(A) Stormwater volume reduction performance goals. Any applicant for a permit resulting in site disturbance that creates one or more acres of new impervious surface or fully reconstructs one or more acres of impervious surface must meet all of the following stormwater performance goals:
(1) New development volume control. For new, nonlinear developments that create more than one acre of new impervious surface on sites without restrictions, stormwater runoff volumes will be controlled and the post-construction runoff volume shall be retained on site for one inch of runoff from all impervious surfaces on the site.
(2) Redevelopment volume control. Nonlinear redevelopment projects on sites without restrictions that create one or more acres of new and/or fully reconstructed impervious surfaces shall capture and retain on site one inch of runoff from the new and/or fully reconstructed impervious surfaces.
(3) Linear development volume control. Linear projects on sites without restrictions that create one acre or greater of new and/or fully reconstructed impervious surfaces, shall capture and retain the larger of the following:
(a) 0.55 inch of runoff from the new and fully reconstructed impervious surfaces on the site; or
(b) One inch of runoff from the net increase in impervious area on the site.
(c) Mill and overlay and other resurfacing activities are not considered fully reconstructed.
(4) Stormwater management rate control. For all development sites (new development, redevelopment and linear developments), site design shall provide on-site treatment during construction and post-construction to ensure no increase in offsite peak discharge for the one year, 24-hour storm event (2.42 inches); two-year, 24-hour storm event (2.81 inches); the ten-year, 24-hour storm event (4.12 inches); and the 100-year, 24-hour storm event (6.72 inches) or as defined by the NOAA Atlas 14 Precipitation Frequency Estimates for Minnesota.
(5) All detention facilities shall be designed with a minimum of 24 inches of freeboard measured to the lowest opening of adjacent structures. A spillway and overflow route must be able to safely pass overflows through the structure without creating damaging conditions downstream of the facility nor inhibit stormwater runoff from upstream development.
(B) Flexible treatment options for sites with restrictions (as found in the MIDS design sequence flowchart). The MIDS design sequence flowchart can be found in the Minnesota Stormwater Manual: http://stormwater.pca.state.mn.us/index.php/Flexible_treatment_options. Applicant shall fully attempt to comply with the appropriate performance goals described above. Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site. If full compliance is not possible due to any of the factors listed below, the applicant must document the reason. If site constraints or restrictions limit the full treatment goal, the following flexible treatment options shall be used:
(1) Alternative #1. Applicant attempts to comply with the following conditions:
(a) Achieve at least 0.55-inch volume reduction from all impervious surfaces if the site is new development or from the new and/or fully reconstructed impervious surfaces for a redevelopment site;
(b) Remove 65% of the annual total phosphorous (TP) load from all impervious surfaces if the site is new development or from the new and/or fully reconstructed impervious surfaces for a redevelopment site; and
(c) Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site.
(2) Alternative #2. Applicant attempts to comply with the following conditions:
(a) Achieve volume reduction to the maximum extent practicable;
(b) Remove 60% of the annual total phosphorous (TP) load from all impervious surfaces if the site is new development or from the new and/or fully reconstructed impervious surfaces for a redevelopment site; and
(c) Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site.
(3) Alternative #3: off-site treatment. Mitigation equivalent to the performance of 1.1 inches of volume reduction for new development or redevelopment as described above in this section (including banking or cash) can be performed off-site to protect the receiving water body. Off-site treatment shall be achieved in areas selected in the following order of preference:
(a) Locations that yield benefits to the same receiving water that receives runoff from the original construction activity;
(b) Locations within the same Department of Natural Resource (DNR) catchment area (Hydrologic Unit 08) as the original construction activity;
(c) Locations within the next adjacent DNR catchment area upstream; and
(d) Locations anywhere within the city's jurisdiction.
(4) Applicant shall document the flexible treatment options sequence starting with Alternative #1. If Alternative #1 cannot be met, then Alternative #2 shall be analyzed. Applicants must document the specific reasons why Alternative #1 cannot be met based on the factors listed below. If Alternative #2 cannot be met then Alternative #3 shall be met. Applicants must document the specific reasons why Alternative #2 cannot be met based on the factors listed below. When all of the conditions are fulfilled within an alternative, this sequence is completed.
(5) Volume reduction techniques considered shall include infiltration, reuse and rainwater harvesting, and canopy interception and evapotranspiration and/or additional techniques included in the MIDS Calculator and the Minnesota Stormwater Manual.
(6) Higher priority shall be given to BMPs that include volume reduction. Secondary preference is to employ filtration techniques, followed by rate control BMPs.
(7) Factors to be considered for each alternative will include:
(a) Karst geology;
(b) Shallow bedrock;
(c) High groundwater;
(d) Hotspots or contaminated soils;
(e) Drinking water source management areas or within 200 feet of drinking water well;
(f) Zoning, setbacks or other land use requirements;
(g) Excessive cost; and
(h) Poor soils (infiltration rates that are too low or too high, problematic urban soils).
(Ord. 20220120-01, passed 1-20-22)