A. MODELING REQUIREMENTS FOR STORMWATER MANAGEMENT PLANS.
a. A hydrograph method or computer program based on NRCS Technical Release #20 (TR- 20) and subsequent guidance must be used to analyze stormwater runoff for the design or analysis of discharge and water levels within and off the project site. The runoff from pervious and impervious areas within the model shall be modeled separately.
b. In determining Curve Numbers for the post-development condition, the Hydrologic Soil Group (HSG) of areas within construction limits shall be shifted down one classification for HSG C (Curve Number 80) and HSG B (Curve Number 74) and ½ classification for HSG A (Curve Number 49) to account for the impacts of grading on soil structure unless the project specifications incorporate soil amendments in accordance with available Soil Amendment Guidelines, such as the MPCA Minnesota Stormwater Manual, or Rice Creek Watershed District guidance. This requirement only applies to that part of a site that has not been disturbed or compacted prior to the proposed project.
c. The Analysis of flood levels, storage volumes, and discharge rates for waterbodies and stormwater management basins must include the NOAA Atlas 14 values, as amended, using a nested rainfall distribution (e.g., MSE 3), for the 2-year, 10-year and 100-year return period, 24-hour rainfall events and the 10-day snowmelt event (Curve Number 100), in order to identify the critical duration flood event. The City Engineer may require analysis of additional precipitation durations to determine the critical duration flood event. Analysis of the 10-day snowmelt event is not required for stormwater management detention basins with a defined outlet elevation at or below the 100-year return period, 24-hour rainfall event elevation.
B. STORMWATER MANAGEMENT PLAN FRAMEWORK.
a. When an existing regional BMP is proposed to manage stormwater runoff, the proposed total impervious surface area must be equal to or less than the impervious surface allocated within the original approved stormwater plan for that site. If an impervious surface area was not specified within the original approved stormwater plan for the site, the applicant shall show that the BMP was designed and constructed to manage the stormwater runoff from the project site, the applicant has permission to utilize any remaining capacity in the BMP, the BMP is subject to maintenance obligations enforceable by the City, and it is being maintained to its original design.
b. Stormwater management plans, with the exception of those for single family residential developments, must specify the proposed impervious surface area draining to each BMP for each land parcel.
c. A combination of Stormwater BMPs may be used to meet the requirements of paragraphs C, D, and E of this section.
d. The proposed project must not adversely affect off-site water levels or resources supported by local recharge, or increase the potential for off-site flooding, during or after construction.
e. A landlocked basin may be provided an outlet only if it:
i. Conforms with Section 7D-709 and all other wetland requirements, as applicable.
ii. Provides sufficient dead storage volume to retain the runoff resulting from back-to-back 100-year, 24-hour rainfall events.
iii. Does not create adverse downstream flooding or water quality conditions as a result of the change in the rate, volume or timing of runoff or a change in drainage patterns.
C. WATER QUALITY TREATMENT.
a. Development creating or reconstructing impervious surface shall apply Better Site Design (BSD) techniques as outlined in the MPCA Minnesota Stormwater Manual as amended (www.stormwater.pca.state.mn.us).
b. Sediment shall be managed on-site to the maximum extent practicable before runoff resulting from new or reconstructed impervious surface enters a waterbody or flows off-site.
c. WATER QUALITY TREATMENT STANDARDS.
i. The required water quality treatment volume standards for all projects, except Public Linear Projects, is determined as follows:
Required Water Quality Treatment Volume (ft3) | = | Area of New or Reconstructed Impervious Surface (ft2) | x | 1.1 (in) | ÷ | TP Removal Factor from Table C1 | ÷ | 12 (in/ft) |
ii. The required water quality treatment volume standard for Public Linear Projects is determined as follows:
Required Water Quality Treatment Volume (ft3) | = | {Greater of} | ||||
Area of New Impervious Surface (ft2) | x | 1.0 (in) | ÷ | 12 (in/ft) | ||
{OR} | ||||||
Sum Area of New and Reconstructed Impervious Surface (ft2) | x | 0.5 (in) | ÷ | 12 (in/ft) |
iii. For alternative Stormwater BMPs not found in Table C1 or to deviate from TP Removal Factors found in Table C1, the applicant may submit a TP Removal Factor, expressed as annual percentage removal efficiency, based on supporting technical data, for City approval.
iv. Stormwater runoff treated by the BMP during a rain event will not be credited towards the treatment requirement.
