Analysis of factors influencing the large wood transport and block-outburst in debris flow based on physical model experiment

Abstract

Large wood carried by debris flow is easy to block the slit-check dam and form unstable temporary blockage. When the blockage suddenly bursts, a larger debris flow will form, resulting in a more serious disaster. To better understand the disaster-caused characteristics of large wood, the chain reaction characteristics of large wood transportation, blockage, blocking and block-outburst in debris flow were investigated through a field investigation and a series of physical model experiments by taking the Keze gully in Sichuan Province of China as the prototype. In addition, some fundamental dimensionless parameters such as “blocking ratio”, “clogging coefficient”, and “discharge magnification ratio” were defined to determine a rational criterion for evaluating the characteristics of large wood transport and block-outburst. The results showed that four transport regimes of large wood passing through the slit-check dam were observed: uncongested, semi-congested, congested and block-outburst. The increase in the blocking ratio of large wood became more pronounced with the number of large wood N = 180 and the relative length of large wood L/B = 0.75, indicating that N = 180 and L/B = 0.75 are the inflection points. The blocking ratio of large wood increased with an increase in debris-flow density. The results demonstrated that the clogging coefficient was closely associated with the relative length of large wood, the number of large wood and the density of debris flow. The maximum clogging coefficient was 3.09 in the experiment, with the relative length of large wood, the quantity of large wood and the density of debris flow of 1.0, 180 and 1500 kg/m3, respectively. In essence, the clogging coefficient correlations were developed as a power function of the discharge magnification ratio based on comparative analysis.