All subdivision design shall incorporate adequate provisions for stormwater runoff consistent with the WRMP, water resources management plan as amended, and with established city policies, and conform to the following standards:
A. Plan Required: The proposed provisions for stormwater runoff shall be documented in a runoff water management plan including a narrative, prepared by a Minnesota licensed engineer to the minimum standards described in subsection B of this section.
B. Minimum Standards For Runoff Water Management Plans: A runoff water management plan shall include the following items:
1. A map containing a delineation of the subwatershed contributing runoff from off site, and proposed and existing subwatersheds on site. The delineation shall conform to the nomenclature of the water resources management plan and shall indicate any significant departures from the watershed delineation of the water resources management plan.
2. Delineation of existing on site "wetlands", as defined in the wetland conservation act, lakes, watercourses, shoreland, and/or floodplain areas.
3. For water bodies and channels, a listing of normal (runout) and calculated 10-year and 100-year elevations on site for both existing and proposed conditions.
4. Stormwater runoff volumes and rates for existing and proposed conditions.
5. All hydrologic and hydraulic computations completed to design the proposed stormwater management facilities. Reservoir routing procedures and critical duration runoff events shall be used for design of water storage areas and outlets.
6. A checklist of best management practices to demonstrate that, to the maximum extent practical, the plan has incorporated the structural and nonstructural best management practices described in the book "Protecting Water Quality In Urban Areas", published by the Minnesota pollution control agency or the applicable publications.
7. A grading plan identifying stormwater overflow routes along streets or drainage easements designed to protect structures from damage due to:
a. Storms in excess of the design storm, or
b. Clogging, collapse or other failure of the primary drainage facilities.
8. An assessment of the potential for construction or contribution to regional detention basins, as opposed to the construction of on site stormwater basins. The water resources management plan identifies potential regional stormwater basins for control of rate and nutrient loading. The following criteria shall be used to determine whether on site storage is required within the subdivision:
a. If the runoff water management plan indicates construction or enlargement of a storage site or water quality storage in a wetland or other water body, the facility or its equivalent shall be constructed to meet the goals of the water resources management plan.
b. If a proposed subdivision is indirectly tributary to a water body and intervening regional detention storage is inadequate, in the opinion of city staff, to meet the water quality goals for downstream water bodies as described in the water resources management plan, additional on site stormwater basin construction will be required to meet these goals.
c. If a proposed subdivision will increase rates of runoff and where downstream stormwater storage or conveyance facilities are inadequate to protect downstream riparian owners from effects of these increases, then on site stormwater storage must be constructed to restrict stormwater rates to the predevelopment rates.
9. Where on site water quality detention basins are required, copies of the calculations determining the design of the basins. The size and design considerations will be dependent on the receiving water body's water quality category, the imperviousness of the development and the degree to which on site infiltration of runoff is encouraged. Design of on site detention basins, as described in the site's runoff water management plan, shall incorporate recommendations from the nationwide urban runoff program (NURP) and "Protecting Water Quality In Urban Areas", published by the Minnesota pollution control agency, as adopted by the city, or the applicable publications, as adopted by the city. The following design considerations are required for on site water quality detention basins based on the receiving water's water quality category. These designs include permanent detention for water quality treatment; extended detention designs may be substituted provided that they provide treatment equivalent to the requirements below:
a. A permanent pool (dead storage) volume below the normal outlet shall be greater than or equal to the runoff from a two and one-half inch (2.5") 24-hour storm over the entire contributing drainage area assuming full development.
b. A permanent pool average depth (basin volume/basin area) which shall be greater than four feet (4'), with a maximum depth of less than ten feet (10').
c. An emergency spillway (emergency outlet) adequate to control the 100-year frequency critical duration rainfall event.
d. Basin side slopes above the normal water level should be no steeper than three to one (3:1) when possible, and preferably flatter. A basin shelf with a minimum width of ten feet (10') and a maximum slope of ten to one (10:1) below the normal water level is recommended to enhance wildlife habitat, reduce potential safety hazards, and improve access for long term maintenance.
e. To prevent short circuiting, the distance between the major inlets and normal outlet shall be maximized.
f. A flood pool (temporary storage) volume above the principal outlet spillway shall be adequate so that the peak discharge rate from the 1-, 10- and 100-year frequency critical duration storm is not greater than the peak discharge for a similar storm and predevelopment watershed conditions.
g. Extended detention of runoff from the more frequent (1-year to 5-year) storms shall be achieved through a principal spillway design which shall include a perforated vertical riser, a small orifice outlet or a compound weir. The spillway must be constructed of a limited maintenance material. The use of treated or naturally decay resistant timber shall not be allowed.
h. Effective energy dissipation devices which reduce outlet velocities to four feet (4') per second or less shall consist of riprap, stilling pools or other such measures to prevent erosion at all stormwater outfalls into the basin and at the detention basin outlet.
i. Trash and floatable debris skimming devices shall be placed on the outlet of all on site detention basins to provide treatment up to the critical duration 10-year storm event. These devices can consist of baffled weirs, submerged inlets or other such measures capable of restricting the overflow of floatable materials, including litter, oil and grease. The skimming device must be constructed of a limited maintenance material. The use of treated or naturally decay resistant timber shall not be allowed. Computations for the design of such devices shall be included. The maximum velocity through the skimming device shall be less than one foot (1') per second (fps) on the 2-year 24-hour event.
j. For purposes of erosion control, vegetation protection and wildlife habitat enhancement, the 10-year flood level of the basin shall be no more than two feet (2') above the normal level of the basin.
