Laboratory Flume Research Articles

Large-Scale Laboratory Experiments On The Wave Generation Due To The Collapse Of Partially And Fully Submerged Granular Columns

Landslides that occur in coastal environments can drive cascading consequences such as wave forces, flooding, and infrastructure damage to coastal communities. It can be difficult to classify these slides as subaerial or submarine, and the mechanics of wave generation associated with partially submerged failures are not well understood. Limited physical modelling has been conducted that encompasses both the triggering of granular landslides and subsequent waves associated with partially and fully submerged mass movements. To date, laboratory work investigating tsunamis generated by submarine landslides has focused on the wave formed in the direction of the mass movement (seaward direction) for rigid block experiments (eg. Rzadkiewicz, 1997) and deformable slide masses (e.g. Grilli et al., 2017, Takabatake, 2020, Bullard et al., 2023). From these experimental data sets, predictive relationships connecting slide acceleration, mass, and initial submergence depth to the amplitude of the wave formed have been presented for the seaward wave. Such relationships have not been presented for the landward directed wave, which propagates in the opposite direction of the submarine landslide motion. Further, not all landslides are easily classified as either subaerial or submarine. Consider the 2018 Anak Krakatoa landslide in which the sliding surface was estimated to be 100 m below sea level (Pakoksung et al., 2020), resulting in one third of the total collapse being submerged. In comparison to the end-member conditions of subaerial and submarine failures, the mechanics of wave generation associated with partially submerged failures is much less clear. Granular column collapse experiments provide an idealized experimental framework to explore momentum transfer processes and the resulting waves generated in partially submerged and fully submerged conditions. Work by Cabrera et al., (2020) made use of granular collapse experiments of partially to fully submerged columns to derive a continuous momentum-based function to estimate the maximum seaward wave amplitude based on the initial column submergence ratio (Hw/Ho). However, these experiments were conducted at a small-scale (Ho = 0.15 m) with a width of one particle (2.4 mm diameter). To address this research gap, a series of 22 large-scale granular collapse experiments were conducted by releasing columns of river stone (0.75 m and 0.50 m high) into a laboratory flume reservoir with water depths ranging up to 1.10 m to explore the wave generation and runup processes in both seaward and landward directions. The columns were released by a rapid pneumatically actuated vertical rising gate designed to enable the near instantaneous loss of support of the source volumes resulting in granular collapse. The failure mechanics were captured with high-speed cameras (Figure 1a,b) and wave amplitudes were measured using wave capacitance gauges (Figure 1c,d). This work also provides the first experimental data set of the landward propagating wave and runup associated with submerged granular collapse experiments. Overall, the seaward wave amplitudes measured in these highly-instrumented, large-scale physical models agree with empirical relationships developed in a previous study using smaller-scale models.

