Initial Sediment Motion Research Articles

Uncertainties in Models Predicting Critical Bed Shear Stress of Cohesionless Particles

Our data show large scatter in the critical Shields stress values for initial sediment motion. The main sources of dispersion are related to the methodological procedures defining the inception of movement (i.e., visual observations or extrapolation of sediment transport rate) and to the estimation of the bed shear stress. The threshold for sediment motion varies with many factors related not only to grain size but also to bed composition (e.g., presence of fine sediments in a coarse matrix), arrangement (e.g., bed roughness, grain orientation, and characteristic lengths of bed structures) and slope. New models to estimate the critical Shields number are proposed using grain size or bed slope or a combination of both. Model parameters and uncertainty are estimated through Bayesian inference using prior knowledge of these parameters and measured data. Apart from the uncertainty in observations, two types of uncertainty can be evaluated: one related to the parameter estimation (i.e., parametric) and one related to the choice of the model (i.e., structural). Finally, a four-parameter model based on the grain size and bed slope yields the best results and demonstrates potential interaction between these two parameters. Model uncertainty, however, remains large, which indicates that other input parameters may be needed to improve the proposed model.

Accelerating Predictions of Morphological Bed Evolution by Combining Numerical Modelling and Artificial Neural Networks

Process-based models have been employed extensively in the last decades for the prediction of coastal bed evolution in the medium term (1–5 years), under the combined action of waves and currents, due to their ability to resolve the dominant coastal processes. Despite their widespread application, they are associated with high demand for computational resources, rendering the annual prediction of the coastal bed evolution a tedious task. To combat this, wave input reduction methods are generally employed to reduce the sheer amount of sea-states to be simulated to assess the bed level changes. The purpose of this research is to further expand on the concept of input reduction methods by presenting a methodology combining numerical modelling and an Artificial Neural Network (ANN). The trained ANN is tasked with eliminating wave records unable to initiate sediment motion and hence further reduce the required computational times. The methodology was implemented in both an idealized and a real-field case study to examine the sensitivity, and produced very satisfactory predictions of the rates of bed level change, with respect to a benchmark simulation containing a very detailed wave climate. The obtained results have strong implications for further accelerating the demanding morphological simulations while enhancing the reliability and accuracy of model predictions.

Dimensionless critical shear stress for sediment particles under varying slope conditions

One of the most fundamental and practical issues in sedimentology is making predictions on initial sediment motion. Many widely used bedload sediment-transport models are based on the concept that sediment transport either begins at or can be scaled by, a constant value of the dimensionless critical shear stress. Flume experiments were conducted for three different diameters of spherical particles under varying channel slope. A model is presented to determine the dimensionless critical shear stress for the incipient motion of spherical particles on nonhorizontal slopes. The findings of the study have been illustrated with mathematical relationships and graphical representations for various combinations of input variables.

Unraveling bed slope from relative roughness in initial sediment motion

AbstractUnderstanding incipient sediment transport is crucial for predicting landscape evolution, mitigating flood hazards, and restoring riverine habitats. Observations show that the critical Shields stress increases with increasing channel bed slope, and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms, particle interlocking across the channel width, and large bed sediment relative to flow depth (relative roughness). Here we use scaled flume experiments with variable channel widths, bed slopes, and particle densities to separate these effects which otherwise covary in natural streams. The critical Shields stress increased with bed slope for both natural gravel (ρs = 2.65 g/cm3) and acrylic particles (ρs = 1.15 g/cm3), and adjusting channel width had no significant effect. However, the lighter acrylic particles required a threefold higher critical Shields stress for mobilization relative to the natural gravel at a fixed slope, which is unexpected because particle density is accounted for directly in the definition of Shields stress. A comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical Shields stress in our experiments. These changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion. Increased relative roughness can explain much of the observed heightened critical Shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore, pumice, or ice clasts on Titan.

Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

AbstractA major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains. The extent of alluvial cover is typically predicted as a function of relative sediment flux (sediment supply rate over bed load transport capacity, qbs/qbc), yet little is known about how the bed roughness affects the alluvial cover. We performed field experiments with various flow discharges, sediment supply rates, grain sizes, and bed surface topographies. We then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover, so as to include the effect of roughness change. The results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments. We explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness (bedrock hydraulic roughness height over grain size, ksb/d). The results suggest the following: (1) a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion; (2) at a given sediment supply rate, the extent of alluvial cover is larger when the relative roughness is larger; (3) when the sediment supply rate and the relative roughness are small, throughput bed load moves over (and can abrade) a purely bedrock channel with no alluvial cover; and (4) the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness. The experimental results and analysis provide a framework for treating the (a) incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover, (b) incisional/alluvial morphodynamics of mixed bedrock‐alluvial rivers, and (c) purely alluvial morphodynamics, as well as the transition between these states.

