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This study investigates the local scour around the cylindrical pier mounted on a gravel bed for a low coarseness ratio (d/d50), which is defined as the ratio of pier diameter d to the median particle size (d50) of the bed material. An experimental study was conducted in a flume with varying pier diameters and flow conditions. The effect of flow depth on the scour was observed for a limiting condition, i.e. the ratio of flow depth (h) to the pier diameter (d) is less than 4.33. The critical limit of h/d for affecting the scour depth for the gravel bed was found to be three times more than that of the sand bed. From the experimental results, an empirical relationship is proposed based on the dimensional analysis for the estimation of scour depth for a low coarseness range (d/d50 < 25). The proposed relationship estimates the scour depth with an error of less than ±15%, within the acceptable limit. Furthermore, it is also performing well for the coarseness ratio up to 100 with an error within the range of ± 30%. The results from this study can be useful for scour prediction and for the design of a bridge pier in a gravel bed with a low range of coarseness value. [ABSTRACT FROM AUTHOR]

The present study uses results of eddy‐resolving numerical simulations to investigate the open channel flow over large clusters of freshwater mussels (Unio elongatulus) partially buried in a rough, gravel bed. The density of the mussels forming the array varies from 26 to 500 mussels/m2. The flow structure is analyzed at large distances from the leading edge of the mussel bed, where the flow can be considered fully developed. The effects of changing the mussel bed density, the filtering discharge, the burial level and the roughness of the bed surface in which mussels are burrowed, are investigated in terms of flow field, turbulent structures, drag forces, and bed shear stresses. It is found that strong interactions occur between energetic eddies generated by the larger gravels on the exposed bed surface and by the mussel shells. Simulations results show that for a burial depth close to 50% and a ratio between the average gravel size and the mussel protruding height of 0.13, the shell induced turbulence becomes dominant for mussel bed densities around 50 mussels/m2. The influence of the bed roughness becomes less relevant with increasing mussel density, as the generation of energetic eddies is mostly controlled by mussel‐to‐mussel interactions. For fixed bed roughness, burial level and filtering velocity, the mean streamwise drag force and the associated drag coefficient for the exposed part of each mussel decrease with increasing mussel density, even if strong variations are observed for individual mussels. For constant mussel bed density and burial level, the mean streamwise drag force and the mean drag coefficient decrease slightly with increasing bed roughness. Increasing the burial level decreases not only the drag forces but also the drag coefficients because of the more streamlined shape of the top of the mussels. Strong active filtering acts toward decreasing the mean streamwise force and the mean drag coefficient. The spanwise drag forces contribute significantly to the total drag force, especially for high mussel bed densities. Based on smooth bed calculations, bed‐averaged shear stresses are reduced in highly dense clusters. Key Points: Mussel‐to‐mussel interactions are important for dense arrays and influence flow structure and turbulenceEddy resolving simulations showed that the effect of bed roughness become less significant with increasing mussel bed densityIn dense clusters of mussels, forces on the shells and bed shear stresses are reduced thus favoring mussel stability [ABSTRACT FROM AUTHOR]

In order to reveal the mechanical characteristics of gravel bed of immersed tube tunnel in heterogeneous ground, a quantitative analysis was conducted through finite element analysis and theoretical calculation on the distribution law, parameter sensitivity, and structural longitudinal stress response of the vertical and horizontal foundation settlement and stiffness of the immersed tunnel. The results show that the pressure distribution at the base of the rock foundation section is small in the middle and large on both sides, and hollowing occurs in the middle. The soft foundation section is small at both ends and large in the middle, and hollowing occurs at both ends of the structure. The base stress value of the rock foundation and the soft foundation are relatively close, but the stiffness of the foundation differs greatly, and the rock foundation section is14 times of the soft foundation section. The longitudinal stiffness of the rock foundation section is greatly affected by the parameters such as the thickness, modulus and preload of the cushion. The design thickness of the cushion layer in the rock foundation section should be used as the control standard for the entire line. The average shear force of the joint of the soft foundation section is about 3 times that of the rock foundation section. The study on the mechanical characteristics of gravel beds for immersed tube tunnels in heterogeneous ground provides an important reference significance for the promotion and application of immersed tube tunnels in complex geological conditions. [ABSTRACT FROM AUTHOR]

In view of the special geological conditions of the Dalian Bay Immersed Tunnel Project, the foundation bed is leveled using the leveling ship " Jinping 1" with full floating leveling, although the pressure of the pile legs on the foundation can be completely avoided, but to consider the influence of tides, waves, wind and other factors on the stability and leveling accuracy of the hull. It is necessary to analyze the self -stability of the leveling ship under the condition of fully floating. Through the analysis and verification of the feasibility of full floating leveling, it is concluded that the process parameters have great reference significance for similar projects in the later stage. [ABSTRACT FROM AUTHOR]

The construction of immersed tunnel is characterized by high precision requirements for the installation of pipe segments. The installation accuracy of pipe segments often depends on the accuracy of laying the gravel bed. The curve radius of pipe segments of Dalian Bay immersed tunnel is small and the curvature is large, so the traditional gravel ridge arrangement is not suitable for the installation of curved section pipe segments. In this paper, the key contents such as the length of the gravel ridge, the arrangement of the ridge body, and the position of the ship were studied, so as to successfully complete the leveling construction of the curved section of the gravel bed, and meet the requirements of the installation and docking of immersed pipes. [ABSTRACT FROM AUTHOR]

