بررسي آزمايشگاهي تاثير حضور دو مانع بر رفتار جريان گل آلود

Experimental Investigating on the Effects of Two Obstacles on Behaviour of Turbidity Current

جريان هاي گل آلود عامل عمده و مهم انتقال رسوبات بداخل مخازن سدها، درياها و اقيانوس ها مي باشند. شناخت ديناميك اين جريان ها به منظور پروسه هاي رسوبگذاري و فرسايش بسيار مهم و اساسي مي باشد. در اين تحقيق تاثيرات حضور دو مانع بر ساختار حركت جريان گل آلود به صورت آزمايشگاهي مورد بحث و بررسي قرار خواهد گرفت. پروفيل هاي سرعت و غلظت جريان در شرايط شبه پايا با استفاده از دستگاه سرعت سنج صوتي vectrino در بالادست مانع اول، فضاي بين دو مانع و پايين دست مانع دوم اندازه گيري شده و با پروفيل هاي سرعت و غلظت در حالت بدون مانع مقايسه شده اند. مقادير دبي در واحد عرض جريان و نرخ انتقال بار معلق بر اساس مقادير سرعت و غلظت اندازه گيري شده، محاسبه شده اند. تاثير تغييرات عدد فرود چگالي ورودي بر دبي جريان عبوري در واحد عرض و نرخ انتقال بار معلق بررسي شده است. نتايج نشان مي دهد احداث موانع در مسير جريان سبب شكل گيري نواحي جديد در پروفيل هاي سرعت خواهد شد و باعث مي شود يك حوضچه آرام كننده جريان در بالادست مانع اول و فضاي بين دو مانع شكل گيرد و محيط مناسب براي ته نشيني ذرات فراهم گردد. روند تغييرات نرخ انتقال بار معلق و دبي جريان عبوري در واحد عرض تابعي از موقعيت نسبي موانع هستند. بگونه اي كه روند تغييرات اين پارامترها با تغييرات عدد فرود چگالي ورودي در بالادست مانع اول متناسب و در فاصله بين دو مانع و پايين دست مانع دوم معكوس خواهد بود.

Turbidity currents account for transporting sediments into reservoirs, seas, and oceans. Therefore, understanding dynamics of these currents for sedimentation and erosion is very important. In this paper, the effects of two obstacles on the behaviour of turbidity currents investigated experimentally. An 11 m long rectangular channel (11 m×0.6 m×1.0 m) with the bottom slope of 0.25% was used to run the experiments and a 3 m3 tank along with a constant head tank were served as the turbid water supplier. Two triangular obstacles were installed at predefined locations from the sluicegate. Then the experiments were carried out and the results compared with those from without obstacle condition. Velocity and concentration profiles at the upstream of first obstacle and between the first and second obstacles are measured by Vectrino at quasi-steady conditions and compared to those of without obstacle conditions showing a significant decrease of velosity in the presence of the two obstacles specially between the two obstacles and also at the downstream of the second obstacle. Fluid volume discharge per unit width and suspended sediment transport rate are calculated based on measured velocity and concentration. Also, the effects of inlet Froud number on the fluid volume discharge per unit width and suspended sediment transport rate was investigated. The results show that presence of obstacles introduces new regions to velocity profiles and two ponds of turbidity currents are formed at the upstream of the first obstacle and between the two obstacles. The hydraulic conditions at these ponds make a suitable condition for the suspended particles to be trapped and hence the sedimentation. Variation of the suspended sediment transport rate and the fluid volume discharge per unit width depend on obstacle location. These parameters at the upstream of the first obstacle are directly in proportion to the inlet Froud number while at the downstream the second obstacle and between the obstacles are inversely proportional. By decreasing the inlet Froud number, the volume discharge per unit width increases at the upstream of the first obstacle wheras, the amount decreases between the obstacles. Also, as the inlet Froud number decreases, the suspended sediment transport rate increases at the the upstream of the first obstacle but the value decreases between the obstacles and downstream of the second obstacle resulting the increase of the trap efficiency. The obstacles become more effective in controlling the turbidity currents when the inlet Froud number decreases. The first obstacle is 1.8 times more effective on reduction of local sedimentation rate than the second obstacle. These parameters at the upstream of the first obstacle are directly in proportion to the inlet Froud number while at the downstream the second obstacle and between the obstacles are inversely proportional. By decreasing the inlet Froud number, the volume discharge per unit width increases at the upstream of the first obstacle wheras, the amount decreases between the obstacles. Also, as the inlet Froud number decreases, the suspended sediment transport rate increases at the the upstream of the first obstacle but the value decreases between the obstacles and downstream of the second obstacle resulting the increase of the trap efficiency. The obstacles become more effective in controlling the turbidity currents when the inlet Froud number decreases. The first obstacle is 1.8 times more effective on reduction of local sedimentation rate than the second obstacle.