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

AbstractPlants 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.

Incipient sediment motion across the river to debris-flow transition

Sediment transport in mountain channels controls the evolution of mountainous terrain in response to climate and tectonics and presents major hazards to life and infrastructure worldwide. Despite its importance, we lack data on when sediment moves in steep channels and whether movement occurs by rivers or debris flows. We address this knowledge gap using laboratory experiments on initial sediment motion that cross the river to debris-flow sediment-transport transition. Results show that initial sediment motion by river processes requires heightened dimensionless bed shear stress (or critical Shields stress) with increasing channel-bed slope by as much as fivefold the conventional criterion established for lowland rivers. Beyond a threshold slope of ∼22°, the channel bed fails, initiating a debris flow prior to any fluvial transport, and the critical Shields stress within the debris-flow regime decreases with increasing channel-bed slope. Combining theories for both fluvial and debris-flow incipient transport results in a new phase space for sediment stability, with implications for predicting fluvial sediment transport rates, mitigating debris-flow hazards, and modeling channel form and landscape evolution.

Slope Correction for Calculation of Bedload Sediment Transport Rates in Steep Channels

AbstractClassical theories developed for investigating bedload sediment transport are applicable largely for channel beds of a gentle slope. Examples are the well-known Shields diagram for the evaluation of the critical condition for initial sediment motion and the classical bedload transport formulas derived with deterministic or probabilistic approaches. How to apply the existing bedload formulas for steep channels remains challenging. In this study, a simple procedure is proposed for correcting slope effects so that the existing bedload formulas can be extended for steep channels. To verify the proposed correction factor, two representative bedload formulas are modified and checked against published data of sediment transport rates that were significantly affected by steep slopes.

Experimental evidence for the effect of hydrographs on sediment pulse dynamics in gravel‐bedded rivers

Gravel augmentation is a river restoration technique applied to channels downstream of dams where size‐selective transport and lack of gravel resupply have created armored, relatively immobile channel beds. Augmentation sediment pulses rely on flow releases to move the material downstream and create conditions conducive to salmon spawning and rearing. Yet how sediment pulses respond to flow releases is often unknown. Here we explore how three types of dam releases (constant flow, small hydrograph, and large hydrograph) impact sediment transport and pulse behavior (translation and dispersion) in a channel with forced bar‐pool morphology. We use the term sediment “pulse” generically to refer to the sediment introduced to the channel, the zone of pronounced bed material transport that it causes, and the sediment wave that may form in the channel from the additional sediment supply, which can include input sediment and bed material. In our experiments, we held the volume of water released constant, which is equivalent to holding the cost of purchasing a water volume constant in a stream restoration project. The sediment pulses had the same grain size as the bed material in the channel. We found that a constant flow 60% greater than the discharge required to initiate sediment motion caused a mixture of translation and dispersion of the sediment pulse. A broad crested hydrograph with a peak flow 2.5 times the discharge required for entrainment caused pulse dispersion, while a more peaked hydrograph >3 times the entrainment threshold discharge caused pulse dispersion with some translation. The hydrographs produced a well‐defined clockwise hysteresis effecting sediment transport, as is often observed for fine‐sediment transport and transport‐limited gravel bed rivers. The results imply a rational basis for design of water releases associated with gravel augmentation that is directly linked to the desired sediment behavior.

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force‐balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed‐roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain‐roughness layer is proposed based on a 1‐D eddy viscosity with wake mixing. In addition, the magnitude of near‐bed turbulent fluctuations is shown to depend on the depth‐averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

Sediment grains moved by passing tsunami waves: Tsunami deposits in deep water

Tsunamis propagating in the open ocean have associated horizontal particle velocities that do not change with depth — yet the limiting water depth where a tsunami of given characteristics will initiate sediment motion remains unknown. Based upon linear wave theory and a parametrization of the Shields curve, equations are derived and solved, using an iterative scheme, to address the topic of grain movement by tsunami waves as a function of water depth and wave amplitude. The focus is on waves in deep water where tsunami waves behave linearly and on non-cohesive sediment grains. Furthermore, the question is addressed of which grain sizes are picked up on a sloping beach as the wave shoals. According to the results, even the Boxing Day tsunami in 2004 was incapable of moving fine sand in water deeper than 985 m in the Bay of Bengal and 335 m in the Indian and Pacific oceans. The results suggest that tectonic tsunamis of size equal to or smaller than the Boxing Day tsunami cannot initiate motion of deep-water cohesionless sediments that can be correlated on an oceanic basin-wide scale.