Local scour around bridge piers is the common cause of undermining of bridge foundation. Many researchers have studied local scour around bridge pier in finer sediment bed. The literature survey reveals a limited information is available on scour around a group of bridge piers with and without collar founded in gravel bed. Keeping this in view, an attempt is made to examine experimentally the efficacy of the collar around a group of three piers placed in a staggered triangular arrangement in the gravel bed of 3 mm effective size. The data on temporal variation of scour depth, scour hole characteristics, and areal extent of scour hole were collected. The collected experimental results infer very prominent information to the bridge engineers. The obtained results on equilibrium scour depth and areal extent of scour help in knowing the extent up to which bridge pier protection can be provided against scour. It is found that at longitudinal pier spacing (Z) equals to 8.45 times the pier diameter (b), the equilibrium scour depth at nose of the piers was maximum than that of at other pier spacing. At pier spacing Z/b > 8.45, the equilibrium scour depth at downstream pier without collar decreases with increasing pier spacing and reaches to minimum at Z/b = 40. At pier spacing 8.45 10. The present study reveals that bridge piers without collar became free of mutual interference effect on local scour at centre-to-centre (c/c) pier spacing of 60 times the pier diameter. It is worth mentioning that for piers with collar, the effect of mutual interference vanished at c/c longitudinal pier spacing of 20 times the pier diameter. The results show that additional shielding of collar makes the piers free from mutual interference effect at lesser spacing than the pier group without collar. This concludes that the present investigation is very useful when limited space is available between the existing and newly proposed bridges.

The present study aims to quantify experimentally the relative role of sediment depositions on near-bed flows and turbulence in a gravel-bed stream. Time-averaged velocity was measured over a sand-filled gravel-bed stream with four cases of sediment depositions and compared with those over a gravel-bed stream without sediment depositions. An acoustic Doppler velocimeter was used to measure the instantaneous velocity of flows. The progressive infilling of void spaces in the gravel-bed stream forms distinct bimodal depositions that alter the mean flows characterised by increasing zero-velocity levels and massive damping in the bed shear stresses. The data plots of turbulent intensity depict enhancement of streamwise turbulent intensity in the near-bed flow region with increasing sand depositions. Moreover, the opposite nature of streamwise and vertical turbulent kinetic energy fluxes in the interfacial sublayer leads to slowing down the time-average Reynolds shear stresses at the vicinity of the gravel-bed surface. At the vicinity of the crest, the ejection and sweep events contributed approximately 86 and 56%, respectively, to the total Reynolds shear stress production in the case of gravel bed under clear-water flow conditions. By contrast, the contributions of turbulent sweep events increased over the sand-filled gravel bed at the same location. HIGHLIGHTS Sediment particle entrainment and interstices within the pore spaces of a gravel bed.; Results are compared with the flows over an immobile gravel-bed stream under clear-water flow conditions.; A higher level of sand deposition depicts massive damping in the near-bed Reynolds shear stress profiles.;

Despite the environmental significance and ecological importance of cohesive sediment (<63 μm), improved knowledge of how effective particle size distributions (EPSDs) change due to flocculation under different conditions of shear stress and bed configuration is required to better understand in situ transport and storage properties and refine existing sediment transport models. Here, a rotating annular flume was used to (i) evaluate EPSDs under different shear stress and bed types (plane-impermeable and -porous gravel bed) for deposition and erosion experiments; (ii) assess flocculation processes with EPSDs; and (iii) compare flume and field EPSDs observations with respect to measured shear stress. While deposition experiments over the impermeable bed led to an EPSD equilibrium in all shear conditions (constant EPSD percentiles), the ingress experiment over the gravel bed resulted in varying EPSDs, and no equilibrium was observed. During the erosion experiment, deposited flocs became coarser due to bed consolidation, and no particle breakage was observed once particles were resuspended. The ingress experiment showed high efficiency in entrapping suspended particles (~95% of initial suspended sediment), and no exfiltration or resuspension was recorded. Flocculation ratios calculated using EPSDs showed negative correlations with shear stress, indicating that increasing flow energy promoted flocculation for flume and field observations. Our results showed that both suspended and bed sediments can flocculate into coarser flocs that, in turn, are preferentially ingressed and stored in the substrate when in suspension. These findings have important implications regarding legacy impacts, as substrate-stored particles can potentially extend the effects of upstream landscape disturbances. [ABSTRACT FROM AUTHOR]

Fine sediment stored in the gravel bed is an important component of river systems. Current field protocols usually allow evaluation of the silt–clay fraction of fine sediment stocks only and neglect the sand fraction. This study proposes a new protocol to quantify fine sediment stocks, including the sand fraction inside the gravel bed matrix. Fine sediment stocks were sampled within patches of 0.30 m × 0.30 m on the dry gravel bed surface, separating the surface layer and the subsurface layer. The grain‐size distribution of the samples was obtained by field sieving (10 mm, 2 mm, 500 μm and 100 μm) over a bucket, using a known volume of water. The mass of the fraction below 100 μm was measured based on the concentration within the bucket. The local stocks were then integrated over the whole river reach by assigning local stocks to facies, in which fine sediment stocks were assumed to be homogeneously distributed. The methodology was applied to a 1 km long reach of the River Galabre (Southern French Alps), characterized by significant fine sediment stocks and upstream sediment input. Results from local measurements show a large amount of sand in both surface and subsurface layers. The quantity of sand can reach up to three times the quantity of silt–clay. An estimation of porosity showed that fine material may play an important role in structuring the bed, since porosity increases with increasing fine sediment content. The potential fine sediment stock that can be resuspended due to channel migration is found to be of the same order of magnitude as the sediment budget estimated from the measured flux in the upstream hydrometric station of the studied reach. [ABSTRACT FROM AUTHOR]

