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<br />_'.~-. ..-......_""... u~i.lrJ~-""'rC""",..,:~t..:..!!.~,J.~ ,,~....j.,,!,::; :;::'~~":.;.'''"''';-";~_:''''~17'-- .~~~ ';~ -~..!!....~~.,. ....:..... ;.';';~..., <br /> <br />'Ur ''--''~':S''~X~'1. :,~"-,,,: <br />! <br /> <br />000926 <br /> <br />4.2 <br /> <br />The volume of debris expected during a flood is important in evaluating <br /> <br /> <br />ways to handle the debris, such as debris basins. Debris volumes were esti- <br /> <br />mated uSing the assumptions belew: <br /> <br />1. <br /> <br />The portion of the flood contributing large amounts of debris lasts 30 <br />minutes (based on descriptions of residents). <br /> <br />2. <br /> <br />The average debris content during the 30 minutes is 50 percent by weight. <br />(This produces a multiplier of 1.37 to convert water volumes to water and <br />debris volumes using the method in the previous paragraph. From this, it <br />is deduced that the flow is 26 percent debris by volume. ) <br /> <br />3. The hydrograph during the 30-i!linute period is triangular in shape. <br /> <br />4. When the debris deposits, its porosity is 40 percent. <br /> <br />Using these assumptions and the discharges from the A&S Consultants' report, <br /> <br />the estimated debris volumes for the 10-, 50- and lOO-year storms were <br /> <br /> <br />calculated. An example calculation of the deposited debris volume is produced <br /> <br />below for the 10-year storm on Portland Creek. <br /> <br /> <br />Peak Discharge = 590 cfs <br /> <br />Volume of Water = <br /> <br />sec <br />590 cfs . 30 min * 60 -:- <br />rnJ.n <br /> <br />2 * <br /> <br />3 <br />43 560 .f!... <br />I A-F <br /> <br />= 12.2 A-F <br /> <br /> <br />The total volume of the debris flow is found by multiplying by 1.37. <br /> <br />Volume debris flow = 12.2 . 1.37 = 16.7 A-F <br /> <br />The unbulked debris is 26 percent of this volume. <br /> <br />Volume debris (unbulked) = 16.7 . 0.26 = 4.3 A-F <br /> <br /> <br />Using a porosity of 0.40 the bulking factor is 1.67. This yields a bulked <br /> <br /> <br />volume of debris of <br /> <br />Volume debris (bulked) = 1.67 . 4.3 " 7.2 A-F <br /> <br />The 7.2 acre-feet is the value chosen to represent the volume of deposited <br /> <br />debris for Portland Creek's 10-year flood. <br /> <br /> <br />The results of the hydrologic analysis were plotted on log-probability <br /> <br /> <br />paper to develop frequency curves. The frequency curves for Portland Creek <br /> <br />and Cascade Creek are presented on Figures 4.1 and 4.2, respectively. <br /> <br />Frequency curves for peak water discharge, peak water and debris discharge, <br /> <br />and debris volume are presented on these figures. It should be noted that <br /> <br />.;A <br />,. , <br />',~ <br />!j <br />'.",,-. <br />\it <br />" <br />:4 <br />... <br /> <br />. <br />.li.i..:..!.!.. <br /> <br />0' <br />01 <br /> <br />., <br />" <br />, <br /> <br />" <br /> <br /> 0 <br /> g <br /> 0 <br /> '" <br /> on <br /> N <br /> 9 <br />.. on <br />0 <br />.. <br />... <br />.:> <br />0 <br />- <br />- N <br />W <br />:J. <br />- <br />~ <br />:r <br />- <br />- <br />w <br />:r <br /> <br />o~ <br />o~ <br />o~ <br />~c: <br />