San Francisco State University
Earth & Climate Sciences
The linkages between fluvial geomorphology and aquatic ecosystems are commonly conceptualized as a one-way causal chain in which geomorphic processes create the physical template for ecological dynamics. In streams with a travertine... more
The linkages between fluvial geomorphology and aquatic ecosystems are commonly conceptualized as a one-way causal chain in which geomorphic processes create the physical template for ecological dynamics. In streams with a travertine step-pool morphology, however, biotic processes strongly influence the formation and growth of travertine dams, creating the potential for numerous feedbacks. Here we take advantage of the decommissioning of a hydroelectric project on Fossil Creek, Arizona, where restoration of CaCO 3 -rich baseflow has triggered rapid regrowth of travertine dams, to explore the interactions between biotic and abiotic factors in travertine morphodynamics. We consider three conceptual frameworks, where biotic factors independently modulate the rate of physical and chemical processes that produce travertine dams; combine with abiotic factors in a set of feedback loops; and work in opposition to abiotic processes, such that the travertine step-pool morphology reflects a dynamic balance between dominantly-biotic constructive processes and dominantly-abiotic destructive processes. We consider separately three phases of an idealized life cycle of travertine dams: dam formation, growth, and destruction by erosive floods. Dam formation is catalyzed by abiotic factors (e.g. channel constrictions, and bedrock steps) and biotic factors (e.g. woody debris, and emergent vegetation). From measurements of changes over time in travertine thickness on a bedrock step, we find evidence for a positive feedback between flow hydraulics and travertine accrual. Measurements of organic content in travertine samples from this step show that algal growth contributes substantially to travertine accumulation and suggest that growth is most rapid during seasonal algal blooms. To document vertical growth of travertine dams, we embedded 252 magnets into nascent travertine dams, along a 10 km stretch of river. Growth rates are calculated from changes over time in the magnetic field intensity at the dam surface. At each magnet we record a range of hydraulic and travertine composition variables to characterize the dominant mechanism of growth: abiotic precipitation, algal growth, trapping of organic material, or in situ plant growth. We find: (1) rapid growth of travertine dams following flow restoration, averaging more than 2 cm/year; (2) growth rates decline downstream, consistent with loss of dissolved constituents because of upstream travertine deposition, but also parallel to a decline in organic content in dam surface material and a downstream shift in dominant biotic mechanism; (3) biotic mechanisms are associated with faster growth rates; and (4) correlations between hydraulic attributes and growth rates are more consistent with biotic than abiotic controls. We conclude that the strong influence of living organisms on rates of travertine growth, coupled with the beneficial effects of travertine on ecosystem dynamics, demonstrate a positive feedback between biology and geomorphology. During our two-year study period, erosive flood flows occurred causing widespread removal of travertine. The temporal distribution of travertine growth and erosion over the study period is consistent with a bimodal magnitudefrequency relation in which growth dominates except when large, infrequent storms occur. This model may be useful in other systems where biology exerts strong controls on geomorphic processes.
- by Leonard Sklar and +1
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- Geomorphology, Remote Sensing
1] Field studies suggest that bedrock incision by granular flows may be the primary process cutting valleys in steep, unglaciated landscapes. An expression has been proposed for debris flow incision into bedrock which posits that erosion... more
1] Field studies suggest that bedrock incision by granular flows may be the primary process cutting valleys in steep, unglaciated landscapes. An expression has been proposed for debris flow incision into bedrock which posits that erosion rate depends on stresses due to granular interactions at the snout of debris flows. Here, we explore this idea by conducting laboratory experiments to test the hypothesis that bedrock erosion is related to grain collisional stresses which scale with shear rate and particle size. We placed granular material in a 56-cm-diameter rotating drum to explore the relationship between erosion of a synthetic bedrock sample and variables such as grain size, shear rate, water content, and bed strength. Grain collisional stresses are estimated as the inertial stress using the product of the squares of particle size and vertical shear rate. Our uniform granular material consisted of 1-mm sand and quartzite river gravel with means of 4, 6, or 10 mm. In 67 experimental runs, the eroded depth of the bed sample varied with inertial stresses in the granular flow to a power less than 1.0 and inversely with the bed strength. The flows tended to slip on smooth boundaries, resulting in higher erosion rates than no-slip cases. We found that lateral wall resistance generated shear across the channel, producing two cells whose widths depended on wall roughness. While the hypothesized inertial stress dependency is supported with these data, wear mechanics needs to account for grain dynamics specifically at the snout and possibly to include lateral shear effects.