Estimation of local scour depth at a bridge pier must be accurate and precise for the safety assessment of pier foundation. Many deterministic scour depth prediction models for bridge piers in sands and gravels have already been reported in the literature. However, the various types of uncertainties may be involved in the deterministic scour depth prediction models and thus affect the accuracy of the scour depth. To account for such uncertainties, a reliability analysis method is generally adopted. A sizable amount of laboratory and field data on scour has been procured in the present study to assess the performance of existing scour prediction models for gravel beds and develop a probabilistic scour prediction model using a First-Order Reliability Method (FORM) based on the object-oriented constraints optimization spreadsheet algorithm for the most appropriate selected deterministic scour prediction model. A new relationship of the silt factor has also been developed to estimate the scour depth in gravel bed as per Indian Practice. A safety factor based on the target reliability index has also been proposed in this study. [ABSTRACT FROM AUTHOR]

Turbulent flow around a circular pier on a gravel bed is investigated experimentally, using particle image velocimetry. The turbulent kinetic energy (TKE), Reynolds shear stress (RSS), and vorticity are examined to understand the effect of flow depth (h) to pier diameter (d) ratio and flow intensity. Additionally, spectrum analysis is carried out to examine the vortex intensity over various frequencies of eddies. Mixing of the horseshoe vortex and surface roller is observed in the upstream of pier for h/d<4.33. Consequently, two opposite rotating vortices reduced the turbulence strength. The critical limit of h/d on a gravel bed is found to be three times more than that of a sand bed. Both TKE and RSS increased approximately linearly with flow intensity. Further, vortex shedding frequency and energy of vortices increased with h/d which attributed to an increase in the normalized equilibrium scour depth. The present study will be resourceful for researchers in developing scour models for gravel bed by giving more insights into the flow hydrodynamics with respect to approach flow parameters. [ABSTRACT FROM AUTHOR]

The present study uses eddy‐resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially‐buried mussel (Unio elongatulus) aligned with the incoming flow. The rough‐bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed, d50, and the height of the exposed part of the mussel, h, varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near‐wake region. Increasing the bed roughness and/or reducing h intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti‐symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness. Key Points: Numerical simulations resolving flow and eddies over a gravel bed and past a partially‐buried freshwater musselThe effects of varying the burial level of the mussel, the discharge through the two mussel siphons, and the bed‐roughness height are investigatedIncreasing the bed roughness produces significant differences in the flow field, turbulence structure, and drag force compared to a smooth bed [ABSTRACT FROM AUTHOR]

A bifurcation in an engineered river system (i.e., fixed planform and width) has fewer degrees of freedom in its response to interventions and natural changes than a natural bifurcation system. Our objective is to provide insight into how a bifurcation in an engineered river responds to peak flows and human interventions. To this end, we analyze the change in hydraulics, bed level, and bed surface grain size in the region of two bifurcations in the upper Rhine delta in the Netherlands over the last century. We show that, over the last two decades, the water discharge in one bifurcate (the Waal branch) has steadily increased at the expense of the other. This gradual increase in the water discharge of the first branch is associated with its erosion rate being larger than the other branch. The quick succession of two or three peak flow events (1993, 1995, and 1998) caused rapid sediment deposition over the upstream part of the bifurcate that has gradually lost discharge, which seems to have triggered the slow change in flow partitioning. Plain Language Summary: A river bifurcation is where a river splits into two branches. Water and sediment from the upstream channel are divided between the bifurcation's downstream branches. This division is important for flood risk, freshwater supply, and navigation. Here we investigate the flow division and related changes in bed level and bed surface grain size in the bifurcation region in the upper Rhine delta in the Netherlands over the last century. We find that, over the last two decades, one branch (the Waal branch) has gradually received a larger share of the water discharge at the expense of the other branch. This gradual increase in the water discharge of the first branch seems to be associated with its erosion rate being larger than the other branch. This slow change in the division appears to be triggered by two or three successive peak flows (1993, 1995, and 1998), which caused sediment to deposit at the upstream end of the branch that subsequently started losing discharge gradually. Our analysis shows that peak flows can play a large role in bifurcation development in engineered river systems. Key Points: Water discharge in one branch at a Dutch Rhine bifurcation has slowly increased at the expense of the other bifurcateThis slow change in flow partitioning is associated with one bifurcate eroding faster than the other bifurcateSudden deposition in one bifurcate (due to rapid succession of peak flows and sediment flux coarsening) likely triggered these changes [ABSTRACT FROM AUTHOR]

