The intent of the Ordinance is to encourage infiltration, or “stay-on”. Even on sites with clay soils, designing practices to increase stay-on is possible. Traditional infiltration practices that rely on ponding water, like bioretention, may need an underdrain and a shallower depth to meet drawdown requirements. Permeable pavement works best by infiltrating a distributed depth of water, which can work well on soils with lower infiltration rates. Other practices, such as green roofs or water re-use, do not depend on in-situ soils at all to keep runoff on the site.

It is more difficult to meet the 90% infiltration standard on sites with clay soils, so an alternative approach may be used, detailed below.

Case 1 - Sites with 5+ feet of clay-textured soils (or with measured infiltration rate <0.6 in/hr) over entire site must meet the following requirements:

1. Dedicate 2% of the site to infiltration practices that conform to applicable standards, e.g. maximum drawdown times;
2. Provide infiltration calculations and associated summary table.

Case 2 – Sites with areas of 5+ feet of clay-textured soils (or with measured infiltration rate <0.6 in/hr) must meet the following requirements:

1. Provide a map delineating exempt and non-exempt areas;
2. Infiltrate 90% of the predevelopment volume from non-exempt areas;
3. Dedicate 2% of the site to infiltration practices that conform to applicable standards, e.g. maximum drawdown times;
4. Provide infiltration calculations and summary table for both non-exempt areas and full site.

It's recommended that designers wanting to request an exemption first discuss with staff for an official determination. Exemption requests must provide a preliminary site plan and soils testing results in accordance with the DNR's Site Evaluation for Stormwater Infiltration. The evaluation will need to cover all of the site to be considered for exemption.

Please note there are no exemptions for areas with shallow groundwater, so these sites will typically need to meet the 90% infiltration standard. Using shallow practices to infiltrate roof water is recommended, as there is no separation requirement. Bioretention can also be used, as the soil filtering layer qualifies as a “filtering layer” and satisfies the required separation for any runoff source. For small sites and practices infiltrating relatively small volumes of water, groundwater can be one foot beneath bottom of practice and still function. Underdrains are highly recommended in these situations, as groundwater can fluctuate with the season and over time.