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Infiltration Guidance

This page is intended to provide guidance on infiltration design, modeling and construction, based on approved policy and frequently asked questions.


Site Design

  • Removal of up to 5 feet of undesirable material in order to reach appropriate infiltrating soils is expected before attempting to claim an exemption.

  • Design infiltration rates should be determined using test pits and soil textural classifications from Table 2 of the DNR Conservation Practice Standard 1002. In-field testing of infiltration rates may be desirable to verify assumed infiltration values.

  • Topographical depressions areas exceeding 1 foot in depth and 20,000 square feet in area should be modeled as ponds in the predevelopment model. Smaller depressions should be modeled using the “ponded area” routines in TR55 and similar models.

  • As-built certification must be completed by a Professional Engineer stating that the stormwater facility is functional and consistent with the approved design

Facility Design

  • Underground infiltration facilities are similar to infiltration basins in function and require both pretreatment and a 72 hour total device drawdown.

Modeling Assumptions

  • Engineered soil hydraulic conductivity is 3.6 in/hr, consistent with sand textured soil and the maximum allowable design infiltration rate.

  • Infiltration facilities modeled in WinSLAMM should have the "side seepage rate" value set to 0.01.

  • Effective infiltration areas and stormwater basin areas should be modeled as impervious for infiltration calculations. Effective infiltration areas cannot be "double counted" as both an infiltration facility and pervious area.


Practices that rely on infiltration to properly function require extra care while being constructed and continual maintenance to prevent failure. The Infiltration Practice Construction Guidance page was created to provide guidance on how to ensure these practices are installed properly, function well and require minimal maintenance.

Additional Information and Guidance