Understanding the streamflow hydrodynamics is important for forecasting flooding phenomena in mountainous rivers. Accordingly, based on long‐term runoff observations obtained using diverse runoff techniques (i.e., rating curves and fluvial acoustic tomography (FAT)), the authors (1) evaluated and explored flood phenomena, (2) discussed the effects of dams on flood hydrograph, (3) and revealed backwater effects caused by river channel bends, sand bars, and floodplains. The duration and slope of the falling limb of the hydrograph were directly influenced by the catchment characteristics that affected runoff recession, whereas the behaviour of the rising limb of the hydrograph was directly influenced by rainfall. A weaker correlation between discharge and rainfall in higher flood events indicates that rainfall events were strongly altered by water released from dams in the basin. Additionally, because of the backwater effects, the rate at which the flow velocity and boundary shear stress changed with the increasing discharge was reduced, potentially slowing the pace of erosion. Our findings are relevant for managing mountain rivers, particularly in river bend reaches prone to backwater effects. The FAT method could help increase the ability to monitor short‐term fluctuations in floods and associated hazards. [ABSTRACT FROM AUTHOR]

Flow resistance in gravel-bed channels is not only affected by the shape and size of the roughness elements, but also by their arrangement on the channel bed surface (position to flow streamlines, spacing between elements, and their protrusion from the channel bed). Many investigations proved that open channel flow resistance can be obtained by integrating the power velocity profile. For a macro-scale roughness condition, this study aims to investigate the effect of different boulder arrangements on flow resistance. First, for each arrangement, the equation relating Γ function of the power velocity profile, the Froude number, and the channel slope was calibrated using available measurements performed in a flume covered by pebbles with "Random", "Transversal stripe", and "Longitudinal stripe" arrangements. For each arrangement, the experimental datasets were divided to consider the effects of boulder concentration Ch. Moreover, the relationship obtained for the "Random" arrangement and element concentration lower than 48% was tested using literature measurements performed in a flume covered by coarse elements randomly arranged. Finally, the effects of the different boulder arrangements on the flow resistance law were investigated by imposing a concentration threshold. The results demonstrated that (i) the Darcy–Weisbach friction factor can be accurately estimated by the proposed flow resistance equation, (ii) the flow resistance increases with Ch for low values (<48%) of concentration, while it does not depend on Ch for high concentrations (≥48%), and (iii) the effect of the boulder arrangement on flow resistance law is more evident for low element concentrations. [ABSTRACT FROM AUTHOR]

The −1 power region of the turbulence spectrum is a constant energy region responsible for energy production. However, its scaling and their implications are underreported. Experiment results show that the production region comprises 42% of turbulence kinetic energy for open-channel flows over fine gravel and very coarse gravel beds. Energy of the production region scales similarly for these gravel bed types, and enables its prediction using logarithmic and exponential equations. Similarity scaling is evidenced for the production region's low and high wavenumber boundaries, defined as the macroturbulence and bursting wavenumbers. The macroturbulence and bursting wavenumbers have similar functions, suggesting that the production region is controlled by the same phenomena across its extent. These results are contrary to those of previous research, in which low and high wavenumber boundaries were hypothesized to be scaled by outer and inner scales, respectively. The authors introduce the macroturbulence streampower, which is derived using the energy similarity and macroturbulence wavenumber. The macroturbulence streampower is distributed across the flow depth, is dependent on roughness element, and agrees with Bagnold's streampower near the channel half-height. [ABSTRACT FROM AUTHOR]

In the current study experiments in a valley (a composite cross-sectional tank consisting of a rectangular upper cross section and a lower trapezoidal) were performed. The purpose of this research is to study the gravel bed in gravity currents with large salinity. The tank is separated in two parts by a vertical gate, which separates the fresh water with the gravel bed from the salt water with the impermeable bed. When the gate opens, a flow is created due to density difference. The salinity densities are large (from 1,050 to 1,090 kg/m3). These specific densities have been chosen to simulate liquid wastes that affect the environment. Compared to experiments in the same tank without a gravel bed, it is observed that the gravel bed reduces the propagation of currents and mainly reduces the current with the smallest density difference at the lowest height. The main conclusion of this study is that the kind of channel bed plays a key role in the propagation of gravity currents. The gravel bed affects significantly the velocity and the distance traveled by the currents in comparison with impermeable bed. HIGHLIGHTS Lock-exchange experiments were performed in a tank of composite cross section.; The tank had a trapezoidal cross-section at its lower part and rectangular in its upper part.; It was discovered that the kind of channel plays a key role in the propagation of gravity currents.; The motion of gravity current in the case of an impermeable bed differs significantly from the case of a permeable (gravel) bed.;

Flow measuring locations and its influence on the dynamics of macro-turbulent flow structures has received substantial interest of research in gravel bed hydrodynamics. Present study aims to elucidate the near-bed turbulent flow fields and bursting events over the stoss side, crest and leeside cavities in a gravel bed stream applying double averaging (DA) methodology. Time averaged flow velocities are measured at thirty six vertical planes with an acoustic Doppler velocimeter. The DA velocity distributions for the lee side cavities resemble S-shape profile expressed by polynomial series below the crest level otherwise, velocity profile over the stoss side and crest follow the logarithmic law of wall. The DA Reynolds shear stress distributions reveal damping at the crest level of lee side cavities and compensated by enhanced DA form induced shear stress (FISS). The experimental results are further explored in order to delineate different turbulent bursting events over the stoss side, crest and lee sides. The quadrant analysis of DA Reynolds shear stress production confirmed that within the interfacial sub-layer the sweep is the dominant event towards the RSS production. [ABSTRACT FROM AUTHOR]

