(a) Runoff Rate: The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations proving no increase in the runoff rates from the one (1), two (2), five (5), ten (10), twenty-five (25), fifty (50) and one hundred (100) year storms.
(b) Runoff Volume: Increases in the runoff volume shall be offset by further restricting runoff rates. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the one (1) year pre- development storm runoff rate. The critical storm shall be calculated as follows:
(1) Determine the total volume of runoff from a one-year frequency, twenty-four hour storm, occurring on the development area before and after development.
(2) From the volumes in paragraph (1) determine the percent of increase in volume of runoff due to development according to the equation (Q after divided by the Q before) X 100 and, using this percentage, select the critical storm from this table:
Table 4: Critical Storm Selection
The Percentage Increase in Volume Of Runoff is: | ||
Equal To Or Greater Than | And Less Than | The 24-hour "Critical Storm" For Discharge Will Be |
0 | 10 | 1 Year |
10 | 20 | 2 Years |
20 | 50 | 5 Years |
50 | 100 | 10 Years |
100 | 250 | 25 Years |
250 | 500 | 50 Years |
500 | --- | 100 Years |
(c) Detention Or Retention Basin Exemption For Redevelopment Or For Expansion Of Existing Facilities:
(1) For any development regulated by this ordinance, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for a 100 year frequency twenty-four (24) hour storm increases the existing peak discharge by one (1) cubic foot per second or less using the TR-55 method of calculation or other method approved by the Community Engineer. The Community Engineer can waive this requirement if existing storm sewers and drainage structures can safely handle the expected increase in flow.
(2) Only one (1) exemption will be allowed per parcel. Any subsequent expansion must provide for detention or retention and must include the previously exempted area.
(d) Where the Community Engineer determines that site constraints exist in a manner that compromises the intent of this ordinance to improve the management of storm water runoff as established in this section, practical alternatives may be used to result in an improvement of water quality and/or a reduction of storm water runoff. Such alternatives must be in keeping with the intent and likely cost of those measures that would otherwise be required to meet the objectives of this section. When possible, all practical alternatives shall be implemented within the drainage area of the proposed development project. Practical alternatives can include, but are not limited to:
(1) Fees shall be paid in an amount specified by the Community Engineer. These fees shall be applied by the Community to storm water management practices that reduce existing storm water runoff.
(2) Implementation of off-site storm water management practices.
(3) Watershed or stream restoration.
(4) Retrofitting of an existing storm water management practice.
(5) Other practices approved by the Community Engineer in keeping with the intent of this section.
(e) Table 4b: Infiltration Post Construction Practices with Maximum Drain Times
Table 4b Infiltration Post-Construction Practices with Maximum Drain Times
Infiltration Practices | Maximum Drain Time of WQv |
Bioretention Area/Cell 1,2 | 24 hours |
Infiltration Basin 2 | 24 hours |
Infiltration Trench 3 | 48 hours |
Permeable Pavement - Infiltration 3 | 48 hours |
Underground Storage - Infiltration 3,4 | 48 hours |
1. Bioretention soil media shall have a permeability of approximately 1 - 4 in/hr. Meeting the soil media specifications in the Rainwater and Land Development manual is considered complaint with this requirement. Bioretention cells must have underdrains unless in-situ condition allow for the WQv (surface ponding) plus the bioretention soil (to a depth of 24 inches) to drain completely within 48 hours.
2. Infiltrating practices with the WQv stored aboveground (bioretention, infiltration basin) shall fully drain the WQv within 24 hours to minimize nuisance effects to standing water and to promote vigorous communities of appropriate vegetation.
3. Subsurface practices designed to fully infiltrate the WQv (infiltration trench, permeable pavement with infiltration, underground storage with infiltration) shall empty within 48 hours to recover storage for subsequent storm events.
4. Underground storage systems with infiltration must have adequate pretreatment of suspended sediments included in the design and documented in the SWP3 in order to minimize clogging of the infiltrating surface. Pretreatment shall concentrate sediment in a location where it can be readily removed. Examples include media filters situated upstream of the storage or other suitable alternative approved by Ohio EPA. For infiltrating underground systems, pretreatment shall be eighty percent (80%) effective at capturing total suspended solids according to the testing protocol established in the Alternative Post-Construction BMP Testing Protocol.
(Ord. 2022-17. Passed 3-22-22.)