TABLE C1. TP REMOVAL FACTORS FOR PROPERLY DESIGNED BMPS.
BMP | BMP Design Variation | TP Removal Factor * |
BMP | BMP Design Variation | TP Removal Factor * |
Infiltration ** | Infiltration Feature | 1.00 |
Water Reuse ** | Irrigation | 1.00 |
Biofiltration | Underdrain | 0.65 |
Filtration | Sand or Rock Filter | 0.50 |
Stormwater Wetlands | Shallow Wetland | 0.40 |
Pond/Wetland | 0.55 | |
Stormwater Ponds *** | Wet Pond | 0.50 |
Multiple Pond | 0.60 | |
Source: Adapted from Table 7.4 from the Minnesota Stormwater Manual, MPCA. *Refer to MPCA Stormwater Manual for additional information on BMP performance. Removal factors shown are average annual TP percentage removal efficiencies intended solely for use in comparing the performance equivalence of various BMPs. **These BMPs reduce runoff volume. ***Stormwater ponds must also provide 2.5" of dead storage as required by Section 7D-708(F)(d)(ii). | ||
d. BMP LOCATIONAL SITING.
i. BMPS shall be located either on-site to treat runoff at the point of generation, or regionally within the primary drainage area.
ii. If infiltration is feasible on site (see Table C2), on-site or regional BMPs must provide volume control to meet the standard of Section 7D-708(C)(c).
iii. If infiltrations is not feasible, any BMP may be used.
iv. Off-site and/or regional BMPs must be sited in the following priority order:
1. In a downstream location that intercepts the runoff volume leaving the project site prior to the Resource of Concern.
2. Anywhere within the same primary drainage area that results in no greater mass of Total Phosphorus reaching the resource of concern than on-site BMPs.
TABLE C2. SPECIFIC CONDITIONS THAT MAY RESTRICT INFILTRATION.
Type | Specific Project Site Conditions | Required Submittals |
Type | Specific Project Site Conditions | Required Submittals |
Potential Contamination | Potential Stormwater Hotspots (PSH) | PSH Locations and Flow Paths |
Contaminated Soils | Documentation of Contamination Soil Borings | |
Physical Limitations | Low Permeability Soils (HSG C & D) | Soil Borings |
Bedrock within three vertical feet of bottom of infiltration area | Soil Borings | |
Seasonal High Water Table within three vertical feet of bottom of infiltration area | Soil Borings High Water Table | |
Karst Areas | Geological Mapping or Report | |
Land Use Limitations | Utility Locations | Site Map |
Nearby Wells (Private and/or Municipal) * | Well Locations | |
*Refer to Minnesota Stormwater Manual or the Minnesota Department of Health for setback requirements. | ||
e. Stormwater runoff from all new and reconstructed impervious surface must be captured be captured and treated for total phosphorus if feasible. Notwithstanding, runoff from undisturbed impervious surface not otherwise being treated prior to the Resource of Concern may be treated in lieu of treating new or reconstructed impervious surface, provided the runoff from that surface drains to the same surface water resource as the new/reconstructed surface not being treated. Except for Public Linear projects, the area not treated for phosphorus may not exceed 15 percent of all the new or reconstructed impervious surface. For all untreated surface, TSS must be removed to the maximum extent practicable. Total water quality treatment volume for the project must be provided in aggregate pursuant to Section 7D-708(C)(c) Section 7D-708(C)(d) except that for a Public Linear Project, water quality treatment volume for reconstructed impervious surface, if required by Section 7D-707(B)(c) must be provided only to the extent feasible.
f. For single-family residential development, the runoff from impervious surface other than parking or driving surface that, in the City's judgment. cannot reasonably be routed to a stormwater BMP is considered effectively treated for water quality if:
i. The length of the flow path across the impervious surface is less than the length of the flow path across the pervious surface to which it discharges; and
ii. The pervious surface is vegetated and has an average slow of five percent or less.
