There are few tools available to predict the character and caliber of bed sediments in steep chan... more There are few tools available to predict the character and caliber of bed sediments in steep channels. Coarse bed sediments influence bedrock incision rates and the routing of sediment through drainage networks. Additionally, beds dominated by cobble- and boulder-sized particles provide over-wintering habitat to salmonids who use the interstitial space as refuge from high flows. Here we report results of an ongoing study of the controls on cobble-boulder occurrence in Pescadero Creek, a forested coastal watershed draining the north-western side of the Santa Cruz Mountains in central California. We aim to develop and test an empirical model to predict the occurrence and distribution of cobble-boulder channels. Median grain size has previously been predicted from channel slope and hydrologic geometry with the assumption that gravel beds are commonly at the threshold of motion at bankfull discharge. In steep channels, however, this approach may breakdown because bed conditions are controlled by hillslope sediment supply processes and the characteristics of local rock types. For example, debris flows both sweep away sediments and deposit coarse material along their runout path; weak lithologies may not produce boulders-sized particles, although rapid uplift rates may force sufficiently steep channel slopes for boulders to be dominant if present. Based on lithologic stratification of the study watershed, we walked sample reaches in different sub-watersheds to identify cobble-boulder channels. Using a combination of 10m USGS DEMs and 1.4m gridded ASLM data, we extracted channel slope, drainage area and estimated reach-scale bankfull threshold-of-motion grain size. Using SHALSTAB, a coupled steady-state rainfall-runoff and hillslope stability model, we developed a distance-weighted basin-instability metric for all observed cobble-boulder mantled reaches. We compared channel steepness index and hillslope curvature to erosion rate estimates to assess the influence of erosion rate on the distribution of cobble-boulder channels. In the laboratory, we measured the strength of rock samples from coarse clasts in cobble-boulder channels to test for the influence of rock strength on cobble-boulder occurrence. Within the Pescadero Creek basin, cobble-boulder channels occur at slopes between 0.06 to 0.20 and at drainage areas between 1 to 5 sq. km. Plotting channel slope versus drainage area yields a cobble- boulder ``sweet spot'' that is the basis of our empirical model. The characteristic slope and drainage area of cobble-boulder channels is consistent with where debris flows deposit. Basins where SHALSTAB predicts widespread instability are more commonly associated with cobble- boulder channels. Subwatersheds dominated by shales and mudstones with tensile strength less than 1.2 MPa tend to have bedrock, rather than cobble-boulder beds; these channels also tend to have lower channel slopes for a given drainage area. The utility of this empirical model, based on slope-area relationships, lithology, and distance-weighted upstream hillslope instability, will be tested in other channels in similar landscapes, and will help guide research into bedrock incision, sediment routing, and aid planners in assessing availability of salmonid over-wintering habitat.
Gravel augmentation is an increasingly common river restoration strategy downstream of dams where... more Gravel augmentation is an increasingly common river restoration strategy downstream of dams where selective transport and lack of gravel resupply have created armored, relatively immobile channel beds. A fundamental challenge in the design of gravel augmentation is predicting the temporal and spatial extent of beneficial bed response to coarse sediment additions, given the range of sediment transport conditions that occur in seasonal and storm-driven hydrographs. Here we report preliminary results of an ongoing series of laboratory flume experiments in which we simulate the addition of pulses of gravel to an armored bed downstream of a dam. These experiments build on the results of early phases of our experimental program, in which we have used constant discharge conditions to explore the translation and dispersion of sediment waves, and mobilization of armored beds by supply of finer sediments, in channels with both suppressed and forced bar pool morphology. Here we use the forced ...
