9-5-8: STORMWATER MANAGEMENT PLAN STANDARDS AND APPROVAL:
   A.   Approval: No stormwater management plan which fails to meet the standards contained in this section shall be approved. All proposed projects required to submit a stormwater management plan, as per subsection 9-5-5A of this chapter, shall incorporate the erosion and sediment control plan requirements set forth in sections 9-5-6 and 9-5-7 of this chapter into the stormwater management plan.
The city may prohibit or restrict the use of stormwater infiltration practices when soil conditions, groundwater supply issues, safety issues, snow removal, and other concerns would show such practices to be impractical or unsafe. All such exceptions must be approved by the director of public works. Specific prohibitions on infiltration practices are:
      1.   Infiltration systems are prohibited:
         a.   Where industrial facilities are not authorized to infiltrate industrial stormwater under an NPDES/SDS industrial stormwater permit issued by MPCA.
         b.   Where vehicle fueling and maintenance occur.
         c.   Where the bottom of the infiltration basin is less than three feet (3') to bedrock or seasonally saturated soils.
         d.   Within areas designated as very high vulnerability and high vulnerability within a drinking water supply management area (DWSMA).
         e.   In an ERA within a DWSMA classified as moderate vulnerability unless the permittee performs or approves a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.
         f.   Outside an ERA within a DWSMA classified as high or very high vulnerability unless the permittee performs a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.
         g.   Where soil infiltration rates are more than 8.3 inches per hour, unless soils are amended to slow the rate to below 8.3 inches per hour
         h.   Low permeability soils (i.e., hydrologic soil group D soils) or where a confining layer exists below the proposed basin.
         i.   Within one thousand feet (1,000') up gradient or one hundred feet (100') down gradient of active karst features.
      3.   For linear projects where the lack of right of way precludes the installation of volume control practices that require volume control in accordance with subsection D1 of this section, the city may allow a lesser volume control on the construction site provided a reasonable attempt has been made to obtain right of way during the project planning process and:
         a.   One or more of the prohibited site conditions listed above exists; and
         b.   The owner implements other practices (e.g., evapotranspiration, reuse, conservation design, green roofs, etc.) on the site that may not fully meet the volume control requirements.
   B.   Compliance With Other Plans: All stormwater management plans must be prepared in accordance with the city plans, city permits, TMDL allocation plans, and other special district plans as adopted and amended from time to time.
   C.   Stormwater Management Criteria for Permanent Facilities: The applicant shall install or construct, on or for the proposed land disturbing or development activity, all stormwater management facilities necessary to manage runoff in compliance with the following design standards:
      1.   Water quality volume: site stormwater practices shall be designed to infiltrate the required water quality volume calculated as follows:
         a.   For projects located within the Northwest Area, volume control practices shall meet the standards set forth in Chapter 7 of the "Inver Grove Heights Stormwater Manual - Northwest Area" (2006) and all subsequent revisions and as directed by the director of public works.
         b.   For projects located outside the Northwest Area that are not defined as linear projects, the water quality volume shall be calculated as one (1.0) inch times the sum of the new impervious surfaces and the fully reconstructed impervious surfaces.
         c.   For linear projects, the water quality volume must be calculated as the larger of one (1.0) inch multiplied by the new impervious surfaces or one-half (0.5) inch multiplied by the new plus fully reconstructed impervious surfaces. Where treatment of water quality volume is found to be cost-prohibitive, the director of public works may allow a water quality volume between the two above-calculated volumes, provided volume reduction practices have been considered and implemented into the project where feasible.
      2.   Pollutant Removal Requirements:
         a.   For projects that have met the infiltration/volume control requirements above, the pollutant removal requirements are considered to be met.
         b.   For projects where infiltration is prohibited, the following pollutant removal standards apply prior to site discharge reaching a downstream receiving water: achieve a minimum eighty five percent (85%) removal of total suspended solids and minimum fifty five percent (55%) removal of total phosphorus.
         c.   For redevelopment projects, BMPs shall be such that there is a net reduction in phosphorus and total suspended solids loading from the redevelopment site when compared to the existing (currently developed) site.
         d.   Where projects propose multiple BMPs in a treatment train approach to meet the pollutant removal requirements, at least fifty percent (50%) of the project area shall be treated in BMPs located in the upstream areas of the site.