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. HIGHLIGHTS Water velocity is higher at the near bed in a sand bed channel.; A gravel bed channel increases the velocity fluctuations.; Flow momentum exchange is higher in a gravel bed channel.; Riverbeds are usually composed of sand and gravel, therefore analyzing the turbulent characteristics of flow is important.; There are benefits for the river engineer since the turbulence governs the morphological changes in the riverbeds.;

The wadis in Algeria organize the beds as gravel by transporting fine sediments form through hydrosedimentary dynamic of intermittent flood events along the time. In this paper, a new flow resistance equation is proposed for gravel bed bedload regime. Due to the field measurements lack in wadis, the study is based on 2,147 flume data with friction factor measurements. The development of convenient equation is achieved through theoretical analysis by taking into account wadi dominant parameters based on dimensional analysis. Hence, deduced relationships are considered between friction coefficient f, the flow characteristics, bed grains size, geometric parameter and bed slope. The proposed equation gives satisfactory results; the estimated mean normalized error is less than 17%. [ABSTRACT FROM AUTHOR]

Present study aims to illustrate the role of form-induced shear stress on turbulent kinetic energy (TKE) budget distributions in permeable gravel bed streams. In order to achieve the objective of this study, some significant experimental works on turbulent flow characteristics over rough beds are reviewed first and compared with the results of present studies. Uniform gravels (d50 = 4 cm) are arranged randomly in five layers on a 15 cm long open channel. The bed topography and porosity induces spatially heterogeneous flow fields in the near-bed region and to resolve the spatial heterogeneity of flows above and within the flow-gravel-bed interface the traditional time-averaging approach is supplemented by the double-averaging (DA) concept. The form induced shear stress (FISS) depicts significant role in order to obtain total turbulent stress and found to be 40% of the DA Reynolds shear stress (RSS) occurring at the virtual bed level. Also, TKE budget distributions reveal that the TKE production, dissipation and pressure energy diffusion attains maximum values above the virtual bed level and decreases sharply below it. Importantly, the pressure energy diffusion shows a negative magnitude below the gravel crest. Moreover, the TKE dissipation rate is considerably smaller than its production rate above and within the flow-gravel-bed interface. [ABSTRACT FROM AUTHOR]

We use flume experiments to better understand how gravel‐bed channels maintain bed surface stability in response to pulses of sediment supply. Bed elevations and surface imagery at high spatial resolutions were used to quantify the co‐evolution of surface grain‐size distribution (GSD), bed roughness statistics, and bed surface structures (clusters, cells and transverse features). Using a new semi‐automated method, we identified individual stone structures over a 2 m × 1 m area throughout the experiments. After an initial coarsening, surface GSD and armouring ratio remained nearly stable as sediment pulses caused net bed aggradation. In contrast, individual grain structures continued to form, increase or decrease in size, and disappear throughout the experiments. The response of the bed to sediment pulses depended on the history of surface roughness evolution and bed surface structure development, as these factors changed much more in response to supply perturbations earlier in the experiments compared to later, even as the bed continued to aggrade. We interpret that the dynamic production and destruction of bed surface structures can act as a 'buffer' to sediment supply pulses, maintaining a stable bed surface during aggradation with minimal change in grain size or armouring. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Gravel riverbeds naturally present a range of sediment size that can span several orders of magnitude. However, fine sediment (i.e., size <2 mm) additions affect sediment transport and are potentially harmful to the river ecology. Limited research has been done to understand the effects of sand addition on gravel bed topography. For this study, we conducted flume experiments testing the effectiveness of topography remote sensing to measure the response of a water‐worked gravel bed to varying sand additions and bed flushing. Repeated measurements of the bed topography with through‐water photogrammetry show that sand deposits primarily in low‐lying areas of the bed, where it is sheltered. Observed changes in bed elevation standard deviation, skewness, and inclination index indicate a smoother bed, as sand fills the depressions on the surface and conceals gravel imbrication. The overall bed morphology (e.g., small bedforms and coarse grain arrangement), however, is preserved. Increasing the sand supply resulted in a gravel bed buried by sand and the formation of sand dunes. Bed flushing with a flow sufficient to move sand allowed a return of the bed roughness to the prepulse conditions, while flushing the bed with a discharge equivalent to the armoring discharge reworked the normally stable surface layer without removing infiltrated sand from the subsurface. This new data set contributes to providing missing information on the small‐scale topographic effects of sand addition upon gravel beds. It might be used with measurements of near‐bed flow hydraulics obtained in other studies to explain variations of sediment transport. Key Points: The topographic imprint of sand addition and bed flushing on water‐worked gravel beds is analyzed using high‐resolution photogrammetrySand addition affects the roughness characteristics such as bed elevation standard deviation, skewness, and inclination indexBed flushing can clean the gravel bed surface from sand, with a return of roughness parameters to prepulse conditions [ABSTRACT FROM AUTHOR]