Advp Measurements Of Flow Over Low-Angle Bedforms In A Laboratory Flume Setup

In many coastal and estuarine environments, bedforms develop due to the complex interactions between hydrodynamics, sediment transport and morphology which affect hydro-morphodynamic processes at various spatio-temporal scales. Furthermore, bedforms are considered main drivers of flow resistance via turbulence and affect near bottom processes such as bed roughness, bottom shear stress and turbulent structures. A detailed study and characterisation of flow over bedforms is therefore significant to many fundamental and engineering applications such as sediment transport calculation and prediction, channel managements, or burial of submarine cables. Asymmetric dunes with an angle-of-repose (30°) lee side have been well studied (Best, 2005; Venditti, 2013). They are representative of dunes commonly found in laboratory flumes and small rivers. Over such bedforms, flow is characterised by a permanent flow separation zone and intense turbulence over their lee side. Recently, it was shown that bedforms in large rivers are mainly low- to intermediate-angle dunes (mean lee side ca. 5-20°) having their steepest slope located close to the trough (Cisneros et al., 2020). Over such low- and intermediate-angle river dunes, flow separation is inexistent or intermittent and only little turbulence is generated (e.g. Kwoll et al., 2016; Lefebvre and Cisneros, 2023). Estuarine bedforms also possess mostly low- to intermediate-angle mean lee slopes between 5° and 20° (Dalrymple and Rhodes, 1995; Lefebvre et al., 2021). However, contrary to river bedforms, estuarine bedforms usually have a sharp pointed crest with the steepest slope situated near the crest and a relatively flat trough (Aliotta and Perillo, 1987; Lefebvre et al., 2021). It is not clear yet how much flow properties vary between high-angle flat-crested dunes and low-angle sharp-crested dunes. In particular, it is unknown whether a permanent or an intermittent flow separation can be observed over some segments of the lee side. Furthermore, the relation between the reversing tidal flow and natural estuarine morphology is not well understood yet. In this study, laboratory flume experiments will be conducted to measure the mean flow and turbulence over estuarine bedforms. Previous experiments performed by Carstensen and Holzwarth (2023) demonstrated the potential of high-resolution measurements over a large-scale estuarine dune in a laboratory flume. Building on this, experiments will be carried out over a fixed concrete low- and intermediate- angle estuarine bedform field. The results from this study can be used to characterise in detail the flow dynamics over estuarine bedforms.

Drag Coefficient of Emergent Vegetation in a Shallow Nonuniform Flow Over a Mobile Sand Bed

AbstractWidely distributed in natural rivers and coasts, vegetation interacts with fluid flows and sediments in a variable and complicated manner. Such interactions make it difficult to predict associated drag forces during sediment transport. This paper investigates the drag coefficient for an emergent vegetated patch area under nonuniform flow and mobile bed conditions, based on an analytical model solving the momentum equation following our previous work (Zhang et al., 2020, https://doi.org/10.1029/2020WR027613). Emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different vegetation coverage ∅. Laboratory flume tests were conducted to measure variations in both the water and bed surfaces along a vegetated patch on a sand bed. Based on the experimental and theoretical analyses, a dimensionless drag model integrating both terms of flow properties and bed effects is proposed to predict the drag coefficient Cd over a mobile bed. The calculated values of Cd exhibit two different trends, that is, nonmonotonically or monotonically increasing along the streamwise direction, due to the combined effect of water surface gradient and bed slope. The morphodynamic response of the mobile bed to nonuniform flow manifests as an evolution in the bed slope within the vegetated patch. Ongoing scouring directs the flow's energy toward overcoming the rising Cd and bed slope, leading to a relatively stable stage with a low sediment transport rate. This study advances the existing understanding of the drag coefficient's role over a mobile bed within nonuniform flows. It also enhances the applicability of vegetation drag models in riverine restoration.

Abiotic origins of self‐organized ridge‐runnel patterns on tidal flats

AbstractStriking large‐scale spatial patterns in ecosystems, generated by self‐organization through biotic and abiotic feedback processes, influence ecosystem functioning and response to global environmental change. A remarkable example of this are the regular ridge‐runnel patterns found on tidal flats, which play an important role in mudflat‐marsh transitions. Yet the mechanisms driving their formation, and whether they are abiotic or biotic in origin, have not been elucidated. The underlying mechanisms are unraveled in this study through a combination of field measurements and targeted laboratory experiments. In the field, we find that on the ridges of the pattern, the sediment bed level is less dynamic and more resistant to erosion than in the runnels. In laboratory flume experiments, we find that erosion‐resistant surfaces, like those found on the ridges, can arise on time scales of hours to days due to the drying of the cohesive sediment bed, while this is prevented in waterlogged sediments in runnels. A disturbance‐recovery experiment on benthic algae then confirms that biological processes require a longer developmental period than the time scale at which we have observed drying‐induced erosion resistance to develop. Together, these results demonstrate that ridge‐runnel patterns begin from an abiotic initiation point that can subsequently provide a template for further biological establishment and self‐organization. Recognition of abiotic processes as catalysts of self‐organization can improve our understanding of ecosystem responses to environmental changes.

