Variations In Erosion Research Articles

Multiscale Estimates of Soil Erodibility Variation under Conditions of High Soil Cover Heterogeneity in the Northern Forest-Steppe of the Central Russian Upland

Multiscale Estimates of Soil Erodibility Variation under Conditions of High Soil Cover Heterogeneity in the Northern Forest-Steppe of the Central Russian Upland

Controls on topography and erosion of the north-central Andes

Abstract We present 17 new 10Be erosion rates from southern Peru sampled across an extreme orographic rainfall gradient. Using a rainfall-weighted variant of the normalized channel steepness index, ksnQ, we show that channel steepness values, and thus topography, are adjusted to spatially varying rainfall. Rocks with similar physical characteristics define distinct relationships between ksnQ and erosion rate (E), suggesting ksnQ is also resolving lithologic variations in erodibility. However, substantial uncertainty exists in parameters describing these relationships. By combining our new data with 38 published erosion rates from Peru and Bolivia, we collapse the range of compatible parameter values and resolve robust, nonlinear ksnQ–E relationships suggestive of important influences of erosional thresholds, rock properties, sediment characteristics, and temporal runoff variability. In contrast, neither climatic nor lithologic effects are clear using the traditional channel steepness metric, ksn. Our results highlight that accounting for spatial rainfall variations is essential for disentangling the multiple influences of climate, lithology, and tectonics common in mountain landscapes, which is a necessary first step toward greater understanding of how these landscapes evolve.

The influence of subsurface architecture on scour hole formation in the Rhine–Meuse delta, the Netherlands

Abstract Scour holes are common features in deltaic rivers which can destabilise embankments through oversteepening of the river bed. Their development has been studied extensively from the hydraulic perspective, but another important control is the erodibility of the river bed which varies considerably due to thickening of heterogeneous deltaic substrate towards the coast. Therefore, we assessed the influence of delta-scale geological heterogeneity and local subsurface architecture on scour hole formation in addition to the hydrodynamic controls. We (1) created an inventory of 165 scour hole locations in the Rhine–Meuse delta, (2) assessed the hydrodynamic conditions at the locations, (3) extracted geometric characteristics and (4) determined the subsurface architecture from geological data. Central and lower delta branches have 0.6–0.7 scours per km while upper delta branches have less than 0.2. Downstream, 58% of scour holes were related to architectural elements, notably sand bodies from former Holocene channel belts and Early Holocene cohesive beds. These scours have steeper slopes due to higher proportions of cohesive sediments near the river bed. Furthermore, scours related to channel belt sand bodies are limited in downstream length and depth, up to maximum of approximately two times the water depth. From our results, we provide a delta-scale explanatory framework that relates the position of present-day river channels with respect to Pleistocene river deposits and Holocene fluvio-deltaic deposits to scour hole formation. Upstream rivers are incised in Pleistocene deposits showing less local variation in erodibility. The majority of scour holes here relate to engineering works. In central and lower delta branches, geologically inherited heterogeneity of the Holocene substrate at critical depths near the channel bottom adds to anthropogenic induced scours and results in high abundances. This demonstrates that downstream variation in subsurface architecture should be considered as a key control on scour locations and characteristics for management purposes.

Quantitatively identify the factors driving loess erodibility variations after ecological restoration

AbstractThe soil erodibility factor is the key index to evaluate regional potential erosion degree and predict soil erosion. Therefore, it is very meaningful to explore the response mechanism of soil erodibility to external environmental factors on the Loess Plateau after ecological restoration. In this study, three soil erodibility factors (K‐USLE, K‐shira, and K‐torri), a comprehensive soil erodibility index (CSEI), different vegetation indices, and five topographic parameters (Topo) in two typical watersheds were selected to achieve this goal. The results revealed that vegetation indices (VI), soil properties (SP), and Topo were significantly affected by regional location and land use types. Vegetation restoration can reduce soil erodibility factors, but the reduction capacity of different land use types varies greatly. SP, VI, and Topo can explain more than 70% of variances in soil erodibility. The partial least squares‐structural equation modelling (PLS‐SEM) illustrated that SP had the largest and significant direct impact on soil erodibility (standard path coefficients (SPC) were −0.935 and −0.895, respectively). Vegetation indirectly inhibited the soil erodibility of the two watersheds by improving SP (SPC were 0.228 and 0.437, respectively), while the indirect impact of Topo mainly interferes with soil erodibility by adjusting vegetation type and spatial distribution (SPC were −0.566 and −0.292, respectively). Furthermore, CSEI was significantly affected by land use, and significantly associated with soil texture, soil organic carbon (SOC), mean weight diameter (MWD), normalized green‐red difference index (NGRDI), and vegetative index (VEG) in both watersheds. The soil in the Xinshui River watershed was more vulnerable to erosion than that in the Zhujiachuan watershed. It is of great significance to explore the response mechanism of soil erodibility to multi‐scale factors to understand the impact of external environmental evolution on soil erosion and sediment yield on the Loess Plateau.

