Sand-gravel Bed Research Articles

Comparison of flow turbulence over a sand bed and gravel bed channel

Abstract Experiments were performed to examine the variations in flow turbulence with respect to sand and gravel bed channel. The 3D instantaneous velocity of water is measured by acoustic doppler velocimeter (ADV) at the test section, which will provide an important data related to the flow turbulence. The parameters of turbulence measured that the vertical profiles of longitudinal velocity in flows with a sand bed channel at the vicinity of the bed surface are higher by 2–6% than flow subjected to gravel bed channel, while away from the boundary surface, it decreases by 5–10%. The variations of Reynolds shear stress increase by 35–50% with gravel bed channel, indicating higher exchange of flow energy towards the boundary and vice versa. The higher value is obtained for shear velocities by 58.5% and the von Karman's constant with gravel bed channel. The gravel bed channel influenced the turbulence intensities with higher magnitude in the streamwise and vertical direction. The present study also analysed the flow anisotropy, correlations coefficient and turbulence diffusivity and compared the results. The outcomes of the current work are beneficial for civil and hydraulic engineers, since the data of turbulence will advance the management of bed materials present in the river.

Soft Sediment Deformation Structures in Quaternary Sediments from Dadra and Nagar Haveli, Western India

Abstract Quaternary sediments are preserved in disconnected patches along the middle and lower reaches of major river valleys like Damanganga, Par, Ratakhadi and Dongarkhadi in the Dadra and Nagar Haveli region of Konkan coastal belt, Western India. These deposits mainly consist of stratified sand, silt, clay and gravel beds. The study area has been earlier affected by moderate earthquakes. The identified soft-sediment deformation structures (SSDS) are mainly developed in the sand silt, sand gravel and clay beds; and includes intrusive sedimentary bodies (dykes and sills), slump structures, suspended clast blocks and convolute structures. The nature, shape and dimension of SSDS suggest that the trigger mechanism and driving forces for the origin of these structures were seismic shock waves. Sediment loading and storm events as a trigger for the SSDS are less likely the reason and the proximity of these structures to the faults support the inference that it may be of seismic origin. Deformation in the Quaternary sediments of Dadra and Nagar Haveli imply the presence of neotectonic activity and points to an earthquake of magnitude >5.5 that struck the study area after sediment deposition.

Riverbed armoring and sediment exchange process in a sand–gravel bed reach after the Three Gorges Project operation

Upstream damming greatly altered the flow and sediment regime entering downstream reaches in the Middle Yangtze River, and the bed material in a sand–gravel bed reach coarsened continuously, which had a significant influence on the sediment transport and bed evolution. In order to study the riverbed armoring, the sediment exchange process (SEP) among bed material, bed load and suspended load in a sand–gravel bed river is firstly clarified, and then, the three-state transition probability model (Markov chain) is proposed in this study, with the hiding-exposure effect of non-uniform sediment being considered. Finally, the equilibrium equation of sediment quantity in an active layer is presented to calculate the grain size distribution of bed material. In this model, the influences of flow and sediment conditions, riverbed erosion and deposition on the SEP are discussed. The results show that the composition of surface bed material at the Zhicheng station became obviously coarse, and the median grain size (d50) of surface bed material increased from 0.230 to 0.424 mm in 2003–2017, with an upward increasing trend. The proposed probabilistic model was validated against field measurements of bed material, and calculated results show reasonable agreement with the measured data at Zhicheng. Accordingly, the probabilistic model can be used to predict the riverbed armoring and to investigate the non-equilibrium transport of non-uniform sediment in a sand–gravel bed river.