Generation Of Scaled Long-Period Ship Waves In A Pump-Driven Flume

Experimental investigations on the generation of long-period, primary ship waves using a pump-driven technique in a closed-circuit flume are presented. Long-period ship waves can have a major impact on river banks and engineering structures. As larger ship dimensions accompanied by a corresponding rise in loads are expected in the future, the characterisation of long-period ship waves and understanding their impact on bank protection and engineering structures is crucial, especially for appropriate design. Our study combines a closed-loop PID controlled wave generation with a numerical finite difference method (FDM) model. This setup offers the advantage of generating repeatable waves at arbitrary positions along the test section of the laboratory flume. The experiments conducted demonstrate a good reproduction of a scaled long-period ship wave signal (1:10) in a distance of approximately 9 m from the flume inlet. The combination of the PID controlled pump-driven technique with a numerical model provides a solid basis for further studies on ship-induced loads, with the aim to predict future wave loads, better understand the wave-structure-interaction and to mitigate potential damage.

Pro+: Automated protrusion and critical shear stress estimates from 3D point clouds of gravel beds

AbstractThe dimensionless critical shear stress (τ*c) needed for the onset of sediment motion is important for a range of studies from river restoration projects to landscape evolution calculations. Many studies simply assume a τ*c value within the large range of scatter observed in gravel‐bedded rivers because direct field estimates are difficult to obtain. Informed choices of reach‐scale τ*c values could instead be obtained from force balance calculations that include particle‐scale bed structure and flow conditions. Particle‐scale bed structure is also difficult to measure, precluding wide adoption of such force‐balance τ*c values. Recent studies have demonstrated that bed grain size distributions (GSD) can be determined from detailed point clouds (e.g. using G3Point open‐source software). We build on these point cloud methods to introduce Pro+, software that estimates particle‐scale protrusion distributions and τ*c for each grain size and for the entire bed using a force‐balance model. We validated G3Point and Pro+ using two laboratory flume experiments with different grain size distributions and bed topographies. Commonly used definitions of protrusion may not produce representative τ*c distributions, and Pro+ includes new protrusion definitions to better include flow and bed structure influences on particle mobility. The combined G3Point/Pro+ provided accurate grain size, protrusion and τ*c distributions with simple GSD calibration. The largest source of error in protrusion and τ*c distributions were from incorrect grain boundaries and grain locations in G3Point, and calibration of grain software beyond comparing GSD is likely needed. Pro+ can be coupled with grain identifying software and relatively easily obtainable data to provide informed estimates of τ*c. These could replace arbitrary choices of τ*c and potentially improve channel stability and sediment transport estimates.

Field Observations Reveal How Plunging Mixing and Sediment Resuspension Affect the Pathway of a Dense River Inflow Into a Deep Stratified Lake

AbstractThe pathway of dense river inflows into lakes, which affects the lake water quality, is not accurately predicted by existing models. The pathway of a dense riverine inflow in a lake with a submerged canyon is analyzed based on measurements during a 4‐month period of weakening lake stratification and weakening density excess between river and epilimnion. In line with models, the dense riverine inflow plunges upon entering the lake, continues as an underflow on the sloping lake bottom, and finally intrudes at its level of neutral buoyancy. Underflow and interflow velocities are O(0.1 m s−1). The river inflow is finally trapped in the pycnocline most of the time, even when the river's density excess and the lake's stratification are marginal. This trapping in the pycnocline is explained by the reduction of the inflow density excess due to the intense plunging mixing, which is an order of magnitude larger than that obtained in confined laboratory flumes. The pathway of the dense riverine inflow is affected by interactions of the underflow with the lake bottom and sedimentary processes. A canyon carved by the underflows confines and accelerates the underflow, which enhances its capacity to entrain and carry sediment. The entrainment of sediment that was previously deposited on the canyon bottom accelerates the underflow. Due to both effects, the underflow can temporarily break through the pycnocline and reach the hypolimnion. Existing models explain these observations qualitatively, but a quantitative prediction would require better parameterizations of the plunging mixing and the sedimentary processes.

