Debris flows are a type of natural disaster induced by vegetation-water-soil coupling under external dynamic conditions. Research on the mechanism by which underground plant roots affect the initiation of gulley debris flows is currently limited. To explore this mechanism, we designed 14 groups of controlled field-based simulation experiments. Through monitoring, analysis, calculation, and simulation of the changes in physical parameters, such as volumetric water content, pore-water pressure, and matric suction, during the debris flow initiation process, we revealed that underground plant roots change the pore structure of soil masses. This affects the response time of pore-water pressure to volumetric water content, as well as hydrological processes within soil masses before the initiation of gully debris flows. Underground plant roots increase the peak volumetric water content of rock and soil masses, reduce the rates of increase of volumetric water content and pore-water pressure, and increase the dissipation rate of pore-water pressure. Our results clarify the influence of underground roots on the initiation of gulley debris flows, and also provide support for the initiation warning of gully debris flow. When the peak value of stable volumetric water content is taken as the early warning value, the early warning time of soil with underground plant roots is delayed by 534 to 1253 s. When the stable peak value of pore-water pressure is taken as the early warning value, the early warning time of soil with underground plant roots is delayed by 193 to 1082 s. This study provides a basis for disaster prevention and early warning of gully debris flows in GLP, and also provides ideas and theoretical basis under different vegetation-cover conditions area similar to GLP.

Forests play an important role in controlling the formation and movement processes of debris flows. They contribute to soil stabilization, regulation of soil water content, and act as robust structures impeding the downstream progression of debris flows. On the positive side, trees, to some extent, can intercept debris flows and effectively mitigate their velocity by increasing flow resistance. On the negative side, trees may suffer damage from debris-flow hazards, characterized by the generation of substantial quantities of wood fragments and consequential ramifications such as river channel blockage, resulting in backwater rise. In extreme cases, this blockage collapse can lead to instantaneous discharge amplification, thereby adversely impacting urban safety and impeding sustainable development. Therefore, in order to grasp the effects of tree characteristics on tree failure modes, the tree failure modes and corresponding parameters, diameters at breast height (DBH) and root-soil plate size, were identified and recorded through the post-event field investigation in Keze Gully, a region prone to debris-flow events in Sichuan, China, respectively. To investigate the impact of spatial variability in tree root distribution on tree failure modes, the root cross-sectional area ratio (RAR), root density (RD), root length density (RLD) and soil detachment rate (SDR) were obtained. The findings indicated that: (1) Tree characteristics reflect the interactions of debris flows and trees, and influence the tree failure modes ultimately. The root distribution characteristics influence the size and shape of the root-soil plate to affect the resistance of trees. (2) Compared to burial and abrasion, stem breakage and overturning are the predominant modes of tree failure in debris-flow hazards. Trees with a smaller DBH primarily experience stem breakage and bending, and trees with a larger DBH mostly experience overturning. (3) The root-soil plate shapes of overturned trees, affected by the root architecture and root growth range, are generally semielliptical or semicircular, and the horizontal and vertical radii increase with DBH, but the correlation between the root-soil plate's breadth-depth ratio and DBH is low. (4) The biomass and RAR decrease with distance. The RAR distribution exhibit the order of upslope direction > downslope direction > lateral direction. The coarse root biomass significantly increases with DBH, but no clear trend in fine root biomass. (5) The roots can significantly enhance the soil erosion resistance, but the erosion resistance of coarse roots is not as significant as that of fine roots. The erosion resistance increases with DBH, and follows the order of upslope direction > downslope direction > lateral direction. The results could provide new insights into the influences of tree and root distribution characteristics on tree failure modes during debris flows.

Forests, serving as natural barriers in mountainous regions, can reduce surface erosion, enhance water flow resistance and facilitate sedimentation. While extensive research has been conducted for forests and their capacity to withstand debris flows, there has been relatively little investigation into the interactions between riparian forests and debris flows. In this paper, a field investigation of Keze gully was conducted through surveys, quadrat surveys and unmanned aerial vehicle field investigations. Based on the flow calculation results, analysis of sediment characteristics, gully morphology change and analysis of stand characteristics, the interaction model between the riparian forest and debris flow in the complex channel was discussed. The results further emphasise the role of forests in mitigating debris flows. Furthermore, the results show that the identified critical slope for the study area provides a threshold to explain two different flow interaction dynamics with riparian forests. Above the critical slope, debris flows predominantly follow the main channel, where the root-soil complex of riparian forests reduces channel bank erosion. However, this erosion can also lead to the uprooting and destruction of trees. Below the critical slope, floodplains are vital in accommodating overflow. Debris flows interact not only with the roots of trees near the riverbank but also with the trunks of trees on floodplains. Trees with a diameter less than 30 cm may also suffer damage due to broken stems.

