646 - I

DRAFT

NATURAL RESOURCES CONSERVATION SERVICE

CONSERVATION PRACTICE STANDARD

VEGETATIVE BARRIER

(Feet)

CODE 646

DEFINITION

Permanent strips of stiff, dense vegetation along the general contour of slopes.

Establishment of Vegetative BarrIers

PhysIcal CharacterIstics of Plants:

PURPOSE

This practice will be applied as part of conservation management systems to support one or more of the following:

1. Reduce sheet and riIl erosion.

2. Reduce ephemeral gully erosion.

3. Manage water flow.

4. Stabilize steep slopes.

5. Trap sediment.

CONDITIONS WHERE PRACTICE APPLIES

This practice applies to all eroding areas, including but not limited to: cropland, pastureland, rangeland, forestland, farmsteads, mined land, and construction sites. This practice applies only when used in conjunction with other conservation practices as part of a conservation management system. On tilled fields, this practice only applies where soil profiles have sufficient depth to retain productivity where benches will develop.

CRITERIA

General CriterIa Applicable to All Purposes

Alignment. Obstructions, such as trees and debris that interfere with vegetative growth and maintenance, will be removed to improve vegetation establishment and alignment.

Establishment of Vegetative Barriers

Physical Characteristics of Plants:

Vegetative barriers will be planted to vegetation having the minimum Vegetation Stiffness Index (VSI) designated in Table I measured at a point 6 inches above the ground. VSI values reflect the importance of large stems in keeping barriers upright during runoff events.

Table 1. Stem Diameter and Minimum Stem Density Values for Vegetation Stiffness Index (VSI) Values of 0.05 and 0.10

Density:

Gaps between plants will be no greater than 3 inches at the end of the first growing season.

Species SelectIon:

Species will be selected that exhibit characteristics that are required for adequate function such as: emergence though several inches of sediment or resuming growth from buried stem nodes, rhizomatous or stoloniferous growth habit, and stems that remain intact and erect year round. Care will be taken when selecting plants to avoid invasive species.

Method of Establishment:

Barriers may be established vegetatively or from seed.

Seeding dates, depths and rates will be appropriate for the species selected and the conditions of the site. Seeds will be placed to insure good seed-to-soil contact and packed after planting.

Barriers established vegetatively will be planted in a single row at a dense enough spacing to insure a functional hedge in one growing season. For most herbaceous species, this will require a spacing of no more than 6 inches for bareroot seedlings, cuttings, sod chunks or divisions consisting of no less than 5 viable stems. Suckering shrubs or herbaceous species established from 6-inch (gallon) potted material will be established at a spacing of no more than 12 inches.

Optimum planting dates for the species will be used. Plants will be placed to insure good root-to-soil contact and packed after planting.

Barrier Width:

Barriers will be the larger of 3 feet wide or 0.75 times the design vertical interval. Broadcast or drilled seed will be sown in a strip at least 3 feet wide. Seed sown with a row planter will be seeded in a minimum of 2 rows.

CriterIa for Reducing Sheet and Rill Erosion

Erosion reduction by barriers is achieved by diverting flow, which reduces slope length (RUSLE “L”), and/ or through the supporting practice factor (RUSLE “P”) for contouring and grass strips.

Gradient. Gradients along the barrier will be no less than 0.2 percent and no greater than 1.0 percent except where the vegetative barrier crosses divides or concentrated flow areas. Gradients entering a concentrated flow area may be up to 1.5 percent for 100 feet in order to get better row alignment.

All tillage and equipment operations in the interval between barriers will be parallel to the vegetative barrier.

In order to redirect flow, and so reduce slope length, a berm must exist at the upsiope edge of the barrier and/or a channel must exist immediately upslope of the barrier. Minimum berm height/channel depth will be 3 inches. These berm/channels may be created by normal tillage operations along the edge of the grass, but will not form in no-till situations. Where berm/channles are used, a stable conveyance system for control of concentrated runoff must exist in flow areas that receive diverted runoff.

