New development and 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 must include stormwater management BMPs in the stormwater management plan.
Permanent stormwater control facilities included as part of the final design for land development and redevelopment shall be addressed in the stormwater management plan and shall meet the following criteria to the maximum extent practicable (MEP):
A. Postconstruction Stormwater Management: Must incorporate any combination of BMPs designed with accepted engineering practices and in accordance with the applicable Minnesota Pollution Control Agency permits.
1. Treat the water quality volume on any project, except as provided in this article, where the sum of the new impervious surface and fully reconstructed impervious surface equals one or more acres.
2. For non-linear projects, the water quality volume must be calculated as one (1) inch times the sum of the new and fully reconstructed impervious surface.
3. Linear Projects: The water quality volume must be calculated as the larger of one (1) inch times the new impervious surface or one-half (0.5) inch times the sum of the new and the fully reconstructed impervious surface. Volume reduction practices must be considered in accordance with this article. 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 permissions to treat stormwater must be made and documented during the project planning process. Volume reduction practices are not required if the practices are cost prohibitive. If additional right-of-way, easements, or other permission cannot reasonably be obtained, owners of construction activity must maximize the treatment of the water quality volume, where practicable, prior to discharge from the MS4.
B. Volume reduction practices (e.g., infiltration) to retain the water quality volume on-site must be considered first when designing the permanent stormwater treatment system. Wet sediment basins and filtration systems are not considered volume reduction practices. If infiltration is prohibited, other volume reduction practices, a wet sediment basin, or a filtration basin may be considered.
C. Limitations: Infiltration is prohibited or restricted in accordance with the applicable Minnesota Pollution Control Agency permits.
D. Pre- Versus Post-Hydrological Response Of Site: An applicant shall install or construct, on or for the proposed land disturbing activity or development activity, all stormwater management facilities necessary to manage runoff such that increases in flow under the design conditions will not occur that could exceed the capacity of the outlet, or the stormwater management system, into which the site discharges or that would cause the stormwater management system to be overloaded or accelerate channel erosion as a result of the proposed land disturbing activity or development activity. Under no circumstances shall the 2-, 10-, or 100-year developed peak flow exceed the 2-, 10-, or 100-year existing peak flow without prior written approval by the city engineer. For regional detention or stormwater management system, the city engineer shall recommend a proposed system charge or assessment to be approved by the city council based upon an approved watershed master plan and an analysis of required drainage systems, projected costs and flood protection benefits provided to those properties directly or indirectly impacted by the regional detention or stormwater management system.
E. Infiltration: Infiltration options include, but are not limited to, infiltration basins, infiltration trenches, rainwater gardens, bioretention areas without underdrains, swales with impermeable check dams, and natural depressions designed in accordance with the applicable Minnesota Pollution Control Agency permits and/or Minnesota Stormwater Manual.
F. Filtration System: Filtration options include, but are not limited to: sand filters with underdrains, biofiltration areas, swales using underdrains with impermeable check dams and underground sand filters designed in accordance with the applicable Minnesota Pollution Control Agency permits and/or Minnesota Stormwater Manual.
G. Wet Sedimentation Basin:
1. Wet sedimentation basins must be designed in accordance with the applicable Minnesota Pollution Control Agency permits with the following exception.
a. Additional depth below the minimum depth of (3) feet must be provided for long-term sediment storage as established in the applicable maintenance.
H. Regional Wet Sedimentation Basin:
1. When the entire water quality volume cannot be retained onsite for volume reduction purposes, regional wet sedimentation basins can be used or created, provided they are constructed basins, not a natural wetland or water body.
2. The regional basin must conform to all requirements for a wet sedimentation basin and in the applicable Minnesota Pollution Control Agency permits.
I. Off-site Treatment: Where the owner and/or operator of a construction activity cannot cost effectively meet the conditions for postconstruction stormwater management on the site of the original construction activity, the owner shall provide or obtain off-site treatment projects at other site locations within the city.
1. Off-site projects must be selected in the following order of preference with approval by the city:
a. Locations that yield benefits to same receiving water that receives runoff from the original construction site.
b. Locations within the same department of natural resources (DNR) catchment area as the original construction site.
c. Locations in the next adjacent DNR catchment area upstream.
d. Locations anywhere within the city's jurisdiction.
2. Off-site projects must involve the creation of new structural stormwater BMPs, the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP.
3. Routine maintenance of structural stormwater BMPs already required cannot be used to meet mitigation requirements.
4. Off-site projects shall be completed within twenty four (24) months after the start of the original construction activity.
5. If the off-site project is a private structural BMP and the city is not responsible for long term maintenance of the project, the city will require a written and recorded document identifying the responsibility for maintenance.
6. If the city receives payment from the owner/operator of a construction activity for off-site treatment purposes in lieu of meeting postconstruction stormwater management requirements, the city shall apply the payment received to a public stormwater project in compliance with the MS4 permit.
J. Natural Features Of Site: The applicant shall give consideration to reducing the need for stormwater management system facilities by incorporating the use of natural topography and land cover such as wetlands, ponds, natural swales and depressions as they exist before development to the degree that they can accommodate the additional water flow without compromising the integrity or quality of these natural features.
K. Postconstruction Stormwater Management Strategies: The following stormwater management practices shall be investigated when developing a stormwater management plan:
1. Natural infiltration of precipitation and runoff on site, if suitable soil profiles can be created during site grading. The purpose of this strategy is to encourage the development of a stormwater management plan that encourages natural infiltration. This includes providing as much natural or vegetated area on the site as possible, minimizing impervious surfaces, and directing runoff to vegetated areas rather than onto adjoining streets, storm sewers and ditches;
2. Flow attenuation by use of open vegetated swales and natural depressions;
3. Stormwater detention facilities; and
4. Stormwater retention facilities (on a case by case basis).
5. Other facilities requested by the city engineer.
A combination of successive practices may be used to achieve the applicable minimum control requirements specified. Justification shall be provided by the applicant for the method selected.
L. Adequacy Of Outlets: The adequacy of any outlet used as a discharge point for proposed stormwater management system must be assessed and documented to the satisfaction of the city engineer. To the extent practicable, hydraulic capacities of downstream natural channels, storm sewer systems, or streets shall be evaluated to determine if they have sufficient conveyance capacity to receive and accommodate post development runoff discharges and volumes without causing increased property damages or any increase in the established base flood elevation. If a floodplain or floodway has not been established by the federal emergency management agency (FEMA), the applicant shall provide a documented analysis and estimate of the base flood elevation as certified by a professional engineer registered in the state of Minnesota. In addition, projected velocities in downstream natural or manmade channels shall not exceed that which is reasonably anticipated to cause erosion unless protective measures acceptable to the city engineer are approved and installed as part of the stormwater management plan. The assessment of outlet adequacy shall be included in the stormwater management plan.
M. Stormwater Detention/Retention Facilities: Stormwater detention or retention facilities proposed to be constructed in the stormwater management plan shall be designed according to the most current technology as reflected in the manual. (Ord. 2015-01, 2-9-2015; amd. Ord. 2022-14, 9-12-2022)