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MCWD Stormwater Management Rule MINNEHAHA CREEK WATERSHED DISTRICT BOARD OF MANAGERS REVISIONS PURSUANT TO MINNESOTA STATUTES §103D.341 Adopted May 26, 2011 Effective June 1, 2011 STORMWATER MANAGEMENT RULE 1. POLICY. It is the policy of the Board of Managers to: (a) Promote abs traction of precipitation and stormwater runoff where feasible for the purposes of improving water quality , increasing groundwater recharge , reducing flooding , and promoting the health of native and designed plant communities and landscapes ; (b) Preserve, maintain and improve the aesthetic, physical, chemical and biological composition of surface waters and groundwater within the District; (c) Limit or reduce stormwater runoff from drainage within the watershed to decrease the negative effects of land -dis turb ing activities on surface water quality and flooding ; (d) Protect and maintain existing groundwater flow, promote groundwater recharge and improve groundwater quality and aquifer protection; (e) Promote the preservation and use of native vegetation for the purpose of stormwater runoff abstraction and pollutant load reduction ; (f) Promote nondegradation of water quality from new development and improvement in water quality from redevelopment ; and (g) Promote the management of stormwater on site for the purposes of providing local groundwater recharge and maintaining natural hydrology. 2. REGULATION. No one may create new or replace existing impervious surface or change the contours of a parcel of land in a way that affects the direction, peak ra te , volume, or water quality of runoff flows from the parcel or subdivide a parcel of one acre or more in size into three or more lots without first submitting a stormwater management plan to the District and securing a permit from the District approving t he plan . New d evelopment is subject to s ection s 3 and 7 -11 below (see Table 2). Redevelopment is subject to s ection s 3 -5 and 7 -11 below (see Tables 3 and 4). Subdivision of land is subject to s ection 3 -5 and 7 -11, as applicable. Linear Transportation Proj ects are subject to s ection s 3 and 6 -11 below (see Table 5). 2 Activity subject to this rule on adjacent sites under common or related ownership shall be considered in the aggregate , and t he requirements applicable to the activity under this rule will be d etermined with respect to all development that has occurred on the site, or on adjacent sites under common or related ownership, since the date this rule took effect (January 2005). The following activities are exempt from this rule: (a ) SINGLE FAMILY HOMES: Construction or reconstru ction of a single - family home . (b ) NEW DEVELOPMENT: N ew development for a residential , commercial, industrial or institutional use (see Table 2): (1) that will result in less than 20 percent impervious surface over the si te ; or (2) on a site of less than one acre . (c) REDEVELOPMENT: Re development for a residential, commercial, industrial or institutional use (see Table 3 ): (1) on a site that is less than five acres in size that will result in at least a ten percent reducti on in impervious surface ; or (2) on a site of five acres or greater where the proposed activity disturbs less than 40 percent of the site and results in at least a ten percent reduction in impervious surface . (d ) LINEAR TRANSPORTATION PROJECTS: C onstruction of a new or reconstruction of an existing road , trail, sidewalk, utility, or oth er linear transportation project (see Table 5): (1) that will create less than 10,000 square feet of new impervious surface ; or (2) for the construction of sidewalks and tra ils that will not exceed 12 feet in width and will be bordered on the downgradient side (s ) by a pervious buffer averaging at least one -half the width of the sidewalk or trail . 3 . STORMWATER MANAGEMENT PLAN GENERAL REQUIREMENTS. A stormwater management plan submitted to the District must meet the following requirements, subject to the provisions in section s 4 -8 : (a) PHOSPHORUS CONTROL . (1) NEW DEVELOPMENT /LINEAR TRANSPORTATION PROJECTS : Activity subject to this rule for new development or linear transportat ion projects shall result in no net increase in phosphorus loading from existing conditions , except that : 3 i. F or a parcel in existing use for row crop agriculture or feedlot , new development shall result in no net increase in phosphorus loading from the site as modeled in meadow condition. (2) REDEVELOPMENT : Phosphorus control must be provided in accordance with subsection 3(c)(2), where applicable . (b) RATE CONTROL . (1) Activity subject to this rule shall result in no net increase in the peak runoff rate for the 1 -, 10 - and 100 -year design storms where stormwater discharges across the downgradient site boundary, compared to the rate for the site in its existing condition , except that : i. F or a parcel in use for row crop agriculture or feedlot , new development shall r esult in no net increase in the peak runoff rate from the site as modeled in meadow condition. (2) Peak runoff rates for the 1 -, 10 - and 100 -year design storms may not increase within a specific drainage area