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William E Dietrich - One of the best experts on this subject based on the ideXlab platform.
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a bottom up control on fresh Bedrock topography under landscapes
Proceedings of the National Academy of Sciences of the United States of America, 2014Co-Authors: Daniella M Rempe, William E DietrichAbstract:The depth to unweathered Bedrock beneath landscapes influences subsurface runoff paths, erosional processes, moisture availability to biota, and water flux to the atmosphere. Here we propose a quantitative model to predict the vertical extent of weathered rock underlying soil-mantled hillslopes. We hypothesize that once fresh Bedrock, saturated with nearly stagnant fluid, is advected into the near surface through uplift and erosion, channel incision produces a lateral head gradient within the fresh Bedrock inducing drainage toward the channel. Drainage of the fresh Bedrock causes weathering through drying and permits the introduction of atmospheric and biotically controlled acids and oxidants such that the boundary between weathered and unweathered Bedrock is set by the uppermost elevation of undrained fresh Bedrock, Zb. The slow drainage of fresh Bedrock exerts a “bottom up” control on the advance of the weathering front. The thickness of the weathered zone is calculated as the difference between the predicted topographic surface profile (driven by erosion) and the predicted groundwater profile (driven by drainage of fresh Bedrock). For the steady-state, soil-mantled case, a coupled analytical solution arises in which both profiles are driven by channel incision. The model predicts a thickening of the weathered zone upslope and, consequently, a progressive upslope increase in the residence time of Bedrock in the weathered zone. Two nondimensional numbers corresponding to the mean hillslope gradient and mean groundwater-table gradient emerge and their ratio defines the proportion of the hillslope relief that is unweathered. Field data from three field sites are consistent with model predictions.
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episodic Bedrock strath terrace formation due to meander migration and cutoff
Geology, 2011Co-Authors: William E Dietrich, Noah J FinneganAbstract:In order to explore mechanisms of Bedrock terrace formation, we have developed a numerical model that couples vertical river incision and meandering. Model results illustrate that flights of unpaired strath terraces can form purely from the internal dynamics of Bedrock river meandering in vertically incising channels. Specifically, knickpoints that propagate upstream following meander cutoffs enhance vertical incision, whereas channel lengthening and corresponding slope reduction during meander growth suppresses vertical incision. Analysis of topography from the Smith River, Oregon, USA, suggests terrace formation by this mechanism. Our results introduce an alternative mechanism to climatic or tectonic forcing, namely inherent instability triggered by meander growth and cutoff, that explains both oscillations in rates of vertical Bedrock incision and the formation of longitudinally traceable, unpaired Bedrock terraces. In addition, our results point to simple topographic criteria for identifying internally generated fluvial Bedrock terraces.
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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, William E Dietrich, David R Montgomery, Brian D Collins, 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.
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a mechanistic model for river incision into Bedrock by saltating bed load
Water Resources Research, 2004Co-Authors: Leonard S Sklar, William E DietrichAbstract:[1] Abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into Bedrock. Here we develop a model for Bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium. We assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity. Particle impact velocity and impact frequency depend on saltation trajectories, which can be predicted using empirical functions of excess shear stress. The model predicts a nonlinear dependence of Bedrock abrasion rate on both sediment supply and transport capacity. Maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the Bedrock bed. Maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension. Measurements of Bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter, which relates rock strength to rock resistance to abrasive wear. The model results suggest that grain size and sediment supply are fundamental controls on Bedrock incision rates, not only by bed load abrasion but also by all other mechanisms that require Bedrock to be exposed in the channel bed.
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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.
Douglas W Burbank - One of the best experts on this subject based on the ideXlab platform.
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quantifying Bedrock fracture patterns within the shallow subsurface implications for rock mass strength Bedrock landslides and erodibility
Journal of Geophysical Research, 2011Co-Authors: Brian A Clarke, Douglas W BurbankAbstract:[1] The role of Bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying Bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of Bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of Bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform Bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of Bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, Bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of Bedrock landslides to within this geomorphically weakened zone.
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Quantifying Bedrock‐fracture patterns within the shallow subsurface: Implications for rock mass strength, Bedrock landslides, and erodibility
Journal of Geophysical Research, 2011Co-Authors: Brian A Clarke, Douglas W BurbankAbstract:[1] The role of Bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying Bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of Bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of Bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform Bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of Bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, Bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of Bedrock landslides to within this geomorphically weakened zone.
Brian A Clarke - One of the best experts on this subject based on the ideXlab platform.
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quantifying Bedrock fracture patterns within the shallow subsurface implications for rock mass strength Bedrock landslides and erodibility
Journal of Geophysical Research, 2011Co-Authors: Brian A Clarke, Douglas W BurbankAbstract:[1] The role of Bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying Bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of Bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of Bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform Bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of Bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, Bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of Bedrock landslides to within this geomorphically weakened zone.
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Quantifying Bedrock‐fracture patterns within the shallow subsurface: Implications for rock mass strength, Bedrock landslides, and erodibility
Journal of Geophysical Research, 2011Co-Authors: Brian A Clarke, Douglas W BurbankAbstract:[1] The role of Bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying Bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of Bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of Bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform Bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of Bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, Bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of Bedrock landslides to within this geomorphically weakened zone.
