For proposed developments of 800 square feet or less of new impervious area:
For low impact developments, in which the total proposed impervious area added to a site is 800 square feet or less, the storm water management regulations and submittal procedures are stated in the Stormwater Guidelines for Low Impact Development, which can be obtained from the Village Zoning Department.
For proposed developments of greater than 800 square feet of new impervious area:
(a) Hydrologic Design. All drainage ways shall be designed in accordance with the following criteria:
(1) Major Waterways. Major waterways are defined as those with a tributary area in excess of four (4) square miles. Major waterways shall be designed for an average recurrence interval of 100 years.
(2) Secondary Waterways. Secondary waterways are defined as those with a tributary area of between one (1) and four (4) square miles. Secondary waterways shall be designed for an average recurrence interval of fifty (50) years.
(3) Minor Waterways. Minor waterways are defined as those with one (1) square mile or less of tributary area. Minor waterways shall be designed for an average recurrence interval of twenty-five (25) years for open channels and culverts.
(4) Roadway Storm Sewer Networks. All roadway storm sewer shall be designed for an average recurrence interval of ten (10) years.
(b) Hydraulic Design.
(1) The design hydraulic grade line for any closed or open waterway, bridges or culverts shall be designed to contain the twenty-five (25)-year rainfall frequency within the proposed system. Storm sewers designed as part of a road system with curb shall be designed so that the hydraulic grade line shall be no higher than the inlet grates for a design discharge of the twenty-five (25)-year design discharge (Q).
(2) For all roadway design improvements or developments, catch basins shall be so placed along the streets that the width of flow does not impede traffic. For acceptable spread requirements along a roadway, reference Ohio Department of Transportation Location and Design Volume 2, the most current edition.
(3) The depth of flow or ponding for a 100-year average recurrence interval storm shall not exceed a level which would cause inundation or damage to any dwelling constructed within the project area.
(4) Site grading within the development shall be such that all lots will readily drain. Lots shall have a one percent (1.%) minimum slope in grass areas. Overland flow on lots shall be limited to a maximum distance of three hundred (300) feet unless approved by the Village of Cedarville.
(5) Bridges spanning open waterways shall have minimum freeboard above water surface of two (2) feet.
(6) The minimum velocity for any closed or open conduit shall be two and one-half (2.5) feet per second under gravity flow conditions.
(c) Structural Design. Catch basins, manholes, inlet structures, etc. placed within an improvement or development shall conform to standard plans on file with the Village. Structural design of all drainage facilities shall be subject to the approval of the Village of Cedarville.
(1) Channels/Swales.
A. Minimum bottom width of constructed channels shall be two (2) feet.
B. Each channel constructed shall have side slopes of 3: 1 or flatter.
C. All permanent drainage swales shall be shown on the construction plans with easements on the final plat.
D. Maintenance of all drainage swales through a development shall be the responsibility of the developer/owner.
E. Bank stabilization and stream bed stabilization, along constructed or natural channels, will be required if the channel velocities are sufficient to cause bank or invert erosion. The top of bank shall be so graded that side drainage will enter channels only at points where structures are provided to prevent bank erosion. Side drainage flow shall enter the channel as nearly parallel with stream flow as possible. Earth channels shall be seeded or sodded depending on the velocity of flow within the channel.
(2) Closed Conduits.
A. Storm drainage within the development, which is capable of being transmitted in a pipe forty-eight (48) inches or less in diameter, shall be carried in a closed conduit. At the request of the applicant and their design engineer, this requirement may be waived. The reason for this waiver shall be to meet the requirements of the Ohio Environmental Protection Agency Construction Permit. The minimum conduit size shall be twelve (12) inch pipe.
B. Minimum clearance between top of pipe and top of surface shall be two (2) feet. All pipes with clearance less than two (2) feet shall be reinforced concrete pipe. The alignment of closed conduits shall be as nearly straight as practical without bends and angle points; manholes shall be provided at all angle points and at intervals not to exceed 300 feet along the conduit unless submitted for approval with proper documentation. Inverted siphons shall not be permitted except for temporary structures.
C. All pipe, bedding and backfill shall be designed in accordance with the Ohio Department of Transportation Design Manual. Field tile/single wall storm tile is NOT allowed. Ditch protection shall be required if the ten (10)-year flow velocities in a channel or waterway exceed four (4) feet per second for soil ditches or six (6) feet per second in sodded ditches. If the exit velocity from a storm sewer exceeds the allowable velocities, an energy dissipating device (i.e. stilling basing, dumped rock) may be necessary.
(d) Storm Water Runoff Control Criteria for Retention/Detention Basins. Any development which increases the runoff rate and volume shall be required to control the discharge rate of runoff prior to its release to its off-site outlet.
(1) Any increase in the volume of site surface drainage water resulting from accelerated runoff caused by site development shall be controlled so that the post development peak discharge rate does not exceed that of the pre-development peak discharge rate, for all twenty-four (24)-hour storms between a one-year frequency and the critical storm frequency as determined below. The method by which an applicant shall determine changes in rates and volumes of runoff is presented in the U.S. Department of Agriculture, Engineering Division of the Soil Conservation Service, Urban Hydrology for Small Watersheds, Technical Release No. 55, June 1986 or most current edition.
To find the critical storm frequency for which additional control will be needed, the applicant shall:
A. Determine the percent increase in runoff volume for a one-year frequency, twenty-four (24)-hour storm occurring on the development area.
B. Determine the critical storm frequency for which additional control is needed by using the percent increase in runoff volume, derived in (a), in Table 1.
C. Control the post development storms of a frequency between one year and the critical storm determined in (b), so as to be equal to or less than the predevelopment peak runoff rate for a twenty-four (24)-hour one-year frequency storm.
(1) Storms of less frequent occurrence (longer return periods) than the critical storm up to the one-hundred-year storm have peak runoff rates no greater than the five(5)-year frequency peak runoff rates. Consideration of the one, two, five, ten, twenty-five, fifty and one hundred-year storms shall be considered adequate in designing and developing to meet this standard.
(2) In no instance, shall the post developed peak runoff rate exceed half of the capacity of the immediate downstream receiving drainage system.
Critical Year Storm Calculation:
(1-year Post-1-Year Pre) x 100%
(1 Year Pre)
Table 1
DETERMINING STORM FREQUENCY FOR WHICH CONTROL IS NEEDED | ||
Percent Increase in Runoff Volume From a 1 Year Frequency, 24 Hour Storm | ||
equal or greater than (percent) | less than (percent) | Storm Frequency (years) |
- | 10 | 1 |
10 | 20 | 2 |
20 | 50 | 5 |
50 | 100 | 10 |
100 | 250 | 25 |
250 | 500 | 50 |
500 | - | 100 |
(e) Post Construction Runoff Control. The drainage design shall incorporate the following post-construction storm water management requirements. These requirements are necessary in order to reduce increases in non-point pollution caused by storm water runoff, minimize the total runoff volume which flows from a site following increases in impervious area by development, and minimize changes in the watershed hydrology inherent with development.
(1) The post construction requirements should meet or exceed the most current Ohio Environmental Protection Agency Construction.
(2) The site-specific characteristics shall be considered in the design.
A. Minimum orifice sizes should be considered in areas of soils with low permeability rates.
B. Infiltration systems in sites over Well Field areas shall require the approval of the Health Department.
(Ord. 2021-08. Passed 12-13-21.)