Restoration practitioners frequently use gravel augmentation to restore channels impacted by seve... more Restoration practitioners frequently use gravel augmentation to restore channels impacted by severe sediment supply reductions. Yet, the fate of a gravel augmentation pulse in a channel subjected to repeated hydrographs remains unknown. Here we examine the effects of variable flow on the passage of a gravel augmentation pulse in a degrading, two-dimensional system using data collected in laboratory flume experiments. We subject a forced bar morphology channel to repeated hydrographs in order to analyze the evolution of the bed morphology following gravel additions. In order to model the conditions downstream of a dam, the pre-pulse experimental conditions consisted of two phases: 1) pre-dam - consisting of an equilibrium of both the time averaged sediment and water flux at constant flow conditions, and 2) post-dam - consisting of zero sediment supply and a constant flow rate. We examine the progression pattern of two gravel augmentation pulses, which we subjected to two different hy...
Background/Question/Methods Over the past decade, an increasing number of studies have shown that... more Background/Question/Methods Over the past decade, an increasing number of studies have shown that biological structures (e.g. roots) have large impacts on sediment erosion. But while it is now recognized that biological structures influence the transport of sediments, work to date has focused primarily on the impacts of individual, usually dominant, species. Here we ask whether competitive interactions cause multi-species communities to have fundamentally different impacts on sediment mobility than single-species systems. To address this question, we manipulated the presence or absence of two common species of net-spinning caddisfly larvae (Ceratopsyche oslari, Arctopsyche californica) in experimental streams and measured how their silk nets influence the critical shear stress required to initiate sediment motion. Results/Conclusions We show that critical shear stress increases non-additively when species are allowed to interact. Critical shear stress needed to initiate sediment mot...
We derived partial differential equations (PDEs) that describe the process of fine sediment infil... more We derived partial differential equations (PDEs) that describe the process of fine sediment infiltration into coarse sediment deposits based on mass conservation and our intuitive understanding of the physical processes involved. We reasoned that fine sediment trapping efficiency (FSTE), defined as the volumetric fraction of fine sediment trapped in the deposit per unit downward distance traveled, is either independent of fine sediment fraction (FSF) within the deposit or increases monotonically as the FSF increases. Solution to the PDEs under the assumption that FSTE is independent of FSF indicates that the equilibrium FSF decreases exponentially with depth into the deposit following fine sediment infiltration into a deposit initially void of fine sediment. Solutions to the PDEs under the assumption that FSTE is a function of FSF results in non-exponential decay function for fine sediment fraction that decreases faster into the depth compared to the exponential profile under the as...
River bed material grain size distributions and long-term average bedload flux rates depend on th... more River bed material grain size distributions and long-term average bedload flux rates depend on the supply rate and size of sediments delivered to channels by hillslopes, the spatial pattern of convergence of sediment transport pathways, and the size reduction of grains by abrasion during transport. For channel networks actively incising into bedrock, where no net deposition of sediment occurs, we
ABSTRACT River incision into bedrock drives the topographic evolution of mountainous terrain and ... more ABSTRACT 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.
Previous studies have shown that biological structures such as plant roots can have large impacts... more Previous studies have shown that biological structures such as plant roots can have large impacts on landscape morphodynamics, and that physical models that do not incorporate biology can generate qualitatively incorrect predictions of sediment transport. However, work to date has focused almost entirely on the impacts of single, usually dominant, species. Here we ask whether multiple, coexisting species of hydropsychid caddisfly larvae have different impacts on sediment mobility compared to single-species systems due to competitive interactions and niche differences. We manipulated the presence of two common species of net-spinning caddisfly (Ceratopsyche oslari, Arctopsyche californica) in laboratory mesocosms and measured how their silk filtration nets influence the critical shear stress required to initiate sediment grain motion when they were in monoculture versus polyculture. We found that critical shear stress increases non-additively in polycultures where species were allowe...
