Subcritical Flow Conditions Research Articles

Impact of dyke and vegetation on fluid force and moment reduction under sub and supercritical flow conditions

Flooding is the most common natural disaster throughout the world and requires efficient management. Therefore, the current investigation aimed to explore the impact of a composite defense system comprising dyke and vegetation on flow dynamics and velocity reduction. Experiments were conducted in an open channel setup with an adjustable bed slope and transparent sidewalls, and the vegetation model was replicated as real trees such as Eucalyptus trees. The study involved calculating several parameters, including flow velocity, reduction of fluid force index (RFI%), reduction in moment index (RMI%), and hydrostatic and hydrodynamic forces. These calculations were done by changing the channel bed slope and keeping the flow rate (discharge) constant while considering both subcritical and supercritical flow conditions. Moreover, regression analysis was performed for the prediction of RFI% and RMI% under various flow conditions. Also, statistical analyses were performed to assess the effectiveness of the defense system in reducing fluid force and moment indices. The result of the current investigation indicates that the highest values of RFI% and RMI% under subcritical flow conditions were 79% and 88%, while under supercritical flow conditions they were 94% and 78%, respectively. Moreover, a velocity reduction of 69% was observed under subcritical flow, while 84% was observed under supercritical flow conditions. Under subcritical flow conditions, RFI% and RMI% enhanced by enhancing Froude number (Fr) because of an increase in velocity reduction and hydraulic jump formation. Similar trends were observed under supercritical flow conditions, with effective mitigation of high-velocity flows by the composite system. The finding of current research helps in providing effective techniques for flood management.

Effectiveness of Collars and Hooked-Collars in Mitigating Scour around Different Abutment Shapes

Abutment scour is a major cause of bridge failures worldwide, leading to disruptions, economic losses, and loss of life. The present experimental study examines countermeasures against abutment scour using hooked-collar protections on vertical-wall and wing-wall abutments (at 45° and 60°) under different flow conditions. All 60 experiments were performed under sub-critical flow conditions by investigating scour around an abutment 20 cm long, 20 cm wide, and 25 cm tall. Two distinct values of the Froude number, 0.154 and 0.179, and a sediment particle diameter (d50) of 0.88 mm were used throughout the experimental phase. The resulting equilibrium scour around the abutments was compared to those with collar and hooked-collar protections. It was determined that the maximum abutment scour depth reduction was 83.89% when hooked collars were placed on vertical wall abutments beneath the bed surface level, and for wing-wall abutments at 45° and 60°, it was 74.2% and 73.5%, respectively, at the bed surface level. Regression analysis was conducted to assess the non-dimensional scour depth (Ds/Yf) and scour reduction (RDs/Yf), with a high enough coefficient of determination (R2 values of 0.96 and 0.93, respectively), indicating high confidence in the analysis. The sensitivity analysis findings demonstrate that the width of the collar (Wc) and La are the most influencing factors affecting Ds/Yf and RDs/Yf.

Data-based models to investigate protective piles effects on the scour depth about oblong-shaped bridge pier

The primary objective of this study is to examine the suitability of employing a machine learning models (MLMs) comprising Support Vector Machine (SVM), Gene Expression Programming (GEP), and Artificial Neural Network (ANN) algorithms to emulate the influence of geometric and geotechnical parameters of the piles on the scour depth reduction about the oblong-shaped bridge pier. A dataset consisting of 90 laboratory observations was utilized, allocating 70 % of the data for the training and 30 % for the testing the MLMs. An oblong-shaped bridge pier of diameter D, was positioned and subjected to a rectangular flume featuring an erodible bed comprising sand sediment particles of D50 = 1.7 mm under clear water and subcritical flow conditions. The spacing between the piles, LD, the angle of pile orientation with respect to the flow, θ, and the Froude number, Fr, were identified as independent parameters through the dimensional analysis and Buckingham-Π theory. The MLMs’ performance were assessed employing root mean square error (RMSE), mean absolute error (MAE), coefficient of determination (R2), and developed discrepancy ratio (DDR). The outcomes, affirming the predictive capacity of all MLMs included, revealed that the GEP model employing a three-gene configuration exhibited superior accuracy compared to the other two MLMs. Specifically, the performance indicators (RMSE, MAE, R2, DDR) during the training and testing phases were (0.0433, 0.033, 0.981, 6.8) and (0.054, 0.045, 0.958, 4.81) respectively. The sensitivity analysis elucidated the hierarchy of influential variables as follows: Fr, LD and θ.