D. PEAK STORMWATER RUNOFF CONTROL.
a. Peak stormwater runoff rates for the proposed project at the project site boundary, in aggregate, must not exceed existing peak runoff rates for the 2-year, 10-year and 100-year, 24-hour rainfall events, or a different critical event duration at the discretion of the City Engineer. Notwithstanding, peak runoff may be controlled to standard in a regional facility consistent with paragraph (D)(b). Aggregate compliance for all site boundary discharge will be determined with respect to runoff not managed in a regional facility.
b. Any increase in a critical duration flood event rate at a specific point of discharge from the project site must be limited and cause no adverse downstream impact. Table C3 shows the maximum curve numbers that may be utilized for existing condition modeling of those project site areas not covered by impervious surface.
c. Within the Flood Management Zone only (refer to FEMA Special Flood Hazzard Areas and/or other local flood information), peak runoff rates for the 2-, 10- and 100-year 24-hour rainfall events shall be reduced to ?80% of the existing condition. This requirement does not apply if the project is a Public Linear Project.
TABLE C3. CURVE NUMBERS FOR EXISTING CONDITION PERVIOUS AREAS.
Hydrologic Soil Group | Runoff Curve Number * |
A | 39 |
B | 61 |
C | 74 |
D | 80 |
*Curve numbers from NRCS Technical Release #55 (TR-55). | |
TABLE C4. HYDROPERIOD STANDARDS.
Wetland Susceptibility Class | Permitted Storm Bounce for 2- Year and 10-Year Event * | Inundation Period for 2-Year Event * | Inundation Period for 10-Year Event * |
Highly susceptible | Existing | Existing | Existing |
Moderately susceptible | Existing plus 0.5 ft | Existing plus 1 day | Existing plus 7 days |
Slightly susceptible | Existing plus 1.0 ft | Existing plus 2 days | Existing plus 14 days |
Least susceptible | No limit | Existing plus 7 days | Existing plus 21 days |
Source: Adapted from: Stormwater and Wetlands Planning and Evaluation Guidelines for Addressing Potential Impacts of Urban Stormwater and Snowmelt Runoff on Wetlands. *Duration of 24-hours for the return periods utilizing NOAA Atlas 14. | |||
E. BOUNCE AND INUNDATION PERIOD.
a. The project must meet the hydroperiod standards found in Table C4 with respect to all down-gradient wetlands.
b. Wetland Susceptibility Class is determined based on wetland type, as follows:
i. Highly susceptible wetland types include sedge meadows, bogs, coniferous bogs, open bogs, calcareous fens, low prairies, coniferous swamps, lowland hardwood forests, and seasonally flooded water basins.
ii. Moderately susceptible wetland types include: shrub-carrs, alder thickets, fresh (wet meadows, and shallow and deep marshes.
iii. Slightly susceptible wetland types include: floodplain forests and fresh wet meadows or shallow marshes dominated by cattail giant reed, reed canary grass or purple loosestrife.
iv. Least susceptible wetland includes severely degraded wetlands. Examples of this condition include cultivated hydric soils, dredge/fill disposal sites and some gravel pits.
F. DESIGN CRITERIA.
a. Infiltration BMPs must be designed to provide:
i. Adequate pretreatment measures to remove sediment before runoff enters the primary infiltration area;
ii. Drawdown within 48-hours from the end of a storm event. Soil infiltration rates shall be based on the appropriate HSG classification and associated infiltration rates (see Table C5). The least permeable layer of the soil boring column must be utilized in BMP calculations (see Design Criteria (5). Alternate infiltration rates based on a recommendation and certified measurement testing from a licensed geotechnical engineer or licensed soil scientist will be considered. Infiltration area will be limited to horizontal areas subject to prolonged wetting;
iii. A minimum of three feet of separation from the Seasonal High Water Table; and
iv. Consideration of the Minnesota Department of Health guidance document Evaluating Proposed Stormwater Infiltration Projects in Vulnerable Wellhead Protection Areas (dated January 26, 2016, as amended). Documentation shall be submitted to support implementation of this guidance document and will be accepted at the discretion of the City Engineer.
b. Water Reuse BMPs must conform to the following:
i. Design for no increase in stormwater runoff from the irrigated area or project site.
