28.060 Stormwater Management Standards.

(1) Responsible Party. The responsible party shall develop and implement a written post-construction stormwater management plan that incorporates the requirements of this section.

(2) Stormwater Discharge Quantity. Unless otherwise provided for in this chapter, all land development activities subject to this chapter shall establish on-site management practices to control the peak flow rates of stormwater discharged from a site as described in this chapter. The amount of peak discharge control previously required for the site shall not be reduced as a result of the proposed development or land disturbance. Infiltration of stormwater runoff shall be in accordance with Wis. Admin. Code NR 151 and WDNR Technical Standards. On-site management practices shall be used to meet the following minimum performance standards:

(a) No development in the City shall increase peak flow rates of stormwater runoff from that which would have resulted from the same storm occurring over the site with the land in its predevelopment conditions for design rainfall events with recurrence intervals of two, 10, and 100 years.

(b) All stormwater conveyance systems within a proposed development, and receiving surface runoff from a proposed development, shall be designed to completely contain peak storm flows as described in subsections (2)(b)(1) and (2) of this section. Calculations for determining peak flows for conveyance system sizing shall be based on the existing or future proposed land use conditions for off-site areas (whichever results in the highest peak flows), and the future proposed land use conditions for the on-site areas:

1. For open channel conveyance systems, the peak flow from the 25-year storm shall be completely contained within a channel’s bottom and banks.

2. For storm sewer pipes, the peak flow from the 10-year storm shall be completely contained within the pipes with no surcharging or pressurized flow. There shall be incorporated into the design, above-ground emergency flow for the 100-year storm. Easements shall be granted to the City for emergency flow paths that are not wholly contained in the public right-of-way (ROW).

(c) Determination of peak flow rates and volume of runoff for purposes of meeting the requirements of subsections (2)(a) and (b) of this section shall be computed by procedures based on the principles and procedures approved by the Director of Public Works and that are described in the City’s “Engineering Design Manual,” as may be amended from time to time.

(d) The amount of peak discharge control previously required for the site shall not be reduced as a result of the proposed development or disturbance.

(e) An adequate outfall shall be provided for each point of concentrated discharge from the post-construction site. An adequate outfall consists of nonerosive discharge velocities and reasonable downstream conveyance.

(f) More stringent discharge limits may be required at the discretion of the Director of Public Works for reasons such as, but not limited to, insufficient downstream system capacity, potential erosion of stream channels, or impacts on flood stages.

(g) All point discharges will be restricted to public drainage systems (including storm sewers and ditches) or to waters of the State. The applicant is responsible to obtain, from adjacent property owners, any easements or other necessary approvals regarding the flow of water from the proposed development onto private lands.

(h) Increases or decreases in the hydrology of natural wetlands shall be minimized to the fullest extent practical. Where such changes are proposed, the impact of the proposal on wetlands shall be assessed and meet the requirements of Wis. Admin. Code NR 103.

(i) Emergency overland flow for all stormwater facilities shall be provided to prevent exceeding the safe capacity of downstream drainage facilities and prevent endangerment of downstream property or public safety.

(3) Stormwater Discharge Quality. After January 1, 2004, and unless otherwise provided for in this chapter, all land development activities subject to MMC 28.050(1) shall establish on-site management practices to control the quality of stormwater discharge from a site. The following on-site management practices shall be used to meet the following minimum standards:

(a) Stormwater management measures shall be designed to remove, on an average annual basis, a minimum of 80 percent of the total suspended solids load from a proposed on-site development when compared to a proposed on-site development without stormwater management measures.

(b) Stormwater management measures on redevelopment sites shall be designed to remove, on an average annual basis, a minimum of 40 percent of the total suspended solids load from a proposed on-site development when compared to a proposed on-site development without stormwater management measures.

(c) Discharge of urban stormwater pollutants to natural wetlands shall have pretreatment and vegetative buffers, unless otherwise exempted by the Director of Public Works. Where pretreatment meets the standards described herein, the impacts of the proposal on wetland functional values shall be assessed using a method acceptable to the Director of Public Works. Significant changes to wetland functional values due to stormwater pollutant loads shall be avoided.

