Straight Compound Research Articles

Stage–discharge relationship in an erodible compound channel with overbank floods

Due to environmental changes and anthropogenic activities, a dramatically diminished upstream sediment supply can promote riverbed erosion and alter the stage–discharge relationship in a channel with overbank flows, which can impact flood control. It is important to understand how the stage–discharge relationship in an erodible compound channel is affected and how to predict this relationship. In this study, we performed laboratory experiments in a straight compound channel with a mobile main channel under clear water scour. Stage–discharge relationships were measured before the destruction of the armor layer and after the formation of a new armor layer at the riverbed surface. The results indicated that the impact of riverbed erosion on the stage–discharge relationship cannot be ignored. Under the same discharge, the flow depth in the main channel increased, while the flow depth in the floodplains decreased after the new armor layer was formed relative to the case before armor layer destruction. Considering the impacts of the erosion depth and the variation in the bed resistance, we proposed a new method for predicting the stage–discharge relationship in an erodible compound channel after riverbed erosion. The predicted stage–discharge relationships agreed with the measurements, indicating that the proposed method could be used to precisely predict the stage–discharge relationship after riverbed erosion occurrence. The comparison of the results of the proposed method with and without considering the erosion depth supported the idea that the impact of riverbed erosion on the stage–discharge relationship must be included. Finally, the proposed method was further used to examine the influence of floodplain encroachment on flood control and riverbed erosion. Floodplain encroachment produced a larger flood flow depth on floodplains and a higher mean velocity in the main channel, suggesting that floodplain encroachment should be avoided to eliminate possible increases in flood disasters and riverbed erosion risks.

Modeling velocity in a compound channel with co-existing emergent and submerged vegetation

Vegetation on a floodplain, which contains both emergent vegetation and submerged vegetation, has a considerable influence on the velocity profile of the channel. In this study, a modified analytical model, which considers interactions within the vegetation, is developed based on the Shiono and Knight method (SKM) and the concept of the two-layer model to obtain the transverse distribution of the depth-averaged streamwise velocity in a compound channel with emergent and submerged vegetation. This analytical model includes the influence of secondary flows, lateral shear stress, bed friction, and the drag force caused by the vegetation. The aforementioned model is then employed in a straight compound channel, which contained various types of vegetation. Using suitable boundary conditions, the calculated data are found to be consistent with the experimental data. Subsequently, the effects of dimensionless eddy viscosity coefficient (λ), the depth-averaged secondary flows coefficient, and the impact coefficient β on the model are analyzed. It finds that the λ has a clear effect on the main channel region, but the depth-averaged secondary flows coefficient (K¯) has only a definite effect in the slope sub-region. The β value is significantly less than 1 for the condition of the vegetation with a variable frontal width. The model proposed in the present work can provide a guidance for the investigation of the flow characteristics of the various vegetated channel.

Combined effect of a mobile bed and floodplain edge vegetation on compound channel conveyance

This technical note presents results from flume experiments conducted in a straight prismatic asymmetric compound channel with two different floodplain roughnesses and rigid emergent vegetation along the floodplain edge in combination with a mobile and fixed main channel bed, respectively. It is found that the contribution of floodplain roughness and vegetation along the floodplain edge to the bulk friction factor is considerably reduced when studied in combination with a mobile main channel bed featuring bedforms. This in turn means that the effect of vegetation along the floodplain edge on the conveyance capacity and bulk friction factor may be overestimated if it is studied solely with fixed bed conditions. The presented results reveal the need to improve our understanding of the complex interplay between mobile beds and vegetation along the floodplain edge and its combined effect on compound channel conveyance capacity and hydraulics.