Flume experiments on the equilibrium of sand-gravel bed rivers

Se describe una instalacion de laboratorio que permite realizar ensayos en canal con alimentacion separada de arena y grava, como es caracterisitico de los lechos de muchos rios. Se hacen ensayos de gran duracion, en que el transporte como carga de fondo llega a valer 1 kg/s. El material solido que sale del canal se envia hacia la cabecera en donde se separa la grava de la arena. La alimentacion se hace con dos tolvas, una para cada material, cuyos pesos se miden continuamente para conocer los caudales de alimentacion. Los ensayos persiguen alcanzar el equilibrio, es decir la permanencia del lecho sin erosion ni crecimiento a largo plazo. Se ofrecen ejemplos reales de varios tipos de ensayo: recirculacion con erosion del lecho y con crecimiento del lecho, recirculacion pura y alimentacion pura. Los cuatro son ejemplos de transporte solido con movilidad igualada. Se hace una discusion del significado de los ensayos

Geomorphic and retention responses following the restoration of a sand-gravel bed stream

Restoration is applied to reinstate geomorphic processes and complexity in degraded streams, but case studies of responses remain rare. A 236 m long reach of the Logie Burn, a straightened sand-gravel bed stream in north-east Scotland was restored by reconnecting its pre-modification channel to improve habitat and reduce nutrient transfer. Morphological, sedimentary and retention (wood, phosphorous and organic matter) responses were monitored over 3 years. Monitoring over 2 years was also conducted in a nearby control reach. A net deposition response of 64.3 ± 32.1 m3, a near two-fold enlargement of channel cross section area and a reduction of bed slope from 0.0008 m m−1 to 0.00014 m m−1 were observed in the restored reach over 3 years. The control reach in contrast was relatively stable. After 3 years, relative pool volume and morphological complexity were similar to the control reach and its pre-restoration state. Substrate diversity (Shannon-Wiener index: H’) after an initial increase, declined but at the end of the monitoring period was higher (H’ = 1.26) than the control reach (H’ = 1) and pre-restoration reach (H’ = 0.8). Following restoration, the median sediment size (D50) of the majority of the restored main channel reduced from 3.4 mm to 1.5 mm – a similar value to pre-restoration (D50 = 1.6 mm) – reflecting the net deposition response although the overall grain size distribution was not statistically different. No relationship between geomorphically effective flows and morphological change was found reflecting geomorphic sensitivity due to the reconnection work and wood recruitment mainly driven by tree collapse in response to channel readjustment. The retention of organic matter (OM) and total phosphorous (TP) did not increase in the main channel but did within the backwaters which also exhibited a statistically significant fining response. Overall, these results suggest that the physical habitat improvement value of the scheme so far has been limited but the creation of the backwaters may have increased the capacity of the reach to retain nutrients and fine sediment.

The Acoustic Properties of Suspended Sediment in Large Rivers: Consequences on ADCP Methods Applicability

The use of echo-levels from Acoustic Doppler Current Profiler (ADCP) recordings has become more and more common for estimating suspended bed-material and wash loads in rivers over the last decade. Empirical, semi-empirical and physical-based acoustic methods have been applied in different case studies, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. These methods entail different assumptions regarding sediment heterogeneity in the ensonified volume (e.g., particle size distribution (PSD) and spatial concentration gradient). Our work was to compare acoustic backscatter and attenuation properties of suspended sediments, sampled in the rivers Parana and Danube that represented rather different hydro-sedimentological conditions during the surveys. The Parana represents a large sandy river, characterized through a huge watershed and the typical bimodal PSD of sediment in suspension, while the Danube represents in the investigated reach an exposed sand-gravel bed and clay-silt particles transported in the water column in suspension. Sand and clay-silt concentrations clearly dominate the analyzed backscattering strength in the rivers Parana and Danube, respectively, with an effect of PSD level of sorting in the latter case. This comparison clarifies the extent of assumptions made, eventually advising on the actual possibility of applying certain ADCP methods, depending on the expected concentration gradients and PSD of suspended sediment to be investigated.

Effects of roughness elements distribution on overland flow resistance

Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.

