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A. Application: Every owner, prior to engaging in any land disturbing activity of one acre or greater, must complete a construction stabilization plan as well as a stormwater pollution prevention plan (SWPPP) in accordance with the general permit authorization to discharge stormwater associated with construction activity under the National Pollutant Discharge Elimination System/State Disposal System Permit Program (MN R100001) (MPCA permit) and the City criteria outlined below. The SWPPP shall identify Best Management Practices (BMPs) to be used to prevent stormwater pollution.
B. Inspections: The City will conduct inspections on a regular basis to ensure that the plan is properly installed and maintained. In all cases the inspectors will attempt to work with the builder or developer to maintain proper erosion and sediment control at all sites. In cases where cooperation is withheld, after a verbal and written warning, the City shall issue construction stop work orders, until erosion and sediment control measures meet the requirements of this chapter. An inspection must follow before work can commence. Inspections are required as follows:
1. Before any land disturbing activity begins.
2. At the time of footing, framing and final inspections.
3. At the completion of the project.
4. Prior to the release of any financial securities, if applicable.
5. Random inspections during the course of the project to ensure compliance with the SWPPP.
A qualified representative for the owner/operator is responsible for inspecting the site for compliance on a weekly basis and after every storm event greater than 0.5 inch in twenty four (24) hours. A record of each inspection must be made and kept available on site for review by City inspectors. At all times the applicant shall be responsible for implementing and maintaining the BMPs as provided in the approved SWPPP.
C. Requirements Of The Stormwater Pollution Prevention Plan: The SWPPP must, at a minimum, and to the extent applicable, contain the items required by the MPCA permit. Nevertheless, due to the diversity of sites, each site will be individually evaluated and, where additional information and/or BMPs are needed, they will be specified at the discretion of the City. Where MPCA criteria differs from this chapter, this chapter will govern.
D. Minimum Control Measures: Minimum control measures are required where one acre or greater of bare soil is disturbed or exposed. Construction operations must, at a minimum, comply with any applicable Federal or State permit and stormwater management plan in addition to the following Best Management Practices:
1. Permit: The owner and contractor are required to apply for and meet the conditions of the Minnesota NPDES construction permit (MN R100001).
2. Site Erosion And Sediment Control Measures: Erosion and sediment control measures must be properly installed by the applicant before construction activity begins. Such measures may be adjusted during dry weather to accommodate short-term activities, such as those that require the passage of very large vehicles. As soon as this activity is finished or before rainfall, the erosion and sediment control structures must be returned to the original configuration.
3. Site Dewatering: Water pumped from the site shall be treated by temporary sedimentation basins, grit chambers, sand filters, upflow chambers, hydrocyclones, soil concentrators or other appropriate controls as deemed necessary. Water may not be discharged in a manner that causes erosion, sedimentation, or flooding on the site, on downstream properties, in the receiving channels, or in any wetland.
4. Waste And Material Disposal: All waste and unused building materials (including garbage, debris, cleaning wastes, wastewater, petroleum based products, paints, toxic materials, or other hazardous materials) shall be properly disposed of off-site and shall not be allowed to be carried by runoff into a receiving channel, storm sewer system, or wetland.
5. Tracking Management: Each site shall have roads, access drives and parking areas of sufficient width, length and surfacing to minimize sediment from being tracked onto public or private roadways. Any material deposited by vehicles or other construction equipment onto a public or private road shall be removed (not by flushing) before the end of each working day.
6. Water Quality Protection: The construction contractor, including the general contractor and all subcontractors, shall be required to control oil and fuel spills and chemical discharges to prevent such spills or discharges from entering any watercourse, sump, sewer system, water body, or wetland.
7. Concrete Washout Area: All liquids and solid waste generated by concrete washout operations must be contained in a leak-proof containment facility or impermeable liner. A compacted clay liner that does not allow washout liquids to enter groundwater is considered an impermeable liner. A sign must be installed adjacent to each washout facility to inform concrete equipment operators to utilize the proper facilities.