The notion of a threshold of dangerous climate change has been central to national and internatio... more The notion of a threshold of dangerous climate change has been central to national and international efforts to address climate risks. However, the focus on a single target has now become an obstacle because it reinforces three key problems: it frames climate change as a distant abstract threat, it impedes integration of mitigation and adaptation, and it fails to recognize the diversity of values and risk perceptions of people around the globe. We present an alternative framework that considers both biophysical science and social values in characterizing the broad spectrum of climate risks. The framework also presents the options for managing these risks within four quadrants defined by the inherent limits to mitigation and adaptation. This quadrant-based approach to managing the spectrum of climate risks restructures the climate change problem from avoiding a distant catastrophe to minimizing collective suffering.
Journal of Geophysical Research: Earth Surface, 2014
Plants and animals affect stream morphodynamics across a range of scales, yet including biologica... more Plants and animals affect stream morphodynamics across a range of scales, yet including biological traits of organisms in geomorphic process models remains a fundamental challenge. For example, laboratory experiments have shown that silk nets built by caddisfly larvae (Trichoptera: Hydropsychidae) can increase the shear stress required to initiate bed motion by more than a factor of 2. The contributions of specific biological traits are not well understood, however. Here we develop a theoretical model for the effects of insect nets on the threshold of sediment motion, τ * crit , that accounts for the mechanical properties, geometry, and vertical distribution of insect silk, as well as interactions between insect species. To parameterize the model, we measure the tensile strength, diameter, and number of silk threads in nets built by two common species of caddisfly, Arctopsyche californica and Ceratopsyche oslari. We compare model predictions with new measurements of τ* crit in experiments where we varied grain size and caddisfly species composition. The model is consistent with experimental results for single species, which show that the increase in τ * crit above the abiotic control peaks at 40-70% for 10-22 mm sediments and declines with increasing grain size. For the polyculture experiments, however, the model underpredicts the measured increase in τ * crit when two caddisfly species are present in sediments of larger grain sizes. Overall, the model helps explain why the presence of caddisfly silk can substantially increase the forces needed to initiate sediment motion in gravel-bedded streams and also illustrates the challenge of parameterizing the behavior of multiple interacting species in a physical model.
There are few tools available to predict the character and caliber of bed sediments in steep chan... more There are few tools available to predict the character and caliber of bed sediments in steep channels. Coarse bed sediments influence bedrock incision rates and the routing of sediment through drainage networks. Additionally, beds dominated by cobble- and boulder-sized particles provide over-wintering habitat to salmonids who use the interstitial space as refuge from high flows. Here we report results of an ongoing study of the controls on cobble-boulder occurrence in Pescadero Creek, a forested coastal watershed draining the north-western side of the Santa Cruz Mountains in central California. We aim to develop and test an empirical model to predict the occurrence and distribution of cobble-boulder channels. Median grain size has previously been predicted from channel slope and hydrologic geometry with the assumption that gravel beds are commonly at the threshold of motion at bankfull discharge. In steep channels, however, this approach may breakdown because bed conditions are controlled by hillslope sediment supply processes and the characteristics of local rock types. For example, debris flows both sweep away sediments and deposit coarse material along their runout path; weak lithologies may not produce boulders-sized particles, although rapid uplift rates may force sufficiently steep channel slopes for boulders to be dominant if present. Based on lithologic stratification of the study watershed, we walked sample reaches in different sub-watersheds to identify cobble-boulder channels. Using a combination of 10m USGS DEMs and 1.4m gridded ASLM data, we extracted channel slope, drainage area and estimated reach-scale bankfull threshold-of-motion grain size. Using SHALSTAB, a coupled steady-state rainfall-runoff and hillslope stability model, we developed a distance-weighted basin-instability metric for all observed cobble-boulder mantled reaches. We compared channel steepness index and hillslope curvature to erosion rate estimates to assess the influence of erosion rate on the distribution of cobble-boulder channels. In the laboratory, we measured the strength of rock samples from coarse clasts in cobble-boulder channels to test for the influence of rock strength on cobble-boulder occurrence. Within the Pescadero Creek basin, cobble-boulder channels occur at slopes between 0.06 to 0.20 and at drainage areas between 1 to 5 sq. km. Plotting channel slope versus drainage area yields a cobble- boulder ``sweet spot'' that is the basis of our empirical model. The characteristic slope and drainage area of cobble-boulder channels is consistent with where debris flows deposit. Basins where SHALSTAB predicts widespread instability are more commonly associated with cobble- boulder channels. Subwatersheds dominated by shales and mudstones with tensile strength less than 1.2 MPa tend to have bedrock, rather than cobble-boulder beds; these channels also tend to have lower channel slopes for a given drainage area. The utility of this empirical model, based on slope-area relationships, lithology, and distance-weighted upstream hillslope instability, will be tested in other channels in similar landscapes, and will help guide research into bedrock incision, sediment routing, and aid planners in assessing availability of salmonid over-wintering habitat.
