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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />In each case, a 500-year value was estimated by extrapolating the <br /> <br />Table 3 <br /> <br />selected Atlas values on semi logarithmic paper. <br /> <br />Adjustments for <br /> <br />Rainfall Depths for 3-Hour Storm Duration <br />at Various Frequencies - Lefthand Creek <br />Reach - Spring Gulch to James Creek <br /> <br />Frequency <br /> <br />lO-year <br />50-year <br />lOa-year <br />500-year <br /> <br />Depth (inches) <br />1.6 <br />2.3 <br />2.7 <br />3.6 <br /> <br />depth-area were based on the depth-area relationships in the NOAA <br />publication. Adjustments for length of record were made on logarithmic <br />probabi 1 ity paper us i ng Beard I stab 1 e (Reference g) of expected <br /> <br />probability adjustments for various lengths of record. <br /> <br />A uniform <br /> <br />la-year <br />50-year <br />lOa-year <br />500-year <br /> <br />Depth (i nches) <br /> <br />1.9 <br />2.7 <br />3.1 <br />4.0 <br /> <br />infiltration rate of 1.0 inch per hour was used for the Lefthand Creek <br />basin. Detention storage was 0.3 inches for the 6-hour rainfall, 0.4 <br />inches for the 3-hour rainfall, and 0.5 inches for the 1-l1our rainfall. <br />Discharge probability analyses were conducted for both gaging stations on <br />Lefthand Creek in order to provide a guide for the calibration of the <br />SWMM model. The analytical methods presented in Bulletin No. 17, <br />publ ished by the Water Resources Council (Reference 10) were used to <br />analyze the stream gage records. Table 6 lists these stream gaging <br />stations. <br /> <br />Table 4 <br /> <br />Rainfall Depths for 6-Hour Storm Duration <br />at Various Frequencies - Lefthand Creek <br />Reach - James Creek Downstream to Mouth <br /> <br />Frequency <br /> <br />Table 6 <br /> <br />Location <br /> <br />Lefthand Creek <br />Stream Gaging Stations <br /> <br />Drainage Area <br />(square miles) <br /> <br />Gaging <br />Records <br /> <br />Table 5 <br /> <br />Near Boulder <br />(2.5 miles above <br />Foothills Highway) <br /> <br />52 <br /> <br />1929 - 1931 <br />1947 - 1953 <br />1955 .;; 1957 <br />1976 - 1980 <br /> <br />1927 - 1942 <br />1953 - 1955 <br /> <br />Rainfall Depths for l-Hour Storm Duration <br />at Various Frequencies - James Creek <br /> <br />At Longmont (mouth) <br /> <br />72 <br /> <br />Frequency <br /> <br />10-year <br />50-year <br />lOa-year <br />500-year <br /> <br />Depth (i nches) <br /> <br />1.3 <br />1.8 <br />2.1 <br />2.8 <br /> <br />The lOO-year flood discharges attenuate from 6,700 cfs at the upstream <br />study limit to 4,610 cfs at the mouth. The reduction of overbank storage <br />by future development would tend to reduce the attenuation effect, thus <br />causing an increase in floodflows in the lower reaches of the study. <br /> <br />6 <br />