GRNN-based models for hydraulic jumps in a straight rectangular compound channel

GRNN-based models for hydraulic jumps in a straight rectangular compound channel

Analytical solution for depth-averaged velocity and boundary shear in a compound channel

Estimation of streamwise depth-averaged velocity and boundary shear stress are important requisites for modelling of river flows associated with flood events. Recently, many methods have emerged to predict these flow variables with great accuracy, but they provide unsatisfactory results in shear layer regions. This paper presents an improved methodology to predict depth-averaged velocity and bed shear stress for a straight compound channel flow. An analytical solution to the depth-integrated turbulent form of the Navier–Stokes equation is obtained. The transverse shear stress in the mixing region is modelled using an effective eddy viscosity concept that contains horizontal coherent structures and three-dimensional bottom turbulence. The secondary flow term is modelled by considering the log-law profile for streamwise velocity and half cosine curve for the transverse velocity component. The analytical solution is successfully applied to a wide range of experimental compound channels and field cases. The efficacy of the present solution has been successfully tested by comparing with observed values.

Contribution to the study of the semi theoretical approach of the hydraulic jump in a rectangular channel of compound section with rough major bed

The purpose of this study is interested in the semi-theoretical analysis of the hydraulic jump evolving in a rectangular straight compound channel with rough major bed. A functional relationship, non-dimensional terms, linking together the different characteristics of the projection, is obtained.

EXPERIMENTAL STUDY OF THE SEQUENT DEPTH RATIO OF THE HYDRAULIC JUMP IN A STRAIGHT COMPOUND RECTANGULAR CHANNEL

The aim of this experimental study is to analyze the sequent depth ratio of the hydraulic jump evolving in a straight compound rectangular channel. A functional relationship, in non-dimensional terms, linking together the different characteristics of the jump, is obtained.

Nonuniform Flows in a Compound Open‐Channel: Assessment of a Hybrid RANS‐LES Approach

AbstractOverbank flows in rivers, also termed compound open‐channel flows, are rarely uniform in the streamwise direction. Flow nonuniformity involves transverse currents directed from the river main channel (MC) to adjacent floodplains (FPs) or vice versa. Depending on their direction and magnitude, the transverse currents can laterally displace the planform shear layer that forms at the interface between MC and FP. They can also modify the cross‐sectional distribution of the helical secondary currents (SCs). This paper reports a numerical study that focuses on assessing the ability of a hybrid RANS (Reynolds Averaged Navier‐Stokes) ‐ LES (Large Eddy Simulation) approach, namely, the Scale Adaptive Simulation (SAS), to capture the evolving structure of both depth‐uniform and nonuniform flows in a straight compound open‐channel. The simulated flows have recently been studied in an 18 m long and 3 m wide laboratory flume by Proust and Nikora (2020, https://doi.org/10.1017/jfm.2019.973). The originality of the study lies in the modeling of flows over the entire flume length and in the estimate of the length‐scales of the large Kelvin‐Helmholtz‐type coherent structures (KHCSs). The SAS approach can accurately predict the longitudinal evolution of: (i) the streamwise time‐averaged flow, including the planform shear layer width, (ii) the SC patterns, and (iii) the turbulence statistics. The emergence and development of the KHCSs are qualitatively retrieved and are confirmed to be controlled by dimensionless velocity shear irrespective of flow depth. The simulations also show that SC patterns depend on the magnitude and direction of the transverse currents and also on the development of KHCSs.

Modeling depth-averaged streamwise velocity in straight trapezoidal compound channels with ice cover

This study considers the flow of water in ice-covered compound channels with straight trapezoidal cross-sections. For describing the lateral distribution of depth-averaged streamwise velocity in aforementioned channels, a modified analytical model, which incorporates the influences of channel bed friction, ice cover resistance, lateral shear turbulence and secondary currents, is derived by the Shiono and Knight method and a two-layer hypothesis. Then the analytical model is applied to straight compound channels with full ice cover and symmetrical shore ice. The modeled results of the two cases, which employ appropriate boundary conditions, indicate good predictions for the transverse distribution of depth-averaged streamwise velocity when compared with captured experimental results. Subsequently, the intensities and rotational directions of secondary flows and the dimensionless comprehensive eddy viscosity effect are exhaustively discussed, finding that the intensities of secondary flows in the main channel domain for the two cases present opposite changing laws as flow depth increases, and the signs of secondary flow coefficients are verified to be consistent with experimental observations. The theoretical approach used herein can supply some guidance for investigating the flow characteristics of ice-covered channels.

