Chapter 14.12
DESIGN CRITERIA AND STANDARDS
Sections:
14.12.020 Water quality criteria.
14.12.030 Stormwater discharge criteria.
14.12.040 Stormwater volume criteria.
14.12.010 Design methodology.
A. The site-specific stormwater management system plan required under the provisions of this title shall be designed in accordance with good engineering practice. The specific methods to be used in the calculation of peak rates of discharge, volumes and water quality conditions and of the hydraulic capacities of storage and conveyance facilities shall be left to the judgment of the professional engineer preparing the plan subject, however, to the approval of the city.
B. The site-specific stormwater management system plan shall be designed such that natural topography and land cover features such as swales, natural depressions, native soil infiltrating capacity, and natural ground water recharge areas shall be preserved and used to the extent practicable.
C. Guiding Principles for Stormwater Management. To satisfy the requirements of this section, a stormwater management plan shall, to the maximum extent practicable, adhere to the following guiding principles:
1. Preserve natural watershed boundaries and drainage patterns;
2. Reserve adequately sized areas for stormwater infiltration, detention and treatment early in the site planning process;
3. Locate stormwater BMPs prior to runoff leaving the site or entering waters of the state, and outside of wetlands, floodplains, primary or secondary environmental corridors or isolated natural areas;
4. Minimize soil compaction and maintain pre-development ground water recharge areas;
5. Minimize impervious surfaces and have them drain to vegetated areas for pollutant filtering and infiltration;
6. Emphasize vegetated swales, warm season and wetland plantings, and low flow velocities for stormwater conveyance, treatment and infiltration, especially for transportation related projects;
7. Allow for different stormwater management strategies for cleaner runoff (i.e., roofs) versus more polluted runoff (i.e., heavily used streets and parking lots);
8. Provide for emergency overflow in all stormwater BMP designs;
9. Distribute stormwater bioretention and infiltration BMPs throughout the site plan for large land developments.
D. Total Suspended Solids. By design, each stormwater management plan shall meet the following post-development total suspended solids reduction targets, based on average annual rainfalls, as compared to no runoff management controls:
1. For new land development, eighty (80) percent reduction in total suspended solids load;
2. For redevelopment, forty (40) percent reduction of total suspended solids load;
3. For in-fill development that occurs prior to October 1, 2012, forty (40) percent reduction total suspended solids load;
4. For in-fill development that occurs after October 1, 2012, eighty (80) percent reduction of total suspended solids load.
E. Infiltration. BMPs shall be designed, installed, and maintained to infiltrate runoff in accordance with the following requirements, except as provided in subsections 1 through 8 below.
1. Residential. For residential developments one of the following shall be met:
a. Infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least ninety (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 project site is required as an effective infiltration area,
b. Infiltrate twenty-five (25) percent of the post-development runoff volume from the two-year, twenty-four (24) hour design storm with a type II distribution. Separate runoff curve numbers for pervious and impervious surfaces shall be used to calculate runoff volumes, not composite curve numbers, as prescribed in NR 151. However, when designing appropriate infiltration systems to meet this requirement, no more than one percent of the project site is required as an effective infiltration area;
2. Nonresidential. For nonresidential development, including commercial, industrial and institutional development, one of the following shall be met:
a. Infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least sixty (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 two percent of the project site is required as an effective infiltration area,
b. Infiltrate ten percent of the post-development runoff volume from the two-year, twenty-four (24) hour design storm. Separate curve numbers for pervious and impervious surfaces shall be used to calculate runoff volumes, not composite curve numbers, as defined in TR-55. However, when designing appropriate infiltration systems to meet this requirement, no more than two percent of the project site is required as an effective infiltration area;
3. Modeling. Refer to NR 151 for details on calculating runoff volumes and pre-development conditions;
4. Pretreatment. Pretreatment shall be required before infiltrating parking lot and road runoff from commercial, industrial and institutional areas. The pretreatment shall be designed to protect the infiltration system from clogging prior to scheduled maintenance and to protect ground water quality in accordance with subsection 8 below. Pretreatment options may include, but are not limited to, oil/grease separators, sedimentation or bioretention basins, filtration swales or filter strips. All designs shall comply with NR 151;