Tsunami Debris Damming Drag Forces And Associated Coefficients On Elevated Coastal Structure Columns

Tsunami overland flow induces hydrodynamic loads on coastal structures and may transport various objects located within the inundation zone, which could become debris and exacerbate hydrodynamic loading. In the process of adopting “the first national, consensus-based standard for tsunami resilience” (Chock, 2016) in the form of ASCE 7-16 Chapter 6: Tsunami Loads and Effects, emphasis was placed on evaluating debris transport and impact forces. This is evidenced by the robust body of literature regarding physical model experiments of these processes and thorough design procedures for both load considerations in current structural engineering standards (ASCE, 2022). Debris damming forces, resultant of debris being transported and accumulating against structures, are less thoroughly studied, having only recently begun to transition from steady flow experiments to transient flow conditions representative of coastal inundation events. A recent pair of experiments bridges this gap, comparing debris damming via steady-state, subcritical flow conditions to that caused by a dam-break style hydraulic bore (Stolle et al., 2018). That paper aimed to study debris dam formation, stability, and loading as well as runup of the flow onto idealized structural columns. Another study varied debris quantity, orientation, and arrangement to determine the effect had on damming and impact loads (Shekhar et al., 2020), however neither compared findings to current standards. The experimental work presented herein represents initial findings of a multi-year experimental campaign to better understand the mechanisms that lead to debris damming and increased structural loading. This work builds upon previous studies by using larger scale debris elements, more numerous debris fields, and more trials to better model such a stochastic process as debris damming. Three different incident wave conditions also led to varied hydrodynamics at the column specimen. In later phases, this campaign will also investigate the debris damming consequences of heterogeneous debris, which more accurately represent highly variable debris fields observed in post-event site surveys (Nistor et al., 2017). This paper aims to compare experimental debris dam loading parameters to those in the current ASCE 7-22 standard (ASCE, 2022). Namely, evaluating conservatism of ASCE 7-22 design values for: overall drag force on buildings, minimum closure ratios used in load determination, and empirical rectilinear structure drag coefficients during both debris accumulation and quasi-steady debris damming phases.

Experimental investigation of the total flow resistance in emergent and submerged rigid canopy flows

In canopy flows, flow resistance mainly originates from vegetation drag and depends on vegetation characteristics and flow conditions. In the present study, a series of experiments were performed in various hydraulic scenarios with high stem Reynolds numbers (2641 ≤ Red≤ 17333) using relatively sparse rigid canopies, represented with four different dimensionless vegetation densities (0.0044, 0.0098, 0.0174 and 0.0392), on a smooth bed. A novel drag plate mechanism was developed to measure the total flow resistance due to the emergent and submerged vegetation arrays in a staggered pattern under subcritical flow conditions. Manning’s roughness coefficient and Darcy–Weisbach friction factor were adopted to represent the total flow resistance in the analyses. Simple empirical relationships based on roughness concentration and submergence ratio were derived to determine the total flow resistance parameters within a broad range of stem Reynolds numbers. Although relationships were proposed in a simple form to be used for direct practical applications, they show similar or better performance in the prediction of total flow resistance parameters than the existing equations in the literature, which require considerable computational effort. Additionally, analyses demonstrated that the results of the present study and those of similar studies regarding canopy flow resistance are in good agreement. Accordingly, the novel drag plate looks promising for measuring flow resistance due to vegetation and bed conditions similar to those in nature.

