(a) All developments requiring a stormwater plan shall be designed, constructed, and maintained to prevent flooding, minimize stream channel impacts, protect water quality, and achieve the purposes of this chapter, as stated above. The city has adopted performance standards to meet the objectives of managing the quantity and quality of stormwater runoff from a site as detailed below and/or in the guidance manual.
(b) Designers may select any combination of stormwater BMPs which meet the performance standards provided the selections:
(1) Comply with the requirements identified in this chapter and the guidance manual;
(2) Comply with all other local, county, state, or federal requirements; and
(3) Do not conflict with the existing local stormwater management and watershed plans.
(c) The particular facilities and measures required on-site shall take into consideration the natural features, upland areas, wetlands, and watercourses on the site; the potential for on-site and offsite adverse stormwater impacts, water pollution, and erosion; and the size of the site.
(d) The following on-site stormwater management concepts shall be followed:
(1) Natural topography and site drainage shall be preserved and site grading shall be minimized to the maximum extent reasonably achievable considering the nature of the development.
(2) The preferred conveyance strategy is to transport, wherever possible, untreated and treated runoff in conveyance facilities open to the atmosphere (e.g. swales, vegetated buffer strips, energy-dissipating structures, etc.), rather than through enclosed pipes, so as to decrease runoff velocity, allow for natural infiltration, allow suspended sediment particles to settle, and to remove pollutants.
(3) Watercourses shall not be deepened, widened, dredged, cleared of vegetation, straightened, stabilized, or otherwise altered without applicable permits or approvals from the public works director, relevant county agencies and the applicable State of Michigan department(s).
(4) The following channel protection criteria shall be met. There shall be no net increase in runoff rate and volume from storm events up to the two-year, 24-hour event from predevelopment conditions. Specific details for compliance with this requirement are outlined in the guidance manual.
(5) The following peak rate/flood control criteria shall be met. The peak discharge rate from all storms up to the 100-year, 24-hour event shall not be greater than predevelopment discharge rates. Where the runoff volume is not increased from the predevelopment condition, the peak rate corresponding to the same storms is considered controlled. If specific watershed conditions require additional peak rate control, the city may, in its discretion, upon the recommendation of its public works director:
a. Restrict the peak discharge from the 100-year, 24 hour event to a fixed release rate; or
b. Require additional runoff volume reduction.
Specific details for compliance with this requirement are outlined in the guidance manual.
(6) The following water quality criteria shall be met. Water quality criteria are met when:
a. The minimum treatment volume control criteria of one inch of runoff from the entire site is detained and
b. When the following total suspended solids criteria is met:
1. A minimum of 80% of total suspended solids has been removed as compared with uncontrolled runoff, or
2. A maximum discharge concentration of TSS not to exceed 80 mg/l is met.
If the required minimum TSS levels are not met by the capture of one inch of runoff from the entire site, then additional treatment methods must be designed to meet the minimum TSS removal requirements. Specific details for compliance with this requirement are outlined in the guidance manual.
(7) Under certain conditions, the city, upon recommendation by the public works director, may impose the following additional restrictions on stormwater discharges:
a. Peak discharge may be further restricted when it can be shown that a probable risk to downstream structures or unique natural areas exists or that existing severe flooding problems could be negatively impacted.
b. Measures may be imposed to protect against ground or surface water pollution where the nature of the soils or bedrock underlying a stormwater management structure constitutes substantial risk of contamination, such as might be the case in limestone formations. Special provisions to be followed in these cases will be provided by the public works director.
c. Where groundwater yields are very low or where a groundwater supply already is heavily used, the city may require that the entire volume of the two-year, 24-hour rainfall event be retained and infiltrated. If substantial irrigation needs are anticipated, portions of stored stormwater may be reused for irrigation purposes.
(8) Stormwater "credits" for onsite stormwater management may be considered in accordance with the provisions provided in the guidance manual.
(Ord. No. 203, 10-17-2016)