(A) Stormwater BMP design.
(1) Design standards. BMPs shall be designed according to the design standards included in Appendix A: Volume and Water Quality Calculations. Compliance with the water quality treatment standard will be calculated by the applicant using Appendix A: Volume and Water Quality Calculations or industry standard water quality models.
(2) The Minnesota Stormwater Manual, Minnesota Pollution Control Agency, as amended from time to time is a supplemental resource, which provides examples of BMPs and design, construction and maintenance guidelines.
(3) The county spreadsheet calculator tool may be used to track and document stormwater treatment requirements and credits. Other stormwater calculator tools may also be used.
(B) Treatment volume requirements.
(2) Non-linear projects. The treatment volume must be calculated as one inch times the sum of new and fully reconstructed impervious surface.
(3) Linear projects. The treatment 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 fully reconstructed impervious surface.
(C) Treatment locations/sequencing.
(1) Treatment locations.
(a) Water quality. Water quality treatment must be provided on-site prior to discharging stormwater runoff to a receiving waterbody.
1. Non-linear projects and new linear projects. If it is not feasible to provide full water quality treatment prior to discharge to a receiving waterbody, structural treatment for TSS removal must be provided at a minimum. Full water quality treatment shall then be provided at a two-to-one ratio at a discharge point to a different receiving waterbody.
2. Linear reconstruction projects. Additional right of way, easements or permissions should be obtained to provide treatment within the drainage area to the same waterbody. If additional right of way, easements, or permissions cannot be obtained, full water quality treatment shall then be provided to a discharge point to a different receiving waterbody within the project area. If it is not feasible to provide full water quality treatment prior to discharging to a receiving waterbody, structural treatment for TSS removal must be provided at a minimum.
(b) Volume control. Volume control must be provided on-site within the same DNR catchment area as the construction activity.
(c) Offsite treatment sequencing. If water quality and/or volume treatment cannot be fully provided on-site in a cost-effective manner per divisions (C)(1)(a) and (b), the offsite treatment may be utilized to provide remaining treatment at locations in the following order of preference. The combination of on-site and off-site treatment must yield the minimum required stormwater benefit and be approved by the county prior to construction.
(d) Order of preference for offsite treatment:
1. To a location that yields benefits to the same receiving water that receives runoff from the original construction activity.
2. Within the same DNR catchment area as the impact.
3. In the next adjacent DNR catchment area upstream.
4. Any feasible location within the CCWMO.
(D) Alternative compliance options. If specific site conditions may make water quality treatment and/or volume control treatment difficult, undesirable or impossible and offsite treatment is not feasible, the applicant may submit a request to the county for alternative compliance treatment. Alternative compliance options are only available following the sequencing steps outlined above (§ 153.56 (C)(1)(c)). All requests for alternative compliance shall address the treatment location sequencing steps, indicate the specific site conditions and limitations present, and include the applicable submittals per §§ 153.42 through 153.43. The county may request site-specific information, such as soils data, local water table data and correspondence from parties affected by the project when deciding on alternative compliance eligibility.
(1) Banked credits. Water quality treatment and volume reduction credits may be purchased from qualifying banks. To qualify, the bank must be within the CCWMO and must be reviewed and approved by the county per the water rules in place at the time of approval. Banked credit locations will be reviewed by the county according to the order of preference for alternative compliance treatment locations. Applicants shall submit a letter to the county outlining the conditions of the transfer and confirming the volume of the transfer. The county must review and approve all credit transfers.
(2) In-lieu fee. The applicant may choose to pay into the county's Stormwater Treatment In-Lieu Fund to cover the cost of implementing equivalent stormwater treatment elsewhere in the CCWMO.
(a) The required amount to contribute to the In-Lieu Fund shall be based on the cost of planning, designing, constructing and maintaining stormwater BMP(s) that provide the required water quality treatment and/or volume reduction credit. This amount shall be calculated as:
1. The flat fee per cubic foot of stormwater treatment required based on the Carver County fee schedule, as update from time to time; or
2. Eighty percent (80%) of the estimated cost provided by the applicant, provided as an itemized list, and as reviewed and approved by the county.
(b) Money contributed to the Stormwater Treatment In-Lieu Fund shall be allocated to water quality treatment and/or volume reduction projects by the county according to the water plan, local stormwater management plans and the county's list of stormwater improvements as updated from time to time. The treatment achieved by these projects will offset the treatment that was not achieved on the permitted development.
(3) Credit banking. Water quality treatment credit and volume reduction credit provided in excess of the treatment requirement may be banked for use on another project. Excess banked credit amounts shall not exceed stormwater credit awarded for the BMP by the county. The county must review and approve all banking credits generated.
(E) Rate control standard.
(1) Peak rates. The peak rates shall not increase from existing conditions for the two-, ten-, 100-year storm events, and the 100-year, ten-day snowmelt event. Peak rates shall be calculated using Atlas 14 precipitation depths and storm distributions.
(2) Conveyance system. At a minimum, the storm sewer conveyance system shall be designed for a ten-year, 24-hour storm event. The pond and pond outlet structure shall handle the 100-year, 24-hour storm event.