(d) Infiltration of stormwater runoff shall be in accordance with Wis. Admin. Code NR 151 and WDNR Technical Standards. When infiltration basins, biofiltration or bioinfiltration basins or other such practices are proposed as a BMP for a site, a soils investigation following WDNR Technical Standard 1002 shall be prepared.

(e) Stormwater discharges shall have pretreatment prior to infiltration, to prolong maintenance of the infiltration practice and to prevent discharge of stormwater pollutants at concentrations that will exceed groundwater preventive action limits or enforcement standards established by the Department of Natural Resources in Wis. Admin. Code NR 140. Stormwater infiltration is prohibited under the following circumstances:

1. Stormwater is generated from highly contaminated source areas at manufacturing industrial sites;

2. Stormwater is carried in a conveyance system that also carries contaminated, nonstormwater discharges; or

3. Stormwater is generated from active construction sites.

(f) Petroleum products in runoff from fueling and vehicle maintenance areas shall be controlled with a properly designed and maintained oil and grease separator, or other equivalent practice. The structure or practice shall remove all visible sheen from the runoff prior to discharge to waters of the State or the City’s stormwater system.

(g) Stormwater ponds and infiltration devices shall not be located closer to water supply wells than indicated below, without prior written approval by the Director of Public Works:

1. One hundred feet from a nonpublic water supply well;

2. One thousand two hundred feet from a City water supply well;

3. The boundary of a recharge area to a well identified in a wellhead area protection plan, if it exists.

(h) The amount of total suspended solids control previously required for the site shall not be reduced as a result of the proposed development or disturbance.

(i) Pollutant loading models such as SLAMM, P8 or equivalent methodology may be used to evaluate the efficiency of the design in reducing total suspended solids.

(j) If the design cannot achieve the applicable total suspended solids reduction specified, the stormwater management 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.

(k) More or less stringent treatment limits may be required at the discretion of the Director of Public Works.

(4) Technical Standards. The following methods shall be used in designing and maintaining the water quality, peak discharges, infiltration, protective areas, and fueling/vehicle maintenance components of the stormwater practices needed to meet the water quality standards of this chapter:

(a) Technical standards identified, developed or disseminated by the Wisconsin Department of Natural Resources under subchapter V of Wis. Admin. Code NR 151.

(b) Technical standards and guidance identified within the City’s “Engineering Design Manual,” as may be amended from time to time.

(c) Where technical standards have not been identified or developed by the WDNR, other technical standards may be used; provided, that the methods have been approved by the Director of Public Works.

(5) Protective Areas. Stormwater runoff through and around protective areas shall be in conformance with Wis. Admin. Code NR 151.

(a) 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 the following widths, as measured horizontally from the top of the channel or delineated wetland boundary to the closest impervious surface. However, in this subsection, protective area does not include any area of land adjacent to any stream enclosed within a pipe or culvert such that runoff cannot enter the enclosure at this location.

1. For outstanding resource waters and exceptional resource waters, and for wetlands in areas of special natural resource interest as specified in Wis. Admin. Code NR 103.04, 75 feet.

2. For perennial and intermittent streams identified on a United States geological survey 7.5-minute series topographic map, or a County soil survey map, whichever is more current, 50 feet.

3. For lakes, 50 feet.

4. For highly susceptible wetlands, 50 feet. Highly susceptible wetlands include the following types: fens, sedge meadows, bogs, low prairies, conifer swamps, shrub swamps, other forested wetlands, fresh wet meadows, shallow marshes, deep marshes and seasonally flooded basins.

5. 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 wetlands dominated by invasive species such as reed canary grass.

6. In subsections (5)(a)(1), (4) and (5) of this section, 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 Wis. Admin. Code NR 103.03.

