Maximum Bed Shear Stress Research Articles

Understanding the Role of Sharp Edges in the Propagation of Surface Gravity Waves

AbstractUltra‐rough oceanic surfaces, such as oyster reefs, are characterized by densely‐packed, sharp‐edged roughness elements that induce high frictional resistance on the ambient flows. To effectively employ, for example, oyster reefs as a nature‐based solution in coastal protection, a detailed understanding of the frictional wave energy dissipation processes is necessary. This work reports on an experimental study in which six surrogates of very to ultra‐rough oceanic bed surfaces were subjected to regular waves. The influences of different sharpness' of roughness elements (bluntly‐shaped, sharp‐edged, and a combination thereof) and relative spacing between elements compared to the near‐bed horizontal excursion amplitude, λ/ab, on the wave attenuation have been investigated. Turbulence is 2–27 times larger for sharp‐edged surfaces and 1 to 18 times larger for mix surfaces than those of bluntly‐shaped surfaces. Maximum bed shear stresses, hydraulic roughness lengths, and wave friction factors are likewise significantly larger for sharp‐edged compared to bluntly‐shaped surfaces. These observations indicate that the sharp edges are crucial for frictional energy dissipation. Comparing the maximum bed shear stresses determined from wave height reductions to those determined from velocity measurements indicates that in addition to turbulent kinetic energy (TKE), periodic form‐induced stresses significantly contribute to the overall bed shear stresses. This study provides new insight into the frictional dissipation processes of oscillating flows encountering ultra‐rough surfaces.

Processes Towards Sedimentation around a Reed Island in a Shallow Lake

There are many studies on the hydromorphological impacts of emergent aquatic vegetation, but many of these focus on gravity-driven flows in riverine or tidal environments. However, an added effect of emergent vegetation in lakes is that it reduces the main external force (wind) significantly and in a spatially coherent way. We performed long Monte Carlo simulations using a 2D hydrodynamic model and a spectral wind wave model to study resuspension dynamics around a 100-m-wide reed island in southern Lake Fertő/Neusiedl. Wave-current interaction, computed in a post-processing step, was found to enhance maximum bed shear stresses only on the leeward side of the reed island, and even there to a small extent. We have found evidence that the present bed topography is close to an equilibrium where simulated combined wave-current shear stresses have a reasonably uniform exceedance probability over the critical shear stress of the bed sediment. We also found that an artificially flattened lakebed around the reed island would start evolving towards the present bed topography and grain size distribution (both of which were surveyed in the field). Despite the limitations of our modelling framework, our results illustrate the importance of how wind climate is translated to an uneven distribution of sediment entrainment around the reed island, explaining tendencies of sediment accumulation and sorting. Our results lead to a better understanding of the complicated processes involving interaction among wind, vegetation, circulation, wind waves, sediment and bathymetry.

Numerical Investigation of Bed Shear Stress and Roughness Coefficient Distribution in a Sharp Open Channel Bend

The helical flow leads to the redistribution of the bed shear stress and roughness coefficient in open-channel bends, influencing the transport of sediment and riverbed evolution process. To explore the mechanisms underlying this redistribution, a 3D numerical simulation of a 180° sharp bend was performed by solving the Reynolds-averaged Navier–Stokes (RANS) equations using the Reynolds stress equation model (RSM) as the anisotropic turbulence closure approach. The results indicate that the high bed shear stress zone appeared in the inner bank region from the 50° to 110° sections, and the section maximum bed shear stress gradually shifted outwards, owing to the advective momentum transport by the circulation cells. The quantitative analyses of the terms in depth-averaged Navier-Stokes equations indicate that the contributions of the cross-stream circulation and cross-flow to the downstream bed shear stress were of the same order of magnitude, and the overall contribution rate of the cross-stream circulation was 20%. The contribution rates of the cross-flow, cross-stream circulation, turbulence, and pressure gradient to the transverse bed shear stress were approximately 30%, 7%, 3%, and 60%, respectively, indicating that the pressure gradient term arising from the transverse water surface slope played a dominant role. The Chezy resistance coefficient showed an overall decreasing trend along the bend. Therefore, an effective expression considering the streamwise variation along the centreline and transverse variation was successfully established to predict the uneven distribution of the Chezy resistance coefficient.

