Experimental Investigation the Effects of Gabion Obstacle's Height, Shape and distance of obstacle from entrance to Control the Turbidity Current

The process by which a heavy flow of sediments move under a clear flow and transfers the sediment to the lake and the oceans is said turbidity current. For heavy flow between the fluid density changes occur and one of the most important factors in sediment transport and erosion in the reservoir. Turbulence of flows is known to be responsible for suspension of particles in the turbidity currents and volumetric concentration of sediments in such currents is less than 10%. The occurrence of the turbidity currents in dam reservoirs transfers sediments to near body of dams posing substantial threats to water release facilities like intakes and bottom outlet. The control of such currents has always been a problem in dam operation. The location of sediment accumulation is another factor affecting the operational lifetime of a dam greatly. There are different methods to control or divert turbidity current in the reservoir, such as construction of permeable and solid obstacles, jet screen and bubble screen and using obstacle with roughness. In the present study we investigated the effect of height and shape of obstacle and the distance of obstacle from entrance of flow by using the gabion obstacle as an obstacle with low permeability to control the turbidity current. In this research using flume with variable bed slope in 10 m long, 30 cm width and 45 cm height and in two slope of 0 and 2.5 percent. The head velocity of turbidity current during the test was measured with chronometer. The video camera was used to control the computation. 2 sampler flushing before and after the barrier was used to measure the concentration of turbidity current. The porosity of obstacle was approximately 35 percent. Input rate constant in all tests was 0.7 liters per second and the feed concentration of 20 grams per liter. Sediment discharge of turbidity current’s head before and after Gabion obstacle was calculated. The results showed that depending on the initial conditions of flow, some of turbidity current after the collision with the barrier Gabion passed over it and at high altitudes very little flow passed through the barrier. The results showed the height of Gabion obstacle and its proximity to the current input, increase its impact in reducing speed and sediment flow. According to the investigation the best form of Gabion obstacle between gabion walls, stair stepping one way in upstream, stair stepping one way in downstream two-wayand stair stepped to decrease velocity and discharge passing over obstacle is two-wayed stair steep.