Terrestrial laser scanning (TLS) and structure-from-motion photogrammetry (SfMp) offer rapid, non-invasive surveying of in situ gravels. Numerous studies have used the point clouds derived from TLS or SfMp to quantify surface layer characteristics, but direct comparison of the methods for grain-scale analysis has received relatively little attention to date. Comparing equivalent products of different data capture methods is critical as differences in errors and sampling biases between the two methods may produce different outputs, effecting further analysis. The sampling biases and errors related to SfMp and TLS lead to differences in the point clouds produced by each method. The metrics derived from the point clouds are therefore likely to differ, potentially leading to different inputs for entrainment threshold models, different trends in surface layer development being identified and different trajectories for physical processes and habitat quality being predicted. This paper provides a direct comparison between TLS and SfMp surveys of an exposed gravel bar for three different survey periods following inundation and reworking of the bar surface during high flow events. The point clouds derived from the two methods are used to describe changes in the character of the surface layer between bar inundation events, and comparisons are made with descriptions derived from conventional pebble counts. The results found differences in the metrics derived using each method do exist, but the grid resolution used to detrend the surfaces and identify spatial variations in surface layer characteristics had a greater impact than survey method. Further research is required to understand the significance of these variations for quantifying surface texture and structure and for predicting entrainment thresholds and transport rates. [ABSTRACT FROM AUTHOR]

The Hongkong-Zhuhai-Macao bridge island & tunnel engineering encountered sudden sedimentation problems in the installation of immersed pipes. To overcome the engineering difficulties, a set of desilting device is added to the existing platform riprapping leveling vessel, which can quickly and efficiently solve the problem of gravel bed hack silting, and become the only platform desilting leveling vessel in the world. The desilting construction has achieved gratifying results and has become another important tool in the island and tunnel engineering construction of the Hongkong-Zhuhai-Macao Bridge. [ABSTRACT FROM AUTHOR]

Highlights • Traveling surface waves drive solute into a gravel bed more quickly and deeply than in wave-free conditions. • Hyporheic exchange in the presence of traveling surface waves is more dispersive in character than without waves. • Wave-induced pressure fluctuations at the water-sediment interface are a likely cause of the apparent dispersive behavior. Abstract Solute exchange between the pore water of a streambed and overlying flowing surface water can have important effects on the chemical mass balance and biological productivity in the aquatic environment. The base mechanism for driving this hyporheic exchange in a moving current is turbulence at the water- sediment interface and the water surface slope. Many previous studies have shown that the nature of the exchange can be significantly modified when spatial pressure variations, induced by standing surface waves or bed forms, are present in the system. In contrast to these studies, we consider how transient pressure variations induced by moving waves modify hypothetic exchange. This is achieved experimentally by using a vertical array of electrical conductivity probes to track the movement of a conservative solute tracer in the gravel bed of a recirculating laboratory flume under a variety of flow conditions. Our experimental observations indicate that the addition of traveling waves greatly increases the initial rate and depth of solute penetration into the bed. When the analytical solution of a one-dimensional (vertical) advection–dispersion model is fit to the experimental measurements it appears that the presence of traveling surface waves results in a more dispersive hyporheic transport. Through experimental observation, model fitting, and direct simulation we demonstrate that this apparent dispersion dominated transport is due to the vertical "pumping" of interstitial fluid in the presence of traveling surface waves. From these simulations we develop a relation that for a given set of flow and wave characteristics allows us to estimate the apparent dispersion due to traveling waves. [ABSTRACT FROM AUTHOR]

Previous studies showed that integrating a power velocity profile, deduced applying dimensional analysis and the incomplete self-similarity condition, the flow resistance equation for open channel flow can be obtained. At first, in this paper the relationship between the Γ function of the power velocity profile, the channel slope and the Froude number, which was already empirically introduced in a previous paper, is now theoretically deduced. Then this relationship is calibrated using the field measurements of flow velocity, water depth and bed slope carried out in 101 reaches of gravel bed rivers available by literature. The proposed relationship for estimating Γ function and the theoretical flow resistance equation are also tested by an independent dataset of 104 reaches of some gravel bed rivers (Fiumare) in Calabria region. Finally, the theoretically-based relationship for estimating the Γ function is calibrated by the overall available database (205 reaches). In this way the three coefficients of the theoretically based Γ function are estimated for a wide range of slopes (0.1%-6.19%) and hydraulic conditions (Froude number values ranging from 0.08 to 1.25). In conclusion, the analysis shows that the Darcy-Weisbach friction factor for gravel bed rivers can be accurately estimated by the approach based on a power-velocity profile and the theoretically-based relationship proposed for estimating Γ function. The analysis also points out a performance in estimating mean flow velocity better than that obtained in a previous study carried out by the authors.

The foundation should be treated before the installation of the immersed tube, and the laying of the gravel bed is a key process. The control precision of the gravel bed laying directly affects the installation precision of the immersed tube in the later stage, and it is also the guarantee procedure for the successful installation of the immersed tube. Combined with the IIZMB island and tunnel project, we expounded the various control technologies used in the laying of the gravel bed, so as to meet the requirements of the high precision docking of the immersed tube, which has a certain reference significance for the subsequent offshore deep-water projects. [ABSTRACT FROM AUTHOR]

