Laserfiche WebLink
<br />The Worrall report used data from eight u.s. Geological Survey <br />gages (Table 2) to develop flood discharges versus drainage-area <br />curves for several recurrence intervals. Seven of the gages are <br />located in the Fraser River bas~n and one is in the nearby Williams <br />Fork basin. All the gage recorqs are affected by diversions. <br />Worrall's hydrologic analysis added the recorded diversion flows, <br />obtained from records of the Denver Water Board, to the U.S. <br />Geological Survey gage records for each peak flow date to estimate <br />the total flow produced. This assumes that the diversion structures <br />are non-operational during peak ,runoff events. This is a <br />conservative assumption. These:combined annual flow peaks were <br />analyzed according to the log-p~arson Type III procedures described <br />by the Water Resources Council in Bulletin l7A (Reference 3). <br />Floodflows for the 2-year, 10-YElar, SO-year, lOa-year, and 500- <br />year recurrence intervals were calculated for all the gages. These <br />values were then used to develop the final regression equations. <br /> <br />To check Worrall's assumption of snowmelt peak flows, the month in <br />which annual peaks have been reeorded were summarized and strip <br />charts from U.S. Geological Survey gaging stations were examined. <br />The snowmelt assumption was fOund to be valid. <br /> <br />Since the Worrall study was completed in early 1981, it did not <br />use the most current Water Resources Council guidelines (Reference 4). <br />In addition, floodflows in the years 1981, 1982, and 1983 since <br />the study was completed have been significantly high. FOr instance, <br />the June 27, 1983, peak flow of 618 cfs on the Fraser River at <br />Winter park is the second highest flow of record. <br /> <br />To determine whether these chan~ed conditions would affect the <br />results significantly, data fro~ one of the eight U.S. Geological <br />Survey gaging stations were ana+yzed using flow data through 1983 <br />and Bulletin l7B procedures. A decrease of approximately 5 percent <br />from the lOa-year peak flow of 679 cfs was used by Worrall, utilizing <br />Bulletin 178 and the extended d.ta base results in a flow value of <br />648 cfs. <br /> <br />The Worrall regressions were uSed for this study for the following <br />reasons: although the Worrall s~udy was completed before the Water <br />Resources Council published Bul~etin l7B, the differences are not <br />considered significant enough t9 materially affect the results of <br />the study; adding the diversion! flows to the USGS gage records is <br />a conservative assumption; the Worrall discharge values agree well <br />with other studies in the Frase. River basin; and worrall flow <br />estimates have been widely used, by developers. <br /> <br />Peak discharge-drainage area re~ationships for streams studied by <br />detailed methods are shown in Table 3. <br /> <br />12 <br />