Slope erosion induced by surges of debris flow: insights from field experiments

Data availability: All data generated during the study are available from the corre- sponding author by request.Copyright © The Author(s) 2022. We conducted field observations and experiments to explore debris flow dynamics, sediment transportation and slope erosion at an active natural debris flow gully in the headwaters of Jiangjia Ravine (Dongchuan region, Southwest China). In this region, the hillslopes were heavily jointed, weathered and sparsely vegetated, providing continuous and rich sediment supplies for initiating debris flows. The debris flow propagated in the channel as a sequence of surges, with periodical changes of flow flux, velocity, water content, and viscosity as controlled mainly by the conditions of erodible sediments and water supplies from the upstream. The water content of bank sediments ranged from 5 to 8%, while it was 16 to 26% for debris surges in the channel. The particle size distribution of sediments on the alluvial fan followed the Weibull’s cumulative distribution and the mean size was in the range of 2 ~ 4 mm. The coarse particles were primarily elongated or prismoidal and aspect ratios followed well a normal distribution with the mean value of 0.4. The angular particles entrained in dense viscous debris flow surges could effectively abrade and groove the channel bed and banks, increasing the intensity of slope erosion. The incised slope had a sequence of terraced depositional layers on both banks. The layer thickness decreased as the erosion depth moved deeper into the stratum where hard bed soil/rock layers existed. The water-soil mixture of debris flow exhibited a clear shear-thinning behavior with its viscosity decreasing gradually with the increase of shear rate following the widely accepted power-law model. The dense viscous debris flow can facilitate the transportation of coarse gravels in channel and contribute to slope erosion.Royal Society, Sino-British Fellow- ship Trust International Exchanges Award (No. IES\R2\202023); open funding of the State Key Laboratory of Geomechanics and Geotechnical Engineering (No. Z019004)

Discrete element analysis of dry granular flow impact on slit dams

Code availability: The code used in this study is the open-source DEM code ESyS-Particle, available at: https://launchpad.net/esys-particleCopyright © 2020 The Author(s). Slit dam is an open-check barrier structure widely used in mountainous regions to resist the destructive impacts of granular flows. To examine the dynamics of granular flow impact on slit dams, a numerical study by discrete element method (DEM) is presented in this article. The study considers dry granular materials flowing down a flume channel and interacts with slit dams installed at the lower section of the flume. The particle shape is explicitly considered by particle clumps of various aspect ratios. The slit dams are modeled as rigid and smooth rectangular prisms uniformly spaced at in the flume. Four key stages of granular flow impact on the slit dams have been identified, namely, the frontal impact, run up, pile up, and static deposition stages. In the impact process, the kinetic energy of the granular flow is dissipated primarily by interparticle friction and damping. The trapping efficiency of the slit dams decreases exponentially with the relative post spacing, while it increases with the particle clump aspect ratio. The numerical results can provide new insights into the optimization of relative post spacing for slit dam design.National Key R&D Program of China (Grant No.2018YFC1505004); International Science & Technology Cooperation Program of China (No. 2018YFE0100100); open funding of the Key Laboratory of Mountain Hazards and Earth Surface Process (Chinese Academy of Sciences & Ministry of Water Conservancy)

Using PIV to measure granular temperature in saturated unsteady polydisperse granular flows

The motion of debris flows, gravity-driven fast moving mixtures of rock, soil and water can be interpreted using the theories developed to describe the shearing motion of highly concentrated granular fluid flows. Frictional, collisional and viscous stress transfer between particles and fluid characterizes the mechanics of debris flows. To quantify the influence of collisional stress transfer, kinetic models have been proposed. Collisions among particles result in random fluctuations in their velocity that can be represented by their granular temperature, T. In this paper particle image velocimetry, PIV, is used to measure the instantaneous velocity field found internally to a physical model of an unsteady debris flow created by using “transparent soil”—i.e. a mixture of graded glass particles and a refractively matched fluid. The ensemble possesses bulk properties similar to that of real soil-pore fluid mixtures, but has the advantage of giving optical access to the interior of the flow by use of plane laser induced fluorescence, PLIF. The relationship between PIV patch size and particle size distribution for the front and tail of the flows is examined in order to assess their influences on the measured granular temperature of the system. We find that while PIV can be used to ascertain values of granular temperature in dense granular flows, due to increasing spatial correlation with widening gradation, a technique proposed to infer the true granular temperature may be limited to flows of relatively uniform particle size or large bulk

Effects of clay content on the volumetric behavior of loess under heating-cooling cycles

Title in Traditional Chinese: 循環溫度荷載下黏粒含量對黃土變形特性的影響202310 bcchAccepted ManuscriptSelf-fundedPublishe

Stress effects on soil freezing characteristic curve : equipment development and experimental results

201909 bcrcVersion of RecordRGC16204817Publishe

Photoluminescent arrays of nanopatterned monolayer MoS2

Monolayer 2D MoS 2 grown by chemical vapor deposition is nanopatterned into nanodots, nanorods, and hexagonal nanomesh using block copolymer (BCP) lithography. The detailed atomic structure and nanoscale geometry of the nanopatterned MoS 2 show features down to 4 nm with nonfaceted etching profiles defined by the BCP mask. Atomic resolution annular dark field scanning transmission electron microscopy reveals the nanopatterned MoS 2 has minimal large-scale crystalline defects and enables the edge density to be measured for each nanoscale pattern geometry. Photoluminescence spectroscopy of nanodots, nanorods, and nanomesh areas shows strain-dependent spectral shifts up to 15 nm, as well as reduction in the PL efficiency as the edge density increases. Raman spectroscopy shows mode stiffening, confirming the release of strain when it is nanopatterned by BCP lithography. These results show that small nanodots (≈19 nm) of MoS 2 2D monolayers still exhibit strong direct band gap photoluminescence (PL), but have PL quenching compared to pristine material from the edge states. This information provides important insights into the structure-PL property correlations of sub-20 nm MoS 2 structures that have potential in future applications of 2D electronics, optoelectronics, and photonics

Photoluminescent arrays of nanopatterned monolayer MoS2

Monolayer 2D MoS 2 grown by chemical vapor deposition is nanopatterned into nanodots, nanorods, and hexagonal nanomesh using block copolymer (BCP) lithography. The detailed atomic structure and nanoscale geometry of the nanopatterned MoS 2 show features down to 4 nm with nonfaceted etching profiles defined by the BCP mask. Atomic resolution annular dark field scanning transmission electron microscopy reveals the nanopatterned MoS 2 has minimal large-scale crystalline defects and enables the edge density to be measured for each nanoscale pattern geometry. Photoluminescence spectroscopy of nanodots, nanorods, and nanomesh areas shows strain-dependent spectral shifts up to 15 nm, as well as reduction in the PL efficiency as the edge density increases. Raman spectroscopy shows mode stiffening, confirming the release of strain when it is nanopatterned by BCP lithography. These results show that small nanodots (≈19 nm) of MoS 2 2D monolayers still exhibit strong direct band gap photoluminescence (PL), but have PL quenching compared to pristine material from the edge states. This information provides important insights into the structure-PL property correlations of sub-20 nm MoS 2 structures that have potential in future applications of 2D electronics, optoelectronics, and photonics

Experimental study on the regulation function of slit dam against debris flows

The authors received financial support from the National Natural Science Foundation of China (grant no. 11672318), the Youth Innovation Promotion Association, CAS, and the Chinese Academy of Sciences (CAS) Pioneer Hundred Talents Program, and from the research grant T22-603/15-N provided by the Research Grants Council of the Government of Hong Kong SAR, China