Plant community near-surface characteristics as drivers of soil erodibility variation along a slope gradient in a typical semiarid region of China

Near-surface characteristics of plant communities (i.e., microbiotic soil crusts (MSCs), plant litter and root, and soil properties) in semiarid regions are strongly controlled by the slope gradient due to limited availability of water, which conversely affects the variation of soil erodibility. However, few studies have thoroughly quantified the variation in soil erodibility with slope gradients from different aspects using many soil erodibility indicators in semiarid region. Therefore, this study evaluated the impacts of slope gradients on soil erodibility characterized by soil cohesion (CH), saturated conductivity (Ks), mean number of water-drop impacts (MDI), mean weight diameter (MWD), penetration resistance (PR), erodibility K factor (K) in the Universal Soil Loss Equation, and an integrated soil erodibility index (ISEI), which considers the six indicators mentioned earlier on the Loess Plateau. Nine vegetation-restored slopes belonging to three slope classes (i.e., gentle, medium, and steep slope) were used as test sites. The results revealed that CH, PR, K, and ISEI increased as a sigmold curve with the slope gradient, while the variations in Ks, MDI, and MWD were opposite to those of CH, PR, and K. Additionally, significant differences were found between the three slope classes for almost all indicators. Compared to that of cropland, the ISEIs of vegetation-restored slopes with slopes ranging from gentle to medium and steep slope classes decreased by 80.7%, 54.2%, and 36.1%, respectively. Significant differences in MSCs thickness, plant root (RMD) and litter (LMD) densities, soil bulk density (BD), clay and silt contents, and organic matter content (SOM) were found between the different slope classes. The variations in these indices were induced by the variations in MSCs thickness, RMD, LMD, BD, soil texture, and SOM with the slope gradient. The results are helpful for soil erosion control on medium and steep slopes in semiarid regions.

New Erodibility Parameterization for Applying WEPP on Rangelands Using ERMiT

HighlightsA practical parameterization approach to estimate erodibility was developed for WEPP applications on rangelands.Soil texture had a greater effect on rill erodibility than vegetation cover.Vegetation cover had a greater effect on interrill erodibility than soil texture.A new set of erodibility input parameters was defined for ERMiT applications on rangelands.Abstract. The USDA Water Erosion Prediction Project (WEPP) is a process-based soil erosion prediction model. WEPP uses three soil erodibility parameters: rill erodibility (Kr), interrill erodibility (Ki), and critical hydraulic shear stress (tc). In this study, a new parameterization approach for estimating erodibility was developed for WEPP applications on rangelands. Data from overland flow experiments on disturbed and undisturbed rangelands were used to develop empirical equations to predict rill erodibility variation as a function of vegetation cover and soil texture. Data from rainfall simulation experiments were analyzed by piecewise regression to develop empirical equations for predicting the variability of interrill erodibility before and after disturbance and across a wide range of soil textures as a function of vegetation cover and soil texture. Critical shear values corresponding to the developed rill and interrill erodibility parameters were proposed. Our results show that the new erodibility approach predicts erosion at the plot scale with a satisfactory range of error (PBIAS =35.6 and Nash-Sutcliffe efficiency = 0.49). The new approach was used to provide soil erodibility values for the Erosion Risk Management Tool (ERMiT), which uses WEPP as the runoff and erosion calculation engine. Keywords: ERMiT, Erosion, Interrill, Rangeland management, Rill, Shrub, Soil burn severity, WEPP.