Analysis of flow at the vicinity of vegetation stems in the floodplain of a compound open channel

ABSTRACT An analysis of flow was carried out in this study to find the effects of trees on the flow structure in a straight compound open channel. To resemble the vegetation stem, two rows of vertical rods were installed on the floodplain of an asymmetrical compound channel. Two-dimensional flow velocity was measured using Particle Image Velocimetry. The results showed a decreasing trend in turbulent kinetic energy and turbulent intensity along the floodplain. At low flow depth, a significant decrease was observed in mean velocity and kinetic energy. Similar reduction was observed at the intersection of floodplain and main channel and around the stems. Under this condition, the direction of velocity vectors changed from floodplain to the main channel. However, this trend was reversed at higher flow depths. This reversal was due to the changes in momentum transfer and shear stress at the intersection of floodplain and main channel. Additionally, a numerical analysis showed that vortices are formed at the junction of the floodplain and main channel.

A circular cylinder in the main-channel/floodplain interface of a compound channel: effect of the shear flow on drag and lift

The interface between the main channel and the floodplain of a compound channel is often populated by trees or buildings that normally remain emergent during floods. This study investigates how drag on an emergent cylinder is affected by the shear flow that develops at the interface. The experimental set-up features a circular cylinder at the interface of a straight compound channel under uniform flow conditions. The integral form of the equation of conservation of momentum was used to calculate the magnitude and direction of the time-averaged drag force per unit submerged-length of the cylinder. All terms were experimentally determined, except for those associated with fluid interaction with the cylinder. The same method was also applied to a cylinder under symmetrical flow conditions. It is concluded that the existence of the shear layer leads to an asymmetrical drag force and to a reduced drag coefficient.

Apparent Friction Coefficient Used for Flow Calculation in Straight Compound Channels

Water flow in channels with a compound cross-section involves an exchange of water mass and momentum between the slower flowing water in the floodplains and the faster water in the main channel. This process is called the streams interaction. As a result, the water velocity in the main channel decreases, and at the same time the velocity and depth of flow increase in the part of the floodplains adjacent to the main channel. Diversification of the surface roughness of the main channel and floodplains significantly affects the form of interactions. The results of laboratory experiments were used to characterize the influence of interactions on the discharge capacity of the channel with diversified roughness. The reduction in velocity of the main channel caused by the stream interactions is described with the apparent friction coefficients introduced at the boundary between the main channel and the floodplain. The obtained values of resistance coefficients, supplemented with the values from experiments reported in the literature, were used to establish a relationship useful in assessing the discharge capacity of such channels.

A new apparent shear stress-based approach for predicting discharge in uniformly roughened straight compound channels

Accurately predicting flow discharge in a compound river channel becomes increasingly important when in river eco-environment design and flood risk management, in which the conventional methods, such as divided channel method (DCM) and single channel method, do not meet precision requirement. In the present paper, a new approach is proposed to improve the prediction of flow based on the concept of the apparent shear stress at the diagonal interface between main channel and floodplains. The new approach is compared with a wide range of the author's experimental data and the data available in the literature. The 27 datasets used in this study cover homogenous symmetric channels (22 datasets) and asymmetric channels (5 datasets), which have various width ratios [channel total width (B) at bankfull / main channel bottom (b) = 1.5 ~ 15.8] and bed slopes (So = 2.6 × 10−4 ~2.5 × 10−3). It was found that the new approach can significantly improve the accuracy of flow prediction compared with the conventional DCM for all the datasets, particularly for relatively low flow depths of floodplain (e.g. Dr < 0.15) where the flow discharge was very difficult to predict correctly. The new approach predicts the total discharge well for straight homogeneously symmetric and asymmetric channels, within a mean absolute percentage error (MAPE) of 5%. Furthermore, compared with the DCM, the new approach can also have a better prediction of zonal discharge percentage and it has less variation of discharge prediction error over the flow depth.