      3.   LID Or Green Infrastructure Design Concepts: Low impact development (LID), or green infrastructure, design concepts shall be incorporated into development projects located in the northwest area and in other landlocked basins in the city. In all other parts of the city, LID design concepts must be implemented wherever possible. Specific LID related requirements for the northwest area and other landlocked basin areas include:
         a.   Postdevelopment runoff volume must match predevelopment runoff volume for the 5-year, 24-hour event.
         b.   Proposed developments must use infiltration rain gardens, vegetated swales, parking lot bioretention, infiltration basins/trenches, disconnection of impervious surfaces, green roofs, and other LID techniques.
         c.   Mass grading should be avoided to reduce compaction of natural/open space areas.
         d.   Joint parking and shared driveway arrangements are encouraged.
         e.   Pervious materials may be used for parking lot surfaces and are encouraged for single-family residential driveways.
         f.   Parking lot curbing generally must be flat (ribbon curb) or have breaks at regular intervals (curb cuts) to convey runoff into the stormwater system.
         g.   Residential downspouts and sump pumps must discharge to cisterns and/or permeable surfaces. Nonresidential downspouts and sump pumps must meet this requirement if reasonably possible.
         h.   Narrower street widths are allowed, with restrictions.
      4.   Impervious Surface Reduction: Applicants shall include methods for reducing the amount of impervious surface on their sites. Methods to use include:
         a.   Reducing road widths, such as allowing parking on only one side of a residential street.
         b.   Reducing sidewalk widths.
         c.   Allowing and providing for shared parking.
         d.   Creating a smaller building footprint (e.g., building two- story houses instead of one-story houses).
         e.   Installing semipermeable/permeable paving, where feasible.
      5.   Storm Sewer Conveyance Systems: Storm sewer conveyance systems will be designed to provide discharge capacity for the 10-year frequency runoff event. The city may allow variances to this standard in areas where a new storm sewer system would connect to an existing storm sewer system that does not have and is not expected in the future to have a ten (10) year capacity. The portions of the system that convey outflows from ponding areas will be sized to convey the critical 10-year storm flow or the required 100-year outflow from upstream ponding areas, whichever is greater. The storm sewer systems shall be designed for 10-year storm events and their performance shall be analyzed for storms exceeding the design storm.
      6.   Postdevelopment Peak Discharge Rates: Postdevelopment peak discharge rates shall not exceed existing discharge rates for the 2-year, 5-year, 10-year, and 100-year (50 percent, 20 percent, 10 percent, and 1 percent probability), 24-hour duration storm events. Hydrologic modeling to calculate the flood levels resulting from the 2-year, 5-year, 10-year, and 100-year, 24-hour duration storm events shall be submitted for review.
      7.   Precipitation And Storm Distribution Data: Atlas 14 precipitation and storm distribution data shall be used for all hydrologic/hydraulic analyses.
      8.   Emergency Overflow: The city will require the incorporation of emergency overflows (e.g., swales, spillways), where feasible, into non-landlocked basin outlet structure designs to prevent undesired flooding resulting from storms larger than the 100-year (1 percent) event or plugged outlet conditions.
      9.   Pond Flow: All ponds shall use multistage outlets where needed to control flows from smaller, less frequent storms and help maintain base flows in downstream open channels. Pipes entering wet ponds shall have the invert elevation set 0.5 feet below the pond normal water level.
      10.   Culvert Outlet Velocities: For culvert outlet velocities less than or equal to four (4) fps, check shear stress to determine if vegetation or riprap will be adequate. If vegetation is used, temporary erosion control during and immediately following construction shall be used until vegetation becomes established. For velocities greater than four (4) fps, energy dissipaters shall be designed in accordance with MnDOT design criteria.
      11.   Skimming Devices: The placement of skimming devices at the outlet of all on site detention basins to capture trash and floatable debris is required.
      12.   Landlocked Basins: For landlocked basin areas only the existing tributary area will be allowed to discharge to a landlocked basin, unless provisions have been made for an outlet from the basin. The water quality and flooding impacts of proposed outlets from landlocked basins on downstream water resources will be evaluated. Easements obtained for landlocked basin outlets should include continuous lands along the flow path between the natural overflow (NOF) location and the downstream water body or storm sewer inlet.
