A stormwater management plan shall describe how the permit holder
and other responsible party will meet the stormwater management requirements
of this article and other related requirements in this chapter. All
stormwater management plans and associated BMPs shall comply with
the planning, design, implementation and maintenance requirements
described in this chapter to achieve a no net increase in runoff from
the disturbed site.
A stormwater management plan shall, to the maximum extent practicable,
adhere to the following principles:
A. Preserve natural watershed boundaries and drainage patterns;
B. Reserve adequately sized areas for stormwater infiltration, detention
and treatment early in the site planning process;
C. Locate stormwater BMPs prior to runoff leaving the site or entering
waters of the state, wetlands, floodplains, primary or secondary environmental
corridors or isolated natural areas;
D. Minimize soil compaction and maintain predevelopment groundwater
recharge areas;
E. Minimize impervious surfaces and have them drain to vegetated areas
for pollutant filtering and infiltration;
F. Emphasize vegetated swales, warm season and wetland plantings, and
low-flow velocities for stormwater conveyance, treatment and infiltration,
especially for transportation-related projects;
Note: Tall, dense, deep-rooted vegetation and low-flow velocities
in open channels encourage infiltration and increase their effectiveness
for runoff pollutant removal. Check dams may also be included in the
swale design to slow runoff flows and improve pollutant removal. Soil
amendments such as compost can help reduce soil compaction and increase
infiltration.
G. Allow for different stormwater management strategies for cleaner
runoff (i.e., roofs) versus more polluted runoff (i.e., heavily used
streets and parking lots);
H. Provide for emergency overflow in all stormwater BMP designs;
I. Distribute stormwater bioretention and infiltration BMPs throughout
the site plan for large land developments.
All stormwater management plans and associated BMPs shall meet
the following minimum requirements to the maximum extent practicable.
It is highly recommended that the applicant meet with the Designated
City Representative prior to preparing a stormwater management plan
to determine the applicability of these requirements early in the
site planning process.
A. Peak discharge.
(1) Minimum requirement. To minimize erosion and the failure of conveyance systems, the calculated post-development peak stormwater discharge rate shall not exceed the calculated predevelopment discharge rates for the two-year, ten-year, and one-hundred-year, twenty-four-hour design storms. Modeling requirements for this provision are further described in Article
VI.
(2) Release rate per acre. The Designated City Representative may establish a maximum allowable release rate on a per acre basis that would supersede the requirements of Subsection
A(1) above for certain watersheds after the necessary hydrologic modeling is completed and the maximum release rate is established.
Note: A detailed watershed-based hydrologic analysis can generate
a more accurate peak discharge rate for the protection of downstream
properties from increased flooding due to the addition of impervious
surfaces.
(3) Peak discharge exemptions. Certain sites or portions of sites may be exempted from the peak discharge requirements of this subsection in accordance with §
367-27 below.
B. 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, 80% reduction in total suspended solids
load;
(2) For redevelopment, 40% reduction of total suspended solids load;
(3) For infill development that occurs prior to October 1, 2012, 40%
reduction total suspended solids load;
(4) For in-fill development that occurs after October 1, 2012, 80% reduction
of total suspended solids load.
Note: The first flush of stormwater runoff from an urban landscape contains the vast majority of pollutants, which tend to be associated with suspended solids. 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 under Subsection
B(1) above.
C. Infiltration. BMPs shall be designed, installed, and maintained to infiltrate runoff in accordance with the following requirements, except as provided in Subsection
C(5) 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 100% of the predevelopment infiltration
volume, based on an average annual rainfall. However, when designing
appropriate infiltration systems to meet this requirement, no more
than 1% of the project site is required as an effective infiltration
area.
(b)
Infiltrate 25% of the post-development runoff volume from the two-year, twenty-four-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 Article
VI. However, when designing appropriate infiltration systems to meet this requirement, no more than 1% 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 60% of the predevelopment 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.
(b)
Infiltrate 10% of the post-development runoff volume from the
two-year, twenty-four-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 2% of the project site is required as an effective infiltration
area.
(3) Modeling. Refer to §
367-30 for details on calculating runoff volumes and predevelopment 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 groundwater quality in accordance with Subsection
C(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 the technical standards in §
367-31.
Note: To achieve the infiltration requirement for the parking
lots or roads, "maximum extent practicable" should not be interpreted
to require significant topography changes that create an excessive
financial burden. 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.