In order to study llie feasibility of first bedding method of the gravel bed and the silling capacity of the gravel bed in Shenzhen-Zhongshan passageway project, we carried out the gravel bed leveling and siltation observation tests in the test trench of Shenzhen-Zhongshan passageway. In this paper, we mainly introduced the leveling of the base bed with different gradations and the furrow width in the test tunnel, and used the silting box, scuba diving exploration and multi beam to monitoring the siltation, and then studied and analyzed the siltation strength of the foundation trench, the silting capacity of the gravel bed and the effect of the flat vessel inserting pile on the stability of the slope. Through the experimental study, it is shown that the silting capacity of different graded gravel beds is more than 13 days. The gravel bed scheme of the first bedding method is feasible in the Shenzhen-Zhongshan passageway, the silting capacity of the gravel bed can meet the requirements of the period of the installation window of the immersed tube, and has great guiding significance for the design and construction of the later cushion plan. [ABSTRACT FROM AUTHOR]

Low submergence is usually observed over a channel bed of a steep slope. It is equivalent to large-scale roughness characterized by high ratios of grain diameter to flow depth, d50/h. This study shows that the critical Shields stress θc for incipient sediment motion can be theoretically formulated either as a function of d50/h or the channel bed slope S for fully rough beds. Different from previous studies, the derivation is conducted by involving the friction factor that applies for open-channel flows subjected to large-scale roughness. The analysis also takes into account other factors including grain density, sediment uniformity, and intergrain friction. The results show that for natural gravel beds, if d50/h<1 or S<0.6 , the incipient sediment motion is controlled by the bed resistance and therefore θc increases with increasing d50/h or S. Otherwise, the incipient motion is dominantly driven by the streamwise component of the grain gravity, which causes a reduction in θc with increasing d50/h or S. The theoretical formula agrees reasonably with experimental data collected from eight sources in the literature. [ABSTRACT FROM AUTHOR]

Previous studies showed that integrating a power velocity profile, deduced applying dimensional analysis and the incomplete self-similarity condition, the flow resistance equation for open channel flow can be obtained. At first, in this paper the relationship between the Γ function of the power velocity profile, the channel slope and the Froude number, which was already empirically introduced in a previous paper, is now theoretically deduced. Then this relationship is calibrated using the field measurements of flow velocity, water depth and bed slope carried out in 101 reaches of gravel bed rivers available by literature. The proposed relationship for estimating Γ function and the theoretical flow resistance equation are also tested by an independent dataset of 104 reaches of some gravel bed rivers (Fiumare) in Calabria region. Finally, the theoretically-based relationship for estimating the Γ function is calibrated by the overall available database (205 reaches). In this way the three coefficients of the theoretically based Γ function are estimated for a wide range of slopes (0.1%-6.19%) and hydraulic conditions (Froude number values ranging from 0.08 to 1.25). In conclusion, the analysis shows that the Darcy-Weisbach friction factor for gravel bed rivers can be accurately estimated by the approach based on a power-velocity profile and the theoretically- based relationship proposed for estimating Γ function. The analysis also points out a performance in estimating mean flow velocity better than that obtained in a previous study carried out by the authors. [ABSTRACT FROM AUTHOR]

In this paper a new flow resistance equation for open channel flow, based on the integration of a power velocity profile, was tested for gravel bed channels. First this flow resistance equation, theoretically deduced by dimensional analysis and incomplete self-similarity condition, was reported. Then a relationship between the Γ function of the velocity profile, the channel slope and the Froude number was calibrated by the available laboratory measurements of flow velocity, water depth and bed slope carried out in 416 flume experimental runs with a gravel bed. Then the relationship for estimating Γ function and the theoretical resistance equation was tested by 83 independent flume measurements. The analysis also showed that the proposed flow resistance equation allows an estimate of the Darcy-Weisbach friction factor which is more reliable and accurate than that obtained by a semi-logarithmic flow resistance law or a variable-power resistance equation, calibrated with the same gravel bed measurements. For testing the applicability of the proposed Γ function (Eq. (17)), whose coefficients were estimated by flume measurements, available fields measurements were used. The analysis demonstrated that a scale factor (equal to 0.7611) is necessary to convert Γ values obtained by flume measurements into those corresponding to gravel bed rivers. The similitude between flow resistance in a gravel bed flume and in a gravel bed river is governed by the Γ function and a scale factor, equal to 1.6, is required to upscale the Darcy-Weisbach friction factor values obtained by flume measurements to the river case. In conclusion, the analysis showed that the Darcy-Weisbach friction factor for gravel bed channels can be accurately estimated by the proposed theoretical approach based on a power-velocity profile. [ABSTRACT FROM AUTHOR]

A simple model for the front of unimpeded infiltrated sand, moving as sediment transport through immobile coarse gravel particles is proposed. The theoretical celerity of the front computed by the model matched the celerity in novel experiments made in a laboratory flume. The dimensionless infiltrated sand load through gravel, estimated from the present experiments and published data, is a function of the Shields parameter and the ratio of the sand front height minus the gravel height to the gravel diameter. No effect of the ratio of sand and gravel diameters was observed. Finally, the experimental bed resistance relationship changes from hydraulically macro-rough of the gravel particles, when the infiltrated sand front is much less than the gravel layer, to a mixture of the sand and gravel roughness, when the sand layer is on the same order of magnitude of the gravel. A new resistance law is proposed. [ABSTRACT FROM AUTHOR]

