§ VI. Major system.
The "major" drainage system consists of all possible routings of runoff from storms greater than the designated "design storm". The major system includes the formal system hydraulically designed to convey the design runoff to receiving streams or channels. The major system also includes side and rear yard swales, streets, and other natural or man-made avenues for large storm runoff to reach large watercourses. The major system will always exist whether considered in design of the development or not. Safe routing of major runoff through the project area must be considered in the initial development of project layout and grading. Details of the major routings in the form of designating flowways and swales must be submitted for review concurrent with submittal of storm drainage facilities conveying the design storm.
Estimates of the runoff rates from a 100-year rainfall should be made for the various reaches of the probable "major" system. The probable hydraulic behavior of the various flowways and swales should be examined.
Several studies over the years have suggested that during major rainfall events, runoff is significantly greater than produced by the Rational Formula. To account for antecedent precipitation conditions producing this excess runoff, the Rational Formula for use with major storms should be adjusted by multiplying the right side of the Rational Formula by a frequency factor, C f . The formula now becomes:
Q = CiAC f
The following table of C f should be used. The product of C × C f should not exceed 1.0.
Recurrence Interval
YearsC f 50 1.20 100 1.25
(Ord. No. 4119, § 1(Exh. A), 6-16-08)