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Soil Loss and Sediment Discharge

Permittees can demonstrate compliance with the sediment discharge standard of 5.0 tons/ac/year in Chapter 14.50(3)(b) by using the Wisconsin DNR's Soil Loss & Sediment Discharge Calculation Tool. This Excel spreadsheet projects soil loss and sediment discharge from a site using the Universal Soil Loss Equation (USLE), based on the timing of land disturbance, restoration, and the implementation of erosion control BMPs.

Note that the spreadsheet estimates soil loss from sheet and rill erosion only. It does not predict soil loss resulting from concentrated flow, high channel velocities, gully erosion or streambank erosion; if those areas exist on a site, they should be addressed through the use of prescriptive erosion control measures.

The WDNR has developed a guidance document that describes how to use the USLE spreadsheet and the erosion control page provides some additional information specific to County erosion control permitting.

Additional Guidance

General

  • If the slope or schedule that is required for the calculation is not representative of the whole site, more than one calculation may be submitted. The area with the higher expected soil loss would then need to receive different erosion control treatment than other areas on the site.
  • If land disturbing activities will occur for longer than 12 months, multiple USLE calculations should be done, so that all 12 month periods starting at the start date and ending at the end date, are modeled. For example, if a project starts April 1, 2020, and will be completed on October 31, 2021, the first USLE calculation should run from April 1, 2020 to March 31, 2021, and the second should model from November 1, 2020 to October 31, 2021.
  • Dates used in the USLE calculation must be consistent with the construction schedule on the plan. The construction schedule should be determined by the plan developer through consultation with the contractor. The USLE calculation should be updated if the construction schedule changes at any time before or during construction.
  • Dane County erosion control permits expire on the stabilization (e.g., seed, mulch, sod, etc.) date provided in the spreadsheet.

Soil Disturbing Activities

  • To meet the standard, a representative “worst case” slope/slope length should be chosen.
  • When a slope and/or slope length is changed by grading during the bare soil period, select another bare soil row under "Activity" and enter its corresponding date. Then enter a new slope and/or slope length in the appropriate column(s). This can be used to represent topsoil stripping, followed by mass grading.
  • While the WDNR guidance states that the USLE spreadsheet should not be used on slopes over 20%, for submittals to the County, include slopes up to 33% in your USLE calculations.
  • In addition to the USLE calculation, erosion control plans should include a map, clearly showing the location of the flow paths modeled in the spreadsheet(s).

Restoration

  • When sites are to be temporarily seeded and mulched, enter activities of bare ground and seed with mulch or erosion mat. Then in subsequent rows, select bare ground for when grading is expected to resume, and seed with mulch or erosion mat for the final stabilization date. When modeling temporary stabilization in this way, there should be no soil disturbance between the temporary seed and mulch date and the date of the next bare ground activity.
  • The growing season is from March 15 through September 15. If seeding will occur during the non-growing season, a May 15th establishment date may be assumed. See the Seeding page for more guidance on seeding outside the growing season.

Sediment Control Practices

  • Any sediment control practices selected in the spreadsheet should be in place prior to the bare ground activity and should be designed to the appropriate WDNR technical standards and include supporting documentation (e.g., show calculations for sediment basin/traps for appropriate size and depth).

References