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<br />000925 <br /> <br />4.1 <br /> <br />, <br /> <br />~.t. A)I:.a!..YS::S <br />~"d..(\ted in this chapter are analyses of the f loading and debris <br /> <br />. on Portland and Cascade Creeks. lU1alyses conducted include hydrol- <br />,t-'..)~..= . <br />.... tllics identification of problems, and damage estimation. The <br />O~J. ~r..r4 , <br />1 from th;s chanter was used in formulating and evaluating the <br />:.~t~t 0:1 .... C" <br /> <br />.;..rnatives in Chapter V. <br /> <br />4. I Hydrology <br /> <br />As presented in Chapter III, the 10-. 50- and 100-year flood peaks for <br /> <br />"'rtlAnd and cascade Creeks were determined by A&S Consultants in the <br /> <br />lI)'<lrology Report for the Floodplain Information Study. However, these peaks <br /> <br />considered only water and not debris. Because the problems surrounding <br /> <br />!looding in Ouray have occurred from floods accompanied by debris flows, it is <br /> <br />very important that debris be considered in the hydrologic analysis. The ori- <br /> <br />ginal A&S Consultants' report was modified by SLA to account for debris. Peak <br /> <br />discharges for the various return periods were increased by a factor of 1.87. <br /> <br />This factor is based on a fla~ consisting of 70 percent by weight of sediments <br /> <br /> <br />and rocks. Johnson, in Physical Processes in Geology, reports solids to <br /> <br />coq>rise 60 to 90 percent of the total debris flow mass by weight. Similar <br /> <br />values are found in additional literature. A value of 70 precent solids by <br /> <br />weight was chosen for corrputing volumes of debris associated with the peak <br /> <br /> <br />discharges in the hydrograph. Since the debris flows occur in surges and do <br /> <br /> <br />not occupy the entire hydrograph, a reduced value of 50 percent solids oy <br /> <br />weight was used to calculate the total volume of debris from the associated <br /> <br />volume of water runoff. <br /> <br /> <br />To convert the peak water discharges to debris and water discharges. the <br /> <br /> <br />peak water discharge was multiplied by a factor of 1.87. This factor was <br /> <br />derived mathematically fro,. the 70 percent solids content by weight by <br /> <br />starting with 100 pounds of debris flow. <br /> <br />Of the <br /> <br />100 pounds. 70 pounds are <br />gravity of 2.65, the unit <br /> <br />solids and 30 pounds are water. <br /> <br />weight of <br />3 <br />I bslft . <br /> <br />the solids <br /> <br />With a <br />3 <br />is 165.4 lbs/ft . <br /> <br />specific <br /> <br />and water <br /> <br />Dividing these unit weights into the respective weights of <br />3 3 <br />yieldS a total volume of solids of 0.423 ft and 0.481 ft <br /> <br />solids <br /> <br />I <br />! <br />i <br />j <br />I <br />I <br />I <br /> <br />The unit weight of water is 62.4 <br /> <br />of water. <br /> <br />The total volume of the 100 pound mixture is .then 0.904 pounds. The ratio of <br />the volwne of water to total debris flow mixture is 0.904 to 0.481 which <br />yields the f actor of 1.87. <br /> <br />'. <br /> <br />I <br />I <br />i <br />J <br /> <br />~~"'''', <br /> <br />7-""'-"=-- .'. .- ".0"' ..""'- .~. '. -.}"'.~ .... <br /> <br />- .T'.~""'.!'",,,,, <br /> <br />~~'''''-'JI.l'''''''.~'-''_. ~.'. ~~ --'--' -" - - <br />~ .~~.. . ......'- ;1-;:...,:.,. .:;- :' ~~-_~ '~'"-;::""li~~~~~~~~.'l~~'n.l~~7$t"V....t1:::r:~~ <br />" ~-'. ::~ '--~ -' ~ '~>::I~::-:~~~~~=53i.:~t{:\%:~:~~:~liftl;i~t~~-; . <br />