چكيده به لاتين
The initial movement of sediment particles is a fundamental aspect of river mechanics, which is used for many issues, such as the design of stable channels, scouring of river beds, and sediment transport. Since the erosion and sedimentation patterns change in the places where the width of the rivers changes and also in the presence of vegetation, therefore, the study of the initial movement of sediment particles in the presence of vegetation and the channel-width change is of importance. The aim of the present study is to investigate the simultaneous effect of changing the channel width and emergent vegetation on the threshold movement of sediment particles. For this purpose, laboratory modeling was carried out and the effect of changes in channel width and wall vegetation on flow characteristics, including velocity, shear stress and turbulence components under conditions of threshold movement of sediment particles was investigated. In order to change the width, semi-elliptical barriers were used on both sides of the flume, symmetrically, and array of reeds was employed as emergent vegetation. Experiments were conducted using three classes of sediment particles and three longitudinal slopes of the bed. The measurements of the velocity components were taken in 20 different test series by Acoustic Doppler velocimetry. The results showed that the vegetation on the channel wall causes the formation of various shear layers and the condition of these layers is very complex especially near the vegetation; so the distribution of Reynolds shear stress has four regions and the distribution is Z-shaped. This is despite the fact that in the absence of vegetation, the Reynolds shear stresses do not have a Z-shaped distribution and the maximum values of the shear stress occur at a lower depth from the bed. The simultaneous presence of wall vegetation and channelwidth change caused the Z-shaped distribution of shear stress even in the central axis of the channel, while in the presence of wall vegetation in the straight channel, this type of distribution is limited only to the axis close to the wall. In the current research, shear velocity and bed shear stress were calculated and compared using different methods (including boundary layer characteristics, Reynolds and Clauser). The results indicate the relative consistency of the results of these methods with each other and the validity of estimating the values of threshold shear velocity and shear stress. Also, by extrapolating the velocity profiles towards the bed, the threshold velocity values near the bed were obtained. In experimental tests on the variable width and in both the presence and absence of the wall vegetation, the threshold velocity and shear stress increased in all three bed slopes, by increasing the particle size. On the other hand, in experiments on the channel with variable width in the presence of vegetation in all three sediment particles, the threshold values of velocity and shear stress decreased, by increasing the longitudinal bed slope. Contrary to these observations, in the experiments related to the channel without wall vegetation, the threshold values of velocity and shear stress for two particles with a smaller diameter increase, with the increase of the bed slope. The main reason of this phenomenon can be the predominance of the pressure gradient effect over the gravity effect. Also, the results indicate greater values of threshold velocity and shear stress in the state without vegetation compared to the presence of vegetation. This is due to the turbulent flow effects on particles movements, caused by the presence of vegetation. In addition, in the experiment on a straight channel without the presence of vegetation, higher velocity and shear stress values are needed to move the bed particles, due to the absence of a turbulence factor. Briefly, under the same conditions (the same particle diameter and zero longitudinal slope), the presence of both width changes and vegetation leads to an increase in the intensity of turbulence and a decrease in the threshold velocity of the movement of sediment particles. Quadrant analysis represents that in all experimental series and in the condition of initial movements of bed particles, the possibility of the Sweep event increases by approaching the bed surface. In this regard, in z/h<0.05, the Sweep event has become the dominant bursting event in the conditions of the the threshold movement.