Experimental investigation of three-dimensional flow dynamics in a laboratory-scale meandering channel under subcritical flow condition

This study investigates the three-dimensional turbulence flow properties in a meandering channel under subcritical flow conditions, specifically focusing on a few critical parameters such as three-dimensional velocity behaviour, Turbulent kinetic energy, skewness, and kurtosis. A model study has been attempted by considering the same sinuosity, Froude number, and bed material from an Indian peninsular river to understand the basic hydrodynamics of a meandering channel. Understanding these critical parameters is crucial for comprehending the complex flow behaviour and its implications in meandering channels. Experimental measurements were conducted to collect data on Velocity, Turbulent Kinetic energy, skewness, and kurtosis in all three directions. The objective was to analyse their variations and behaviour along the meandering path, providing insights into the flow dynamics and turbulence characteristics. Advanced measurement techniques, such as Acoustic Doppler Velocimeters (ADV), were utilized to obtain accurate and detailed data. The study revealed distinct patterns and trends in the velocity, Turbulent Kinetic energy, skewness, and kurtosis along the flow path of a sand bed meandering channel. These parameters were found to vary significantly in different sections, highlighting the influence of channel geometry and flow conditions. Graphical representations of the flow profiles were employed to visualize the spatial distribution and understand the variations in these turbulence properties. The findings of this study enhance our understanding of the three-dimensional turbulence flow properties in sand bed meandering channels under subcritical flow conditions. The results offer valuable information for designing hydraulic structures, flood control measures, and river restoration projects.

Double sill stilling basin to enhance energy dissipation for a strong hydraulic jump with a high Froude number

Roller Compacted Concrete (RCC) is a new and viable construction material for concrete gravity dams. It is gaining popularity due to its low cost and fast deployment. The design of a stilling basin, part of a dam that controls the hydraulic jump and dissipates energy, is crucial. One of the important considerations is the Froude number at the spillway toe (F1), which determines the type of hydraulic jump that will occur. An energy dissipator is also required to prevent scour caused by the change in flow conditions from supercritical to subcritical. Double sills installed downstream of the steep chute way channel can be used as an energy dissipator. This study uses hydraulic modeling to evaluate the flow energy reduction and subcritical flow conditions in a stilling basin with double sills. The investigation is expected to provide better information on hydraulic jump control.

Effects of Reynolds number and blockage ratio on the turbulence characteristics of open channel flow passing through trash rack

Trash racks are extensively used to prevent debris from being transported to down-stream reaches of waterways. However, debris accumulation at the screens can significantly affect their hydraulic performance, resulting in negative economic and environmental consequences. Basic research related to the mechanism of these physical processes is still rather limited. In this experimental study, the flow fields, including the turbulence intensities, Reynolds stresses, and turbulent kinetic energy near a trash rack in a fixed-bed open channel for subcritical flow conditions are investigated for two Reynolds numbers and two blockage ratios. The experimental study investigates the important physical mechanisms relating to turbulence generation and possible sediment deposition induced by the vertical trash rack. Reynolds number effect on time mean streamwise velocity is predominant in the trash rack’s downstream free surface layer as a higher Reynolds number flow has higher normalized streamwise velocity in the free surface layer and lower in the inner layer. In both the Reynolds number effect case and the blockage ratio effect case peak normalized turbulent kinetic energy was observed immediately downstream of the trash rack and with the increment of longitudinal distance downstream of the trash rack, normalized Turbulent kinetic energy is decreased.

