The erosion of sediment through the exchange of momentum and energy transfer within the debris flow affects the unstable motion of the debris flow. Many models have been established to study the ...impact of erosion on debris flow motion, but most of the models were based on bed erosion. This paper analyzed the process of unstable motion of debris flows through an experimental flume to contrast bank erosion-dominated conditions and bed erosion-only conditions. The experiments showed that bank erosion enhanced the formation and propagation of debris flows. The volume of sediments eroded by water and the debris flow mass for the bank erosion-dominated conditions was much greater than that for the bed erosion-only conditions. The height and velocity of the debris flows fluctuated, and the total basal normal stress and pore pressure increased unsteadily along the path under both conditions. However, bank erosion increased the velocity of the debris flow and made the motion fluctuation more obvious. Physical equations were established and the analyses suggested that bank erosion-dominated debris flows had increased resistance and gradient enhancement than that of bed erosion-only debris flows.
•The excess topography ZE is the dominant control parameter in debris-flow gullies.•The sediment supply during debris flow events mainly comes from bank collapse.•Debris flow gullies of intermediate ...size have the highest ZE and are the most active.
An investigation on 152 gullies along the Daheba River in the Tongde sedimentary basin was performed. Debris flows develop in gullies with an excess topography ZE, which represents the sediment availability, above a critical threshold value. Debris flows in the Daheba watershed are supply-unlimited, i.e sediment is abundantly available from the steep erodible gully banks. Debris flows consist of a head and a body. The body propagates faster than the head and constantly supplies it with sediment. The body and head propagate in an intermittent way through the transient storage of sediment on the riverbed and its subsequent remobilization. Although the main sediment supply is provided by bank collapse, debris-flow events also incise the gully bed. The growth and incision of debris-flow gullies in supply-unlimited watersheds is mainly controlled by the frequency of occurrence of debris flows, which is closely related to ZE. With growth of the gully drainage area, ZE and the debris-flow frequency initially increase, until they reach maximum values in gullies with a drainage area of intermediate size, which are assumed to be the morphologically most active gullies. With further growth of the gully drainage area, ZE and the debris-flow frequency decrease, which opposes the development of debris flows and leads to a more stable gully morphology.
•Debris flows near the dam and barrier lake complicated river sedimentation rate.•Vegetation growth disturbance was consistent well with river sedimentation rate.•Trees recovery time lagged behind ...debris flow event 1–3 years in glacial valley.
In incised mountain valleys, debris flows often cause damage to vegetation, especially when debris flows form fans or block receiving rivers. In such contexts, riparian trees may be partially buried in aggrading fans or temporary lakes. The Guxiang gully is the largest debris flow system along the Palongzangbo River on the Tibetan Plateau and at least 13 debris flows (volume > 104 m3) have occurred since 1953. Four of these debris flows (in 1953, 1965, 1975 and 2005) dammed the river forming a barrier lake which is now more than 25 km in length. The repeated damming by debris flows has increased the dam height and thereby affected the sedimentation rates in the barrier lake. The frequent debris flow activity has prevented vegetation recovery on the fan, while sedimentation in the barrier lake has allowed vegetation to colonize the deltas. This study investigated the effects of frequent debris flow damming on sedimentation rates as well as on revegetation and tree disturbance along the river. We documented the landscape evolution and measured the sedimentation thickness and grain size of deposited material at different locations of the barrier lake using cores. We also collected and analyzed growth-ring records from disturbed trees on the fan and the deltas of the barrier lake. We found that frequent debris flows had created a stepped river morphology and greatly increased sedimentation rates in the lake, affecting vegetation on the fan and behind the dam.
In the Tibet Plateau, Glacial Lake Outburst Floods (GLOFs) and associated debris flow events have been gradually increasing in the alpine regions of West- and Trans-Himalayas due to a notable rise in ...both the frequency of extreme rainstorms and the effects of climate warming. This article documents GLOFs in Rongdui watershed, Western Himalayas (China), on October 5, 2021. Data records were extracted from multiple resources to thoroughly examine the climatic conditions. These data are from a local meteorological station, the Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) thermal bands, pre- and post-flow high-resolution sensing maps, the small baseline subset InSAR time-series technique (SBAS-InSAR), water release, eroded moraine debris, and the topographic change. The released water approximately reached 59.5 × 10
4
m
3
, strongly affecting at least 158.6 × 10
4
m
3
of moraine debris to form debris flows. The potential cause of this change is high temperatures in the summer season in 2020 and late September 2021, leading to a glacier lake expansion and the corresponding GLOFs events. Though a large amount of moraine debris was entrained to form debris flows, the majority of it has been deposited in the wide and low-gradient valley floor area. Results from SBAS-InSAR prove that the moraine dam failure and the glacial till exhibit obvious subsidence before GLOFs events. Importantly, a direct correlation between moraine dam displacements and temperatures was observed, which could be further examined for hazard warning in the future. This case study reveals that the characteristic of deposits is finer in midstream and on fan areas than in other locations. The interior porewater pressure is substantial, with slower dissipation downstream than upstream, which may result in a long-runout behavior of debris flows. Lastly, the possibility of long-runout debris flows could increase due to the lag effect of GLOFs after years of high temperatures, highlighting the necessity for extra attention to the potential risks in West- and Trans-Himalayas.
