Methods to control bed erosion at 90° river confluence: an experimental study
10 Apr 2017-International Journal of River Basin Management (Taylor & Francis)-Vol. 15, Iss: 3, pp 297-307
TL;DR: In this article, vanes and circular piles are proposed as scour mitigation measures in a distorted mobile bed model with 90° confluence angle and three discharge ratios (Qr = 0.33, 0.50 and 0.75) are used.
Abstract: Confluence is a common occurrence in rivers. The convergence of flows often leads to erosion of the river bed and formation of a deep scour-hole at the confluence. In the present experimental study, vanes and circular piles are proposed as scour mitigation measures. Experiments are performed in a distorted mobile bed model (d50 = 0.28 mm) with 90° confluence angle. Three different discharge ratios (Qr = ratio of lateral to main flow discharge) of 0.33, 0.50 and 0.75 are used. Vanes (1.5 cm width and 1 mm thick) or piles (ɸ = 8 mm and 12 mm) are arranged in a row perpendicular to the lateral flow at a spacing of 5, 10 or 15 cm. Three vane angles of 15°, 30° and 60° with respect to the main flow are used. The experimental results show that scour depth (Sd) increases with an increase of Qr. Sd reduces by 33%, 50% and 47% with vanes for Qr = 0.33, 0.50 and 0.75, respectively. Sd reduces by 43%, 55% and 55% with 12 mm piles and by 70%, 60% and 59% with 8 mm piles, for the corresponding discharge ratios...
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01 Jan 2004
TL;DR: In this article, a simple one-dimensional formulation was proposed to predict the transitional flow at an open-channel junction, based on energy and continuity equations, and an empilical relation between the junction losses, the junction angle, and the discharge ratio was suggested which agrees well with the experimental results.
Abstract: On the basis of energy and continuity equations a simple one-dimensional formulation was proposed to predict the transitional flow at an open-channel junction. An empilical relation between the junction losses, the junction angle, and the discharge ratio was suggested which agrees well with the experimental results. The results calculated by the present formulation for the depth ratio were compared with the results of earlier one-dimensional formulations and experiments. It is found that the present results coincide better with experiments than those of others.
24 citations
TL;DR: In this article, a study was conducted at the point of convergence of the River Deo with the River Manu using simple geospatial technique and field work to justify the hydrological and morphological changes in downstream of the confluence point.
Abstract: Tributaries play a significant role in changing flow structure of the main river through their additional discharge. A study was conducted at the point of convergence of the River Deo with the River Manu using simple geospatial technique and field work to justify the hydrological and morphological changes in downstream of the confluence point. The spatio-temporal change of both the river channels for 10 km length in upstream and downstream of the confluence point was considered. Moreover, field measurements and post-field work were carried out to examine the spatial variation of hydrodynamic characteristics like flow velocity, depth, water discharge, wetted perimeter and hydraulic radius of the selected stretches. Sieving method was applied for grain size analysis of river bed sediment samples. The result revealed that both the aggradation and degradation processes were equally active in the upstream segment of the Manu River but in downstream segment aggradation exceeded the degradation activity, though all the hydrodynamic variables were boosted up in downstream, except flow velocity. The present research highlighted that the steeper gradient of the R. Deo had enhanced its competency to transport medium-sized grains to the R. Manu, where fine grains were commonly found. Moreover, increased wetted perimeter in downstream specified more friction between channel bed and its flow consequent upon reduced flow velocity with extra sediment load accumulation.
9 citations
TL;DR: In this paper, experiments were conducted to control sediment entry into an intake channel using submerged vanes in a physical model with a rectangular mobile-bed main channel and a trapezoidal rigid-bed intake channel diverting at an angle of 45°.