ii. Required design submittal packages for water reuse BMPs must include:
1. An analysis using Metropolitan Council Stormwater Reuse Guide 'Water Balance Tool Irrigation Constant Demand' spreadsheet for irrigation practices or 'Water Balance Tool Non-Irrigation Constant Demand' spreadsheet for non-irrigation practices. The tools are available download at: http://www.metrocouncil.org/wastewater-water/planning/water-supply-planning/studies-projects-workgroups-(1)/completed-studies- projects/stormwater-reuse-guide.aspx;
2. Documentation demonstrating adequacy of soils, storage system, and delivery system; and
3. Operations plan.
iii. Approved capacity of an irrigation practice will be based on:
1. An irrigation rate of 0.5 inches per week over the irrigated pervious area(s) or the rate identified through the completion of the Metropolitan Council Stormwater Reuse Guide 'Water Balance Tool Irrigation Constant Demand' Spreadsheet (whichever is less); or as approved by the City; and
2. No greater than a 26 week (April 15th to October 15th) growing season.
3. An additional water quality treatment capacity beyond 0.5 inches per week may be recognized under a stormwater management plan or a phased development permit based on an average of three consecutive years of monitoring records of volume irrigated and pursuant to a monitoring plan approved by the City.
iv. Approved capacity of a non-irrigation practice shall be based on the rate identified through the completion of the Metropolitan Council Stormwater Reuse Guide 'Water Balance Tool Non-Irrigation Constant Demand' spreadsheet, or as approved by the City.
c. Biofiltration/filtration BMPs must be designed to provide:
i. Adequate pretreatment measures to remove sediment before runoff enters the primary biofiltration area;
ii. Drawdown within 48-hours from the end of a storm event;
iii. A minimum of 12-inches of organic material or sand above the rock trench or drain tile system; and
iv. Drain tile system must be designed above the Seasonal High Water Table.
TABLE C5. SOIL TYPE AND INFILTRATION RATES.
Hydrologic Soil Group | Soil Textures | Corresponding Unified Soil Classification | Infiltration Rate (in/hr) | |
Hydrologic Soil Group | Soil Textures | Corresponding Unified Soil Classification | Infiltration Rate (in/hr) | |
A | Gravel Sandy Gravel Silty Gravels | GW | Well-graded gravels, sandy gravels | 1.63 |
GP | Gap-graded or uniform gravels, sandy gravels | |||
GM | Silty gravels, silty sandy gravels | |||
SW | Well-graded gravelly sands | |||
Sand Loamy Sand Sandy Loam | SP | Gap-graded or uniform sands, gravelly sands | 0.8 | |
B | Loam Silt Loam | SM | Silty sands, silty gravelly sands | 0.45 |
MH | Micaceous silts, diatomaceous silts, volcanic ash | 0.3 | ||
C | Sandy Clay Loam | ML | Silts, very fine sands, silty or clayey fine sands | 0.2 |
D | Clay Loam Silty Clay Loam Sandy Clay Silty Clay Clay | GC | Clayey gravels, clayey sandy gravels | 0.06 |
SC | Clayey sands, clayey gravelly sands | |||
CL | Low plasticity clays, sandy or silty clays | |||
OL | Organic silts and clays of low plasticity | |||
CH | Highly plastic clays and sandy clays | |||
OH | Organic silts and clays of high plasticity | |||
Source: Adapted from the "Design infiltration rates" table from the Minnesota Stormwater Manual, MPCA, (January 2014). | ||||
d. Stormwater ponds must be designed to provide:
i. Water quality features consistent with NURP criteria and accepted design standards for average and maximum depth;
ii. A permanent wet pool with dead storage at least equal to the runoff volume from a 2.5-inch rainfall over the area tributary to the pond;
iii. An outlet structure capable of preventing migration of floating debris and oils for at least the one-year storm;
iv. An identified emergency overflow spillway sufficiently stabilized to convey flows greater than the 100-year critical storm event; and
v. An outlet structure to control the 2-year, 10-year and 100-year frequency events.
e. Soil borings (utilizing ASTM D5921 and D2488, as amended) shall be considered for design purposes, and provided to the City, for each proposed BMP. The soil borings must be taken to a depth of at least 5 feet below the bottom of the proposed feature.
f. An outfall structure discharging directly to a wetland, public water or public water wetland must incorporate a stilling-basin, surge-basin, energy dissipater, placement of ungrouted natural rock riprap or other feature to minimize disturbance and erosion of natural shoreline and bed resulting from stormwater discharges. Where feasible, outfall structures are to be located outside of the natural feature.