of the site so as to create or exacerbate drainage or erosion problems. (c) VOLUME CONTROL. (1) The stormwater management plan must provide for the abstraction of the first one inch of rainfall from the site’s impervious surface. Credit toward compliance with the one inch volume control standard will be calculated by the applicant using industry accepted hydrologic models and Appendix A: Volume Abstraction Credit Schedule, following guidance provided in the Minnesota Pollution Control Agency’s Minnesota Stormwater Manual . (2) Where an applicant demonstrate s that it is infeasible to meet the one inch abstraction requirement through use of volume control credits pursuant to subsection 3(c)(1), the stormwater management plan must provide for abstraction of runoff to the greatest extent feasible , and at least 0 .5 inches, and phosphorus control in an amount equivalent to that which would be achieved through abstraction of one inch of rainfall from the site’s impervious surfaces. To demonstrate infeasibility of providing abstraction pursuant to 3(c)(1), the a pplic ant must submit a completed Abstraction Analysis contain ing at a minimum the following information : i. A narrative that lists and explains the variables that limit the feasibility of providing one inch of volume control for runoff from the sites imperviou s surface. These variables may include but are not limited to unified soil classification, soil contamination, proximity to bedrock, 4 proximity to groundwater, proximity to existing utilities, spatial constraints, zoning requirements, and financial conside rations. ii. A narrative and conceptual plan(s) that describe s and discuss es how reasonable modifications to the size, scope, configuration or density of the project would influence the feasibility of providing one inch of volume control for runoff from t he sites impervious surface. iii. An explanation of efforts undertaken by the applicant to accommodate or remove the constraints that influence the feasibility of providing one inch of volume control for runoff from the site ’s impervious surface. (3) The vol ume of runoff draining to a landlocked receiving area may not increase due to a project unless the applicant can demonstrate that any additional runoff volume from the project will be effectively abstracted . In addition, the applicant shall either own or h ave proper rights over the landlocked property receiving runoff from the project area . Back -to -back 100 -year runoff events will be used to analyze holding capacity and high -water elevation for landlocked areas. (d) BEST MANAGEMENT PRACTICES (BMPs). (1) BMPs addressing the potential water resource impacts associated with the proposed activity must be incorporated to limit creation of impervious surface, maintain or enhance on -site infiltration and peak flow control and limit pollutant generation on and dischar ge from the site. BMPs may include site design, structural and non -structural practices. (2) BMPs must be designed and installed in accordance with generally accepted design practices and guidance contained in the Minnesota Pollution Control Agency’s M inn esota Stormwater Manual and its subsequent revisions. (e) HIGH WATER ELEVATION . (1) All applications shall provide at least two vertical feet of separation between low openings of structures and the 100 -year high water elevations of stormwater BMPs and wate rbodies. 4 . REDEVELOPMENT REQUIREMENTS – DECREASE OR NO CHANGE IN IMPERVIOUS SURFACE. A stormwater management plan submitted to the District that proposes through redevelopment to decrease or result in no net increase in impervious surface must meet the following requirements (s ee Table 3 ): (a) For sites that are one acre or less, Best Management Practices are required in accordance with subsection 3(d); 5 (b ) For sites that are between one acre and five acres and the proposed activity disturbs less than 40 percent of the site, Best Management Practices are required in accordance with subsection 3(d); (c ) For sites that are between one acre and five acres and the proposed activity disturbs 40 percent or more of the site, the stormwater management plan mu st meet the volume control requirement in subsection 3(c) and the p hosphorus c ontrol requirement in subsection 3(a)(2), where applicable ; (d ) For sites that are greater than five acres and the proposed activity disturbs less than 40 percent of the site, B est Management Practices are required in accordance with subsection 3(d); (e) For sites that are greater than five acres and the proposed activity disturbs 40 percent or more of the site , the stormwater management plan must meet the volume control require ment in subsection 3(c) and the phosphorus control requirement in subsection 3(a)(2), where applicable . 