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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 />- HYDROLOGY AND HYDRAULIC DETERMINATIONS - <br /> <br />100000 <br /> <br />Hydrologic Analysis <br /> <br />The hydrologic analysis for Boulder Creek consisted of determining the magni- <br />tude of the 10-, 50-, 100-, and 500-year floods. Since the fragmentary gaging <br />record for Boulder Creek at Boulder was considered too short to be a reliable <br />basis for estimating the discharge-probability at Boulder and since the long- <br />term records for Boulder Creek at Orodell does not include the flows on Four- <br />mile Creek, which, according to historical information, has been the major <br />source of flooding in Boulder, the discharge-probability relationships for Boulder <br />Creek were developed using the Environmental Protection Agency's Storm Water <br />Management Model (SWMM). This model was calibrated using data collected from <br />the flood hydrograph of May, 1973, for the uncontrolled area of Cherry Creek <br />located downstream from Cherry Creek Dam. Based upon this calibration, the <br />hydrologic model was applied to the Boulder Creek basin. Tributary hydrographs <br />developed by SWMM were routed along Boulder Creek using the Corps of Engineers' <br />version of Harder's diffusion routing model. The resulting discharge-proba- <br />bility relationships were compared with that from the short record on Boulder <br />Creek at Boulder and with the larger record on Boulder Creek at Longmont and <br />with regional criteria prepared by the U.S. Geological Survey. These compari- <br />sons were considered good (Figure 2). <br /> <br />50000 <br /> <br />[ ISC HAR E IIALU ~S 08 AI NE ~ ITI S,^ MM ---- <br /> .......... '. <br /> / <br />U by! DE YEL bPEC Ff O~ f E( 10~ AL j / <br />RI h-E IA RE' ENT tD IN Tr # I . <br /> / <br /> / <br /> /' -r <br /> E STI~ ATE D V. Jl,LU -. F R n E j <br /> ,-_S <br /> r.:T" . ..".,. ""'" cue VI.:.. ., --- ------ rr <br /> ~ l) <br /> ,&,.- E'i E F T LI E <br /> ~AC" '1\ <br /> (! <br /> t;\..; <br /> I;)~ <br /> V <br /> ./ ......0 ~ REC RD ED F LO NS 18 89 0 908 WI H <br /> 1893 I~ 4, 19 )2 19 D3 I\ND 19C 4 ~ IS IN p <br /> <br />o <br />z <br />o <br />u <br />w <br />CJ) <br />Q:: 10000 <br />W <br />a.. <br /> <br />~ <br />W 5000 <br />W <br />LL <br />U <br />lD <br />::l <br />U <br />Z <br /> <br />Rainfall input for the hydrologic model was derived from the National Oceanic <br />and Atmospheric Administration's Atlas 2, Vol. III, for Colorado, published <br />in 1973. This data was adjusted for expected-probability. Values for the <br />500-year flood were extrapolated. After study of the various rainfall dura- <br />tions contained in the NOAA Atlas, a 6-hour duration was selected. A 30- <br />minute distribution of the 6-hour storm was developed from a study of hourly <br />precipitation data recorded for major streams in the South Platte River basin and <br />refined to 30-minute values. Rainfall values for a 6-hour duration are 3.8 <br />inches, 2.8 inches, 2.5 inches, and 1.7 inches for the 500-, 100-, 50-, and 10- <br />year floods respectively. Adjustments for depth area were based on the depth- <br />area relationship presented in the NOAA Atlas. <br /> <br />W <br />~ 1000 <br /><l: <br />J: <br />U <br />CJ) <br />o 500 <br /> <br />100 <br />99.9 99.5 9998 95 90 80 70 6050 40 30 20 10 5 2 I ,5 <br /> <br />,I <br /> <br />EXCEEDANCE PROBABILITY IN PERCENT <br /> <br />FIGURE 2 - DISCHARGE PROBABILITY CURVE <br /> <br />7 <br />