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<br />I <br /> <br />In each case, a SOC-year value w~s estimated by extrapolating the <br /> <br />TableJ <br /> <br />selected Atlas values on semllogarithmic paper. <br /> <br />Adjustments for <br /> <br />Rainfall Depths for J-Hour Storm Duration <br />at Various Frequencies - Lefthand Creek <br />Reach - Spring GulCh to James Creek <br /> <br />depth-area were based on the depth-area relationships in the NOAA <br /> <br />publication. Adjustments for length of record were made on logarithmic <br /> <br />I <br />I <br />[ <br />I <br /> <br />Frequency <br />lO-year <br />50-year <br />lOO-year <br />50D-year <br /> <br />Depth (Inches) <br />1., <br />1.3 <br />1.7 <br />3.' <br /> <br />probability paper using Beard's table {Reference 9) of expected <br /> <br />probability adjustments for various lengths of record. A uniform <br />infiltratfon rateaf 1.0 inch per hour was used for the Lefthand Creek <br />basin. Detention storage was 0.3 inches for the 6-l1our rainfall, 0.4 <br /> <br />Table 4 <br /> <br />inches for the 3-hour rainfall, and 0.5 inches for the l-hourrainfall. <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 />published by the Water Resources Council (Reference 10) were used to <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 />10-year <br />50-year <br />lOO-year <br />50D-year <br /> <br />Depth (inches) <br />1.9 <br />1.7 <br />3.1 <br />4.0 <br /> <br />analyze the stream gage records. Table 6 lists these stream gaging <br />stations. <br /> <br />Table 6 <br /> <br />lefthandCreek <br />Stream Gaging Stations <br /> <br />location <br /> <br />Drainage Area <br />(square miles) <br /> <br />Gaging <br />Records <br /> <br />TableS <br />Rainfall Depths for l-Hour Storm Duration <br />at Various Frequencies - James Cr€ek <br /> <br />Near Boulder <br />(2.5 miles above <br />Foothills Highway) <br /> <br />51 <br /> <br />1929 <br />1947 <br />1955 <br />1976 <br /> <br />1931 <br />1953 <br />1957 <br />19BO <br /> <br />At Lon9mant (mouth) <br /> <br />72 <br /> <br />1927 1942 <br />1953 1955 <br /> <br />Frequency <br /> <br />Depth (inches) <br /> <br />lD-year <br />50-year <br />100-year <br />SOO-year <br /> <br />1.3 <br />1.8 <br />1.1 <br />1.8 <br /> <br />The 100-yearflood discharges attenuate from 6,700cfs at the upstream <br />study limit to 4,6l0cfs at the mouth. The reduction of overbank storage <br />by future development would tend to reduce the attenuation effect, thus <br />causin9 an increase in floodflows in the lower reaches of the study. <br /> <br />, <br />