Stormwater Management Procedure

MAC Local Government Unit (LGU) Municipal Permit Requirements

The Metropolitan Airports Commission (MAC) has been granted a Municipal Permit with LGU status by the Lower Minnesota River Watershed District (LMRWD) for Minneapolis / St. Paul International Airport (MSP). This section outlines MAC’s stormwater management policy to meet the intent of LMRWD’s Rule D: Stormwater Management Rule while considering unique aspects of airport operations. This policy will be enforced for new development, redevelopment, and drainage alterations at MSP that exceed one (1) acre of new impervious surfaces. The MAC intends to meet with the Lower Minnesota Watershed District annually to review upcoming projects.

An overall hydraulic and hydrologic stormwater model using XP-SWMM software was developed by TKDA and is updated periodically to reflect existing conditions. The model consists of nodes (manholes, catch basins, diversion structures) and links (storm sewer, ditches, weirs, overland flow) with definitions based on survey data. As of January 2024, there are 1,092 individual drainage areas totaling 2,920 acres. Drainage area delineations, impervious coverage and land use are based on survey data, aerial imagery, and LIDAR topography. Outer boundaries of the model coincide with the XP-SWMM model boundaries of Minnehaha Creek Watershed District to the North and the MnDOT I-494 design-build model to the South and West. The model includes storage definitions for Pond 1, Pond 2, Pond 3, Pond 4, 494 Pond, MnDOT Almaz Pond, runway medians, surface depressions, other ponds, and infiltration basins. Special parameters include the runway 17-35, 12R-30L, Y-3, and W-Y tunnels and pumps, pond diversion structures and high flow bypasses. Outfalls include three discharge points to the Minnesota River, including from the 494 Pond, Highway 5 Outfall, and from Pond 4. Using actual rainfall and flow monitoring data, the model was calibrated to better reflect observed conditions. The MAC will update this model as projects that have potential impacts to drainage systems or land cover are completed.

One of the key differences between this stormwater management policy for MSP and LMRWD’s Rule D is that new infiltration and new detention/retention basins are not encouraged for MSP in the policy. In early 2020, the Federal Aviation Administration (FAA) published Advisory Circular (AC) No: 150/5200-33C – Hazardous Wildlife Attractants on or near Airports. The advisory states that detention/retention ponds encourage wildlife to enter an airport’s approach/departure airspace or aircraft operations area and increase the risk of aircraft collisions. Aircraft collisions with birds and other wildlife are a serious economic and public safety problem. During the past century, wildlife-aircraft strikes have resulted in the loss of hundreds of lives worldwide and billions of dollars in aircraft damage.

An overall water quality model using P8 Urban Catchment Model (P8) software was developed by TKDA for each major watershed at MSP. The stormwater treatment system at MSP includes stormwater management best management practices such as vegetated buffer strips, wet detention ponds, and infiltration basins. Most of the runways at MSP are considered indirectly connected impervious land cover, because runoff flows through grass medians before entering storm sewers. Most of the treatment is provided by wet detention Ponds 1 and 2. Accumulated sediment in the ponds is removed by dredging every four years on average. P8 simulates the generation and transport of stormwater runoff pollutants using continuous water-balance and mass-balance calculations. Simulations are driven by existing data for hourly precipitation and daily air temperature at MSP. Runoff contributions from snowmelt are also simulated. Predicted water quality components include total suspended solids (TSS) and total phosphorus (TP) expressed in terms of percentage reduction and accumulated loading. Hydrology and hydraulic parameters were imported from the overall MSP XP-SWMM model and used as a basis for the watershed definitions. The MAC will update this model as projects that have potential impacts to drainage systems or land cover are completed.

