The Experts below are selected from a list of 4959 Experts worldwide ranked by ideXlab platform
Leonard S Sklar - One of the best experts on this subject based on the ideXlab platform.
-
new insights into the mechanics of fluvial bedRock erosion through flume experiments and theory
Geomorphology, 2015Co-Authors: Michael P Lamb, Noah J Finnegan, Joel S Scheingross, Leonard S SklarAbstract:River incision into bedRock drives the topographic evolution of mountainous terrain and may link climate, tectonics, and topography over geologic time scales. Despite its importance, the mechanics of bedRock erosion are not well understood because channel form, river hydraulics, sediment transport, and erosion mechanics coevolve over relatively long time scales that prevent direct observations, and because erosive events occur intermittently and are difficult and dangerous to measure. Herein we synthesize how flume experiments using erodible bedRock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory. We also build on this work by providing new theory for Rock resistance to abrasion, thresholds for plucking by vertical entrainment, sliding and toppling, and by assessing bedRock-analog materials. BedRock erosion experiments in the last 15 years reveal that the efficiency of Rock abrasion scales inversely with the square of Rock Tensile Strength, sediment supply has a dominant control over bed roughness and abrasion rates, suspended sediment is an efficient agent of erosion, and feedbacks with channel form and roughness strongly influence erosion rates. Erodibility comparisons across Rock, concrete, ice, and foam indicate that, for a given Tensile Strength, abrasion rates are insensitive to elasticity. The few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed, and the dominance of block sliding and toppling at knickpoints. These observations are consistent with new theory for the threshold Shields stress to initiate plucking, which also suggests that erosion rates in sliding- and toppling-dominated rivers are likely transport limited. Major knowledge gaps remain in the processes of erosion via plucking of bedRock blocks where joints are not river-bed parallel; waterfall erosion by toppling and plunge-pool erosion; feedbacks between weathering and physical erosion; erosional bedforms; and morphodynamic feedbacks between channel form and erosion rates. Despite scaling challenges, flume experiments continue to provide much needed tests of existing bedRock-erosion theory, force development of new theory, and yield insight into the mechanics of landscapes.
-
field measurements of incision rates following bedRock exposure implications for process controls on the long profiles of valleys cut by rivers and debris flows
Geological Society of America Bulletin, 2005Co-Authors: Jonathan D Stock, Brian D Collins, David R Montgomery, William E Dietrich, Leonard S SklarAbstract:Until recently, published rates of incision of bedRock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedRock lowering at geologically unsustainable rates. We report observations of bedRock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary Rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedRock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact Rock into plates or equant fragments that are removed by higher fl ows. Consequent incision rates are proportional to the square of Rock Tensile Strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosionrate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term Rock-uplift rates. This rate disparity makes it unlikely that the long profi les of these rivers are directly adjusted to either bedRock hardness or Rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profi les are well fi t by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedRock hardness or Rock-uplift rate, controls channel slope in weak bedRock lithologies with Tensile Strengths below ~3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields’ threshold transport concept with measured hydraulic relationships and downstream fi ning rates. In contrast to fl uvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedRock by debris fl ows indicate any postevent fl uvial lowering. These results are consistent with episodic debris fl ows as the primary agent of bedRock lowering in the steepest parts of the channel network above ~0.03–0.10 slope.
-
sediment and Rock Strength controls on river incision into bedRock
Geology, 2001Co-Authors: Leonard S Sklar, William E DietrichAbstract:Recent theoretical investigations suggest that the rate of river incision into bedRock depends nonlinearly on sediment supply, challenging the common assumption that incision rate is simply proportional to stream power. Our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion, but inhibits erosion at high supply rates by burying underlying bedRock beneath transient deposits. Maximum erosion rates occur at a critical level of coarse-grained sediment supply where the bedRock is only partially exposed. Fine-grained sediments provide poor abrasive tools for lowering bedRock river beds because they tend to travel in suspension. Experiments also reveal that Rock resistance to fluvial erosion scales with the square of Rock Tensile Strength. Our results suggest that spatial and temporal variations in the extent of bedRock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in Rock uplift rate and climate. Furthermore, we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedRock channel gradients and topographic relief.
He Liu - One of the best experts on this subject based on the ideXlab platform.
-
extended finite element simulation of fracture network propagation in formation containing frictional and cemented natural fractures
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Xiaolong Wang, Fang Shi, Chuang Liu, He LiuAbstract:Abstract Shale gas reservoirs often need hydraulic fracturing treatments to create complex fracture network to enhance production. Frictional and cemented natural fractures are often contained in shale formations. The interactions between the hydraulic fractures and these two types of pre-existing natural fractures are different. In this study, we established a two-dimensional fluid-solid coupled hydraulic fracturing model using the extended finite element method (XFEM) to simulate the interactions between hydraulic fractures and natural fractures, and further the formation of fracture network. The results show that when a hydraulic fracture intersects with a natural fracture, the hydraulic fracture may be arrested and propagate along the direction of natural fracture, or cross the natural fracture without being affected. For the frictional natural fractures, the intersection angle, frictional coefficient, stress anisotropy and Rock Tensile Strength have a significant influence on creating fracture network. It is found that decreasing stress difference and interfacial friction, or increasing Rock Tensile Strength may lead to more complex fracture network. For the cemented natural fractures, the intersection angle and the ratio of cement toughness and Rock toughness play critical roles in the creation of fracture network. Smaller intersection angle and cement toughness of natural fractures and larger Rock fracture toughness often lead to more complex fracture network. In addition, for the same initial geometrical configuration of natural fractures, hydraulic fracturing often leads to more complex fracture network in formations containing frictional natural fractures compared with formations containing cemented natural fractures. These findings offer new insights into the nature and degree of fracture complexity, helping to optimize hydraulic fracturing design in shale gas reservoirs.