      13.   LFE Of Structure Adjacent To Inundation Area With Outlet: The lowest floor elevation (LFE) of any structure adjacent to an inundation area with an outlet shall be at least two feet (2') above the 100-year flood level. For the purposes of this section, "structure" means a walled and roofed building, including gas or liquid storage tanks, which are principally above ground. The term includes recreational vehicles and travel trailers on site for more than one hundred eighty (180) days.
      14.   LFE Of Structure Adjacent To Landlocked Basin: The lowest floor elevation (LFE) of any structure (as described in subsection C13 of this section) adjacent to a landlocked basin or inundation area shall meet the following standards and as illustrated in figure 1 of this section:
         a.   Scenario 1: Where the 100-year flood level is zero to six feet (6') below the natural overflow elevation (NOF) the LFE shall be the greater of the 100-year elevation plus two feet (2') or the NOF plus one foot (1'). The NOF must be maintained and an easement obtained over the NOF. Easements obtained for landlocked basin outlets should include continuous lands along the flow path between the NOF location and the downstream water body or storm sewer inlet.
         b.   Scenario 2: Where the 100-year flood level is six (6) to eighteen feet (18') below the NOF, the LFE shall be the 100-year elevation plus six feet (6'). The overflow shall consist of a drop inlet having a minimum pipe diameter of eighteen inches (18") and an invert set four feet (4') above the 100-year flood level. The overflow and overflow pipe routing shall include an easement corridor between the NOF and the downstream water body or storm sewer inlet.
         c.   Scenario 3: Where the 100-year flood level is more than eighteen feet (18') below the NOF, the LFE shall be the 100-year elevation plus ten feet (10'). The overflow shall include an easement corridor and contingency for a future gravity outlet or lift station outlet along a path for future downstream force main, gravity sewer, or overland routing.
         d.   For all landlocked basins, the LFE shall be set at least one foot (1') above the greater of the back to back 100-year storm event and a 100-year, 10-day snowmelt.
   FIGURE 1
   LOWEST FLOOR ELEVATION STANDARDS FOR LANDLOCKED BASINS
 
      15.   Structures Or Fill In Inundation Area: The city may allow nonbuilding structures or fill within an inundation area, provided the structure or fill is placed in an area where the 100-year flood level is not more than eighteen inches (18") in depth in the location of the activity, and the activity does not result in any loss of flood storage volume or result in an increase in the regulatory flood elevation.
   D.   Mitigation For Permanent Facilities On Non-Linear Projects: If the applicant, in consultation with the director of public works, believes that the requirements for volume control, TP and/or TSS cannot be cost-effectively met on the site of the original construction activity, the applicant may request to pursue mitigation off site for all or a portion of the volume requirement that cannot be created on site, and provide appropriate documentation to the city as support for a request to pursue mitigation. The proposed mitigation must meet the following criteria:
      1.   Mitigation project areas should be selected in the following order of preference and in consultation and with approval by the city:
         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 (or city subwatershed area shown in the WRMP) as the original construction activity.
         c.   Locations in the next adjacent DNR catchment area (or city subwatershed area shown in the WRMP) upstream.
         d.   Priority locations within the city.
      2.   Mitigation projects must involve the creation of new structural stormwater BMPs, the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP.
      3.   Routine maintenance of structural stormwater BMPs required by this section cannot be used to meet mitigation requirements.
      4.   Mitigation projects must be completed within twenty four (24) months after the start of the original construction activity.
      5.   If the mitigation project is a private structural stormwater BMP and the city is not responsible for long term maintenance of the project, the city will require written and recorded documentation of maintenance responsibilities.
   E.   Models/Methodologies/Computations: Hydrologic models and design methodologies used for determining runoff characteristics and analyzing stormwater management structures shall be as set forth in the "Inver Grove Heights Stormwater Manual - Northwest Area" (2006) and all subsequent revisions and as directed by the director of public works.
   F.   Legal Documents: Legal documents for securing permanent easements as necessary shall be submitted for review. Easements extending up to at least the 100-year flood elevation over floodplains, detention areas, wetlands, ditches, and all other parts of the stormwater system shall be conveyed to the city. (Ord. 1326, 11-28-2016; amd. Ord. 1435, - -2022)