Equilibrium or regime models based on a single formative (channel‐forming) discharge have been instrumental in developing a quantitative understanding of river channel dynamics. However, alternative paradigms can be used to ask fundamentally different questions about river channel behavior. In this paper, we present the Stochastic Channel Simulator (STOCHASIM), a simple biogeomorphic model that models the interplay between erosion and vegetation encroachment through changes in channel geometry. Results for a range of flood distributions are compared to predictions from a similar model based upon a traditional regime approach. Flood variability strongly influences the mean channel geometry and channel stability; the regime model and the stochastic model predict the same channel width when flow variability is low but diverge as flow variability increases. The return period of the formative flow required to match the geometry generated by a traditional regime approach increases systematically, from about 2 years for flow regimes in humid regions to nearly 8 years for more variable flow regimes, like those typical of arid regions. While the traditional regime approach provides a reasonable simplification for streams with little variability in the flood distribution, stochastic modeling may provide more realistic estimates of channel size as flood variability increases (e.g., in arid streams or small watersheds). The success of STOCHASIM in replicating realistic patterns of erosion, as well as the historical contingency often observed in natural streams, suggests that adopting a stochastic dynamics paradigm could advance geomorphology, just as it has done in hydrology, ecology, and other natural sciences. Key Points: Unlike traditional deterministic models, stochastic models can be used to explore the effects of variability in key governing parametersStreams with higher variability in year‐to‐year flood size have higher rates of bank erosion and a wider average channel geometryTraditional regime models are likely to underpredict the width of streams with high flow variability and fail to characterize bank erosion [ABSTRACT FROM AUTHOR]

The friction coefficient between the reinforced concrete structure immersed tube and gravel bed has not been confirmed in the actual operation process, mostly just simulated data of physical model test. Combined with the data such as current forces, cable forces in shipboard, pulling in jack of the pull-in construction of pipe sections, we calculated the friction coefficient between the reinforced concrete structure immersed tube and gravel bed, which provides the reference for domestic immersed tube project. [ABSTRACT FROM AUTHOR]

Abstract: The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

This paper presents basic tests to develop an airborne photogrammetric methodology that derives grain size characteristics of gravel bed rivers. The data acquisition was done using a lightweight action cam and a hand-held digital single lens reflex camera. Image processing comprised the structure from motion technique and multiview-stereo algorithms to obtain digital elevation models of non-cohesive gravel beds. Laboratory results indicate that the method accuracy is about four to six times lower than laser-scan data when based on action cam data. The accuracy of digital elevation models computed via photos taken by the reflex camera is almost of the same range as the laser data. Field experiments were done to test the performance of image based gravelometry against manual surface sampling. For this application the action cam was mounted to a low-cost quadrocopter, while the reflex camera was operated by hand. Results indicate that this combination has a high potential to generate data from which characteristic grain size parameters can be estimated. [ABSTRACT FROM AUTHOR]

Abstract: Pulses of fine sediment composed of sand, silt, and clay can be introduced to gravel bed rivers through runoff from burn‐impacted hillslopes, landslides, bank failure, or the introduction of reservoir sediment as a result of sluicing or dam decommissioning. Here we present a study aimed at quantifying exchange between suspensions of clay and gravel beds. The questions that motivate the work are: how do bed roughness and pore space characteristics, shear velocity ( u ∗), and initial concentration (C0) affect clay deposition on or within gravel beds? Where does deposition within these beds occur, and can deposited clay be resuspended while the gravel is immobile? We examine these questions in a laboratory flume using acrylic, open‐framework gravel, and armored sand‐gravel beds under conditions of varying u ∗ and C0. Deposition of clay occurred to all beds (even with Rouse numbers ∼ 0.01). We attribute deposition under full suspension conditions to be an outcome of localized protected zones where clay can settle and available pore space in the bed. For smooth wall cases, protection came from the viscous wall region and the development of bed forms; for the rough beds, protection came from separation zones and low‐velocity pore spaces. Bed porosity was the strongest influencer of nondimensional deposition rate; deposition increased with porosity. Deposition was inversely related to u ∗ for the acrylic bed runs; no influence of u ∗ was found for the porous bed runs. Increases in discharge resulted in resuspension of clay from acrylic beds; no resuspension was observed in the porous bed runs. [ABSTRACT FROM AUTHOR]

• The density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD). • Keystones tend towards a random spatial distribution regardless of their initial configuration. • Sediment transport has a positive correlation to the proportion of dislodged keystones. Large particles strongly influence flow resistance, energy dissipation, bed stability, and channel morphology in mountain streams. We conducted flume experiments to evaluate effects of the density and spatial distribution of coarse particles (i.e., keystones) on bed stability, surface texture and sediment transport in steep channels. Keystones were placed on the channel bed surface at the beginning of each experiment, both in clustered and in uniformly spaced (i.e., anti-clustered) configurations and in different numbers (i.e., for different values of keystone density). The flow rate was increased by 20 % every hour and each experiment continued until the bed was completely scoured. Sediment transport was measured with a sediment trap located at the flume outlet. Topographic data from high-resolution surface imagery and second-order structure functions (SSF) of bed elevations, were used to characterize bed surface structuring while the Ripley's K function was used to assess the degree of clustering of keystones relative to random spatial distributions. Our results indicate that (1) the density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD), (2) for increasing flow rates, the spatial distribution of keystones naturally evolves towards a random distribution, regardless of the initial spatial arrangment, and (3) sediment transport has a higher correlation with the proportion of dislodged keystones than with flow discharge. [ABSTRACT FROM AUTHOR]