Моделирование эволюции песчано-гравийного дна канала в одномерном приближении

Моделирование эволюции песчано-гравийного дна канала в одномерном приближении

Grain-size distribution in suspension over a sand-gravel bed in open channel flow

Grain-size distributions of suspended load over a sand-gravel bed at two different flow velocities were studied in a laboratory flume. The experiments had been performed to study the influence of flow velocity and suspension height on grain-size distribution in suspension over a sand-gravel bed. The experimental findings show that with an increase of flow velocity, the grain-size distribution of suspended load changed from a skewed form to a bimodal one at higher suspension heights. This study focuses on the determination of the parameter βn which is the ratio of the sediment diffusion coefficient to the momentum diffusion coefficient of n th grain-size. A new relationship has been proposed involving βn, the normalizing settling velocity of sediment particles and suspension height, which is applicable for widest range of normalizing settling velocity available in literature so far. A similar parameter β for calculating total suspension concentration is also developed. The classical Rouse equation is modified with βn and β and used to compute grain-size distribution and total concentration in suspension, respectively. The computed values have shown good agreement with the measured values of experimental data.

Mathematical model on grain-size distribution in suspension over sand-gravel bed

In this study, the grain-size distribution in suspension over a sand-gravel bed in an open channel turbulent flow is investigated from theoretical point of view. On the basis of diffusion equation, a model on grain-size distribution is developed incorporating the effect of stratification and hindered settling due to increased suspension concentration. The hydrodynamic diffusion related to particle–particle interaction is considered in the computation of reference level and the influence of incipient motion probability, non-ceasing probability and pick-up probability of the sediment particles are considered in the computation of reference concentration. Due to inclusion of several factors, the proposed model predicts well the grain-size distribution of suspended load when compared with the experimental data and also shows its prediction superiority with respect to other existing models.

Near bed turbulence measurement with acoustic doppler velocimeter (ADV)

Acoustic Doppler Velocimeter (ADV) has become prominent device in flow and turbulence measurement. The in situ turbulence measurement requires laboratory-quality data to be obtained in the field. Thus, ADV vector type provides a better choice for flow measurement at field scale. A field study was undertaken at sand-gravel bed rivers to observe the flow field and turbulence characteristics in the presence of protuberant clast. Profile measurements were executed at near bed region to characterize the flow and turbulence characteristics at the proximity of clast reference. Four major areas were identified as sampling location relative to clast reference namely stoss side, adjacent area, wake area and the downstream area. Besides, bed load measurement was carried out concurrently to elucidate the possible relation between flow variation and transport rates. Physical meaning of flow field is expressed in term of mean velocity, turbulent intensity, turbulent kinetic energy, Reynolds stresses and horizontal-vertical interaction, local bed shear stress and quadrant analysis. It is postulated that the presence of protuberant clast modifies flow field and affect transport rates. Possible occurrences of transport rates are subject to accurate estimation of acting forces on moving particles. Estimation of critical Shields stress from Reynolds extrapolation provides better comparison with actual transport rates. Besides, contribution rate to Reynolds stress analysis compliment the importance of sweeps and ejection events to occurrence of bed load transport.

Measurements of riverbed hydraulic conductivity in a semi-arid lowland river system (Murray–Darling Basin, Australia)

Riverbed hydraulic conductivity (Kr) was measured along one river reach in four tributaries of the Murray–Darling Basin (MDB) in south-eastern Australia. Two techniques were trialled: in-river falling-head tests in high Kr sediments, and laboratory evaporation tests on intact riverbed cores for low Kr sediments. In-river falling-head tests were conducted using two types of permeameter: a steel-base permeameter or a stand-pipe permeameter. Kr was found to range from 10–10 to 10–3 m s–1, corresponding to a range in riverbed sediment textures from clay to silty gravels, respectively. Although the within-reach variability in Kr was also large, in general the river reaches could be divided in two groups, those with a low Kr (<10–8 m s–1) or a high Kr (>10–5 m s–1). The low Kr reach (Billabong Creek) was a clay-lined bed, whereas the others had silty sand or silty gravel beds. Thus, regional-scale assessments of Kr in the MDB could be made using a stratified sampling process in which reaches would be first classified into low or high Kr classes, and then Kr measurements made in a subsample of low and high Kr reaches. This would be an improvement over the current practice whereby riverbed Kr is estimated either from regional soil maps or through the calibration of groundwater models.

