Design storms for control of discharges from storm or combined sewer systems are typically stated in terms of duration/recurrence-interval, e.g. the system must be designed for the 5-year 24-hour storm or the 1-year 6-hour storm. Selection of the design storm under these conditions normally means selecting it from standard curves or charts prepared by a government agency for the specific geographic area involved. When control requirements are stated in terms of allowable overflows or discharges, rather than in terms of containment or conveyance of a specific storm event, selection of the design storm or storms is not so easily determined. This chapter presents a methodology for selecting design storms to limit overflow from a combined sewer system to not more than a specified number of occurrences per year.

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