One-dimensional numerical sediment transport models ͑DREAM-1 and DREAM-2͒ are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with... more
One-dimensional numerical sediment transport models ͑DREAM-1 and DREAM-2͒ are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with measured data indicate that DREAM-1 and -2 closely reproduce the sediment transport flux and channel bed adjustments following the introduction of fine and coarse sediment pulses, respectively. The cumulative sediment transport at the flume exit in a DREAM-1 simulation is within 10% of the measured values, and cumulative sediment transport at flume exit in a DREAM-2 simulation is within a factor of 2 of the measured values. Comparison of simulated and measured reach-averaged aggradation and degradation indicates that 84% of DREAM-1 simulation results have errors less than 3.3 mm, which is approximately 77% of the bed material geometric mean grain size or 3.7% of the average water depth. A similar reach-averaged comparison indicates that 84% of DREAM-2 simulation results have errors less than 7.0 mm, which is approximately 1.7 times the bed material geometric mean grain size or 11% of the average water depth. Simulations using measured thalweg profiles as the input for the initial model profile produced results with larger errors and unrealistic aggradation and degradation patterns, demonstrating that one-dimensional numerical sediment transport models need to be applied on a reachaveraged basis.
1] River beds are often arranged into patches of similar grain size and sorting. Patches can be distinguished into ''free patches,'' which are zones of sorted material that move freely, such as bed load sheets; ''forced patches,'' which... more
1] River beds are often arranged into patches of similar grain size and sorting. Patches can be distinguished into ''free patches,'' which are zones of sorted material that move freely, such as bed load sheets; ''forced patches,'' which are areas of sorting forced by topographic controls; and ''fixed patches'' of bed material rendered immobile through localized coarsening that remain fairly persistent through time. Two sets of flume experiments (one using bimodal, sand-rich sediment and the other using unimodal, sand-free sediment) are used to explore how fixed and free patches respond to stepwise reductions in sediment supply. At high sediment supply, migrating bed load sheets formed even in unimodal, sand-free sediment, yet grain interactions visibly played a central role in their formation. In both sets of experiments, reductions in supply led to the development of fixed coarse patches, which expanded at the expense of finer, more mobile patches, narrowing the zone of active bed load transport and leading to the eventual disappearance of migrating bed load sheets. Reductions in sediment supply decreased the migration rate of bed load sheets and increased the spacing between successive sheets. One-dimensional morphodynamic models of river channel beds generally are not designed to capture the observed variability, but should be capable of capturing the time-averaged character of the channel. When applied to our experiments, a 1-D morphodynamic model (RTe-bookAgDegNormGravMixPW.xls) predicted the bed load flux well, but overpredicted slope changes and was unable to predict the substantial variability in bed load flux (and load grain size) because of the migration of mobile patches. Our results suggest that (1) the distribution of free and fixed patches is primarily a function of sediment supply, (2) the dynamics of bed load sheets are primarily scaled by sediment supply, (3) channels with reduced sediment supply may inherently be unable to transport sediment uniformly across their width, and (4) cross-stream variability in shear stress and grain size can produce potentially large errors in width-averaged sediment flux calculations.
A theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits. The governing equations are derived from mass conservation and the assumption that the amount of fine sediment... more
A theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits. The governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction. Model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment, which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth. Comparisons of the theory with flume data indicate that the model adequately reproduced the weighted-averaged fine sediment fraction values from experiments. An early model developed by Sakthivadivel and Einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration. Applying a correction to the Sakthivadivel and Einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model.
