Table of Contents
Infiltration Guidance
This page is intended to provide guidance on infiltration design, modeling and construction, based on approved policy and frequently asked questions.
The current version of WinSLAMM (10.5.0) overestimates evapotranspiration (ET) volume. Until a fix is implemented, ET may not be turned on in the model.
Site Consideration
- 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 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.
Practice Design
- Underground infiltration practices 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 practices modeled in WinSLAMM should have the “side seepage rate” value set to 0.01.
- Infiltration practices and permanent pools should be modeled as impervious for infiltration calculations. Effective infiltration areas cannot be “double counted” as both an infiltration facility and pervious area.
- Void storage below the basin bottom may not be used to model peak rates in HydroCAD. With the engineered soil typically being more restrictive than the underdrain, it acts as the effective outlet control with the underground storage layer “downstream” and not part of the same level pool as the pond storage. It's not realistic to assume water entering the practice would instantly start filling from the bottom of the storage layer, which is what occurs if modeled as one storage area. Therefore, infiltration practices modeled in HydroCAD must be set up using one of the options shown in the figure below, unless the flow out of the underdrain is more restrictive than flow through the engineered soil. Explanation of this guidance can be found here.
Construction
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.