(A) Submittal of site plans consisting of post-construction plans.
(1) Site plans must be submitted for review and confirmation that chapter requirements have been met, prior to start of construction activity.
(2) Site plans must consist of:
(a) All calculations for the permanent stormwater treatment system;
(b) The water quality volume that will be treated through volume reduction practices;
(c) Rationale and documentation supporting the location of any off-site permanent stormwater treatment projects;
(d) If applicable, the amount paid to the city for in lieu of off-site treatment; and
(e) All legal mechanisms related to long-term maintenance.
(B) Post-construction storm water management BMPs must meet the following criteria:
(1) Designed with accepted engineering practices and in accordance with permanent storm water management system design criteria.
(2) Treat the water quality volume on any project where the sum of the new impervious surface and the fully reconstructed impervious surface equals one or more acres.
(3) For non-linear projects, water quality volume (calculated as an instantaneous volume) must be calculated as 1 inch times the sum of the new and the fully reconstructed impervious surface.
(4) For linear projects, water quality volume (calculated as an instantaneous 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 the fully reconstructed impervious surface. 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 permission to treat the stormwater during the project planning process must be made. Volume reduction practices must be considered first. Volume reduction practices are not required if the practices cannot be provided cost effectively. If additional right of-way, easements, or other permission cannot be obtained, the owner/operator of construction activity must maximize the treatment of the water quality volume prior to discharge from the city's MS4.
(5) Volume reduction practices (e.g., infiltration or other) to retain the water quality volume on-site must be considered first when designing the permanent storm water treatment system. Wet sedimentation basins and filtration systems are not considered volume reduction practices. If infiltration is prohibited, as described in infiltration system, other volume reduction practices, a wet sedimentation basin, or a filtration basin may be considered.
(6) Off-site treatment.
(a) For non-linear projects, where the water quality volume cannot cost effectively be treated on the site of the original construction activity, the remaining water quality volume must be addressed through off-site treatment and meet the following requirements (must be selected in the following order of preference):
1. Locations that yield benefits to the same receiving water that receives runoff from the original construction activity.
2. Locations within the same DNR catchment area as the original construction activity.
3. Locations in the next adjacent DNR catchment area up-stream.
4. Locations anywhere within the city's jurisdiction.
(b) Off-site treatment projects must involve the creation of new structural storm water BMPs or the retrofit of existing structural storm water BMPs, or the use of a properly designed regional structural storm water BMP. Routine maintenance of structural storm water BMPs owned or operated by the city cannot be used to meet this requirement.
(c) Off-site treatment projects must be completed no later than 24 months after the start of the original construction activity.
(d) The applicant may provide payment to the city in lieu of off-site treatment.
(C) Long-term maintenance. The applicant must enter into a long-term maintenance agreement with the city that documents all responsibilities for long-term operation and maintenance of storm water treatment practices that are not owned or operated by the city. At a minimum, the long-term maintenance agreement must include provisions that:
(1) Allow the city to conduct inspections of structural storm water BMPs not owned or operated by the city, perform necessary maintenance, and assess costs for those structural storm water BMPs when the city determines the owner of that structural storm water BMP has not ensured proper function;
(2) Are designed to preserve the city's right to ensure maintenance responsibility, for structural stormwater BMPs not owned or operated by the city, when those responsibilities are legally transferred to another party; and
(3) Are designed to protect/preserve structural storm water BMPs. If structural stormwater BMPs change, causing decreased effectiveness, new, repaired, or improved structural storm water BMPs must be implemented to provide equivalent treatment to the original BMP.
(D) Permanent storm water management system design criteria.
(1) Infiltration system.
(a) 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;
(b) To determine if an infiltration system is suitable, either the MPCA's contamination screening checklist must be completed or an assessment must be conducted. The checklist or assessment must be documented in the site plan. For more information and to access the MPCA's "contamination screening checklist" see the Minnesota Stormwater Manual;
(c) Must be designed such that pre-existing hydrologic conditions of wetlands in the vicinity are not impacted (e.g., inundation or breaching a perched water table supporting a wetland);
(d) Must not be excavated to final grade, or within 3 feet of final grade, until the contributing drainage area has been constructed and fully;
(e) When excavating to within 3 feet of final grade, the applicant must stake off and mark the area so heavy construction vehicles or equipment do not compact the soil in the infiltration area;
(f) When excavating to within 3 feet of final grade, the applicant must stake off and mark the area so heavy construction vehicles or equipment do not compact the soil in the infiltration area;
(g) A pretreatment device such as a vegetated filter strip, forebay, or water quality inlet (e.g., grit chamber) to remove solids, floating materials, and oil and grease from the runoff, to the maximum extent practicable, must be used before the system routes stormwater to the infiltration system;
(h) Designed to provide a water quality volume;
(i) Designed to discharge all stormwater (including stormwater in excess of the water quality volume) routed to the system through the uppermost soil surface or engineered media surface within 48 hours. Additional flows that cannot infiltrate within 48 hours must bypass the system through a stabilized discharge point;
(j) Must provide a means to visually verify the infiltration system is discharging through the soil surface or filter media surface within 48 hours or less;
(k) Must provide at least one soil boring, test pit or infiltrometer test in the location of the infiltration practice for determining infiltration rates;
(l) For design purposes, divide field measured infiltration rates by two as a safety factor or use soil-boring results with the infiltration rate chart in the Minnesota Stormwater Manual to determine design infiltration rates. When soil borings indicate type A soils, field measurements should be performed to verify the rate is not above 8.3 inches per hour.
