Dane County Stormwater Manual

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====== Temporary Diversion ======

A temporary diversion is a channel constructed across a slope that is designed to intercept and collect stormwater, diverting it down slope to an area that is less susceptible to erosion.  Temporary diversions reduce runoff velocities, allowing sediment and soluble pollutants to settle out before leaving the site.

Temporary diversions are primarily used for construction sites and include graded surfaces to redirect sheet flow, dikes, and berms.  Diversions may be constructed upslope to protect exposed sites from upland runoff or down slope of sites to collect any sediment that occurs from erosion.

**Advantages**
  *Can significantly reduce from a site
  *Removes sediment and soluble pollutants
  *Low cost
  *Existing fill may be used for construction

**Disadvantages**
  *Requires a relatively large land area
  *Ineffective on sites with slopes greater than 15%
  *Has a maximum life span of 12 months

=====Design=====
====Capacity====
Temporary diversions should be designed, at a minimum, to convey the runoff from a 2 year, 24-hour storm event with at least 0.3 foot of additional capacity (freeboard).  Temporary diversions may drain an area of no greater than 5 acres.
===Shape and Slope===
Temporary diversions may be parabolic, trapezoidal, or V-shaped with a minimum ridge width of 2 feet.

Side slopes should be flatter than 3:1, as steeper slopes may be unstable and make maintenance and repair activities more frequent.  Channel slopes 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.  Channel slopes that exceed 2% require stabilization from vegetation or erosion matting.  In addition, the slope behind the dike must be great enough to assure proper drainage, but must be flat enough so that erosion of the structure does not occur during high volume periods of runoff.

====Outlet====
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====
If temporary diversions are used longer than 30 days, vegetation must be established. Diversions used less than 30 days may only be mulched.  Plant species selected should meet the following criteria:
  *[[Native plants|Native species]] may be used with careful selection
  *Species should be tolerant to frequent inundation as well as extended dry periods
  *Species should form a dense cover
  *Avoid exotic, noxious, and invasive species

=====Construction and Maintenance=====
  *Diversions should be mulched immediately after grading to prevent erosion of the structure
  *Temporary diversions should be inspected for damage after each rainfall
  *Routine inspections should be performed weekly - any damage should be repaired immediately

=====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==
  *[[https://dnr.wisconsin.gov/sites/default/files/topic/Stormwater/1066ConstructionSiteDiversion.pdf|DNR Construction Site Diversion Technical Standard 1066]]