A new approach to define surface/sub-surface transition in gravel beds

The vertical structure of river beds varies temporally and spatially in response to hydraulic regime, sediment mobility, grain size distribution and faunal interaction. Implicit are changes to the active layer depth and bed porosity, both critical in describing processes such as armour layer development, surface-subsurface exchange processes and siltation/ sealing. Whilst measurements of the bed surface are increasingly informed by quantitative and spatial measurement techniques (e.g., laser displacement scanning), material opacity has precluded the full 3D bed structure analysis required to accurately define the surface-subsurface transition. To overcome this problem, this paper provides magnetic resonance imaging (MRI) data of vertical bed porosity profiles. Uniform and bimodal (σ g = 2.1) sand-gravel beds are considered following restructuring under sub-threshold flow durations of 60 and 960 minutes. MRI data are compared to traditional 2.5D laser displacement scans and six robust definitions of the surface-subsurface transition are provided; these form the focus of discussion.

Estimating bedload transport in a large sand–gravel bed river from direct sampling, dune tracking and empirical formulas

Three methods used to estimate river bedload discharges were compared in a large sand–gravel bed river. The unit and total bedload transport rates estimated by sediment sampling, dune tracking method and the equations of Van Rijn and Meyer-Peter and Müller were compared. The analysis was based on a large data set obtained from field surveys that combined sediment sampling, acoustic doppler profiler (ADP) measurements, and multibeam echosoundings of the middle reach of the River Loire (France) for contrasting flow conditions ranging from low flows to 2-year floods. For transport stages between 2 and 6 (i.e. between the annual mean discharge and the discharge for a 2-year flood), the tested equations predicted fairly well the unit and total bedload discharges and roughly estimated the temporal variability of the bedload during floods. For these transport conditions, the best agreement with the sediment sampling measurements was observed with the Van Rijn formula. For lower flow conditions, the tested formulas provide lower estimates than those sampled. To improve the accuracy of the equations for low flows, potential solutions include integrating near-bed turbulences and non uniformity of bed particles into the models. The ability of the formulas to predict the temporal variability of the bedload can be improved by using the bedform roughness to adjust the estimates. The bedload transport rates calculated with dune tracking method are lower than the fluxes sampled because a substantial fraction of the bedload does not participate to the dune migration. Finally, a first rating curve that was implemented for the River Loire estimates the annual bedload discharge for 2010 to be 480,000t.

Unified View of Sediment Transport by Currents and Waves. III: Graded Beds

The problem of suspended load and bed load transport in river and coastal flows over graded beds is addressed. Two effects are important: the degree of exposure of the sediment particles of unequal size within a mixture (hiding of smaller particles resting or moving between the larger particles) and the nonlinear dependence of transport on particle diameter. The former effect can be modeled by modifying the critical bed-shear stress through a correction factor and by modifying the effective grain roughness through another correction factor. The modeling of the effective bed-shear stress parameter is studied by using various alternative methods. Based on comparison with suspended load and bed load transport data for graded beds in steady and oscillatory flow, the most promising method is selected. The proposed prediction method is found to work well for the fine sand bed range as well as the coarse sand-gravel bed range.

The response of subaqueous dunes to floods in sand and gravel bed reaches of the Dutch Rhine

Abstract The branches of the River Rhine in the Netherlands, characterized by a sand–gravel bed in the upstream part and a sand bed in the downstream part of the river system, show migrating dunes, especially during floods. In the last 20 years, these dunes have been studied extensively. High‐resolution echo‐sounding measurements of these dunes, made with single and multibeam equipment, were analysed for three different sections of the Rhine river system during several floods. This analysis was done to quantify the growth, decay and migration rates of the dunes during floods. In addition, the migrating dunes were used to calculate bedload transport rates with dune tracking. The results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods, and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain. The relations are used to show that the growth and migration rate of dunes, and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations.

