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The Arctic is a water-rich region, with freshwater systems covering about 16 % of the northern permafrost landscape. Permafrost thaw creates new freshwater ecosystems, while at the same time modifying the existing lakes, streams, and rivers that are impacted by thaw. Here, we describe the current state of knowledge regarding how permafrost thaw affects lentic (still) and lotic (moving) systems, exploring the effects of both thermokarst (thawing and collapse of ice-rich permafrost) and deepening of the active layer (the surface soil layer that thaws and refreezes each year). Within thermokarst, we further differentiate between the effects of thermokarst in lowland areas vs. that on hillslopes. For almost all of the processes that we explore, the effects of thaw vary regionally, and between lake and stream systems. Much of this regional variation is caused by differences in ground ice content, topography, soil type, and permafrost coverage. Together, these modifying factors determine (i) the degree to which permafrost thaw manifests as thermokarst, (ii) whether thermokarst leads to slumping or the formation of thermokarst lakes, and (iii) the manner in which constituent delivery to freshwater systems is altered by thaw. Differences in thaw-enabled constituent delivery can be considerable, with these modifying factors determining, for example, the balance between delivery of particulate vs. dissolved constituents, and inorganic vs. organic materials. Changes in the composition of thaw-impacted waters, coupled with changes in lake morphology, can strongly affect the physical and optical properties of thermokarst lakes. The ecology of thaw-impacted lakes and streams is also likely to change; these systems have unique microbiological communities, and show differences in respiration, primary production, and food web structure that are largely driven by differences in sediment, dissolved organic matter, and nutrient delivery. The degree to which thaw enables the delivery of dissolved vs. particulate organic matter, coupled with the composition of that organic matter and the morphology and stratification characteristics of recipient systems will play an important role in determining the balance between the release of organic matter as greenhouse gases (CO2 and CH4), its burial in sediments, and its loss downstream. The magnitude of thaw impacts on northern aquatic ecosystems is increasing, as is the prevalence of thaw-impacted lakes and streams. There is therefore an urgent need to quantify how permafrost thaw is affecting aquatic ecosystems across diverse Arctic landscapes, and the implications of this change for further climate warming.

Fault slip inevitably causes the multiscale wear damage of asperities, ranging from nanometers to meters. However, the nanoscale asperity wear mechanism remains poorly understood. While plastic wear has been inferred as one of the dominant wear modes, the dynamic wear mechanism of plastic wear has not been thoroughly investigated. Here, we explicitly present a series of nanoscale 3‐D plastic wear processes of α‐quartz asperities by using molecular dynamics method, where asperity climbing mode dominates during the sliding. We identify a transition from atom‐by‐atom wear damage to layer removal of α‐quartz asperities with increasing normal forces. Moreover, nanoscale wear volume evolution depends on the normal force and loading velocity and shows sublinear increase with loading distance. We confirm that the tangential shear work can well predict the nanoscale plastic wear volume under various loading conditions due to the proportional relation.

The multi-row stabilizing piles have been applied in the stabilization of large-scale reservoir landslides in recent years. However, the mechanical behavior and deformation characteristics of the multi-row stabilizing piles reinforced reservoir landslides have rarely been investigated. This study takes the Taping landslide, a large-scale reservoir landslide in China, as a prototype. Two centrifuge tests were conducted to study the deformation and failure characteristics of the multi-row stabilizing piles reinforced reservoir landslide with two different row spacings. The result shows that the reservoir water level (RWL) drawdown operation induced the soil movement and high downslope driving force, further causing a significant increase in bending moments at the lower section of the piles, with peaking near the sliding zone; eventually, bending deformation and failure occurred more easily near the sliding zone. The downslope part of the piles can change the mechanical transmission behavior of the multi-row stabilizing piles in reservoir landslides. Small row spacing can enhance the mechanical connection between the rows of piles and raise the overall reinforcement capacity of the piles. The large row spacing weakens the mechanical connection between the rows of piles, and the mechanical states of the pile in different rows are relatively independent. As a result, the piles are easily damaged one by one from the first row to the last row, and the overall reinforcement capacity of the multi-row stabilizing piles is poor.

