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<br />meets the debris dams formed in the 10"'101' channels. <br /> <br />The internal shear <br /> <br />strength of the debris dams is reduced as flood rN'aters infiltrate the material. <br /> <br />This enables the debris dam to flow down the channel 8S a viscous fluid <br /> <br />mass which is mantled with large boulder's. Additional solid material can <br /> <br />be entrained into the flow through addition of channel lag deposits that <br /> <br />normally accumulate in the drainages between major debris-flood events. <br /> <br />This process of avalanehing, damming, and debris-flow formation can <br /> <br />occur within a basin several times during any partic:ular episode of erosion <br /> <br />and can produce several distinct flo" surges that may reach the upper debris <br /> <br />fan at intervals of several minutes or less.. Torrent ial flood waters wi 11 <br /> <br />follow the debris surges and cause erosion and random dispersion of material <br /> <br />on the fans. <br /> <br />The entire process continues until there is insufficient <br /> <br />free water available to provide mobility to the flo"s. Debris-flow surges <br /> <br />that become immobile due to loss of water were found in the channels at <br /> <br />several locations (Figure 4). <br /> <br />This sequence of events (Figure 5) requires only the conditions described <br /> <br />~n the previous chapter, all oJ: which are available during an intense rainstorm. <br /> <br />Although parts of the upper basins experie,nc:ed a forest fire in 19"16, this <br /> <br />probably did not affect the severity of the debris flow significantly. <br /> <br />The partially denuded slopes may have increased the runoff rate, but if <br /> <br />- 17 - <br />