Gravel augmentation is an increasingly common river restoration strategy downstream of dams where... more Gravel augmentation is an increasingly common river restoration strategy downstream of dams where selective transport and lack of gravel resupply have created armored, relatively immobile channel beds. A fundamental challenge in the design of gravel augmentation is predicting the temporal and spatial extent of beneficial bed response to coarse sediment additions, given the range of sediment transport conditions that occur in seasonal and storm-driven hydrographs. Here we report preliminary results of an ongoing series of laboratory flume experiments in which we simulate the addition of pulses of gravel to an armored bed downstream of a dam. These experiments build on the results of early phases of our experimental program, in which we have used constant discharge conditions to explore the translation and dispersion of sediment waves, and mobilization of armored beds by supply of finer sediments, in channels with both suppressed and forced bar pool morphology. Here we use the forced ...
Restoration practitioners frequently use gravel augmentation to restore channels impacted by seve... more Restoration practitioners frequently use gravel augmentation to restore channels impacted by severe sediment supply reductions. Yet, the fate of a gravel augmentation pulse in a channel subjected to repeated hydrographs remains unknown. Here we examine the effects of variable flow on the passage of a gravel augmentation pulse in a degrading, two-dimensional system using data collected in laboratory flume experiments. We subject a forced bar morphology channel to repeated hydrographs in order to analyze the evolution of the bed morphology following gravel additions. In order to model the conditions downstream of a dam, the pre-pulse experimental conditions consisted of two phases: 1) pre-dam - consisting of an equilibrium of both the time averaged sediment and water flux at constant flow conditions, and 2) post-dam - consisting of zero sediment supply and a constant flow rate. We examine the progression pattern of two gravel augmentation pulses, which we subjected to two different hy...
Background/Question/Methods Over the past decade, an increasing number of studies have shown that... more Background/Question/Methods Over the past decade, an increasing number of studies have shown that biological structures (e.g. roots) have large impacts on sediment erosion. But while it is now recognized that biological structures influence the transport of sediments, work to date has focused primarily on the impacts of individual, usually dominant, species. Here we ask whether competitive interactions cause multi-species communities to have fundamentally different impacts on sediment mobility than single-species systems. To address this question, we manipulated the presence or absence of two common species of net-spinning caddisfly larvae (Ceratopsyche oslari, Arctopsyche californica) in experimental streams and measured how their silk nets influence the critical shear stress required to initiate sediment motion. Results/Conclusions We show that critical shear stress increases non-additively when species are allowed to interact. Critical shear stress needed to initiate sediment mot...
We derived partial differential equations (PDEs) that describe the process of fine sediment infil... more We derived partial differential equations (PDEs) that describe the process of fine sediment infiltration into coarse sediment deposits based on mass conservation and our intuitive understanding of the physical processes involved. We reasoned that fine sediment trapping efficiency (FSTE), defined as the volumetric fraction of fine sediment trapped in the deposit per unit downward distance traveled, is either independent of fine sediment fraction (FSF) within the deposit or increases monotonically as the FSF increases. Solution to the PDEs under the assumption that FSTE is independent of FSF indicates that the equilibrium FSF decreases exponentially with depth into the deposit following fine sediment infiltration into a deposit initially void of fine sediment. Solutions to the PDEs under the assumption that FSTE is a function of FSF results in non-exponential decay function for fine sediment fraction that decreases faster into the depth compared to the exponential profile under the as...