(5) Infiltration exclusions. Infiltration of runoff shall not be credited
toward meeting the requirements of this subsection, and is prohibited
to minimize the potential for groundwater contamination for the following:
(a)
Runoff from outdoor material storage and loading docks for tier
1 and tier 2 industrial facilities, as identified in § NR
216.21(2), Wis. Adm. Code.
(b)
Runoff from fueling and vehicle maintenance areas, not including
rooftops and canopies.
(c)
Infiltration of runoff within 1,000 feet upgradient or within
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 groundwater 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 groundwater or the top of bedrock.
(f)
Areas within 400 feet of a community water system well as specified
in § NR 811.12(5), Wis. Adm. Code, or as required by applicable
wellhead protection ordinances, or within 100 feet of a private well
as specified in § 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 § NR
720.03(2), Wis. Adm. Code, are present in the soil through which infiltration
will occur.
(h)
Areas associated with Tier 1 industrial facilities identified
in § NR 216.21(2)(a), Wis. Adm. Code, including storage,
loading, rooftop and parking.
(i)
Any area where the soil does not exhibit one of the following
characteristics between the bottom of the infiltration system and
the seasonal high groundwater and top of bedrock: at least a three-foot
soil layer with 20% fines or greater or at least a five-foot soil
layer with 10% fines or greater.
Note: This subsection does not apply where the soil medium within
the infiltration system provides an equivalent level of protection
and does not prohibit infiltration of roof runoff.
(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 inch per hour and the Designated City Representative determines it would be impracticable to modify existing soil conditions. Other sites may be exempted in accordance with §
367-27 below.
(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) Groundwater 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 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)
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 Ch. NR 812, Wis. Adm. Code.
(d)
All stormwater BMPs shall comply with the provisions of any
applicable wellhead protection plan for a community water supply under
Ch. NR 811, Wis. Adm. Code.
D. Protective areas.
(1) "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 (minimums listed below).
(a)
For perennial and intermittent streams, 75 feet.
(c)
For highly susceptible wetlands, as determined by the Designated
City Representative, 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.
(d)
For less susceptible wetlands, 10% 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.
(e)
In Subsection
D(1)(a),
(c) and
(d), 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 Ch. NR 103, Wis. Adm. Code.
(f)
For concentrated flow channels with drainage areas greater than
130 acres, 10 feet.
(2) Requirements. The following requirements shall be met for all land
development activity located within a protective area:
(a)
Impervious surfaces shall be kept out of the protective area,
except for boathouses and walkways authorized under Ch. NR 115, Wis.
Adm. Code, and applicable shoreland and/or floodplain zoning. The
erosion control plan shall contain a written site-specific explanation
for any parts of the protective area that are disturbed during construction.
(b)
Where land disturbing activity occurs within a protective area,
and where no impervious surface is present, adequate sod or self-sustaining
vegetative cover of 70% 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: It is recommended that seeding of nonaggressive native
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."
(c)
Best management practices such as filter strips, swales, or
wet detention basins, which are designed to control pollutants from
nonpoint sources, may be located in the protective area as reviewed
and approved by Designated City Representative on a site-by-site basis,
but shall not encroach into wetlands, floodplains or primary or secondary
environmental corridors.
Note: Other regulations, such as Ch. 30, Wis. Stats., and Chs.
NR 103, NR 115, NR 116 and NR 117, Wis. Adm. Code, and their associated
review and approval process may apply in the protective area.
(3) Protective area exemptions. The protective area requirements of this subsection may be exempted in accordance with §
367-27 below and do not apply to the following:
(a)
Structures that cross or access surface waters such as boat
landings, bridges and culverts;
(b)
Structures constructed in accordance with § 59.692(1v),
Wis. Stats.; and
(c)
Sites where runoff does not enter the surface water, except
to the extent that vegetative ground cover is necessary to maintain
bank stability.
Note: A vegetated protective area to filter runoff pollutants from post-construction sites described in Subsection
D(3) above 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.
E. 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.
Note: A combination of the following BMPs may be used: oil and
grease separators, canopies, petroleum spill cleanup materials, or
any other structural or nonstructural method of preventing or treating
petroleum in runoff.
F. 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 stormwater flow paths and permanent stormwater BMP locations. Recorded 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. Recorded covenants shall also specify maintenance responsibilities and authorities in accordance with Article
VII.
(2) Site grading. Site grading shall ensure positive flows away from
all buildings, roads, driveways and septic systems, be coordinated
with the general stormwater drainage patterns for the area, and minimize
adverse impacts on adjacent properties.
