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, such as bioretention, may need an underdrain and a shallower depth to meet drawdown requirements. Permeable pavement works best by infiltrating a small depth of water, so can work well on soils with lower infiltration rates. Other practices do not depend on the site's soils at all, such as green roofs or water re-use.

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 less than 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 less than 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.