Apparent shear stress-based method on an inclined interface plane for predicting discharge in straight compound channels

Accurately predicting flow discharge in a compound river channel becomes increasingly important for flood risk management and river eco-environment design. This paper proposes a new general approach based on the concept of the apparent shear stress at an inclined interface plane between main channel and floodplains. The new approach with a diagonal plane is applied with a wide range of the author’s experimental data and the data available in the literature, which include 59 datasets. Among them, 27 are homogenous channels of symmetric channels (22 datasets) and asymmetric channels (5 datasets) whereas 32 are heterogeneously roughened channels of symmetric channels (22 datasets) and asymmetric channels (10 datasets). It was found that the new approach improves the accuracy of discharge compared with the DCM for all datasets. The predicted total discharge for straight homogeneous channels has a mean absolute percentage error (MAPE) of 5%, whereas the MAPE error is about 6.7% for heterogeneously roughened channels.•A general approach of discharge prediction is presented based on apparent shear stress on an inclined interface.•Both zonal and total discharge can be calculated using the proposed method.•The predicted results are compared with 59 sets of experimental data along with the DCM.

TRANSPORTATION OF BED LOAD AND ITS FORMATION FOR INBANK FLOW: A PHYSICAL MODELLING APPROACH

Floods hit various states in Malaysia and caused damage to properties, infrastructures, human suffering and even loss of lives. Frequent flood incidents and soil erosion are reported which lead to sedimentation problems in the drainage and river systems due to urbanisation. It is important to understand the sedimentation process and the behaviour of bed forms in the water course for post-flood events. However, it is risky and dangerous to conduct field study during occurrence of flooding. Laboratory study has been chosen as an alternative to fulfill the objectives. Experimental investigations on the bed load transport and bed forms in rectangular open channels have been undertaken. Small and large scale physical models are used in the laboratory. The effects of flow on mobile sandy bed channel and bed load transportation are studied by using a modified flumes with an asymmetric straight compound channel. However, the findings on rate of transport and bed formation for non-flooding cases are presented in this paper. It is found that the bed formation profiles for both channels are different due to scale and flume characteristics. The bed forms, erosion and deposition processes are significantly influenced by the water velocity in the channel. The bed forms observed for large flume are repeating ripples and dunes; meanwhile variable of ripples are observed in small flume.

Influence of riparian vegetation on flow resistance in mobile bed straight compound channels

Riparian vegetation is an important feature in rivers and essential in hydraulic engineering including flood management and river restoration. Deforestation activities have been pointed out as a contributing factor to the severity of flood damages. This shows that the importance of riparian vegetation in river restoration schemes in order to lessen the issues of flood risk and flood protection. The riparian vegetation creates additional resistance to flood flow which resulted in velocity reduction, increasing of flow depth and transportation of sediment in rivers. The hydraulics of flood flows in vegetated floodplain of mobile bed straight channel was experimented in the Hydraulics Laboratory, Faculty of Civil Engineering, Universiti Teknologi Malaysia. Two-lined steel rods with tandem and staggered arrays to simulate as rigid emergent vegetation were placed along the riparian zone of an asymmetrical channel. The flow resistance including Manning’s n, Darcy’s f and drag force, FD were studied. The findings showed that the riparian vegetation had increased the Manning’s n by 20%. Meanwhile, staggered arrangement of vegetation was found to generate the highest Darcy’s f of about 16% as compared to tandem arrangement. The presence of riparian vegetation also found to induce 20% more drag force compared to the main channel roughness effect. The dramatically changes of bed morphology also led to the increase of the flow resistance in the channel.