(3) 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 center line for
on-street drainage shall not exceed six inches during the peak flows
generated by the one-hundred-year, twenty-four-hour design storm,
using planned land use conditions for the entire contributing watershed
area.
(4) 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 center line for all crossings shall not exceed six inches during the peak flows generated by the one-hundred-year, twenty-four-hour design storm, using planned land use conditions for the entire contributing watershed area. All predevelopment 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 Article
III for all new or modified structures that are located within a mapped floodplain or that the Designated City Representative determines to be necessary to maintain floodplain modeling for the applicable watershed.
(5) 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 Designated City Representative shall be notified of
any drain tiles that are uncovered during construction, which the
Designated City Representative may require to be restored or connected
to other drainage systems. No discharge of groundwater from tile lines,
sump pumps or other means shall be allowed onto another person's land
or any public space without the written approval of the owner or unit
of government.
(6) Open channels. All open channel drainage systems shall at a minimum
be designed to carry the peak flows from a ten-year, twenty-four-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 Designated City Representative for unique site conditions.
Open channels that carry runoff from more than 130 acres shall, at
a minimum, be designed to carry the peak flows from a twenty-five-year,
twenty-four-hour design storm.
(7) Storm sewers. All storm sewers shall be designed in accordance with
applicable City technical standards and specifications.
(8) Structure protection and safety. Flows generated by the one-hundred-year,
twenty-four-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:
(a)
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-year, twenty-four-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
(b)
The structure shall be set back at least 50 feet horizontally
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-year, twenty-four-hour design storm.
G. Additional requirements. The Designated City Representative may establish
more stringent requirements than the minimums set forth in this section,
such as addressing thermal impacts of stormwater or chronic wetness
conditions, if the Designated City Representative determines that
an added level of protection is needed to protect:
(1) Low-impact development for all new development or additional impervious
areas. The first one inch of runoff from impervious areas shall be
treated from drainage areas no larger than one acre. Treatment options
include and may be limited to bioinfiltration, infiltration, rain
gardens, stormwater reuse and buffers. Small practices shall be employed
in large sites to the greatest extent possible.
(2) An environmentally sensitive area.
Preliminary stormwater management plans shall contain the following
applicable items:
A. Drafting date and contact information for the project engineer with
all other mapping elements and scale consistent with the site plan
map.
B. Delineation of existing and proposed watersheds, subwatersheds and
major flow paths within the site and draining into the site from adjacent
properties.
C. Location, type and preliminary design of proposed stormwater BMPs
needed to comply with this chapter.
D. Location and type of major stormwater conveyance systems proposed
for the site.
E. Existing and proposed stormwater discharge points.
F. Location and preliminary dimensions of proposed drainage easements.
G. Location of soil borings and soil profile evaluations with surface
elevations and unique references to supplemental data sheets, as needed
to determine feasibility of any proposed stormwater BMP and to comply
with applicable BMP technical standards.
Note: The required location, depth and type of soil evaluations
will depend on the stormwater BMPs proposed for the site. In general,
soil profile evaluations usually need to extend to a depth of three
to 10 feet below the proposed bottom elevation of stormwater BMPs.
Refer to BMP technical standards for details.
H. Preliminary location of access lanes for maintenance of stormwater
BMPs. (Exemptions may be made by Designated City Representative. Access
lanes for maintenance intended for larger BMPs.)
I. Support documentation, including:
(1) A preliminary plan narrative describing site drainage, ultimate receiving
water body for off-site discharges, major site restrictions, and how
the preliminary stormwater management plan will meet the requirements
of this chapter and other objectives identified by the project engineer.
(2) Summary of watershed, subwatershed and land use data in acres and
the preliminary results of any hydrology calculations.
(3) Soil profile evaluation data in accordance with BMP technical standards.
(4) Proposed ownership and maintenance responsibilities for all proposed
stormwater BMPs.
Note: Mapping elements may be included in the site plan map.
Final stormwater management plans shall contain the following
applicable items:
A. Drafting date and contact information for the project engineer, with
all other mapping elements and scale consistent with the site plan
map;
B. Location of existing and proposed stormwater discharge points;
C. Delineation and labeling of all proposed impervious areas and accompanying
area computations;
D. Final design drawings of all proposed stormwater BMPs with unique
references to support documentation, prepared in accordance with minimum
Designated City Representative standards and of sufficient clarity
for those responsible for site grading, including:
(1) Plan views showing the location of proposed BMPs in combination with
the site plan map at a scale of one inch equals no more than 100 feet;
(2) Additional detail plan view drawings at a scale of one inch equals
no more than 40 linear feet, showing proposed two-foot contours and
all critical design features and elevations;
(3) Detailed cross-sections and profiles of each BMP showing all critical
design features, side slopes, structures, soil profiles and applicable
elevations, including seasonal high water table; and
(4) Detailed drawings or material specifications for inlets or outlets.