(F) Water quality standard.
(1) Total Phosphorus (TP) removal standards. The stormwater management plan must remove 90% of the TP generated by the site under developed conditions.
(2) Total Suspended Solids (TSS) removal standards. The stormwater management plan must remove 90% of the TSS generated by the site under developed conditions.
(3) BMP sizing. Treatment areas must be sized appropriately for the area draining to the feature.
(G) Volume control standards.
(1) Volume control standard. The stormwater management plan must provide volume control for:
(a) One inch from the impervious surface for unconstrained sites.
(b) One-half inch from the impervious surface for constrained sites.
(2) BMP sizing. Volume control areas must be sized appropriately for the area draining to the feature.
(3) Volume control standard for constrained sites. Specific site conditions may make volume control difficult, undesirable, or impossible. If these conditions are present, the site may qualify as "constrained," subject to county review and determination. Some of these conditions are listed in Table 1. The applicant may also submit a request to the county for constrained site eligibility for site conditions not listed below. All requests for constrained site eligibility shall indicate the specific site conditions present and include a grading plan, utility plan and the submittal requirement listed in Table 1.
Table 1. Constrained Site Conditions.
Type | Specific Site Condition | Volume Reduction Limitation | Submittal Requirement |
Potential contamination | Potential stormwater hotspots/industrial facilities | Infiltration prohibited | - - |
Contaminated soils | Infiltration prohibited | Soil analysis | |
Vehicle fueling and maintenance areas | Infiltration prohibited | Site map with vehicle fueling/maintenance areas shown | |
Physical limitations | Low permeability soils | Infiltration restricted | 1) Carver County soil survey data showing greater than 50% of site is hydrologic group C and D soils; or 2) Carver County soil survey data showing greater than 50% of site has the following unified soil classifications: MH, ML, GS, SC, CL, OL, CH, OH 3) Documentation that site has been previously disturbed by construction activity that resulted in compaction and/or other significant reduction in permeability as shown by field infiltration test rate showing infiltration rate of less than 0.3 inches per hour; or 4) Documentation of field infiltration tests showing infiltration rate of less than 0.3 inches per hour. |
Bedrock or groundwater within three vertical feet of bottom of volume control practice | Infiltration restricted | Soil borings required; Piezometer data may be required. | |
Land use limitations | Wellhead protection areas | Infiltration restricted | Site map with wellhead protection areas shown
|
(H) High water elevation standard.
(1) As described below, all applications shall provide vertical separation between low openings of new and existing structures and high water elevations of local existing and proposed stormwater facilities and surface waters. The high water elevations used should be the greater between the 100-year, 24-hour or 100-year, ten-day high water elevations. Emergency overflows are required for all ponds and basins.
(a) Low opening of new and existing structures must have a minimum of two feet of separation from local high water level.
(b) Low floor of new and existing structures must have a minimum of one foot of vertical separation from local high water level.
(c) In rare cases where an emergency overflow (overland or pipe) is not feasible, the low opening vertical separation is increased to three feet.
(2) The requirements described above can be waived for non-habitable structures if an LGU allows for less vertical separation from high water elevations based on flood-proofing standards included in a building code.
(3) If side or rear yard overflow swales are constructed, the cities should document through the building permitting and inspection process that high water levels for side or rear yard overflow swales are below the low openings of structures.
(I) Requirements for maintenance and access.
(1) Maintenance of stormwater facilities. All stormwater management structures and facilities must be designed to allow access for maintenance and must be properly maintained in perpetuity to ensure that they continue to function according to the approved design.
(2) Maintenance agreement. No stormwater plan may be approved unless a maintenance agreement is provided that defines maintenance responsibilities following completion of the project, specifies types and frequency of inspection and maintenance activities, and specifies who will conduct inspections and maintenance activities. A sample agreement and list of inspection/maintenance activities are included in the Carver County Water Resource Management Ordinance and BMP guidelines.
(a) Prior to project close out return of the financial security, an agreement shall be in place regarding maintenance responsibilities.
(b) Maintenance responsibilities must be assumed by either the local government unit (LGU) or by the responsible party. If the LGU is assuming maintenance responsibilities, written acknowledgment and acceptance is needed from the LGU.
(c) If the Local Government Unit (LGU) is assuming maintenance responsibilities, a single memorandum of agreement for each LGU may be used to cover all stormwater management structures and facilities required by this ordinance within the LGU's jurisdiction.
(d) The agreement must be executed and recorded in a format acceptable to the county. The recordable executed agreement must be submitted to the county prior to release of financial security for the project.
(3) Drainage and utility easement. A drainage and utility easement is required in the following situations:
(a) When a stormwater management BMP must be placed on private property.
(b) When changes caused by a land disturbing activity result in alterations to flow paths which impound or slow down water on an adjacent property.
(c) When changes caused by a land disturbing activity reroute water onto a neighboring property, where it wasn't previously routed.
(Ord. 57-2005, passed 1-10-06; Am. Ord. 75-2012, passed 6-26-12; Am. Ord. 83-2016, passed 9-20-16; Am. Ord. 99-2022, passed 6-28-22)