The effect of building arrangement on the flow characteristics in meandering compound channels

Due to the fact that most of the cities and villages are built on the banks of the rivers, the hydraulic characteristics of water flow always undergo changes during floods, so it is necessary to study the interaction of the flood flows of meandering compound channel in these areas. In this paper, the effect of building arrangement on the flow characteristics in meandering compound channels was investigated by using laboratory models. For this purpose, a smooth floodplain (without building) and three different types of building arrangement on the floodplain including structural obstacles perpendicular and parallel to the floodplain flow and checkered structural obstacles have been used. The tests have been conducted at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. All experiments were performed under a quasi-uniform and fully developed flow condition. The measurement data were recorded at different sections along the meandering main channel. Stage-discharge curves, Manning's n, velocity distribution, boundary shear stress and Darcy-Weisbach friction factor are determined over 12 test runs. The results of this research show that the arrangement of buildings has a great effect on the longitudinal velocity of the main channel. At a relative depth of 0.49, in a meandering compound channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the dimensionless longitudinal velocity of the main channel compared to the smooth floodplain increases by 113%, but with the arrangement of structural obstacles parallel to the floodplain flow, the dimensionless longitudinal velocity increases by 27%. Also, the building arrangement is effective in reducing the discharge capacity of the meandering compound channel, so that in a building density of 18.5%, structural obstacles perpendicular to the floodplain flow reduce the discharge capacity of the channel by 50% compared to the smooth floodplain. In the channel with structural obstacles arranged perpendicular to the floodplain flow, Manning's roughness coefficient increases sharply with increasing the relative depth, but in the channel with the structural obstacles parallel to the floodplain flow, Manning's roughness coefficient decreases with increasing the relative depth. In the channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the bed shear stress of the main channel is higher than the other building arrangements, so that at the relative depth of 0.49, the average maximum bed shear stress in all sections increases by 42% compared to the smooth floodplain.

Numerical Study of Road Embankment Type Action on Shear Stress Around Skewed Bridge Abutment

This paper investigates the effect of different geometries of approach embankments and guides banks on the flow pattern and bed shear stress values in the skewed bridges in the compound channel using three-dimensional numerical modelling. First, the numerical model was evaluated based on the results of existing laboratory studies. After ensuring its proper performance, the elliptical guide bank and the three types of abutments: vertical-wall, spill-through, and wing-wall, at different skew angles are examined. Investigation of the values of maximum velocity and bed shear stress at the flow-conducting embankment and the flow-splitting embankment showed that in the flow-conducting embankment, the best performance is assigned to the elliptical guide bank. In contrast, the performance of various abutments is different for the flow-splitting embankment depending on the skew angle of the bridge. Then, different patterns based on streamlines for the geometry plan of the guide bank were proposed and studied. The results show that the most suitable pattern for the guide bank reduces the maximum flow velocity by up to 15% and reduces the maximum bed shear stress by up to 80% around the flow-splitting embankments.

An experimental evaluation of the blocks in floodplain on hydraulic characteristics of flow in a meandering compound channel

