Infiltration is a highly effective stormwater practice that reduces runoff volume, increases ground water recharge, improves surface water quality, provides thermal benefits and helps to mimic predevelopment hydrology. While other practices may address stormwater quality and rate control, limiting increased volumes of runoff from development and redevelopment is the most effective way to reduce the cumulative impacts on downstream water resources.
This fact sheet addresses effective tools and strategies to assess existing ordinances and develop consensus for the adoption of an infiltration standard. In addition, this fact sheet includes discussion on the importance of mimicking natural or predevelopment hydrology, and provides a range of example infiltration standards adopted across the state with links to example municipal ordinances and watershed district rules. Benefits / Pollution Reduction
The main goal of an infiltration standard is to mimic the natural hydrology of the landscape by allowing water to soak into the ground close to where it falls. This generally means defining an infiltration standard that limits post-development runoff volume to pre-development runoff volume. The multiple objectives that can be realized with infiltration (volume control) as part of a stormwater management plan include:
The infiltration standard of your MS4 should be based primarily on the local geology/soils, existing and planned land use, stormwater goals and stakeholder interests. Attaining a balance among these sometimes competing interests will determine what infiltration standard is feasible.
Infiltration standards vary across the state based on the factors previously discussed. Watershed Districts, Municipalities and Water Management Organizations all enact standards that are designed to protect their waters. The standards generally fall into two categories: flat standards and pre-to-post standards.
Flat standards Flat standards are typically expressed as the volume of runoff generated by a certain rainfall depth, typically 0.5- or 1.0-inch. These standards are usually applied only to