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Page 3 <br />August 20, 1998 <br />Mr. Michael B. Long <br />o.~ss <br />P = 0.5+ T 24.04 Equation (2.31a) from Barfield, Warner and Haan; 1983. <br />P,a 24 2~ T~+0.04 <br />where T =Time - 12 in hours, P =total cumulative precipitation, and Pia =total 24 hour <br />precipitation. This equation fits the Type ll curve with only slight discrepancy on either side of <br />12 hours as peak precipitation intensity is shifred to 12 hours. The discrepancy causes no <br />noticeable difference in resulting runoff. <br />To calculate the peak I-hour event one applies this equation to one-half hour either side of 12 <br />hours. The difference between the two numbers is the proportion of the 24-hour event that is <br />attributable to the peak 1-hour period. Inserting 1 l.5 into the equation for T yields 0.277. <br />Inserting 12.5 into the equation for T yields 0.723. The difference being 0.446 or 44.6% of the <br />24-hour event attributable to the peak 1-hour period. <br />The 1-hour event is applicable to the design and function of the culverts in question since the <br />time to concentration is calculated to be less than 0.1 hours for both CMP #2 and CMP #4 using <br />SEDCAD+ Ver. 3.1. <br />On July 27, 1998, between the hours of 1530 and 1630, 0.82 inches of rainfall were recorded at <br />the D Portal Area. This equates to a maximum l-hour intensity expected from a 1.84-inch 24- <br />hour event. This event would have generated peak runoff well exceeding the design <br />requirements for the culverts. Excessive flow was the basic cause for the culverts plugging <br />during the storm of July 27, 1998. <br />The 1-hour intensity of the storm of July 27, 1998 was equivalent to the 16.5-year 24-hour event. <br />This return interval is based on the log-normal relationship of storm return periods verse <br />precipitation intensities. Regressing the isopluvials from NOAA Atlas 2, Volume III for the 10, <br />25, and 100-year events (1.6, 2.1, and 2.4 inches respectively) yields the following equation: <br />RI = 1 Ot~P cvsl <br />where: <br />RI =the return interval expressed in years <br />P =the precipitation of the storm in question (1.84 equivalent 24-hour inches) <br />C =equation constant (0.896) <br />S =slope of the equation (0.7756) <br />RI = 10«1.84-o.89e>io.nse) = 16.5 years <br />This return interval exceeds the design requirements to safely pass peak runoff from a 10-year, <br />24-hour precipitation event per Rule 4, Section 4.03.2(4). Even with the excessive amount of <br />runoff little or no damage occurred beyond the clogging of the two culverts in question. <br />