Abstract: Intake canals are used to withdraw water from rivers for various purposes. Sedimentation in the intake canal reduces the quality and quantity of water being delivered. In this study, experiments were conducted to control sediment entry into an intake channel using submerged vanes in a physical model with a rectangular mobile-bed main channel and a trapezoidal rigid-bed intake channel diverting at an angle of 45°. The variables in the study included vane angle, number of vane rows, and vane spacing in terms of mean flow depth in the main channel. In addition to the commonly used vane array with uniform vane heights, three other vane-height configurations were also tested. The least local scour around vanes and highest sediment reduction (~70%) were observed for vanes oriented at a 15° vane angle with an increasing vane-height configuration placed in two rows. It was also observed that control of sediment entry into the intake canal increased with an increase of both vane spacing and number of vane rows.
7 citations
14 Jun 2018
TL;DR: A confluence is a place where two flows with different flow and sediment characteristics merge together as discussed by the authors, which is a common occurrence along the natural rivers as well as artificial open channels.
Abstract: A confluence is a place where two flows with different flow and sediment characteristics merge together. Confluences are common occurrences along the natural rivers as well as artificial open channels. In general, a lateral flow confluences into a main flow at various angles. The confluence angle influences the flow and sediment transport at the confluence region. The bed erosion occurs because of turbulence at the confluence. Sometimes, the bank opposite to the direction of lateral flow fails due to the increase in lateral momentum. In addition, the main flow width in the downstream of the confluence increases due to increase of discharge. A confluence is characterized by the presence of a stagnation zone, a separation zone, a mixing layer and the recovered flow in the downstream. A secondary circulation (helicoidal flow cells) induced by the centrifugal action of the lateral flow when merging with the main flow leads to formation of a scour-hole along the central portion of the confluence. The eroded soil from the confluence poses problems by deposition in the downstream locations such as check dams, barrages and reservoirs resulting in reduction of water storage capacity as well as water quality. Hence, this necessitates studies on control of bed erosion at the confluence.
6 citations
TL;DR: In this article, a 2D numerical model is used in simulating hydromorpho dynamics in the rivers confluence to mitigate the erosion and deposition zones by adopting vanes as control structures.
Abstract: Controlling the flow and bed morphology in a river confluence is important in training and navigation works. The flow in river confluence is highly complex due to crucial and rapid changes associated with flow dynamics, sediment transport, and geomorphology. The flow in Malaysia’s rivers has many confluence junctions in natural drains of catchment areas. The confluence between Kurau and Ara Rivers, in Perak, Malaysia, is selected to investigate the scour hole that usually forms in the erosion zone and the bar that forms in the deposition zone. A 2D numerical model is used in simulating hydro-morpho dynamics in the rivers confluence to mitigate the erosion and deposition zones by adopting vanes as control structures. Simulation results suggest that the most effective location, dimension, and angle of vanes can be decided based on their performance in scouring and deposition zones. The distribution velocity and flow vectors can help in deciding the location of the vanes.
5 citations
Cites background from "Methods to control bed erosion at 9..."
...Two recent studies were conducted on a laboratory scale with 60◦ and 90◦ confluences [38,39], in which a set of vanes and piles were proposed to control bed erosion....
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References
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TL;DR: In this article, a method is presented which enables the computation of the bed-load transport as the product of the saltation height, the particle velocity and the bed load concentration.
Abstract: A method is presented which enables the computation of the bed-load transport as the product of the saltation height, the particle velocity and the bed-load concentration. The equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity. Experiments with gravel particles (transported as bed load) are selected to calibrate the mathematical model using the lift coefficient as a free parameter. The model is used to compute the saltation heights and lengths for a range of flow conditions. The computational results are used to determine simple relationships for the saltation characteristics. Measured transport rates of the bed load are used to compute the sediment concentration in the bed-load layer. A simple expression specifying the bed-load concentration as a function of the flow and sediment conditions is proposed. A verification analysis using about 600 (alternative) data shows that about 77% of the predicted bed-load-transport rates are within 0.5 and 2 times the observed values.
1,653 citations
"Methods to control bed erosion at 9..." refers background in this paper
...Critical shear velocity (u*c) and particle Reynolds number (Re*) describe various conditions for incipient motion of the particles and ensure the sediment entrainment in the model for a given flow and sediment characteristics (van Rijn 1984)....
