(a) So that a receiving stream's physical, chemical and biological characteristics are protected and stream functions are maintained, post-construction storm water practices shall provide perpetual management of runoff quality and quantity. To meet the post-construction requirements of this regulation, the SWP3 must contain a description of the post-construction BMPs that will be installed during construction for the site and the rationale for their selection. The rationale shall address the anticipated impacts on the channel and floodplain morphology, hydrology and water quality.
(b) Detail drawings and maintenance plans shall be provided for all post-construction BMPs. Maintenance plans shall be provided by the applicant to the post-construction operator of the site (including property owner associations) upon completion of construction activities. Applicants may be required to develop and implement a maintenance plan to comply with regulations for municipal separate storm sewer systems. Maintenance plans must ensure that pollutants collected within structural post-construction practices are disposed of on accordance with local, state and federal regulations.
(c) Large Construction Activities: For all large construction activities (involving the disturbance of five (5) or more acres of land or will disturb less than five (5) acres, but is a part of a larger common plan of development or sale which will disturb five (5) or more acres of land0, the post-construction BMPs chosen must be able to detain storm water runoff for protection of the stream channels, stream erosion control and improved water quality. Structural (designed) post-construction storm water treatment practices shall be incorporated into the permanent drainage system for the site. The BMPs chosen must be sized to treat the water quality volume (WQv) and ensure compliance with Ohio's Water Quality Standards in OAC Chapter 3745-1. The WQv shall be equivalent to the volume of runoff from a 0.75-inch rainfall and shall be determined according to one of the two following methods:
(1) Through a site hydrologic study approved by the City's authorized reviewing agent that uses continuous hydrologic simulation and local long-term hourly precipitation records or
(2) Using the following equation:
WQv = C * P * A/12 where:
WQv = water quality volume in acre-feet
C = runoff coefficient appropriate for storms less than 1 inch
(See Table 4)
P = 0.75 inch precipitation depth
A = area draining into the BMP in acres
Table 4: Runoff Coefficients Based on Land Use
Land Use | C |
Industrial and Commercial | 0.90 |
High Density Residential (5 to 8 Dwellings/Acre) | 0.70 |
Medium Density Residential (2 to 4 Dwellings/Acre) | 0.55 |
Low Density Residential (1 Dwelling/Acre) | 0.30 |
Open Space and Recreational Areas | 0.25 |
Agricultural | 0.15 |
(3) Where land uses will be mixed, the runoff coefficient should be calculated using a weighted average.
(4) An additional volume equal to 20 percent of the WQv shall be incorporated into the BMP for sediment storage and/or reduced infiltration capacity. It is recommended that BMPs be designed according to the methodology included in the most recent edition of the Rainwater and Land Development Manual. BMPs 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 Table 5.
Table 5: Target Drain Times for Post-Construction BMPs
Best Management Practice (BMP) | Drain Time of WQv (Hours) |
Infiltration | 24 to 48 |
Vegetated Swale and Filter Strip | 24 |
Extended Detention Basin (Dry Basin) | 48 |
Retention Basin (Wet Basin) | 24 |
Constructed Wetland (Above Permanent Pool) | 24 |
Media Filtration, Bioretention | 40 |
(5) The applicant may request approval from the City's authorized reviewing agent to use alternative structural post-construction BMPs if the applicant can demonstrate that the alternative BMPs are equivalent in effectiveness to those listed in Table 5. Construction activities shall be exempt from this condition if it can be demonstrated that the WQv is provided within an existing structural post-construction BMP that is part of a larger common plan of development or if structural post-construction BMPs are addressed in a regional or local storm water management plan. For redevelopment projects (i.e., developments on previously developed property), post-construction practices shall either ensure a 20 percent net reduction of the site impervious area, provide for treatment of at least 20 percent of the WQv, or a combination of the two.
(d) Small Construction Activities. For all small land disturbance activities (which disturb one (1) acre or more, but less than five (5) acres of land and are not a part of a larger common plan of development or sale which will disturb five (5) or more acres of land), a description of measures that will be installed during the construction process to control pollutants in storm water discharges that will occur after construction operations have been completed must be included in the SWP3. Structural measures should be placed on upland soils to the degree attainable.
(1) Such practices may include, but are not limited to: storm water detention structures (including wet basins); storm water retention structures; flow attenuation by use of open vegetated swales and natural depressions; infiltration of runoff onsite; and sequential systems (which combine several practices). The SWP3 shall include an explanation of the technical basis used to select the practices to control pollution where flows exceed pre-development levels.
(2) Velocity dissipation devices shall be placed at discharge locations and along the length of any outfall channel to provide non-erosive flow velocity from the structure to a watercourse so that the natural physical and biological characteristics and functions are maintained and protected (e.g., no significant changes in the hydrological regime of the receiving water).
(Ord. 06-16. Passed 5-9-16.)