A.
Stormwater BMPs shall be designed to manage stormwater flow within
the available capacity of the downstream conveyance system as determined
by the Village.
In order to achieve the goals and purposes of this chapter,
the following stormwater management performance standards are hereby
established;
A.
Water quality treatment: Treat the calculated site runoff for the
entire project site from the 90% annual non-exceedance storm, which
is approximately equal to one inch of rain (i.e., on average, 90%
of the storm in a given year produces one inch or less). The treatment
volume specified is based on capturing and treating the volume of
stormwater that is the first to run off in a storm and expected to
contain the majority of pollutants. This volume of runoff is often
referred to as the "first flush." The water quality treatment standard
is required for all sites.
(1)
Total suspended solids (TSS). The methods selected to treat the volume
of water calculated for the water quality treatment performance standard
shall be designed on a site-specific basis to achieve either a minimum
of 80% removal of TSS, as compared with uncontrolled runoff, or discharge
concentration of TSS that does not exceed 80 milligrams per liter
(mg/l). This performance standard is based on TSS as a surrogate for
other pollutants normally found in stormwater runoff. Control of TSS
to meet this standard is expected to achieve control of other pollutants
to an acceptable level that protects water quality.
B.
Channel protection: Maintain the post-development project site runoff
volume and peak flow rate at or below pre-development levels for all
storms up to the two-year, twenty-four-hour event. At a minimum, pre-development
is defined as the last land use prior to the planned new development
or redevelopment. The channel protection standard is required for
stormwater discharges to surface waters of the MS4.
C.
Flood control: Control the volume of site runoff from the flood control
rainfall event with a maximum allowable release rate to reduce the
potential for property damage for overbank flooding and preserve existing
floodplains. The flood control event and maximum allowable release
rate shall be determined by the Village. The flood control standard
is required for all sites.
(1)
Overflow routes: Acceptable overflow routes for the 100-year flood
shall be identified for the site and for downstream areas between
the site and the nearest acceptable floodway or outlet. Stormwater
conveyance systems are usually designed to handle flows generated
by the ten-year storm. When larger storms generate higher flows, the
conveyance system is expected to surcharge resulting in stormwater
accumulating on the surface of the ground. Gravity will cause such
stormwater to flow overland to lower elevations. By carefully managing
the shape of the land surface such overland stormwater flow can be
directed to locations that will not cause property damage. Adequate
emergency overland flow-ways will direct stormwater flows generated
by the 100-year storm to avoid damage to structures and facilities.
D.
Site-specific requirements:
(1)
Pretreatment: Pretreatment of site runoff is required on a site-specific
basis prior to discharging to certain stormwater BMPs. Pretreatment
provides for the removal of fine sediment, trash and debris, and preserves
the longevity and function of the BMP.
(2)
Hot spots and groundwater contamination: Some land use activities
have a potentially greater risk of polluted runoff than others. Project
sites with these types of activities are referred to as "hot spots"
and include uses such as gas stations, commercial vehicle maintenance
and repair, auto recyclers, recycling centers, and scrap yards. Hot
spots also include areas with the potential for contaminating public
water supply intakes. Pretreatment of stormwater runoff to address
pollutants associated with hot spots is required for the site. Stormwater
management strategies and BMPs that reduce the potential to mobilize
existing soil and groundwater contaminants, or that capture and treat
stormwater runoff and/or accidental spills to protect groundwater
or nearby surface waters are required.
(3)
Coldwater streams: Stormwater management strategies and BMPs that
minimize thermal impacts from site runoff and maximize groundwater
recharge are required for sites with a surface water discharge to
a coldwater stream as determined by the Michigan Department of Natural
Resources (MDNR).
A.
The water quality treatment and channel protection performance standards
focus on maintaining or restoring stable hydrology. However, potential
physical constraints may limit the ability to fully meet the post-construction
requirement at the project site. When physical constraints limit the
feasibility of maintaining or restoring hydrology, off-site mitigation
and/or payment-in-lieu may be approved by the Village. Off-site mitigation
will only be considered if there is a planned or constructed private
mitigation bank or project immediately downstream. The Village will
only consider payment-in-lieu if the Village has a planned or constructed
water quality improvement project immediately downstream. The cost
of payment-in-lieu will be considered on a case-to-case basis and
will represent the actual cost of implementing public downstream water
quality enhancements. Any request for payment-in-lieu shall be accompanied
by a development agreement that clearly states a public/private partnership
has been formed for the site development. Payment-in-lieu will be
considered, at the discretion of the Village, when reviewing a project
of regional significance.