Spacing. Horizontal spacing between the vegetative barriers will be determined using the lessor of: a vertical interval of no more than 6 feet, or the allowable L^m that achieves soil loss tolerance in RUSLE considering the planned practices in the conservation management system.

Crop strip width will be planned in multiples of widths of planting, tillage, spraying, and harvest equipment This spacing may be adjusted up to 10 percent between the barriers.

Vegetation. The vegetation will be of species to provide the designated minimum stem density with the designated stem diameter and have a vegetation stiffness index (VSI) of 0.05.

Criteria for Reducing Gully Erosion

Alignment. Many fields have too much undulation to allow alignment on the contour across a concentrated flow area at angles convenient for the operation of farm equipment. in this case, vegetative barriers flay be installed across concentrated flow areas perpendicular to the direction of water flow. Vegetative barriers, when used to control ephemeral erosion, do not need to extend across the ridge top where water does not flow into the vegetative barrier.

Width and Length. Vegetative barriers will consist of a minimum of 2 rows. If a step is created across the barrier, one row of the barrier should be located at the bottom of the step and the other near the top Vegetative barrier length will vary depending on the topography. At a minimum, each strip will extend far enough to provide 1.5 feet of elevation from the center of the flow area to the end of the vegetative barrier (Figure 1).

Spacing. This practice functions by having backwaters from one barrier extending up to the base of the next barrier up slope. This backwater dissipates energy and causes sediment deposition in the gully channel.

Spacing between the vegetative barriers will be based on the vertical interval of 1.5 feet for conditions where no tillage is performed between the barriers and 3 feet for all other conditions where sediment deposition and bench development is anticipated.

Adjustments of 10 percent in the width of the crop strip between the barriers will be allowed for wildlife habitat or maintenance concerns. Crop strip width will be in multiples of widths of planting, tillage, spraying, and harvest equipment.

Maximum Watershed. The total watershed in a vegetative barrier system will be the smaller of: the size that will provide runoff to impound 1 foot of water upslope of the lowest barrier in the system,, or the size that will generate velocities upslope of that barrier greater than allowable for the soil and slope conditions as determined in Chapter 7, Grassed Waterways and Outlets, in the Engineering Field Manual. The last barrier should be located on the foot slope so that the existing velocity does not exceed the critical velocity for soil condition.

Vegetation. The vegetation will be of species to provide the designated minimum stem density with the designated stem diameter and have a vegetation stiffness index (VSI) of 0.05 for areas diverting runoff and VSI of 0.1 for areas retarding and ponding runoff.

Criteria for Managing Water Flow

For this purpose, barriers are designed to slow runoff by increasing path length and by retarding and spreading runon water for subsequent treatment within filter strips and flow channels to remove contaminants by ponding, filtration, infiltration, and exposure to sunlight.

Gradient. In order to divert flow, gradients along the barrier will be no less than 0.2 percent nor no greater than 1.0 percent except where the vegetative barrier crosses a divide or a draw (a concentrated flow area). Gradients entering a concentrated flow area may be up to 1.5 percent for 100 feet in order to get better row alignment.

In order to redirect flow, a berm must exist at the upslope edge of the barrier and/or a channel must exist immediately upslope of the barrier. Minimum berm height/channel depth will be 3 inches. These berm/channels may be created by normal tillage parallel to the vegetative barrier, but would need to be preformed in no-till situations

Width and Length. Vegetative barriers may consist of I or 2 rows. Vegetative barriers may be wider to adjust for planter and/or sprayer width, or for improved contour alignment. Vegetative barrier length will vary depending on the topography. At a minimum, each strip will extend far enough from concentrated flow areas to provide 1.5 feet of elevation from the center of the flow area to the end of the vegetative barrier (Figure 1).

