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<br />3.5 <br /> <br />~.~. <br /> <br />o <br /> <br />slopes at approximately eight to ten percent to the Uncompahgre from the apex <br /> <br />of the debris fan. Cascade Creek, on the other hand, slopes at 18 to 13 per- <br /> <br />cent the majority of its distance from the fan apex to the Uncompahgre River. <br /> <br /> <br />The Cascade Creek slope is steeper since its material does not flow as easily <br /> <br /> <br />and a steeper slope has developed as a consequence in order to-transport the <br /> <br /> <br />material. <br /> <br />,,' <br /> <br />) <br /> <br />The characteristics discussed in this section are summarized in <br /> <br />" <br /> <br />Table 3. 1. <br /> <br />I <br /> <br />The potential sediment supply for both creeks is extremely extensive due <br /> <br /> <br />to the erodible nature of the volcanics in the upper watershed and to the con- <br /> <br />tinuous supply of rockfall material and glacial drift. Therefore the sediment <br /> <br /> <br />yield to debris flows is difficult to estimate. The supply at any specific <br /> <br /> <br />time is a function of the weathering ~ates through time as well as the proba- <br /> <br /> <br />bility of a precipitation event occurring over the watershed. <br /> <br />'\ <br /> <br />./ <br /> <br />3.2 Hydrology' <br /> <br /> <br />The climate of the Ouray area is characterized by relatively cold ~~nters <br /> <br /> <br />with significant snowfalls, especially at the higher elevations and mild sum- <br /> <br /> <br />mers with frequent thunderstorms. This study is most concerned with the thun- <br /> <br /> <br />derstorms, since they trigger the debris flows which have caused flood damages <br /> <br /> <br />in Ouray. Debris flows are not associated with snowmelt runoff or the more <br /> <br /> <br />general, less intense rain storms. <br /> <br /> <br />Utilizing the NOAA "Precipitation Frequency Atlas of the Western United <br /> <br />States," (Volume III, Colorado), estimates of the rainfall volum.e for 30 minu- <br /> <br />-' <br /> <br />tes and 1 hour storms were made. These are presented in Table 3.2. These <br /> <br /> <br />values, all less than two inches, are typical of mountainous areas of <br /> <br /> <br />Colorado. Values of the 24-hour prectpitation were taken from the Hydrology <br /> <br /> <br />Report for the Floodplain Information Study. <br /> <br /> <br />The peak discharges associated wtth the 10-, 50- and 100-year storms for <br /> <br /> <br />Portland and Cascade Creeks were presented in a separate unpublished Hydrology <br /> <br /> <br />Report for the Floodplain Information Study. The associated peak discharges <br /> <br /> <br />are presented in Table 3.3. These peak discharges are based on the 24-hour <br /> <br />storm. The discharges do not consider the effect of debris. This will be <br /> <br /> <br />taken into account in later analyses. In addition, it would be more <br /> <br /> <br />appropriate to consider peak discharges from the 30-minute or 1-hour storm for <br /> <br /> <br />this stUdy since the debris flows are characterized by these types of storms <br /> <br />- <br /> <br />~. <br /> <br />- <br />,- <br /> <br />,;./ <br />, <br />-, <br />'.1 <br />