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<br />(S) Closely spaced trees are quite effective in stopping <br /> <br />boulders and large racks on debris fans (see Figure 7). <br /> <br />(6) Debris flow surges sre often followed by muddy, <br /> <br />debris-laden flood waters. <br /> <br />These waters can erode and <br /> <br />redistribute the debris but probably will not cause <br /> <br />impact forces as large as the debris flows. <br />. <br /> <br />Because of these characteristics, strict channelization of the flows <br /> <br />cannot be recommended as a nlitigat ion method. <br /> <br />Such channels can quickly <br /> <br />become blocked, as illustrated in Figure 4, csusing subsequent surges to <br /> <br />flow in new directions. <br /> <br />This occurred in a small debris flow basin on <br /> <br />the west side of the Roaring Fork River during the July 24 storm. A flow <br /> <br />surge blocked the existing channel aod advanced in a new direction. Channelization <br /> <br />of water runoff is necessar)' as part of the overall drainage plan in the <br /> <br />area but will prove ineffective without additional structural control. <br /> <br />Two types of control are discussed below. These are direct protection <br /> <br />for buildings against debris impact, and arresting and breaking structures <br /> <br />which will lessen the hazard to both buildings and surrounding property. <br /> <br />C. Direct Protection Against :~mpaE.~ <br /> <br />Very little structural damage from debris impact to structures occurred <br /> <br />during the recent flows, even though lnoderate to large boulders were pushed <br /> <br />(presumably quite slowly) against buildings. However, it must be stressed <br /> <br />that this will not necessarily be the behavior of future flows. <br /> <br />- 34 .- <br />