Trans 401.106(3)(3) Total suspended solids. Best management practices shall be designed, installed and maintained to control total suspended solids carried in runoff from the transportation facility as follows: Trans 401.106(3)(a)(a) For transportation facilities first constructed on or after January 1, 2003 by design, reduce the suspended solids load to the maximum extent practicable, based on an average annual rainfall, as compared to no runoff management controls. A reduction in total suspended solids by at least 80% meets the requirements of this paragraph. Trans 401.106(3)(b)(b) For highway reconstruction and non-highway redevelopment, by design, reduce to the maximum extent practicable the total suspended solids load by at least 40%, based on an average annual rainfall, as compared to no runoff management controls. A 40% or greater total suspended solids reduction shall meet the requirements of this paragraph. In this paragraph, “redevelopment” means the construction of residential, commercial, industrial or institutional land uses and associated roads as a substitute for existing residential, commercial, industrial or institutional land uses. Trans 401.106(3)(c)(c) Notwithstanding pars. (a) and (b), if the design cannot achieve the applicable total suspended solids reduction specified, the design plan shall include a written and site-specific explanation why that level of reduction is not attained and the total suspended solids load shall be reduced to the maximum extent practicable. Trans 401.106(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 or to the 2-year design storm with a duration equal to the time of concentration 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 below shall be used. Trans 401.106 NoteNote: 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.
Trans 401.106(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 or for a 2-year design storm with a duration equal to the time of concentration. Trans 401.106 NoteNote: Hydraulic models, such as HEC-2 or an equivalent methodology, may be used to determine the change in surface water elevations.
Trans 401.106(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: Trans 401.106(5)(a)1.1. Infiltrate sufficient runoff volume so that the post-construction infiltration volume shall be at least 60% of the pre-construction 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. Trans 401.106(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. Trans 401.106(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 and grease separation, sedimentation, biofiltration, filtration, swales or filter strips. Trans 401.106 NoteNote: 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.
Trans 401.106(5)(d)(d) The following are prohibited from meeting the requirements of this subsection, due to the potential for groundwater contamination: Trans 401.106 NoteNote: Runoff from tier 2 parking and rooftop areas may require pretreatment before infiltration.
Trans 401.106(5)(d)4.4. Areas within 1000 feet upgradient or within 100 feet downgradient of karst features. Trans 401.106(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. Trans 401.106(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. Trans 401.106(5)(d)7.7. Areas within 400 feet of a well serving a community water system as specified in ch. NR 811 or within 100 feet of a well serving a non-community or private water system as specified in ch. NR 812 for runoff infiltrated from commercial, industrial and institutional land uses or regional devices for residential development.
