VEGETATION ALONE
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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:

6. PLANNING

Useful for Erosion Processes:
  Toe erosion with upper bank failure
Scour of middle and upper banks by currents
  Local scour
Erosion of local lenses or layers of noncohesive sediment
  Erosion by overbank runoff
  General bed degradation
  Headcutting
  Piping
  Erosion by navigation waves
  Erosion by wind waves
  Erosion by ice and debris gouging
General bank instability or susceptibility to mass slope failure

Spatial Application:
  Instream
Toe
Midbank
Top of Bank

Hydrologic / Geomorphic Setting
Resistive
  Redirective
Continuous
Discontinuous
Outer Bend
Inner Bend
  Incision
  Lateral Migration
  Aggradation

Conditions Where Practice Applies:

Vegetation can be used from the channel to the upland riparian zone of a stream. A schematic drawing showing the type of riparian vegetation suitable for different locations (zones) on a streambank is shown in Figure 2. Only aquatic plants should be used below mean low water. Successful applications of vegetation for bank protection along perennial streams depend on structural protection of the toe. Vegetation is suitable for slopes from flat up to 1V:2H. Slopes of 1V:3H are the typical maximum for grasses that will be mowed. See Technique: Slope Flattening for guidelines on recommended slopes for different treatments.


Figure 2. Location of riparian vegetation according to bank zone( Maryland, 2000).

Complexity:

Low to High. Complexity will vary depending on the specific soil and climatic conditions. In all cases, the soil must be prepared properly and must be a suitable horticultural medium. So-called "engineered-soils" are soils that have had organic material removed and are compacted to a dense, hard, airless condition. This is the opposite of the typical horticultural needs of any plant, whether it is grass or an oak tree; this must be taken into account. A typical engineered-compacted slope must be loosened and amended to allow plant roots to penetrate and for the plant to receive air and nutrients to prosper. Many construction site soils are subsoils and contain no topsoil at all. Subsoils are typically dense, poorly developed soils that lack air, good drainage, and any nutrients or organic matter, all of which are vital to successful plant growth. Note that different groups of plants have different nutrient and water needs.

Design Guidelines / Typical Drawings:

Slopes steeper than 1V:3H and vegetated channel boundaries that will be subject to flows greater than 4 cm (3 in) deep at rates of more than 1.5 m/s (5 ft/s) should have additional netting or stronger techniques to hold seed, plants, and mulch in place. See the special topic, Combining Techniques regarding combinations of vegetation and structures. See Techniques Turf Reinforcement Mats, Erosion Control Blankets, and Geocellular Containment Systems. Allowable velocities and shear stresses for several types of vegetation have been compiled from several sources by Fischenich (2001); relevant data is presented in Table 1. Relevant parts of a compilation prepared by Fripp (2002) are reproduced in Table 2 in the following section on Hydraulic Loading.

TABLE 1: Permissible Shear and Velocity for Selected Lining Materials
(Note: Ranges of values generally reflect multiple sources of data or different testing conditions) (adapted from Fischenich, 2001)

Vegetation Type

Permissible Shear Stress
(Kg/m2 (lb/ft2))

Permissible Velocity
(m/s (ft/sec))

Citation(s)

Class A turf

18.1 (3.7)

1.8 – 2.4 (6 – 8)

Gray & Sotir (1996); Data from Author (2001)

Class B turf

10.25 (2.1)

1.2 – 2.1 (4 – 7)

Gray & Sotir (1996); Data from Author (2001)

Class C turf

4.9 (1.0)

1.0 (3.5)

Gray & Sotir (1996); Data from Author (2001)

Long native grasses

5.9 – 8.3 (1.2 – 1.7)

1.2 – 2.4 (4 – 6)

Kouwen, Li, & Simons (1980); Norman (1975); Temple (1980); Data from Author (2001)

Short native and bunch grass

3.4 – 4.6 (0.7 – 0.95)

0.9 – 2.4 (3 – 4)

Kouwen, Li, & Simons (1980); Norman (1975); Temple (1980); Data from Author (2001)

Reed plantings

0.5 – 2.1 (0.1 – 0.6)

N/A

Gray & Sotir (1996); Data from Author (2001)

Hardwood tree plantings

2.0 – 12.2 (0.41 – 2.5)

N/A

Gray & Sotir (1996); Data from Author (2001)

Bank zone location and environmental factors must be taken into account when designing vegetation establishment projects. Conceptual designs showing the proper placement and location according to bank zone and water elevations are shown in Figure 3 for herbaceous plants, and in Figure 4 for woody plant cover. Purple osier willow (Salix purpurea), red osier dogwood (Cornus siricea), crown vetch (Coronilla varia), reed canary grass (Phalaria arundinacea,) and tall fescue (Festuca arundinacea) have been found to perform well in the Ohio River area, while black willow (Salix nigra), Alamo switchgrass (Panicum virgatum), and sericia lespedeza (Lespedeza cuneata) performed well when used in combination with structural toe protection and grade control along small streams in northern Mississippi (Shields et al., 1995). Native plants are often the best adapted to site conditions, and normally provide broad-based environmental benefits, such as food or nesting sites for particular species. Native plants are usually well adapted to local weather extremes, such as prolonged drought or flooding.


