Effect Of Bed Roughness Research Articles

Bed Roughness Effects on the Turbulence Characteristics of Flows through Emergent Rigid Vegetation

During floods, the riparian vegetation in a watercourse significantly changes the velocity distribution and the turbulence structures of the flow. However, a certain influence on them is always exerted by the bed sediments. The aim of the present work is to study the bed roughness effects on the turbulence characteristics in an open-channel flow with rigid and emergent vegetation. Toward this end, an experimental campaign was conducted and consisted of three runs with different bed roughness conditions. The study is based on the analysis of the velocity, Reynolds shear stress, and viscous stress distributions. The results show that, in the region below the free surface region, the flow is strongly influenced by the vegetation. However, moving toward the bed, the flow is affected by a combined effect of vegetation, firstly, and bed roughness, secondly. This flow zone becomes more extended, as the size of the bed sediments increases. The shear stress distributions confirm the distinction between the two flow regions. In fact, the shear stresses are practically negligible in the upper zone of the water depth influenced by vegetation, whereas, owing to the bed roughness, they reach the maximum value near the bed surface. Finally, the analysis of the turbulent kinetic energy (TKE) revealed high values below the crest level and in the near-bed flow zone in the streamwise direction, whereas a strong lateral variation of TKE from the flume centerline to the cylinder occurred in the intermediate region.

Structure functions and invariants of the anisotropic Reynolds stress tensor in turbulent flows on water-worked gravel beds

In this paper, a statistical description of turbulence in flow on a water-worked gravel-bed is presented by applying the laws of turbulence in conjunction with the double-averaging methodology. To this end, a laboratory experiment was performed, in which the gravel-bed was worked by flowing water. From the Taylor frozen-in approximation, the energy spectra of streamwise velocity fluctuation reveal the existence of the inertial subrange (lying between the energy containing and the dissipation ranges), where the turbulent kinetic energy (TKE) dissipation rate is constant. It is revealed that, in this range, Kolmogorov’s 4/5-law for the spatial increments of streamwise velocity is valid, thus allowing an accurate estimation of the TKE dissipation rate. Although Monin–Yaglom’s 4/3-law for the third-order mixed structure function provides a behavior similar to that of Kolmogorov’s 4/5-law, the estimation of the TKE dissipation rate by Monin–Yaglom’s 4/3-law is not quite accurate, owing to the departure from the isotropic turbulence at large scales. Therefore, the present study demonstrates the validity of this statistical approach (Kolmogorov’s 4/5-law) to investigate the turbulence at small scales. Besides, the data plots for an anisotropy invariant map suggest that near-bed anisotropic turbulence tends to reduce to the three-dimensional isotropic turbulence with an increase in the vertical distance, indicating a relaxation of the effects of bed roughness on anisotropic turbulence toward the free surface.

Direct numerical simulations of turbulence and hyporheic mixing near sediment–water interfaces

The effects of bed roughness, isolated from those of bed permeability, on the vertical transport processes across the sediment–water interface (SWI) are not well understood. We compare the statistics and structure of the mean flow and turbulence in open-channel flows with a friction Reynolds number of 395 and a permeability Reynolds number of 2.6 over sediments with either regular or random grain packing at the SWI. The regular sediment interface is formed by cubic packing of spheres aligned with the mean-velocity direction. It is shown that, even in the absence of any bedform, the subtle details of the particle roughness alone can significantly affect the dynamics of turbulence and the time-mean flow. Such effects translate to large differences in penetration depths, apparent permeabilities, vertical mass fluxes and subsurface flow paths of passive scalars. The less organized distribution of mean recirculation regions near the interface with a random packing leads to a more isotropic form-induced stress tensor. The augmented wall-normal form-induced fluctuations play a significant role in increasing mixing and wall-normal mass and momentum exchange.

Experimental study on the effects of artificial bed roughness on turbidity currents over abrupt bed slope change

A series of experimental observations are presented in the current study to discuss the effects of artificial bed roughness on the turbidity current flowing in a rectangular channel with an abrupt change in bed slope. For this purpose, two different types of elements, sinusoidal and trapezoidal, with various heights and arrangements are considered as artificial bed roughness. A Vectrino velocity meter was used to measure the velocity and sediment concentration profiles. The effects of inlet sediment concentration on front velocity, body velocity, unit discharge, sediment concentration, and suspended load transport rate also were investigated. Accurate equations were developed for estimation of the velocity of a turbidity current over smooth and rough beds. The unexpected experimental results showed that unlike the effect of roughness height, a change in the roughness arrangement has no significant influence on the velocity of a turbidity current. Also, the effect of bed roughness on the front velocity of a denser current is more significant.