Comparison of Methods for Determining Erosion Threshold of Cohesive Sediments Using a Microcosm System

Erosion of cohesive sediments is a ubiquitous phenomenon in estuarine and intertidal environments. Several methods have been proposed to determine the surface erosion threshold (τc0), which are still debatable because of the numerous and uncertain definitions. Based on erosion microcosm experiments, we have compared three different methods using (1) eroded mass (EM), (2) erosion rate (ER), and (3) suspended sediment concentration (SSC), and suggested a suitable method for revealing the variation of erodibility in intertidal sediments. Erosion experiments using a microcosm system were carried out in the Muuido tidal flat, west coast of South Korea. The mean values of τc0 for three methods were: 0.20 ± 0.08 Pa (EM); 0.18 ± 0.07 Pa (ER); and (3) 0.17 ± 0.09 Pa (SSC). The SSC method yielded the lowest τc0, due to the outflow of suspended sediment from the erosion chamber of the microcosm. This was because SSC gradually decreased with time after depleting the erodible sediment at a given bed shear stress (τb). Therefore, the regression between SSC and applied τb might skew an x-intercept, resulting in the underestimation (or “not-determined”) of τc0. The EM method yielded robust and accurate (within the range of τb step at which erosion begins) results. The EM method represents how the erodible depth thickens as τb increases and therefore seems better suited than the SSC and ER methods for representing depth-limited erosion of cohesive sediments. Furthermore, this study identified the spatiotemporal variations of τc0 by EM method in an intertidal flat. The τc0 in mud flat was about two times higher than that in mixed flat. Compared to the end of tidal emersion, the sediment was 10–40% more erodible at the beginning stage.

Growing topography due to contrasting rock types in a tectonically dead landscape

Abstract. Many mountain ranges survive in a phase of erosional decay for millions of years following the cessation of tectonic activity. Landscape dynamics in these post-orogenic settings have long puzzled geologists due to the expectation that topographic relief should decline with time. Our understanding of how denudation rates, crustal dynamics, bedrock erodibility, climate, and mantle-driven processes interact to dictate the persistence of relief in the absence of ongoing tectonics is incomplete. Here we explore how lateral variations in rock type, ranging from resistant quartzites to less resistant schists and phyllites, and up to the least resistant gneisses and granitic rocks, have affected rates and patterns of denudation and topographic forms in a humid subtropical, high-relief post-orogenic landscape in Brazil where active tectonics ended hundreds of millions of years ago. We show that catchment-averaged denudation rates are negatively correlated with mean values of topographic relief, channel steepness and modern precipitation rates. Denudation instead correlates with inferred bedrock strength, with resistant rocks denuding more slowly relative to more erodible rock units, and the efficiency of fluvial erosion varies primarily due to these bedrock differences. Variations in erodibility continue to drive contrasts in rates of denudation in a tectonically inactive landscape evolving for hundreds of millions of years, suggesting that equilibrium is not a natural attractor state and that relief continues to grow through time. Over the long timescales of post-orogenic development, exposure at the surface of rock types with differential erodibility can become a dominant control on landscape dynamics by producing spatial variations in geomorphic processes and rates, promoting the survival of relief and determining spatial differences in erosional response timescales long after cessation of mountain building.

Geological settings and controls of fluid migration and associated seafloor seepage features in the north Irish Sea

Shallow gas accumulation in unconsolidated Quaternary sediments, and associated seepage at the seafloor, is widespread in the north Irish Sea. This study integrates high-resolution seafloor bathymetry and sub-surface geophysical data to investigate shallow gas accumulations and possible fluid (gas and/or liquids) migration pathways to the seafloor in the northern part of the Irish Sea. Shallow gas occurs broadly in two geological settings: the Codling Fault Zone and the Western Irish Sea Mud Belt. The gas has been recognised to accumulate in both sandy and muddy Quaternary marine near-surface sediments and is characterised by three characteristic sub-bottom acoustic features: i) enhanced reflections, ii) acoustic turbid zones, and iii) acoustic blanking. The seepage of shallow gas at the seafloor has resulted in the formation of morphological features including methane-derived authigenic carbonates, seabed mounds and pockmarks. In many instances, the evidence for this gas as biogenic or thermogenic in origin is inconclusive. Two distinct types of pockmarks are recorded in the Western Irish Mud Belt: pockmarks with a relatively flat centre, and pockmarks with a central mound. Based on our observation and existing models, we infer that the formation of a carbonate crust at the seabed surface is needed as a precursor for the creation of such mounds within pockmarks. The formation processes are interpreted to be different for sandy versus muddy sediments, due to variability in erodibility and sealing capacities of the substrate. We suggest that the origin of these features is linked to the presence of deeper hydrocarbon source rocks with existing and reactivated faults forming fluid migration pathways to the surface. This in turn could indicate a mixed thermogenic-biogenic origin for seep-related structures in the study area. These features have significant implications for the future development of offshore infrastructure including marine renewable energy as well as for seabed ecology and conservation efforts in the Irish Sea.