Influence of shear stress fluctuation on bed particle mobility

Whether or not a sediment particle is entrained from a channel bed is associated with both average bed shear stress and shear stress fluctuation, the latter being flow-dependent and also related to bed irregularities. In the first part of this study, a preliminary analysis of possible fluctuations induced by bed roughness is presented for the case of an immobile plane bed comprised of unisized sediments. The result shows that the roughness-induced variation is generally comparable to that associated with near-bed turbulence, and both variations can be approximated as log-normal in terms of probability density distribution. The bed particle mobility is then analyzed by considering the effects of shear stress fluctuations. The relevant computations demonstrate that with increasing shear stress fluctuations, the probability of the mobility of a bed particle may be enhanced or weakened. For the case of low sediment entrainment, the probability is increased by turbulence. However, the probability may be reduced by the shear stress fluctuation if the average bed shear stress becomes relatively higher. This study shows that the critical condition for initial sediment motion could be overestimated if the Shields diagram is applied for the condition of flows with high turbulence intensities.

A note on tidally generated sand waves

The process leading to the formation of sand waves in tide dominated coastal areas is investigated by means of the linear stability analysis of a flat sandy bottom subject to oscillatory tidal currents. The conditions for the decay or amplification of small bottom perturbations are determined for arbitrary values of the parameters of the problem. According to field observations, the initial growth of sand waves requires a minimum amplitude of the tidal current, even when the critical bed shear stress for the initial motion of sediment is set equal to zero. Moreover the minimum amplitude depends on sediment characteristics. In particular, the analysis shows that sand waves appear only for a sandy bottom and their growth does not take place when a coarse sediment covers the sea bed. The solution procedure extends the truncation method which is often used to describe the flow generated by the interaction of bottom perturbations with the oscillatory tidal current. The obtained results show that the truncation method describes the mechanism inducing the growth of sand waves, but values of the parameters exist for which its results are not quantitatively accurate. Finally, the asymptotic approach for large values of both r, which is the ratio between the amplitude of the horizontal tidal excursion and the wavelength of the bottom perturbations, and of the stress parameter s is modified in the bottom boundary layer to describe cases characterized by values of s of order one, which is the order of magnitude suggested by an analysis of field data.

A simple model of sediment initiation under waves

A simple model is developed to study the initial motion of sediment on a horizontal bed under non-breaking waves. The model is derived to be A=C( T− T 0) based on a wide range of experimental data collected in different flow regimes, where A is the nearbed semi-excursion of wave motion, T is the wave period, and C and T 0 are the coefficients dependent on sediment properties only. For a given sediment, the onset velocity of sediment motion derived from the model is shown to initially increase sharply with wave period T and then approach a constant. The flow Reynolds number Re corresponding to an initiated sediment is also calculated from the simple model and found to be a function of sediment properties and wave period. For the completeness of this study, the initial motion of light sediment under very short waves is also investigated. The present model agrees well with the available laboratory and field data.

Two-fraction model of initial sediment motion in gravel-Bed rivers

The prediction of sediment transport in gravel-bed rivers is essential to the management of land, water, and ecological resources in mountain regions. Dividing the bed sediment into two populations-sand and gravel-permits realistic and useful predictions of the onset of sediment transport. The critical flow initiating grain motion decreases rapidly with sand content over the transition from a gravel-framework bed to a sand-matrix bed. The two-fraction model provides a simple means of forecasting the movement of excess fine sediment supply. The model also helps to explain the development of the abrupt gravel-sand transition commonly observed in natural rivers.

Initial Motion of Sediment in Oscillatory Flow

A new model is developed to study the initial motion of sediment in oscillatory flow over a flat bottom on the basis of available experimental data. It is found that the initial motion of sediment in oscillatory flow is uniquely defined by the simple model of A/d=KA2ω/ν+B, where A= semiexcursion of wave orbital motion near the bed; ω= angular frequency; d= grain diameter; ν= kinematic viscosity; and K and B= dimensionless coefficients determined by the immersed sediment weight. For a given sediment, the onset velocity derived from the model is found to initially increase with the wave period T and then approach a constant. A practical formula is also presented to calculate the onset velocity of sediment in the coastal zone.

