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Storm Water released from any part of a small Development site of 1 acre or greater but less than 5 acres shall implement post construction Best Management Practices (BMPs)/Storm Water Control Measures (SCMs). Structural post construction BMP/SCM methods and design parameters shall be commensurate with the impacts on the watershed and follow the current version of the State of Ohio's Rainwater and Land Development manual. A description of the measures that will be installed during the construction process to control pollutants in Storm Water Discharges that will occur after construction operation has been completed must be included in the (SWP3) for review & approval. The SWP3 shall include an explanation of the technical basis used to select the practices to control pollution where flow exceeds pre-development levels.
Storm Water released from any part of a large Development site of 5 or more acres or which will disturb less than 5 acres, but is a part of a larger common plan of Development or sale which will disturb 5 or more acres of land, shall include the post-construction BMP(s)/SCM(s) that will be able to detain storm water for protection of the stream channels, stream erosion control, and improved water quality, and shall meet the most restrictive of the following criteria as well as the current requirements of the Ohio EPA:
(a) The rationale for BMP/SCM selection must address the anticipated impacts on channel and floodplain morphology, hydrology, water quality and riparian form (habitat). The BMP(s)/SCM(s) chosen must be compatible with site and soil conditions.
(b) Post construction BMPs/SCMs must achieve the following goals:
(1) Water Quality Volume (WQv): For all large Development on previously undeveloped property, structural (designed) post-construction Storm Water treatment practices shall be incorporated into the permanent drainage system for the site. These practices must be sized to treat the Water Quality Volume (WQv). The WQv shall be the maximized water quality capture volume for the site, as defined in "Urban Runoff Quality Management," WEF Manual of Practice No. 23 and ASCE Manual and Report on Engineering Practice No. 87 (WEF and ASCE, 1998).
(2) The WQv shall be determined, through a site hydrologic study approved by the Development Engineer, that uses continuous hydrologic simulation and local long-term hourly precipitation records, or by using the following equation:
WQv = C* P*A/12
WQv = Water Quality Volume in acre-feet
C = Runoff Coefficient appropriate for storms less than 1 inch (see the most current Ohio EPA NPDES Storm Water Permit Table 1)
P = 0.75 inch precipitation depth
A = area draining into the BMP in acres
Where the land use will be mixed, the Runoff Coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the Storm Water treatment structure is Low Density Residential, 30% is High Density Residential and 10% is Open Space, the Runoff Coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) = 0.35.
The following alternative equation may also be used:
WQv = (0.858i3 - 0.78i2 + 0.774i + 0.04) * PA/12
where: WQv = Water Quality Volume in acre-feet I = watershed impervious ratio, namely, percent total imperviousness divided by 100;
P = mean storm presentation volume in inches
= regression constant from least-squares analysis (see the most current Ohio EPA NPDES Storm Water Permit
A = area draining into the facility in acres
(3) An additional volume equal to 20 percent of the Water Quality Volume shall be incorporated into the facility for sediment storage and/or reduced infiltration capacity. Facilities shall be designed according to the methodology included in the WEF and ASCE manual of practice, State of Ohio's Rainwater and Land Development manual, or in another design manual acceptable for use by the Director and Ohio EPA.
(4) The BMP's listed in the most current Ohio EPA NPDES Storm Water Permit below shall be considered standard BMP's approved for general use. BMP's listed in the most current Ohio EPA NPDES Storm Water Permit shall also be incorporated in this list. BMP's shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage available for successive rainfall events as described in the most current Ohio EPA NPDES Storm Water Permit below and avoid the creation of nuisance conditions. The outlet structure must not discharge more than the first half of the WQv or extended detention volume (EDv) in less than one-third of the drain time. The EDv is the volume of Storm Water runoff that must be detailed by a structural post-construction BMP. The EDv is equal to 75 percent of the WQv for wet extended detention basin, but is equal to the WQv for all the other BMP's listed in the most current Ohio EPA NPDES Storm Water Permit.
(5) Facilities shall be cleaned and maintained such that the full Water Quality Volume is available and that the facility functions as designed.
(6) All construction activities shall maintain or improve ecological function of watercourses by protecting or improving the stream and riparian form. Ecological functions include pollution assimilation, flood attenuation, maintenance of the sediment regime, base flow, moderation of temperature and habitat to the maximum extent practicable (MEP);
(7) For all construction activities immediately adjacent to Surface Waters of the state, a minimum Riparian and Wetland setback, pursuant to Chapter 1201 : Riparian and Wetland Buffers, shall be maintained in its natural state as a permanent buffer. Where impacts within this setback area are unavoidable due to the nature of the construction activity (e.g., stream crossings for roads or utilities), the project shall be designed so the number of stream crossings and the width of the Disturbance within the setback area are minimized.
(8) For all redevelopment projects: Post-construction practices shall assure a net reduction of 20% of the impervious area of the site, or provide for treatment of 20% of the WQv
(9) Transportation improvement projects of existing facilities located predominantly within existing rights-of-way may provide post construction water quality measures in accordance with the Ohio Department of Transportation's Location & Design Manual. The above is permissible if significant right-of-way impacts are required to meet the provision of the case as determined by the Director.
(10) Transportation Projects: The construction of new roads and roadway improvement projects by public entities may implement post construction BMP's in compliance with the current version of the Ohio Department of Transportation's Location and Design Manual, Volume Two Drainage Design.
(11) Offsite Mitigation of Post-Construction: Offsite mitigation of postconstruction BMP requirements where the standard methods listed in the most current Ohio EPA NPDES Storm Water Permit Table 2 are not feasible, must be approved by the Ohio EPA.
(Ord. 2017-17. Passed 3-15-17.)