جستجوی مقالات مرتبط با کلیدواژه « meander » در نشریات گروه « آب و خاک »

Meandering rivers are among the most dynamic features on earth's surface. A meandering river exhibits progressive change in position as it migrates across its floodplain. The planform dynamics of meandering rivers result from the interactions among flow, sediment transport, and evolving channel morphology that collectively scale with planform curvature. The view that planform curvature has a major influence on the rate and character of meander migration is supported by field observations and theoretical considerations. Morphological river changes (bed topography and/or plane-shape changes) occur as result of natural and/or of human interventions so that alluvial meandering streams strive to achieve an equilibrium configuration. As documented by many experimental and theoretical works the plane shape of a meander wave is the result of the complex interplay flow pattern-bed deformation bank erosion. The geometric shape of the meander wave gives rise to a complicated flow field which, in turn, modifies the bed topography and reshapes the meander wave through the bank erosion. In fact, in nature, different meandering streams exhibit different geometric characteristics so that the stream conditions may vary from one meander loop to another. Thus, many laboratory and field studies, apart numerical researches show that the evolution of a meander wave is mainly governed by the bed deformation that drives the erosion process at the channel banks. Particularly, outer-banks are considerably vulnerable to erosion processes. On the other side, the evaluation of the bank erosion and the consequent migration of meandering rivers are fundamental, both because of hazards associated with them and because of their effects in riparian ecosystem dynamics.
The modeling of meandering-river migration requires the simulation of the following processes: hydrodynamics, sediment transport, bed morphodynamics, and bank erosion. Furthermore, in this paper plane form of a meander wave was determined by the mathematical model. Under the conditions prevailing in natural streams, the channel centerlines follow sine-generated curves, with an assumed steady-state turbulent and subcritical flow, of large width-to-depth ratio and small Froude number. The plane deformation of the channel was caused by the action on the banks of the convective vertically-averaged meandering flow. It has to be pointed out and as it is well known that, the regime development of the flow width goes much faster than that of the channel slope. Therefore, in this model the regime development of slope (i.e., the loop expansion), a nearly constant flow width was assumed, which can be identified as the regime width. Accordingly, in this article, the flow width (with as the flow depth) was assumed to be constant along the channel length. Since the width-to-depth ratio was large, the effect of the cross-circulation was negligible. In overall, the channel deformation, as the result of both channel down valley migration and lateral expansion, is entirely due to the action of the convective vertically-averaged meandering flow. The growth (expansion and expansion) of meander loops is attributed to the regime-trend the loops continually grow in their amplitude (i.e., their length increases) and thus the channel slope progressively decreases to the regime state. This model was able to predict the migration and expansion of meander loops.
The following computational procedure was adopted to determine the migration and expansion speed of the meander loop under various values of the deflection angle along the channel centerline. (1) Set regime channel conditions, (2) Compute of meandering channel lengths, (3) Calculate regime slope, (4) Compute water depth, (5) Compute the average specific volumetric sediment transport rate (sediment transport is assumed to be the only bed-load in this article), (6) Compute the angle between the streamline and the coordinate line, (7) Compute the migration speed of meander loops and (8) Compute the expansion speed of meander Loops. The model was then applied to a sine-generated meandering channel and the migration and expansion component of bank speed deformation was calculated. Modeling results showed that, for small deflection angle migration was the main deformation in the meandering channels, on the other hand, for large deflection angles both migration and expansion became smaller in the meandering channels. Furthermore the effect of deflection angle along the channel centerline and channel-averaged specific volumetric sediment transport rate, have been proposed by model. The result showed that the specific volumetric sediment transport rate had significant effect on channel migration and expansion rates.