• Flow behavior in meandering channels with 2-sided blocks in floodplains examined experimentally. • The presence of blocks in floodplains significantly enhances Q mc / Q ). • In cases with blocks densities of 6.2% and less, the values of Manning's n decrease by increasing Dr. • For all cases of blocks densities, the maximum bed shear stress occures near the convex side of CS1. • With high blocks density, apparent lateral velocity gradients seen in central region of CS3-CS5. In this research, the main flow behaviors in the meandering compound channels with different densities of two side blocks on the floodplains were investigated by using laboratory experiments at the three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the blocks density on the floodplain has a significant effect on the flow behavior in the main channel. So that the smooth channel efficiently conveys 100% more water on average than that conveyed in the case with maximum blocks density. For the case with smooth floodplains, the Q mc /Q ratios of main channel discharge to total discharge increases continuously by reducing the flow depth, reaching 47% for Dr = 0.29 (Dr is the relative depth). By contrast, Q mc /Q in the channel with maximum blocks density peaks at 70% (>50%), showing that the main channel controls the capacity of compound channel conveyance because the blocks occupy most of the cross-sectional area of the floodplain when Dr = 0.29. In cases with blocks densities of 6.2% and less in floodplain, the Manning n decreases by increasing the relative depth (Dr). In cases with blocks densities of 9.7% and 23.7%, the Manning n increases by increment the relative depth. In addition, in cross-over sections of the case with smooth floodplains, a steep gradient of velocity in the vertical direction appears from the dense contour lines around the bankfull level near the right-hand sidewall indicating that a considerable effect occurs from the upstream flow. Instead, for channel with maximum blocks density, an apparent lateral velocity gradient was located in the central region in cross-over sections. The maximum shear stress of the bed at the apex section for all channels occurs near the convex side and the amount of bed shear stress at all sections decreases with reducing the relative depth. In the cross-over sections, due to the increase of the flow angle between the main channel and the floodplain, the effect of the secondary flow is stronger and the flow velocity decreases more, so the Darcy–Weisbach coefficient increases more in this area.

The Erosional and Depositional Potential of Holocene Tibetan Megafloods Through the Yarlung Tsangpo Gorge, Eastern Himalaya: Insights From 2D Hydraulic Simulations

AbstractProfound effects of episodic megafloods (≥106 m3/s) have been observed on Earth and Mars. Quaternary megafloods sourced from valley‐blocking glaciers on the Tibetan Plateau likely play an important role in the geomorphic evolution of the Yarlung‐Tsangpo Gorge and mountain landscape of the eastern Himalaya. We use the first 2D numerical simulation of a megaflood sourced from a reconstructed 81 km3 Tibetan lake to analyze flood hydraulics and examine the erosional and depositional potential of megafloods in mountain landscapes. The simulated flood has a duration >60 hr and a peak discharge of 3.1 × 106 m3/s. We find that the extent of inundated features like terraces, narrow valley sections, tight meander bends, and overtopped ridges influences locations of observed maximum depth (370 m), speed (76 m/s), and bed shear stress (>100 kPa), creating dynamic patterns of erosive potential. Consequently, it is difficult to predict local (≤1 km) patterns of megaflood erosional potential from either unit stream power or flood power from smaller magnitude outburst floods. However, both are useful when predicting regional (≥25 km) order‐of‐magnitude shifts in megaflood flood power. Portions of the flood domain downstream of the Gorge experience lower bed shear stresses and flood power <5 kW/m2, indicating potential for significant deposition. We suggest widespread deposition of boulders within the modern channel and fine‐grained particles on hillslopes during a megaflood likely impedes subsequent erosion and affects channel width and longitudinal form throughout the flood pathway. Our findings show the legacy of megaflooding in mountainous terrain includes both extensive erosion and deposition.

On the distribution of sand banks in the southwestern North Sea and an approach of predicting them

The morphology of the southwestern North Sea is greatly influenced by sand banks. With this paper these features have been identified by analysing bathymetric data and estimating their geometric properties. This allowed an assessment of the dimensions of the structures and enabled an economical evaluation of the associated sand resources. In fact, there are several aspects that are of economic interest, which include the sand as a potential exploitable resource and the interaction of the sand banks with submarine infrastructures, such as cables, pipelines and shipping channels. Therefore, this study aimed to evaluate and compare several oceanographic and sedimentary factors on the formation and distribution of the sand banks in the southwestern North Sea to create a prediction model for the bank occurance. The main focus was on the relation between critical shear stress of the bedload and the shear stress of currents and waves and it's effects on the sand bank formation. It has been figured out that the maximum bed shear stress needs to exceed the critical shear stress by a factor of nine to allow sand bank formation to a greater extend. Thus, this study contributes new knowledge about sand banks in the southern North Sea.