Development of innovative groynes to establish fish passability of regulated rivers based on the example of the Wien River, Austria. Part I: Impact of groyne parameters on water depth and velocity

AbstractRivers in Europe have been heavily modified over the last 200 years, with a significant impact on their ecology and environment. This also applies to rivers like the Wien River, Austria, which are designed as overwide concrete channels for the benefit of flood protection. To achieve a good ecological potential in such heavily modified water bodies, one key element is fish passability. This requires an increase in the water level at low flow conditions and a reduction of the flow velocity. The aim of this study is to assess whether groynes are suitable for this application. A design study was conducted to examine the effect of individual groyne parameters on water depths and velocities. Physical experiments were carried out in a laboratory flume at a scale of 1:8. In addition, a 2D numerical model was used. It was found that the groynes had to be submerged and the alignment had to be repelling to achieve both requirements. The configuration of the groyne height, distance and degree of obstruction parameters were crucial. The groyne angle and shape had a minor effect and can be used for fine‐tuning. The best groyne design created a passable section for fish. Thereby, and through sedimentation, the best design contributed to an ecological improvement. However, it did not create habitats and did not constitute a fullfledged restoration. In general, submerged groynes can fulfil the hydraulic requirements for fish passage in heavily modified water bodies with a fixed bed.

Granular-fluid avalanches: the role of vertical structure and velocity shear

Field observations of debris flows often show that a deep dry granular front is followed by a progressively thinner and increasingly watery tail. These features have been captured in recent laboratory flume experiments (Taylor-Noonan et al., J. Geophys. Res.: Earth Surf., vol. 127, 2022, e2022JF006622). In these experiments different initial release volumes were used to investigate the dynamics of an undersaturated monodisperse grain–water mixture as it flowed downslope onto a horizontal run-out pad. Corresponding dry granular flows, with the same particle release volumes, were also studied to show the effect of the interstitial fluid. The inclusion of water makes debris flows much more mobile than equivalent volumes of dry grains. In the wet flows, the formation of a dry front is crucially dependent on the heterogeneous vertical structure of the flow and the velocity shear. These effects are included in the depth-averaged theory of Meng et al. (J. Fluid Mech., vol. 943, 2022, A19), which is used in this paper to quantitatively simulate both the wet and dry experimental flows using a high-resolution shock-capturing scheme. The results show that velocity shear causes dry grains (located near the free surface) to migrate forwards to create a dry front. The front is more resistant to motion than the more watery material behind, which reduces the overall computed run-out distance compared with debris-flow models that assume plug flow and develop only small dry snouts. Velocity shear also implies that there is a net transport of water to the back of the flow. This creates a thin oversaturated tail that is unstable to roll waves in agreement with experimental observations.

Characterizing Erosion and Deposition in and Around Riparian Vegetation Patches: Complex Flow Hydraulics, Sediment Supply, and Morphodynamic Feedbacks

AbstractRiparian vegetation plays a fundamental role in alluvial channel evolution by modifying flow and sedimentation dynamics. To elucidate the roles of sediment supply, flow‐dependent transport capacity, and morphodynamic feedbacks on the evolution of emergent vegetation patches over a single hydrograph, we conducted two experiments with different reach‐scale sediment supplies: a high‐supply experiment (HSE) and low‐supply experiment (LSE). We measured flow velocities, bedload transport, and topographic changes around a full‐scale patch of live emergent willows in an outdoor laboratory flume. Erosion occurred in the patch‐adjacent channel areas irrespective of the sediment supply, whereas deposition within the patch interior was suppressed in the LSE and enhanced in the HSE. The magnitude of patch deposition in each experiment was controlled by the local sediment supply to the patch and local sediment mobility during each discharge in the hydrograph. The local sediment supply was affected by bed morphodynamics at the patch head, which modulated the reach‐scale sediment supply by redistributing the bedload in the channel. Sediment mobility within the patch was flow‐dependent and a function of velocity and turbulent kinetic energy. For different discharges, the velocity in the patch changed proportionally with the freestream velocity; however, the turbulent kinetic energy was more sensitive, being elevated compared with the freestream during high flows and inhibited at low flows. Therefore, deposition and erosion within vegetation patches are not simply functions of the reach‐scale sediment supply or patch flow characteristics, as is often assumed, but additionally depend on the local sediment supply to the patch interior.