The saltation–abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply, grain size, boundary shear stress and rock strength. Here we use this experimentally calibrated model to explore the... more
The saltation–abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply, grain size, boundary shear stress and rock strength. Here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady-state topography. Over a wide range of rock uplift rates we find a characteristic downstream trend, in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed, while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure. Profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining. Concavity is also sensitive to the supply rate of coarse sediment, which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class. Variations in rock strength have little influence on profile concavity. Profile relief is most sensitive to grain size and amount of runoff. Rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift. Analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size. Similarly, covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form. Copyright © 2008 John Wiley & Sons, Ltd.
1] Bed load sediment particles supplied to channels by hillslopes are reduced in size by abrasion during downstream transport. The branching structure of the channel network creates a distribution of downstream travel distances to a given... more
1] Bed load sediment particles supplied to channels by hillslopes are reduced in size by abrasion during downstream transport. The branching structure of the channel network creates a distribution of downstream travel distances to a given reach of river and thus may strongly influence the grain size distribution of the long-term bed load flux through that reach. Here we investigate this hypothesis, using mass conservation and the Sternberg exponential decay equation for particle abrasion, to predict bed material variability at multiple scales for both natural and artificial drainage networks. We assume that over a sufficiently long timescale, no net deposition occurs and that grains less than 2 mm are swept away in suspension. We find that abrasion during fluvial transport has a surprisingly small effect on the bed load sediment grain size distribution, for the simple case of spatially uniform supply of poorly sorted hillslope sediments. This occurs because at any point in the channel network, local resupply offsets the size reduction of material transported from upstream. Thus river bed material may essentially mirror the coarse component of the size distribution of hillslope sediment supply. Furthermore, there is a predictable distance downstream at which the bed load grain size distribution reaches a steady state. In the absence of net deposition due to selective transport, largescale variability in bed material, such as downstream fining, must then be due primarily to spatial gradients in hillslope sediment production and transport characteristics. A second key finding is that average bed load flux will tend to stabilize at a constant value, independent of upstream drainage area, once the rate of silt production by bed load abrasion per unit travel distance is equal to the rate of coarse sediment supply per unit channel length (q). Bed load flux equilibrates over a distance that scales with the inverse of the fining coefficient in the abrasion rate law (a) and can be approximated simply as q/3a. Thus the efficiency of particle abrasion sets a fundamental length scale, shorter for weaker rocks and longer for harder rocks, which controls the expression in the river bed of variability in sediment supply. We explore the role of the abrasion length scale in modulating the influence of sediment supply variability in a number of channel network contexts, including individual tributary junctions, a sequence of tributary inputs along a main stem channel, and variable basin shapes and network architecture as expressed by the width function. These findings highlight the need for both data and theory that can be used to predict the grain size distributions supplied to channels by hillslopes.
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... more
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.
We present results and analyses from flume experiments investigating the infiltration of sand into immobile clean gravel deposits. Three runs were conducted, each successive run with the same total sediment feed volume, but a 10-fold... more
We present results and analyses from flume experiments investigating the infiltration of sand into immobile clean gravel deposits. Three runs were conducted, each successive run with the same total sediment feed volume, but a 10-fold increase in sand feed rate. The highest sand feed rate produced less sand infiltration into the subsurface deposits than the other two runs, which had approximately equivalent amounts of sand infiltration. Experimental data, combined with simple geometric relations and physical principles, are used to derive two relations describing the saturated fine sediment fraction in a gravel deposit and the vertical fine sediment fraction profile resulting from fine sediment infiltration. The vertical fine sediment fraction profile relation suggests that significant sand infiltration occurs only to a depth equivalent to a few median grain diameters of the bed material.
