بررسی آزمایشگاهی اثر زاویه‌ی قرارگیری سری آبشکن‌های‌ نفوذ‌پذیر بر روی ابعاد چاله‌ی آبشستگی در شرایط غیر مستغرق در قوس ملایم 90 درجه

Interaction between both centrifugal force and pressure gradient in river bends and the resulted vortex and spiral flows, causes erosion in outer bank and deposition in inner bank, which in turn, leads to gradual change in river form. Spur-dikes are among those structures which, through improving flow pattern and reducing secondary flow power, control and reduce erosion in river bends. But, as a result of reduced flow cross section and making strong vortices, they themselves are subject to scour. The rate of scour around spur-dikes depends on various factors such as permeability and their position angle, with respect to flow direction. In this study, the effect of the successive permeable spur-dikes position angle in three states of absorbing (۱۲۰ degree), vertical (۹۰ degree) and repulsive (۶۰ degree) for mild ۹۰° bend has been investigated. In order to achieve this goal, experiments were conducted on ۳۳% permeability spur-dikes and effective length equal to ۲۰% of flume width with four Froude numbers ۰.۲۱, ۰.۲۳, ۰.۲۶ and ۰.۲۸. Distance between the spur-dikes in all experiments was constant and set at ۴ times of the spur-dikes effective length. Flow was induced in the flume after structures were installed and sediment bed was graded. To start the flow, first, the sliding valve at the end of the flume and drain valve were tightly closed. When the water level was raised, the water depth was maintained at ۱۴ cm by opening the end sliding valve and increasing the flow to the expected amount. The starting point of the experiment was when the water level was steady at the specified depth. After three hours, the end valve was closed, the flow was stopped, and the sediment bed was completely drained. Then, the bed topography was surveyed via laser gauge. Gathered data from the experiments and surveying of the bed topography, were analyzed using “Surfer” software. To investigate changes in the scour dimensions due to position angle, the spur-dike which the highest sour occurred on its peripheral sides, was considered as the critical spur-dike. In all experiments, the critical spur-dike position was at an angle between ۸۰ to ۹۰ degrees of the bend or at the straight path toward the outlet. After the critical spur-dike was indicated, the scour dimensions on its peripheral sides were studied. Results indicated that among three angles of ۶۰, ۹۰, and ۱۲۰ degrees, the ۶۰ degrees (repelling) spur-dike had a better performance regarding the spur depth, comparing with ۹۰ degrees and ۱۲۰ degrees, and in all cases had the lowest scour depth, in a way that the maximum scour depth in Froud number ۰.۲۸ was ۰.۵۴ times of the flow depth in absorbing condition, where this value was ۷.۴% more than vertical position, and ۱۱.۱ times more than repelling condition. The maximum scour length belonged to ۶۰ degrees (repelling), and the minimum length was related to ۱۲۰ degrees (absorbing). The maximum scour length in Froud number of ۰.۲۸ was calculated ۱.۹۲ times of the flow depth for repelling condition, where this was ۹.۲ times more than vertical position, and ۱۴.۸% more than repelling condition. The highest value of scour width belonged to ۶۰ degrees (repelling), and the lowest amount of width was related to ۱۲۰ degrees (absorbing). The maximum scour width in Froud number of ۰.۲۸ for repelling case was calculated ۲.۳۵ times of flow depth, which was ۶% more than vertical position and ۱۵.۱% more than absorbing condition. It was observed that with an increase in Froud number, the scour dimensions in all three repelling, vertical and absorbing states were also increased. This could be explained by the fact that since greater Froud number implies increase in flow velocity and thereby higher shear stress exerted by the flow, hence the flow impinges more rigorously and at a higher velocity on the spur-dikes, resulting in more severe scour. Based on the results of this study, it can be concluded that the maximum scour depth was occurred when the spur-dike was positioned in absorbing condition, and the minimum depth was related to placing the spur-dike in repelling condition. Also, the maximum scour length and width belonged to repelling condition, where the minimum values referred to absorbing condition. Finally, higher Froud number in all cases resulted an increase in the scour dimensions.