E. Type, size, location and cross-sections of all pipes, open channels,
grade stabilization structures and other proposed stormwater conveyance
systems, with references to support documentation;
F. Location and dimensions of proposed drainage easements;
G. Location, dimensions and surfacing material or soils data of proposed access lanes and delineation of easements needed to allow future maintenance of all stormwater BMPs in accordance with §
367-38 below. The minimum width of any access easement shall be 15 feet; (Exemptions may be made by Designated City Representative for access lanes and easements or outlot designations. Access lanes and easements for maintenance intended for larger BMPs.)
H. Location of soil borings and soil profile evaluations with surface
elevations and references, as needed to determine feasibility of any
proposed stormwater BMP and to comply with technical standards;
I. Detailed construction notes explaining all procedures to implement
the plan, including planting and landscaping specifications, timing
and sequencing of construction and any temporary measures needed to
protect BMPs during the construction phase;
Note: Some BMPs, such as infiltration and bioretention practices,
are susceptible to sedimentation and may need to be protected during
construction or planned for construction later in the project sequence.
J. A detailed construction inspection plan, outlining the critical elements
in the plan that need to be surveyed or inspected by a representative
of the project engineer, and the Designated City Representative, and
the timing and notification requirements involved.
Note: Examples of critical elements for a construction inspection
plan include, but are not limited to: checking subgrade elevations
or the placement of footings, pipes or other structures prior to covering,
soil testing, material inspections and final grade checks before seeding.
Any inspection conducted by the Designated City Representative(s)
or the City does not waive the permit holder's responsibility for
construction oversight and verification.
K. A final stormwater BMP maintenance agreement in accordance with Article
VII;
L. Support documentation summarized in accordance with Designated City
Representative standards, including but not limited to:
(1) A narrative summary of the stormwater management plan, briefly explaining any unique information that led to the selection of BMPs, how the proposed plan meets the guiding principles under §
367-24 above, and the specific stormwater planning requirements under §
367-26 above;
Note: The narrative can be combined with the narrative for erosion control planning under Article
IV above. Some provisions may also be included in the construction notes under Subsection
I above.
(2) Maps of existing and proposed watersheds, subwatersheds, Tc/Tt flow
paths, soil types, hydrologic soil groups, land uses/cover type and
accompanying runoff curve numbers within the site and draining into
the site from adjacent properties, with unique references to hydrology
data summaries and a description of the ultimate receiving water body(ies)
for off-site discharges;
(3) Predevelopment and post-development hydrology and pollutant loading (if applicable) data for each watershed, such as peak flows and runoff volumes, as needed to meet the requirements of this chapter. All major assumptions used in developing input parameters shall be clearly stated and cross-referenced to the maps under Subsection
L(2) above;
(4) Impervious surface maps and calculations of runoff volumes and effective infiltration areas, in accordance with §
367-26C above;
(5) Hydraulic and hydrologic data summaries for all existing and proposed pipes, open channels, grade stabilization structures and other stormwater conveyance systems, and the necessary documentation to demonstrate compliance with the site drainage requirements under §
367-26F above;
(6) BMP design data for each proposed BMP, showing how it complies with
applicable technical standards and the requirements of this chapter;
(7) Soil evaluation reports, following the standards in §
367-34, with matching references to map features showing their location and elevations;
(8) A cover sheet stamped and signed by a professional engineer registered
in the State of Wisconsin indicating that all plans and supporting
documentation have been reviewed and approved by the engineer and
certifying that they have read the requirements of this chapter and
that, to the best of their knowledge, the submitted plans comply with
the requirements;
(9) Cost estimates for the installation of proposed stormwater BMPs, which shall serve as a basis for the financial assurance under §
367-15 above. The applicant may use average costs for BMP installations in the City rather than specific estimates, upon approval by the Designated City Representative; and
(10)
For sites where changes are proposed in stormwater flow paths,
or where proposed stormwater discharges may otherwise have a significant
negative impact on downstream property owner(s), the Designated City
Representative may require the applicant to submit written authorization
or complete other legal arrangements with the affected property owner(s);
and
M. Other items deemed necessary by the Designated City Representative
to ensure compliance with the requirements of this chapter.