Rock erodibility and the interpretation of low-temperature thermochronologic data

Rocks vary significantly in strength and erodibility. Here we evaluate if rock erodibility variations should be considered when interpreting thermochronologic datasets. We do this by applying 1D thermo–kinematic numerical models that exhume two lithologies of contrasting erodibility. For thick layers (>2 km), soft over hard layering causes earlier cooling and therefore older thermochronologic dates than no layering, with the opposite true for hard over soft layering. In some circumstances, even 2–10x erodibility contrasts substantially influence the results, and a 10x erodibility contrast can be nearly as important as contrasts several orders of magnitude greater. Thinner alternating layers (<0.5 km) dramatically reduce the effect. The results imply that rock erodibility variations should not substantially influence thermochronologic data from most continental sedimentary packages, which are dominated by lithologic layering <0.5 km-thick. However, the effect may be important for data from basement samples exhumed beneath softer sedimentary rocks. For example, the abrupt cooling and erosion rate decrease recorded by thermochronologic data from Rocky Mountain basement uplifts of the western U.S. coincides with when erosion-resistant Precambrian basement was exposed after removal of softer sedimentary cover. These data may largely record a change in exposed rock type erodibility rather than a dramatic change in external erosional forcing. Our results suggest that in some cases, variations in rock erodibility should be considered when interpreting cooling and erosion histories from thermochronologic datasets.

An alternative method for interpreting jet erosion test (JET) data: part 1. Theory

AbstractThe jet erosion test (JET) is a widely applied method for deriving the erodibility of cohesive soils and sediments. There are suggestions in the literature that further examination of the method widely used to interpret the results of these erosion tests is warranted. This paper presents an alternative approach for such interpretation based on the principle of energy conservation. This new approach recognizes that evaluation of erodibility using the jet tester should involve the mass of soil eroded, so determination of this eroded mass (or else scour volume and bulk density) is required. The theory partitions jet kinetic energy flux into that involved in eroding soil, the remainder being dissipated in a variety of mechanisms. The energy required to erode soil is defined as the product of the eroded mass and a resistance parameter which is the energy required to entrain unit mass of soil, denoted J (in J/kg), whose magnitude is sought. An effective component rate of jet energy consumption is defined which depends on depth of scour penetration by the jet, but not on soil type, or the uniformity of the soil type being investigated. Application of the theory depends on experimentally determining the spatial form of jet energy consumption displayed in erosion of a uniform body of soil, an approach of general application. The theory then allows determination of the soil resistance parameter J as a function of depth of scour penetration into any soil profile, thus evaluating such profile variation in erodibility as may exist. This parameter J has been used with the same meaning in soil and gully erosion studies for the last 25 years. Application of this approach will appear in a companion publication as part 2. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Complexities of landscape evolution during incision through layered stratigraphy with contrasts in rock strength