Reservoir landslides greatly threaten reservoir safety. Understanding the deformation characteristics and mechanism of reservoir landslides can help evaluate their stability and prevent secondary disasters. A detailed analysis of the deformation characteristics and landslide reactivation mechanism of the Wangjiashan (WJS) ancient landslide during the initial impoundment of the Baihetan Reservoir region was performed using comprehensive in situ monitoring and drilling data. The WJS landslide slowly deformed before impoundment. Reservoir impoundment was the main factor driving the intensifying deformation of the WJS landslide. The rise in reservoir water resulted in bank collapse at the landslide toe. After the reservoir water flooded the sliding zone of the landslide toe, creep deformation occurred along the deep sliding zone, which developed into overall sliding on July 7. The further rise in the reservoir water level has led to the rapid sliding of the landslide. The WJS landslide is a buoyancy weight-reducing landslide. When the reservoir water rises to a high level, the buoyancy force of the reservoir water acts on the resisting section, which reduces the resisting force and leads to the rapid sliding of the landslide. When the reservoir water level drops from the high level, the buoyancy acting on the resisting section decreases gradually, and the stability of the landslide can be restored. At present, the WJS landslide deformation rate gradually decreases with the reservoir water level, and the probability of large-scale landslides is low. However, WJS landslide monitoring needs to be strengthened to more closely study its deformation mechanism.

The empirical rate‐ and state‐dependent friction law is widely used to explain the frictional resistance of rocks. However, the constitutive parameters vary with temperature and sliding velocity, preventing extrapolation of laboratory results to natural conditions. Here, we explain the frictional properties of natural gouge from the San Andreas Fault, Alpine Fault, and the Nankai Trough from room temperature to ∼300°C for a wide range of slip‐rates with constant constitutive parameters by invoking the competition between two healing mechanisms with different thermodynamic properties. A transition from velocity‐strengthening to velocity‐weakening at steady‐state can be attained either by decreasing the slip‐rate or by increasing temperature. Our study provides a framework to understand the physics underlying the slip‐rate and state dependence of friction and the dependence of frictional properties on ambient physical conditions. Plain Language Summary The physics of friction is crucial to understanding fault mechanics, impacting virtually every aspect of earthquake initiation, propagation, and associated hazards. The mechanics of active fault zones exhibit a complex dependence on temperature and sliding velocity among other factors. The frictional resistance of natural gouge can be explained by empirical rate‐ and state‐dependent friction laws for a limited range of conditions. However, explaining the non‐stationary frictional behavior of gouge friction and extrapolation of laboratory constraints to natural conditions remains challenging. In this study, we describe a constitutive law that predicts the velocity of sliding of natural gouge based on applied shear stress, effective confining pressure, and the ambient temperature of the fault. The transition from stable to unstable sliding is controlled by the competition between micro‐mechanisms of deformation within the gouge that dominate in distinct ranges of temperature and slip‐rate. Once calibrated to mechanical data for a specific lithology and confining pressure, the model explains the temperature and slip‐rate control on fault stability, allowing extrapolation of laboratory data to natural conditions. Key Points The dependence of natural gouge friction on temperature and velocity cannot be captured by empirical laws with constant coefficients The competition of healing mechanisms explains a velocity‐ and temperature‐controlled transition between velocity‐weakening and hardening The constitutive law explains the mechanics of natural gouge from various tectonic settings, allowing scaling up from laboratory to nature

As one of the most common failure forms of discontinuity-controlled rock slopes, wedge failure is likely to occur in a wide range of geologic and geometric conditions. In this study, the wedge failure of rock slopes and the movement and disaster processes after failure are investigated using 3D discontinuous deformation analysis (DDA). Compared with the analytical solutions derived from a typical rock wedge model, the performance of the original 3D DDA for analyzing the wedge stability under different geometrical and physical parameters is presented. The deficiency of the joint contact model in 3D DDA under critical state is improved. The improved 3D DDA is used to simulate a rock slope subjected to wedge failure in Tibet Autonomous Region, and the failure of the dangerous rock masses and movement of the formed blocks under different discontinuity cutting are discussed. The improved 3D DDA has high accuracy in calculating wedge critical stability and sliding after failure. The actual wedge slope presents sliding failure along the intersection line of structural planes, and tensile and shear failure and downward dislocations can be observed among blocks. The lateral deviation and deflection of wedge blocks occur constantly, showing 3D kinematic characteristics. With the increase of secondary discontinuities, the influence range of sub-blocks due to wedge failure becomes larger, constituting the geological disaster of the G318 national road. 3D DDA can evaluate wedge stability and analyze kinematic characteristics of wedge blocks, which lay a foundation for formulating disaster prevention countermeasures and reducing human casualties.

