All development shall include provision for safe and efficient removal of surface and subsurface waters from the site. Technical standards and specifications for measures to be taken to meet the following criteria are found in the current version of the EPA NPDES General Permit For Construction Site Stormwater and Rainwater and Land Development: Ohio's Standards for Stormwater Management, Land Development and Urban Stream
Protection.
(a) Drainageways to Remain Clear of Obstruction. Except for stormwater control facilities approved by the City, no new temporary or permanent structure to fill and no storage of materials or equipment (other than watercraft) shall be permitted within the channels or floodways of permanent or intermittent streams, drainage ditches, or any other drainage facility or system, unless the developer submits an engineering analysis that demonstrates:
(1) That such development will not impair the capacity of such facility or system, either by itself or by catching or collecting debris carried by flowing water.
(2) That such structures, materials, or equipment will not be subject to damage by flowing water or materials carried by flowing water at velocities likely to be encountered during floods; and,
(3) That no such structure or material or equipment shall be susceptible to being entrained and transported by flowing water, thereby to pose a threat to downstream property.
(b) Ponds Less Than 4,000 Square Feet. Ponds with a surface area less than 4000 square feet are exempt from the prohibitions in Section 1154.02. Ponds with surface area greater than 4000 square feet are exempt from the prohibitions of Section 1154.02 provided the design is inspected and approved by the City Engineer.
(c) Disposal of Stormwater. All drainage facilities shall be designed and maintained to carry surface and/or subsurface waters to the nearest practical storm drain or natural watercourse approved by the City as a safe place to deposit such waters. Concentrated stormwater shall not be allowed to flow down the face of a slope unless it is contained within an adequate channel, flume, or slope drain structure with approved energy dissipation. The grade of drainage facilities shall be sufficient to maintain sediment in suspension.
(d) Stormwater Structures to be Related to Downstream Discharge Facilities. Stormwater retention and detention structures and drainage facilities shall be designed and maintained so as not to overload downstream discharge facilities.
(e) Erosion and Flood Control in Outfalls and Channels. Storm outfalls and constructed or modified channels shall be designed and constructed to withstand the expected velocity of flow from a post development, twenty-five-year frequency, 24-hour duration storm without eroding or flooding. Where such conduits may cross unstable slopes or slopes capable of becoming unstable with the addition of significant water, the conduits shall be lined with a water- resistant material.
(f) Storm water Control for Development Areas Disturbing One Acre or More or Within a Common Plan of Development. Shall meet the requirements for earth disturbed area (EDA) as described in the current edition of the EPA NPDES General Permit For Construction Site Stormwater. To control stream channel erosion and sediment pollution caused by increased storm water runoff rates and volumes, storm water rnnoff from development areas shall be restricted so that development does not increase peak runoff rates so that any increase in runoff volume consequent to development is compensated for by a reduction in runoff rates, as follows:
(1) Development shall not increase rate of runoff. The peak rates of runoff from any area following development shall be no greater than the peak rates of runoff from equivalent-size storms in the same area prior to that development for all twenty-four (24) hour storms from one-to-one-hundred-year frequency. Design and development to match the peak rate of runoff for the one-, five-, ten-, twenty-five-, fifty-, and one-hundred-year storms shall be considered adequate to meet this standard.
(2) Increase in volume of runoff requires reduction in rate of runoff. If the volume of runoff from an area is projected to be greater after development than before development, the volume increase shall be compensated for by reducing the peak rate of runoff from the critical storm and all more frequent storms occurring on the development area to the peak rate of runoff from a one-year-frequency, twenty-four-hour storm occurring on the same area under pre-development conditions.
For storms of less frequent occurrence (longer return periods) than the critical storm, up to the one-hundred-year storm, development need only comply with paragraph (f)(1) above.
The critical storm for a specific development area is determined as follows:
A. Calculate the total volumes of runoff from a one-year-frequency, twenty-four hour storm occurring on the development area both before and after development.
B. From the volumes calculated in paragraph (2)A. above, calculate the percent increase in volume of runoff that will result from the proposed development. Using this percentage, select the appropriate critical storm from the following table:
Percentage Increase in “Critical Storm”
Volume of Runoff Discharge Limitation
(at least) (but less than)
00 10 1 year
10 20 2 years
20 50 5 years
50 100 10 years
100 250 25 years
250 500 50 years
500 (or more) 100 years
(3) Points at which runoff is to be calculated. The requirements of this Section for runoff rates and volumes shall be satisfied at each location where runoff leaves the development area.
(4) Points at which runoff is to be calculated. The requirements of this Section for runoff rates and volumes shall be satisfied at each location where runoff leaves the development area.
(5) Access easements. Permanent access and access easements, a minimum of 20-feet wide, shall be provided to allow inspection and maintenance of stormwater control structures and stormwater conveyance systems.
(g) Drainage Facility Design. Where a development area is large enough to require the preparation of a drainage analysis, all drainage facilities shall be designed by a professional engineer in accordance with their commendations of that analysis.
(Ord. 280(22-23). Passed 6-15-23.)