AbstractVariation in the erodibility of rock units has long been recognized as an important determinant of landscape evolution but has been little studied in landscape evolution models. We use a modified version of the Channel‐Hillslope Integrated Landscape Development (CHILD) model, which explicitly allows for variations in rock strength, to reveal and explore the remarkably rich, complex behavior induced by rock erodibility variations in even very simple geologic settings with invariant climate and tectonics. We study the importance of relative contrasts in erodibility between just two units, the order of these units (whether hard rocks overlie soft or soft rocks overlie hard) and the orientation of the contact between the two units. We emphasize the spatial and temporal evolution of erosion rates, which have important implications for basin analysis, detrital mineral records, and the interpretation of cosmogenic isotope concentrations in detrital samples. Results of the landscape evolution modeling indicate that the stratigraphic order of units in terms of erodibility, the gross orientation of the contact (i.e. dipping away or toward the outlet of the landscape) and the contact dip angle all have measurable effects on landscape evolution, including significant spatial and temporal variations in erosion rates. Steady‐state denudation conditions are unlikely to develop in landscapes with significant contrasts in rock strength in horizontal to moderately tilted rock layers, at least at the scale of the entire landscape. Additionally, our results demonstrate that there is no general relation between rock erodibility and erosion rates in natural settings. Although rock erodibility directly controls the erosion rate constant in our models, it is not uncommon for higher erosion rates to occur in the harder, less erodible rock. Indeed erosion rates may be either greater or less than the rock uplift rate (invariant in time and space in our models) in both hard and soft rocks, depending on the local geology, topography, and the pattern of landscape evolution. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Numerical modelling of mixed-sediment consolidation

Sediment transport modelling in estuarine environments, characterised by cohesive and non-cohesive sediment mixtures, has to consider a time variation of erodibility due to consolidation. Generally, validated by settling column experiments, mud consolidation is now fairly well simulated; however, numerical models still have difficulty to simulate accurately the sedimentation and consolidation of mixed sediments for a wide range of initial conditions. This is partly due to the difficulty to formulate the contribution of sand in the hindered settling regime when segregation does not clearly occur. Based on extensive settling experiments with mud-sand mixtures, the objective of this study was to improve the numerical modelling of mixed-sediment consolidation by focusing on segregation processes. We used constitutive relationships following the fractal theory associated with a new segregation formulation based on the relative mud concentration. Using specific sets of parameters calibrated for each test—with different initial sediment concentration and sand content—the model achieved excellent prediction skills for simulating sediment height evolutions and concentration vertical profiles. It highlighted the model capacity to simulate properly the segregation occurrence for mud-sand mixtures characterised by a wide range of initial conditions. Nevertheless, calibration parameters varied significantly, as the fractal number ranged from 2.64 to 2.77. This study investigated the relevance of using a common set of parameters, which is generally required for 3D sediment transport modelling. Simulations were less accurate but remained satisfactory in an operational approach. Finally, a specific formulation for natural estuarine environments was proposed, simulating correctly the sedimentation-consolidation processes of mud-sand mixtures through 3D sediment transport modelling.

Sediment texture, erodibility, and composition in the Northern Gulf of Mexico and their potential impacts on hypoxia formation

A region of hypoxic waters has formed annually over the past several decades in the northern Gulf of Mexico. This has motivated the studies of mechanisms controlling the development of hypoxia. Both field and laboratory approaches were used to examine sediment texture, erodibility, and composition. Sediment texture analyses show that grain size relates to the proximity to the Mississippi and Atchafalaya river deltas and to the remnants of shifts in the Mississippi and Atchafalaya deltaic lobes. Temporal variability in erodibility relates to seasonal weather patterns, with more energetic wave conditions in winter and spring setting up an active bottom layer that increases erodibility, compared to quiescent summers that allow for seabed consolidation. The amount of eroded material is fairly low until shear stress levels in the bottom boundary layer exceed 0.4 Pa. An organically enriched fluff layer was found at the sediment–water interface, which is highly erodible under low shear stress levels. Eroded volatile suspended solids (a proxy for organic material) vs. increasing levels of shear stress revealed a distinct pattern at all sample areas; higher concentrations of organic material were eroded at the lowest (0.01 Pa) and highest (0.6 Pa) applied shear stresses, and there was a higher ratio of the volatile to total suspended solids at 0.01 Pa. Based on erodibility experiments and modeling data analysis, the low shear stress levels during the quiescent periods in summer were sometimes high enough to resuspend this fluff layer, but not underlying sediment, thereby potentially facilitating the development of bottom water hypoxia.