Effect of bed roughness on tsunami bore propagation and overtopping

The accurate modelling of overtopping of coastal defences by tsunami waves is of vital importance for the formulation of disaster management strategies. To improve knowledge of this phenomena the authors conducted experiments on coastal structure overtopping using bores that were generated by a dam-break mechanism. Three types of structures were tested, namely a coastal dyke, a wall, and a wall of infinite height. The results highlight the necessity to consider the energy present in a bore to determine if a structure will be overtopped or not. As a result of these experiments an empirical formula to determine the height of overtopping given the incident bore height and velocity was validated. The study highlights the importance of clearly modelling the velocity and Froude number of a tsunami. Such experiments should be conducted on rough beds, for which a suitable Manning's n seems to be around 0.06 sm-1/3. The study also contrasted the results obtained to those of the ASCE7 method, and concludes that the Manning's n values recommended in ASCE7 are probably too low.

Effect of bed roughness on tsunami-like waves and induced loads on buildings

Tsunami, impulse-waves and dam-break waves afflict humanity with casualties and damages. An insight into the flow mechanisms of these waves is important to provide safety and reduce reconstruction costs. This experimental study focuses on the effect of bed roughness on the main hydrodynamic properties of surges propagating on dry bed. In addition, the resulting wave impact forces on buildings with and without openings are studied. Results pointed out that dry bed surges on a rough bed had a lower front celerity and a higher flow depth, resulting into inundation depths during the impact around 20% higher as compared to the smooth bed. Furthermore, a rough bed induced a lower momentum flux during wave propagation, resulting into lower impact forces exerted on the building. The rough bed configuration also caused shorter impact durations, leading to lower impulse values transferred to the building. Results pointed out that even on rough bed, openings within the buildings linearly reduced impact forces, thus providing some helpful information for the design of safer coastal structures.

Coupled modeling of bed deformation and bank erosion in the Jingjiang Reach of the middle Yangtze River

Recent significant channel evolution in the Jingjiang Reach has raised much attention, particularly the remarkable bank erosion. A coupled model for simulating bed deformation and bank erosion has been proposed in this study, which focuses on the erosion of the bank with a composite structure in the Lower Jingjiang Reach. In order to cover three contributing processes that may interact with each other, the proposed model integrates a one-dimensional morphodynamic module with a two-dimensional module of ground water flow and a bank erosion module for the cantilever failure of a composite riverbank. Model performance was evaluated through a detailed simulation of channel evolution along a 150.8-km subreach in the Jingjiang Reach over the 2005 hydrological year. Satisfying results were obtained from the simulation, showing relatively close agreement between the calculations and measurements in terms of hydrological data at the outlet section, bank erosion sites, longitudinal channel profile and typical cross-sectional profiles. In addition, investigations into temporal changes in bank soil properties and critical overhanging width at cantilever failure demonstrate that there was a seasonal variation in the volumetric water content of bank soil, which increased during the rising and flood periods and then decreased during the recession period, showing an impact on the occurrence timing of cantilever failures. The tensile strength and critical overhanging width had an inverse relationship with the water content, whereas the critical width sharply increased and then decreased during high flows affected by a rapid change in river stage. The temporal distribution of cantilever failure events indicates that cantilever failure primarily occurred in the flood and recession periods. The effects of bed roughness, water content variation and secondary flow on bank erosion were also discussed.

Oxygen Transfer at the Sediment/Water Interface for Sediment Bed With Rough Surface

AbstractA model of sediment oxygen demand (SOD) for a sediment bed with a rough surface was presented. The equivalent sand roughness commonly used in hydraulics was chosen to express the size of roughness elements, and SOD was expressed as a function of the intensity of near‐bed turbulence described by the shear velocity; microbial oxygen uptake rate, that is, the maximum oxidation rate; and roughness height. The modeling results show that bed roughness enhances oxygen transfer from the water column to the sediment surface. The effects of bed roughness on SOD depend on both the shear velocity and microbial oxygen uptake rate. At low microbial oxygen uptake rate, SOD approaches a constant value rapidly as the shear velocity increases. Bed roughness barely influences SOD regardless of the shear velocity because SOD can substantially be sediment side limiting. The roughness height plays an important role in determining SOD at high microbial oxygen uptake rate, because SOD can be controlled by oxygen transfer from the water column to the sediment surface, that is, transfer limiting. Increasing roughness height makes the oxygen transfer rate from flowing water to the sediment surface faster as well as increasing shear velocity, resulting in raised SOD. The proposed mode can substantially reproduce the experimental SOD for rough bed.