Under cover ice transport processes under equilibrium and non-equilibrium conditions

The transport of ice particles beneath a stable ice cover (referred to as cover load transport), and the processes leading to ice deposition and erosion are highly dynamic and complex. Based on the strong analogous relationship between bed load and cover load transport processes, knowledge from bed load transport research can be used when developing experiments for cover load transport processes. This paper presents a detailed experimental study of cover load transport rate in a laboratory flume with simulated ice particles and a simulated ice cover. The effect of the particle shape and size on cover load transport rate is quantified using the buoyant velocity and its velocity coefficient. The critical dimensionless flow strength is identified as an important parameter which determines the incipient motion of the under cover ice particles, and was calculated using hydraulic measurements obtained through laboratory experiments. A new cover load transport formula is introduced based on regression analysis of experimental data for a wide range of dimensionless flow strength conditions. The impact of incoming ice supply rate on the under cover ice transport rate was also investigated by altering the ice supply rate to the flume. A control volume analysis of ice discharge underneath the ice cover is used as the approach to quantify the dimensionless ice transport rate at different sections of the flume. The concept of equilibrium ice transport rate is introduced based on the experimental observations with variable incoming ice supply rates. The experimental observations on the impact of incoming ice supply were supported by an analysis of field data from Hequ Reach, China (Sun et al., 1986; Wang, 1992). The introduction of a cover load transport formula developed exclusively with simulated ice particles that have similar physical properties to ice forming in a natural environment, and the discussion on equilibrium ice transport rate are important contributions to advance the present understanding on under cover ice transport processes.

Quantifying microplastic dispersion due to density effects

An experimental study was conducted on how polymer density affects the transport and fate of microplastics in aquatic flows. For the first time, polypropylene (PP), polyethylene (PE), polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), and polyvinyl chloride (PVC) were chemically stained and tested using solute transport techniques and velocities found among rivers in the natural environment (0.016 – 0.361 m/s). The movement of 3D-polymers with densities ranging from 0.9 – 1.4 g/cm³ was quantified in a laboratory flume scaled to simulate open-channel flows in fluvial systems. Except for PP, in most conditions microplastics exhibited similar transport characteristics to solutes regardless of density and established solute transport models were successfully implemented to predict their transport and fate. Mass recoveries and ADE routing model demonstrated microplastic deposition and resuspension was associated with polymer density below critical velocity thresholds ≤ 0.1 m/s. When density becomes the dominant force at these slower velocities, concentrations of denser than water microplastics will be momentarily or permanently deposited in channel beds and microplastics follow the classical Shields sediment transport methodology. This data is the first to provide microplastic suspension and deposition thresholds based on river velocity and polymer density, making a key contribution to research predicting microplastic fate and organismal exposure.

Capture of zooplankton by site-attached fish: striking dynamics under different flow speeds and prey paths