Talus slopes play an important role in landscape evolution in some mid-altitude mountains. In this study, we analyzed talus slope activity on Xiaowutai Mountain in the Northern Taihang Mountain Belt, ...China, using a combination of aerial map interpretation, topographic analysis, field investigation, and climate monitoring. Most of the identified talus slopes were located in the elevation range 1501–2882 m, on south-facing slopes with a gradient of 20.1–45°, and in areas with sparse vegetation and underlying rhyolite geology. Their number and area significantly increased between 2003 and 2017 in association with a warming climate. The annual change in the area covered by talus slopes was positively and negatively correlated to thawing and freezing indexes, respectively, with the largest coverage increase recorded in 2016. Furthermore, the average area of high-roughness talus slopes increased more between 2015 and 2016 compared to low-roughness talus slopes. We show that surface runoff is the primary factor driving talus slope expansion in this area, which is secondary to freeze-thaw cycling. These results provide a new understanding of the development of talus slopes in similar mid-altitude regions in comparison to alpine and polar areas, where ice melting is considered the dominant formation mechanism.
The Dadu River Basin (DRB) is an important economic corridor on the Chuanxi Plateau. The DRB is located in the highly changeable terrain of the eastern Qinghai-Tibet Plateau and is often threatened ...by debris flows. To mitigate the damage of debris flow disasters to the construction and operation of large infrastructures such as railways, roads and hydropower stations in the DRB, it is necessary to evaluate debris flow susceptibility. The connectivity of potential material sources (CPMS), which proxies the possibility of potential material transfer from the source to the outlet, was calculated using an improved index of connectivity, excess topography and brittle fracture index to identify and quantify debris flow susceptibility. The results show that the IC and the gully scale have a Logartihmic function in the DRB, in which IC decreases as the catchment area increases. In 3967 gullies of the Dadu River and its tributaries, the total connectivity of potential material sources (TCPMS) could be used to distinguish between debris flow gullies and non-debris flow gullies (AUC = 0.748). The TCPMS and average connectivity of potential material sources (ACPMS) were used to classify the scale and activity intensity of the 2398 debris flow gullies into four levels. The moderate- and high-activity intensity and large- and extra large-scale debris flows are mainly distributed in Danba, Luding and Kangding. The results of this study provide reference and evidence for the identification and evaluation of debris flow susceptibility.
•A new method for assessing debris flow susceptibility based on potential material sources and sediment connectivity.•The quantitative relationship between the scale of debris flow and conditioning factors.•The contribution of excess topography was considered.
The critical rainfall of runoff-initiated debris flows is utmost importance for local early hazard forecasting. This paper presents research on the critical rainfall of runoff-initiated debris flows ...through comparisons between slope gradients and three key factors, including topographic contributing area, dimensionless discharge, and Shields stress. The rainfall amount was estimated by utilizing in-situ rainfall records and a slope-dependent Shields stress model was created. The created model can predict critical Shields stress more accurately than the other two models. Furthermore, a new dimensionless discharge equation was proposed based on the corresponding discharge-gradient datasets. The new equation, along with factors such as contributing area above bed failure sites, channel width, and mean diameter of debris flow deposits, predicts a smaller rainfall amount than the in-situ measured records. Although the slope-dependent Shields stress model performs well and the estimated rainfall amount is lower than the in-situ records, the sediment initiation in the experiments falls within sheet flow regime due to a large Shields stress. Therefore, further sediment initiation experiments at a steeper slope range are expected in the future to ensure that the sediment transport belongs to mass failure regime characterized by a low level of Shields stress. Finally, a more accurate hazard forecast on the runoff-initiated debris flow holds promise when the corresponding critical slope-dependent dimensionless discharge of no motion, fluvial sediment transport, mass flow regime, and sheet flow regime are considered.
