Thresholds and Sediment Transport Dynamics in an Interbedded Shale and Limestone Controlled Urban Watercourse

Sediment transport is a fundamental component of research into river morphology and related engineering practices. The relationship between flow and sediment particle entrainment underpins many of the empirical models used to estimate sediment transport dynamics. The scientific literature reports a research gap specific to the thresholds of mobility of different sized particles in non-gravel bed systems, including those in bedrock channels. Particle tracer technology was used to study coarse sediment entrainment and transport dynamics in an urban, bedrock controlled stream channel in Toronto, Ontario, Canada. Passive integrated transponders were inserted in constrained and unconstrained particles within an incised reach of stream. The distribution of particles transport distances conformed to a two-parameter gamma distribution model, which assumes integrations of the travelled series of steps and rests. Size selective dependency of path length was found to increase for coarser clasts, as compared to observed conditions for gravel-bed systems. Coarser particles were also found to transport in an unconstrained mode, as compared to finer grains. A force exceedance model was applied to further test the performance of reported size selective transport relationships for the study site. Many particles were found to transport at critical shear ratios less than 1, when assuming a modified Shields’s based model for entrainment. Field data was then used to determine a reference shear based on the smallest magnitude competent storm. The results show that, when compared to alluvial gravel-bed conditions, finer particles require larger thresholds to mobilize and the inverse is true for coarser particles. Using the reference shear conditions, rates of sediment transport were calculated and compared to common models for coarse particle transport. The results confirm size selectivity by grain class and indicate differentiations between fine and coarse transport relationships for the site. This research confirms non-conformity of particle entrainment and transport relationships for the study site, when compared to common empirical model for gravel-bed rivers. The results may be used to obtain critical entrainment parameters and sediment transport relationships, which can then be used to inform design criteria for regional watercourses having like lithology and morphology

The science behind scour at bridge foundations : a review

Foundation scour is among the main causes of bridge collapse worldwide, resulting in significant direct and indirect losses. A vast amount of research has been carried out during the last decades on the physics and modelling of this phenomenon. The purpose of this paper is, therefore, to provide an up-to-date, comprehensive, and holistic literature review of the problem of scour at bridge foundations, with a focus on the following topics: (i) sediment particle motion; (ii) physical modelling and controlling dimensionless scour parameters; (iii) scour estimates encompassing empirical models, numerical frameworks, data-driven methods, and non-deterministic approaches; (iv) bridge scour monitoring including successful examples of case studies; (v) current approach for assessment and design of bridges against scour; and, (vi) research needs and future avenues

Incipient motion of streambeds

Results of an experimental study of the incipient motion of streambeds are reported. The experiments were conducted in an 8 m long, and 0.30 m wide by 0.30 m deep glass-walled tilting flume and an 18 m long, 0.80-1.10 m wide by 0.15 m deep trapezoidal concrete channel. The purpose of the experiments was to determine flow conditions associated with the initiation of bed sediment motion and to investigate near-to-threshold bedload transport. Uniform and graded natural sands and gravels were used as bed material. A new approach to the description of critical state of the bed based on the intensity of sediment motion, or transport intensity, is proposed. This approach describes the state of the streambed mobility in terms of the fraction of bed particles mobilized in unit time, which eliminates the subjectivity in defining threshold conditions and provides a probabilistic description of the process of sediment entrainment. On the basis of the flume data obtained for uniform sediment, a relationship between the intensity of particle motion and sediment transport rate is established. This relationship allows any measured transport rate to be expressed in terms of the probability description of the process of sediment entrainment. The experiments reveal that critical bed shear stress for incipient motion of uniform sediment depends not only on the grain size, but also on the bed slope. This is explained by the effect of relative depth (depth to grain size ratio) on overall flow resistance. It is also shown that the value of critical dimensionless bed shear stress is not coarser gravel. The same conclusion follows from the measurements of turbulence characteristics near the bed. A revised Shields diagram relating critical stress, grain (or alternatively, different probabilities of sediment entrainment). The experimental results are formalized to provide a generalized method for calculating the bedload transport rate and critical Shields stress of coarse uniform sediments

A mechanistic model linking insect (Hydropsychidae) silk nets to incipient sediment motion in gravel‐bedded streams

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. Key Points Caddisfly silk nets are incorporated into a model of incipient sediment motion Silk nets increase critical shear stress in gravel‐bedded streams Species‐specific silk and behaviors control the range of grain sizes affectedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109329/1/jgrf20303.pd

Observations of pockmark flow structure in Belfast Bay, Maine

Vertical current and CTD profiles were acquired over a small, spherical pockmark and a larger, more elongated pockmark in Belfast Bay, Maine in July 2011. These observations showed evidence for mixing within the pockmarks, a rotational pattern that resembles open cavity flow, and incipient motion along the rims. Over the center of each pockmark, observations of uniform temperature properties below 12 m are indicative of mixing within the pockmark. The observed complex rotational structure over each pockmark shows significant rotation with depth and a greater degree of rotation during ebbing tide. These observations are qualitatively consistent with circulation patterns predicted by cavity flow models. Critical Shields parameters for cohesive sediment were estimated at the rim and center of each pockmark and were only exceeded along the rim. During the infrequently observed upwelling events, and in the absence of flocculation, suspended sediment would be unable to settle through the water column

Scoping the impact of tidal and wave energy extraction on suspended sediment concentrations and underwater light climate