Consumption of pelagic zooplankton plays a vital role in the functioning of benthic communities such as coral reefs and kelp forests. Many fish that consume zooplankton in those habitats are site attached, foraging for drifting prey while maintaining a fixed position close to a shelter such as a branching coral or a perforated rock. Therefore, the flow, in which their planktonic prey drifts, is expected to affect their foraging movements. However, most attributes of those movements are poorly understood- a gap that our study seeks to fulfil. Our experiments were carried out in a laboratory flume with 4 common coral-reef site-attached species. Their movements were recorded in 3D, using two orthogonal video cameras. Different fishes exhibited similar trends despite noticeable differences in their body size, their morphology, the type of shelters they use, and the typical size of the groups in which they reside. In all species, the strike distance decreased with increasing flow speed. Similarly, the distance between the fish and prey at the moment of strike initiation (“Reactive Distance”) decreased with increasing flow speed, as well as the angle between that “Reactive Distance” and flow direction. Surprisingly, striking speeds (relative to Earth) remained nearly unchanged under different flows speeds. However, faster strikes occurred when oriented at wider angles relative to the flow. Taken together, the fish appear to determine the speed and angle of their strikes based on a cognitive ability to assess the prey’s drifting speed and path in order to reach on time the intercepting point. A rough estimate of the time it takes the fish to decide on the strike’s orientation and speed, would suggest a few hundred of milliseconds. Using published data on the fishes’ feeding rates, we found that the fish significantly differed in their feeding efficiencies, defined as the percent of prey they captured from those passing through their actual foraging space. That difference may explain inter-specific differences in the habitats the fish use and their group size.

Leaves stimulate aquatic phosphorus uptake by dark‐grown but not by light‐grown microbial communities in sediments: A laboratory study

Abstract It is a widely discussed topic in restoration ecology to use organic carbon (OC) management to increase nutrient uptake in chronically loaded agricultural streams. However, although nitrate uptake has been shown to correlated closely with the availability of dissolved OC, this relationship is less clear for phosphate. Likewise, the role of particulate OC (POC) on stimulating nutrient uptake has to be clarified. We aimed at investigating the effects of leaf additions on the net uptake of soluble reactive phosphorus and nitrate‐nitrogen from the water column by benthic biofilms in combined laboratory flume and batch experiments. Fine sediments were colonised under nutrient‐enriched conditions in the dark (dark‐grown heterotrophic biofilms) and under a 12/12‐hr dark/light cycle (light‐grown photoautotrophic biofilms) in flumes. In each light treatment, half of the biofilms were grown under strong OC‐limitation, the other half under near‐optimal C:N:P ratios. For the uptake experiments, the colonised sediments were incubated in nutrient‐enriched water to which (1) fresh alder leaves, (2) pre‐leached leaves, or (3) no leaves were added. Net nutrient uptake (or release) rates were determined via the change in nutrient concentrations in the water column over time. Net phosphorus uptake by dark‐grown biofilms was significantly increased by the addition of leaves. Despite their high initial nutrient leaching, fresh leaves showed stronger stimulating effects on the net P uptake than leached leaves over the entire experiment. Furthermore, biofilms grown under near‐optimum C:N:P ratios responded stronger to the POC supply than biofilms grown under C‐limited conditions. In contrast to dark‐grown biofilms, leaf additions had no significant effects on the net P uptake by light‐grown biofilms. We also found no effects of leaf additions on net nitrate uptake. Our study shows that an increased POC supply in nutrient‐impacted streams may boost heterotrophic phosphate uptake. However, the stimulating effect of the POC addition depends on a variety of POC‐dependent (e.g. respiration, heterotrophic assimilation, denitrification) and independent (e.g. adsorption, photoautotrophic assimilation) processes, making nutrient control via POC management challenging. We suggest application of management approaches that focus on restoration of riparian areas and wetlands to increase the amount of natural, complex POC with a moderate degradability, thus keeping the heterotrophic metabolism low but potentially stimulating in‐stream N and P retention.

