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<br />~c
<br />
<br />Jarrett (1987) developed re8ional rainfall flood-frequency rela-
<br />tions for the foothills area of the South PIette River basin in
<br />Color.do. The analysis. used only the drainage area of a basin below
<br />2,440 m (believed to be . conservative elevation selection). These
<br />regIonal flood:frequeney equations provide more reliable estimates
<br />a! both common and TaTe floods (Jarrett, 1987; Jarrett and Costa.
<br />1988). The regional flood. frequency relations. supported by the
<br />paleoflood information, indicate that the 1976 Big Thompson River
<br />flood, downstream fr~m the center of the rainstorm, had a recurrence
<br />interval of approximately 10,000 years.
<br />
<br />The recurrence intervals of the probable maximum flood (PMF) at
<br />aeveyal aites in Colorado were estimated using the regional rela.
<br />tions (Jarrett and Co.ta, 1988). These results indicate that for
<br />aitea at pr upatream from about 2,300 m (inclUding the PKF for Olym-
<br />pus Dam), PKF recurrence intervals far exceed 10,000 years.
<br />However, at lower elevations, PMF recurrence intervals range from
<br />2,000 to 3,000 years'. The differences in recurrence intervals sug-
<br />gest varying risks associated with the PMF values in different loca-
<br />tions.
<br />
<br />during the last 8,000 to 10,000 years (Jarrett and Coata.
<br />The present capacity for the Olympus Dam spillway ia 637
<br />
<br />runoff,
<br />1988).
<br />mS/s.
<br />
<br />The methods developed for this study are applicable to other
<br />regions. Although many paleohydrologic techniques are available,
<br />paleohydrology has not yet reached its full potential; additional
<br />paleohydrologic research is needed. Ke.teoTologic research is needed
<br />to understand the causes of the elevation limit of 2,300 m on sig-
<br />nificant rainfall-produced flooding in Colorado.
<br />
<br />REFERENCES
<br />
<br />4 SIl!lKARY AND DISCUSSION
<br />
<br />Baker, V,R., R,D. Kochel, and P.C. Patron (eds.) 1988. Flood Geomor-
<br />phology. New York: Wiley, 503 P.
<br />Costa, J.C., 1987. A history of paleoflood hydrology in the United
<br />States 1800-1970. In E.R. Landa., and S. Ince (eds.), The history
<br />of hYd~ology, p. 49-53. Washington, D.C.: American Gaophyaical
<br />Union.
<br />Fritts, H.C. 1976. Tree rings and climate. London: Academic Preaa,
<br />567 p.
<br />Jarretc, R,D, 1987, Flood hydrology' of foochill and mountain atreama
<br />in Colorado. Ph.D. Dissertation,' Colorado State University, Fort
<br />Collins, Colorado, 239 p.
<br />Jarrett, R.D., and J.E. Costa 1988. Evaluation of the flood hydrol-
<br />ogy in the Colorado Front Range using precipitation. streamflow.
<br />end paleoflood data. U.S, Geol. Sur. WRIR 87-4117, 37 p.
<br />Patton, P.C. 1987. Measuring the rivers of the past--A history of
<br />fluvial paleohydrology. In E.R. ,Landa and S. Ince (eds,) , The hia-
<br />tory of hydrology, p.SS-67. Washington, D.C.: American Geophysical
<br />~~. '
<br />Stedinger, J,R., and V.R. Baker 1987: Surface water hydrology, His-
<br />torical and paleoflood information. Reviews of Geophysics
<br />25(2):119-124.
<br />
<br />A multidisciplinary study was conducted to improve 'the understanding
<br />of flood hydrometeorology in Colorado. Interpretation of streamflow
<br />(Figures 1, 2, and 3) and precipitation (figure 4) data and
<br />paleoflood information indicates that snowmelt flows predominate
<br />above 2.300 m, and rainfall generated floods predominate below 2,300
<br />m in Colorado. Above 2,300 _, maximum unit discharge has not ex-
<br />ceeded 1.1 mS/s/km'. Maximum 6-hour rainfall has not exceeded 50
<br />mm above 2,440 m. ,Paleoflood hydrology provides important supplemen-
<br />tal information about" the spatial occurrence, magnitude, and fre-
<br />quency of flooding, Paleoflood investigations indicate that large
<br />but infrequent floods have occurred in all basins below 2,300 m. No
<br />evidence of wa~~r floods much higher than bankfull discharge has
<br />been found in any stream valley above 2.300 m in Colorado (a
<br />paleoflood record.of about the last 10,000 years). Together the
<br />hydrologic results and the paleoflood information indicate that the
<br />1976 Big Thompson River flood had a recurrence inte~al of ap-
<br />proximately 10,000 yeara.
<br />
<br />The results of this" study are useful in assessing flood hazards
<br />and in c:le.creaslng the uncertainty in the design of hydraulic struc-
<br />tures OT for other flood-plain studies. These results have impoT-
<br />rant implications for the evaluation of the apillways of dams, such
<br />as Olympu8 Dam located at Estes Pa~k, Colorado (elevation 2.300 m),
<br />in the Big Thompson River basin. The absence of any paleoflood
<br />evidence of large floods in the upper Big Thompson River basin thar
<br />drains into Lake Estes indicates that significant floods have not
<br />occurred during'post-glacial times. Paleoflood investigations in
<br />the Big Thompson River besin upstream from Olympus Dam indicate
<br />there has not been a natural flow greater than 85 to 140 mS/s,
<br />which is consistent with peak discharges expected from snowmelt
<br />
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