(A) Purpose. This appendix provides assistance to applicants in calculating volume and water quality credits for best management practices (BMPs) to meet the requirements outlined in Chapter 153. For additional information on designing BMPs, please see the Carver County Water Resource Management Ordinance and BMP Guidelines.
(B) Calculation for volume and water quality treatment volume. The treatment volume is a storm event of 1.0 inch. The treatment is divided into volume reduction and water quality treatment as described below.
(1) Volume reduction. The volume to be controlled on site is 1.0 inch from the site impervious.
(2) Water quality treatment. The water quality treatment volume is calculated as 1.0 inch from the site impervious.
(C) Volume credits. Practices that can be used to meet the 1.0 inch volume requirement are described below. Methods for calculating the volume retained are included for each practice.
(1) Amended soils. The volume retained is calculated using 0.5 inches over the amended area.
(2) Bioretention basins. The volume retained is calculated as 40% of the ponded volume. No volume credit will be given for bioretention practices within three feet of vertical separation from the seasonally high groundwater or sited immediately adjacent to wet ponds and/or bioretention practices controlled by the same outlet as a wet pond (e.g. bioretention shelves or benches).
(3) Dry swale. The volume retained is calculated as 40% of the ponded volume. No volume credit will be given for bioretention practices within three feet of vertical separation from the seasonally high groundwater or sited immediately adjacent to wet ponds and/or bioretention practices controlled by the same outlet as a wet pond (e.g. bioretention shelves or benches).
(4) Stormwater reuse (irrigation). To meet the volume requirement, the volume to be retained on-site must be utilized on-site once per week for a period of 20 weeks during the growing season. The volume reduction for stormwater reuse is calculated by the area irrigated times the irrigation rate.
(5) Preservation or restoration of upland vegetation.
(a) Volume retained. The volume retained is calculated using 0.5 inches over the area preserved.
(b) The area to be preserved should consist of existing trees or predominantly native vegetation. Areas to be restored must be restored to predominantly native vegetation.
(c) The area to be preserved must be placed under easement to ensure that it continues to provide treatment in perpetuity.
(6) Green roof. The area of the green roof is excluded from the total impervious calculation, thereby reducing the total treatment volume.
(7) Pervious pavement. The volume reduction for pervious pavement is calculated as 50% of the volume below the tile outlet elevation (assumes 50% pore space below tile). The area of pervious pavement is also excluded from the total impervious calculation.
(8) Infiltration. The volume retained is calculated as 80% of the ponded volume.
(9) Biorentention shelf/bench. No volume credit will be given for biorentention practices sited immediately adjacent to wet ponds and/or biorentention practices controlled by the same outlet as a wet pond (e.g. bioretention shelves or benches).
(10) Disconnecting impervious surfaces. Volume reduction for disconnecting impervious surface is dependent upon the pervious area being routed and must have amended soils to receive volume reduction credits. Volume reduction is 0.5 inches over the area of amended soils. Impervious areas must be discharging to the amended soils as a uniform sheet flow, with a max flow path of 100 feet.
(D) Water quality credits. Practices that can be used to meet the water quality treatment requirement are described below. Methods for calculating the amount of water quality treatment provided are included for each practice. In order to be eligible for the water quality credit described below, BMPs must meet the design standards in the Carver County BMP guidelines.
(1) Removal requirements. The following removal percentages are required for the site.
(a) 90% Total Phosphorus (TP) removal for 1.0 inch off site impervious.
(b) 90% Total Suspended Sediment (TSS) removal for 1.0 inch off site impervious.
(2) TP and TSS removal percentages. The CCWMO assumes the following removal percentages for total phosphorus:
(a) Stormwater pond. For stormwater ponds designed to meet NURP design criteria, 60% TP removal can be assumed and 80% TSS removal can be assumed.
(b) Bioretention basin. For bioretention basins designed to meet the design criteria in the Carver County BMP Guidelines, 75% TP removal can be assumed and 90% TSS removal can be assumed.
(c) Iron-enhanced sand filter. For iron-enhanced sand filter systems designed to meet design criteria in the Carver County BMP Guidelines, 90% TP removal can be assumed and 90% TSS removal can be assumed.
(d) Hydrodynamic separator. TSS removal is 50% of the watershed being treated by the device. No credit is given for TP removal.
(e) Disconnecting impervious surfaces. 45% TP reduction and 75% TSS reduction with a maximum area equal to the area of impervious area discharging to the area. Discharge must be uniformed sheet flow with a max flow path of 100 feet.
(f) Volume practices.
1. Stormwater reuse. 50% TP and 50% TSS requirements can be met through reuse when treating one inch of stormwater.
2. Infiltration. 100% TP and TSS removal can be assumed for the volume naturally infiltrating.
(E) Alternative design approaches.
(1) Alternative designs. Alternative designs may be approved if, upon review, the county determines the design will provide treatment equal to or greater than the practices described in this appendix. Applicants wishing to utilize an alternative design must submit plans and specifications for the proposed design along with calculations showing compliance with the stormwater standards of Chapter 153 of this code. Calculations should be generated using industry standard models. In order to be eligible for the volume credit described below, BMPs must meet the design standards in the Carver County BMP guidelines.
(2) Site specific soil determinations. On-site information may be accepted in lieu of the Hydrologic Soil Group determinations found in the Carver County Soil Survey. The on-site determination must be completed by a state-licensed soil scientist. Information submitted must present a detailed soil profile description including, but not limited to, horizon depths, Munsell colors, USDA textural classifications, bulk density analysis, and saturated hydraulic conductivity tests to a minimum depth of eight feet. Geotechnical soil borings alone are not acceptable. If requested, an on-site meeting can be held for further determination. If the initial determination is disputed, the applicant must submit a minimum number of soil profile locations in the disputed area based on one per soil group and one per five acres or as agreed upon between the county and the applicant in the disputed area. A final determination of on-site soils will be made by a consensus of the county, Carver SWCD and the LGU following review by state soil licensed staff and/or consultant.
(3) Infiltration rates. Design infiltration rates from the most recent version of the Minnesota Stormwater Manual shall be used to calculate the area and draw-down time period for infiltration BMPs. Percolation tests can be conducted and submitted to determine the actual rate of infiltration after the sub-grading is established.
(F) References. Additional information and design guidelines are available in the following documents:
(1) "Carver County Water Resource Management Ordinance and BMP Guidelines," as amended from time to time.
(2) MPCA's "Minnesota Stormwater Manual", as amended from time to time.
(Ord. 75-2012, passed 6-26-12; Am. Ord. 83-2016, passed 9-20-16)