5. REDEVELOPMENT REQUIREMENTS – INCREASED IMPERVIOUS SURFACE. A stormwater management plan submitted to the District that proposes to increase imperv ious surface through redevelopment must meet the following requirements (s ee Table 4 ): (a) For sites that are one acre or less, Best Management Practices are required in accordance with subsection 3(d); (b ) For sites that are greater than one acre and th e proposed activity disturbs less than 40 percent of the site and results in an increase in impervious surface of less than 50 percent, the phosphorus control requirements of subsection 3 (a), rate control requirements of subsection 3(b) and volume control requirements of subsection 3(c) apply to the area of increased impervious surface; (c ) For sites that are greater than one acre and the proposed activity disturbs 40 percent or more of the site , or results in a n increase in impervio us surface of 50 perce nt or more , the phosphorus control requirements of subsection 3(a), rate control requirements of subsection 3(b), and volume control requirements of subsection 3(c) apply to the entire site . 6 . LINEAR TRANSPORTATION PROJECT REQUIREMENTS (see Table 5). (a) The c onstruction of a new road, trail, sidewalk, utility, or other linear transportation project that will create 10,000 square feet or more of impervious surface must meet the phosphorus control requirements in accordance with subsection 3(a), rate c ontrol requirements in accordance with subsection 3(b) and volume control requirement s in accordance with subsection 3(c); 6 (b) Linear R econstruction P rojects that will increase the impervious area within the project limits by between 10,000 square feet an d one acre from existing conditions must meet the phosphorus control requirements in accordance with subsection 3 (a) and rate control requirements in accordance with subsection 3(b) for the area of increased impervious surface; (c) Linear R econstruction P rojects that will increase the impervious area within the project limits by one acre or more from existing conditions must meet the phosphorus control requirements in accordance with subsection 3(a), rate control requirements in accordance with subsection 3(b), and volume control requirements in accordance with subsection 3(c) for the area of increased impervious surface. 7. REGIONAL STORMWATER MANAGEMENT . (a) An applicant may comply with this rule by providing equal or greater phosphorus control, rat e control, or volume control through a regional or subwatershed plan approved by the District; such a plan must provide for an annual accounting to the District of treatment capacity created and utilized by projects or land -disturbing activities within the drainage and treatment area of the plan. (b) District approval of a regional or subwatershed plan will be based on a determination that : (1) the use of a regiona l facility in place of onsite stormwater management will not result in adverse impacts to l ocal groundwater or natural resources located upstream of the regional facility , including , but not limited to, reduced water quality, altered wetland hydrology, changes to stream velocities or baseflow , erosion, or reduced groundwater recharge ; and (2) the plan incorporates on site BMPs as necessary to mitigate impacts and provide local benefits not provided by the regional facility. (c) Individual project sites utilizing a regional facility to meet phosphorus, rate, or volume control requirements must i ncorporate BMPs on the project site in accordance with subsection 3(d). (d) The applicant, before commencing any land -altering activity, must demonstrate that it holds the legal rights necessary to discharge to the stormwater facility or facilities in the plan, and that the facility or facilities are subject to a maintenance document satisfying the requirements of s ection 11 . 8. IMPACT ON DOWNSTREAM WATERBODIES . (a) No new point source may discharge to a waterbody without pretreatment for sediment and nutri ent removal. Pretreatment may be provided by non -structural means. An activity changing flow that discharges from an existing point source is not a new point source. 7 (b) No activity subject to this rule may alter a site in a manner that results in a (n ): (1) Increase in the bounce in water level for any downstream lake or wetl and beyond the limit s specified in Table 1 below based on management classification, during a rainfall event of critical duration with a return frequency of 1, 10, or 100 years . (2) In crease in the duration of inundation for any downstream lake or wetland beyond the limit s specified in Table 1 below based on management classification , during a precipitation event of critical duration with a return frequency of 1, 10, or 100 years. (3) Change in the elevation of the runout control of any lake or wetland beyond the limits specified in Table 1 below based on management classification. Table 1 : Impacts on downstream waterbodies Wetland Management Class / Waterbody Permitted Bounce fo r 1 -, 10 -, and 10 0 -Year Event Inundation Period for 1 -Year Event Inundation Period for 10 - and 100 -Year Event Runout Control Elevation Preserve Existing Existing Existing No change Manage 1 Existing plus 0.5 feet Existing plus 1 day Existing plus 2 days No change Manage 2 Existing plus 1.0 feet Existing plus 2 days Existing plus 14 days 0 to 1.0 ft above existing runout Manage 3 No limit Existing plus 7 days Existing plus 21 days 0 to 4.0 ft above existing runout Lakes Existing N/A N/A No change 9 . FINANCIAL ASSURANCE. (a) A performance bond, letter of credit or other financial assurance, consistent with the District Financial Assurance Rule, may be required for any project that requires the installation of stormwater best management practices. The financial assurance shall be maintained until the stormwater best management practice has been constructed and stabilized in accordance with District rules and as shown on a set of as built drawings submitted to the District . 