Effects of Periodic Fluctuation of Water Level on Solute Transport in Seasonal Lakes in Poyang Floodplain System

AbstractSeasonal lakes appear in floodplain systems due to the seasonal variation of water level of rivers or main lakes and play an important ecological function. Previous studies have focused on exchange flux between the surface water and groundwater in seasonal lakes, but the process of solute transport at the sediment‐water interface under the effect of seasonal water level fluctuations remains unclear. In this study, laboratory flume experiments and numerical simulations were conducted to investigate non‐reactive solute transport and exchange in a seasonal lake under effects of hydrological connectivity, water level amplitude, sediment permeability and solute source. The results show that the recharge pattern between seasonal lakes and rivers or main lakes is determined by the seasonal water level. The solute transports from the groundwater of seasonal lakes to the main lake during the dry season, while the reverse applies during the wet season. The perennial hydrological isolation increases the concentration in the overlying water and pore water of the seasonal lake by 20.5% and 28.7%, respectively. The solute concentration in the overlying water and pore water of the seasonal lake decreases by 55.2% and 51.5%, respectively, when solute source occurs in the main lake compared with that occurs in the seasonal lake. The amplification of water level amplitudes leads to an increase of 6.46% and 10.55% in the solute concentration drop from the overlying water, respectively. These findings contribute to improving water resource utilization and protecting aquatic ecological environment in floodplain systems.

Effects of the angle between the rock strata and slope on flow hydraulics and sediment yield in karst trough valley: Laboratory scour experiment

The angle between the exposed bedrock and slope in a karst trough valley strongly affects overland flow behavior, thus, altering the generation of runoff and sediment. However, the directions and magnitudes of these effects remain unclear. The current study aimed to investigate the effects of the angle between the rock and slope on flow hydraulics, runoff, and sediment yield, as well as their linkages. A laboratory flume scour experiment was done under different combinations of angles between rock strata and slope (0°, 30°, 60°, 90°, 120°, and 150°), flow discharge rates (5, 7.5, and 10 L/min) and slope gradients (10°, 15°, and 20°) to simulate the field environment in a karst trough valley. The flow hydraulics (Reynolds number, flow velocity, flow depth, and Darcy–Weisbach friction factor), sediment yield, sediment concentration, and runoff rate were determined. The results showed that with increasing scour duration, the Reynolds number and flow velocity decreased but the flow depth and Darcy–Weisbach friction factor increased. The angle between the rock strata and slope significantly affected the flow velocity and Darcy–Weisbach friction factor, while slightly affecting the flow depth and Reynolds number. Over the scour duration, the runoff rate first increased and remained at a steady state but the sediment concentration and sediment yield rate first increased and then sharply decreased to a steady state. The sediment concentration and sediment yield rate both significantly decreased as the angle between the rock strata and slope increased. The runoff rate was closely linked to the flow velocity and Darcy–Weisbach friction factor as well as the sediment yield rate and sediment concentration. The current results indicate that the angle between the rock strata and the slope should be considered an important parameter when developing a soil erosion prediction model for karst trough valleys.

Experimental and numerical investigations on the performance and wake characteristics of a tidal turbine under yaw

This paper presents an extensive examination of the wake characteristics and hydrodynamic performance of a 1:60 scale horizontal axis tidal stream turbine under yawed inflow conditions. Experimental investigations were conducted in a laboratory flume, while numerical simulations utilized the actuator line method and large-eddy simulation technique. The turbine was subjected to an inflow turbulence intensity of 6% and was tested at five different yaw angles (5°, 10°, 15°, 20°, and 30°). The findings demonstrate a substantial reduction in the power coefficient as the yaw angle increases, with a notable drop of 47.1% recorded at a yaw angle of 30°. Furthermore, numerical simulations of wake characteristics, including mean velocity and turbulence intensity, were examined and compared with measurements, yielding satisfactory agreement. The yawed wake flow exhibited asymmetrical distributions of mean velocity and deviations from the centerline. Additionally, distinct deformations of the yawed wake in the y-z cross sections were observed, indicating a discernible curled kidney shape that intensified with larger yaw angles. Moreover, interactions between the turbine wake and support structures affected the width and turbulence intensity of the near wake, introducing additional unsteadiness that accelerated wake recovery.