Refined hydro-environmental modelling for tidal energy generation: West Somerset Lagoon case study

An accurate assessment of the hydro-environmental impacts of tidal range energy schemes, where the performance of the scheme has an impact on the marine environment and ecology, is crucial in optimising the design and development of such schemes. A proposal for a new coastally-attached impoundment, namely West Somerset Lagoon, has been investigated in this research and the numerical model TELEMAC-2D has been refined to model theimpacts of this scheme on the Bristol Channel and Severn Estuary. Domain decomposition was applied and full momentum conservation between the subdomains was included in the model by implementing momentum source terms at the turbine locations. The results have confirmed the importance of including full momentum conservation in modelling the effects of turbo-machinery in tidal lagoons. It was found that the operation of the scheme decreased the high water level slightly in the Bristol Channel and Severn Estuary, while there was a decrease in the low intertidal areas. The maximum velocity and bed shear stress were predicted to increase in the inner Bristol Channel, while they decreased noticeably across most of the interior of the lagoon, away from the turbine wakes. Furthermore, the operation of the lagoon significantly improved the water renewal in the region.

Numerical study of the dam-break waves and Favre waves down sloped wet rigid-bed at laboratory scale

The bed slope and the tailwater depth are two important ones among the factors that affect the propagation of the dam-break flood and Favre waves. Most previous studies have only focused on the macroscopic characteristics of the dam-break flows or Favre waves under the condition of horizontal bed, rather than the internal movement characteristics in sloped channel. The present study applies two numerical models, namely, large eddy simulation (LES) and shallow water equations (SWEs) models embedded in the CFD software package FLOW-3D to analyze the internal movement characteristics of the dam-break flows and Favre waves, such as water level, the velocity distribution, the fluid particles acceleration and the bed shear stress, under the different bed slopes and water depth ratios. The results under the conditions considered in this study show that there is a flow state transition in the flow evolution for the steep bed slope even in water depth ratio α = 0.1 (α is the ratio of the tailwater depth to the reservoir water depth). The flow state transition shows that the wavefront changes from a breaking state to undular. Such flow transition is not observed for the horizontal slope and mild bed slope. The existence of the Favre waves leads to a significant increase of the vertical velocity and the vertical acceleration. In this situation, the SWEs model has poor prediction. Analysis reveals that the variation of the maximum bed shear stress is affected by both the bed slope and tailwater depth. Under the same bed slope (e.g., S0 = 0.02), the maximum bed shear stress position develops downstream of the dam when α = 0.1, while it develops towards the end of the reservoir when α = 0.7. For the same water depth ratio (e.g., α = 0.7), the maximum bed shear stress position always locates within the reservoir at S0 = 0.02, while it appears in the downstream of the dam for S0 = 0 and 0.003 after the flow evolves for a while. The comparison between the numerical simulation and experimental measurements shows that the LES model can predict the internal movement characteristics with satisfactory accuracy. This study improves the understanding of the effect of both the bed slope and the tailwater depth on the internal movement characteristics of the dam-break flows and Favre waves, which also provides a valuable reference for determining the flood embankment height and designing the channel bed anti-scouring facility.