1] The effectiveness of gravel augmentation as a river restoration strategy depends on the extent and duration of the topographic and bed texture changes created by the pulse of added sediment. Previous work has emphasized the strong... more
1] The effectiveness of gravel augmentation as a river restoration strategy depends on the extent and duration of the topographic and bed texture changes created by the pulse of added sediment. Previous work has emphasized the strong tendency for natural sediment waves to propagate primarily by dispersion; however, pulse translation may occur for gravel additions to armored channels downstream of dams where added sediments are finer than the preexisting bed material. Here we report results of a laboratory investigation in which we created an immobile, armored bed and documented the spatial and temporal evolution of the bed topography and bed texture in response to gravel pulses of various volumes and grain sizes. The introduced sediment waves evolved by a combination of translation and dispersion, with a significant translational component. Pulse translation and dispersion can be readily discerned on a graph of the time evolution of the downstream cumulative distribution of elevation differences from the preexisting bed topography. Translation was most evident for smaller volumes of added sediment. Pulses of finer-grained gravel moved through the flume more rapidly, resulting in a larger magnitude but shorter duration of bed fining. More work is needed to understand the influence of bar-pool topography and flow magnitude and duration before the grain size and volume of gravel additions can be selected to achieve optimal patterns of pulse propagation.
Landscape evolution models are widely used to explore links between tectonics, climate, and hillslope morphology, yet mechanisms of hillslope erosion remain poorly understood. Here we use a laboratory hillslope of granular material to... more
Landscape evolution models are widely used to explore links between tectonics, climate, and hillslope morphology, yet mechanisms of hillslope erosion remain poorly understood. Here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion. In our experimental hillslope, disturbance-driven sediment transport rates increase nonlinearly with slope due to dilation-driven granular creep, and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding. We use spectral analysis to quantify the variability of sediment flux and estimate the slopedependent transition from creep to landsliding. The power spectrum of sediment flux steepens with hillslope gradient, exhibiting fractal 1/f scaling just below the creep-landsliding transition. By evolving the experimental hillslope under fixed base-level boundary conditions, we demonstrate how disturbance-driven transport generates hillslope convexity. The transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model, as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep-dominated transport.
- by Leonard Sklar and +1
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- Earth Sciences, Geology
The saltation-abrasion model is a mechanistic model for river incision into bedrock by saltating bedload, which we have previously derived and used experimental data to constrain all parameter values. Here we develop a method for applying... more
The saltation-abrasion model is a mechanistic model for river incision into bedrock by saltating bedload, which we have previously derived and used experimental data to constrain all parameter values. Here we develop a method for applying the saltationabrasion model at a landscape scale, and use the model as a reference for evaluating the behavior of a wide range of alternative incision models, in order to consider the implications of the saltation-abrasion model, as well as other models, for predicting topographic steady-state channel slope. To determine the single-valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock, we assume all runoff can be partitioned between a low-flow and a high-flow discharge, in which all bedload sediment transport occurs during high flow. We then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency, due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply. Incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for. Using the predictions of the saltation-abrasion model as a reference, we find that the threshold of motion is the most important effect that should be represented explicitly, followed in order of decreasing importance by the cover effect, the tools effect and the threshold of suspension effect. Models that lack the threshold of motion over-predict incision rate for low shear stresses and under-predict the steady-state channel slope for low to moderate rock uplift rates and rock strengths. Models that lack the cover effect over-predict incision rate for high sediment supply rates, and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage. Models that lack the tools effect over-predict incision rate for low sediment supply rates, and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value. Overall, the saltation-abrasion model predicts that steady-state channel slope is most sensitive to changes in grain size, such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible, but as yet poorly understood, influence on the size distribution of sediments delivered to channel networks by hillslopes.