Spacing. Horizontal spacing between the vegetative barriers intended to redirect runoff will be determined using the lessor of: a vertical interval of no more than 6 feet, or the allowable “L” that achieves soil loss tolerance in RUSLE considering the planned practices in the conservation management system.

For barriers intended to retard and spread runon, the maximum vertical interval will be 1 foot.

Crop strip width will be planned in multiples of widths of planting, tillage, spraying, and harvest equipment. This spacing may be adjusted up to 10 percent between the barriers.

Maximum Watershed. The total watershed in a vegetative barrier system will be the smaller of: the size that will provide runoff to impound 1 foot of water upslope of the lowest barrier in the system,, or the size that will generate velocities upslope of that barrier greater than allowable for the soil and slope conditions as determined in Chapter 7, Grassed Waterways and Outlets, in the Engineering Field Manual.

Vegetation. The vegetation will be of species to provide the designated minimum stem density with the designated stem diameter and have a vegetation stiffness index (VSI) of 0.05 for areas diverting runoff and VSI of 0.1 for areas retarding and ponding runoff.

Criteria for Stabilizina Steep Slopes

Alignment. The barrier will be installed on the contour.

Spacing. Horizontal spacing between the vegetative barriers will be the spacing that results in a vertical interval of no more than 6 feet.

Vegetation. The vegetation will be of a deeply rooting species that provide the designated minimum stem density with the designated stem diameter and have a vegetative stiffness index (VSI) of 0.05.

Criteria for Trpping Sediment as Field Borders at the Bottom of Fields and/or the Ends of Furrows

Barriers intended only to trap sediment at the end of fields will not be credited with additional in-field erosion reduction credit and should therefore be used as field boarders in fields already within soil loss tolerance. They will effectively reduce sediment delivery to surface water downslope of the barrier and are desirable additions to conservation management systems.

Alignment. Vegetative barriers may be used as field borders at the bottom of fields and/or the ends of furrows whether the furrows are aligned up and down the slope, across the slope or on the contour.

Width. Vegetative barriers used as field borders will be a minimum of 3 feet wide. There is no maximum crop strip width or slope length.

CONSIDERATIONS

During the planning process, consider the following:

General Considerations

Management practices such as conservation cropping rotation and residue management must be considered in designing the conservation management system on cropland. Associated structural practices such as water and sediment control basins, subsurface drainage, and underground outlets must be considered to adequately handle surface and subsurface water. This practice may improve the efficiency of other practices such as stripcropping, filter strips, ripanan buffer zones, grassed waterways, and diversions, and terraces.

Soils in fields where barriers are being established will be evaluated for their potential to create field access problems by ponding water behind the barriers. Subsurface drains perpendicular to or along the length of the barrier may improve drainage of the area upsiope of the barrier.

Water Quantity

1. Effects on the water budget, especially on volumes and rates of runoff, infiltration, evaporation, transpiration, deep percolation, and ground water discharge.
2. Variability effects caused by seasonal or climatic changes
3. Effects of snowmelt on water budget components
4. Potential for a change in plant growth and transpiration because of changes in the volume of soil water.
5. Effects on the downstream or aquifers that could affect other water uses or users.
6. Effect on the water table of the field to ensure that it will provide a suitable depth across the field for the anticipated land uses.

Water Quality

1. Effects on erosion and the movement of sediment, pathogens, and soluble and sediment-attached substances that would be carried by the runoff.
2. Effects of nutrients and pesticides on surface and ground water quality.
3. Effects on the visual quality of onsite and downstream water.
4. Short-term and tillage related effects on the quality of onsite and downstream water.
5. Potential for development of saline seeps or other salinity problems resulting from increased infiltration in soils that have restrictive layers.
6. Potential for uncovering or redistributing toxic materials such as saline soils.
7. Effects on the movement of dissolved substances below the root zone and to the ground water.
8. Effects on wetlands and water related wildlife habitats.

Wildlife Habitat

Effects on availability of food and nesting and escape cover.

