3. Off-Site Drainage. When designing BMPs, runoff draining to the BMP from off-site shall be taken into account in determining the treatment efficiency of the practice. Any impact on the efficiency shall be compensated for by increasing the size of the BMP accordingly.
(b) PEAK DISCHARGE.
1. By design, BMPs shall be employed to maintain or reduce the 1-year, 24-hour; the 2-year, 24-hour post-construction peak runoff discharge rates to the 1-year, 24-hour; and the 2-year, 24-hour pre-development peak runoff discharge rates respectively, or to the maximum extent practicable. The runoff curve numbers in Table 2. shall be used to represent the actual pre-development conditions. Peak discharges shall be calculated using TR-55 runoff curve number methodology, Atlas 14 precipitation depths, and the appropriate NRCS Wisconsin MSE3 or MSE4 precipitation distribution. On a case-by-case basis, the [administering authority] may allow the use of TP-40 precipitation depths and the Type II distribution.
Note to Users: For determining compliance with the peak flow requirement, the Wisconsin Department of Natural Resources recommends use of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 Precipitation Frequency Estimates for precipitation depth. The Natural Resources Conservation Service (NRCS) – Wisconsin has calculated county-specific Atlas 14 precipitation depths and they are to be used in combination with the appropriate NRCS MSE3 or MSE4 precipitation distribution. The NRCS calculated county-specific Atlas 14 precipitation depths and MSE3 and MSE4 precipitation distributions are available at: http://www.nrcs.usda.gov/wps/portal/nrcs/detail/wi/technical/engineering/?cid=nrcs142p2_025417. Note to Users: When the pre-development condition is a combination of woodland, grassland, or cropland, the runoff curve number should be pro-rated by area.
2. This subsection of the ordinance does not apply to any of the following:
a. A post-construction site where the discharge is directly into a lake over 5,000 acres or a stream or river segment draining more than 500 square miles.
b. Except as provided under S. 07 (3), a redevelopment post-construction site.
c. An in-fill development area less than 5 acres.
Note to Users: The intent of the peak discharge standard is to minimize stream bank erosion, under bank-full conditions. For water quantity concerns, the post-development peak flow rate for the 10-, 25-, 50-, and 100-year – 24-hour storm events should also be controlled either at or below pre-development discharge rates. This has not been addressed in this model ordinance but may need to be included in the local ordinance to address local flood control issues.
(c) INFILTRATION.
1. Best Management Practices. BMPs shall be designed, installed, and maintained to infiltrate runoff in accordance with the following or to the maximum extent practicable:
a. Low imperviousness. For development up to 40 percent connected imperviousness, such as parks, cemeteries, and low density residential development, infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 90 percent 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 one percent of the post-construction site is required as an effective infiltration area.
b. Moderate imperviousness. For development with more than 40 percent and up to 80 percent connected imperviousness, such as medium and high density residential, multi-family development, industrial and institutional development, and office parks, infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 75 percent 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 percent of the post-construction site is required as an effective infiltration area.
c. High imperviousness. For development with more than 80 percent connected imperviousness, such as commercial strip malls, shopping centers, and commercial downtowns, infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 60 percent 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 percent of the post-construction site is required as an effective infiltration area.
2. Pre-development. The pre-development condition shall be the same as specified in Table 2 of the Peak Discharge section of this ordinance.
3. Source Areas.
a. Prohibitions. Runoff from the following areas may not be infiltrated and may not qualify as contributing to meeting the requirements of this section unless demonstrated to meet the conditions identified in S. 07 (4) (c) 6.:
i. Areas associated with a tier 1 industrial facility identified in s. NR 216.21 (2) (a), Wis. Adm. Code, including storage, loading, and parking. Rooftops may be infiltrated with the concurrence of the regulatory authority. ii. Storage and loading areas of a tier 2 industrial facility identified in s. NR 216.21 (2) (b), Wis. Adm. Code. Note to Users: Runoff from the employee and guest parking and rooftop areas of a tier 2 facility may be infiltrated but runoff from the parking area may require pretreatment.
iii. Fueling and vehicle maintenance areas. Runoff from rooftops of fueling and vehicle maintenance areas may be infiltrated with the concurrence of the regulatory authority.
b. Exemptions. Runoff from the following areas may be credited toward meeting the requirement when infiltrated, but the decision to infiltrate runoff from these source areas is optional:
i. Parking areas and access roads less than 5,000 square feet for commercial development.
ii. Parking areas and access roads less than 5,000 square feet for industrial development not subject to the Prohibitions under par a.
iii. Except as provided under S. 07 (3), redevelopment post-construction sites.
iv. In-fill development areas less than 5 acres.
v. Roads on commercial, industrial, and institutional land uses; and arterial residential roads.