Leonard S Sklar - One of the best experts on this subject based on the ideXlab platform.
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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, William E Dietrich, David R Montgomery, Brian D Collins, 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.
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a mechanistic model for river incision into Bedrock by saltating bed load
Water Resources Research, 2004Co-Authors: Leonard S Sklar, William E DietrichAbstract:[1] Abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into Bedrock. Here we develop a model for Bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium. We assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity. Particle impact velocity and impact frequency depend on saltation trajectories, which can be predicted using empirical functions of excess shear stress. The model predicts a nonlinear dependence of Bedrock abrasion rate on both sediment supply and transport capacity. Maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the Bedrock bed. Maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension. Measurements of Bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter, which relates rock strength to rock resistance to abrasive wear. The model results suggest that grain size and sediment supply are fundamental controls on Bedrock incision rates, not only by bed load abrasion but also by all other mechanisms that require Bedrock to be exposed in the channel bed.
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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.
John M. Esch - One of the best experts on this subject based on the ideXlab platform.
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DETERMINING Bedrock DEPTHS USING THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD - EXAMPLES FROM MICHIGAN
2016Co-Authors: John M. EschAbstract:The Bedrock surface is a fundamental surface for many geological, environmental, and engineering investigations. Drilling to Bedrock or running geophysical surveys to determine Bedrock depth can often be cost prohibitive. The Horizontal-to-vertical spectral ratio (HVSR) passive seismic method has a number of advantages including low cost, ease of use, short sampling times, and minimal data processing. Additionally it is portable, noninvasive, and only requires one man operation and single station readings. The HVSR method has been successfully used in Michigan to determine the glacial drift thickness and configuration of the Bedrock surface. The HVSR method has generally yielded good results in determining Bedrock depths across Michigan. Although the HVSR method may not work everywhere and occasionally less than optimum results occur, useful data can still be gathered. Sometimes other geological inferences can be made with the data in addition to the depth to Bedrock estimation. Eleven local and regional HVSR calibration curves as well as a statewide compilation curve have been generated from readings at wells of known Bedrock depth. Areas where a well-defined HVSR calibration curve already exists can be used to quickly gather exploration readings, process the data and determine Bedrock depth while still in the field. In areas where local HVSR calibration data isn't available yet, a statewide calibration curve is available to estimate Bedrock depths at exploration readings. This not the preferred option because using the statewide calibration curve can sometimes significantly overestimate or underestimate the Bedrock depths. For small sites with a limited number of Bedrock depth control points for calibration curve generation, readings can be taken at the Bedrock control points available and an average shear wave velocity calculated at each reading using the equation Vs=fo*4z, where Vs=average shear wave velocity, fo=resonance frequency, z=depth to rock. The mean of these shear wave velocities is calculated for the site. Exploration readings are then collected with the resonance frequency at each reading inserted into the equation solving for z, Bedrock depth.Several examples will show its use to determine Bedrock depth for different applications.
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Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for Bedrock depth determination in Michigan
Abstracts with Programs - Geological Society of America, 2015Co-Authors: John M. Esch, William A. SauckAbstract:The horizontal-to-vertical spectral ratio (HVSR) passive seismic geophysical method allows one to determine glacial drift thickness (Bedrock depth) if there are strong enough acoustic impedance contrasts between the drift and underlying Bedrock. The HVSR method uses naturally occurring seismic noise (wind, waves, flowing water, distant weather) and man-made noise (vehicles, industry) as an energy source. A single station, three-component seismometer (two horizontal and one vertical) is used to record the ambient seismic noise. Michigan has the thickest drift on land in North America, but the thickness is quite variable and the underlying Bedrock surface is very irregular. Many places have a poor distribution or quality of Bedrock depth control points. The HVSR method has been evaluated to Michigan for the last two years. HVSR calibration readings at wells of known Bedrock depth have been gathered in several areas of the state. The calibration readings are compiled for a given area with the same geologic setting resulting in local and regional calibration curves and curve equations. HVSR exploration readings are taken at locations of unknown Bedrock depths and the data inserted into the local calibration equation solving for Bedrock depth. The HVSR method has been successfully used in several parts of the state to determine Bedrock depths, map Bedrock topography, fill in data gaps, to confirm or deny anomalous Bedrock depths, and to better define Bedrock valleys, highs and scarps. The HVSR method has many advantages including low cost, ease of use, one man operation, single station, portability, noninvasive, quick, minimal data processing, its specificity to a single interface (Bedrock surface) and its ability to be used in culturally noisy areas. The HVSR technique is currently being used in geological mapping, groundwater investigations, and mineral exploration. It has great potential in geotechnical and engineering investigations and utility excavations. Additionally it can be used as an independent depth calibration for modeling with other geophysical survey methods. The HVSR technique may aid in petroleum exploration by supplying an independent Bedrock elevation profile along seismic reflection lines providing higher quality Bedrock elevations to assist in traditional static corrections.