Stability of streambeds is dependent on different factors some of which maybe destabilized at any moment. Dam erection on the riverbed is the most important among those factors. Some morphological changes of a meandering river, the most obvious of which are the gradient change and the area deducted or added to the riverbed, were studied downstream of the Karkheh Dam. To delineate the areas, which had been taken from or added to the riverbed due to the lateral and longitudinal displacement of the meanders, morphology of the downstream river bed prior to and after the dam construction were studied employing satellite imagery, and their extent was calculated using the CCHE2D model. Slope reduction, as expected, follows an increase in the riverbed breadth. The average width of the riverbed was reduced from 273 m to 60 m (78% reduction) after the dam construction. The land taken off the streambed was almost 21 hectares for each kilometer of the length of the river. Mean, maximum and minimum horizontal thalweg displacements were 340,768 and 53 meters, respectively; 56% of the displacements were towards right (west) and 59% of the displacement took place out of the original streambed. The mean annual lateral displacement of the riverbed was 34 meters after the dam construction. This finding illustrates the instability of the terrain through which the river flows.

In This study using CCHE2D, to simulate flow and sediment transport pattern meandering river reachlocated in the downstream of the Minab dam (Hormozgan province, Iran). First, the geometry model and after that a numerical mesh systemwere calculated using the topographic surveysof the study area. Second, the required parameters for running CCHE2D were collected through field works. Finally, the model outputs including flow depth and velocity and changes in suspended load and bed sediment were obtained for the considered riverreach. In the end, the two statistical criteria RMSE and MAPE, the data obtained from numerical simulation model against observational data were compared. Model error parameters, RMSE of velocity, depth and energy slope, respectively, 0.075, 0.116 and 0.0008 and the index of MAPE respectively, 2.7, 6.2 and 4.4 percent. The results comparison indicated that the simulated values matched well with the field measurements.
In This study using CCHE2D, to simulate flow and sediment transport pattern meandering river reachlocated in the downstream of the Minab dam (Hormozgan province, Iran). First, the geometry model and after that a numerical mesh systemwere calculated using the topographic surveysof the study area. Second, the required parameters for running CCHE2D were collected through field works. Finally, the model outputs including flow depth and velocity and changes in suspended load and bed sediment were obtained for the considered riverreach. In the end, the two statistical criteria RMSE and MAPE, the data obtained from numerical simulation model against observational data were compared. Model error parameters, RMSE of velocity, depth and energy slope, respectively, 0.075, 0.116 and 0.0008 and the index of MAPE respectively, 2.7, 6.2 and 4.4 percent. The results comparison indicated that the simulated values matched well with the field measurements.
In This study using CCHE2D, to simulate flow and sediment transport pattern meandering river reachlocated in the downstream of the Minab dam (Hormozgan province, Iran). First, the geometry model and after that a numerical mesh systemwere calculated using the topographic surveysof the study area. Second, the required parameters for running CCHE2D were collected through field works. Finally, the model outputs including flow depth and velocity and changes in suspended load and bed sediment were obtained for the considered riverreach. In the end, the two statistical criteria RMSE and MAPE, the data obtained from numerical simulation model against observational data were compared. Model error parameters, RMSE of velocity, depth and energy slope, respectively, 0.075, 0.116 and 0.0008 and the index of MAPE respectively, 2.7, 6.2 and 4.4 percent. The results comparison indicated that the simulated values matched well with the field measurements.
In This study using CCHE2D, to simulate flow and sediment transport pattern meandering river reachlocated in the downstream of the Minab dam (Hormozgan province, Iran). First, the geometry model and after that a numerical mesh systemwere calculated using the topographic surveysof the study area. Second, the required parameters for running CCHE2D were collected through field works. Finally, the model outputs including flow depth and velocity and changes in suspended load and bed sediment were obtained for the considered riverreach. In the end, the two statistical criteria RMSE and MAPE, the data obtained from numerical simulation model against observational data were compared. Model error parameters, RMSE of velocity, depth and energy slope, respectively, 0.075, 0.116 and 0.0008 and the index of MAPE respectively, 2.7, 6.2 and 4.4 percent. The results comparison indicated that the simulated values matched well with the field measurements.
In This study using CCHE2D, to simulate flow and sediment transport pattern meandering river reachlocated in the downstream of the Minab dam (Hormozgan province, Iran). First, the geometry model and after that a numerical mesh systemwere calculated using the topographic surveysof the study area. Second, the required parameters for running CCHE2D were collected through field works. Finally, the model outputs including flow depth and velocity and changes in suspended load and bed sediment were obtained for the considered riverreach. In the end, the two statistical criteria RMSE and MAPE, the data obtained from numerical simulation model against observational data were compared. Model error parameters, RMSE of velocity, depth and energy slope, respectively, 0.075, 0.116 and 0.0008 and the index of MAPE respectively, 2.7, 6.2 and 4.4 percent. The results comparison indicated that the simulated values matched well with the field measurements.

