Movement and intercept characteristics of driftwood in debris flow
WANG Daozheng, CHEN Xiaoqing, ZHAO Wanyu, ZUO Linyong
1. Institute of Mountain Hazards and Environment, Key Laboratory of Mountain Hazards and Surface Processes, Chinese Academy of Sciences, 610041, Chengdu, China;
2. Shaanxi Institute of Engineering Prospecting, 710068, Xi'an, China;
3. Sichuan Institute of Nuclear Geology, 610052, Chengdu, China
Abstract：[Background] Debris flow with woody could often have great impact destructive force, and threaten the safety of debris dams for woody debris may clog densely in the spillway. Judging from the results of past experiments related to driftwood in debris, we found that driftwood size, driftwood specific gravity, water depth, channel drop, and driftwood volume had a direct impact on the driftwood transport.[Methods] The model test tank device was selected as the rectangular section. A total of 75 kinds of test conditions under three kinds of debris flow bulk density, five kinds of driftwood number, and five kinds of L/B combination were designed. The experiment was conducted through 5 steps of 1) the model preparation 2) material configuration 3) test 4) change the debris flow bulk density, the number of driftwood (N) and the length of the driftwood, and repeat above 3 steps and 5) record the number of blocked driftwood.[Results] 1)Driftwood tends to be parallel to the upstream debris flow direction due to the uneven forces acting on both sides of it, and transport to the downstream with a stable angle, this phenomenon is more obvious with the debris flow density becoming smaller. 2)The relative size of the driftwood has a certain effect on the intercept rate of the driftwood, and the effect on the intercept rate of the driftwood is small when relative length of driftwood L/B ≤ 0.75 and driftwood intercept type is dead zone type. The intercept rate increases when 0.75< L/B ≤ 1, but the intercept rate decreases due to the occurrence of blocking phenomenon when L/B=1, and then the intercept rate is gradually increasing because the blocking rate of driftwood is weakening. At this time, the intercept type is mainly key driftwood, and the theoretical analyses are consistent with experimental flume results. 3)The number of driftwood also has a great impact on the intercept rate, in general, the driftwood intercept rate increases with the number of driftwood increase. Driftwood intercept rate is very small and the size of the driftwood has little impact on the intercept rate when N ≤ 120. With the increase of the number of driftwood, the probability of multiple driftwood reaching the channel shrinkage increases at the same time, and the intercept rate of driftwood increases when N>120.[Conclusions] The bulk density of debris, the number of driftwood and the relative size of the driftwood cause the great impact on the intercept rate. The results show that studying pattern of the movement and intercept rate of driftwood in debris flow is of great significance for understanding the characteristics of driftwood disaster and engineering control.
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2017, Vol. 15 