B.
Consideration of either off-site mitigation or payment-in-lieu will
be entirely the discretion of the Village.
C.
"Off-site mitigation" refers to MBPs implemented at a location different
from the original project site.
D.
"Payment-in-lieu" refers to the developer paying a fee to the Village
that is applied to a public stormwater management project. The stormwater
management project may be either a new BMP or a retrofit to an existing
BMP and developed in accordance with the Stormwater Standards Manual.
E.
The location for off-site mitigation and payment-in-lieu projects
shall be within the same watershed and sewershed as the original project,
and within the Village's jurisdictional boundaries. The watershed
is the area represented by the State of Michigan, Department of Environment,
Great Lakes and Energy (EGLE), or successor organization, ten-digit
Hydrologic Unit Code (HUC). The sewershed is the area where stormwater
is conveyed by an MS4 to a common outfall or point of discharge.
F.
The determination to approve off-site mitigation or payment-in-lieu
will be based on multiple criteria and not solely on the difficulty
of cost of implanting BMPs on site. Conditions under which the option
to move off site would become available may include:
(1)
Limited size of the lot outside of the building footprint to create
the necessary infiltration capacity even with amended soils.
(2)
Soil instability as documented by a thorough geotechnical analysis.
(3)
A site use that is inconsistent with capture and reuse of stormwater.
(4)
Too much shade or other physical conditions that preclude adequate
use of plants.
(5)
The potential water quality impact from the original project site
and the benefits realized at the off-site location.
G.
The Village may approve off-site mitigation or payment-in-lieu if
the developer demonstrates that site constraints preclude sufficient
treatment and restoration of hydrology on site. At a minimum the Village
requires:
(1)
Off-site ratio. The off-site ratio for the amount of stormwater not
managed on site in relation to the amount of stormwater required to
be mitigated at another site, or for which in-lieu payments will be
made as follows:
(a)
First Tier: Manage a minimum of 0.4 inch of stormwater runoff
on site and provide a 1:1.5 offset ratio for the remaining amount
of stormwater managed off site.
(b)
Second Tier: If it is completely infeasible to manage the minimum
on site, provide a 1:2 offset ratio for the amount of stormwater managed
on site.
(2)
Schedule. Off-site mitigation and payment-in-lieu projects shall
be completed within 24 months after the start of the original site
construction.
(3)
Assurances. Off-site and in-lieu projects shall be preserved and
maintained in perpetuity through the procedures and tracking system
administered by the Village.
A.
In many cases, infiltration will likely be used as the primary means
of retention. It is not, however, the sole means of providing on-site
retention, and the developer must include consideration of stormwater
reuse, interception, evapotranspiration, and other vegetative (nonstructural)
BMPs at the project site. Site constraints that limit the use of infiltration
may include:
(1)
Poorly draining soils (less than 0.24 inch per hour; typically, hydrologic
soil groups C and D).
(2)
Bedrock.
(3)
High groundwater, or the potential of mounded groundwater to impair
other uses.
(4)
Wellhead protection areas.
(5)
Stormwater hot spots.
(6)
Part 201 and 213 sites, and areas of soil or groundwater contamination.
B.
The Village may grant a waiver of the on-site retention criteria for channel protection described in § 320-46 and allow an alternative approach to meet the channel protection performance standard if the developer demonstrates that site constraints preclude sufficient retention on site. If a waiver is granted, the developer must meet the following extended detention criteria:
(1)
Extended detention: Detain the portion of the channel protection
volume unable to be retained on site for a minimum of 24 hours with
a maximum release rate no greater than the existing one-year peak
discharge, and a drawdown time no greater than 72 hours. A waiver
from the Village must be granted to use this alternative approach.
The Village may adopt a resolution establishing more detailed
design and performance standards for stormwater management facilities,
consistent with the terms of this chapter, and in order to further
implement its goals and purposes.