7. For concentrated flow channels with drainage areas greater than 130 acres, 10 feet.

(b) Wetlands Shall Be Delineated. Wetland boundary delineations shall be made in accordance with Wis. Admin. Code NR 103.08(1m). This subsection (5) does not apply to wetlands that have been completely filled in accordance with all applicable State and Federal regulations. The protective area for wetlands that have been partially filled in accordance with all applicable State and Federal regulations shall be measured from the wetland boundary delineation after fill has been placed.

(c) This subsection (5) applies to post-construction sites located within a protective area, except those areas exempted pursuant to subsection (5)(f) of this section.

(d) The following shall be met:

1. Impervious surfaces shall be kept out of the protective area to the maximum extent practicable. The stormwater management plan shall contain a written site-specific explanation for any parts of the protective area that are disturbed during construction.

2. Where land disturbing construction activity occurs within a protective area, and where no impervious surface is present, adequate sod or self-sustaining vegetative cover of 70 percent or greater shall be established and maintained. 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. Nonvegetative 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 nonaggressive vegetative cover be used in the protective areas. Vegetation that is flood and drought tolerant and can provide long-term bank stability because of an extensive root system is preferable. Vegetative cover can 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.”

3. Best management practices such as filter strips, swales, or wet detention basins that are designed to control pollutants from nonpoint sources may be located in the protective area.

Note to Users: Other regulations, such as Wis. Stat. Ch. 30 and Wis. Admin. Code NR 103, NR 115, NR 116 and NR 117, and their associated review and approval process, may apply in the protective area.

(e) A protective area established or created after the effective date of the ordinance codified in this chapter shall not be eliminated or reduced, except as allowed in subsection (5)(f)(1), (2), (3) or (4) of this section.

(f) Exemptions. The following areas are not required to meet the protective area requirements of this subsection (5):

1. Redevelopment and routine maintenance areas provided the minimum requirements within this subsection (5) are satisfied.

2. Structures that cross or access surface waters such as boat landings, bridges and culverts.

3. Structures constructed in accordance with Wis. Stat. § 59.692(1v).

4. Post-construction sites from which runoff does not enter the surface water, 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 subsection (5)(d) of this section is not necessary since runoff is not entering the surface water at that location. Other practices necessary to meet the requirements of this section, such as a swale or basin, will need to be designed and implemented to reduce runoff pollutants before the runoff enters a surface water of the State.

(6) Exceptions. The Director of Public Works may waive the minimum requirements for on-site stormwater management practices established in subsections (2) and (3) of this section upon written request of the applicant, in which a site-specific explanation as to why that level of reduction is not attained and the total solids load shall be reduced to the maximum extent practicable; and provided, that at least one of the following conditions applies:

(a) Alternative minimum requirements for on-site management of stormwater discharges have been established in a stormwater management plan that has been approved by the Director of Public Works.

(b) Provisions are made to manage stormwater by an off-site facility. This requires that a off-site facility is in place, is fully functional, and is designed and adequately sized to provide a level of stormwater control that is equal to or greater than that which would be afforded by on-site practices meeting the requirements of this chapter, and has a legally obligated applicant responsible for long-term operation and maintenance of the stormwater practice.

(c) The Director of Public Works finds that meeting the minimum on-site management requirements is not feasible due to site restrictions. The Director of Public Works shall not use financial hardship as the sole criterion in determining feasibility.

(7) Fee in Lieu of On-Site Stormwater Management Practices. Where the Director of Public Works waives all or part of the minimum on-site stormwater management requirements under subsection (3)(c) of this section, or, where the waiver is based on the provision of adequate stormwater facilities provided by the City downstream of the proposed development, as provided for under subsection (3)(b) of this section, the applicant shall be required to pay a fee, or alternative consideration, in an amount determined by resolution of the Common Council. In setting the fee for land development projects, the City shall consider an equitable distribution of the cost of land, engineering design, construction, and maintenance. The minimum fee shall be as determined by the Common Council through resolution. Partial detention will be credited towards the fee, if approved by the Director of Public Works.

[Ord. 08-522 §§ 6 – 17, 2008; Ord. 06-064 §§ 2 – 4, 2006. Prior code § 28.06]