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in ~ lo~er liffs On ~ng1o_ :'ocn 'oUnd_ ;ged Iheds Y of L an '.ng r1y ed In- Jer it 000904 3.5 i tely eight to ten percent to the Uncompahgre from the apex . ~..~~ 4~ 'l'prox .... . Cascade Creek, on the other hand, slopes at 18 to 13 per- I~! ~!'A deI!).:1. ..4n- i of its distance from the fan apex to the Uncompahgre River. ~c~~ ~~ ~,or ty ~.. C4.:od4 Creek slope is steeper since its material does not flow as easily .~ . .~oe~r slope has developed as a consequence in order to. transport the e.s~.f'!.l.. ~. characteristics discussed in this section are summarized in ,,:..b~. J.1. ~. potential sediment supply for both creeks is extremely extensive due 10 t..~. erodible nature of the volcanics in the upper watershed and to the con- 1~~8 supply of rockfall material and glacial drift. Therefore the sediment yield to debris flows is difficult to estimate. The supply at any specific .~ 18 a function of the weathering rates through time as well as the proba- bll~ty ot a precipitation event occurring over the watershed. l.2 Ilvdroloqy The climate of the Ouray area is characterized by relatively cold winters with significant snowfalls, especially at the higher elevations and mild sum- ....ro with frequent thunderstorms. This study is most concerned with the thun- derstorms, since they trigger the debris flows which have caused flood damages 1n OUray. Debris flows are not associated with snowmelt runoff or the more qeneral, less intense rain storms. Utilizing the NOAA .Precipitation Frequency Atlas of the Western United States,. (Volume III, Colorado), estimates of the rainfall volume for 30 minu- tes and 1 hour storms were made. These are presented in Table 3.2. These value., all less than two inches, are typical of mountainous areas of Colorado. Values of the 24-hour precipitation were taken from the Hydrology Report for the Floodplain Information Study. The peak discharges associated with the 10-, 50- and 100-year storms for Portland and Cascade Creeks were presented in a separate unpublished Hydrology Report for the Floodplain Information Study. The associated peak discharges are presented in Table 3.3. These peak discharges are based on the 24-hour storm. The discharges do not consider the effect of debris. This will be taken into account in later an~lyses. In addition, it would be more appropriate to consider peak discharges from the 3u-minute or 1-hour storm for this study since the debris flows are characterized by these types of storms -:;e.~:"'.."r''''''~ ___~~4'!:;'. 'e "" t"~~"~:""~ ,._-,~- .........~, ., - <. -'\..~-:"'I'f:':.t'''<':"'''', ) -.p,~ - -"-~::.~-..