William E Dietrich - One of the best experts on this subject based on the ideXlab platform.
-
field measurements of incision rates following bedRock exposure implications for process controls on the long profiles of valleys cut by rivers and debris flows
Geological Society of America Bulletin, 2005Co-Authors: Jonathan D Stock, Brian D Collins, David R Montgomery, William E Dietrich, Leonard S SklarAbstract:Until recently, published rates of incision of bedRock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedRock lowering at geologically unsustainable rates. We report observations of bedRock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary Rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedRock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact Rock into plates or equant fragments that are removed by higher fl ows. Consequent incision rates are proportional to the square of Rock Tensile Strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosionrate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term Rock-uplift rates. This rate disparity makes it unlikely that the long profi les of these rivers are directly adjusted to either bedRock hardness or Rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profi les are well fi t by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedRock hardness or Rock-uplift rate, controls channel slope in weak bedRock lithologies with Tensile Strengths below ~3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields’ threshold transport concept with measured hydraulic relationships and downstream fi ning rates. In contrast to fl uvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedRock by debris fl ows indicate any postevent fl uvial lowering. These results are consistent with episodic debris fl ows as the primary agent of bedRock lowering in the steepest parts of the channel network above ~0.03–0.10 slope.
-
sediment and Rock Strength controls on river incision into bedRock
Geology, 2001Co-Authors: Leonard S Sklar, William E DietrichAbstract:Recent theoretical investigations suggest that the rate of river incision into bedRock depends nonlinearly on sediment supply, challenging the common assumption that incision rate is simply proportional to stream power. Our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion, but inhibits erosion at high supply rates by burying underlying bedRock beneath transient deposits. Maximum erosion rates occur at a critical level of coarse-grained sediment supply where the bedRock is only partially exposed. Fine-grained sediments provide poor abrasive tools for lowering bedRock river beds because they tend to travel in suspension. Experiments also reveal that Rock resistance to fluvial erosion scales with the square of Rock Tensile Strength. Our results suggest that spatial and temporal variations in the extent of bedRock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in Rock uplift rate and climate. Furthermore, we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedRock channel gradients and topographic relief.
Xiaolong Wang - One of the best experts on this subject based on the ideXlab platform.
-
extended finite element simulation of fracture network propagation in formation containing frictional and cemented natural fractures
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Xiaolong Wang, Fang Shi, Chuang Liu, He LiuAbstract:Abstract Shale gas reservoirs often need hydraulic fracturing treatments to create complex fracture network to enhance production. Frictional and cemented natural fractures are often contained in shale formations. The interactions between the hydraulic fractures and these two types of pre-existing natural fractures are different. In this study, we established a two-dimensional fluid-solid coupled hydraulic fracturing model using the extended finite element method (XFEM) to simulate the interactions between hydraulic fractures and natural fractures, and further the formation of fracture network. The results show that when a hydraulic fracture intersects with a natural fracture, the hydraulic fracture may be arrested and propagate along the direction of natural fracture, or cross the natural fracture without being affected. For the frictional natural fractures, the intersection angle, frictional coefficient, stress anisotropy and Rock Tensile Strength have a significant influence on creating fracture network. It is found that decreasing stress difference and interfacial friction, or increasing Rock Tensile Strength may lead to more complex fracture network. For the cemented natural fractures, the intersection angle and the ratio of cement toughness and Rock toughness play critical roles in the creation of fracture network. Smaller intersection angle and cement toughness of natural fractures and larger Rock fracture toughness often lead to more complex fracture network. In addition, for the same initial geometrical configuration of natural fractures, hydraulic fracturing often leads to more complex fracture network in formations containing frictional natural fractures compared with formations containing cemented natural fractures. These findings offer new insights into the nature and degree of fracture complexity, helping to optimize hydraulic fracturing design in shale gas reservoirs.
Dingjian Wang - One of the best experts on this subject based on the ideXlab platform.
-
local failure probability of the anti dip slope susceptible to flexural toppling
Stochastic Environmental Research and Risk Assessment, 2019Co-Authors: Dingjian Wang, Huiming Tang, Yongquan Zhang, Peiwu ShenAbstract:Anti-dip slope that fails in the form of flexural toppling is commonly encountered in Rock engineering. This paper presents an approach to calculate the local failure probability of the anti-dip slope susceptible to flexural toppling. The realization of this approach consists of four major steps: (1) establishing geo-mechanical model of the slope and generating physical parameters randomly using Monte Carlo simulation, (2) determining the demarcation of the toppling and sliding failure zones, (3) evaluating the stability state of each single column, and (4) deriving the local failure probability by an iterative calculation. A case study of the Mari landslide is analyzed by employing the proposed approach, and the Maxwell model is applied to reduce physical parameters with time. The results show that the instability grade increases gradually throughout the whole evolution process of the slope. In the incipient stage, particular attention should be paid to the middle–upper columns, which have the highest failure probabilities. However, as the slope deformation increases, all the columns should be taken into account since they have approximately the equal possibility of failure. A simplified slope model is employed to perform a parametric analysis. The results show that (1) the local failure probability of the slope tends to decrease with increases in the Rock friction angle, Rock Tensile Strength or interface friction angle, while increasing the Rock unit weight leads to a higher probability of local failure; (2) the computation results are more sensitive to the uncertainty in the interface friction angle than that in the other three above-mentioned parameters; and (3) the maximum failure probability depends on the geometric parameters, including the column interface angle, slope angle, crest angle, slope height and column thickness.