(m) Must employ appropriate on-site testing to ensure a minimum of 3 feet of separation from the seasonally saturated soils (or from bedrock) and the bottom of the proposed infiltration system;
(n) Must design a maintenance access, typically 8 feet wide;
(o) Infiltration systems are prohibited in the following areas (See "higher level of engineering review" in the Minnesota Stormwater Manual for more information):
1. Areas that receive runoff from vehicle fueling and maintenance areas;
2. Areas where infiltrating storm water may mobilize high levels of contaminants in soil or groundwater;
3. Areas where soil infiltration rates are field measured at more than 8.3 inches per hour unless the soils are amended to slow the infiltration rate below 8.3 inches per hour;
4. Areas with less than 3 feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock;
5. Areas of predominately hydrologic soil group type D soils (clay);
6. Within a drinking water supply management area (DWSMA) as defined in Minn. Rules 4720.5100, subp. 13, if the system will be located:
a. In an emergency response area (ERA) within a DWSMA classified as having high or very high vulnerability as defined by the Minnesota Department of Health; or
b. In an ERA within a DWSMA classified as moderate vulnerability unless a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater has been approved by the city; or
c. Outside of an ERA within a DWSMA classified as having high or very high vulnerability unless a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater has been approved by the city.
7. Areas within 1,000 feet upgradient or 100 feet downgradient of active karst features; and
8. Areas that receive runoff from the following industrial facilities not authorized to infiltrate stormwater under the NPDES stormwater permit for industrial activities:
a. Automobile salvage yards;
b. Scrap recycling and waste recycling facilities;
c. Hazardous waste treatment, storage, or disposal facilities;
d. Wood preserving facilities; or
e. Air transportation facilities that conduct deicing activities.
(2) Filtration system.
(a) 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;
(b) Must not install filter media until the contributing drainage area is constructed and fully stabilized unless they provide rigorous erosion prevention and sediment controls (e.g., diversion berms) to keep sediment and runoff completely away from the filtration area;
(c) Designed to remove at least 80% of TSS;
(d) Must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, water quality inlet, forebay or hydrodynamic separator to remove settleable solids, floating materials, and oils and grease from the runoff, to the maximum extent practicable, before runoff enters the filtration system;
(e) Designed to provide a water quality;
(f) Designed to discharge all stormwater (including stormwater in excess of the water quality volume) routed to the system through the uppermost soil surface or engineered media surface within 48 hours. Additional flows that the system cannot filter within 48 hours must bypass the system or discharge through an emergency overflow. Designed to provide a means to visually verify the system is discharging through the soil surface or filter media within 48 hours;
(g) Designed to provide a means to visually verify the system is discharging through the soil surface or filter media within 48 hours;
(h) Employ appropriate on-site testing to ensure a minimum of 3 feet of separation between the seasonally saturated soils (or from bedrock) and the bottom of the proposed filtration system;
(i) Construct with an impermeable liner when the system has less than 3 feet of separation between seasonally saturated soils or bedrock;
(j) Designed with a maintenance access, typically eight feet wide.
(3) Wet sedimentation basin.
(a) Permanent volume of 1,800 cubic feet of storage below the outlet pipe for each acre that drains to the basin;
(b) Permanent volume must reach a minimum depth of at least 3 feet and must have no depth greater than 10 feet;
(c) Must be configured to minimize scour or resuspension of solids;
(d) In addition to the permanent volume, the basin must provide the water quality volume as live storage;
(e) Water quality volume discharges at no more than 5.66 cubic feet per second (cfs) per acre of surface area of the basin;
(f) Designed to prevent short-circuiting and the discharge of floating debris;
(g) Basin outlets must have energy dissipation;
(h) Must include a stabilized emergency overflow to accommodate storm events in excess of the basin's hydraulic design;
(i) Must have a maintenance access, typically 8 feet wide, for the basin;
(j) Must be located outside of surface waters and any buffer zones identified in sediment control practices.
(k) Permittees must design basins using an impermeable liner if located within active karst terrain.
(4) Regional wet sedimentation basins.
(a) When the entire water quality volume cannot be retained onsite, regional wet sedimentation basins can be used or created, provided they are constructed basins, not a natural wetland or water body.
(b) The regional basin conforms to all requirements for a wet sedimentation basin as described in wet sedimentation basin.
(c) Must be large enough to account for the entire area that drains to the basin.
(d) Waterways between the project and the regional basin must not be significantly degraded.
(e) Written authorization from the city or private entity that owns and maintains the regional basin.
(Ord. 712, passed 11-14-2022)