A geomorphic transport law is a mathematical statement derived from a physical principle or mechanism, which expresses the mass flux or erosion caused by one or more processes in a manner that: 1) can be parameterized from field... more
A geomorphic transport law is a mathematical statement derived from a physical principle or mechanism, which expresses the mass flux or erosion caused by one or more processes in a manner that: 1) can be parameterized from field measurements, 2) can be tested in physical models, and 3) can be applied over geomorphically significant spatial and temporal scales. Such laws are a compromise between physics-based theory that requires extensive information about materials and their interactions, which may be hard to quantify across real landscapes, and rules-based approaches, which cannot be tested directly but only can be used in models to see if the model outcomes match some expected or observed state. We propose that landscape evolution modeling can be broadly categorized into detailed, apparent, statistical and essential realism models and it is the latter, concerned with explaining mechanistically the essential morphodynamic features of a landscape, in which geomorphic transport laws are most effectively applied. A limited number of studies have provided verification and parameterization of geomorphic transport laws for: linear slope-dependent transport, non-linear transport due to dilational disturbance of soil, soil production from bedrock, and river incision into bedrock. Field parameterized geomorphic transport laws, however, are lacking for many processes including landslides, debris flows, surface wash, and glacial scour. We propose the use of high-resolution topography, as initial conditions, in landscape evolution models and explore the applicability of locally parameterized geomorphic transport laws in explaining hillslope morphology in the Oregon Coast Range. This modeling reveals unexpected morphodynamics, suggesting that the use of real landscapes with geomorphic transport laws may provide new insights about the linkages between process and form.
- by Leonard Sklar and +1
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- Geomorphology
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... more
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. model for river incision into bedrock by saltating bed load, Water Resour. Res., 40, W06301,
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... more
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.
1] A mechanistic model is derived for the rate of fluvial erosion into bedrock by abrasion from uniform size particles that impact the bed during transport in both bed and suspended load. The erosion rate is equated to the product of the... more
1] A mechanistic model is derived for the rate of fluvial erosion into bedrock by abrasion from uniform size particles that impact the bed during transport in both bed and suspended load. The erosion rate is equated to the product of the impact rate, the mass loss per particle impact, and a bed coverage term. Unlike previous models that consider only bed load, the impact rate is not assumed to tend to zero as the shear velocity approaches the threshold for suspension. Instead, a given sediment supply is distributed between the bed and suspended load by using formulas for the bed load layer height, bed load velocity, logarithmic fluid velocity profile, and Rouse sediment concentration profile. It is proposed that the impact rate scales linearly with the product of the near-bed sediment concentration and the impact velocity and that particles impact the bed because of gravitational settling and advection by turbulent eddies. Results suggest, unlike models that consider only bed load, that the erosion rate increases with increasing transport stage (for a given relative sediment supply), even for transport stages that exceed the onset of suspension. In addition, erosion can occur if the supply of sediment exceeds the bed load transport capacity because a portion of the sediment load is transported in suspension. These results have implications for predicting erosion rates and channel morphology, especially in rivers with fine sediment, steep channel-bed slopes, and large flood events.
Sediment management is frequently the most challenging concern in dam removal but there is as yet little guidance available to resource managers. For those rivers with beds composed primarily of non-cohesive sediments, we document recent... more
Sediment management is frequently the most challenging concern in dam removal but there is as yet little guidance available to resource managers. For those rivers with beds composed primarily of non-cohesive sediments, we document recent numerical and physical modelling of two processes critical to evaluating the effects of dam removal: the morphologic response to a sediment pulse, and the infiltration of fine sediment into coarser bed material. We demonstrate that (1) one-dimensional numerical modelling of sediment pulses can simulate reach-averaged transport and deposition over tens of kilometres, with sufficient certainty for managers to make informed decisions; (2) physical modelling of a coarse sediment pulse moving through an armoured pool-bar complex shows deposition in pool tails and along bar margins while maintaining channel complexity and pool depth similar to pre-pulse conditions; (3) physical modelling and theoretical analysis show that fine sediment will infiltrate into an immobile coarse channel bed to only a few median bed material particle diameters. We develop a generic approach to sediment management during dam removal using our experimental understanding to guide baseline data requirements, likely environmental constraints, and alternative removal strategies. In uncontaminated, noncohesive reservoir sediments we conclude that the management impacts of rapid sediment release may be of limited magnitude in many situations, and so the choice of dam removal strategy merits site-specific evaluation of the environmental impacts associated with a full range of alternatives.