Coarse-grained submarine channels: from confined to unconfined flows in the Colombian Caribbean (late Eocene)

Submarine channel mouth settings are hardly preserved in the stratigraphic record. Although they are still poorly known with respect to other segments of turbidite systems, conceptual models are being refined in the light of new discoveries in modern and ancient examples. Still, some questions such as the transition between expansion zones and the traditional Channel-Lobe Transition Zone (CLTZ) remains open in ancient systems. Upper Eocene deposits of the Colombian Caribbean (San Jacinto Fold Belt) are interpreted here as a fan-delta-fed, submarine, coarse-grained channel-lobe system. It displays a well-preserved channel inception stage in the shelf break represented by sigmoidal to lens-shaped gravels, and planar cross-stratified pebbly sandstones (foreset and backset) interpreted as cyclic steps in an expansion zone. In a later stage, a classical channel-levee complex was developed, represented by channel fill elements showing sharp- and erosional-based, fining-upward sequences that are meters thick, having basal massive matrix-supported pebble conglomerates (hard—extrabasinal—clasts, rip-up clasts, coastal bioclasts), vertically evolving to liquefied massive to planar-laminated coarse-grained sandstones with phytodetrital carbonaceous laminae. They are interpreted as concentrated flow deposits (high-density turbidites) coming from continental areas or from coastal systems (i.e., delta reworking). Undifferentiated channel belt thin-bedded turbidites associated with levees and terraces deposits are related to these confined systems. The channel-lobe transition zone is characterized by debrites from cohesionless debris flow in a channel-mouth bar setting, representing bypass processes that developed distally into low-angle, planar cross- and sigmoidally-stratified (upstream antidune) pebble-size to coarse-grained sandstones that fill low-angle scours (cut-and-fill structures) in an antidune field setting with supercritical conditions. When the currents lose channel confinement, the setting is characterized by changes from Froude supercritical to subcritical flow conditions in an inner lobe to lobe off-axis environment. Large seasonal fluctuations in precipitation favor high sediment concentrations, promoting the formation of volumetrically significant fan deltas and coarse-grained submarine channels with high erosive capacity; therefore, their record helps refine interpretations of depositional processes, providing criteria for recognizing areas of the turbiditic systems that are hardly preserved. The particular aggradational conditions for the preservation and stratigraphic characterization of the rare exhumed submarine channel mouth systems make it possible to decipher sediment dispersal patterns and thus connect the models proposed here, from supercritical systems to the traditional models of turbiditic systems.

Assessment of Aerodynamic Plates Subjected to Von Kármán Vortex Street for Enhancing the Wind Energy Generation in Blade-Less Devices

This study explores the feasibility of using an oscillating plate downstream of a cylindrical body to produce mechanical energy from a Von Kármán vortex street under sub-critical flow conditions (Re = 72,500). The study aims to quantify the impact of the plate length, its separation from the cylinder, and a machine damping factor on the power coefficient and the blade’s displacement to identify the optimal configuration. This preliminary assessment assumes that the plate oscillation is small enough to avoid changes in the vortex dynamics. This assumption allows the construction of a surrogate model using Computational Fluid Dynamics (CFD) to evaluate the effect of plate length and separation from the cylinder on the fluctuating lift forces over the plate. Later, the surrogate model, combined with varying machine damping factors, facilitates the description of the device’s dynamics through the numerical integration of an angular momentum equation. The results showed that a plate with a length of 0.52D, a separation of 5.548D from the cylinder, and a damping factor of 0.013 achieved a power coefficient of 0.147 and a perpendicular displacement of 0.226D. These results demonstrate a substantial improvement in the performance of blade-less generators.