Palaeohydraulics revisited: palaeoslope estimation in coarse‐grained braided rivers

Methods for estimating palaeoslope from fluvial deposits have been available for some time, but new data and improved understanding of the relevant physical processes afford the possibility of improving existing methods, and the emerging field of quantitative stratigraphy provides a new context for the results. Here we focus on deriving palaeoslope estimates for coarse‐grained fluvial deposits. These estimates can be used in basin analyses to constrain the magnitude of the slope change necessary for a given deflection of palaeocurrents, to constrain temporal and spatial variation in basin subsidence rate, and to provide a surface datum for use in sediment‐backstripping calculations. The algorithm we derive to estimate palaeoslope applies to rivers that self‐adjust through variations in channel width to maintain a temporally and spatially averaged bed shear stress equal to some constant multiple of the critical shear stress for initial motion of bed sediment. Data from modern coarse‐grained rivers with minimal bank cohesion and form resistance suggest that this boundary shear stress is equal to about 1.4 times the critical shear stress for movement of the median‐sized clast of the surface layer. The key sedimentological criteria for recognition of systems appropriate for this type of analysis are: (1) field relations suggesting that channel banks formed in effectively noncohesive gravel (i.e. free of clay‐size sediment and plant roots); (2) absence of significant volumes of dune‐derived cross‐stratification; and (3) absence of indicators of extremely rapid, flash‐flood‐type deposition. The basic input data for a palaeoslope calculation are spatially averaged estimates of palaeodepth and median grain size. The most important aspect of data collection is that the depth and grain‐size estimates should be determined independently by random sampling over the whole outcrop. Joint analysis of data from appropriate modern rivers and of errors associated with palaeodepth and grain‐size estimates indicates that in coarse‐grained braided‐river deposits, palaeoslope can be estimated to within a factor of 2.

Initial motion of sediment under waves and wave-current combined motions

Recently, one of the authors proposed a very convenient formula to evaluate the friction coefficient induced by wave-current combined motion, which spans all flow regimes including the transitional regime and requires no iteration. This formula is applied to an analysis of existing experimental data regarding initial motion of sediment particles under pure waves and wave-current combined motion. It was found that most of the data analyzed here were located in the transitional regime. Thus, the full-range friction coefficient formula can be conveniently applied as compared with previous implicit formulae and graphical friction laws. The formula of general motion under pure wave motion can be modified to compute the critical water depth of sediment movement. Some discontinuities in the former work in the transitional regime now can be overcome in the present study.

Colonization of freshly deposited barite and silica sediments by marine microorganisms in a laboratory flume flow

The biomass and structure of microbial communities colonizing barite freshly deposited onto natural marine mud was investigated in a laboratory flume. Modern biochemical methods based on the lipid content of microbial cells were used to quantify microbial colonization. The experiment involved the following 3 treatments: (1) fresh barite deposited onto the surface of a natural muddy sediment; (2) fresh silica (utilized as a colonization control) also deposited onto natural mud; and (3) the muddy sediment itself. The treatments were placed in an open-channel flow with 10 pm filtered sea water and sampled every other day for 16 d. The flow was set at a shear velocity (u.) of 0.30 cm S-', less than the critical U. to initiate sediment motion of these particles. After 48 h, the barite and silica treatments contained approximately 60 times less microbial biomass than the natural mud. Little change in microbial biomass was observed in the barite and silica treatments until 13 d of incubation when microbial blomass began to increase. The carbon content of the fresh barite and silica treatments was 60 to 570 times less than in the natural mud and this lower carbon content is the likely proximal cause of the lower microbial biornass. The structure of microbial communities was analyzed after 13 d of incubation. Despite the large difference in total biomass, microbial communities in the natural mud and barite treatments were similar in structure. In the silica treatment, however, community structure was highly variable and dissimilar to the natural mud community. The community in flowing sea water was more similar to communities in natural mud and barite than in silica. These results suggest that natural marine sediments receiving inputs of barite can be expected, given sufficient time, to develop microbial communities similar in structure and possibly biomass to those found in ambient sediments.

INITIAL MOTION IN COMBINED WAVE AND CURRENT FLOWS

Measurements are presented of the conditions for the initial motion of sediment under combined steady and oscillatory flow. The measurements were made in a steady flow flume with an oscillating tray set into its bed. The direction of oscillation of the tray was at right angles to the axis of the steady flow flume. Four different grades of sand were tested. It is found that the critical condition for the initiation of motion is reasonably represented by a critical value of the vector sum of the component shear stresses assuming no nonlinear interaction between the steady and oscillatory flows. The resultant bed shear stress was also calculated with the aid of several combined wave-current models. The results of these various approaches are compared with Shields curve.