Bed roughness effects on characteristics of turbulent confined wall jets

In this paper, effects of bed roughness on the flow pattern and turbulence characteristics, such as velocity contours, shear stresses, Reynolds stresses and turbulence intensities on a three-dimensional wall jet were investigated experimentally. The experiments have been conducted in a channel which is made of metal floor and glass walls, width 0.57 m, height 1 m and length 10 m. Jet was settled using a pipe with internal diameter of 20 mm over a horizontal bed. A velocimeter was used to measure the flow velocity at three directions. The time-averaged values of velocities were used to calculate the turbulence characteristics. The results showed that with increasing surface roughness, the rate of growth length of longitudinal scale decreases, while decay rate and jet velocity increases. Moreover, the maximum value of turbulence intensity is augmented with increasing the surface roughness. When surface roughness increases, the rate of length scale decreases and decay of jet velocity enhances. Whereas u-component of turbulence intensity increases by surface roughness increment. Also, when distance increases from jet exit, bed shear stress decreases and when surface roughness increases, surface shear stress enhances.The vertical distribution of ŵ for various x̂ showed that the vertical direction of velocity has changed from descending to ascending in x/d = 45 to x/d = 55 due to the existence of backflow and consequently the creation of a vortex. Furthermore, with an increase in surface roughness, the decay of W component of velocity has increased.

Effect of bed roughness on velocity profile and water entrainment in a sedimentary density current

In this study, the effect of bed roughness on velocity profile and water entrainment in a sedimentary density current for Richardson numbers of 1.2–7 (subcritical flow conditions) was investigated. Experiments were carried out in a tilting flume with four different bed slopes, four roughness heights, and two fluid densities of sedimentary density currents. The results showed that bed roughness significantly affects the general shapes of velocity profiles, especially in the wall region. Two empirical equations were developed as the functions of the relative roughness for the wall and jet regions of velocity profile using the measured velocities of density currents. Water entrainment was also affected by bed roughness and an empirical equation was developed describing the relationship of this phenomenon with the Richardson number and relative roughness. Sensitivity analysis of this equation by using elasticity coefficient method showed that the effectiveness of the Richardson number is 3.9 times more than the effect of relative roughness on water entrainment.

Effects of roughness and permeability on solute transfer at the sediment water interface

Understanding the mechanisms of solute transfer across the sediment-water interface plays a crucial role in water quality prediction and management. In this study, different arranged particles are used to form typical rough and permeable beds. Large Eddy Simulation (LES) is used to model the solute transfer from the overlying water to sediment beds. Three rough wall turbulence regimes, i.e., smooth, transitional and rough regime, are separately considered and the effects of bed roughness on solute transfer are quantitatively analyzed. Results show that the classic laws related to Schmidt numbers can well reflect the solute transfer under the smooth regime with small roughness Reynolds numbers. Under the transitional regime, the solute transfer coefficient (KL+) is enhanced and the effect of Schmidt number is weakened by increasing roughness Reynolds number. Under the rough regime, the solute transfer is suppressed by the transition layer (Brinkman layer) and controlled by the bed permeability. Moreover, it is found that water depth, friction velocity and bed permeability can be used to estimate the solute transfer velocity (KL) under the completely rough regime.

D-jump in rough sloping channels at low Froude numbers

Hydraulic jump is a phenomenon which usually occurs in rivers and in correspondence with hydraulic structures. It is characterized by a rapid transition from a super- to sub-critical flow conditions, involving a huge energy dissipation. For its characteristics and its relevance in hydraulics, it has been studied since the early years of the last century. Nevertheless, many features still require a further investigation, e.g., scale effects, air entrainment process, effect of bed roughness on conjugate depths, etc. In particular, in the last few decades, the analysis of the main geometric parameters has been further developed. Nevertheless, no studies dealing with D-jump occurring on a rough sloping channel are present in literature. Therefore, the present paper reports the results of experimental investigations, which allowed to develop a semi-theoretical approach in order to evaluate the D-jump conjugate depth in a wide range of channel bed geometric and boundary configurations.