Review of Sorting Out Sand and Gravel: Sediment Transport and Deposition in Sand-Gravel Bed Rivers by Maarten Kleinhans

Bedload transport in sand and gravel mixtures is difficult to describe because two media, sand and gravel, are interacting in a complex set of processes. Both materials can enhance as well as delay each other’s entrainment and transport. As a results, equations commonly employed to compute entrainment and transport of either sand or gravel alone are not applicable to sand-gravel mixtures. In his book, Maarten Kleinhans observes, measures, and analyzes processes of sand-gravel interactions in subaqueous dunes; he offers a modified Meyer-Peter and Muller transport equation and develops a model for predicting sediment stratification and bedload transport caused by dune development and decay during flow events. Given the title of the book, one might expect a wide-ranging, general treatment of rivers with sand-gravel beds. However, the book addresses only large rivers where sand content is high and gravel particles are small, and focuses on how both interact in the formation and propagation of subaqueous dunes. The interaction

Sediment Supply-Limited Bedforms in Sand-Gravel Bed Rivers

The stability of bedforms in mixtures of gravel and sand is not well understood. Two bedform types are characteristic: flow- parallel sand ribbons and flow-transverse barchans. Flume experi- ments and field data presented here show that gradual transitions exist from sand ribbons to barchans, and from barchans to fully de- veloped dunes. Barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully devel- oped ripples or dunes. The reason is that a part of the bed sediment is immobile, e.g., with an armor layer, which limits the sediment sup- ply and thus the volume of sediment available for the formation of bedforms. Bedform stability diagrams are shown to be extendable to sedi- ment supply-limited bedforms in sand-gravel sediment, if the par- ticle parameters of the diagrams are derived from the transported sediment instead of the bed sediment. Barchans and forms transi- tional to fully developed dunes plot in the dune stability fields. Sand ribbons, on the other hand, plot in the ripple, lower plane bed, and dune fields. In the case of sediment supply limitation, bedforms are partly or completely related to the characteristics of the sediment supply from upstream. The sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non-existent. Consequently, bedform char- acteristics in sand-gravel mixtures in supply-limited conditions often are not predictable from the local hydraulics and sediment charac- teristics.

Stochastic Prediction of Sediment Transport in Sand-Gravel Bed Rivers

Classical deterministic bedload transport predictors are applied to sand-gravel bed rivers. The turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion. In extending the predictors to stochastic predictors for nonuniform sediment, many parameters that represent near-bed turbulence and the particle size distribution must be chosen. The parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths. Choosing other combinations of parameters may give equal total bedload transport rates, but at the cost of large errors in fractional transport rates. Attention is given to the hiding-exposure phenomenon and a hindrance effect related to nonuni- form sediment. Validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach, while the deterministic approach gives successful predictions well above incipient motion.

The key role of fluvial dunes in transport and deposition of sand–gravel mixtures, a preliminary note

Abstract Sediment transport and deposition in sand–gravel bed rivers is shown to depend on dunes. The sediment mixture is vertically sorted in avalanches at the lee side of the dunes. Part of the resulting upward fining sets of the largest dunes that occur during a discharge wave, is preserved in the bed. Furthermore the sediment is entrained and deposited size selectively in the dune troughs, which results in an additional upward fining accumulation of lag deposits. This deposit is the source for sediment entrained during the next discharge wave, which will depend on the relict vertical sorting and on the depth from which it is entrained. The entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height. Thus subsequent discharge waves of decreasing magnitude will leave the upward fining cross-bedded sets at depths below the bed surface related to the concurrent dune height. This is demonstrated with flume experiments and with new vibrocores collected from the river Waal (The Netherlands).