The Matmata region, located in the south of Gab & egrave;s (Tunisia), experienced significant damage during the floods of the Beni zelten wadi on November 11, 2017. These floods, exacerbated by the steep slopes and underlying soil conditions, led to the occurrence of debris flows, posing a threat to road infrastructure. The generation of debris flows is closely linked to intense rainfall events that surpass the soil capacity to retain water. To gain insights into the behaviour of the soil samples, various characteristics were analysed, including texture, clay mineralogy, grain size distribution, and Atterberg limits. The results showed that the mean liquid limit values ranged from 38% to 62%, while the mean plasticity index of the materials in the landslide-prone areas varied from 18% to 27.9%. These findings indicate presence of clay formations and highlight a significance of the increased soil clay content as contributing factors to landslide development. The X-ray Diffraction analysis revealed that gypsum, quartz, phyllosilicate and calcite minerals were the most abundant minerals identified in the soil samples. This work shows the importance of clay mineral and geotechnical parameters of the soils in the occurrence of landslides and predicting debris flows occurrences in the Matmata region.

The possible influence of permafrost degradation on the formation of debris flows in an area of the South Tyrolean Alps, Italy, was examined by comparing debris flow activity since 1983 with the modelled contemporary permafrost distribution. The study focused on the spatial congruence of new initiation zones and potentially marginal permafrost, which should be especially sensitive to climatic change and is presumed to be currently degrading. The results show that distinct changes in the spatial position of debris flow initiation areas mainly occurred at elevations above this marginal zone. Consequently, the changes detected in debris flow activity do not appear to have been influenced by atmospheric warming-induced degradation of permafrost. However, a link may exist to the thickening of the active layer caused by the melting of a glacier. Copyright (C) 2011 John Wiley & Sons, Ltd.

A widespread risk in high mountains is related to the accumulation of loose sediments on steep slopes, which represent potential sources of different types of geomorphic processes including debris flows. This paper combines data on 50 yr of permafrost creep at the Ritigraben rock glacier (Valais, Swiss Alps) with magnitude-frequency (M-F) relationships of debris flows recorded in the Ritigraben torrent originating at the rock-glacier front. Debris production and volumetric changes at the rock-glacier front are compared with debris-flow activity recorded on the cone and potential couplings and feedbacks between debris sources, channel processes and debris sinks. The dataset existing for the Ritigraben rock glacier and its debris-flow system is unique and allows prime insights into controls and dynamics of permafrost processes and related debris-flow activity in a constantly changing and warming high-altitude environment. Acceleration in rock-glacier movement rates is observed in the (1950s and) 1960s. followed by a decrease in flow rates by the 1970s, before movements increase again after the early 1990s. At a decadal scale, measured changes in rock-glacier movements at Ritigraben are in concert with changes in atmospheric temperatures in the Alps. Geodetic data indicates displacement rates in the frontal part of the rock glacier of up to 0.6-0.9 m yr(-1) since the beginning of systematic measurements in 1995. While the Ritigraben rock glacier has always formed a sediment reservoir for the associated debris-flow system, annual horizontal displacement rates of the rock-glacier body have remained quite small and are in the order of decimeters under current climatic conditions. Sediment delivery from the rock-glacier front alone could not therefore be sufficient to support the 16 debris flows reconstructed on the cone since 1958. On the contrary, debris accumulated at the foot of the rock glacier, landslide and rockfall activity as well as the partial collapse of oversteepened channel walls have to be seen as important sediment sources of debris flows at Ritigraben and would represent 65-90% of the material arriving on the Ritigraben cone. There does not seem to exist a direct coupling between displacement rates of and sediment delivery by the rock-glacier body and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes clearly exists in the case of active-layer failures. In this case, failure processes at the rock-glacier snout and debris-flow events in the channel occur simultaneously and are both triggered by the rainfall event. (C) 2010 Elsevier B.V. All rights reserved.

Debris-flow activity in a watershed is usually defined in terms of magnitude and frequency. While magnitude-frequency (M-F) relations have long formed the basis for risk assessment and engineering design in hydrology and fluvial hydraulics, only fragmentary and insufficiently specified data for debris flows exists. This paper reconstructs M-F relationships of 62 debris flows for an aggradational cone of a small ( 50 mm) in August and September, when the active layer of the rock glacier in the source area of debris flows is largest. Over the past similar to 150 years, climate has exerted control on material released from the source area and prevented triggering of class XL events before 1922. With the projected climatic change, permafrost degradation and the potential increase in storm intensity are likely to produce class XXL events in the future with volumes surpassing 5 x 10(4) m(3) at the level of the debris-flow cone. (C) 2009 Elsevier B.V. All rights reserved.