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).
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.