants minimize physical soil disturbance from such things as hooves or feet, however, plants used alone are vulnerable to heavy traffic and trampling.

12. MATERIALS AND EQUIPMENT

Environmental factors such as climatic zone, soil type, moisture availability, soil chemistry, light conditions, etc., must be taken into account when developing a planting design. Detailed lists of woody plants recommended for revegetation of riparian corridors are provided elsewhere (USDA, 1996; Washington State, Appendix H, 2003). Locally, native plants are often the best adapted to conditions, and often provide broad-based environmental benefits, such as food or nesting sites for particular species, e.g., certain butterflies, or excellent adaptation to local weather extremes such as prolonged drought or flooding. Plants may need irrigation either for an establishment period or for the life of the project.

Plants always benefit from suitable high quality soil and organic amendments. Plants, whether from seeds, cuttings or rooted container plants, are usually planted in combination with protective surface mulch. Mulch is a layer of organic material that is applied to the soil surface. Examples of mulch include blankets of wood chips or straw, or shredded wood waste or newspaper pulp that is sprayed on the ground by hydraulic mulching pumps. Mulch provides a protective cover against soil erosion and loss of small plants and seed, prevents rapid drying of soil and roots, and discourages growth of weeds that compete for light, growing space, and nutrients.

13. CONSTRUCTION / INSTALLATION

Vegetation can be planted in the form of seed, rooted transplants (also called container plants), or cuttings. Seeding can be accomplished by hydraulic seeding (hydromulching), seed drilling, hand broadcasting, or mechanical broadcasting. Hydromulching sprays are mixtures of seed, mulch, and emulsifying/stabilizing agents that harden and provide temporary protection. Rooted transplants in containers are usually placed in prepared soil dug by hand as shown in Figure 6. Detailed information on planting methods can be found in Washington State, 2003. See Techniques Live Staking and Willow Posts and Poles for examples of propagation with cuttings.

In all cases, the soil must be prepared properly, so that it provides a suitable horticultural medium. So-called "engineered-soils" are soils that have had organic material removed and are compacted to a dense, hard, condition with poor aeration and water transmission properties. This is the opposite of typical horticultural needs of any plant, whether grass or an oak tree. Many construction sites consist of subsoils that contain no topsoil at all.

Figure 6. Woody plant installation using bare-root, ball and burlap, or container plants (from Washington State, 2003).

Sub-soils are typically dense, poorly developed soils that lack sufficient air, good drainage, and nutrients or organic matter, all of which are vital to successful plant growth. A typical engineered-compacted slope must be loosened near the surface and amended to allow plant roots to penetrate and for the plant to receive air and nutrients to prosper. Nutrient and water needs vary by plant type. See the Special Topic, Optimizing Soil Compaction and Other Strategies, for different ways of ameliorating or circumventing adverse conditions associated with engineered or compacted soils.

Optimal application of straw mulch to prevent surficial rainfall erosion ranges from 2.2-4.5 metric tons/ha (1-2 tons/ac). The following publications provide useful information and advice on soil and site preparation to maximize plant establishment and survival: Gray and Leiser, 1992; Washington State, 2003.

14. COST

Revegetation is usually an important component of most environmentally sensitive bank protection projects. Revegetation materials include seed, cuttings, and plants that are either bare-rooted, balled, burlapped, or potted (containerized). Mulch is normally part of revegetation. Plant-material costs vary widely depending upon the species, maturity of plants purchased, and whether the stock is containerized or in a bare-root condition. Seed and tubeling stock are usually sold at a fraction of the cost of more mature stock in gallon size containers. Cost is also affected by other factors, such as the lead time a nursery is given to acquire and/or cultivate the materials ordered. The costs shown in Table 3 were assembled by Washington State, 2003.

TABLE 3: Range of costs for plant materials applied in streambank-protection projects
(after Washington State , 2003).

Plant Material	Unit of Measure	Unit Cost
Soil preparation	Square meter (Square yard)	$2.80 ($2.25) (includes tilling and grading)
Live cuttings	Each	$2-$5 (planted)
Tubelings	Each	$1-$4 (planted)
Conservation plugs	Each	$1-$4 (planted)
Grass seed	Hectare ( Acre )	$1,853 ($750)
Evergreen trees (1 m (3 ft) height)	Each	$15
Deciduous trees	Each	$20
Shrubs (1-2 gallon)	Each	$8-$12
Ground cover (1 gallon)	Each	$8-$10
Mulch	Square meter (Square yard)	$2.40-$6 ($2-$5)
Hydroseeding	Square meter (Square yard)	$0.54 ($0.45)

15. MAINTENANCE / MONITORING

Revegetation efforts do not end with initial installation and planting; monitoring and maintenance are crucial to project success as well. Some follow-up planting may be required to replace plants that did not survive the initial planting. Plants may need irrigation for an initial establishment period (see Figure 7), but are not likely to, and should not require, continuous irrigation. Irrigation alternatives are described by Fischenich (2000). Plantings should be checked for browsing damage and if necessary measures taken (see Figure 8) to protect young saplings and emergent vegetation. Weed control may be necessary in some cases as well.

Figure 7. Drip irrigation ring used around a small conifer tree.	Figure 8. Wire cage used to protect transplants against browsing.

Monitoring should be conducted monthly during the first full growing season after installation, and can be reduced to single, annual visits thereafter (Washington State, 2003). Initially, survival of installed plants can be monitored by a physical count, but later on, as cover density increases, it may be necessary to use percent cover as an indicator of plant health and survival. Survival monitoring in a riparian zone should also take into account sediment deposition, which can affect survival of installed plants, but in the long run may be beneficial to the establishment of desirable native riparian species.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

Many factors can contribute to or cause failure of plants to establish, thrive, and survive. Inadequate soil moisture (either too little or too much), insufficient soil nutrients, toxic soil conditions (high alkalinity or acidity), and inadequate light are all soil and site conditions that can influence plant health. Other common causes of failure include incorrect planting locations (see Figure 2), inability of plant material to reach the summer water table, damage by wildlife and livestock, excessive pedestrian traffic, and inability of installed plants to compete with naturally establishing riparian vegetation.

17. CASE STUDIES AND EXAMPLES

During implementation of a riparian mitigation project on Sulphur Creek in Redding, California, workers planted both container plants and poles of Salix species (both Willows and Cottonwood). Although most of the plants grew well, it was noted that the trees grown from poles were much hardier and had a higher likelihood of survival. One explanation for this may be the very hot summers in the region; just after installation, the poles are closer to the water table than the container trees.

Figure 9. Using heavy equipment to install willow poles.

Figure 10. Three growing seasons after installation.

Manatawny Creek

Two J-hooks were designed to restore the section of Manatawny Creek that had experienced severe erosion. Prior to installation, the banks were regraded and shaped in preparation for vegetation establishment and structure installation. The structures were carefully constructed in the live stream. The J-Hooks were installed in succession on the right bank of the stream to direct flow away from the newly graded bank and into the center of the channel. The rocks used were 0.6 to 1 m (2 to 3.3 ft) angular rocks that were placed carefully and then backfilled at the key. Coir matting was installed upstream of the first structure to provide additional support to the bank. All disturbed areas were then seeded and mulched with straw. The J-hooks were installed over the course of two weeks in November 2002 and have successfully stabilized this section of Manatawny Creek. The project has sustained two winters, included snowfall and freezing conditions.

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