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<br />As mentioned earlier, debris-flow velocity can fluctuate greatly on <br /> <br />the fan. During the last events the velocities at the buildings at the <br /> <br />instant of impact evidently wer'2 fairl). small. This may have been true <br /> <br />for several reasons includirlg (1) varying water content ..ithin the flows, <br />(2) dispersal of flows by flood waters so that the actual debris flows <br /> <br />did not come into contact with the buildings, (3) deposition of mud and <br /> <br />fine-grained material against buildings just prior to arrival of large <br /> <br />boulders, thereby providing a "cushion" effect, or (4) denection of debris-flow <br /> <br />surges by previously deposi,:ed d"bris lob"s. How,aver, none of of these <br /> <br />factors can be depended upon to randomly combine in such a way that damage <br /> <br />from future flows will be avoided. It was simply fortunate that it happened <br /> <br />this way the last time. Significant dynamic pressures against buildings <br /> <br />from debris flows must be anticipated in the future. <br /> <br />The magnitude of the clynamic pressure that llhould be designed for <br /> <br /> <br />depends on the unit weight of th" flowing debris and the flow velocity. <br /> <br /> <br />It is assumed that the flow unit ,,-eight is 125 Ibs/ft3 (2 gm/cm3) which <br /> <br /> <br />is characteristic of some measured flows; however. as discussed earlier. <br /> <br /> <br />velocity varies over the channel and fan. It can be safely assumed that <br /> <br />the velocity decreases with distance from the mouths of the channels. <br /> <br />Calculations in the channels and eyewitness reports suggest that it is <br /> <br />reasonable to assume a design veJ.ocity of 15 it/sec (4.5 m/sec) for the <br /> <br />- 35 - <br />