River bed material grain size distributions and long-term average bedload flux rates depend on th... more River bed material grain size distributions and long-term average bedload flux rates depend on the supply rate and size of sediments delivered to channels by hillslopes, the spatial pattern of convergence of sediment transport pathways, and the size reduction of grains by abrasion during transport. For channel networks actively incising into bedrock, where no net deposition of sediment occurs, we
ABSTRACT River incision into bedrock drives the topographic evolution of mountainous terrain and ... more ABSTRACT 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.
Previous studies have shown that biological structures such as plant roots can have large impacts... more Previous studies have shown that biological structures such as plant roots can have large impacts on landscape morphodynamics, and that physical models that do not incorporate biology can generate qualitatively incorrect predictions of sediment transport. However, work to date has focused almost entirely on the impacts of single, usually dominant, species. Here we ask whether multiple, coexisting species of hydropsychid caddisfly larvae have different impacts on sediment mobility compared to single-species systems due to competitive interactions and niche differences. We manipulated the presence of two common species of net-spinning caddisfly (Ceratopsyche oslari, Arctopsyche californica) in laboratory mesocosms and measured how their silk filtration nets influence the critical shear stress required to initiate sediment grain motion when they were in monoculture versus polyculture. We found that critical shear stress increases non-additively in polycultures where species were allowe...
The notion of a threshold of dangerous climate change has been central to national and internatio... more The notion of a threshold of dangerous climate change has been central to national and international efforts to address climate risks. However, the focus on a single target has now become an obstacle because it reinforces three key problems: it frames climate change as a distant abstract threat, it impedes integration of mitigation and adaptation, and it fails to recognize the diversity of values and risk perceptions of people around the globe. We present an alternative framework that considers both biophysical science and social values in characterizing the broad spectrum of climate risks. The framework also presents the options for managing these risks within four quadrants defined by the inherent limits to mitigation and adaptation. This quadrant-based approach to managing the spectrum of climate risks restructures the climate change problem from avoiding a distant catastrophe to minimizing collective suffering.
Journal of Geophysical Research: Earth Surface, 2014
Plants and animals affect stream morphodynamics across a range of scales, yet including biologica... more Plants and animals affect stream morphodynamics across a range of scales, yet including biological traits of organisms in geomorphic process models remains a fundamental challenge. For example, laboratory experiments have shown that silk nets built by caddisfly larvae (Trichoptera: Hydropsychidae) can increase the shear stress required to initiate bed motion by more than a factor of 2. The contributions of specific biological traits are not well understood, however. Here we develop a theoretical model for the effects of insect nets on the threshold of sediment motion, τ * crit , that accounts for the mechanical properties, geometry, and vertical distribution of insect silk, as well as interactions between insect species. To parameterize the model, we measure the tensile strength, diameter, and number of silk threads in nets built by two common species of caddisfly, Arctopsyche californica and Ceratopsyche oslari. We compare model predictions with new measurements of τ* crit in experiments where we varied grain size and caddisfly species composition. The model is consistent with experimental results for single species, which show that the increase in τ * crit above the abiotic control peaks at 40-70% for 10-22 mm sediments and declines with increasing grain size. For the polyculture experiments, however, the model underpredicts the measured increase in τ * crit when two caddisfly species are present in sediments of larger grain sizes. Overall, the model helps explain why the presence of caddisfly silk can substantially increase the forces needed to initiate sediment motion in gravel-bedded streams and also illustrates the challenge of parameterizing the behavior of multiple interacting species in a physical model.
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Papers by Leonard Sklar