Scour around the single emergent cylinder due to subcritical and supercritical approach flow conditions

ABSTRACT Flume experiments with a uniform cohesionless sand with d 50 = 0.9 m m were utilized in the current study to examine the local scour process around a single emergent cylinder under various geometrical circumstances and with a various flow characteristic that simulates an inundating tsunami flow. This single emergent cylinder represents a piloti-type column located over a movable bed material because piloti-type columns are used to elevate the building structures from the ground level. In the experiments, it was noted that the maximum scour depth probably occurs at the cylinder’s upstream face, resembling local scour around a scaled-down bridge pier in an open channel. Additionally, the scour hole around the cylinder is shallower the smaller they are, as are the horseshoe vortices that surround them. Furthermore, the effects of the surface bow-wave, hydraulic jump detachment and the wall-jet over the front face of the emerging cylinder were used to identify the relationship concerning the scouring pattern and selected flow characteristics over a piloti-type emergent column under tsunami flow. Empirical formulae have been finally developed to determine the maximum scour depth in front of the cylinder, which is beneficial to characterize the stability of the piloti-type columns, considering the specified Froude number conditions.

Explaining the Lack of Mesh Convergence of Inviscid Adjoint Solutions near Solid Walls for Subcritical Flows

Numerical solutions to the adjoint Euler equations have been found to diverge with mesh refinement near walls for a variety of flow conditions and geometry configurations. The issue is reviewed, and an explanation is provided by comparing a numerical incompressible adjoint solution with an analytic adjoint solution, showing that the anomaly observed in numerical computations is caused by a divergence of the analytic solution at the wall. The singularity causing this divergence is of the same type as the well-known singularity along the incoming stagnation streamline, and both originate at the adjoint singularity at the trailing edge. The argument is extended to cover the fully compressible case, in subcritical flow conditions, by presenting an analytic solution that follows the same structure as the incompressible one.

Experimental research on aerosol scrubbing characteristics under steam-air mixture submerged jet condition

The occurrence of severe accidents in nuclear power plants may lead to the leakage of radioactive aerosol. And the scrubbing efficiency of aerosol contained in the air-steam mixture submerged jet is important for assessing the radioactivity in the containment during a severe accident. Several experiments were carried out to analyze the effect of inlet pressure, aerosol diameter, submergence, and steam fraction on the scrubbing efficiency of aerosol in the jet regime. Then, the scrubbing effects of steam and different flow regimes under submerged jet conditions were investigated by comparing the experimental results. The scrubbing efficiency increased with the increase of the aerosol diameter in the air jet regime while the most penetrating region of the aerosol in the steam-air mixture jet regime is 0.2–0.4 μm. When the gas flow characteristic is subcritical flow, as the inlet pressure increases, the scrubbing efficiency increases rapidly. When the critical flow is reached, the inertial effect was the dominant factor in aerosol scrubbing due to the sufficient gas velocity, the scrubbing efficiency under different steam fractions is close to 100%. The scrubbing efficiency increases with the increase of the steam fraction, which may be caused by the increase of the temperature gradient field under subcritical flow conditions. Additionally, the increase the submergence can increase the residence time and improve the scrubbing efficiency. However, due to the decrease of the gas velocity during the gas floating, the scrubbing efficiency in the flow region of the same height is gradually decreasing.

Investigation of Blockage Upstream Box Coverage on the Scour Pattern in the Open Channel

Under sub-critical flow conditions, the presence of blockage (solid wastes and weeds) upstream of box coverage and the scours pattern downstream of the coverage was explored in this study. The upstream blocking was simulated using 48 runs in an artificial trapezoidal cross-section with three square box coverages of side dimensions 8.80, 10.40, and 12.90 cm, four water flows of 2, 5, 8, and 11 L/s, and four blockage ratios of 0, 10, 20, and 30% relative to the coverage cross-section area. To estimate the scour hole characteristics, a 2.00 m long, 0.60 m bed width, and 0.30 m deep sand basin filled with D50 = 0.50 mm bed material was constructed directly downstream of the coverage outlet. In each run, the water level, velocity, and scoured hole parameters downstream of the coverage were measured. According to the analysis, the Non-blocked coverage has less scour depth and length than the partially blocked coverage, where the maximum scour depths and length of the Non- blocked coverage for cases 1, 2, and 3 at the discharge of 11 l/s were 73, 72, and 17 % respectively, and 77.56, 77.34, and 83.66 % respectively relative to the maximum scour depth and length of partially blocked coverage at the discharge of 11 l/s and blockage ratio 30 %. The depth and length of the scour hole downstream coverage are increased with the increment of the coverage's blockage ratio and discharge and are reversely proportional to the inlet area of coverage. Increased relative scour depth and length by 3.60 percent and 11.80 percent respectively, by increasing the downstream Froude number of flow (Frd) by 0.01 while the coverage’ relative wetted area (Ar) is constant. The study suggested applying coverage with a suitable area and water discharge, and protection techniques downstream of the coverage to reduce the influence of coverage and blockage on the open channel's hydraulic efficiency and the scour pattern downstream of the coverage. Also, install a trash rack upstream of the covering and remove aquatic weeds and solid wastes periodically upstream and inside the coverage.