4. Location of Practices.
a. Prohibitions. Infiltration practices may not be located in the following areas:
i. Areas within 1000 feet upgradient or within 100 feet downgradient of direct conduits to groundwater.
ii. Areas within 400 feet of a community water system well as specified in s. NR 811.16 (4), Wis. Adm. Code, or within the separation distances listed in s. NR 812.08, Wis. Adm. Code, for any private well or non-community well for runoff infiltrated from commercial, including multi-family residential, industrial and institutional land uses, or regional devices for one- and two-family residential development. iii. Areas where contaminants of concern, as defined in s. NR 720.03 (2), Wis. Adm. Code, are present in the soil through which infiltration will occur. b. Separation distances.
i. Infiltration practices shall be located so that the characteristics of the soil and the separation distance between the bottom of the infiltration system and the elevation of seasonal high groundwater or the top of bedrock are in accordance with Table 3:
ii. Notwithstanding subd 4. b. ii, applicable requirements for injection wells classified under ch. NR 815, Wis. Adm. Code, shall be followed. c. Infiltration rate exemptions. Infiltration practices located in the following areas may be credited toward meeting the requirements under the following conditions, but the decision to infiltrate under any of the following conditions is optional:
i. Where the infiltration rate of the soil measured at the proposed bottom of the infiltration system is less than 0.6 inches per hour using a scientifically credible field test method.
ii. Where the least permeable soil horizon to 5 feet below the proposed bottom of the infiltration system using the U.S. Department of Agriculture method of soils analysis is one of the following: sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, or clay.
5. Alternate Use. Where alternate uses of runoff are employed, such as for toilet flushing, laundry, or irrigation or storage on green roofs where an equivalent portion of the runoff is captured permanently by rooftop vegetation, such alternate use shall be given equal credit toward the infiltration volume required by this section.
6. Groundwater Standards.
a. Infiltration systems designed in accordance with this section shall, to the extent technically and economically feasible, minimize the level of pollutants infiltrating to groundwater and shall maintain compliance with the preventive action limit at a point of standards application in accordance with ch. NR 140, Wis. Adm. Code. However, if site specific information indicates that compliance with a preventive action limit is not achievable, the infiltration BMP may not be installed or shall be modified to prevent infiltration to the maximum extent practicable. b. Notwithstanding subd. 6. a., the discharge from BMPs shall remain below the enforcement standard at the point of standards application.
7. Pretreatment. 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 subd. 6. Pretreatment options may include, but are not limited to, oil and grease separation, sedimentation, biofiltration, filtration, swales, or filter strips.
8. Maximum Extent Practicable. Where the conditions of subds. 3. and 4. limit or restrict the use of infiltration practices, the performance standard of S. 07 (4) (c) shall be met to the maximum extent practicable.
(d) PROTECTIVE AREAS.
9. Definition. In this paragraph, “protective area” means an area of land that commences at the top of the channel of lakes, streams, and rivers, or at the delineated boundary of wetlands, and that is the greatest of any of the following widths, as measured horizontally from the top of the channel or delineated wetland boundary to the closest impervious surface. However, in this section, “protective area” does not include any area of land adjacent to any stream enclosed within a pipe or culvert, so that runoff cannot enter the enclosure at this location.
a. For outstanding resource waters and exceptional resource waters, 75 feet.
b. For perennial and intermittent streams identified on a U.S. Geological Survey 7.5-minute series topographic map, or a county soil survey map, whichever is more current, 50 feet.
c. For lakes, 50 feet.
d. For wetlands not subject to subd. 1. e. or f., 50 feet.
e. For highly susceptible wetlands, 75 feet. Highly susceptible wetlands include the following types: calcareous fens, sedge meadows, open and coniferous bogs, low prairies, coniferous swamps, lowland hardwood swamps, and ephemeral ponds.
f. For less susceptible wetlands, 10 percent of the average wetland width, but no less than 10 feet nor more than 30 feet. Less susceptible wetlands include: degraded wetland dominated by invasive species such as reed canary grass; cultivated hydric soils; and any gravel pits, or dredged material or fill material disposal sites that take on the attributes of a wetland.
g. In subd. 1 d. to f., determinations of the extent of the protective area adjacent to wetlands shall be made on the basis of the sensitivity and runoff susceptibility of the wetland in accordance with the standards and criteria in s. NR 103.03, Wis. Adm. Code. h. Wetland boundary delineation shall be made in accordance with s. NR 103.08 (1m), Wis. Adm. Code. This subd. 1. h. does not apply to wetlands that have been completely filled in compliance with all applicable state and federal regulations. The protective area for wetlands that have been partially filled in compliance with all applicable state and federal regulations shall be measured from the wetland boundary delineation after a fill has been placed. Where there is a legally authorized wetland fill, the protective area standard need not be met in that location. i. For concentrated flow channels with drainage areas greater than 130 acres, 10 feet.
j. Notwithstanding subd. 1. a. to i., the greatest protective area width shall apply where rivers, streams, lakes, and wetlands are contiguous.
Note to Users: A stream or lake is not eligible for a lower protective area width even if contiguous to a less susceptible wetland.
