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Permanent Diversion

General

A permanent diversion is a vegetated channel that is designed to intercept and collect runoff, diverting it down slope to an area that is less susceptible to erosion. Diversions are constructed upslope of areas where erosion is likely to occur and, by reducing runoff velocities, allow sediments and soluble pollutants to settle out.

Permanent diversions can be used in residential, commercial, and industrial areas and include: graded surfaces to redirect sheet flow, dikes, berms, and conveyance structures such as swales, channels, gutters, and drains.

Advantages

  • Can significantly reduce erosion from a site
  • Removes sediment and soluble pollutants
  • Can be aesthetically pleasing if designed properly, which can increase adjacent property values

Disadvantages

  • Requires a relatively large land area
  • Are not recommended down slope of high-sediment producing areas
  • Ineffective on sites with slopes greater than 15%

Design

Capacity

Permanent diversions should be designed, at a minimum, to convey the runoff from a 10 year, 24-hour storm event with at least 0.3 foot of additional capacity (freeboard). However, it is recommended that diversions that protect roads and urban areas have a capacity sufficient to transport runoff from the 25 year, 24-hour storm event. In addition, the designed capacity must take into account any soil settling that may occur. While the amount of settling that occurs will depend upon the type of soils present on site, a minimum value of 10% should be used.

Shape and Slope

Permanent diversions may be parabolic, trapezoidal, or V-shaped with a minimum ridge width of 4 feet.

Side slopes should be flatter than 3:1, as steeper slopes may be unstable and make maintenance activities more difficult. Channel slopes will depend upon the topography of the site, but should be designed so that sheet flow is sustained and water velocities are maintained below 5.0 ft/s.

Permissible Velocities for Diversion (FPS)

Soil TextureBare ChannelPoor VegetationFair VegetationGood Vegetation
Sand, silt, sandy loam, and silty loam1.51.52.03.0
Silty clay loam and sandy clay loam2.02.03.04.0
Clay2.52.54.05.0

Outlets

The outlet selected for each diversion will vary upon the needs of each site. Outlets should be stable and non-erosive and may be vegetated, paved, rock-lined with geotextile fabric, or drain tiled. If a vegetated outlet is chosen, it must be constructed before the rest of the diversion to allow time for the vegetation to become established. Outlets may also incorporate riprap or gabions to further prevent erosion and reduce the velocity of outflows.

Vegetation

Plant species selected for permanent diversions should meet the following criteria:
  • Native species may be used with careful selection
  • Species should be tolerant to frequent inundation as well as extended dry periods
  • Species should be resistant to matting
  • Species should form a dense cover
  • Avoid exotic, noxious, and invasive species

Construction and Maintenance

  • Vegetation should be established immediately after grading is complete to prevent erosion of the structure
  • Until vegetation is established, diversions should be inspected after each rainfall for signs of erosion
  • After establishment, permanent diversions should be inspected annually to ensure that they are operating properly and to check for any potential problems
  • Mowing should be performed only during dry periods using light equipment to prevent soil compaction

Method to Determine Practice Efficiency

Diversions effectively reduce the slope length by diverting runoff away from slopes and other areas that are prone to erosion. The efficiency for this practice is thus derived from the reduction in slope length that it provides. To calculate the efficiency, simply use the new, reduced slope length in place of the pre-existing one in the USLE and recalculate. The difference is the efficiency for the practice.

References