TABLE C6. LOW FLOOR AND LOW ENTRY FREEBOARD REQUIREMENTS.
Freeboard | 100-Year Flood Elevations | Detention Basins, Wetlands & Stormwater Ponds | Infiltration and Biofiltration Basins | Rain Gardens* | ||||
100-yr | EOF | 100-yr | EOF | Bottom | 100-yr | EOF | EOF | |
Low Floor | 2.0 ft | 1.0 ft | 0.0 ft | NA | 0.0 ft | NA | NA | NA |
Low Entry | NA | NA | 2.0 ft | 1.0 ft | NA | 2.0 ft | 1.0 ft | 0.5 ft |
g. All new residential commercial, industrial, and other habitable or non-habitable structures, and all stormwater BMPs, must be constructed so that the lowest floor and lowest entry elevations comply with Table C6. The low entry freeboard criterion of Table C6 may be deemed met when the structure does not have the required vertical separation but is protected from surface flooding to the required elevation by a berm or other natural or constructed topographic feature capable of providing flood protection.
h. Within a landlocked basin, minimum low floor elevations must be at least one foot above the surveyed basin run out elevation. Where a structure is proposed below the run out elevation of a land-locked basin, the low floor elevation will be a minimum of two feet above the highest water level of either the 10-day snowmelt event or back-to-back 100-year, 24- hour rainfalls. Aerial photos, vegetation, soils, and topography may be used to derive a "normal" water elevation for the purpose of computing the basin's 100-year elevation.
i. All stormwater management structures and facilities must be designed for maintenance access and be properly operated and maintained in perpetuity to assure that they continue to function as designed. The maintenance responsibility must be memorialized in a document executed by the property owner in a form acceptable to the City and filed for record on the deed. Alternatively, a public permittee may meet its perpetual maintenance obligation by executing a programmatic or project-specific maintenance agreement with the City. Regional ponds owned by public entities that are only used to meet the runoff rate requirements of the City Code do not need a maintenance agreement with the City.
j. The permitee must use construction best practices so that the facility as constructed will conform to design specifications and the soil and surrounding conditions are not altered in a way adverse to facility performance.
k. Before work under the permit is deemed complete, the permittee must submit as-built plans demonstrating that at the time of final stabilization, stormwater facilities conform to design specifications. If at any time the City finds that the stormwater facility is not performing as designed, on City request the permittee must undertake reasonable investigation to determine the cause of inadequate performance.
G. EASEMENTS.
a. Before permit issuance, the permittee must, submit a copy of any plat or easement required by the local land use authority establishing drainage or flowage over stormwater management facilities, stormwater conveyances, ponds, wetlands, on-site floodplain up to the 100-year flood elevation, or any other hydrologic feature.
b. Before permit issuance, the permittee must convey to the City an easement to the public drainage system specifying a City right of maintenance access over the right of way of the public drainage system as identified within the public drainage system record. If the right of way of the public drainage system is not described within the record, then the easement shall be conveyed with the following widths:
i. For tiled/piped systems, 40 feet wide perpendicular to the direction of flow, centered on the tile line or pipe;
ii. For open channel systems, a width that includes the channel and the area on each side of the channel within 20 feet of top of bank. For adequate and safe access, where top of bank is irregular or obstruction exists, the City may specify added width.
c. Public Linear Projects are exempt from the public drainage system easement requirement of Section G(2).
d. Refer to local, state, and federal wetland rules for additional easement requirements near wetlands.