The depth to which sunlight penetrates below the sea surface is one of the key factors determining the species composition and productivity of marine ecosystems. The effects range from the rate and fate of primary production, through the performance of visual predators such as fish, the potential for refuge from predators by migrating to depth, to the scope for seabed stabilisation by algal mats. Light penetration depends partly on spectral absorption by seawater and dissolved substances, but mainly on the scattering caused by suspended particulate material (SPM). Some of this SPM may be of biological origin, but in coastal waters the majority is mineral material originating ultimately from seabed disturbance and land erosion, the latter being deposited in the sea by rivers and aerial processes. SPM is maintained in the water column or deposited on the seabed depending on combinations of hydrodynamic processes including baroclinic (density-driven) or barotropic (mainly tidal and wind driven) currents, and wave action (Ward et al. 1984; Huettel et al. 1996). Since tidal and wave energy extraction must alter these hydrodynamic properties at some scales depending on the nature of the extraction process, we can expect some kind of impact on the concentration of the SPM. If these are large enough, we may have to consider the extent to which these may impact the underwater light environment and the local or regional ecology. Whilst several coupled hydrodynamic-sediment models exist to predict SPM distributions in aquatic systems, their skill level in open coastal and offshore marine waters is acknowledged to be relatively low. This is largely because the processes are not well understood and the formulations are largely based on empirical relationships rather than fundamental physical principles. The models are also highly demanding in terms of calibration data and computational resources. Hence their utility for predicting relatively subtle effects arising from changes in flow or wave environments due to energy extraction devices seems rather low. Here, we summarise the key mathematical functions describing the processes involved in sediment suspension, and propose a lightweight one-dimensional (vertical) model which can be used to scope the effects of changes in flow and wave energy on SPM

Lecture notes on sediment transportation and channel stability

These notes have been prepared for a series of lectures on sediment transportation and channel stability given by the authors to a group of engineers and geologists of the U. S. Department of Agriculture assembled at Caltech on September 12-16,1960. The material herein is not intended to serve as a complete textbook, because it covers only subjects of the one-week sequence of lectures Due to limitation of space and time, coverage of many subjects is brief and others are omitted altogether. At the end of each chapter the reader will find a selected list of references for more detailed study

Low transport stages by water streams of fine, cohesionless, granular and flakey sediments

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DEVELOPMENT OF A SUNKEN OIL TRANSPORT TOOL USING MESOSCALE EXPERIMENTS

Oil spilled into fresh or saline water can float, become submerged in the water column (i.e., submerged oil), or sink to the bottom (i.e., sunken oil). Once introduced to the environment, oil can negatively impact ecological and public health, and the economy. Non-floating oil spills pose unique challenges to responders including the complexity of trajectory modeling; the inability to detect, track and recover oil due to limited visibility; the lack of readily deployable response technology; and limited understanding of how bottom substrate dynamics influence its fate and behavior. This dissertation research determined that the driving factors used to predict sunken oil transport are the oil’s kinematic viscosity (v_o) and the median sediment size (d_50). The stages of oil transport were characterized based on v_o, and empirical relationships using v_o and d_50 were derived to predict the oil’s critical shear stress (CSS). For v_o\u3c 2x104 cSt, thresholds of movement were defined as: (1) gravity dispersion, (2) rope formation, (3) ripple formation, and (4) break-apart/resuspension. For v_o \u3e 6x104 cSt, the stages include: (1) type II erosions, and (2) bedload transport. Using the experimentally derived oil transport equations, a prototype sunken oil transport tool (SOTT) was developed to predict sunken oil transport in a current driven environment. In the event of a non-floating oil spill, responders can input the spilled oil’s characteristics (i.e., density, viscosity) and in-situ environmental conditions (e.g., water velocity, temperature, sediment type) to evaluate if oil will transport along the bottom, resuspend into the water column, or be buried by sediments

Sediment Transport Impacts Upon Culvert Hydraulics

Sedimentation buildup and accumulation can cause serious impediments to the hydraulic capacity of culvert systems. There has not been any significant research to date regarding the behavior of bed load transport nor the implications of bed forms upon the hydraulics associated with culvert flow. The primary objective of this study was to investigate how sediment transport occurs in a culvert and to then develop a methodology and test setup to successfully investigate this sediment transport. The investigation was limited to studying culvert and pipeline transport of alluvial material in sand and gravel sizes. This dissertation develops a semi-empirical bed load transport equation from existing open channel flow models to be used in predicting sediment yields in culvert applications. Incipient motion and critical shear stresses were investigated for application into eight empirically based models. The methods analyzed include the Meyer-Peter Müller, Engelund and Hansen, Shields, Toffaleti (as seen in the United States Army Corps of Engineers program HEC RAS), Schoklitsch, DuBoy, Yang, and Rottner methods. These methods were tested for predictive accuracy to physically modeled bed load transport data obtained from a 304.8 mm (11.89 in) diameter culvert. Tests involved fully pressurized, partially pressurized inlet controlled, and open channel flow regimes for a variety of bed elevations and bedforms. Bedform regime and associated resistance impacts on flow energy were presented to better understand their hydraulic consequence in culvert applications. An extensive literature review regarding sediment transport in both open channel and closed conduit applications is provided to develop a foundation of knowledge to pursue further research in this area. This literature review summarizes the current body of scientific knowledge that is applicable to sediment transport in culverts. Investigations into both historical and current works are cited throughout this studies literature review. A tested methodology is presented for the investigation of sediment bed load transport in culvert applications. Development of a procedure for the testing of critical shear limits and bed load transport is outlined. A detailed application example is provided. Recommended changes in testing techniques and physical model are made for the next generation of culvert sediment transport research