Establishing An Infiltration Standard

Creating an infiltration standard for development and redevelopment

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:

  • Reduced stormwater pollutants
  • Increased ground water recharge
  • Decreased runoff volume and peak flow rates
  • Preserved base flow in streams
  • Reduced thermal impacts of runoff
  • Lowered infrastructure costs
This image shows rain garden used to infiltrate street runoff
Pervious pavers in Minneapolis, MN. Image Courtesy of Emmons & Olivier Resources, Inc.

Program Development & Implementation

The Right Infiltration Standard for Your MS4=

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.

  • Geology/soils – Defines the infiltration rates and connectivity of the predevelopment landscape. MS4s that contain diverse geology should implement a pre-to-post development event standard. Homogeneous landscapes may be able to implement a flat standard for the entire MS4. Note that although some level of infiltration can occur in any soil, other volume reduction techniques might be better when extremely low infiltration rates prevent achieving a community’s volume reduction goal.
  • Existing and planned land use – A fully developed MS4 will want to focus on retrofit and redevelopment stormwater standards, whereas new construction infiltration standards should be the focus of a developing MS4.
  • Stormwater goals –Stormwater issues need to be defined and the infiltration requirement should be tailored to address these issues. Water quality and quantity can be addressed partially, if not fully, through infiltration practices.
  • Stakeholder Interests – Working with all stakeholders in the MS4 to garner support for any ordinance is very important to its success. The best approach is to get feedback early and often and work through any issues or misconceptions that may exist.

Infiltration Standard Categories

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

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