NR 151.24(4)(a) (a) By design, BMPs shall be employed to maintain or reduce the peak runoff discharge rates, to the maximum extent practicable, as compared to pre-development site conditions for the 2-year, 24-hour design storm applicable to the transportation facility. Pre-development conditions shall assume “good hydrologic conditions” for appropriate land covers as identified in TR-55 or an equivalent methodology. The meaning of “hydrologic soil group” and “runoff curve number” are as determined in TR-55. However, when pre-development land cover is cropland, rather than using TR-55 values for cropland, the runoff curve numbers in Table 2 of subch. III shall be used.

NR 151.24 Note Note: The curve numbers in Table 2 represent mid-range values for soils under a good hydrologic condition where conservation practices are used and are selected to be protective of the resource waters.

NR 151.24(4)(b) (b) This subsection does not apply to:

NR 151.24(4)(b)1. 1. A transportation facility where the change in hydrology due to development does not increase the existing surface water elevation at any point within the downstream receiving surface water by more than 0.01 of a foot for the 2-year, 24-hour storm event.

NR 151.24 Note Note: Hydraulic models such as HEC-RAS or another methodology may be used to determine the change in surface water elevations.

NR 151.24(4)(b)2. 2. A highway reconstruction site.

NR 151.24(4)(b)3. 3. A transportation facility that is part of a redevelopment project.

NR 151.24 Note Note: The intent of sub. (4) is to minimize streambank erosion under bank full conditions.

NR 151.24(5) (5) Infiltration.

NR 151.24(5)(a) (a) Except as provided in pars. (d) to (g), BMPs shall be designed, installed and maintained to infiltrate runoff to the maximum extent practicable in accordance with one of the following:

NR 151.24(5)(a)1. 1. Infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 60% of the pre-development infiltration volume, based on an average annual rainfall. However, when designing appropriate infiltration systems to meet this requirement, no more than 2% of the project site is required as an effective infiltration area.

NR 151.24(5)(a)2. 2. Infiltrate 10% of the post-development runoff volume from the 2-year, 24-hour design storm with a type II distribution. Separate curve numbers for pervious and impervious surfaces shall be used to calculate runoff volumes and not composite curve numbers as defined in TR-55. However, when designing appropriate infiltration systems to meet this requirement, no more than 2% of the project site is required as an effective infiltration area.

NR 151.24(5)(b) (b) Pre-development condition shall be the same as specified in sub. (4) (a).

NR 151.24 Note Note: A model that calculates runoff volume, such as SLAMM, P8 or an equivalent methodology may be used. Information on how to access SLAMM and P8 is available from the storm water coordinator in the runoff management section of the bureau of watershed management at dnr.wi.gov.

NR 151.24(5)(c) (c) Before infiltrating runoff, pretreatment shall be required for parking lot runoff and for runoff from new road construction in commercial, industrial and institutional areas that will enter an infiltration system. The pretreatment shall be designed to protect the infiltration system from clogging prior to scheduled maintenance and to protect groundwater quality in accordance with par. (g). Pretreatment may include, but is not limited to, oil/grease separation, sedimentation, biofiltration, filtration, swales or filter strips.

NR 151.24 Note Note: To minimize potential groundwater impacts it is desirable to infiltrate the cleanest runoff. To achieve this, a design may propose greater infiltration of runoff from low pollutant sources such as roofs, and less from higher pollutant source areas such as parking lots.

NR 151.24(5)(d) (d) The following are prohibited from meeting the requirements of this subsection:

NR 151.24(5)(d)1. 1. Areas associated with tier 1 industrial facilities identified in s. NR 216.21 (2) (a), including storage, loading, rooftop and parking.

NR 151.24(5)(d)2. 2. Storage and loading areas of tier 2 industrial facilities identified in s. NR 216.21 (2) (b).

NR 151.24 Note Note: Runoff from tier 2 parking and rooftop areas may be infiltrated but may require pretreatment.

NR 151.24(5)(d)3. 3. Fueling and vehicle maintenance areas.

NR 151.24(5)(d)4. 4. Areas within 1000 feet upgradient or within 100 feet downgradient of karst features.

NR 151.24(5)(d)5. 5. Areas with less than 3 feet separation distance from the bottom of the infiltration system to the elevation of seasonal high groundwater or the top of bedrock.

NR 151.24(5)(d)6. 6. Areas with runoff from industrial, commercial and institutional parking lots and roads and residential arterial roads with less than 5 feet separation distance from the bottom of the infiltration system to the elevation of seasonal high groundwater or the top of bedrock.

NR 151.24(5)(d)7. 7. Areas within 400 feet of a community water system well as specified in s. NR 811.16 (4) or within 100 feet of a private well as specified in s. NR 812.08 (4) for runoff infiltrated from commercial, industrial and institutional land uses or regional devices for residential development.

NR 151.24(5)(d)8. 8. Areas where contaminants of concern, as defined in s. NR 720.03 (2), are present in the soil through which infiltration will occur.

NR 151.24(5)(d)9. 9. Any area where the soil does not exhibit one of the following characteristics between the bottom of the infiltration system and seasonal high groundwater and top of bedrock:

NR 151.24(5)(d)9.a. a. At least a 3-foot soil layer with 20% fines or greater.

NR 151.24(5)(d)9.b. b. At least a 5-foot soil layer with 10% fines or greater.