8 10 . REQUIRED EXHIBITS. (a) Plans certified by a professional engineer registered in the State of Minnesota and reflecting the following items shall accompany the permit application (one set of plans must be full size; one set must be reduced to a maximum size of 11" x 17"; provide e lectronic ArcGIS or CADD files when available ): (1) Property lines and delineation of lands under ownership of the applicant. (2) Delineation of the subwatershed contributing runoff from off -site and proposed and existing subwatersheds on -site. (3) Pro posed and existing location s , alignment s , and elevation s of stormwater facilities . (4) Delineation of existing on -site wetland, shoreland, and/or floodplain areas. (5) Existing and proposed normal, and 100 year high water elevations on -site. (6) Existin g and proposed site contour elevations at two foot intervals, related to National Geodetic Vertical Datum (NGVD ), 1929 datum. (7) Construction plans and specifications for all proposed stormwater management facilities. (8 ) Stormwater runoff volume and r ate analyses for the 1 -, 10 - and 100 - year design storms for existing and proposed conditions. (9 ) All hydrologic, water quality, and hydraulic computations completed to design the proposed stormwater management facilities including runoff volume abstract ions. (10) Delineation of any flowage easements or other property interests dedicated to stormwater management purposes, including, but not limited to, county or judicial ditches. (b ) For applications proposing infiltration, a s oil sampling plan and the resulting identification, description, permeability, and approximate delineation of site soils . Investigation methods shall include soil pits or hand augers. Borings at the location of the infiltration facility must extend at least five feet deeper than t he proposed bottom elevation of the infiltration facility. (c ) For applications proposing tree preservation or planting, a s ite map showing existing trees larger than six inches in diameter, including species, diameter, and associated drip lines (canopy area). Tree map must designate trees to be removed and trees to be added. 9 (d ) For applications proposing soil amendments, a soil amendment plan following guidance from the Minnesota Pollution Control Agency ’s Minnesota Stormwater Manual . (e ) For applic ations proposing capture and reuse, an o perating plan and calculations that quantify the benefits of the proposed stormwa ter reuse system . (f ) Documentation indicating conformance with an existing municipal stormwater management plan. When a municipal pla n does not exist, documentation that the municipality has reviewed the project. (g ) Documentation that the applicant has applied for a National Pollutant Discharge Elimination System (NPDES) Permit if required by the Minnesota Pollution Control Agency (MP CA ). (h ) Abstraction analysis (if applicable) in accordance with subsection 3(c )(2 ). (i ) A declaration and maintenance agreement in conformance with s ection 1 1 . 11 . MAINTENANCE . (a) All stormwater management structures and facilities must be designed for maintenance access and properly maintained in perpetuity to assure that they continue to function as designed. Permit applicants must provide a maintenance plan that identifies and protects the design, capacity and functionality of onsite and offsite s tormwater management facilities; specifies the methods, schedule and responsible parties for maintenance; provides for the maintenance in perpetuity of the facility; and contains at a minimum the requirements in the District’s standard maintenance declarat ion. The plan will be recorded on the deed in a form acceptable to the District. A public entity assuming the maintenance obligation may do so by filing with the District a document signed by an official with authority. 10 Table 2: Stormwater management requirements for new development Site Size Impervious Surface Requirements < 1 acre N/A None ≥ 1 acre < 20% of site None ≥ 20% of site Phosphorus Control , Rate Control, and Volume Control Table 3 : Stormwater management requirements for r edevelopment resulting in a decrease or no change in impervious surface Site Size Site Dis turbance Impervious Surface Reduction Requirements ≤ 1 acre N/A 10% reduction in impervious surface None 0 - 9% reduction in impervious surface Incorporate BMPs 1 - 5 acre s < 40 % site disturbance 10% reduction in impervious surface None 0 - 9% reduction in impervious surface Incorporate BMPs ≥ 40 % site disturbance 10% reduction in impervious surface None 0 - 9% reduction in impervious surface Volume control required for site ’s impervious surface ≥ 5 acre s < 40 % site distur bance 10% reduction in impervious surface None 0 - 9% reduction in impervious surface Incorporate BMPs ≥ 40 % site disturbance