Effect of lateral bed roughness variation on particle suspension in open channels

In this paper the effect of variation of bed roughness along lateral direction on suspension concentration distribution in open channel turbulent flows was investigated. Starting from the mass and momentum conservation equations, this study demonstrates that both the Reynolds shear stress \((-\overline{u'v'})\) and sediment diffusivity depends on bed roughness. From the theoretical analysis, it is found that both the Reynolds shear stress and the sediment diffusivity increase over smooth bed surfaces and decrease over rough bed surfaces. At the junction of smooth and rough bed surface, the effect of bed roughness on the Reynolds shear stress and sediment diffusion is almost negligible. Including this effect, suspension concentration distribution is also studied and from the Hunt’s diffusion equation, an analytical model for predicting suspension concentration is proposed. Apart from this effect, the effects of moveable bed roughness and stratification are also considered in the model. It is observed that the Rouse equation is obtained from the proposed model as a special case when the flow is considered as single phase and there is no effect of secondary current, stratification and bed roughness variation. On the basis of experimental data available in literature, the proposed model is validated and also compared with the Rouse equation. To get a quantitative idea about the goodness of fit, weighted relative error is calculated. The comparison results and calculated errors indicate that the present model is capable of describing the suspension concentration distribution more accurately than Rouse model throughout the flow depth in open channel flow.

Fluctuations and time scales for bed-load sediment motion over a smooth bed

Results are presented for experiments of bed-load sediment transport over a plane, smooth bed. The smooth-bed configuration, though not adequate for mimicking natural streams, enables the effects of bed roughness to be filtered out, thus, highlighting the role of flow turbulence for particle dynamics. Sediments were individually tracked along their paths, measuring position and velocity of the individual grains. A number of analyses were then applied to the data: probability density function, auto-correlation, and spectra of the grain velocity. Several Lagrangian time scales of particle motion were obtained and compared to available data for the turbulent flow field to determine a phenomenological interpretation of the process.

Flow structure and mean residence times of lateral cavities in open channel flows: influence of bed roughness and shape

Natural lateral cavities in open channels are important because their lower water velocities promote water quality and provide refugia for organisms. Little is known about the influence of natural cavity shapes and roughness on flow structure and exchange dynamics. We investigated the effects of cavity shape (semi-circular, backward conic, and forward conic) and bed roughness on the flow structure and mean residence time (MRT) of a lateral cavity in a flume. All cavity shapes have a flow field dominated by a one-gyre recirculation pattern, contrasting results of rectangular cavities at similar Reynolds number and aspect ratios. Transverse velocity energy spectra indicate the flow is dominated by large-scale quasi-2D coherent structures. Fundamental frequencies of mixing layer vortex shedding are fastest for forward conic cavities and slowest for backward conic cavities. Fluid enters cavities at shallower mixing layer depths, and fluid exits cavities at deeper mixing layer depths. MRTs are smaller for hydraulically smooth cases and forward conic cavities due to higher recirculation velocities. MRTs are larger for rough bed cases and backward conic cavities. Rough flow cases have a strong correlation to a predictive MRT relationship derived by Jackson et al. (Water Resour Res: 10.1002/wrcr.20272 , 2013) (R 2 = 0.77); however, this predictive model does not work well for smooth cavities. Two MRT relationships were derived for smooth lateral cavities and both have strong power-law correlations to normalized MRT. Understanding cavity shape and bed roughness effects will provide a guideline for designing lateral embayments in stream restoration projects.

Effect of Bed Roughness on Submerged Hydraulic Jumps

Free and submerged hydraulic jumps are important phenomena in open channel flow. The basic characteristics of these jumps formed on smooth, horizontal channel beds were covered well in literature both experimentally and theoretically. However, not many studies are reported on submerged hydraulic jumps generated on rough beds. This study mainly aims to investigate the characteristics of submerged hydraulic jumps over rough, horizontal beds. But, as the submerged hydraulic jumps are created from free jumps, analyses of free jumps are also required to proceed with the present study. So, altogether 110 experimental runs were carried out under both smooth and rough bed conditions. All the experiments were carried out in a rectangular flume and for the rough bed condition. Bed of the flume was roughened by placing rectangular wooden strips at four different spacing. The hydraulic parameters such as initial water depths, subcritical sequent depth, tail water depth, back-up depth and flow rates were measured for different roughness. Using these available data, required data analyses were carried out and several relationships were developed in order to express the submerged hydraulic jump characteristics. Here, free and submerged hydraulic jumps were considered separately. ENGINEER, Vol. 47, No. 04, pp. 33-39, 2014

