A. Complete Design Required: A complete and adequate drainage system design shall be required for the subdivision and may include a storm sewer system or system of open ditches, culverts, pipes, catch basins and ponding areas, or a combination thereof.
B. Design: Drainage system utilities shall be designed in accordance with standards established by the City in the "City's standard detail plates", the requirements of the Small Municipal Separate Storm Sewer Systems (MS4) General Permit (MNR040000), or as otherwise specified by the City Engineer.
1. A plan shall be developed that requires the use of any combination of Best Management Practices, with highest preference given to green infrastructure techniques and practices (e.g., infiltration, evapotranspiration, reuse/harvesting, conservation design, urban forestry, green roofs, etc.), necessary to meet the conditions on the site of a construction activity to the maximum extent practicable.
C. Rate Control: The applicant shall install or construct or pay the City fees for all stormwater management facilities necessary to manage increased runoff, so that the 2-year, 10-year, and 100-year storm peak discharge rates existing before the proposed development shall not be increased. Precipitation depths using NOAA Atlas 14 volume 8 precipitation frequency estimates, published June 2013, or its successor, shall be used for each project location. Also, accelerated channel erosion shall not occur as a result of the proposed land disturbing or development activity. An applicant may also make an in-kind or a monetary contribution to the development and maintenance of community stormwater management facilities designed to serve multiple land disturbing and development activities undertaken by one or more persons, including the applicant.
1. All property within the County ditch no. 9 watershed shall not drain or discharge more than one-half (1/2) of the predevelopment rate of runoff.
D. Volume Control: The owner of a construction activity must treat the water quality volume on any project where the sum of the new impervious surface and fully reconstructed impervious surface equals one or more acres. The design shall explicitly address the use of BMPs to limit the loss of pervious area, and limit runoff volume increases from impervious areas to the extent feasible considering site specific conditions. At a minimum, volume control practices shall comply with the following:
1. The infiltration/filtration practice shall be capable of draining down within forty eight (48) hours or less.
2. When using infiltration for volume control, infiltration volumes and facility sizes shall be calculated using the appropriate hydrological soil group. Site specific infiltration or hydraulic conductivity measurements completed by a licensed soil scientist or geotechnical engineer shall be used, or the design criteria in the Minnesota Stormwater Manual shall be followed.
3. The bottom area of treatment ponds shall not be accepted as part of an infiltration practice; however, properly designed slopes and safety shelves may be accepted.
4. Before infiltrating runoff, pretreatment shall be required for gravel and paved surface lot runoff that will enter the infiltration system. The pretreatment shall be designed to protect the infiltration system from clogging and to protect ground water quality. Pretreatment options may include, but are not limited to, oil/grease separation, sedimentation, biofiltration, filtration, swales or filter strips. To minimize potential groundwater impacts it is desirable to infiltrate the cleanest runoff. To achieve this, a design may propose greater infiltration of runoff from low pollutant sources such as roofs, and less from higher pollution source areas such as parking lots.
5. Infiltration systems shall be designed to bypass higher flows without incurring damage from erosion or loss of top soils.
6. Infiltration areas shall be fenced or otherwise protected from compaction and/or disturbance before and during all land disturbing activities.
7. Constructed infiltration practices, such as infiltration ponds and trenches are discouraged in favor of other appropriate volume control practices. Constructed infiltration practices are prohibited in the areas identified in the Post-Construction Stormwater Management section of the MS4 General Permit (MNR040000):
8. Infiltration facilities to be maintained by the City shall be located in outlots dedicated to the City.
9. For linear projects that lack right-of-way for the installation of volume control practices, a reasonable attempt must be made to obtain right-of-way during the planning process. Exceptions may be made for lesser volume on site if any limitations are encountered as allowed per the MS4 General Permit (MNR040000).
10. If the owner and/or operator of a construction activity believes that the water quality volume cannot be met on the site of the original construction activity, the owner and/or operator must provide appropriate documentation to the City as support. The stormwater requirements for the water quality volume not met on the site of the original construction activity may be mitigated off-site at the City's discretion. If off-site mitigation is allowed, the mitigation provisions of the MS4 General Permit (MNR040000) must be followed.
E. Information Submitted: All calculations and hydrologic models/information used in determining peak flows shall be submitted along with the stormwater pollution prevention plan. A delineation of all ponds, infiltration facilities, ditches, public waters and wetlands located on and adjacent to the site, including the normal water level (NWL), the 100-year high water level (HWL), and the ordinary high water level (OHW).
F. Incorporation Of Natural Topography: The applicant shall consider reducing the need for stormwater management facilities by incorporating the use of natural topography and land cover such as natural swales and depressions as they exist before development to the degree that they can accommodate the additional flow of water without compromising the integrity or quality of the wetland or pond.
G. Minimum Design Standards For Stormwater Wet Detention Facilities: The stormwater wet detention facilities shall conform to the most current technology as reflected in the current version of the Minnesota Stormwater Manual, and the current requirements found in the MPCA's Construction Stormwater Permit (MNR100001).
H. Protection For Natural Wetlands: Wetlands must not be drained or filled, wholly or partially, unless replaced by either restoring or creating wetland areas of at least equal public value pursuant to the Wetland Conservation Act.
I. Models/Methodologies/Computations: Hydrologic models and design methodologies used for determining runoff characteristics and analyzing stormwater management structures shall be approved by the City Engineer. Plans, specifications and computations for stormwater management facilities submitted for review shall be signed by a registered professional engineer.
J. Access Routes To Ponding Areas: Show or define access routes for maintenance purposes to all ponding areas (must be maximum of eight percent (8%) grade, two percent (2%) cross slope and ten feet (10') wide). Paving or pavers on the access routes is required with a design load to able support maintenance vehicles.
K. Emergency Overflow: An emergency overflow, sufficient to convey the incoming flow from a 100-year storm event, shall be included with the design and construction of all ponding facilities.
L. Long-Term Maintenance Of Structural Stormwater BMPs: All stormwater management structures and facilities shall be maintained in perpetuity to assure the structures and facilities function as originally designed. The responsibility for maintenance shall be assumed by either the City with jurisdiction over the structures and facilities, or by the applicant entering into a maintenance agreement with the City.
1. Maintenance Of Private Structural Stormwater BMPs: Any structural stormwater BMP that the City determines to be private shall meet the following requirements:
a. A permanent public easement shall be provided to the City for access for inspection and/or maintenance purposes. Costs incurred by the City for any maintenance of private systems will be billed and/or assessed to the owner/operator;
b. Recorded inspection and maintenance agreements that define inspection and maintenance responsibilities are required. A minimum annual inspection for private systems shall be required. These requirements are transferrable to any party that becomes the owner/operator of the site; and,
c. If site configurations or structural stormwater BMPs change, causing decreased BMP effectiveness, new or improved structural stormwater BMPs must be implemented to meet the requirements of this section. (Ord. 2018-02, 1-16-2018; amd. Ord. 2023-03, 3-20-2023)