Determining the peak rate of storm water run-off reaching the point of consideration is fundamental to the design of the storm water management system. Careful selection of variables is prerequisite to adequate design.
(A) Rational method. The rational method shall be used to determine peak rate of run-off from areas not larger than 50 acres. The basic formula for the rational method is Q = ciA where:
(1) Q = Peak rate of run-off in cubic feet per second (cfs).
(2) c = Run-off coefficient representing the characteristics of the drainage area.
(3) i = Average intensity of rainfall in inches per hour for a duration equal to the time of concentration (tc) for a selected rainfall frequency.
(4) A = Area drained in acres.
(B) Run-off coefficient. The following table presents values for use with the rational method. A weighted run-off coefficient must be individually computed for each drainage area contributing to a catch basin, an inlet, a culvert, an open channel, or any other drainage facility. The Village Engineer may permit a general weighted run-off coefficient to be used within a subdivision, land development, or for portions of the area. The calculations will be part of the drainage and grading plan submission.
Character of Surface Run-off Coefficient
Pavements, roofs, and
other hard surfaces 0.90
Lawns, yards, and other
unpaved areas, except
wooded areas
Flat - less than 4:1 slope 0.40
Steep - 4:1 slope or greater 0.70
Natural dense wooded areas 0.1 to 0.40
(1) The use of the above run-off coefficients for natural dense wooded areas is restricted to areas 50 feet or more away from a building which will remain undisturbed and in their natural state; and where engineering data verifying the coefficient has been presented to and approved by the Village Engineer.
(2) Time of concentration. The time of concentration in minutes is the estimated time for storm run-off from the most remote part of the drainage area to reach the point being analyzed. Time for flow in open channels, gutters, pipes, and the like will be estimated using appropriate hydraulic methods. The total time of concentration in minutes is the summation of overland flow, open channels, gutters, pipe flow, and the like. The time of concentration to any drainage facility or point of design is the greater of the summation as defined above or the minimum time of ten minutes.
(3) Intensity of rainfall. The intensity of rainfall will be obtained from the most recent edition of the Hamilton County Storm Drainage Standards, Plate II. The frequency of storm to use is specified in the section of these criteria covering the type of facility being designed.
(Ord. C-604, passed 9-8-86)