5. Infiltration Exclusions. Infiltration of runoff shall not be credited toward meeting the requirements of this subsection for the following:
a. Runoff from outdoor material storage and loading docks for tier 1 and tier 2 industrial facilities, as identified in NR 216(2) Wis. Admin. Code,
b. Runoff from fueling and vehicle maintenance areas, not including rooftops and canopies,
c. Infiltration of runoff within one thousand (1,000) feet upgradient or within one hundred (100) feet downgradient of karst features,
d. Infiltration of runoff from any area except rooftops with less than three feet separation distance from the top of the filtering layer to the elevation of seasonal high ground water or the top of bedrock,
e. Infiltration of runoff from industrial, commercial and institutional parking lots and roads and residential arterial roads with less than five feet separation distance from top of the filtering layer to the elevation of seasonal high ground water or the top of bedrock,
f. Areas within four hundred (400) feet of a community water system well as specified in s. NR 811.16(4), Wis. Adm. Code and City Code 13.24.040(c), or within one hundred (100) feet of a private well as specified in s. NR 812.08(4), Wis. Adm. Code, for runoff infiltrated from commercial, industrial and institutional land uses or regional devices for residential development, not including rooftop runoff,
g. 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;
6. Infiltration Exemptions. The infiltration requirements of this subsection do not apply to frozen soil conditions and may be exempted if soils have a measured infiltration rate of less than 0.6 inches per hour and the city determines it would be impracticable to modify existing soil conditions;
7. Alternate Runoff Uses. Where storage and reuse of runoff are employed, such as to support green roofs, landscape watering, toilet flushing, laundry or irrigation, such alternate uses shall be given equal credit toward the infiltration volume required by this section;
8. Ground Water Protection.
a. Infiltration systems designed in accordance with this subsection shall, to the extent technically and economically feasible, minimize the level of pollutants infiltrating to ground water and shall maintain compliance with the preventive action limit at a point of standards application in accordance with Chapter 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. The discharge from BMPs shall remain below the enforcement standard at the point of standards application.
c. No stormwater BMP shall be installed that meets the definition of an injection well under Chapter NR 812 Wis. Admin. Code.
d. All stormwater BMPs shall comply with the provisions of any applicable wellhead protection plan for a community water supply under Chapter NR 811 Wis. Admin. Code and in accordance with Title 13.24 Wellhead Protection Ordinance.
F. Protective Areas. “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 section, “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 Chapter NR 103 Wis. Admin. Code, seventy-five (75) feet.
2. For perennial and intermittent streams identified on the Waukesha County GIS system, fifty (50) feet. If there is a discrepancy between the Waukesha County GIS system and the applicable United States Geological Survey 7.5-minute series topographic map, the more stringent stream identification shall apply.
3. For lakes, fifty (50) feet.
4. For highly susceptible wetlands, as determined by the Wisconsin DNR, fifty (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. Wetland boundary delineations shall be made in accordance with Chapter NR 103 Wis. Admin. Code. This paragraph 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.
5. For less susceptible wetlands, ten percent of the average wetland width, but no less than ten feet nor more than thirty (30) feet. Less susceptible wetlands include degraded wetlands dominated by invasive species such as reed canary grass.
6. In subdivision (F)(1), (4) and (5), 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 Chapter NR 103 Wis. Admin. Code.
7. For concentrated flow channels with drainage areas greater than one hundred thirty (130) acres, ten feet.
G. Requirements. The following requirements shall be met for all land development activity located within a protective area:
1. Impervious surfaces shall be kept out of the protective area, except for boathouses and walkways authorized under shoreland and floodland zoning. The erosion control plan shall contain a written site-specific explanation for any parts of the protective area that are disturbed during construction;
2. Where land disturbing activity occurs within a protective area, and where no impervious surface is present, adequate sod or self-sustaining vegetative cover of seventy (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. 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;
3. Best management practices such as filter strips, swales, or wet detention basins, that are designed to control pollutants from non-point sources may be located in the protective area, but shall not encroach into wetlands, floodplains or primary or secondary environmental corridors.