1] In transient landscapes, adjustments in river channel width, roughness, and alluvial cover, in addition to slope, provide potentially important but poorly understood mechanisms by which bedrock channels accommodate changes in external... more
1] In transient landscapes, adjustments in river channel width, roughness, and alluvial cover, in addition to slope, provide potentially important but poorly understood mechanisms by which bedrock channels accommodate changes in external forcing. We used a laboratory flume to investigate experimentally how bedrock channel slope, width, roughness, alluvial cover, and incision rate collectively adjusted during the transient incision of an initially smooth channel with a varying bed load supply rate. When the channel was free of alluvial cover, incision was focused over a fraction of the bed width that varied strongly with both bed load supply and bed load transport capacity. Nondimensionalization yields a relationship for the width of active incision that explicitly incorporates bed load supply rate, sediment grain size, and bed shear stress, which suggests that in natural channels, width may respond dynamically to accommodate changes in bed load sediment supply. Because increases in sediment supply widened the band of active bed load sediment transport and thus the width over which incision took place, mass removal from the bed scaled with sediment supply when the bed was free of cover, consistent with incision being limited by the availability of erosive tools. However, bed roughness growth due to the spatial variation of incision during the experiment eventually inhibited bed load transport efficiency. This, in turn, led to deposition of alluvial cover and the suppression of incision on the bed at high sediment supply rates, consistent with incision being limited by the extent of alluvial cover deposited on the bed. The dynamics of roughness creation and alluvial cover deposition can therefore drive both negative and positive feedbacks on incision rate change following sediment supply perturbations. These experimental results offer several potentially field-testable hypotheses that together may help explain variability in the width, slope, and bed roughness of bedrock river channels in transient landscapes. Citation: Finnegan, N. J., L. S. Sklar, and T. K. Fuller (2007), Interplay of sediment supply, river incision, and channel morphology revealed by the transient evolution of an experimental bedrock channel,
1] Field data from channels in the Henry Mountains of Utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds (the cover effect). We compare several small channels that share... more
1] Field data from channels in the Henry Mountains of Utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds (the cover effect). We compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit (Navajo Sandstone) but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources. Bedrock channels that contain abundant clasts (diorite-rich) have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment (diorite-poor). The diorite-poor tributaries have incised more deeply to lower average slopes and have more reach-scale slope variability, which may reflect bedrock properties, longitudinal sediment sorting, and incision at lower sediment supply. Diorite-rich channels have less bedrock exposed and smoother longitudinal profiles than diorite-poor channels. We find that (1) coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision, validating the hypothesized cover effect in fluvial incision models; (2) the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock, suggesting that the slope of incising bedrock channels can become adjusted to the sediment load; (3) when abundant sediment is available, transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope; and (4) cover effects can be important even when moderate amounts of bedrock are exposed in channel beds.
- by Leonard Sklar
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Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft fl ybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent... more
Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft fl ybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fl uvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan's fl uvial networks imply a variety of mechanical controls, including structural infl uence, on channelized fl ows. At the reach scale, the various morphologies of individual fl uvial features, implying a broad range of fl uvial processes, suggest that (paleo-)fl ows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland fl ows, which are not visible in lowerresolution SAR data. This high-resolution snapshot suggests that some fl uvial features observed in SAR data may be river val-leys rather than channels, and that uplands elsewhere on Titan may also have fi ne-scale fl uvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fi ne-scale fl uvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fl uvial sediment. Flow suffi cient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fl uvial processes . With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to fl ow on Titan for fully turbulent fl ow and rough boundaries. For low-Reynolds-number fl ow over smooth boundaries, however, knowledge of fl uid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively infl uenced by fl uvial processes, for which appropriate terrestrial analogs and formulations may provide insight.
- by Leonard Sklar and +1
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