Ligand and precatalyst offerings expand

The bed slope and the tailwater depth are two important ones among the factors that affect the propagation of the dam-break flood and Favre waves. Most previous studies have only focused on the macroscopic characteristics of the dam-break flows or Favre waves under the condition of horizontal bed, rather than the internal movement characteristics in sloped channel. The present study applies two numerical models, namely, large eddy simulation (LES) and shallow water equations (SWEs) models embedded in the CFD software package FLOW-3D to analyze the internal movement characteristics of the dam-break flows and Favre waves, such as water level, the velocity distribution, the fluid particles acceleration and the bed shear stress, under the different bed slopes and water depth ratios. The results under the conditions considered in this study show that there is a flow state transition in the flow evolution for the steep bed slope even in water depth ratio α = 0.1 (α is the ratio of the tailwater depth to the reservoir water depth). The flow state transition shows that the wavefront changes from a breaking state to undular. Such flow transition is not observed for the horizontal slope and mild bed slope. The existence of the Favre waves leads to a significant increase of the vertical velocity and the vertical acceleration. In this situation, the SWEs model has poor prediction. Analysis reveals that the variation of the maximum bed shear stress is affected by both the bed slope and tailwater depth. Under the same bed slope (e.g., S0 = 0.02), the maximum bed shear stress position develops downstream of the dam when α = 0.1, while it develops towards the end of the reservoir when α = 0.7. For the same water depth ratio (e.g., α = 0.7), the maximum bed shear stress position always locates within the reservoir at S0 = 0.02, while it appears in the downstream of the dam for S0 = 0 and 0.003 after the flow evolves for a while. The comparison between the numerical simulation and experimental measurements shows that the LES model can predict the internal movement characteristics with satisfactory accuracy. This study improves the understanding of the effect of both the bed slope and the tailwater depth on the internal movement characteristics of the dam-break flows and Favre waves, which also provides a valuable reference for determining the flood embankment height and designing the channel bed anti-scouring facility.

Three-Dimensional Numerical Investigations of the Flow Pattern and Evolution of the Horseshoe Vortex at a Circular Pier during the Development of a Scour Hole

In the current study, a three-dimensional CFD model is utilized to investigate the variation of the flow structure and bed shear stress at a single cylindrical pier during scour development. The scour development is presented by seven solidified geometries of the scour hole, collected during previous experimental work at different scour stages. Different turbulence models are evaluated and the (k-ω) model is chosen due to its relative accuracy in capturing the flow oscillation and vortex shedding at the pier downstream side with personal computer computational and storage resources. The numerical results are verified against dimensionless parameters from different previous experimental works. This research describes in detail the flow structure and bed shear stress variations through seven stages of the scour hole development. The dimensionless area-averaged circulation coefficient (Ψi) is developed to evaluate the changes in the vortex strength through the scouring process by eliminating the calculation area effect. It was concluded that the circulation in the (Y) direction is the main driving factor in the development of the scour hole more than the circulation in the (X) direction. The ratio between the horseshoe vortex (HV) mean size and the scouring depth (DV/dS) in addition to the location of the maximum bed shear stress are investigated during different stages of the scour development.

Flow Field in a Sloped Channel with Damaged and Undamaged Piers: Numerical and Experimental Studies

This paper analyzed the flow pattern around damaged and undamaged bridge piers in a channel with a mild slope. The experiments were carried out on a straight channel with a length of 12 meters and a slope of 0.008. Acoustic Doppler velocimeter (ADV) was employed to collect three-dimensional flow velocity data, and the results were analyzed and compared with particle image velocimetry (PIV) data. Sediment Simulation in Intakes with Multiblock option (SSIIM) was incorporated for the numerical simulation of the flow in this study. Generally, the results obtained from the comparisons referred to the appropriate agreement between the numerical and the experimental data. The results showed that an undular hydraulic jump occurred at a distance of two meters from the channel entrance in every case; the maximum bed shear stress in the sloped channel was 12 times that in a horizontal channel for installing two damaged and undamaged piers. With this position of the piers in the sloped channel, the upstream water level underwent a 72.5% reduction compared to similar conditions in a horizontal channel, while the amount of this water level decrease was equal to 8.3% compared to the other cases in a sloped channel. In addition, with the presence of both piers, the maximum Froude number was 5.7 times that in a horizontal channel.