Effect of bed roughness on 1-D entropy velocity distribution in open channel flow

A theoretic-analytical formulation, based on entropy velocity profile law and classical relationships for uniform flow and friction factor, is proposed enlightening the general logarithmic relationship existing between the parameter Φ(M), defined as mean cross section velocity over maximum velocity, and the ratio water depth/bed roughness (D/d). The relationship Φ(M)–D/d has been applied to a relevant set of experimental velocity measurement data collected both in laboratory and in field, showing different behaviour between small scale and large-intermediate roughness flows. In particular, the roughness influence becomes remarkable whenever shallow water flow conditions occur, that is when the ratio between the flow depth and the roughness height is less than 4, while Φ(M) tends to be constant as the value of D/d increases.

Effect of bed roughness on grain-size distribution in an open channel flow

Grain-size distributions of suspended load were studied in a laboratory flume over five sediment beds having different values of bed roughness at three different flow velocities. The experiments had been performed to investigate the influence of bed roughness, flow velocity and suspension height on the grain-size distribution in suspension. This study focuses on the determination of the proportionality parameter βn in suspension which is the ratio of sediment diffusion coefficient to the momentum diffusion coefficient of the nth grain-size class. An empirical equation for βn has been proposed which is valid for a wide range of normalizing settling velocity of sediment particles and bed roughness. Also, the bed roughness effect is studied on the parameter β for total concentration in suspension and on the reference concentration, which is very important in suspension studies. The Rouse equation with modified βn and β was surveyed to know the grain-size distribution and total concentration in suspension that agreed well when compared with the experimental data.

Flow Field Around Single and Tandem Piers

The present study provides a comparison between the flow pattern around two circular piers in tandem and a single pier set up on a moderately rough flat bed in a laboratory flume. Velocities are measured by an Acoustic Doppler Velocimeter (ADV). The contours of the time-averaged velocity components, Reynolds shear stresses, turbulence intensities and turbulence kinetic energy at different planes are presented. Streamlines and vectors are used to study the flow features. The analysis of power spectra around the piers is also presented. The results show that the presence of downstream pier changes the flow structure to a great extent, particularly in the near-wake region. Within the gap between the two piers, a stronger and substantial upflow is shaped. However, a weaker transverse-deflection is formed in comparison with that in the single pier. Near the bed, the velocity of flow approaching the downstream pier decreases to 0.2–0.3 times of the approach mean velocity (U 0) due to the sheltering effect of the upstream pier. In the wake of downstream pier, the flow structure is completely different from the one in the wake of single pier. In comparison with the single pier, the values of turbulence kinetic energy and turbulence intensities show a considerable decrease around the tandem piers. In tandem piers, the high values of turbulence characteristics are found near the downstream pier. There is a recirculation zones just upstream of the sheltered pier close to the bed and another behind that pier near the free surface. The results show a decrease in the strength of vortical structure in the wake of tandem piers in comparison with single pier. It is shown that the formation of flow with different Reynolds number along the flow depth due to the effect of bed roughness, as well as pier spacing, can influence the type of flow regime of tandem case. In addition to enhancing the flow structure indulgence, the present detailed measurements can also be used for verification of numerical models.

Calibration of a 2-D morphodynamic model using water–sediment flux maps derived from an ADCP recording

This study compared numerical simulations of the hydro-morphodynamics of a stretch of the Po River (Italy) to detailed measurements of the river's morphology and water–sediment fluxes. A survey of the 8-km-long, 250-m-wide river section, previously described in former publications, was performed using two vessels. The first was equipped with a multibeam echo sounder for bathymetry and the investigation of bed forms, while the second was equipped with two acoustic Doppler current profilers (ADCPs) to map bed roughness, flow velocity, and suspended sediment fields. This paper focuses on the calibration of the shallow water numerical model MIKE21C implemented by the Danish Hydraulic Institute. Sensitivity analyses were performed to analyze the effects of bed roughness and sediment transport direction on the simulated flow field and morphology. The results fixed the predominant flow and morphology parameters in the model, and show how the velocity and sediment field maps that were derived from the ADCP recording could be used for calibration and validation of the existing software.