The Grand Canyon of the Nu River is the upper reach of the Salween River, and it is located on the eastern margin of the Tibetan Plateau. There were 187 debris flow gullies along the Grand Canyon of ...the Nu River, and debris flows from 53 of these gullies completely blocked the river. Debris flows have carried a large volume of sediment into the river since the Holocene and have formed high-density dams on the riverbed, with the maximum dam height exceeding 30 m. The Nu River comprises the repeated sections of straight flow connecting lakes and the debris flow dams that changed the movement and distribution of sediment in the river. In this paper, terrain data for debris flow dams were collected by Lidar, water depth data for the lakes were collected by an unmanned ship with a depth sounder, and the thicknesses of sediment deposits, both in the dams and in the lakes, were measured using a geophysical exploration method (EH4). The debris flow dams were stratified and had multi-stage stepped structures with high resistance because of repeated historical breaches of older dams and river coarsening, different from the structure of alluvial fans constructed by floods. The construction of debris flow dams and the sedimentation depths and volumes in the Grand Canyon were analyzed. The research shows that the high density of debris flow dams made the Grand Canyon stepped and the sedimentation rate greatly increased, reversing the Nu River Grand Canyon’s historical cutting trend.
Glacial debris flows occurring on the Tibetan Plateau consistently result in significant property damage and loss of human life. A comprehensive field investigation was conducted in Tianmo valley ...along the Sichuan‐Tibet Highway to reveal the dynamics of a debris flow that occurred on 11 July 2018. Furthermore, a depth‐averaged multiphase debris flow model was proposed and employed to reconstruct the characteristics of the debris flow. The model derivation, implementation, evaluation, and application were presented to demonstrate its performance. The Voellmy model was chosen because it adequately accounts for both basal frictional effects and the entrainment phenomenon. The entrainment processes, the ice melting, and the lubrication effect, were also taken into consideration. Based on the numerical results combined with field investigation data, the kinetic characteristics of the glacial debris flow were analyzed. The Tianmo valley has a small area, but the volume and erosion rate of debris flows were much larger than that of two‐phase debris flows in the same location due to ice melting. The simulation results demonstrated that the glacial debris reached a peak velocity of 20 m/s. Additionally, the volume of the debris flow increased by 50% due to the erosion over a short runout distance of approximately 4,000 m. This increase was a result of the high velocity and abundant entrainment sources on the slope. This study aims to improve understanding of the high velocity and destructive potential of debris flows in the Tianmo valley.
Plain Language Summary
Debris flows frequently affect people or infrastructure due to their large volumes and high velocities. A detailed field investigation was taken in Tianmo valley, Tibet Plateau, China in this paper to reveal the dynamics of debris flow hazard chains. In addition, an optimized numerical model was proposed and used to analyze the kinetic characteristics of a glacial debris flow that occurred on 11 July 2018. The basal friction, debris erosion, and ice melting were taken into account in the numerical simulation. Based on the numerical results combined with field investigation data, the kinetic characteristics of the glacial debris flow were obtained. Simulated results indicated that the movement process of a glacial debris flow moved approximately 4,000 m, reaching a maximum velocity of 20 m/s and 50% more materials through erosion along its path. It is expected that this study can help to better understand the significant destructive force in the Tianmo valley and establish evacuation and rescue plans for communities in Tibet Plateau.
Key Points
The kinetic characteristics of a glacial debris flow in Tibet were investigated
An optimized depth‐averaged multiphase debris flow model was derived, implemented, and evaluated
The Tianmo debris flow exhibited high velocity, large volume, and high erosion rates due to ice melting and sufficient entrainment sources
An evaluation of the measurement uncertainty of on-road NOx emissions using portable emission measurement system (PEMS) based on real local testing data collected in China was carried out as per the ...type B method defined in the EN 17507 standard. The aim of this work was to quantify the “absolute” measurement uncertainty of PEMSs, which excluded “PEMS relative to laboratory constant volume sampler (CVS)” uncertainty from the calculation of on-road NOx measurement uncertainty using PEMSs. PEMS instruments from three mainstream manufacturers were employed. The zero drift of the NOx analyzers was evaluated periodically during the real driving emissions (RDE) test, and it was noticed that there was neither a linear nor step model of zero drift, with no correlation with the boundary conditions or measurement principle. Additionally, from the 256 valid RDE tests, the zero drift always ranged from 3.8 ppm to −3.8 ppm, and more than 95% of the span drifts were within a range of 1.5%. Based on the laboratory testing of ten vehicles using the worldwide harmonized light-duty vehicle test cycle (WLTC), the type B uncertainty of PEMS NOx measurements corresponding to China-6a and China-6b limits was assessed. An uncertainty of 26.5% for China-6a was found (NOx limit = 60 mg/km over the WLTC), which is very close to the 22.5% from the EU evaluation results (NOx limit = 80 mg/km over the WLTC); the uncertainty with respect to China-6b was found to be 42.8% because the type-I limit was tuned down to 35 mg/km. This result indicates that, with the ever-tightening regulatory limits of vehicle NOx emissions, big challenges will be posed in terms of the reliability of PEMS measurements, which requires PEMS manufacturers to improve the performance of the instruments and policymakers to refine the test procedures and/or result calculation method to minimize the impacts.
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