10. Applicability. This section applies to post-construction sites located within a protective area, except those areas exempted pursuant to subd. 4.
11. Requirements. All of the following requirements shall be met:
a. Impervious surfaces shall be kept out of the protective area entirely or to the maximum extent practicable. If there is no practical alternative to locating an impervious surface in the protective area, the storm water management plan shall contain a written, site-specific explanation.
b. Where land disturbing construction activity occurs within a protective area, adequate sod or self-sustaining vegetative cover of 70 percent or greater shall be established and maintained where no impervious surface is present. The adequate sod or self-sustaining vegetative cover shall be sufficient to provide for bank stability, maintenance of fish habitat, and filtering of pollutants from upslope overland flow areas under sheet flow conditions. Non-vegetative materials, such as rock riprap, may be employed on the bank as necessary to prevent erosion such as on steep slopes or where high velocity flows occur.
Note to Users: It is recommended that seeding of non-invasive vegetative cover be used in the protective areas. Some invasive plants that should not be used are listed in ch. NR 40, Wis. Adm. Code. Flood and drought-tolerant vegetation that can provide long-term bank stability because of an extensive root system is preferable. Vegetative cover may be measured using the line transect method described in the University of Wisconsin extension publication number A3533, titled “Estimating Residue Using the Line Transect Method.” c. BMPs such as filter strips, swales, or wet detention ponds, that are designed to control pollutants from non-point sources, may be located in the protective area.
Note to Users: Other laws, such as ch. 30, Wis. Stats., and ch. NR 103, 115, 116, and 117, Wis. Adm. Code, and their associated review and approval processes may apply in the protective area. 12. Exemptions. This section does not apply to any of the following:
a. Except as provided under S. 07 (3), redevelopment post-construction sites.
b. In-fill development areas less than 5 acres.
c. Structures that cross or access surface water such as boat landings, bridges, and culverts.
d. Structures constructed in accordance with s. 59.692 (1v), Wis. Stats. e. Areas of post-construction sites from which the runoff does not enter the surface water, including wetlands, without first being treated by a BMP to meet the local ordinance requirements for total suspended solids and peak flow reduction, except to the extent that vegetative ground cover is necessary to maintain bank stability.
Note to Users: A vegetated protective area to filter runoff pollutants from post-construction sites described in par. (e) is not necessary since the runoff at that location is treated prior to entering the surface water. Other practices necessary to meet the requirements of this section, such as a swale or pond, will need to be designed and implemented to reduce runoff pollutants prior to runoff entering a surface water of the state.
(e) FUELING AND MAINTENANCE AREAS. Fueling and vehicle maintenance areas shall have BMPs designed, installed, and maintained to reduce petroleum within runoff, so that the runoff that enters waters of the state contains no visible petroleum sheen, or to the maximum extent practicable.
Note to Users: A combination of the following BMPs may be used: oil and grease separators, canopies, petroleum spill cleanup materials, or any other structural or non-structural method of preventing or treating petroleum in runoff.
Note to Users: The following paragraph, “Swale Treatment for Transportation Facilities,” is optional.
(f) SWALE TREATMENT FOR TRANSPORTATION FACILITIES.
1. Requirement. Except as provided in subd. 2., transportation facilities that use swales for runoff conveyance and pollutant removal are exempt from the requirements of local ordinance requirements for peak flow control, total suspended solids control, and infiltration, if the swales are designed to do all of the following or to the maximum extent practicable:
a. Swales shall be vegetated. However, where appropriate, non-vegetative measures may be employed to prevent erosion or provide for runoff treatment, such as rock riprap stabilization or check dams.
Note to Users: It is preferred that tall and dense vegetation be maintained within the swale due to its greater effectiveness at enhancing runoff pollutant removal.
b. Swales shall comply with sections V.F. (Velocity and Depth) and V.G. (Sale Geometry Criteria) with a swale treatment length as long as that specified in section V.C. (Pre-Treatment) of the Wisconsin Department of Natural Resources technical standard 1005 “Vegetated Swale,” dated September 2016, or a superseding document. Transportation facility swale treatment does not have to comply with other sections of technical standard 1005.
2. Other requirements.
i. Notwithstanding subd. 1., the [administering authority] may, consistent with water quality standards, require that other requirements, in addition to swale treatment, be met on a transportation facility with an average daily traffic rate greater than 2,500 and where the initial surface water of the state that the runoff directly enters is any of the following:
ii. An outstanding resource water.
ii. An exceptional resource water.
iii. Waters listed in section 303 (d) of the Federal Clean Water Act that are identified as impaired in whole or in part, due to non-point source impacts.
iv. Water where targeted performance standards are developed pursuant to s. NR 151.004, Wis. Adm. Code. b. The transportation facility authority shall contact the [administering authority] to determine if additional BMPs beyond a water quality swale are needed under this paragraph.
(5) GENERAL CONSIDERATIONS FOR STORM WATER MANAGEMENT MEASURES. All of the following considerations shall be observed in on-site and off-site runoff management:
