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<br /> Table 4,--Peak flow data at selected cross sections
<br />Di stance Cross
<br />downstream Average Maximum section
<br />from Lawn ve I oc ity , a depth, Top area, in
<br />Lake dam, in feet in width, square
<br />in miles per second feet in feet feet
<br />0,55 b b b
<br />b 8,0 b23,8 185 b2,070
<br />1. 50 b 11. 3 b18,6 97 b1,340
<br />3,83 9.9 14,0 348 1,270
<br />5.36 3.6 9.0 927 2,980
<br />5,78 3,3 7.9 1,112 2,250
<br />6,50 b 4,6 b10.1 328 b1,560
<br />7.68 11.2 10,8 148 1,170
<br />7,74 12,6 9,9 227 1,020
<br />8,78 12.1 10,6 170 910
<br />10,28 12,0 7.8 175 710
<br />11,45 7,4 6,4 336 880
<br />12,50 6,8 10.5 99 810
<br />
<br />~Based on the peak discharge profile in figure 11,
<br />Severe channel erosion may have influenced value,
<br />
<br />natural floods of this magnitude. A summary of flood frequencies for the
<br />Roaring River and the Big Thompson River is given in table 5,
<br />
<br />Floods in the mountainous regions of Colorado generally are from three
<br />meteorologic causes: (1) Snowmelt floods, (2) rainfall floods, and (3) rain-
<br />on-snow floods, Many times, runoff peaks on a given stream originate from all
<br />three causes, but convent i ona I hydro I ogi c ana lys i s fa i I s to account for the
<br />mixed population of runoff peaks contributing to the total population of flood
<br />peaks in mixed popul ation fl ood areas, When snowmelt- and rai n-generated
<br />peaks are examined separately, flood-frequency analysis shows different trends
<br />based on elevation (Jarrett and Costa, 1983), In the Estes Park area above
<br />about 7,500 ft, snowmelt dominates, with rainfall generally not contributing
<br />to the flood potential for recurrence intervals greater than the 100-year
<br />flood. Where rainfall does contribute significantly to flooding above about
<br />7,500 ft, unit discharges are small (generally less than 20 ft3/s/mi2) (cubic
<br />feet per second per square mile), compared with lower elevation floods result-
<br />ing from rainfall (which commonly exceed 1,000 ft3/s/mi2). Below about 7,500
<br />ft, rainfall-produced floods predominate.
<br />
<br />The fl ood- frequency curve for the Big Thompson Ri ver at Estes Park was
<br />analyzed for mixed population flows by the methods described in Jarrett and
<br />Costa (1983) (fig, 13), Inspection of the plotted rainfall and snowmelt
<br />flood-frequency curves indicates that snowmelt flood peaks predominate in the
<br />
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