10. For subdivisions that create one or more acres of new impervious surface, the following volume control practices shall apply:
a. Postconstruction Water Quality Criteria:
(1) Postconstruction stormwater runoff quality measures shall meet the standard for the general permit authorization to discharge stormwater associated with construction activity under the national pollutant discharge elimination system/state disposal system permit program permit MN R100001 (NPDES general construction permit) issued by the Minnesota pollution control agency, August 1, 2008, as amended; except where more specific requirements are provided in subsections B10a(2), B10a(3) and B10a(4) of this section.
(2) Infiltration/filtration options, and credits described under runoff volume control criteria are the preferred approach to satisfying the water quality treatment requirements of the NPDES general construction permit in areas that drain to the trout stream portions of the Vermillion River and its tributaries where such areas do not first drain to a water body with ten (10) or more acres of open water.
(3) The water quality control volumes necessary to meet the NPDES general construction permit that are satisfied using infiltration or filtration technologies can count toward the volume control requirements of this section.
(4) Stormwater basins with permanent wet pools are allowed if the applicant demonstrates:
(A) That the wet stormwater basin is designed for pretreatment prior to discharge into an infiltration/biofiltration area, or
(B) That the volume control requirements of this section are met and the following measures are used to the extent practical in order of decreasing preference:
(i) The wet stormwater basin is designed with a combination of measures such as shading, filtered bottom withdrawal, vegetated swale discharges, or constructed wetland treatment cells that will limit temperature increases.
(ii) Additional volume control measures and credits are used beyond that required to meet the runoff volume standards as a means of limiting the frequency and duration of discharges from the stormwater basin.
b. Peak Runoff Rate Control Criteria:
(1) A hydrograph method based on sound hydrologic theory as approved by the city engineer will be used to analyze runoff for the development design or analysis of flows and water levels.
(2) Runoff rates for proposed activities and development shall:
(A) Not exceed existing runoff rates for the 1-year and 10-year critical duration storm events.
(B) Not exceed the existing rate for the 100-year critical duration storm event.
c. Runoff Volume Control Criteria:
(1) All new development and redevelopment, where feasible, must provide for the infiltration or retention on site of 0.5 inch of runoff from all new impervious surfaces on the developed site, except for 1.5 inches in the South Creek drainage district.
(2) Credits for site design to meet the volume control requirements may be considered and must be approved by the city engineer prior to the design of infiltration or filtration facilities.
(3) Other credits as may be approved by the city engineer.
(4) The water quality control volumes necessary to meet the NPDES general construction permit that are satisfied using infiltration or filtration technologies (filtration only on type C and D soils), can count toward the volume control requirements of this section.
(5) When using infiltration for volume control:
(A) Infiltration volumes and facility sizes shall be calculated:
(i) Using the appropriate hydrological soil group classification and saturated infiltration rate from the table below:
HydrologicSoil Type | Infiltration Rate | Soil Texture |
A | 0.30 inches/hour | Sand, loamy sand, or sand loam |
B | 0.15 inches/hour | Silt loam or loam |
C | 0.07 inches/hour | Clay loam, silty clay loam, silty clay or clay |
Source: Urban Hydrology Or Small Watersheds (SCS 1986), as amended.
or
(ii) Using documented site specific infiltration or hydraulic conductivity measurements completed by a licensed soil scientist or engineer, or
(iii) Using the method provided in the "Minnesota Stormwater Manual" volume 2 (MPCA 2005), pages 18 through 21 of chapter 12-INF; and
(B) The design shall consider the infiltration rates of the least permeable horizon within the first five feet (5') below the bottom of the infiltration practice; and
(C) The system shall be capable of infiltrating the required volume in forty eight (48) hours, except in the South Creek drainage area where the system shall infiltrate the required volume in seventy two (72) hours.
(6) Constructed infiltration facilities, such as infiltration basins and trenches:
(A) Must include pretreatment of stormwater runoff designed to protect the infiltration system from clogging with sediment and to protect groundwater quality. Pretreatment for these facilities must be designed to remove at least fifty percent (50%) of the inflow sediment loads. Pretreatment can consist of vegetative swales, filter strips, sediment forebays/traps, grit chambers or other measures;
(B) Cannot be used within four hundred feet (400') of a municipal or other community supply well or within one hundred feet (100') of a private well unless specifically allowed by an approved wellhead protection plan;
(C) Cannot be used for runoff from fueling and vehicle maintenance areas and industrial areas with significant exposed materials;
(D) Cannot be used on areas with less than three feet (3') vertical separation from the bottom of the infiltration system and the seasonal high water table, except for regional infiltration basins with the approval of the city engineer;
(E) Cannot be used in type D soils.