Figure 3. Herbaceous cover conceptual design (Washington State, 2003).

Select plant materials with project goals in mind (see Glossary: Vegetative Techniques and Revegetative Techniques). Grass lined channels require low, flat, tough grasses that withstand inundation. In contrast, a habitat restoration and bank stabilization stream project might require a mixture of herbaceous and woody plants varying from sedges to large riparian trees. A prospective user should not only have clear goals for the planting, but also work with an experienced local horticulturist to specify soil, soil amendments, fertilizers, plants, planting techniques, mulches, and irrigation needs.


Figure 4. Woody plantings conceptual design (Washington State, 2003)

Vegetation Alone Typical Drawing

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.


Figure 5. Allowable velocity for vegetation and other types of streambank cover (Gray & Sotir, 1996).

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.

Please visit the Photo Gallery for more pictures.

18. RESEARCH OPPORTUNITIES

Research regarding the architecture, distribution, and tensile strength of the root systems of different riparian plant species would be useful. Protocols should be developed for incorporating this information into geotechnical slope stability analyses. Another important research issue is the extent to which plant roots can penetrate across buried geotextiles used as separation layers or filters in drainage applications. Further research is needed regarding the amount of vegetative cover needed to achieve desired levels of hydraulic and environmental performance. This would provide guidance on determining planting success and effectiveness.

19. REFERENCES

Allen, H. & Leech, J. R. (1997). Bioengineering for Streambank Erosion Control; Report 1, Guidelines. TR EL-97-8. 90 pp. (pdf)

Austin & Theisen. (1994). BMW extends vegetation performance limits. Geotechnical Fabrics Report, April/May

Chen Y. H. & Cotton, G. K. (1988). Design of roadside channels with flexible linings. Hydraulic Engineering Circular No. 15. Publication No. FHWA-IP-87-7 (pdf)

Fischenich, J. C. (2000). Irrigation systems for establishing riparian vegetation. EMRRP Technical Notes Collection (ERDC TN - EMRRP-SR-12), U.S. Army Engineer Research and Development Center, Vicksburg, MS (pdf)

Fischenich, J. C. (2001).  Stability thresholds for stream restoration materials.  EMRRP Technical Notes Collection (ERDC-TN-EMRRP-SR-29), U.S. Army Engineering Research and Development Center, Vicksburg, MS. (pdf)

Fripp, J. (2002). Compilation of Allowable Shear and Velocity Data. USDA Natural Resources Conservation Service, Fort Worth, TX . Unpublished table, personal communication.

Gray, D. H. & Leiser, A. (1982).   Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold, New York, N. Y.

Gray, D. H. & Sotir, R.  (1996).  Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Kouwen, N., Li, R. M., & Simons, D.B. (1980). A stability criteria for vegetated waterways. Proceedings, International Symposium on Urban Storm Runoff. University of Kentucky, Lexington, KY, 28-31 July 1980, 203-210.

Maryland Department of the Environment, Water Management Administration (Follweiler, J eds.) (2000). Maryland’s Waterway Construction Guidelines, Section 3 Channel Stabilization and Rehabilitation Techniques, Baltimore, MD. (pdf)

Norman, J. N. (1975). Design of stable channels with flexible linings. Hydraulic Engineering Circular 15, U.S. Dept. of Transportation, Federal Highway Administration, Washington, DC.

Parsons, (1963). Vegetative Control of Streambank Erosion. Federal Interagency Sedimentation Conference, Washington, DC, USDA Miscellaneous Publication 970:130-136

Schiechtl, H. M. & Stern, R. (1996). Water Bioengineering Techniques for Watercourse Bank and Shoreline Protection. Blackwell Science, Inc. 224 pp.

Schoklitsch, A. (1937). Hydraulic structures; a text and handbook. Translated by Samuel Shulits. The American Society of Mechanical Engineers, New York.

Shields, F. D., Jr., Bowie, A. J., & Cooper, C. M.  (1995).  Control of streambank erosion due to bed degradation with vegetation and structure.  Water Resources Bulletin 31(3):475-489.  (pdf)

Temple, D.M. (1980). Tractive force design of vegetated channels. Transactions of the ASAE, 23:884-890. (pdf)

U. S. Army Corps of Engineers. (1981).  Final Report to Congress, The Streambank Erosion Control Evaluation and Demonstration Act of 1974, Section 32, Public Law 93-251. Main Report and Summary and Conclusions. Washington, D.C.

USDA, 1947 (revised 1954). SCS Handbook of Channel Design for Soil and Water Conservation, SCS TP-61

USDA Soil Conservation Service. (1996). Chapter 16: Streambank and Shoreline Protection. Part 650, 210-EFH, Engineering Field Handbook, 88 pp. (pdf)

Washington Dept of Fish & Wildlife (2003). Integrated Streambank Protection Guidelines, published in co-operation with Washington Dept. of Transportation and Washington Dept. of Ecology, June 2002. (Chapter 6 pdf) (Appendix L pdf) (Appendix H pdf) http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (April 2003)

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