I am trying to determine what numbers to use for the "average rainfall event" and "rainfall event interval" to calculate the volume captured via our collection facilities. Since the volume of runoff is calculated by using the 2 year 24 hour rainstorm INTENSITY (inches/hour), should I use the 2 year 24 hour rainstorm PRECIPITATION DEPTH (inches) for the "average rainfall event"? If so, what, then, is my rainfall event interval? 24 hours? 2 years? I need to convert this to seconds to calculate the minimum drawdown rate, but don't know what the interval is. With real numbers, the 2 year 24 hour intensity is 0.13 in/hr and the 2 year 24 hour depth 3.14 inches.
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Bradley Buck
1 thumbs up
December 15, 2012 - 4:56 pm
Based on the research I've done and the example calculation within SSc6 in the Reference Guide 2009 Ed., Updated April 2010, this is based on the 'watershed type" your site is located within. The 'average rainfall event' is based on the 90th percentile of average rainfall within the type of watershed. There are three watershed types: humid (1" / >40" annually), semi-arid (0.75" / 20-40" annually), and arid (0.5" / <20" annually). The design storm figures you provided are used instead for Equation 3: Determining Rainwater Runoff.