(7) Infiltration areas must be fenced or otherwise protected from disturbance during construction as outlined in the state of "Minnesota Stormwater Manual" (MPCA 2005), chapter 12-INF IV.
(8) Infiltration facilities must be designed so that volumes in excess of the design volume are safely conveyed into the downstream stormwater system.
(9) Volume control may be waived by the city for sites with impermeable soils or where a shallow water table prevents construction of infiltration systems. If the applicant claims that infiltration is not feasible on site, the applicant must provide supporting documentation to the city. If the city agrees that infiltration is not feasible, the applicant shall design alternative stormwater runoff treatment methods including the following in order of decreasing preference:
(A) Credits and site design practices to minimize the creation of connected impervious surfaces are used to the extent practical.
(B) Underdrains to promote filtration instead of infiltration.
(C) Off site infiltration.
(D) On site wet detention with permanent pool volume below the normal outlet that is greater than or equal to the runoff from a 2.5 inch, 24-hour storm over the entire contributing drainage area, assuming full development.
(10) Vegetation used in conjunction with infiltration systems must be tolerant of urban pollutants and the range of soil moisture conditions anticipated.
(11) Infiltration facilities must conform to the minimum setbacks required by the Minnesota department of health, as summarized below:
Setback From | Minimum Distance (Feet) |
Property line | 10 |
Building foundation (with slopes directed away from building) | 10 |
Septic system tank/leach field | 35 |
(12) Infiltration depressions shall not be lined nor shall mining of native material be allowed.
(13) In the South Creek drainage district stormwater basins shall be set back a distance of seventy five feet (75') from the stream centerline to the edge of a stormwater basin. In the case of distributed infiltration areas, this setback may be varied. Where the stormwater design is such that fifty percent (50%) of the infiltration area volume is beyond one hundred feet (100') from the channel centerline, no setback is required.
(14) In the South Creek drainage district effective energy dissipation devices which reduce outlet velocities to four feet (4') per second or less shall consist of riprap, stilling pools or other such measures to prevent erosion at all stormwater outfalls into the basin and at the detention basin outlet.
C. Easements: The applicant shall establish, in a form acceptable to the city, temporary and perpetual easements, or dedicated outlots, for ponding, flowage, and drainage purposes over hydrologic features such as water bodies and stormwater basins. The easements or outlots shall include the right of reasonable access for inspection, monitoring, maintenance, and enforcement purposes.
D. Drainage Alterations: No person or political subdivision shall artificially drain surface water, or obstruct or divert the natural flow of runoff so as to affect a drainage system, or harm the public health, safety, or general welfare, unless the drainage plan complies with the following:
1. Outlets from landlocked basins with a tributary drainage area of one hundred (100) acres or more will be allowed, provided such outlets are consistent with the Lakeville water resources management plan, this code, state and federal regulations, and the downstream impacts, riparian impacts, and habitat impacts of such outlets have been analyzed and no detrimental impacts result. The analysis and determination of detrimental impacts shall:
a. Use a hydrograph method approved by the city engineer to analyze runoff for the design or analysis of flows and water levels;
b. Ensure a hydrologic regime consistent with the peak runoff rate control criteria and the runoff volume control criteria of this section;
c. Ensure the outlet does not create adverse downstream flooding or water quality conditions, or materially affect stability of downstream major waterways;
d. Maintain dead storage within the basin to the extent possible while preventing damage to property adjacent to the basin;
e. Ensure that the low floors of new structures adjacent to the basin are set consistent with the city floodplain standards; and
f. Ensure that proposed development tributary to the landlocked basin has incorporated runoff volume control practices to the extent practical.
2. Artificial drainage, flow obstruction, and diversions involving waterways, public waters, public water wetland, wetlands with drainage areas of six hundred forty (640) acres or more will be allowed, provided such alterations or diversions are consistent with the Lakeville water resources management plan, city ordinances, state and federal regulations, and the downstream impacts, riparian impacts and habitat impacts of such alterations or diversions have been analyzed and no detrimental impacts result. Proposals for drainage alterations and diversions shall demonstrate that:
a. There is a reasonable necessity for such drainage alteration or diversion to improve or protect human health and safety, or to improve or protect aquatic resources;
b. Reasonable care has been taken to avoid unnecessary adverse impacts to upstream and downstream land;
c. The utility or benefit accruing to the land on which the drainage will be altered reasonably outweighs the gravity of the harm resulting to the land receiving the burden; and
d. The drainage alteration or diversion is being accomplished by reasonably improving and aiding the normal and natural system of drainage according to its reasonable carrying capacity, or in the absence of a practicable natural drain, a reasonable and feasible artificial drainage system is being adopted.
3. Drainage alterations, diversions, and landlocked basin outlets shall be provided with stable channels and outfall. (Ord. 866, sec. 36, 5-17-2010)