On June 28, 2016, a landslide occurred in Xinlu Village, Qijiang District, Chongqing, China. The landslide volume was 854,200 m3. Exposed to continuous rainfall, the rocky part of the trailing edge slipped first, and then the soil part of the leading edge was pushed downward. Numerical simulations were used to analyze the formation characteristics of the landslide and to predict the stability of the landslide after the slip. In order to ensure the accuracy and rationality of numerical calculation, we used field surveys, laboratory tests, and back-calculation to restore the typical sections, determine the relevant mechanical parameters, and control boundary conditions. The displacement, stress, effective plastic strain distribution, and safety factor, obtained by numerical calculation, were compared to examine the action of rainfall. According to the evaluation of landslide stability and numerical calculation results, the Xinlu Village landslide remained in a peristaltic stage after the initial overall sliding, and additional sliding could occur under storm conditions.

Bubble growth, departure and sliding in low-pressure flow boiling has received considerable attention in the past. However, most applications of boiling heat transfer rely on high-pressure flow boiling, for which very little is known, as experimental data are scarce and very difficult to obtain. In this work, we conduct an experiment using high-resolution optical techniques. By combining backlit shadowgraphy and phase-detection imaging, we track bubble shape and physical footprint with high spatial ($6\\,\\mathrm {\\mu }{\\rm m}$) and temporal ($33\\,\\mathrm {\\mu }{\\rm s}$) resolutions, as well as bubble size and position as bubbles nucleate and slide on top of the heated surface. We show that at pressures above 1 MPa bubbles retain a spherical shape throughout the growth and sliding process. We analytically derive non-dimensional numbers to correlate bubble velocity and liquid velocity throughout the turbulent boundary layer and predict the sliding of bubbles on the surface, solely from physical properties and the bubble growth rate. We also show that these non-dimensional solutions can be leveraged to formulate elementary criteria that predict the effect of pressure and flow rate on bubble departure diameter and growth time.

We performed a passive seismic monitoring of the La Praz ∼14,000 m3 unstable slope (French Alps) spanning over 10 years. During the last 6 months prior to collapse, we detected a clear 24% decrease in the slope's fundamental resonance frequency, f0, caused by a reduction in overall rock mass stiffness. The combined study of f0 and slope deformation suggested the alternating importance of sudden brittle failure processes versus more ductile phases with possible sliding. Seismic monitoring revealed slope damage that remained ambiguous or undetected with ground surface deformation monitoring, and highlighted critical periods with intense damage. Only some of these critical damage periods could be related to clear external forcing factors such as intense rainfall episodes. These new insights into rock slope's structural condition at depth represent an asset for future monitoring systems. Surface deformation and passive seismic stiffness tracking combined could reveal active slopes with ongoing damage processes. Plain Language Summary Forecasting the time of rockfalls is of critical importance for risk mitigation operators in order to preserve the safety of persons and the integrity of infrastructure. Most monitoring systems are based on ground deformation measurements, which may fail when the surface motion does not accurately reflect changes in slope stiffness with time. In this work, we used a seismic sensor that passively recorded ground vibrations on top of a ∼14,000 m3 unstable slope. We detected a significant decrease in the slope's first resonance frequency (−24%) during the 6 months preceding complete collapse. We also revealed details of slope damage processes acting within the slope, showing sudden breakage phases alternating with smoother deformation and sliding phases. These processes would not have been suspected with slope deformation monitoring alone. The use of a new parameter that combines passive seismic tracking and surface deformation measurements could help revealing active slopes with ongoing damage processes. This inovative approach represents an asset for future practical rock slope monitoring. Key Points We observed a 24% decrease in rock slope fundamental frequency during the 6 months preceding a ∼14,000 m3 collapse New insights into slope degradation processes revealed the alternate control of two damage regimes with contrasting rate Results revealed slope damage phases that would not have been suspected on the basis of slope deformation monitoring alone

We present the result of the third Marine Ice Sheet Model Intercomparison Project, MISMIP+. MISMIP+ is intended to be a benchmark for ice-flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient to model buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments first tests that models are able to maintain a steady state with the grounding line located on a retrograde slope due to buttressing and then explore scenarios where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. The majority of participating models passed the first test and then produced similar responses to the loss of buttressing. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress but also the differences between numerical methods – taking a secondary role.