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<br />values concerning debris are approximate, especially the debris volume. The
<br />
<br />vol~~e of debris supplied by the creeks is dependent on factors other than
<br />
<br />
<br />vater discharge, mainly those relating to the availability of debris. These
<br />
<br />[actors include previous storm history, antecedent moisture condition of
<br />
<br />
<br />suils, distribution of the rainfall over the watersheds, and ,buildup of debris
<br />
<br />in steep side channels. Therefore# the debris volume curves should be taken
<br />
<br />4S conservatively high estimates of the debris volume for a storm considering
<br />
<br />
<br />availability of debris to be high.
<br />
<br />ouray has been hit by major floods in 1908, 1909, 1927, 1929, 1951, 1965,
<br />
<br />
<br />1973, 1981 and 1982. This represents 9 series of floods since the turn of the
<br />
<br />
<br />century. This would point to a return period of approximately nine years as
<br />
<br />the lower limit of floods that create problems in OUray. Considering that
<br />
<br />
<br />both cascade and Portland Creeks did not create problems each time there was a
<br />
<br />major flood, a return period of ten years for the threshold level at which
<br />
<br />
<br />debris and flooding problems inflict significant damages on Portland and
<br />
<br />
<br />Cascade Creeks is an appropriate estimate. The ten-year flood corresponds to
<br />
<br />a peak combined water and debris discharge of 1,100 cfs on Portland Creek and
<br />
<br />500 cfs on Cascade Creek. The corresponding debris volumes would be 7.2 acre-
<br />
<br />
<br />feet and 3.4 acre-feet, respectively.
<br />
<br />A rough estimate of the amount of debris deposited on Cascade Creek
<br />
<br />
<br />after the August 20, 1982, flood was three to five acre-feet. This would make
<br />
<br />the August 20 flood approximately a 10- to 25-year flood on Cascade Creek.
<br />
<br />The August 20, 1982, flood deposited less debris in Portland Creek; however,
<br />
<br />
<br />the flooding that followed the next several days was more severe. No estimate
<br />
<br />
<br />of debris volumes for the severe flooding that occurred was made. Based on
<br />
<br />the amount of damage and qualitative comparison with the extent of flooding
<br />
<br />
<br />from other floods', the return period of the more severe flooding on Portland
<br />
<br />
<br />Creek would be on the order of 25 years.
<br />
<br />4.2 Hydraul ic Analysis
<br />
<br />
<br />Analysis of hydraulic conditions (velocity, depth and discharge) was made
<br />
<br />
<br />for the portland and Cascade flows for the existing flumes. The analysis was
<br />
<br />
<br />performed for the conditions of water only, and water and debris. In addi-
<br />
<br />tion, the capacity of various flume cross sections to transport debris flows
<br />
<br />was determined in order to estimate the best size, if the flumes are totally
<br />
<br />rebuilt. Analysis of the water-only condition utilized Manning's equation and
<br />
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