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Excess water pressure strongly affects the rheology and flow behaviour of debris flows, and the mechanisms for development and persistence of the pressure can be diverse. Using nine experimental slurries prepared with the sediments collected from the deposit of a large‐magnitude debris flow by modifying maximum grain size and solid concentration, the influence of agitation on excess water pressure is investigated with a cylinder apparatus, consisting of a coaxially mounted bucket and impeller driven by a variable‐speed motor. Agitation proceeding in three dynamic stages raises relative excess water pressure (R) by 17%–34% of the peak R in the static stage, showing that agitation can significantly increase excess water pressure. The magnitude of the steady R increases with the level of agitation, which is measured by impeller speed and slurry temperature, and the steady R in a slurry is agitation‐level‐dependent below its potential maximum value. The transfer of particle weight to water in debris flows can occur under zero or very low to extremely low effective stresses. Agitation raises excess water pressure essentially by promoting dispersion of the finest‐grained grains in water and that of progressively larger grains in gradually coarser‐grained slurries. The flow resistance of the tested slurries, characterized by net torque exerted on the impeller by the motor, decreases with increasing liquefaction ratio, demonstrating that agitation enhances debris‐flow mobility by raising excess water pressure. It is of importance to dampen agitation by building proper mitigation measures in remedial practices. [ABSTRACT FROM AUTHOR]

Due to building up excess water pressure, debris flows differ markedly from other water-sediment mixtures in mountain basins. Although a number of mechanisms are known to create excess water pressure, persistent high excess water pressure develops in static-quasistatic debris-flow slurries when possible triggers such as saturation, contraction, and Reynolds stress are absent. This suggests that there exist more fundamental mechanisms for the creation and persistence of high excess water pressure. Using 12 clastic materials with four maximum diameters up to 40 mm from three typical debris flows in Southwest China, we make comparative measurements of excess water pressure, gravity drainage, and decanting of water on 12 pairs of debris-water mixtures. It is found that saturation is not a mechanism for excess water pressure generation, which can be ascribed to the heterogeneity of saturated sediments. Mixing undoubtedly creates persistent high excess water pressure in the mixed saturated sediments. Thorough mixing causes the homogenization of water-saturated sediments through random grain rearrangements. The particles in the homogeneous debris-water mixtures stay suspended in water, resulting in excess water pressure derived from particle weight. More significant is the integration of clasts and water over the timescales for debris-flow durations promoted by mixing, which leads to the persistence of high excess water pressure. The development of persistent high excess water pressure in sediment-water mixtures is controlled by external factor—mixing, and internal factors, including sorting of sediments and water content of mixtures. • Water pressure, gravity drainage and decanting of water are measured. • Mixing generates high and persistent excess water pressure in debris flows. • Excess water pressure results from the transfer of particle weight to water. • Sorting of debris, water content, and mixing control excess water pressure build-up. [ABSTRACT FROM AUTHOR]

Rock avalanches have an important impact on the geomorphological evolution of mountain landscapes. Rock avalanche-debris geometry holds important clues related to the processes of the disaster predisposition, occurrence and rapid runout of rock avalanche-debris flows. The morphological features of the debris of the Touzhai rock avalanche deposit in Southwest China are examined as a specific case with the help of three-dimensional laser scanning, polarization microscopy (for polished sections) and field-emission scanning electron microscopy (FESEM). The unexpectedly high roundness is the most impressive morphological characteristic of the Touzhai rock avalanche-debris, particularly that of sand-plus silt-sized particles. The shapes of 12 scanned metric-sized boulders are closer to cubes than to spheres. 90% of the 114 scanned subangular blocks with a mean particle diameter of 12.04 cm and an average specific surface area of 0.225 cm2/g fall into the spherical domain of Zingg's classification diagram according to their flatness ratio and elongation ratio, and the average sphericity of 0.798 further indicates that they are more akin to cubes compared with spheres. The average form factor of the 1201 sand-sized grains is 0.784 with a standard deviation of 0.044, indicating that they are nearly spherical, and the solidity ranging from 0.951 to 0.974 also strongly shows that these particles are close to smooth spheres. Most of the grains in the size range of <0.075 mm are sub-angular except slaty secondary clay minerals of <0.005 mm based on the visual estimation under FESEM. The detailed qualitative and quantitative contrasts between the morphometric characteristics of the Touzhai rock-avalanche debris and those of the fresh basalt fragments demonstrate that the geometry of the former is dominated by the pre-failure predesign in which the basalt, primary and secondary structures, unloading and weathering acted together. Weathering was the direct contributor to its high roundness. The dynamic fragmentation triggering newly generated ruptures during the post-failure motion was less important, which should be the case in some rock avalanches in other regions. The slope pattern of "large creeping main slide mass + lower intact locked segment" generalized according to this study should be applicable to some other rock avalanches worldwide, and can help the pre-event identification of the potential rock-avalanche sites. Our results support the hypothesis of "aerification of debris" to explain the high mobility of rock avalanches. Highly fragmented source rockmass with enough fine grains should be necessary for the occurrence of rock avalanches and long, rapid runout of the rock avalanche-debris flows. • A combination of 3D laser scanning, optical microscopy and FESEM is uniquely used. • The geometry of the debris from a large rock-avalanche deposit is quantified. • The studied debris has unexpectedly high roundness. • The high roundness was derived from of the predesign of the rock avalanche. • Some of the conclusions should be applicable to similar rock avalanches. [ABSTRACT FROM AUTHOR]

Debris flows with long reaches are one of the major natural hazards to human life and property on alluvial fans, as shown by the debris flow that occurred in the Dongyuege (DYG) Gully in August 18, 2010, and caused 96 deaths. The travel distance and the runout distance of the DYG large-scale tragic debris flow were 11 km and 9 km, respectively. In particular, the runout distance over the low gradient channel (channel slope < 5°) upstream of the depositional fan apex reached up to 3.3 km. The build-up and maintenance of excess pore pressure in the debris-flow mass might have played a crucial role in the persistence and long runout of the bouldery viscous debris flow. Experiments to measure pore pressure and pore water escape have been carried out by reconstituting the debris flow bodies with the DYG debris flow deposit. The slurrying of the debris is governed by solid volumetric concentration (SVC), and the difference between the lower SVC limit and the upper SVC limit can be defined as debris flow index ( I d ). Peak value ( K p ) and rate of dissipation ( R ) of relative excess pore pressure are dependent on SVC. Further, the SVC that gives the lowest rate of dissipation is regarded as the optimum SVC ( C v o ). The dissipation response of excess pore pressure can be characterized by the R value under C v o at a given moment (i.e., 0.5 h, 1 h or 2 h later after peak time). The results reveal that a relatively high level of excess pore pressure developed within the DYG debris-flow mass and had a strong persistence capability. Further research shows that the development, peak value and dissipation of excess pore pressure in a mixture of sediment and water are related to the maximum grain size (MGS), gradation and mineralogy of clay-size particles of the sediment. The layer-lattice silicates in clay particles can be the typical clay minerals, including kaolinite, montmorillonite and illite, and also the unrepresentative clay minerals such as muscovite and chlorite. Moreover, small woody debris can also contribute to the slurrying of sediments and maintenance of debris flows in well vegetated mountainous areas and the boulders suspended in debris flows can elevate excess pore pressure and extend debris-flow mobility. The parameters, including I d , K p , R and etc., are affected by the intrinsic properties of debris. They, therefore, can reflect the slurrying susceptibility of sediments, and can also be applied to the research on the occurrence mechanisms and risk assessment of other debris flows. [ABSTRACT FROM AUTHOR]