8. Storm Drain Protection: All storm drain inlets shall be protected during construction with control measures as contained in the SWPPP. These devices shall remain in place until final stabilization of the site. A regular inspection and maintenance plan shall be developed and implemented to assure these devices are operational at all times. Storm drain protection may conform to the protection alternatives pre-approved by City staff and available at City Hall and on the City website.
9. Soil Stockpiling: All exposed soil areas must be stabilized as soon as possible to limit soil erosion, but in no case later than fourteen (14) days after the construction activity in that portion of the site has temporarily or permanently ceased. If the site is within one mile of a special or impaired water, soil areas shall be stabilized within seven (7) days after the construction activity in that portion of the site has temporarily or permanently ceased. Temporary clean aggregate stockpiles, demolition concrete stockpiles, sand stockpiles and the constructed base components of roads are exempt from this requirement.
10. Channeled Runoff: Diversion of channeled runoff around disturbed areas, if practical, or the protection of the channel.
11. Easements: If a stormwater management plan involves directing some or all of the site's runoff, the applicant or his designated representative shall obtain from adjacent property owners any necessary easements or other property interests concerning the flowing of such water.
12. Impact On Erosion And Sediment: Schedule the site's activities to lessen the impact on erosion and sediment creation, so as to minimize the amount of exposed soil.
13. Parking On Bare Lots: Parking on all bare lots is allowed with a conditional use permit. During construction all traffic on and off site shall use designated construction entrances and exits.
14. Roof Drain Leaders: All newly constructed and reconstructed buildings must route roof drain leaders to pervious areas (not natural wetlands) where the runoff can infiltrate. The discharge rate shall be controlled so that no erosion occurs in the pervious areas.
15. Disturbed Areas Less Than One Acre: For disturbed areas less than one acre, sedimentation basins are encouraged, but not required, unless specifically required by City staff or City Engineer. The applicant shall install erosion and sediment controls at locations directed by the City. Minimum requirements include silt fences, rock check dams, or other equivalent control measures along slopes. Silt fences are required along channel edges to reduce sediment reaching the channel. Silt fences, rock check dams, etc., must be regularly inspected and maintained.
E. Minimum Protection For Natural Wetlands: Runoff must not be discharged directly into wetlands without appropriate quality and quantity runoff control, as required by this chapter, the MPCA and Minnesota Rules 7050.0186, and 7090.2. Minnesota water quality standards as established by law and rule shall govern and be enforceable by the City.
F. Stormwater Design Standards: All permanent stormwater management plans must be submitted to the City Engineer prior to the start of construction activity. Designers are expected to follow the requirements of this section to meet the volume control, water quality, and water quantity requirements of the City of Baxter. Designs should meet the stormwater design standard of this chapter and the Minnesota Stormwater Manual. Deviations from the recommended guidance will require detailed written explanation with discretion given by the City. Stormwater facilities included as part of the final design for a permanent development shall be addressed in the design package submitted to the City and shall meet the following criteria:
1. Design Of Stormwater Storage Facilities: Stormwater storage facilities shall be designed to retain runoff from a 100-year critical duration rainfall event onsite without overtopping. A reduction of the required onsite storage capacity may be granted under the following circumstances:
a. The site or any portion of the site is included as part of a larger City approved regional stormwater management plan.
b. The site is adjacent to a City owned and maintained storm sewer system that drains to a regional stormwater storage facility and the site (or portion of) has been included as part of the contributing area for design of the system.
c. The site contains topographic features that allow stormwater storage outside of the designated stormwater storage facility without inundating wetlands, causing adverse conditions or damage to adjacent properties.
d. Other reasons as determined by the City Engineer.
The designer shall confer with the City Engineer to determine how much, if any, stormwater runoff from the site or any portion of the site has been accounted for in a pre- existing regional stormwater management plan or publicly maintained system.
2. Design Of Trunk, Lateral, And Collector Systems: Trunk, lateral and collector systems shall be designed for the 10-year rainfall event. The following table shall be used for the calculation of peak rates using the rational method:
Parks, open space
3. Clogging Factor: For collection systems not designed to meet rate control standards (e.g., catch basins) a clogging factor of fifty percent (50%) will be utilized in sizing intake structures.
4. Rate Control Diameter: No orifice having a diameter less than four inches (4") is allowed in the design of rate control structures within the City. If a lower discharge rate is required a weir may be used to meet the requirements.
