Entrainment and suspension of sand and gravel

The entrainment and suspension of sand and gravel are important for the evolution of rivers, deltas, coastal areas, and submarine fans. The prediction of a vertical profile of suspended sediment concentration typically consists of assessing (1) the concentration near the bed using an entrainment relation and (2) the upward vertical distribution of sediment in the water column. Considerable uncertainty exists in regard to both of these steps, especially the near-bed concentration. Most entrainment relations have been tested against limited grain-size-specific data, and no relations have been evaluated for gravel suspension, which can be important in bedrock and mountain rivers. To address these issues, we compiled a database with suspended sediment data from natural rivers and flume experiments, taking advantage of the increasing availability of high-resolution grain size measurements. We evaluated 12 dimensionless parameters that may determine entrainment and suspension relations and applied multivariate regression analysis. A best-fit two-parameter equation (r² = 0.79) shows that near-bed entrainment, evaluated at 10 % of the flow depth, decreases with the ratio of settling velocity to skin-friction shear velocity (w_(si)/u_(∗ skin)), as in previous relations, and increases with Froude number (Fr), possibly due to its role in determining bedload-layer concentrations. We used the Rouse equation to predict concentration upward from the reference level and evaluated the coefficient β_i, which accounts for differences in the turbulent diffusivity of sediment from the parabolic eddy viscosity model used in the Rouse derivation. The best-fit relation for β_i (r² = 0.40) indicates greater relative sediment diffusivities for rivers with greater flow resistance, possibly due to bedform-induced turbulence, and larger w_(si)/u_(∗ skin); the latter dependence is nonlinear and therefore different from standard Rouse theory. In addition, we used empirical relations for gravel saltation to show that our relation for near-bed concentration also provides good predictions for coarse-grained sediment. The new relations extend the calibrated parameter space over a wider range in sediment sizes and flow conditions compared to previous work and result in 95 % of concentration data throughout the water column predicted within a factor of 9

Entrainment and suspension of sand and gravel

The entrainment and suspension of sand and gravel are important for the evolution of rivers, deltas, coastal areas, and submarine fans. The prediction of a vertical profile of suspended sediment concentration typically consists of assessing (1) the concentration near the bed using an entrainment relation and (2) the upward vertical distribution of sediment in the water column. Considerable uncertainty exists in regard to both of these steps, especially the near-bed concentration. Most entrainment relations have been tested against limited grain-size-specific data, and no relations have been evaluated for gravel suspension, which can be important in bedrock and mountain rivers. To address these issues, we compiled a database with suspended sediment data from natural rivers and flume experiments, taking advantage of the increasing availability of high-resolution grain size measurements. We evaluated 12 dimensionless parameters that may determine entrainment and suspension relations and applied multivariate regression analysis. A best-fit two-parameter equation (r² = 0.79) shows that near-bed entrainment, evaluated at 10 % of the flow depth, decreases with the ratio of settling velocity to skin-friction shear velocity (w_(si)/u_(∗ skin)), as in previous relations, and increases with Froude number (Fr), possibly due to its role in determining bedload-layer concentrations. We used the Rouse equation to predict concentration upward from the reference level and evaluated the coefficient β_i, which accounts for differences in the turbulent diffusivity of sediment from the parabolic eddy viscosity model used in the Rouse derivation. The best-fit relation for β_i (r² = 0.40) indicates greater relative sediment diffusivities for rivers with greater flow resistance, possibly due to bedform-induced turbulence, and larger w_(si)/u_(∗ skin); the latter dependence is nonlinear and therefore different from standard Rouse theory. In addition, we used empirical relations for gravel saltation to show that our relation for near-bed concentration also provides good predictions for coarse-grained sediment. The new relations extend the calibrated parameter space over a wider range in sediment sizes and flow conditions compared to previous work and result in 95 % of concentration data throughout the water column predicted within a factor of 9

Acoustically derived suspended sediment concentrations and flux in the Fraser River, Canada

In Canada, there is no existing systematic sediment data collection program for river systems and limited resources are available to mount manual measurement programs. Yet, there is a pressing need to understand and predict sediment fluxes because the quantity and caliber of transported sediment controls river channel stability, influences river ecology and should be considered in river management. In the Lower Fraser River, British Columbia, Canada conventional methods for estimating sediment flux are based on historical data that are no longer reliable due to the river\u27s ongoing adjustment to land use practices, climate change, sea level variation and dredging. This research establishes methods to monitor suspended sediment delivery to the Fraser Delta using hydroacoustic signals as a surrogate of suspended sediment concentration (SSC) and grain size. Both single- and multi-frequency sediment detection approaches are evaluated. Acoustic signals from an array of three horizontally-mounted acoustic Doppler current profilers (ADCPs) are coupled with physical bottle samples within the acoustically ensonified volume. Bottle samples are analyzed for SSC and for grain size distribution. Discharge, channel-average SSC and flux are measured. Twenty-five sampling campaigns were carried-out in the Fraser River at Mission between 2012 and 2014. I develop data processing methods for acoustic signals near the ADCP noise-floor and establish threshold concentrations below which attenuation measured in-situ is unreliable. A single-frequency, two-stage acoustic inversion is developed for application in large rivers where the ADCPs cannot penetrate the full channel width. The method involves calibration of ADCPs and a correlation between ADCP SSC and the channel-averaged SSC. Strong calibrations for total SSC, sand SSC and silt/clay SSC are obtained. Good correlations between acoustically derived SSC and channel-average SSC allow for continuous SSC and flux estimates. Annual flux fell within the same order of magnitude as historical flux from the same location, computed with traditional methods, supporting the robustness of the method. Explicit and implicit multi-frequency inversions are explored. Comparisons between the inversion results and sample data show that the implicit method tends to perform best for estimating concentration at all flows. Realistic estimates of particle size are obtained for high flows only using this method

Bullying Prevention in Special Education: a Multimethod Investigation of Evidence-Based Programs and Practices

This study investigated bullying prevention programming in special education populations. Systematic review was used to identify and assess the evidence supporting bullying prevention programming in special education students. The systematic review was supplemented by a qualitative analysis that examined instructional practices and policies endorsed by the American Speech-Language-Hearing Association (ASHA). These instructional practices were compared to components of bullying prevention programs to determine which prevention programs were the most appropriate for students with speech and language delays. Results indicated that very few bullying prevention programs had been evaluated on special education populations. However, several bullying prevention programs were found to have elements that were consistent with ASHA practice policy. Implications and recommendations for bullying prevention programming among special education populations are discussed

Runoff generation mechanisms in a steep first-order British Columbian watershed.

This research was undertaken in the Malcolm Knapp Research Forest in British Columbia Canada and aimed to examine runoff generation mechanisms in a steep forested watershed. The research questions were: (1) can water infiltrate into the bedrock in the study watershed, (2) can bedrock infiltration be described with simple infiltration models that have been developed for soil infiltration, and (3) what is the spatial variation in the relations between discharge and piezometric response on the hillslope? To answer these questions hydrometric data from a 20 m by 18 m hillslope and from bedrock infiltration ponds were collected. The results from this research showed that the bedrock is permeable and that soil infiltration models can represent bedrock infiltration. The hillslope has two distinct water table zones; a hillslope zone and riparian zone. The riparian zone is located 0-8 m from the stream while the hillslope zone is 8-18 m uphill

Mud in rivers transported as flocculated and suspended bed material

Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s⁻¹, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory

Mud in rivers transported as flocculated and suspended bed material

Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s⁻¹, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory