Guideline S.2 2018-02-28T20:27:25+00:00

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Guideline S.2: Stormwater Management

Intent

To minimize the negative impacts of a project, both on- and offsite, by maintaining a more natural hydrologic cycle through infiltration, evapotranspiration, and reuse.

Required Performance Criteria

  1. Runoff Rate and Volume:
    1. Control the rate of runoff from the post-development site to match the runoff rates for the native soil and vegetation conditions for the 2-year and 10-year, 24-hour design storms.
    2. Do not allow discharge from the site for 1.1 inches of runoff from all new or redeveloped impervious (non-vegetated) areas.
  2. Runoff Quality:
    1. Provide treatment systems to remove 80% of the post-development Total Suspended Solids (TSS).
    2. Provide treatment systems to remove 60% of the post-development Total Phosphorus (TP).
  3. Operations and Maintenance Manual:
    1. Stormwater Best Management Practices (BMPs) must have an Operations and Maintenance manual created that outlines maintenance requirements and schedules for completion. Operations and maintenance manuals must be recorded with the county registrar.

Note that all governing rules and regulations for stormwater management apply to work covered by this section. The more stringent regulation takes precedence. Methods to achieve credits in this section must be consistent with those found in the most current edition of the Minnesota Pollution Control Agency’s “Minnesota Stormwater Manual.”

Implementation in the Design Process:

In Predesign and other early design phases, seek direction from the local government unit or authority with jurisdiction over the project’s stormwater management. Applicable rules, regulations, and permitting requirements must be understood and applied.

Perform topographic, utility, boundary, and wetland surveys, as applicable. Identify onsite areas where the site conditions and topography will facilitate stormwater management. Identify onsite areas where site conditions do not allow for stormwater treatment or infiltration, such as groundwater recharge areas or karst topography. Perform geotechnical analysis of the site to determine soil types, infiltration rates, and areas best suited for stormwater management.

Through the design process, complete general calculations to estimate the volume of stormwater that will need to be treated onsite per B3 Guideline requirements. Identify stormwater management techniques that are appropriate for the amount and type of stormwater generated by the developed site.

In the construction documents phase of the project, finalize stormwater calculations to determine the volume of stormwater necessary for onsite treatment. Details and specifications for necessary stormwater management techniques must also be developed based on the stormwater calculations.

As the project moves into and through the construction administration phase, compliance submittals – with plans and details – will be monitored. Bidders must be made aware of specific requirements for stormwater management. A pre-construction meeting should be held to identify requirements for the construction of stormwater management areas and for protection during and after the construction process. Stormwater management features should be constructed in a sustainable manner, according to drawings and specifications.

An operations and maintenance manual should be developed for the ongoing care of stormwater management areas. Stormwater management areas must be maintained per the operations and maintenance manual. As-built records of stormwater systems must be maintained, stormwater management techniques must be monitored, and performance data must be recorded.

The most recent version of the “Minnesota Stormwater Manual” put out by the Minnesota Pollution Control Agency is a valuable resource, available as a wiki at stormwater.pca.state.mn.us/index.php/Main_Page.

Documentation:

The documentation required for the stormwater guideline includes the following:

  • Pre-settlement, existing conditions and proposed runoff rate for the 10-year, 24-hour precipitation event and for 1.1 inches of runoff from impervious surfaces.
  • Existing and proposed conditions for Total Suspended Solids (TSS) and Total Phosphorus Loads (TP) for a 2-year, 24-hour event.
  • Operations and maintenance manual for the ongoing care of stormwater areas. There may also be requirements from local jurisdictions relating to O&M procedures and documentation.

Predesign:

  • S.2A: Pre-settlement and existing condition runoff rates for the 2-year, 24-hour precipitation event and the 10-year, 24-hour precipitation event. Pre-settlement runoff quantity for 1.1 inches of runoff from impervious areas.
  • S.2B: Contaminant load of TSS and TP for a 2-year, 24-hour event before treatment.

Design:

  • S.2A: Pre-settlement, existing condition, and proposed condition runoff rates for the 2-year, 24-hour precipitation event and the 10-year, 24-hour precipitation event. Pre-settlement and proposed condition runoff quantity for 1.1 inches of runoff from impervious areas.
  • S.2B: Contaminant load of TSS and TP for a 2-year, 24-hour event before treatment and proposed contaminant load change.

Final Design:

  • S.2A: Pre-settlement, existing condition, and proposed condition runoff rates for the 2-year, 24-hour precipitation event and the 10-year, 24-hour precipitation event. Pre-settlement and proposed condition runoff quantity for 1.1 inches of runoff from impervious areas.
  • S.2B: Contaminant load of TSS and TP for a 2-year, 24-hour event before treatment and proposed contaminant load change.
  • S.2C: Operational stormwater BMP manuals.

Closeout:

  • 2A: Pre-settlement, existing condition, and installed condition estimated runoff rates for the 2-year, 24-hour precipitation event and the 10-year, 24-hour precipitation event. Pre-settlement and installed condition estimated quantity for 1.1 inches of runoff from impervious areas, and verification that the site contractor understands the requirements and intent of this guideline.
  • 2B: Contaminant load of TSS and TP for a 2-year, 24-hour event before treatment and contaminant load change with installed measures, and verification that the site contractor understands the requirements and intent of this guideline.
  • 2C: Updated operational stormwater BMP manuals and verification that the site contractor understands the requirements and intent of this guideline.

Additional Resources

Minnesota Pollution Control Agency MIDS Calculator: https://stormwater.pca.state.mn.us/index.php/MIDS_calculator

National Resources Conservation Service, Standard Curve Numbers; https://www.nrcs.usda.gov/wps/portal/nrcs/site/national/home/

Environmental Protection Agency: www.epa.gov

Metropolitan Council Urban Small Sites BMP Manual Low-Impact Development Guidelines: https://metrocouncil.org/Wastewater-Water/Planning/Water-Resources-Management/Water-Quality-Management-Key-Roles.aspx

Minnesota Association of Watershed Districts: www.mnwatershed.org

Minnesota Pollution Control Agency: www.pca.state.mn.us

Stormwater Manual Best Management Practices Manual: https://www.pca.state.mn.us/water/stormwater-best-management-practices-manual

Center for Watershed Protection: www.cwp.org

Board of Water Quality and Soil Resources: www.bwsr.state.mn.us

International Stormwater Database: www.bmpdatabase.org

Minnesota Erosion Control Association: www.mnerosion.org

Stormwater Manager’s Resource Center: www.stormwatercenter.net