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TL;DR: In this article, the authors studied the relationship between sediment transport and river morphology by defining sediment transport regimes on the basis of the Shields number, a non-dimensional measure of the capacity of the channel to move sediment of a given caliber.
Abstract: The morphology of an alluvial river channel is the consequence of sediment transport and sedimentation in the river. Morphological style is determined chiefly by the caliber and quantity of sediment delivered to the channel, although modulated by channel scale. Yet the relations between sediment transport and river morphology have received only limited, qualitative attention. In this review, the problem is studied by defining sediment transport regimes on the basis of the Shields number, a nondimensional measure of the capacity of the channel to move sediment of a given caliber. The problem is also approached from an inverse perspective by which the quantity and character of sediment deposits are used to infer details about the variation of sediment transport and sedimentation along a channel. Coupling the two approaches establishes a basis to gain new insights into the origins of alluvial channel morphology.
607 citations
"Methods to control bed erosion at 9..." refers background in this paper
...In alluvial rivers, channel morphology and sediment transport processes are inevitably linked (Church 2006)....
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TL;DR: In this paper, a design method for the estimation of equilibrium depths of local scour at bridge piers is presented, based upon envelope curves drawn to experimental data derived mostly from laboratory experiments.
Abstract: A design method for the estimation of equilibrium depths of local scour at bridge piers is presented. The method is based upon envelope curves drawn to experimental data derived mostly from laboratory experiments. The laboratory data include wide variations in flow velocity and depth, sediment size and gradation, and pier size, shape, and alignment. Local scour depth estimation is based upon the largest possible scour depth that can occur at a cylindrical pier, which is 2.4D, where D=thepierdiameter. According to the method, this depth is reduced using multiplying factors where clear‐water scour conditions exist, the flow depth is relatively shallow, and the sediment size is relatively coarse. In the case of nonrectangular piers, additional multiplying factors to account for pier shape and alignment are applied. The method of estimation of local scour depth is summarized in a flow chart.
565 citations
"Methods to control bed erosion at 9..." refers background in this paper
...Since σg > 1.3 and Vm/Vc 1 (Vc = 0.195 m/s) for all discharge ratios, clear-scour conditions prevailed (Melville and Sutherland 1988)....
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...The flow velocity observed is less than the threshold mean flow velocity (Vc) but it is sufficient (Vm > 0.5Vc) for sediment entrainment (Melville and Sutherland 1988)....
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...In addition, the pile diameter influences the local scour around the piles and the local scour increases with an increase of pile diameter (Melville and Sutherland 1988, Subhasish et al. 2014)....
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Book•
01 Jan 1989
504 citations
"Methods to control bed erosion at 9..." refers methods in this paper
...The critical shear velocity for d50 = 0.28 mm was obtained as 0.011 m/s from Shields number threshold value of 0.056 for present particle Reynolds number, Re* = 233 which indicated sediment entrainment (Re* > 40) (Ponce 1989, Jayaraman 1995)....
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TL;DR: In this paper, the authors present results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence, showing that sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transported around rather than through the centre of the confluence.
Abstract: River channel confluences form important morphological elements of every river system, being points at which rapid changes in flow, sediment discharge and hydraulic geometry must be accommodated. This article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence.
Bed morphology at channel confluences is characterized by three distinct elements: avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone formed at the downstream junction corner. These elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels. As confluence angle and discharge ratio increase, the sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transported around rather than through the centre of the confluence. This segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence, an increase in the scour depth, a change in the orientation of the scour and an increase in the size of the separation zone bar. Field measurements closely replicate the flume simulation.
A model of sediment transport and bed morphology links these features to the fluid dynamics of these sites. An understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record.
412 citations
"Methods to control bed erosion at 9..." refers background in this paper
...For Qr = 0.50, Best (1988) obtained highest values of Srmax, may be due to the small channel width (15 cm) which increases velocity and leads to high scour....
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...Furthermore, the size and location of scour-hole are studied in natural confluences by Rhoads and Sukhodolov (2001), Riley and Rhoads (2012), Riley et al. (2015), and experimental studies are conducted by Best (1988), Biron et al. (1993), Boyer et al. (2006), etc....
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