Impacts of Channel‐Spanning Log Jams on Hyporheic Flow

AbstractIn‐stream wood structures, such as single logs, river steps, and debris dams, are known to drive hyporheic flow, defined as the flow that goes into the subsurface region and then back to the free‐flowing surface water. The hyporheic flow plays an important role in regulating water quality and biogeochemical cycles in rivers. Here, we investigated the impact of a channel‐spanning porous log jam, representing piles of wood logs, on hyporheic flow through a combination of direct visualization and theories. Specifically, we developed a method using refractive index‐matched sediment to directly visualize the hyporheic flow around and below a porous log jam, formed by piles of cylindrical rods, in a laboratory flume. We tracked the velocity of a fluorescent dye moving through the transparent sediment underneath the log jam. In addition, we measured the water surface profile and the spatially varying flow velocity near the log jam. Our results show that the normalized log jam‐induced hyporheic flux remained smaller than 10% at Froude numbers () below 0.06 and increased by a factor of five with increasing at . We combined the mass and momentum conservation equations of surface flow with Darcy's equation to explain the dependency of the log jam‐induced hyporheic flux on . Further, we observed that at , the water surface dropped noticeably and the turbulent kinetic energy increased immediately on the downstream side of the log jam. These findings will facilitate future quantification of hyporheic flow caused by channel‐spanning porous log jams.

Effects of Flow Unsteadiness on the Transport of Bimodal Bed Material

The grain size distribution of the transported bed load was experimentally investigated under unsteady flow conditions with bimodal mixture of sand and gravel in a laboratory flume. Five various triangular hydrographs were generated. A clockwise behavior for the total bed load versus shear velocity was observed meaning that the bed load during rising limb was higher than that of falling limb. It was found that the percent finer at the plateau of bimodal sediment size distribution curve had higher values during the initial and final phases compared to those obtained during the peak time. At all plateaus, the percent finer values related to the hydrograph peak discharge were in the same order of magnitude with that of the bed material. The sand content of the transported bed material initially decreased, then maintained a constant value during a certain time interval and finally returned to its original value. The sand percent of the bed load decreased in the falling limb showing a counterclockwise loop and the duration of the hydrograph did not affect the results considerably. The greater the peak flow rate of the hydrograph, the greater was the hysteresis. The bimodality index was calculated for all transported sediment samples and it was revealed that its initial and final values were less than that of the bed material but it was approximately the same elsewhere. The 5% finer sediment amount was nearly equal during rising and falling limbs. It was revealed that D50 value of the bed load decreased in the rising limb showing a clockwise loop. The hysteresis was not considerably changed according to the hydrograph characteristics. The clockwise type hysteresis was also observed for the size group of D95. The lag increased as the peak flow rate increased. A strong relation was found between the dimensionless total bed load Wt* and the total work index Wk as well as Wk and the ratio WR/WF. The correlations between the dimensionless total bed load and the unsteadiness parameters P, and Pmod were very weak, whereas a high value of determination coefficient was obtained with the unsteadiness parameter Pgt, implying an appreciable interdependence.

Effects of local scouring on the bearing capacity of monopile-wheel composite foundations in sandy deposit

The monopile-friction wheel composite foundation is an innovative offshore wind turbines (OWTs) foundation type for its good bearing properties and anti-erosion advantages. The local scouring is a non-negligible challenge by moving surrounding soils away till even some engineering disasters. To explore this kind of unfavorable effects, we model the response of scouring to the bearing capacity of the composite foundation by means of laboratory flume tests. Furthermore, the influence of different loading sequences (i.e., T→H or H→T) is considered, and the failure envelopes are also obtained. On this basis, a 3D finite element model is established to investigate the impact of key scouring parameters and different loading point heights on the bearing capacity. The conclusions are as follows: local scouring can significantly reduce the bearing capacity; the scouring depth has the most significant effect, followed successively by the scouring angle and extent; compared with pure horizontal foundations, the ultimate torsional bearing capacity of the foundations under the combined T→H loading sequence decreases by about 1.95%–11.1%, while the horizontal bearing capacity under the combined H→T loading sequence decreases by about 0.82%–1% compared with pure torsional foundations; the loading point height has noticeable effects on the lateral resistance for the composite foundation.