Numerical Investigation of 3D Flow Properties around Finite Emergent Vegetation by Using the Two-Phase Volume of Fluid (VOF) Modeling Technique

This study predicts how the Free Surface Level (FSL) variations around finite length vegetation affect flow structure by using a numerical simulation. The volume of fluid (VOF) technique with the Reynolds stress model (RSM) was used for the simulation. Multizone Hexahedral meshing was adopted to accurately track the free surface level with minimum numerical diffusion at the water–air interface. After the validation, finite length emergent vegetation patches were selected based on the aspect ratio (AR = vegetation width-length ratio) under constant subcritical flow conditions for an inland tsunami flow. The results showed that the generation of large vortices was predominated in wider vegetation patches (AR > 1) due to the increase and decrease in the FSL at the front and back of the vegetation compared to longer vegetation patches (AR ≤ 1), as this offered more resistance against the approaching flow. The wider vegetation patches (AR > 1) are favorable in terms of generating a large area of low velocity compared to the longer vegetation patch (AR < 1) directly downstream of the vegetation patch. On the other hand, it has a negative impact on the adjacent downstream gap region, where a 14.3–34.9% increase in velocity was observed. The longer vegetation patches (AR < 1) generate optimal conditions within the vegetation region due to great velocity reduction. Moreover, in all the AR vegetation cases, the water turbulent intensity was maximum in the vegetation region compared to the adjacent gap region and air turbulent intensity above the FSL, suggesting strong air entrainment over this region. The results of this study are important in constructing vegetation layouts based on the AR of the vegetation for tsunami mitigation.

Fish body geometry reduces the upstream velocity profile in subcritical flowing waters

Fish body geometry is highly variable across species, affecting the fluid-body interactions fish rely on for habitat choice, feeding, predator avoidance and spawning. We hypothesize that fish body geometry may substantially influence the velocity experienced by fish swimming. To test this hypothesis, we built nine full-scale physical prototypes of common freshwater fish species. The prototypes were placed in a large laboratory flume and upstream time-averaged velocity profiles were measured with increasing distance from the anterior-most location of each body. The measurements revealed that the body geometry can have a significant influence on the velocity profile, reducing the flow field at a distance of one body length upstream of the fish. Furthermore, it was found that the upstream velocity profiles from the nine fish species investigated in this study can be normalized to a single fit curve based on the freestream velocity and fish body length under subcritical flow conditions. These findings are significant, because they show that conventional point velocity measurements overlook the reducing effect of the fish body on the upstream flow field, creating a systematically biased representation of the velocity experienced by fish in subcritical flowing waters. This bias is illustrated by velocity field maps created with and without the presence of the physical models for three different fish species. Finally, we provide an example of how point velocity measurements can be recalculated to provide upstream velocity field maps closer to “the fish’s perspective”.