H. Protective Area Exemptions. The protective area requirements of this subsection may be exempted and do not apply to the following:
1. Structures that cross or access surface waters such as boat landings, bridges and culverts;
2. Structures constructed in accordance with s. 59.692(1v), Wisconsin Statutes; and
3. Sites where runoff does not enter the surface water, except to the extent that vegetative ground cover is necessary to maintain bank stability.
I. Fueling and Vehicle Maintenance Areas. Fueling and vehicle maintenance areas shall have BMPs designed, installed and maintained to reduce petroleum within runoff, such that the runoff that enters waters of the state contains no visible petroleum sheen.
J. Site Drainage. Measures shall be implemented to ensure proper site drainage, prevent property damage and protect public health and safety, including the following minimum requirements:
1. Drainage Easement. Perpetual drainage easements or other deed restrictions shall be recorded on the property to preserve major stormwater flow paths and permanent stormwater BMP locations. Covenants in these areas shall not allow buildings or other structures and shall prevent any grading, filling or other activities that interrupt or obstruct flows in any way. Covenants shall also specify maintenance responsibilities and authorities in accordance with City Ordinance 14.24.
K. Site Grading. Site grading shall ensure positive flows away from all buildings, roads and driveways, be coordinated with the general stormwater drainage patterns for the area, and minimize adverse impacts on adjacent properties.
L. Street Drainage. All street drainage shall be designed to prevent concentrated flows from crossing the traffic lanes to the maximum extent practicable. Design flow depths at the road centerline for on-street drainage, shall not exceed six inches during the peak flows generated by the one hundred (100) year, twenty-four (24) hour design storm, using planned land use conditions for the entire contributing watershed area.
M. Bridges and Cross-Culverts. All new or modified bridges and cross-culverts shall comply with applicable design standards and regulations, facilitate fish passage and prevent increased flooding or channel erosion upstream or downstream from the structure. Design flow depths at the road centerline for all crossings shall not exceed six inches during the peak flows generated by the one hundred (100) year, twenty-four (24) hour design storm, using planned land use conditions for the entire contributing watershed area. All pre-development runoff storage areas within the flow path upstream of bridges and cross-culverts shall be preserved and designated as drainage easements, unless compensatory storage is provided and accounted for in modeling. As-built documentation shall be submitted in accordance with City Ordinance 14.32.030(G) for all new or modified structures that are located within a mapped floodplain or that the city determines to be necessary to maintain floodplain modeling for the applicable watershed.
N. Subsurface Drainage. Basement floor surfaces shall be built one foot above the seasonal high water table elevation, as documented in the submitted soil evaluations, and shall avoid hydric soils as much as possible. The city shall be notified of any drain tiles that are uncovered during construction, which the city may require to be restored or connected to other drainage systems. No discharge of ground water from tile lines, sump pumps or other means shall be allowed onto another persons land or any public space without the written approval of the owner or unit of government.
O. Open Channels. All open channel drainage systems shall at a minimum be designed to carry the peak flows from a ten-year, twenty-four (24) hour design storm using planned land use for the entire contributing watershed area. Side slopes shall be no steeper than 3h:1v unless otherwise approved by the city for unique site conditions.
P. Storm Sewers. All storm sewers shall be designed in accordance with applicable city technical standards and specifications.
Q. Structure Protection and Safety. Flows generated by the one hundred (100) year, twenty-four (24) hour design storm under planned land use conditions may exceed the design capacity of conveyance systems, but shall not come in contact with any buildings. For buildings designed for human occupation on a regular basis, the following additional requirements shall apply:
1. The lowest elevation of the structure that is exposed to the ground surface shall be a minimum of two feet above the maximum water elevation produced by the one hundred (100) year, twenty-four (24) hour design storm, including flows through any stormwater BMP that may temporarily or permanently store water at a depth of greater than one foot; and
2. The structure shall be set back at least fifty (50) feet from any stormwater BMP that may temporarily or permanently store water at a depth of greater than one foot. Setback distance shall be measured from the closest edge of water at the elevation produced by the one hundred (100) year, twenty-four (24) hour design storm. (Ord. 2112-07 § 5, 2007; Ord. 1749 § 1 (part), 2000)
14.12.020 Water quality criteria.
The stormwater management facilities and measures required to serve land use development, redevelopment and property land division activities subject to this title shall be designed to meet the following minimum standards:
A. Stormwater discharges shall be treated to achieve the levels of pollutant removals specified in the adopted city stormwater management system plan. In areas for which the city has not prepared a stormwater management system plan, the levels of pollutant reduction required shall be specified by the city upon due consideration of recommendations contained in the adopted regional water quality management plan and priority watershed plans. Unless otherwise specified in the above plans, stormwater discharges shall generally be designed to remove, on an average annual basis, a goal of eighty (80) percent of the total suspended solids load based on no controls. To achieve this level of control, stormwater practices shall be designed to accommodate, at a minimum, the runoff volume resulting from 1.5 inches of rainfall.