Seabed erodibility variations on the Louisiana continental shelf before and after the 2011 Mississippi River flood

Erodibility is critical to the sediment resuspension process but has not been measured systematically in large river-dominated muddy continental shelves before. During early summer of 2011, the Mississippi River experienced a major flood event. This flood provided a unique opportunity to examine how shelf seabed erodibility responded to a large river flood, and the ultimate fate of flood deposition is important to geological and biogeochemical processes (e.g., stratal formation, carbon sequestration).A total of 106 sediment cores were collected on the Louisiana shelf during five cruises in 2010 and 2011, and a new dataset was used to evaluate the response of the seabed to the recent conditions. The localized flood deposit was mainly within tens of kilometers of river sources, and little sediment accumulated on the middle Louisiana shelf. Seabed erodibility was measured using a dual-core Gust Erosion Microcosm System. The erodibility of sediment collected in April 2011 exceeded that for August 2010 and August 2011. The springtime increase in erodibility seemed to be related to the recent presence of energetic waves that mobilized the seabed. Erodibility was highest on the inner shelf southwest of Atchafalaya Bay, intermediate on the middle shelf, lowest in the Mississippi Canyon, and highly variable on the Mississippi subaqueous delta. These spatial patterns were influenced by proximity to river sources, flood-deposit thicknesses, intensity of wave-driven bed stresses, and bioturbation. The flood-deposit thickness itself, however, was not sufficient to explain all the spatial variations of erodibility after the peak of the Mississippi flood. Comparing values to published data, the depth-varying erodibility on the Louisiana shelf was close to the “low erodibility” level for the York River of Virginia, and similar to the data collected from Baltimore Harbor in Maryland and the main stem of upper Chesapeake Bay. Our findings promote understanding of the resuspension of fluffy organic-rich layer at the water–sediment interface, which influences sediment oxygen demand on the Louisiana shelf. This dataset is also valuable to observational and modeling studies of large river sediment dispersal systems worldwide.

An assessment of the erodibility of Holocene lithounits comprising streambanks in northeastern Kansas, USA

Streambanks are the primary source of sediment for watersheds in the Midwestern USA. In much of this region, deposits of fine-grained Holocene alluvium comprising streambanks have been assigned to a single lithostratigraphic unit, the DeForest Formation. This study examines the stratigraphic relationships and measures the erodibility of the different members of the DeForest Formation in three watersheds in northeastern Kansas.Distinct differences in erodibility, measured in terms of critical shear stress (τc) by a submerged jet-test device, were observed between the different members of the DeForest Formation. The most erodible member is the Camp Creek Member (average τc=1.0Pa) while the most resistant is the Gunder Member (average τc=10.4Pa). Variability in erodibility between and within the members of the DeForest Formation is attributed to the magnitude of post-depositional soil-forming processes, including the presence of buried soils, as well as the inherent natural variability in the different parent materials. A weak positive correlation was found between percent clay and τc. Resistance to erosion by fluid flow was found to be significantly greater where clay contents exceed 28%.Although the Camp Creek Member was found to be the most erodible, it always occurs, stratigraphically, as the uppermost member. Available bankfull stage indicators suggest that bankfull discharges rarely attain elevations sufficient to erode Camp Creek Member deposits. Therefore, other members of the DeForest Formation are able to exert some control on the rate of bank erosion by hydraulic flow. Furthermore, given the observed differences in lithology, soil development and erodibility, the susceptibility to mass wasting processes is also likely to vary between the different members. Therefore, lithostratigraphic and soil-stratigraphic relationships have important implications for streambank erodibility and are crucial for accurately determining areas prone to streambank erosion in alluvial settings.

Sediment availability on burned hillslopes

[1] Erodibility describes the inherent resistance of soil to erosion. Hillslope erosion models typically consider erodibility to be constant with depth. This may not be the case after wildfire because erodibility is partly determined by the availability of noncohesive soil and ash at the surface. This study quantifies erodibility of burned soils using methods that explicitly capture variations in soil properties with depth. Flume experiments on intact cores from three sites in western United States showed that erodibility of fire-affected soil was highest at the soil surface and declined exponentially within the top 20 mm of the soil profile, with root density and soil depth accounting for 62% of the variation. Variation in erodibility with depth resulted in transient sediment flux during erosion experiments on bounded field plots. Material that contributed to transient flux was conceptualized as a layer of noncohesive material of variable depth (dnc). This depth was related to shear strength measurements and sampled spatially to obtain the probability distribution of noncohesive material as a function of depth below the surface. After wildfire in southeast Australia, the initial dnc ranged from 7.5 to 9.1 mm, which equated to 97–117 Mg ha−1 of noncohesive material. The depth decreased exponentially with time since wildfire to 0.4 mm (or < 5 Mg ha−1) after 3 years of recovery. The results are organized into a framework for modeling fire effects on erodibility as a function of the production and depletion of the noncohesive layer overlying a cohesive layer.