Morphology of rivers constantly changes due to their dynamic features, and these changes can negatively affect the structures in riversides, agricultural lands, etc. Meanders are considered as important indicators of environmental change. This study examines the changes of the sealing degree and meander shapes of the Kashafroud river using 1966 aerial photography, 1987 Landsat TM sensing images, and 2013 Google Earth images. In this study, morphological parameters of 16 meanders in a range of 11 kilometers along the Kashafroud river was determined for each period. The results of the paired t mean comparison showed that changes in the sealing degree ranging from 1966 to 2013 have a significant difference at level of one percent. To determine the effects of changes in some meanders parameters on sealing degree, multivariate linear regression model was used. The results showed that in case of change in the sealing degree, meanders reduce along the river. In addition, Spearman correlation analysis showed that changes in the parameters of sealing level and meander shape are strongly interdependent, so that there is a significant correlation between changes in sealing degree and meander shape at level of one percent. With increasing degree of sealing, meander shapes convert into U and Ω shapes, and meanders tend to close. In meanders with reduced sealing degree, such as meanders number 2, 3, 11, and 12, the path will mainly change into a strait one, and a part of the river will be separated as bovine horn ponds.

The aim of this research was to apply a two-dimensional depth-averaged hydrodynamic model capability to predict and investigate flow field, sediment transport and bank erosion in a natural meandering river. Study area is a meander of Karun River with the length of approximately 5 km between Ahvaz and Farsiat hydrometric stations. Bank protection of the so¬-called meander is so important because the Khuzestan Water and Energy company structures and agricultural farms are located in the vicinity. Mesh generation was performed by CCHE-MESH model. The validated CCHE2D model was applied for flow and sediment simulation in a 20 years period of time. Results showed the meander thalweg was decreased between 4 to 8 m and 5.7 m in average (in compare with initial bed level) and secondary flow caused a higher decrease in outer bank elevation. Spur dike structure was designed in the CCHE-2D model because of its capability to river bank protection. After 20 years of simulation, dikes caused to increase the elevation of outer bank between 1 to 9 m and 4.7 m in average (in compare with 20 years of simulation without dikes), But in some sections thalweg became approximately 4m deeper.

A large number of authors have studied to find out how to control scouring around a pier along direct stream. The objective of this research is then to study a scour pattern around such piers on curves rivers due to the fact that there may be a difference between a flow pattern on a direct line and the meander. Thus, this study tries to check whether the use of cable and collar alone and in a combination of cable and collar affects scouring around bridge pier group constructed on rivers meander. In this study two thickness of cable 0.2D and 0.4D and three steps 0.2D, 0.5D and D are used. The results showed that more scouring depth is decrease as a cable diameter increases and the pitch of helix decrease. About pier groups that are in the direction of water flow, scour depth for first pier is more than single pier, that represents increase of reinforcing phenomenon, also scouring depth for middle and back piers due to sheltering phenomenon, is less than single pier. About piers perpendicular to the flow direction, the maximum scour depth is observed in middle pier, also at this case scour depth is reduced by increasing distance between piers. Using collar is more effective than cable for small distances between piers. Also it is found that using a combination of cable and collar results the most reduction in scoring depth.

The most important indicators of environmental changes are meanders. For the first times, this study was carried out investigation of sealing degree and morphology changes of meander in Karoon River using TM and ETM+ instruments images of Landsat satellite and remote sensing in 1989 and 1995. In this research, morphological parameters of 20 meanders were determined at 128 km of Karoon River in two mentioned years. The results of t student testshowed that sealing degrees have a significant difference at 1% level in years 1989 and 1995. The multiple linear regressions were used for the sealing degree changes and some meander parameters changes. The modeling results showed that the meander length (L) have reduces, if sealing degree changed. Also, the results of Spearman correlation showed that sealing degree and meander shape correlated strongly. Therefore, there are significant correlations between changes of sealing degree and meander shape in the level of 1 percent. By increasing the sealing degree, the meander shape changed to U and Ω shapes and meander tended to be closed.