5. Emergency Spillway: An emergency spillway (emergency outlet) from ponding areas shall be installed a minimum of one foot (1') below the lowest building opening and shall be designed to have a capacity to overflow water at an elevation below the lowest building opening at a rate not less than the anticipated 100-year peak inflow rate to the basin, or three (3) times the 100-year peak discharge rate from the basin, whichever is greater.
6. 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.
7. Water Quality Treatment Standards: Stormwater treatment must be designed to remove eighty percent (80%) of total suspended solids (TSS) on an average annual basis. Treatment must be provided in on-site or regional systems and through a combination of BMPs, with highest preference given to green infrastructure techniques and practices (i.e., infiltration, evapotranspiration, reuse/harvesting, conservation design, urban forestry, green roofs, etc.) that will meet this requirement. This requirement is anticipated to result in 40% - 60% total phosphorus (TP) removal. The stormwater discharges of TSS and TP shall result in no net increase from pre-project conditions for new development projects. The stormwater discharges of TSS and TP shall result in a net reduction from pre-project conditions for redevelopment projects. Where TSS and/or TP reduction requirements cannot be met on the site of the original construction, the applicant will be required to locate alternative sites where TSS and/or 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 up-stream.
d. Locations anywhere within the City of Baxter.
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. Previously required routine maintenance of structural stormwater BMPs cannot be considered mitigation. Mitigation projects must be finished within twenty four (24) months after the original construction activity begins. A maintenance agreement shall be completed, specifying the owner as the responsible party for long-term maintenance. Payments will not be accepted in lieu of the construction project meeting the TSS and TP treatment standards.
8. Infiltration/Volume Control: Volume control measures are required on projects to meet the water quality criteria of the City and to meet the requirements of the City of Baxter's MS4 permit obligations. Except where conditions listed below are not met, stormwater runoff abstraction via infiltration, evapotranspiration, capture, and/or reuse of stormwater runoff is required to treat the water quality volume of one inch (1") (or 1 inch minus the volume of stormwater treated by another system on the site) of runoff from new impervious surfaces created by the project when a development project creates one acre or more new impervious surfaces. All flow from a 100-year rain event will be infiltrated in ponds designed as infiltration ponds. For wet sedimentation pond design, for new development projects, stormwater discharge volume shall result in no net increase from pre-project conditions. For redevelopment projects, stormwater discharge volume shall result in a net reduction from pre-project conditions. For all ponds, water quality runoff must be infiltrated within forty eight (48) hours or less. To simplify the review process, no runoff will be assumed from pervious surfaces from a 1-inch rainfall event.
a. Infiltration will not be required or allowed in areas where there are known groundwater contaminants, in areas that receive discharges from vehicle fueling and maintenance, in areas that receive discharges from industrial facilities which are not authorized to infiltrate industrial stormwater under an NPDES/SDS industrial stormwater permit issued by the MPCA, within a drinking water supply management area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13, where the soils are not suitable for infiltration (hydrologic soil group D), in areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rates below 8.3 inches per hour, or in areas where there is less than three feet (3') of separation between the bottom of the infiltration system and the groundwater or top of bedrock. Prior to infiltration design, a test pit is required to determine depth to mottled soils, indicating groundwater elevations. Mottled soil elevations shall be verified by City staff. Percolation test results from a double ring infiltrometer shall be submitted to the City to verify the infiltration rates of on-site soils following the construction of infiltration BMPs.
b. Pretreatment of stormwater is required prior to discharge to an infiltration system. The pretreatment practice of vegetated filter strips shall follow the Minnesota Stormwater Manual for design. This pretreatment shall collect sediment and be easily accessed for inspection and maintenance. The infiltration/filtration system selected must meet the following criteria:
(1) Remove settleable solids, floating materials, and oils and grease to the maximum extent practicable before runoff enters the system.
(2) Filtration must be designed to remove eighty five percent (85%) of total suspended solids.
(3) Consider the impact of construction and infiltration practices on existing hydrologic features (e.g., existing wetlands) and maintain pre-existing conditions.