Boundary Shear Stress Distribution in Straight Compound Channel Flow Using Artificial Neural Network

Boundary shear stress distribution of a compound channel is generally influenced by the geometric, roughness, and hydraulic parameters. Experiments are performed on both homogeneous and nonhomogeneous compound channels to study the dependency of variables on the boundary shear distribution. This study proposes an artificial neural network (ANN) model for the prediction of boundary shear stress distribution in straight compound channels. The most influential parameters such as width ratio, relative flow depth, aspect ratio, Reynolds number, and Froude number are considered as input parameters. A large number of experimental data sets comprising wide ranges of width ratio, relative flow depth, roughness ratio, Reynolds number, Froude number, bed slope, and aspect ratio with the present experimental data series are used for both training and validation of the model. Previous models can provide good results only for specific ranges of independent parameters, whereas back-propagation neural network (BPNN) models are capable of performing well for global ranges of independent parameters. This is because BPNN is able to perform nonlinear mapping between the dependent and independent variables during the training. The efficacy of the models is verified with the standard statistical error analysis using the global data sets.

Drag determination of an array of square cylinders subjected to shear flow in a compound channel

Overbank flow in rivers threatens integrity of built elements located in the floodplain. Elements of infrastructure close to the interface between main channel and floodplain are subjected to complex hydrodynamic actions resulting from the obstruction of the shear flow that develops in that interface. In the current paper, the drag forces and the drag coefficient of building-like structures positioned in the interface are investigated. The experimental setup in Laboratorio Nacional de Engenharia Civil (LNEC) involves the placement of an array of square cylinders on the floodplain of a straight compound channel, next to the interface with the main channel. Three-component instantaneous-velocity recordings were performed by means of Acoustic Doppler Velocimetry (ADV) within the boundaries of a considered fluid-control volume encompassing the array, while uniform-flow conditions were established in the channel. The equation of momentum conservation was applied in its integral form in the fluid control-volume towards estimation of the time-averaged drag force at a certain elevation from the floodplain. The drag coefficient is estimated accounting for the typical shear layer at the main-channel/floodplain interface and is compared with coefficients strictly valid for isolated cylinders.

A New Antifungal Antibiotic from Bacillus sp. KM5 Isolated from Rice Rhizospheric Soil

Bacterial strain, Bacillus sp. KM5 was recently isolated and characterized by strong antifungal activities against several agronomically important filamentous pathogenic fungi including Rhizoctonia solani Nees. This is a new study to characterize four prominent antibiotics synthesized by Bacillus sp. KM5 (termed M1, M2, M3, and M4), isolated from rice rhizospheric soil. Lipopeptide profiling and spectroscopic characterizations of antibiotics M1 and M2 identified them as Bacillomycin F and Bacillomycin D, respectively. Antibiotic M3 was recognized as another iturinic lipopeptide Bacillopeptin A. Finally, M4 emerged as a novel straight chain polyhydroxy polyether compound with a single double bond, indicating a non-polyene structure with no prior mention in the literature. Furthermore, in vitro bioassay of M4 showed maximum activity against Rhizoctonia solani among five plant pathogenic fungi. This research could help in developing new or improved antifungal formulations or novel strategies to prevent fungal spoilage.

Stage–discharge estimation in straight compound channels using isovel contours

AbstractFor various hydrological applications such as flood control projects, a knowledge of stage–discharge relationship is of particular interest to river engineers. Stage–discharge curves in compound channels cannot be easily predicted in comparison with single channels due to their 3D characteristics of flow. In this paper, the concept of cross‐sectional isovel contours is used for estimation of stage–discharge curves in compound channels. The multivariate Newton's method is applied to the difference between the observed and estimated data to optimize the exponent values of the governing parameters. The accuracy of the proposed model is tested successfully against available experimental results, which are taken from the Flood Channel Facility (FCF) laboratory. Then the results are compared with the Single and Divided Channel Methods (SCM and DCM, respectively), the Weighted Divided Channel Method (WDCM), the exchange discharge method (EDM), and the Coherence Method (COHM). The average values of mean absolute percentage error (MAPE) and normalized root mean square error (NRMSE) in discharge estimation based on each referenced section at any level for 6 sections of the experimental cases are within 3.1% and 0.023, respectively. The biggest advantage of the proposed method is its inherent simplicity, which does not need any calibration.