1. Policy
   1. As the LGU, it is MAC’s policy to:
      1. manage new development, redevelopment, and drainage alternations by performing reviews for each development or land-disturbing activity to confirm that the project will manage its stormwater effectively;
      2. limit discharges of stormwater to downstream receiving waters, including the Minnesota River;
      3. control the rate and volume of stormwater runoff originating from MSP so that surface water and groundwater quantity and quality is protected or improved, soil erosion is minimized, and flooding potential is reduced;
      4. protect and improve natural resources within the watershed to prevent further degradation; and
      5. meet annually with the Lower Minnesota Watershed District to review upcoming projects.
2. Regulation
   1. A formal stormwater management review process, conducted by MAC or their assigned Engineer, is required for any land-disturbing activity involving the development, redevelopment, reconstruction, and drainage alterations that create new impervious areas greater than one (1) acre.
3. Exceptions
   1. A formal stormwater management review process is not required for the following activities:
      1. Rehabilitation of paved surfaces, such as impervious surface mill, reclamation, overlay, or paving of an existing rural section gravel road, where the underlying structural aggregate base is not removed.
      2. Maintenance activities or in-kind replacements, such as catch basin repair and replacement, utility repair and replacement, pipe repair and replacement, lighting, and pedestrian ramp improvements.
      3. Trails, sidewalks, and retaining walls that do not exceed 10 feet in width and are bordered down gradient by a pervious area extending at least half the trail width.
      4. Land-disturbing activities that do not involve creation of new impervious surface, reconstruction of existing impervious surface, or grading that materially alter stormwater flow at a site boundary.
4. Criteria
   1. Rate Control
      1. Stormwater runoff rates will be evaluated using hydraulic and hydrologic stormwater modeling to verify that the proposed activity shall not lead to an increase in the existing runoff rates as measured at the discharge point to the Minnesota River. Where a localized approach may be more appropriate, rate comparisons may be further upstream than the discharge point to the Minnesota River. The design storm events to which this standard applies are the 1 or 2-year, 10-year, and 100-year 24-hour events using NOAA Atlas 14 precipitation depths and MSE type 3 rainfall distribution.
      2. Compliance with this Rate Control criteria may be demonstrated by either of the following options:
         1. Comparison of the existing overall XP-SWMM model with a proposed version of the XP-SWMM model updated to reflect the proposed changes. Confirm runoff rates do not increase at the downstream discharge point to the Minnesota River, including discharges from the 494 Pond, Highway 5 Outfall, or from Pond 4.
         2. A site-specific stormwater model of existing and proposed conditions demonstrating that runoff rates are not exceeded at the discharge point from the site.
   2. Volume Reduction
      1. To the extent practicable, volume reduction practices shall be included in each project. The recommendations of FAA Advisory Circular 150/5200-33C regarding Hazardous Wildlife Attractants on or near Airports shall apply regarding the use of infiltration, detention, and retention basins. Stormwater harvesting for graywater or irrigation use is an example of a volume reduction practice that meets the FAA Advisory Circular criteria.
      2. In addition, an MPCA Alternative Compliance sequencing flow chart will be prepared for each project, including a narrative indicating evidence of proposed volume reduction practices to the extent practicable (see Attachment).
      3. Infiltration practices will not be allowed in the following locations:
         1. Areas that receive discharge from fueling and maintenance activities.
         2. Areas with less than three feet of separation from the bottom of an infiltration system and elevation of seasonally saturated soils or top of bedrock.
         3. Areas that receive discharges from industrial facilities that are not authorized to infiltrate industrial stormwater under an NPDES/SDS Industrial Stormwater Permit issued by the MPCA.
         4. Areas where infiltrating stormwater will mobilize subsurface water pollutants from past industrial activities.
         5. Areas of predominately Hydrologic Soil Group D (clay) soils, unless allowed by an LGU with a current NPDES/SDS Municipal Separate Storm Sewer Systems (MS4) permit.
         6. Areas within 1,000 feet up gradient or 100 feet down gradient of active karst features, unless allowed by an LGU with a current MS4 permit.
         7. Areas within a Drinking Water Supply Management Area (DWSMA), as defined in Minnesota Administrative Rules 4720.5100, subpart 13., unless allowed by an LGU with a current MS4 permit.
         8. Areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour or as allowed by an LGU with a current MS4 permit.
   3. Water Quality
      1. Water quality modeling will be used to verify that the proposed activity does not cause the removal efficiency, as measured at the discharge point to the Minnesota River, to fall below 50% removal of total phosphorus (TP) and 80% removal of total suspended solids (TSS) on an average annual basis. The MAC will update the P8 Urban Catchment model as projects are completed.
   4. Maintenance
      1. Stormwater management elements of the proposed activity shall be maintained in accordance with the following:
         1. All stormwater management structures and facilities must be designed for maintenance access and properly maintained in perpetuity so that they continue to function as designed.
         2. A maintenance plan shall identify and protect the design, capacity, and functionality of stormwater management facilities; specify the methods; and schedule responsible parties for maintenance for every stormwater management facility.
5. Required Information and Exhibits
   1. Narrative
      1. Description of existing and proposed conditions including total disturbance area, existing and proposed impervious surfaces, and any modifications or additions to stormwater management practices.
   2. Stormwater Modeling
      1. Stormwater modeling summary of the existing and proposed conditions to demonstrate compliance with the criteria for rate control (4.1) and water quality (4.3).The MAC will update the XP-SWMM model for each project completed based on the following criteria:
         1. Changes to impervious coverage exceeding 10,000 square feet.
         2. Storm sewer modifications that affect conveyance capacity.
         3. Changes to flow paths that affect drainage patterns.
   3. Construction Plans
      1. Construction plans including the following, as applicable:
         1. Removals Plan
         2. Site Plan
         3. Grading Plan
         4. Storm Sewer Plan & Profile
         5. Stormwater Management Plan
            1. Proposed and existing stormwater facility locations, alignment, and elevation
            2. Delineation of existing wetlands, shoreland, and/or floodplain areas
            3. Design details for outlet control structures
         6. Landscaping / Restoration Plan
         7. Stormwater Erosion and Sediment Control Plan
   4. Soils
      1. If infiltration is proposed, a geotechnical investigation report is required. The report shall include:
         1. Soil borings and infiltration testing in accordance with the Minnesota Stormwater Manual guidelines.
         2. No evidence of groundwater or redoximorphic soil conditions within three (3) feet of the bottom of the facility, practice or system.
         3. Soil conditions within five (5) feet of the bottom of any stormwater treatment facility, practice, or system.
         4. If requested by the engineer, site-specific infiltration capacity of soils at the bottom of the facility, practice or system.
         5. Where there is potential for soil contamination, submission of a Phase I Environmental Site Assessment and/or other documentation.
   5. Stormwater Harvesting
      1. If a stormwater harvesting practice is proposed, the following are required:
         1. Documentation of storage capacity and delivery systems.
         2. Delineation of green space to be irrigated, if applicable.
6. Attachments
   1. MIDS Design Sequence Flow Chart (MPCA Minnesota Stormwater Manual).