H. REQUIRED EXHIBITS. The following exhibits must accompany the permit application. The vertical datum must clearly be labeled on each plan set.
a. An erosion and sediment control plan and, for projects that require an NPDES permit, a Storm Water Pollution Prevention Plan.
b. Property lines and delineation of lands under ownership of the applicant.
c. Delineation of the sub watershed contributing runoff from off-site, proposed and existing sub watersheds onsite, emergency overflows, and drainageways.
d. Geotechnical analysis including soil borings at all proposed stormwater management facility locations utilizing ASTM D5921 and D2488, as amended.
e. Proposed and existing stormwater facilities location, alignment and elevation.
f. Delineation of existing on-site wetland, marshes and floodplain area.
g. Identification of existing and proposed normal, ordinary high and 100-year water elevation on-site.
h. Identification of existing and proposed contour elevations within the project site.
i. Construction plans and specifications of all proposed stormwater management facilities, including design details for outlet control structures.
j. Stormwater runoff volume and rate analyses for the 2- 10- and 100-year critical events, existing and proposed conditions utilizing NOAA Atlas 14.
k. All hydrologic, water quality and hydraulic computations completed to design the proposed stormwater management facilities.
l. Narrative including a project description, discussion of BMP selection, and revegetation plan for the project site.
m. Other project site-specific submittal requirements as may be required by the City.
I. EXCEPTIONS.
a. A permit is not required for single-family residential construction on an individual lot of record, if the proposed impervious surface of the lot is less than 10,000 square feet, excluding the driveway. If the lot is within a development previously approved by the City, the construction must conform to the previous approval.
b. Section 7D-707 requirements do not apply to sidewalks and trails 10 feet wide or less that are bordered down-gradient by vegetated open space or vegetated filter strip with minimum width of 5 feet.
c. Section 7D-707 requirements do not apply to Bridge Spans and Mill, Reclamation and Overlay projects.
d. Section 7D-708(C) and Section 7D-708(D) requirements do not apply to single family residential subdivisions creating seen or few lots that:
i. Establish no new public roadway; and
ii. Include no private roadway/driveway serving three or more lots.
e. Criteria of Section 7D-708(D) may be waived if the project site discharges directly to a water body with large storage capacity (such as a public water), the volume discharged from the project site does not contribute to a downstream flood peak, and there are no downstream locations susceptible to flooding.
f. Section 7D-708(C) and Section 7D-708(D) are waived for a portion of a project that paves a gravel roadway if the right-of-way ditch is maintained and does not discharge a concentrated flow directly to a wetland or another sensitive waterbody.
J. EXTENDED PERMIT TERM AND REGIONAL FACILITIES FOR NON-RESIDENTIAL PHASED DEVELOPMENT.
a. The following definitions apply to this section:
i. "Area Development Permit" (ADP) means a City stormwater management permit for non-residential development that includes construction of a stormwater management facility explicitly intended to serve compliance requirements for a parcel other than that on which the facility is located.
ii. "Phased Development Permit" (PDP) means a City stormwater management permit for non-residential development that includes construction of a stormwater management facility explicitly intended to serve compliance requirements not just for development under the permit, but also for subsequent development on that parcel or a contiguous parcel under common ownership.
b. If an off-site stormwater management facility approved under a prior ADP cannot be used for compliance due to a rule change occurring since the date of ADP approval, the City nevertheless by permit will approve its use, subject to the following:
i. The applicant must demonstrate that the facility was built in compliance with the ADP, that the ADP identified the development site as one that may use the facility, and that the requirements of Section 7D-708 subsection B(a) above, are met.
ii. If the current rule requires a level of peak flow or volume control, or of water quality treatment, beyond that provided by the off-site facility, the applicant must provide for the additional treatment. This does not disallow use of an existing facility on the ground that it does not meet a sequencing requirement with respect to the BMP location or type.
The protection against rule change provided by this subsection J(b) does not apply if the City makes written findings, on the basis of new knowledge or information, that use the facility would have a material adverse impact on water quality, flood management or other specific public interest, or if the approval date of the development permit is more than 10 years after the date of ADP approval.
c. The City may issue a PDP with a permit term of up to 10 years.
i. During the permit term, development using the stormwater management facilities approved under the PDP will not be subject to a rule change occurring after the date of PDP approval, provided the PDP states the design criteria to which subsequent development will conform and the proposed development meets those criteria.
ii. This section does not apply to an ADP or a PDP approved before December 1, 2004.
[§ 7D-708 amended by Ord. No. 07-02, effective March 1, 2007; amended by Ord. No. 23-04, effective August 10, 2023.]