B. Discharge of urban stormwater pollutants to wetlands shall be minimized to the extent practicable. Significant degradation of wetland functional values due to stormwater pollutant loading shall be avoided.
C. Stormwater discharges shall be pretreated prior to infiltration to prolong maintenance of the infiltration capacity and to prevent discharge of stormwater pollutants and concentrations that would result in exceedences of ground water quality standards established by the Wisconsin Department of Natural Resources.
D. Stormwater detention, retention and infiltration facilities shall not be located closer than one hundred (100) feet from a well serving a private water system; or four hundred (400) feet from a well serving a municipal public water supply system; or within the wellhead protection area of a well serving a municipal public water supply system, if such a protection area has been delineated.
E. The design criteria and standards contained in “The Wisconsin Stormwater Manual” prepared and published by the Wisconsin Department of Natural Resources shall be used in the design of stormwater facilities and measures. (Ord. 2112-07 § 6, 2007; Ord. 1749 § 1 (part), 2000)
14.12.030 Stormwater discharge criteria.
A. The conveyance and storage facilities incorporated into the site-specific stormwater management system plan required under this Title 14 shall be designed as an integral part of complementary minor and major subsystem. The minor subsystem shall be designed to avoid nuisance flooding of streets and yards and shall accommodate the peak rate of runoff from rainfall events up to and including the ten-year recurrence interval event. The rainfall intensity shall be determined based on appropriate times of concentration from relationships established and published by the Southeastern Wisconsin Regional Planning Commission. The complementary major subsystem shall consist of the public streets and interconnected flow paths to the streets and from the streets to receiving streams and watercourses. The major system shall be designed to accommodate peak rates of discharge from rainfall events up to and including the one hundred (100) year recurrence interval event without inundation of exposed basements, building basement window wells, basement entryways, or the first floors of buildings, utilizing a one foot freeboard.
B. The peak flow discharge rates of stormwater runoff from the site under the post-development site conditions shall not exceed the rates under the existing, or predevelopment conditions, as calculated in Section 14.08.070(F) or as required by Chapter 13 of the Milwaukee Metropolitan Sewerage District Rules on Surface Water and Storm Water Runoff Management. (Ord. 1902 § 5, 2003; 1749 § 1 (part), 2000)
14.12.040 Stormwater volume criteria.
Water quality control facilities included in the stormwater management system plan required under this Title 14 shall be designed in conformance with the adopted city stormwater management system plan. In areas for which the city has not prepared a stormwater management system plan, the facilities shall be designed to accommodate a stormwater runoff volume specified by the city based upon recommendations contained in the adopted regional water quality management plan and priority watershed plans. (Ord. 1749 § 1 (part), 2000)
14.12.050 Exceptions.
The city may establish stormwater management requirements either more or less stringent than those set forth in this Title 14; provided, that the engineer finds that one or more of the following conditions applies:
A. A higher level of quality in the stormwater discharge is required to protect sensitive environmental resources;
B. A higher level of protection from ponding or flooding is required to protect the public health and safety;
C. Provisions are available to manage the stormwater runoff by off-site facilities; provided, that all of the following conditions are met for the off-site facilities: the facilities are in place; the facilities are adequately sized to provide a level of stormwater runoff control equal to or greater than that which would be afforded by on-site facilities and measures meeting the requirements of this Title 14; and a legal entity exists that is responsible for the maintenance of the facilities.
D. To meet the requirements of Chapter 13 of the Milwaukee Metropolitan Sewerage District Rules on Surface Water and Storm Water Runoff Management. (Ord. 1902 § 6, 2003; 1749 § 1 (part), 2000)