The existence of bridge pier in streamflow causes a complex 3D flow formation، which also causes the scouring around bridge pier. Since rivers are usually curved، it is necessary to investigate the impact of change in flow patterns caused by passage of flow through the curve on the scouring around bridge pier. By developing Computational Fluid Dynamics (CFD)، there is a possibility to simulate the flow pattern around the bridge piers. Therefore، the purpose of this research is modeled a 3D flow stream near the bridge piers in a curved channel. For this purpose a fluent model software was employed، and solved by stream equations using finite volume method of centralism. For discretization of Navier Stocks equation، three turbulence models of K-ε، K-ω، and RSM were used. In order to consider free surface، Fluid Volume Method was applied. The numerical model was validated with measured experimental data around the bridge piers in the meandering flume with 5 sequential curve paths. The results showed that the RSM turbulence model performed well compared to the other two models. When comparing the flow of upstream to downstream of bridge piers it can be observed that the placement of bridge piers in the middle of curved shape channel may lead secondary flow towards the inner curve of a channel. Also، the resulted vortex continues with a 150 degree curve.

The local scour around a bridge pier in recent years has been widely studied by different authors. They proposed different methods to control the scouring depth. Until now, many of authors have studied to find out how to control scouring around a pier along the straight stream. However, it may be required to construct a bridge across a bend of a river due to the some constraints. The objective of this research is to study a scour pattern around such piers on a bend due to the fact that there may be a difference between a flow pattern on a straight stream and the meander. Thus, this study tries to check whether the use of collar and cable together effects on scouring around bridge pier constructed on river meander. The results showed that as cable diameter increases and pitch of helix decrease, scouring depth will be decreased. The reduction of scouring depth by using cable does not exceed from 59% and alone collar reduces scour depth up to 55%. Using collar and cable together is reduced scour depth to 69.7%.

The present paper tries to describe the advantage and improvement of a numerical model when predicting government processes on Flow Rivers. With regard to the important effect of the flow velocity and shear stress forces on river bank erosion, we apply a Two-Dimensional numerical model, named CCHE2D, to simulate river flow pattern at a meandering river Khoshk-e-Rud River of Farsan, 30 Km west of Shahr-e- Kord. Various algorithms and parameters were implemented in a computational fluid dynamic model (CFD) for simulation of two-dimensional (2D) water flow to gain an insight into the capabilities of the numerical model. At this surveying, at first, we applied the topographic maps of the studied location and then, made the model geometry and calculation mesh with diverse dimensions. Finally, using the measured properties of the river flow and the Depth-Average, Two-Dimensional Hydrodynamic Model was run. Then, we obtained the results of model, such as depth and flow velocity at the river meander. Within the scope of the test cases, the model simulated water flow pattern processes at an intake, as well as a steady flow regime in a sine-shaped meandering channel by a 90_channel bend, which is the free-forming meander evolution of an initially straight channel. Because of high accuracy of this numerical model and multiple content of its internal parameters, the evaluation result of model, confirmed the measurement results. Therefore, the parameters gained from the model showed good conformity with measurement parameters at field cross-section. All results matched well with the measurements. The results also showed that using computational fluid dynamics for modeling water flow is one step closer to having a universal predictor for processes in Meandering Rivers

Naturally, rivers are rarely straight and more likely to take a winding course, called meandering. Because of presence of strong secondary currents in meanders, flow in river meanders is a complicated phenomenon, making it interesting for many researchers and engineers to investigate the equations governing this kind of flow. Many studies have been carried out by different researches during the last 45 years, and consequently, different relations have been published to determine the hydraulic parameters of the flow. In this research, a laboratory trapezoidal channel with a central radius curve of 5 m and central angle of 94 degrees was used to measure the hydraulic parameters for 120, 180, and 230 l/sec flows over a sandy movable bed having rigid walls. The results of this experiment were compared to the results of different models developed to estimate the flow characteristics. Finally, the most suitable model to determine the flow characteristics was introduced.