Diversion, Permanent
Description
A permanent ridge or channel, or a combination ridge and channel, constructed across sloping land or at the top or bottom of a steep slope. Used to convey runoff water, this practice is used to reduce slope lengths, break up concentration of runoff, and move water to stable outlets at a non-erosive velocity.
Where Practice Applies
This practice applies to sites where runoff can be diverted and used or disposed of safely to prevent flood damage or erosion and sediment damage, including:
- Above steep slopes to limit surface runoff onto the slope.
- Across long slopes to reduce slope length to prevent gully erosion.
- Below steep grades where flooding, seepage, or sediment deposition may occur.
- Around buildings or areas that are subject to damage from runoff.
Diversions must have stable outlets. The site, slopes, and soils must be such that the diversion can be maintained throughout its planned life.
Permanent diversions are not applicable below high sediment producing areas unless land treatment practices, or structural measures designed to prevent damaging accumulations of sediment in the channels are installed with or before the diversions.
Advantages
Diversions are among the most effective and least costly practices for controlling erosion and sedimentation.
Planning Considerations
Permanent diversions should be planned as a part of initial site development. They are principally runoff control measures that subdivide the site into specific drainage areas.
Permanent diversions can be installed as temporary diversions until the site is stabilized, then completed as a permanent measure, or can be installed in final form during the initial construction. The amount of sediment anticipated and the maintenance required as a result of construction operations will determine which approach should be used.
Stabilize permanent diversions with vegetation or materials such as riprap, paving stone, or concrete as soon as possible after installation.
Base the location, type of stabilization, and diversion configuration on final site conditions. Evaluate function, need, velocity control, outlet stability, and site aesthetics. When properly located, land forms such as landscape islands, swales or ridges can be used effectively as permanent diversions.
Base the capacity of a diversion on the runoff characteristics of the site and the potential damage after development. Consider designing an emergency overflow section or bypass area to limit damage from storms that exceed the design storm. The overflow section may be designed as a weir with riprap protection.
Design Recommendations
Capacity
Peak runoff values should be determined by accepted methods. Recommended minimum design frequencies are shown below. In all cases, the design storm frequency should be chosen to provide protection compatible with the hazard or damage that would occur if the diversion should overtop.
- Homes, schools, industrial buildings, etc.: 50-year design frequency
- Playfields, recreation areas, similar land areas: 25-year design frequency
Permissible Flow Velocity
| Soil Texture | Bare Channel | Vegetated Channel |
| Sand, silty sandy loam | 1.5 feet/second | 2.5 feet/second |
| Silty clay and sandy clay loam | 2.0 feet/second | 3.5 feet/second |
| Clay | 2.5 feet/second | 4.5 feet/second |
Cross Section
The channel may be parabolic or trapezoidal. It should be designed to have stable side slopes.
Side slopes for permanent diversions should not be steeper than 3:1 for maintenance purposes and preferably 4:1. In no case should side slopes be steeper than 2:1.
Back slope of the ridge is not to be steeper than 2:1 and preferably 4:1.
The ridge should include a settlement factor equal to 5 percent of its height.
The minimum top width of the diversion ridge after settlement is to be 4.0 feet at the design elevation.
Freeboard equaling 0.5 foot minimum.
In determining the cross section of temporary diversions, consideration should be given to soil type and frequency and type of equipment that is anticipated to be crossing the diversion.
Grade
Channel grade for diversions may be uniform or variable. The permissible velocity for the soil type and vegetative cover will determine maximum grade. Level diversions with blocked ends may be used, provided pipes of sufficient size and spacing are placed in the embankment to drain the channel after runoff stops.
Outlets
Diversions are to have adequate outlets which will convey runoff without causing erosion.
The following types of outlets are acceptable:
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Natural or constructed vegetated outlets capable of safely carrying the design discharge. The outlet should be established and well vegetated prior to construction of the diversion.
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Properly designed and constructed grade stabilization structures or storm sewers.
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Natural or constructed open channels which are stable and have adequate capacity and depth.
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A stable area having a good sod cover or a woodland area with a deep erosion resistant litter. The outlet end of the diversion channel should be flared in a manner to spread the water over a wide area at a shallow depth.
Level Spreader
A level lip spreader should be considered at diversion outlets discharging onto area already stabilized by vegetation. Spreaders shall be excavated at least 6 inches deep into undisturbed soil. The bottom of the excavation and the downstream lip of edge shall be level. Minimum spreader lengths shall be based on the peak rate of flow from a 10-year frequency storm.
Diversion Dikes
Diversion dikes should be used to divert runoff for temporary or permanent protection of cut or fill slopes. Diverted runoff must be discharged onto a stabilized area or through a slope protection structure.
Recommended criteria
- Drainage area 5 acres or less.
- Top width 2 feet minimum.
- Height (compacted fill) 18 inches unless otherwise noted on the plans. (Height measured from the upslope toe to top of the dike).
- Side slopes 2:1 or flatter.
- Grade dependent upon topography, but must have positive drainage to the outlet; may require vegetative or mechanical stabilization where grades are excessive.
Protection Against Sediment
- Permanent diversions - As a minimum, a filter strip of close growing grass should be maintained above the channel. The width of the filter, measured from the center of the channel, should be one-half the channel width plus 15 feet. The diversion ridge and channel should be vegetated to prevent erosion. Small eroded areas and sediment-producing channels draining into the diversion should be shaped and seeded prior to or at the time the diversion is constructed.
Construction Recommendations
- All trees, brush, stumps, and other objectionable material should be removed so they will not interfere with construction or proper functioning of the diversion.
- All ditches or gullies which must be crossed should be filled and compacted prior to or as part of the construction.
- Fence rows and other obstructions that will interfere with construction or the successful operation of the diversion should be removed.
- The base for the diversion ridge should be prepared so that a good bond is obtained between the original ground and the placed fill.
- Vegetation should be removed and the base thoroughly disked before placement of the fill.
Vegetation
Diversions should be vegetated as soon after construction as practical. Consider jute matting, excelsior matting, or sodding of channel to provide erosion protection.
Seeding, fertilizing, mulching, and sodding should be in accord with applicable vegetative standards for permanent cover. See Permanent Seeding.
Very moist channels are often best vegetated by working rootstocks of reed canary grass into the seedbed.
When soil conditions are unfavorable for vegetation (such as very coarse-textured subsoil material), topsoil should be spread to a depth of 4 inches or more on at least the center half of parabolic shaped channels or on the entire bottom of trapezoidal shaped channels.
Seeded channels should be mulched. For critical sections of large channels, and for steep channels, the mulch should be anchored by cutting it lightly into the soil surface or by covering with paper twine fabric or equivalent material; or jute netting should be used.
Maintenance
If no sediment protection is provided on temporary diversions, periodic cleanout will probably be required.
References
Massachusetts Department of Environmental Protection, Office of Watershed Management, Nonpoint Source Management Manual, Boston, Massachusetts, June, 1993.
North Carolina Department of Environment, Health, and Natural Resources, Erosion and Sediment Control Field Manual, Raleigh, NC, February 1991.
North Carolina Sediment Control Commission, Erosion and Sediment Control Planning and Design Manual, Raleigh, NC, September, 1988.