N/A Volume control required for site ’s impervious surface 11 Table 4 : Stormwater management requirements for r edeve lopment resulting in an increase in impervious surface Site Size Site Disturbance Impervious Surface Increase Requirements Treatment Scope ≤ 1 acre N/A N/A Incorporate BMPs N/A > 1 acre < 40% s ite d isturbance < 50% i ncrease in i mpervious sur face Phosphorus Control, Rate Control, and Volume Control Additional i mpervious s urface ≥ 50% i ncrease in i mpervious surface Entire site’s impervious s urface ≥ 40% s ite d isturbance N/A Phosphorus Control, Rate Control, and Volume Control Entire s ite’s i mpervious s urface Table 5: Stormwater management requirements for linear transportation projects Project Type Impervious Surface Increase Requirements Treatment Scope New Linear Transportati o n Project < 10,000 squ are feet None N/A ≥ 10,000 square feet Phosphorus Control , Rate Control, and Volume Con trol New i mpervious s urface Linear Reconstruction Project < 10,000 square feet None N/A ≥ 10,000 square feet and < 1 acre Phosphorus Control and Rate Control Additional impe rvious surface ≥ 1 acre Phosphorus Control , Rate Control, and Volume Con trol Additional impervious surface APPENDIX A: MCWD Volume Abstraction Credit Schedule Practice Design Guidance Credit Calculation Methods Surface Infiltration Basin Mi nnesota Stormwater Manual Volume provided AV (1) = Volume below overflow elevation (2) Underground Infiltration Trench Minnesota Stormwater Manual Void volume provided AV = Volume below overflow elevation (2) Preservation of tree(s) Not Applicable Percent interception by species AV = % Interception (3) * tree canopy area (4) * 1 inch rainfall Planting of New Tree(s) Not Applicable One -half p ercent interception by species (5) AV = 0.5 * % Interception (3) * tree canopy area (4) * 1 inch rainfall Soil Ame ndment(s) Minnesota Stormwater Manual 0.5 -inch credit over the area of soil amendment area (6) AV = 0.5/12 * area of soil amendment Capture and Reuse of Stormwater Submit pump design plans and hydrologic calculations Volume capacity to capture and reuse runoff from a 1 -inch rainfall event Submit operating plan and calculations for reuse system to document annual volume reuse during dry, wet, and average years Enhancement of Pervious Area (7) (wetland buffer s , forest or prairie conservation or restoration ) Submit vegetation planting and maintenance plan 0.5 -inch credit over the area of enhancement (8) AV = 0.5/12 * area of enhancement Filtration Minnesota Stormwater Manual 50% volume abstraction credit (9) AV = 0.5 * Volume below overflow elevation (filte red volume is not considered) (1) AV = Abstraction Volume (2) Volume infiltrated during a rainfall event shall not be credited towards the abstraction volume requirement. This is a simple approach for designers and for reviewers to verify conformance to th e standard; a stormwater model is not needed for calculations. This is a conservative assumption because infiltration of stormwater in Minnesota is an evolving practice. MCWD will continue to research current trends, collect and analyze monitoring dat a, and utilize modeling and engineering methods to assess the effectiveness of the standards to achieve the water quality goals of the District. (3) Percent rainfall interception shall be determined using results from the City of Minneapolis, Minnesota Mu nicipal Tree Resource Analysis . Percentages for the species studied are listed below. If desired tree species is not listed, the applicant shall use the median value provided below or provide documentation by a certified arborist to support a different p ercent interception. Average Percent Rainfall Interception by Tree Species Species Average Percent Rainfall Interception Green Ash 13 Sugar Maple 8 Norway Maple 8 Littleleaf Linden 12 American Elm 18 Honeylocust 6 American Basswood 10 Northern Hac kberry 6 Ginkgo 4 Silver Maple 16 Elm 21 White Ash 10 Basswood 14 Red Maple 7 Median 10 (4 ) Tree canopy area must be documented as part of the permit application submittal . (5 ) Granting ½ credit for new trees is intended to encourage preservation of trees over tree removal and replacement . (6) For SCS TR -55 cover type “open space (lawns),” compacted soil (HSG C, curve number 74) begins to generate runoff with a 0.9 -inch rainfall. A HSG B soil (curve number 61) begins to generate runoff with a 1.5 -i nch rainfall. Therefore, preserving the infiltration capacity of HSG B soil through the use of soil amendments yields an approximate 0.5 -inch volume reduction credit . (7) Area shall not be subject to motorized vehicle, bicycle, or likely human foot traf fic (i.e., parking lot islands, conventional landscaping). (8 ) For SCS TR -55 cover type “herbaceous mixture,” additional rainfall of approximately 0.5 inches generates no runoff if the hydrologic con dition is improved from “fair” to “good.” C redit will no t be granted for “tree preservation” and “enhancement of pervious area.” The applicant must designate the desired abstraction practice . (9) The Minnesota Stormwater Manual reports that nutrient removal (total phosphorus) is approximately half as effective for filtration as infiltration.