Bridge pier geometry effects on local scour potential: A comparative study

Scour, or the localized loss of soil around the foundation, is a dominant factor contributing to failure. Scour at bridge piers, columns of elevated buildings, and wind turbines with monopile foundations can cause damage after flooding and natural hazards. However, due to the lack of sophisticated modeling, scour is not fully understood. This study utilizes computational fluid dynamic (CFD) simulations to understand the performance of different flow countermeasures in reducing the scour potential. We look at altering the flow around the pier and thereby alleviating the horseshoe vortices and downflow issues. CFD models are developed and validated with available experimental data. The recommended models, with turbulence closures, are employed to predict the bed shear stress for different pier configurations: streamlined shape, tapered sheath, delta vane, and guide wall with slanting vanes. Reductions in the maximum bed shear stress are 30% for both the angled plate and the delta vane, 20% for the tapered sheath, and 15% for the guide wall with slanting vanes. By reducing the bed shear stress, these countermeasures demonstrate their capability to mitigate scour. The proposed solutions have a potential to minimize the accelerated deterioration and protect bridges, elevated buildings, wind turbines, and coastal and offshore infrastructure against scour-induced failures.

Experimental analysis of multi-horizontal submerged jets energy dissipater

ABSTRACT The use of a stilling basin with multiple horizontal submerged jets emerging from two layers has been suggested for use as an energy dissipation structure in hydropower stations. In this study, laboratory velocity measurement is performed to analyze the three-dimensional flow structure of submerged jets and to evaluate the effect of the downstream submergence on the flow field. The results indicate that the Reynolds shear stress and vorticity magnitudes increase with increasing jet submergence. Further, it was found that submergence increase leads to a decrease of 2.2% in maximum bed (floor) shear stress and 7.2% in side-walls shear stress. The Reynolds shear stress quadrant decomposition is also performed for all (H = 0) and high magnitude (H = 2) bursting events, which indicate that the turbulence events of outward and inward interactions made dominant contributions to the total Reynolds stress. Turbulent kinetic energy is also analysed to obtain a better characterization of the flow field.

A multi-parameter design formula for riprap size selection at wing-wall abutments

A numerically-based approach is used to obtain a size selection formula for the riprap stone part of the apron used to protect wing-wall abutments against erosion. Several series of simulations with a given mean riprap diameter are conducted to estimate the maximum bed shear stress over the riprap apron and then the maximum (critical) Froude number at which riprap stones will resist shear failure by the flow for cases where the abutment is placed in straight and curved channels with or without a floodplain. Simulation results show that some of the existing design formulas for wing-wall abutments placed in straight river channels are not conservative enough for relatively large floodplain widths. Using data from the numerical simulations, a modified, two parameter design formula for riprap size selection is proposed that explicitly incorporates the effects of bank curvature and floodplain width.

Flow structure and bottom friction of the nonlinear turbulent boundary layer under stormy waves

Wave environment equivalent to the full-scale nearshore storm was generated in a large wave flume, and the flow within wave boundary layer was measured using an acoustic Vectrino profiler. Analysis were made on various factors including the velocity profile, phase lead, boundary layer thickness, steady streaming, turbulence as well as bottom shear stress, and comparisons were made with existing studies using oscillatory flow tunnels. Results show that the phase lead in the boundary layer is less significant compared to previous tunnel experiments, generally smaller than 20°. Free surface and vertical velocity lead to prominent wave-induced Reynolds stress at the top of the boundary layer, resulting in a thinner boundary layer. Both wave-induced Reynolds stress and wave shape asymmetry contribute to the steady streaming, making its direction either onshore or offshore. Relationship of friction factor versus relative roughness generally agrees with the exponential expression in the literatures for rough turbulent boundary layer, however the value of friction factor is larger. It is stressed that the friction factor would be several times higher if the free stream velocity is calculated using linear wave theory, which is questionable as the measured waves in the large wave flume were shown to be highly nonlinear. To compensate the nonlinear effects which significantly contribute to the maximum bed shear stress, a velocity skewness factor based on second-order Stokes wave theory was introduced. For predicting the intra-wave bed shear stress, a time-varying friction factor was also constructed, which was proved to have acceptable precision compared with experimental data. These formulations have simple, explicit expressions that can be used in practical engineering applications, especially for the nonlinear waves under nearshore storm conditions.

