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A. Specifications: All postconstruction stormwater management plans must be submitted to the city engineer prior to the start of construction activity. At a minimum, applicants shall meet the specifications set forth below and observe the standards established in NPDES construction general permit requirements, the city's surface water management plan, and the city's "Plan Requirements And Design Guidelines".
B. Design Criteria:
1. Volume Control: Volume control measures are required on projects to meet the water quality criteria of the city's surface water management plan and the standard for the general permit authorization to discharge stormwater associated with construction activity under the NPDES construction general permit.
2. Rate Control:
a. For newly developing areas, no discharge or infiltration can be assumed for purposes of establishing the 100-year, 24-hour storm event high water elevation. For events with longer duration, a maximum peak stormwater discharge rate will be limited to 0.05 cfs per acre. Redeveloping areas will be required to meet these standards to the maximum reasonable extent practical.
b. In the event that the city will not be providing a regional system, storage of the runoff from the 100-year, 24-hour storm event is required on site.
c. Landlocked depressions that presently do not have a defined outlet and do not typically overflow may be allowed a positive overflow to prevent damage to adjacent properties. Any overflows from landlocked depressions will comply with the city's rate control, runoff volume control, and low floor requirements including storing runoff from the 100-year, 24-hour storm event for new development and restricting discharge to 0.05 cfs per acre for longer duration storm events.
d. New storm sewer systems shall be designed to accommodate discharge rates from a 10-year storm event.
3. Flood Control: The city requires that for any new or redevelopment, at least three feet (3') of freeboard between the anticipated critical 100-year high water elevation and the minimum building opening be maintained. Any deviation from the three feet (3') freeboard requirement is subject to the following conditions and may be approved by the city engineer if the following can be demonstrated:
a. That within the two foot (2') freeboard area, stormwater storage is available which is equal to or exceeds fifty percent (50%) of the stormwater storage currently available in the basin below the 100-year high water elevation.
b. That a twenty five percent (25%) obstruction of the basin outlet over a twenty four (24) hour period would not result in more than one foot (1') of additional bounce in the basin.
c. An adequate overflow route from the basin is available that will provide assurance that one foot (1') of freeboard will be maintained for the proposed low building opening.
4. Design Criteria Of Permanent Facilities: All permanent stormwater facilities must meet the design criteria as provided in the city's surface water management plan.
C. Limitations And Restrictions:
1. Stormwater control facilities shall not receive discharges from or be constructed in areas where:
a. Industrial facilities are not authorized to infiltrate industrial stormwater under an NPDES industrial stormwater permit issued by the MPCA.
b. Vehicle fueling or maintenance activities occur.
c. There is less than three feet (3') of separation between the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock.
d. There are known groundwater contaminants or groundwater will be mobilized by the construction of infiltration BMPs.
2. For areas where infiltration is prohibited the applicant shall consider alternative volume reduction BMPs and the water quality volume must be treated by a wet sedimentation basin, filtration system, regional ponding or similar method prior to the release of stormwater to surface water.
3. For linear projects with lack of right of way, easements or other permissions from property owners to install treatments systems that are capable of treating the total water quality volume on site, the project must maximize treatment through other methods or combination of methods before runoff is released to nearby surface waters. Alternative treatment options include: grassed swales, filtration systems, smaller ponds, or grit chambers. In all circumstances, a reasonable attempt must be made to obtain right of way during the project planning and all attempts of infeasibility must be recorded.
4. The city may restrict the use of infiltration features to meet postconstruction requirements for stormwater management, without higher engineering review, if the infiltration techniques will be constructed in the following areas where:
a. Soils are predominately hydrologic soil group D (clay) soils.
b. Within one thousand feet (1,000') up gradient, or one hundred feet (100') down gradient of active karst features.
c. Drinking water supply management areas are present, as defined by Minnesota rules 4720.51000, subp. 13, unless precluded by a local unit of government with an MS4 permit.
d. Soil infiltration rates are more than 8.3 inches per hour unless soils are amended to flow the infiltration rate below 8.3 inches per hour.
D. Exceptions: The city may authorize lesser volume control for the following situations:
1. If the project meets one of the limitations outlined above; and
2. If the owner/operator implements to the maximum extent possible other volume reduction practices, besides infiltration, on the site but may not meet the requirements for postconstruction stormwater management.
E. Mitigation Provisions:
1. Under certain circumstances some construction projects cannot meet the total suspended solids (TSS) and/or total phosphorus (TP) reduction requirements for new or redevelopment projects on the site of the original construction. When this occurs, the owner/operator will be required to identify alternative locations where TSS and TP treatment standards can be achieved. Mitigation project locations are chosen 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 as the original construction activity.
c. Locations in the next adjacent DNR catchment area upstream.
d. Locations anywhere within the city of Rosemount.
2. Mitigation projects shall also meet the following criteria:
a. Mitigation projects shall involve the establishment of new structural stormwater BMPs or the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP.
b. Previously required routine maintenance of structural stormwater BMPs cannot be considered mitigation.
c. Mitigation projects must be finished within twenty four (24) months after the original construction activity begins.
d. Monies received for mitigation purposes in lieu of meeting conditions for postconstruction stormwater management shall be applied to a public stormwater project and shall comply with this subsection.
e. A maintenance agreement specifying the responsible party for long term maintenance shall be identified.
F. Maintenance Agreement: The applicant shall enter into a maintenance agreement with the city that documents all responsibilities for operation and maintenance of private long term stormwater treatment BMPs. Such responsibility shall be documented in a maintenance plan and executed through a maintenance agreement. All maintenance agreements must be approved by the city and recorded at the Dakota County recorder's office prior to final plan approval. At a minimum, the maintenance agreement shall describe the inspection and maintenance obligations:
1. The responsible party who is permanently responsible for maintenance of the structural and nonstructural measures.
2. Pass responsibilities for such maintenance to successors in title.
3. Allow the city and its representatives the right of entry for the purposes of inspecting all permanent stormwater management systems.
4. Allow the city the right to repair and maintain the facility, if necessary maintenance is not performed after proper and reasonable notice to the responsible party of the permanent stormwater management system.
5. Include a maintenance plan that contains, but is not limited to, the following:
a. Identification of all structural permanent stormwater management systems.
b. A schedule for regular inspections, monitoring, and maintenance for each practice. Monitoring shall verify whether the practice is functioning as designed and may include, but is not limited to, quality, temperature, and quantity of runoff.
c. Identification of the responsible party for conducting the inspection, monitoring and maintenance for each practice.
d. Include a schedule and format for reporting compliance with the maintenance agreement to the city.
6. The issuance of a permit constitutes a right of entry for the city, its contractors, and agents to enter upon the construction site. The applicant shall allow the city, its contractors, agents and any authorized representatives, upon presentation of credentials, to:
a. Enter upon the permitted site for the purpose of obtaining information, examination of records, conducting investigations or surveys.
b. Bring such equipment upon the permitted development as is necessary to conduct such surveys and investigations.
c. Examine and copy any books, papers, records, or memoranda pertaining to activities or records required to be kept under the terms and conditions of the permit.
d. Inspect the stormwater pollution control measures.
e. Sample and monitor any items or activities pertaining to stormwater pollution control measures.
f. Correct deficiencies in stormwater and erosion and sediment control measures. (Ord. 2015-01, 2-17-2015; amd. Ord. 2015-05, 7-7-2015)