This image shows rain garden used to infiltrate street runoff
Pervious pavers in Minneapolis, MN. Image Courtesy of Emmons & Olivier Resources, Inc.

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 may be realized with infiltration (volume control) as part of a stormwater management plan include the following.

  • 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

Program development and implementation

The right infiltration standard for your MS4

The infiltration standard of your Municipal Separate Storm Sewer System (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 impervious surfaces, either net additional impervious or total impervious. Net additional impervious is calculated by subtracting the total area of existing impervious area from the impervious area under post construction conditions. Total impervious is the total impervious area under post construction conditions. Some standards may also only apply to areas draining to special water such as trout streams, or may include rainfall over the pervious areas of the site as well as the impervious areas. Flat standards do not mimic existing condition hydrology as precisely as a pre-to-post standard, but are considered by most practitioners to be more easily implemented.

Pre-to-post standards Pre-to-post standards require modeling of existing runoff volumes and hold post-development runoff volumes to existing conditions for a return frequency rainfall event, typically the 1- or 2-year storm event. In other words, pre-to-post standards restrict the volume that leaves the site after development, keeping it equal to predevelopment conditions. This requires more in-depth development review, but also provides a better analysis of the land than a flat standard. This method more reliably mimics existing conditions, better protects downstream resources and avoids over- or under-applying a flat infiltration standard.

Example infiltration standards

The following table is a sampling of infiltration requirements for development/redevelopment within the Twin Cities Metropolitan Area (2009). Flat standards are the most prevalent due to the ease of application. Pre-to-post standards are also utilized, particularly in locales that have unique geologic setting and resources of high value.

Example Infiltration Requirements in the Twin Cities Metro Area (2009)
Link to this table

Municipalities
Entity Infiltration Requirement Requirement Applies To
Afton 0.6” Impervious area
Inver Grove Heights NWA 3.6” Pre-Post Entire site
Lakeville 1.5” Impervious area (within areas draining to trout waters)
Woodbury 0.5” Entire site
Maplewood 1” Entire site
Watershed Districts
Entity Infiltration Requirement Requirement Applies To
Brown's Creek 2.8” Rainfall Event Pre-Post, 3.6” Rainfall Event if Landlocked Entire site
Capitol Region 1” Impervious area
Comfort Lake-Forest Lake 2.8” Rainfall Event Pre-Post, 3.6” Rainfall Event if Landlocked Entire site
Minnehaha Creek 1” Impervious area
Nine Mile Creek 1” Impervious area
Prior Lake-Spring Lake 0.5” New Impervious
Ramsey-Washington Metro 1” Impervious area
Watershed Management Organizations
Entity Infiltration Requirement Requirement Applies To
Scott County 0.5” New impervious area
Vermillion River 2.8” Rainfall Event Pre-Post Entire site
Shingle Creek 0.5” Impervious area
West Mississippi 0.5” Impervious area
Upper Rum River 0.5” New impervious area

Existing ordinance compatibility with infiltration standards

It is possible that existing ordinances and regulations currently being used by a community or by another regulatory entity within a community, such as a watershed organization, could work contrary to infiltration concepts or to each other. For example, a community might not recognize the importance of infiltration while a watershed organization’s rules require it. Or a community might have subdivision requirements that mandate an extra-wide street that generates more runoff and consumes land that could be used for infiltration. Or the street design standards of a community may preclude the use of ribbon curb, flat curb or curb cuts to convey runoff surficial to adjacent infiltration features such as rain gardens.

The above examples indicate the need for MS4s to review all of their existing regulations when considering the adoption of infiltration requirements. The community should also review the rules of other entities operating within its borders. Additional resources can be found in the Center for Watershed Protection’s A Handbook for Changing Development Rules in Your Community and in the Minnesota Stormwater Manual.

Contamination caution

Whenever runoff is directed to an infiltration BMP, there is a danger of ground water contamination by the pollutants being carried in the runoff. The Minnesota Stormwater Manual discusses the ways in which potential pollution hotspots can be identified and runoff from them diverted from infiltration areas. The manual also includes a section on potential groundwater impacts from stormwater infiltration. In short, any surface runoff source that exposes or generates toxic or highly contaminating material should not be routed to an infiltration device unless some form of pretreatment is provided to remove the contaminant. Example hotspots include chemical storage (including salt), vehicle maintenance facilities, gasoline service stations, airports (de-icing agents), waste disposal facilities and scrap yards. The manual also provides guidance on the suitability of locating infiltration features within vulnerable wellhead protection areas.

Maintenance considerations

The maintenance of infiltration features can be the responsibility of the private sector or of the MS4. Generally speaking, larger stormwater infiltration features are considered part of the regional stormwater system and maintenance is conducted by the MS4s. Small rain gardens may be maintained privately with agreements in place that assign responsibility when BMPs are being poorly maintained.

Infiltration BMPs initially obtained a bad reputation because of the lack of proper design, installation and/or maintenance. Any one of these three factors could easily lead to system failure. Substantial progress has been made on guidance for each. The element of maintenance is absolutely essential to long-term infiltration BMP success. Some key factors to keep in mind include: adequate pretreatment to remove particles that could clog the infiltration surface; inspection of system overflow to assure free flow around the facility when needed; monitoring of sub-surface water levels to assure proper and timely drainage; and rapid removal of any sediment accumulation on the infiltration surface.

For more information on maintenance, link here.

Typical cost

The costs to set an infiltration requirement will vary based on the complexity of the situation. Costs will include time for MS4 staff to create, propose and implement any requirement. Additional expenditures may include engineering costs to analyze the local geology and determine reasonable standards, legal fees and inspection costs.

This page was last modified on 21 April 2017, at 14:43.

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