(4) Consider potential hotspots, groundwater warning, design measures, maintenance considerations or other retention, detention, and treatment devises as specified in the MN Stormwater Manual.
(5) The infiltration practice shall not be used within fifty feet (50') of a Municipal, community or private well, unless specifically allowed by an approved wellhead protection plan.
(6) The infiltration practice shall not be used for runoff from fueling and vehicle maintenance areas and industrial areas with exposed materials posing contamination risk, unless the infiltration practice is designed to allow for spill containment.
(7) Ensure the area is not compacted while the site is under construction.
(8) When an infiltration system is excavated to within three feet (3') of final grade, the permittee must employ rigorous erosion prevention and sediment controls (e.g., diversion berms) to keep sediment and runoff completely away from the infiltration area. The area must be staked off and marked so that heavy construction vehicles or equipment will not compact the soil in the proposed infiltration area.
(9) To prevent clogging the system shall have a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before stormwater discharges into the system.
(10) Ensure appropriate on-site testing consistent with the MN Stormwater Manual is conducted to verify soil type and to ensure a minimum of three feet (3') of separation from the seasonally saturated soils (or bedrock) and the bottom of the proposed system is maintained.
(11) Ensure filtration systems with less than three feet (3') of separation from seasonally saturated soils or from bedrock are constructed with an impermeable liner.
(12) The infiltration practice shall not be used in hydrologic soil group (HSG) D soils without soil corrections.
(13) Provide an eight foot (8') wide maintenance access.
9. Permanent Wet Sedimentation And Regional Pond Water Quality Standards: If infiltration practices are prohibited, a permanent water quality pond shall be used to meet water quality and rate control requirements as specified herein. The pond is required to meet the following criteria. If a pond is designed using this criteria, it will be assumed to meet the City standard of eighty percent (80%) TSS removal and result in approximately 40% - 60% TP removal.
a. The pond shall have a permanent volume of one thousand eight hundred (1,800) cubic feet of storage below the outlet pipe for each acre that drains to the basin.
b. Permanent pool depth shall be a minimum of three feet (3') and maximum of ten feet (10').
c. The basin must provide live storage for water quality volume of one inch (1") of runoff (or 1 inch minus the volume of stormwater treated by another system on the site) from the new impervious surfaces created by the project.
d. The basin must minimize scour and the suspension of solids.
e. The basin outlet must be designed to prevent short-circuiting and the discharge of floating debris, and the maximum discharge at the water quality volume shall be less than 5.66 cubic feet per second (cfs) per acre of surface area of the pond at the water quality volume elevation.
f. An emergency outlet to control the 100-year storm event.
g. Basin slopes no steeper than 3:1.
h. A basin shelf (10 feet wide and 1 foot below the normal water level) to enhance wildlife habitat, reduce safety hazards, and improve maintenance access.
i. Flood pool volume above the normal outlet so that peak discharge rates from the 2-year, 10-year, and 100-year storm events are no greater than existing conditions.
j. An eight foot (8') wide maintenance access must be provided.
k. Be located outside of surface waters or any buffer zone.
l. Natural wetlands and waterbodies are not considered a regional stormwater pond and construction will not occur within existing wetlands unless they are mitigated in accordance with the State of Minnesota Wetland Conservation Act.
m. Waterways connected to the pond will not be degraded.
n. Safety considerations will be made in the design of permanent water quality ponds.
10. Outlet And Inlet Pipes:
a. Inlet pipes of stormwater ponds shall be extended to the pond normal water level whenever possible.
b. Outfalls with velocities greater than four (4) fps into channels requires energy dissipation or stilling basins.
c. Outfalls with velocities of less than four (4) fps generally do not require energy dissipaters or stilling basins, but will require riprap protection.
d. In the case of discharge to channels, riprap shall be provided on all outlets to an adequate depth below the channel grade and to a height above the outfall or channel bottom. Riprap shall be placed over a suitably graded filter material with filter fabric to ensure that soil particles do not migrate through the riprap and reduce its stability. Riprap shall be placed to a thickness at least two (2) times the mean rock diameter to ensure that it will not be undermined or rendered ineffective by displacement. If riprap is used as protection for overland drainage routes, grouting may be recommended.
e. Discharge velocity into a pond at the outlet elevation shall be six (6) fps or less. Riprap protection, or other appropriate energy dissipation practice, is required at all inlet pipes into ponds from the NWL to the pond bottom.
f. Where outlet velocities to ponds exceed six (6) fps, the design should be based on the unique site conditions present. Submergence of the outlet or installation of a stilling basin approved by the City is required when erosive outlet velocities are experienced.
