§ 20.06 POST-CONSTRUCTION STORMWATER MANAGEMENT.
   Subd. 1. Purpose. This chapter is intended to prevent or reduce water pollution after construction activity is completed, related to new development or redevelopment projects with land disturbance of greater than or equal to one acre, including projects less than one acre that are part of a larger common plan of development or sale with the city and that discharge to the city's MS4.
   Subd. 2. Definitions. See § 20.03.
   Subd. 3. Conditions for Post-Construction Stormwater Management. Plans for drainage design and stormwater management must meet the regulations of the Cedar River Watershed District, the Department of Natural Resources, and the U.S. Army Corps of Engineers. Site plans for new development or redevelopment shall also include any combination of Best Management Practices, with the highest preference given to green infrastructure techniques and practices, necessary to meet the following conditions on the site of a construction activity to the maximum extent practicable:
      A. Owners of construction activity to submit site plans with post construction stormwater management BMPs designed with accepted engineering practices to the permittee for review and confirmation that regulatory mechanism(s) requirements have been met, prior to start of construction activity.
      B. Owners of construction activity must treat the water quality volume on any project where the sum of the new impervious surface and the fully reconstructed impervious surface equals one or more acres.
      C. For construction activity (excluding linear projects), the water quality volume must be calculated as one inch times the sum of the new and the fully reconstructed impervious surface.
      D. For linear projects, the water quality volume must be calculated as the larger of one inch times the new impervious surface or one-half inch times the sum of the new and the fully reconstructed impervious surface.
         1. Where the entire water quality volume cannot be treated within the existing right-of-way, a reasonable attempt to obtain additional right-of-way, easement, or other permission to treat the stormwater during the project planning process must be made.
         2. Volume reduction practices must be considered first, as described in Subd 3. Volume reduction practices are not required if the practices cannot be provided cost effectively.
         3. If additional right-of-way, easements, or other permission cannot be obtained, owners of construction activity must maximize the treatment of the water quality volume prior to discharge from the MS4.
      E. Volume reduction practices (e.g., infiltration or other) to retain the water quality volume on-site must be considered first when designing the permanent stormwater treatment system. This permit does not consider wet sedimentation basins and filtration systems to be volume reduction practices. If this permit prohibits infiltration as described in item 20.9, other volume reduction practices, a wet sedimentation basin, or filtration basin may be considered.
   Subd. 4. Stormwater management limitations and exceptions.
      A. Limitations. The use of infiltration techniques to achieve the conditions for post-construction stormwater management in Subd. 3 is prohibited when the infiltration structural stormwater BMP will receive discharges from, or be constructed in areas:
         1. That receive discharges from vehicle fueling and maintenance areas, regardless of the amount of new and fully reconstructed impervious surface.
         2. Where high levels of contaminants in soil or groundwater may be mobilized by the infiltrating stormwater. To make this determination, the owners and/or operators of construction activity must complete the MPCA's site screening assessment checklist, which is available in the Minnesota Stormwater Manual, or conduct their own assessment. The assessment must be retained with the site plans.
         3. Where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour.
         4. With less than three feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock.
         5. Of predominately Hydrologic Soil Group D (clay) soils.
         6. In an Emergency Response Area (ERA) within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, Subp. 13, classified as high or very high vulnerability as defined by the Minnesota Department of Health.
         7. In an ERA within a DWSMA classified as moderate vulnerability unless you perform or approve a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.
         8. Outside of an ERA within a DWSMA classified as high or very high vulnerability unless you perform or approve a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.
         9. Within 1,000 feet up-gradient or 100 feet down gradient of active karst features.
         10. That receive stormwater runoff from these types of entities regulated under NPDES for industrial stormwater: automobile salvage yards; scrap recycling and waste recycling facilities; hazardous waste treatment, storage, or disposal facilities; or air transportation facilities that conduct deicing activities.       B. Exceptions for stormwater discharge volume. Lesser volume control on the site of the original construction activity than that in Subd. 3. is allowed only under the following circumstances:
         1. The owner and/or operator of a construction activity is precluded from infiltrating stormwater through a designed system due to any of the infiltration related limitations described above; and
         2. The owner and/or operator of the construction activity implements, to the MEP, volume reduction techniques, other than infiltration, (e.g., evapotranspiration, reuse/harvesting, conservation design, green roofs, and the like) on the site of the original construction activity that reduces stormwater discharge volume, but may not meet the conditions for post-construction stormwater management in Subd. 3.
   Subd. 5.   Mitigation provisions. In circumstances where the City or other owners and operators of a construction activity cannot cost effectively meet the conditions for post-construction stormwater management for TSS and/or TP in Subd. 3. on the site of the original construction activity, the city shall identify, or may require owners or operators of a construction activity to identify, locations where mitigation projects can be completed. Any stormwater discharges of TSS and/or TP not addressed on the site of the original construction activity shall be addressed through mitigation and, at a minimum, shall ensure the following requirements are met:
      A. Mitigation project areas are selected in the following order of preference:
         1. Locations that yield benefits to the same receiving water that receives runoff from the original construction activity;
         2. Locations within the same Department of Natural Resource (DNR) catchment area as the original construction activity;
         3. Locations in the next adjacent DNR catchment area up-stream;
         4. Locations anywhere within the permittee's jurisdiction.
      B. Mitigation projects must involve the creation of new structural stormwater BMPs or the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP.
      C. Routine maintenance of structural stormwater BMPs already required by this permit cannot be used to meet mitigation requirements of this part.
      D. Mitigation projects shall be completed within 24 months after the start of the original construction activity.
      E. The city shall determine, and document, who is responsible for long-term maintenance on all mitigation projects of this part.
      F. If the city receives payment from the owner and/or operator of a construction activity for mitigation purposes in lieu of the owner or operator of that construction activity meeting the conditions for post-construction stormwater management in Subd. 3., the city shall apply any such payment received to a public stormwater project, and all projects must be in compliance with requirements in Subd. 4.A. through E. above.
   Subd. 6. Long-term maintenance of structural stormwater BMPs. Legal mechanism(s) shall be established between the city and owners or operators responsible for the long-term maintenance of structural stormwater BMPs not owned or operated by the city, that have been implemented to meet the conditions for post-construction stormwater management in Subd. 3. This includes only structural stormwater BMPs constructed after this chapter is passed, that are directly connected to the city's MS4, and that are in the city's jurisdiction. The legal mechanism shall include provisions that, at a minimum:
      A. Allow the city to conduct inspections of structural stormwater BMPs not owned or operated by the city, perform necessary maintenance, and assess costs for those structural stormwater BMPs when the city determines that the owner and/or operator of that structural stormwater BMP has not conducted maintenance.
      B. Include conditions that are designed to preserve the city's right to ensure maintenance responsibility, for structural stormwater BMPs not owned or operated by the city, when those responsibilities are legally transferred to another party.
      C. Include conditions that are designed to protect/preserve structural stormwater BMPs and site features that are implemented to comply with Subd. 3. If site configurations or structural stormwater BMPs change, causing decreased structural stormwater BMP effectiveness, new or improved structural stormwater BMPs must be implemented to ensure the conditions for post-construction stormwater management in Subd. 3. continue to be met.
(Ord. 622, passed 12-1-14; Am. Ord. 707, passed 8-15-22)