Economics

1. Effects on the production of crops in adjacent fields from shading, and competition for water and nutrients.
2. Relative cost of vegetative barriers to other erosion and sediment control practices.

Considerations to Enhance the Functioning of Other Practices

1. Field Strlpcropping and Contour Buffer Stripcropplng: These strips are similar to vegetative barriers except they are wider, do not have as strict an alignment criteria, and require sediment accumulations to be periodically removed and redistributed on the land. Vegetative barriers established with field strips where they cross concentrated flow areas could reduce the failure of field strips caused by concentrated flow. Barriers used in association with field strips will be 3 feet wide and be located immediately upsiope of the field strip.
2. Filter Strips: Filter strips are areas of vegetation located along field borders or above conservation practices such as terraces or diversions to improve water quality. Vegetative barriers incorporated into the upsiope portion of filter strips will increase filter strip longevity by promoting sediment deposition above the filter strip. Barriers used in association with filter strips will be 3 feet wide and be located immediately upsiope of and/or periodically within the filter strip.
3. Field Borders: Field borders are areas of vegetation located along field borders to provide wildlife habitat or access to the field. Vegetative barriers incorporated into the upsiope portion of field borders at the bottom of slopes will increase field border longevity by promoting sediment deposition above the field border. Vegetative barriers will also additional provide wildlife cover in borders of predominantly sod-forming grasses. Barriers used in association with field borders will be 3 feet wide and be located immediately upslope of the border.
4. Riparian Forest Buffers: Ripanan forest buffers are similar to filter strips but include woody vegetation as well as herbaceous. Vegetative barriers could be used on the upslope edge of the vegetation zones. Barriers used in association with riparian forest buffers will be 3 feet wide and be located immediately upsiope of the buffer. Shading effects on vegetative barrier growth should be considered in selecting species.

PLANS AND SPECIFICATIONS

Plans and specifications will include:

1. Field map with location of vegetative barriers.
2. Width of crop strip.
3. Strip orientation.
4. Width of barrier.
5. Vegetative species and cultivar.
6. Vegetation establishment date, rate or spacing.
7. Operation and maintenance.

OPERATION AND MAINTENANCE

The following actions will be carried out to insure that this practice functions as intended. These actions include normal activities in the application and use of the practice and repair and maintenance of the practice.

1. Establishment failures will be replanted or reseeded immediately, short gaps in seeded barriers may be reestablished more effectively and immediately with transplanted plant material.
2. Mowing of herbaceous barriers may be used as a management practice to encourage the development of a dense stand and prevent shading of crops in adjacent fields. Mowing will not be closer than 15 inches or the recommended height, whichever is taller. Mowing will be scheduled to coincide with access through crops in adjacent fields. Mowing in concentrated flow areas is discouraged because it will lower the vegetative stiffness index (VSI) by reducing average stem diameter.
3. Burning of herbaceous barriers may be used as a management practice to encourage the development of a dense stand and prevent the accumulation of residue in the barrier. Burning will be performed when the vegetation is dormant and with adequate supervision to prevent the fire from damaging surrounding areas.
4. Weed control will be accomplished by mowing or by spraying or wick application of labeled herbicides.
5. Crop tillage and planting operations will be parallel with the vegetative barrier.
6. Pest control in adjacent fields will be performed with techniques and pesticides that will not damage the vegetative barrier.
7. Washouts or rills that develop will be filled and replanted immediately. Short gaps in established barriers will be reestablished with transplanted plant material.
8. Vegetative barriers will not be used as a field road or turn row. Vegetative barriers in concentrated flow areas will not be crossed with machinery.
9. Vegetative barriers will not be crossed with water furrow plows or similar implements to cut drainage ditches to allow the passage of surface and subsurface water. if necessary, water will be drained with subsurface drains installed above the barrier.

Conservation practice standards are reviewed periodically, and updated if needed.To obtain the current version of this standard, contact the Natural Resources Conservation Service.

NRCS, NHCP November 1999