11. Limitations And Restrictions For Permanent Stormwater Management: The City may limit or restrict the construction of permanent management facilities based on the following criteria.
a. Permanent infiltration stormwater management facilities may not receive discharges from or be constructed in areas where:
(1) Industrial facilities are not authorized to infiltrate industrial stormwater under an NPDES industrial stormwater permit issued by the MPCA.
(2) Vehicle fueling or maintenance activities occur.
(3) 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.
(4) There are known groundwater contaminants or groundwater will be mobilized by the construction of infiltration BMPs.
b. For areas where infiltration is prohibited the applicant must 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.
c. For linear projects with lack of right-of-way, easements or other permissions from property owners to install treatment 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.
d. The City may restrict the use of infiltration features to meet requirements for stormwater management, without higher engineering review, if the infiltration techniques will be constructed in the following areas where:
(1) Soils are predominately hydrologic soil group D (clay) soils.
(2) Drinking water supply management areas are present, as defined by Minn. R. 4720.51000, subp. 13, unless precluded by a local unit of government with an MS4 permit.
(3) 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.
12. Exceptions For Permanent Stormwater Management: The City may authorize reduced volume control for the following situations:
a. If the project meets one of the limitations outlined above.
b. If the applicant implements to the maximum extent possible other volume reduction practices, besides infiltration, on the site but may not meet the requirements for stormwater management.
13. Drainage And Utility Easements: New stormwater management BMPs (e.g., ponds, infiltration systems, swales) constructed as part of private development may be covered by drainage and utility easements or outlots that are dedicated to the City. Maintenance responsibilities for these areas will be spelled out in a Stormwater Facilities Maintenance Agreement. All maintenance agreements must be approved by the City and recorded at the Crow Wing County Recorder's Office prior to final plan approval. At a minimum, the maintenance agreement will describe the following inspection and maintenance obligations:
a. No private stormwater facilities may be approved unless a maintenance plan is provided that defines how access will be provided, who will conduct the maintenance, the type of maintenance and the maintenance intervals. At a minimum, all private stormwater facilities shall be inspected annually and maintained in proper condition consistent with the performance goals for which they were originally designed and as executed in the Stormwater Facilities Maintenance Agreement.
b. The party who is permanently responsible for inspections and maintenance of the structural and nonstructural measures.
c. Pass responsibilities for such maintenance to successors in title.
d. Allow the City and its representatives the right of entry for the purposes of inspecting all permanent stormwater management systems.
e. 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. The cost will be assessed to the owner.
f. The agreement shall also stipulate that if site configuration or structural stormwater BMPs change, causing decreased structural stormwater BMP effectiveness, new or improved BMPs shall be installed.
g. Access to all stormwater facilities must be inspected annually and maintained as necessary. The applicant shall obtain all necessary easement or other property interests to allow access to the facilities for inspection or maintenance for both the responsible party and the City of Baxter.
14. Skimmers: The City requires skimmers or other devices, with the intent to remove floatables, in the construction of new pond outlets and the addition of skimmers to existing systems whenever feasible and practical. The designs shall provide for skimmers that extend a minimum of four inches (4") below the water surface and minimize the velocities of water passing under the skimmer to less than 0.5 feet per second for rainfall events having a ninety nine percent (99%) frequency. Wood skimmers are not allowed.
15. Habitat And Aesthetic Enhancement: The City encourages the design of stormwater management features that provide an opportunity to enhance the habitat and aesthetics of the area. This includes providing upland buffers around ponds, planting coniferous and deciduous trees, seeding the area with native vegetation, and designing the slopes equal to or flatter than 4:1.
16. Combination Of Practices: A combination of successive practices may be used to achieve the applicable minimum control requirements specified. (Ord. 2017-012, 11-21-2017)