| Abstrakt: |
Background and Objectives: One of the natural structures in rivers is mid-channel bar that is created due to shear stress decreasing and sediment deposition in the central part of river. Bifurcation and erosion are seen in rivers because of mid-channel bar existence. Also, energy loss is the other impact of mid bars that would intensify the deposition in rivers. Mid-channel bar is known as one of the most important aquatic habitats, so there is necessity to find more information on flow structures around it in operation processes and river restoration. In this research, the flow structures around mid-channel bars are assessed as laboratory study. Materials and Methods: Three mid-channel bars were made in the shape of rhombic, ellipse and lemniscate in this research. These shapes are common in natural alluvial rivers. This study was established in a laboratory flume by length of 9 meter, and width and height of 0.4 meter. The bed slope was fixed of 0.0007. This flume was located in the water and sediment research laboratory in Gorgan University of agricultural sciences and natural resources. Three-dimensional velocity of flow was collected using an electromagnetic current velocity sensor with a frequency of 30 Hz in 39 data points. The flow discharge was fixed as 4.5 liter per seconds using a frequency flow meter. The flow regime was subcritical and turbulent in all examinations. Results: Findings showed that bar shapes have different impacts on flow structures and amounts of energy losses. Comparison between plots implied that there are similar conditions in elliptical and lemniscate shapes but a little different in rhombic case. The variation range of transverse velocity was between 0.4 to 1.6 times of average value for all shapes. According to the results, the existence of elliptical bar led to significant tolerance in longitudinal velocity in all axes (between 0.5 to 1.6 times the average in left and right axes and 0.2 to 1.4 times the average value in central line), but these tolerances were not significant for lemniscate and rhombic bars (between 0.8 to 1.2 times the average value in left and right axes and between 0.8 to 1.4 times the average value for central axis). The variation of vertical velocity in middle axis presented the maximum decrease before facing elliptical bar and similar conditions for this axis in facing with lemniscate bar, except that this decrease is started exactly at facing point here. In addition to the values of the velocity components, the direct effect of encountering bars should also be sought on the values of the flow energy as a cumulative indicator of the erosive power of the flow. A more detailed analysis of the graphs shows left that in the and right axes, encountering the elliptical bar has led to an increase in the relative energy of the current by 1.4 times the average, and after passing through the bar, the energy load has decreased to less than the average value. But in the middle axis, the presence of the bar reduced the relative energy of the flow to 0.6 average value and after passing through the bar, this component has an increasing trend. Conclusion: Exposure to the elliptical bar led to the greatest changes in the relative values of the triple velocity components, compared to the rhombic and lemniscate bars. Also, the changes in the total energy load caused by the presence of two rhombic and lemniscate bars are insignificant, but similar to the changes in all three velocity components, transverse, longitudinal, and vertical, the elliptical bar led to significant changes in the relative values of the total energy load along the channel. [ABSTRACT FROM AUTHOR] |