Seasonal variations in erodibility and sediment transport potential in a mesotidal channel-flat complex, Willapa Bay, WA

Measurements of erodibility, porosity and sediment size were made three times over the course of a year at sites within a muddy, mesotidal flat-channel complex in southern Willapa Bay, WA, to examine spatial and seasonal variations in sediment properties and transport potential. Average critical shear stress profiles, the metric we used for erodibility, were quantified using a power-law fit to cumulative eroded mass vs. shear stress for the flats and channel. Laboratory erosion measurements of deposits made from slurries of flat and channel sediment were used to quantify erodibility over consolidation time scales ranging from 6 to 96h. Erodibility of the tidal flats was consistently low, with spatial variability comparable to seasonal variability despite seasonal changes in biological activity. In contrast, channel-bed erodibility underwent large seasonal variations, with mobile sediment present in the channel thalweg during winter that was absent in the spring and summer, when channel-bed erodibility was low and comparable to that of the tidal flats. Sediment on the northern (left) channel flank was mobile in summer and winter, whereas sediment on the southern flank was not. Seasonal changes in channel-bed erodibility are sufficient to produce order-of-magnitude changes in suspended sediment concentrations during peak tidal flows. Porosity just below the sediment surface was the best predictor of erodibility in our study area.

Concentrated flow erodibility for physically based erosion models: Temporal variability in disturbed and undisturbed rangelands

Current physically based overland flow erosion models for rangeland application do not separate disturbed and undisturbed conditions in modeling concentrated flow erosion. In this study, concentrated flow simulations on disturbed and undisturbed rangelands were used to estimate the erodibility and to evaluate the performance of linear and power law equations that describe the relationship between erosion rate and several hydraulic parameters. None of the hydraulic parameters consistently predicted the detachment capacity well for all sites, however, stream power performed better than most of other hydraulic parameters. Using power law functions did not improve the detachment relation with respect to that of the linear function. Concentrated flow erodibility increased significantly when a site was exposed to a disturbance such as fire or tree encroachment into sagebrush steppe. This study showed that burning increases erosion by amplifying the erosive power of overland flow through removing obstacles and by changing the soil properties affecting erodibility itself. However, the magnitude of fire impact varied among sites due to inherent differences in site characteristics and variability in burn severity. In most cases we observed concentrated flow erodibility had a high value at overland flow initiation and then started to decline with time due to reduction of sediment availability. Thus we developed an empirical function to predict erodibility variation within a runoff event as a function of cumulative unit discharge. Empirical equations were also developed to predict erodibility variation with time postdisturbance as a function of readily available vegetation cover and surface soil texture data.

Modeling the effects of weathering on bedrock-floored channel geometry

[1] Field and modeling studies suggest that bedrock channels equilibrate to base‐level change through geometry and slope adjustment to imposed discharge, sediment supply, and substrate erodibility conditions. In this study we model the influence of bedrock weathering on channel geometry and slope as mean peak discharge (Qm) and uplift rate (U) vary. We find that channels in which weathering is allowed to increase erodibility are wider, deeper, and less steep than nonweathering channels with the same initial conditions. While fixed erodibility channels maintain similar width/depth ratios regardless of Qm or U, the width/depth ratio of weathering channels is sensitive to uplift rate. At low uplift rates, weathering outpaces erosion, and channels obtain similar width/depth ratios but are wider and less steep than fixed erodibility channels with equal initial conditions. At high uplift rates, erosion outpaces weathering and erodibility remains near the unweathered value, with channel shape and slope nearly identical to a fixed erodibility channel with equal initial conditions. Weathering channels differ most from fixed erodibility channels at intermediate uplift rates, with greater width/depth ratios and lower slopes than fixed erodibility channels with the same initial conditions. Our results support the hypothesis that cross‐channel variations in erodibility created by weathering may be an important control on channel geometry and provide guidance for further testing of this hypothesis in natural systems.