(A) Design standards. Storm water BMPs shall be designed to manage storm water flow within the available capacity of the downstream conveyance system as determined by the city. In addition, storm water BMPs shall be designed to meet performance standards as described in division (B) below. Storm water system design shall be in accordance with the latest version of the Storm Water Standards Manual published by the city.
(B) Performance standards. In order to achieve the goals and purposes of this chapter, the following storm water management performance standards are hereby established:
(1) 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 storm water that is the first to runoff 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.
(a) 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 storm water runoff. Control of TSS to meet this standard is expected to achieve control of other pollutants to an acceptable level that protects water quality.
(2) 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, 24-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 storm water discharges to surface waters or the MS4.
(3) 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 Kent County. The flood control standard is required for all sites.
(a) 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. Storm water 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 storm water accumulating on the surface of the ground. Gravity will cause such storm water to flow overland to lower elevations. By carefully managing the shape of the land surface such overland storm water flow can be directed to locations that will not cause property damage. Adequate emergency overland flow-ways will direct storm water flows generated by the 100-year storm to avoid damage to structures and facilities.
(4) Site-specific requirements.
(a) Pretreatment. Pretreatment of site runoff is required on a site-specific basis prior to discharging to certain storm water BMPs. Pretreatment provides for the removal of fine sediment, trash and debris, and preserves the longevity and function of the BMP.
(b) 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 storm water runoff to address pollutants associated with hot spots is required for the site. Storm water management strategies and BMPs that reduce the potential to mobilize existing soil and groundwater contaminants, or that capture and treat storm water runoff and/or accidental spills to protect groundwater or nearby surface waters are required.
(c) Coldwater streams. Storm water 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).
(C) Alternative approach for channel protection. In many cases, infiltration will likely be used as the primary means of retention. It is not, however, the sole means of providing onsite retention, and the developer must include consideration of storm water reuse, interception, evapotranspiration, and other vegetative (non-structural) BMPs at the project site. Site constraints that limit the use of infiltration may include:
(1) Poorly draining soils (<0.24 inches 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) Storm water hot spots; and
(6) Part 201 and 213 sites, and areas of soil or groundwater contamination.
The city may grant a waiver of the onsite retention criteria for channel protection described in division (B) above and allow an alternative approach to meet the channel protection performance standard if the developer demonstrates that site constraints preclude sufficient retention onsite. If a waiver is granted, the developer must meet the following extended detention criteria:
Extended detention. Detain the portion of the channel protection volume unable to be retained onsite 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 city must be granted to use this alternative approach.
(D) Resolution to implement performance and design standards. The City Commission of the City of East Grand Rapids may adopt a resolution establishing more detailed design and performance standards for storm water management facilities, consistent with the terms of this chapter, and in order to further implement its goals and purposes.
(Prior Code, § 2.187) (Ord. effective 1-18-2002; Ord. effective 4-29-2022)