چكيده به لاتين
Rivers change under various factors and parameters in terms of dimensions, form, direction and flow pattern. Factors such as Discharge, Bed grains, bank erosion, etc. effect how rivers change and therefore rivers can be classified based on these factors. One of the most Important classifications is based on the Flat form of Rivers. In this classification, rivers are classified into Straight, Meandering or Braided Rivers. Meandering Rivers follow a non-straight line with twists and turns, this class of rivers are continuously displaced by erosion and sedimentation processes. Presence of vegetation in the water alters the main parameters of the flow and these alterations could provide invaluable environmental services. One of the services of vegetation in meandering rivers is protection of the Outer bank against erosion, increasing stability and preventing long-term meander migration. Presence of vegetation in the flow could also alter secondary flow patterns. In this thesis, field measurements are conducted in 2 meanders of a small stream called the Deryuk River with the aim of exploring the effect of presence of vegetation on the Bed shear stress, erosion and Secondary flow patterns. To investigate the bed stress, the contours of shear stress were plotted using TecPlot software and the changes of this parameter at the back and front of the vegetation patch were investigated. To explore the erosion pattern, the elevation of the bed at the surrounding areas of the patch was studied. The secondary flow patterns were investigated by 2 different methods, in the first of which a coefficient (C) is used to change the power of the secondary flows, and in the second method, the distortion and asymmetry of the velocity isobars are used to confirm the presence of secondary flows. The results show that the bed shear stress at the wake of the vegetation patch in the outer bank is lower (25% less than the average bed shear stress), this could prevent instability in the outer bank of meander in a river. In order to study the effect of vegetation on erosion, Shields parameter was investigated, the value of this parameter at areas around the vegetation patch were compared. The relation between the Shields parameter and the erosion sedimentation pattern shows how the vegetation patch alters the bed stability with changing the velocity locally and the bed shear stress consequently. In studying the secondary flow patterns in the presence of vegetation, it was concluded that the secondary flow has a positive trend before the patch and then it is reduced considerably after passing the vegetation patch.