Mach and Reynolds number dependency of the unsteady shock-induced drag force on a sphere

Shock–particle interaction is an important phenomenon in a wide range of technological applications and natural phenomena, and the development of accurate models for this interaction is therefore of interest. This study investigates the transient forces during shock–particle interaction at particle Reynolds numbers between 100 and 1000, and incident shock wave Mach numbers between 1.22 and 2.51. This is achieved with the aid of particle-resolved large-eddy simulations. The simulation results show that shock–particle interaction differs qualitatively for subcritical and supercritical incident flow conditions. By decomposing the total force, the inviscid and viscous unsteady forces are estimated. The inviscid unsteady component is significantly larger than the viscous contribution, but the magnitude of the viscous component is comparable to steady-state drag. The predictions of current state of the art force models are compared to the computed particle forces. For subcritical flows, the models are quite successful in predicting the drag. For these conditions, the magnitudes of both the inviscid and viscous unsteady force models agree well with the simulation results, but the transient nature of the viscous unsteady force history is not well captured. For supercritical flows, the inviscid unsteady force model is not able to capture the force dynamics. This highlights the need for the development of unsteady force models for supercritical flow conditions.

Experimental and Numerical Investigation on Hydraulic Performance of Slit-check Dams in Subcritical Flow Condition

Experimental and Numerical Investigation on Hydraulic Performance of Slit-check Dams in Subcritical Flow Condition

Numerical Study of Flow Structures Through Horizontal Double-Layered Vegetation Consisting of Combined Submergent and Emergent Vegetations

This study addresses the vivid internal flow structure variations through horizontal double-layered vegetation (HDLV) under subcritical flow conditions for an inland tsunami. The computational domain was built in ANSYS Workbench, while post-processing and simulation were performed using the computational fluid dynamics (CFD) tool FLUENT with the three-dimensional (3D) Reynolds stress model (RSM). Two alternative arrangements of HDLV were considered, namely Configuration 1 (short submergent layer [Formula: see text] emergent layer (Lt)) and Configuration 2 (tall emergent layer [Formula: see text] submergent layer (Ls)) along with varying flow depths. Strong inflections in velocity and Reynolds stress profiles were observed at the interface near the top of Ls, Whereas, these profiles were almost constant from bed to the top of vegetations inside Lt. A shear layer zone was formed above the top of Ls, which extended to the downstream region in Configuration 2 while it was restricted by Lt in Configuration 1. The normal Reynolds stresses at the bed were significantly greater within Ls in Configuration 2 than inside Lt in Configuration 1. Hence, Configuration 1 was performed relatively better than Configuration 2 in terms of reducing velocity within the vegetation, while Configuration 2 played a key role in attenuating the increased velocities and confining the shear layer above the short submergent layer.

Experimental study of obstacle and bed roughness effects on behavior of turbidity current

Laboratory experiments were carried out to study the behavior of turbidity current under subcritical approach flow condition. In order to study the effects of solid obstacle and bed roughness on turbidity current, 24 experiments were conducted. Two trapezoidal macro-roughness elements with 0.01 m and 0.03 m height and three triangular obstacles with 0.1 m to 0.3 m height were used. The parameters such as front velocity, velocity profiles, body height, flow discharge, suspended load transport rate and efficiency of the obstacle were determined. It was found that under subcritical flow regime, the main portion of the turbidity current over the rough bed is controlled, if the obstacle height is 2–3 times of the body height. The growth of the front height depends on the inlet sediment concentration, the roughness elements and the obstacle height. The front velocity upstream of the obstacle over the smooth bed is independent of the obstacle height and significantly influenced by roughness elements. Simultaneous use of the bed roughness and obstacle significantly reduces the front velocity downstream of the obstacle. The flow discharge per unit width significantly decreases downstream of the obstacle. Simultaneous use of an obstacle and roughness elements decreases the flow discharge per unit width by about 90%. An obstacle with height equal to 0.75 of the initial height of the turbidity current with the smooth bed blocks about 65% of the suspended sediment. While, simultaneous use of the obstacle and the roughness elements blocks about 95% of the suspended sediment.