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

• 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's Technical Standard Site Evaluation for Stormwater Infiltration 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

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

• 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.

• Underground storage may not be incorporated in the HydroCAD model, due to the relatively slow rate at which water flows through engineered soil.

• Underdrains modeled in HydroCAD must be set up using one of the options shown in the figure below. Void storage below the basin bottom may not be modeled.

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

• Infiltration Performance Standard Guidance

• Modeling Post-Construction Storm Water Management Treatment

• Owner's Guide to Infiltration