Sand‐Mud Tidal Flat Morphodynamics Influenced by Alongshore Tidal Currents

AbstractTidal flats, where significant land‐ocean interactions take place, are often abstracted as a cross‐shore bed profile with sediment zonation from the lower sand flat to the upper mud flat. However, in addition to cross‐shore tidal currents, the impact of the alongshore components on cross‐shore sediment transport, morphological evolution, and sediment grain‐size change remains unclear. Here we investigated sand‐mud tidal flat morphodynamics in the absence of waves by combining approaches of field observations, numerical modeling, and analytical interpretations, based on the example of the tidal flat on the central Jiangsu coast, China. The results show that the morphodynamic processes are complicated by the interactions of cross‐shore and alongshore tidal currents, cross‐shore flat morphology, and sediment zonation. With amplified phase lags of the alongshore tidal level at the boundaries, alongshore currents become dominant over cross‐shore tidal currents on the lower flat, while the upper flat is always dominated by the cross‐shore currents. Therefore, bed profiles and mud content on the upper flat were independent of the alongshore tidal current magnitude, being convex‐up and consisting of mud. In contrast, the strong alongshore currents can erode mud on the lower flat and promote landward sand transport from the subtidal area to the lower flat, forming a sand flat. The maximum tidal bed shear stress is almost spatially uniform across the muddy area but pronouncedly elevated where the bed sediment coarsens on the lower flat. The contributions of the alongshore tidal currents and sand‐mud sorting processes should be appropriately addressed in similar coastal environments.

Effect of submerged vane on three dimensional flow dynamics and bed morphology in river bend

AbstractA series of hydraulic experiments were undertaken in a 180° bend laboratory flume with parabolic erodible bed to simulate the effect of submerged vanes on the flow field and subsequent scour morphology in a river bend. The objective of the study was to investigate the role of submerged vanes in protecting riverbed erosion in a river bend. Parameters responsible for scour and deposition, including the velocity distribution field, bed shear stress, and kinetic energy, are presented. Results indicate that the presence of submerged vanes (a) reduced the maximum velocity along the outer bank of the parabolic riverbed by approximately 23%, and the location of this velocity field shifted from the outer bank of the 150°‐section to a central zone of 40°; (b) reduced the maximum bed shear stress and turbulence by 32% and 19%, respectively, and their locations also changed; and (c) reduced scour depths by up to 40% at the outer bank of the bend. The data obtained from these experiments will aid in the efficient design of riverbed protection works.

Direct numerical simulations of ripples in an oscillatory flow

Sea ripples are small-scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed. The phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains (i.e. the flow around each individual particle is accounted for). Two sets of parameter values (differing in the amplitude and frequency of fluid oscillations, among other quantities) are adopted which are motivated by laboratory experiments on the formation of laminar rolling-grain ripples. The knowledge of the origin of ripples is presently enriched by insights and by providing fluid- and sediment-related quantities that are difficult to obtain in the laboratory (e.g. particle forces, statistics of particle motion, bed shear stress). In particular, detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests. The ripple amplitude is observed to grow exponentially, consistent with established linear stability analysis theories. Particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells: particles at ripple crests are significantly faster and show larger excursions than those lying in ripple troughs. In analogy with the segregation phenomenon of polydisperse sediments, the non-uniform distribution of the velocity field promotes the formation of ripples. The wider the gap between the excursion of fast and slow particles, the larger the resulting growth rate of the ripples. Finally, it is revealed that, in the absence of turbulence, the sediment flow rate is driven by both the bed shear stress and the wave-induced pressure gradient, the dominance of each depending on the phase of the oscillation period. In phases of maximum bed shear stress, the sediment flow rate correlates more with the Shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress.