Experimental Study on the Effects of the Angle and Location of Submerged Impermeable Plates on Turbidity Current Characteristics

Today, the construction of dams is one of the most important solutions for the storage of surface water. Due to fresh water limitations, the construction of reservoir dams to control the surface water resources in Iran is inevitable and necessary. In dam reservoirs, the turbidity current is usually the cause of sediment transfer and deposition. If the current is completely stopped in the middle areas of the reservoir, the amount of sedimentation will be reduced at the bottom of the dam wall and, as a result, the main functions of the dam will not be disturbed. Therefore, it is necessary to study this phenomenon. One of the methods for hydraulically changing the turbidity current is to roughen it or to use a barrier in the bed. Several studies have been carried out on the velocity of the turbidity current so far, and controlling it by roughening and barriers. However, it seems that no studies have been made on the effects of the angle and location of impermeable submerged plates on the turbidity currents.

Here, 42 experiments were made on the effects of the impermeable submerged plates on the head velocity and the height of the turbidity current head. For this purpose an experimental, flume with 30cm width, 10m length and 46cm height was used. The turbidity current entered the flume with concentrations of 20 and 40 g/l and bed slopes of 1 and 2%. In order to investigate the effects of the angle of the impermeable submerged plates, impermeable plates which came into seven different angle of mounting of the plates with respect to the current axis (0, 15, 30, 45, 60, 75, 90 degrees) and also, impermeable plates with eight locations were located across the current. Measurements of velocity and height of the head turbidity current were made on 6 sections with 50cm distances. Then, based on the data obtained, and by dimensional analysis of Edgar Buckingham’s method (Buckingham π methodology), the non-dimensional graphs of velocity, height and Densimetric Froude Number of the head of turbidity current were plotted.

According to the performed experiments, the results of this study are presented in two sections. In the first section, the results are related to the effect of the plate angles; in the second, the results are related to the effect of the plate positioning. Generally, due to the collision of the turbidity current with the plates, the non-dimensional velocity was reduced along the mounting route. Also, the Densimetric Froude Number was decreasing along the current route. The results showed that with increasing the angle of the plates, the decreasing gradient of the dimensionless velocity increases along the dimensionless route. Therefore, the plates with angles of 75° and 90° which are in the vertical and near-vertical position relative to the current axis, are affecting the current flow velocity more than plates at other angles. In general, the flow velocity of the turbidity current decreased by 8.6 to 27.1% with respect to the control state in case of different angle of mounting of the plates with respect to the current axis. With a concentration of 20 g/l and the slope of 1%, the plates with the mounting angles of 75 and 90° relative to the horizontal axis of the turbidity current flow provide the most significant decrease in the velocity compared to the control, with 26.4% and 27.1%. The analysis of the position of the plates showed when the plates are mounted across the total width of the canal, they provide the most significant reduction in the velocity compared to the control. In general, in different conditions, the flow velocity of turbidity current increases to 6.3% in the worst location and decreases 45.1 percent in the best location relative to the control state (in bed without submerged impermeable plates). With a concentration of 20 g/l, the plates at the position of 2, with 45.1% and the slope of 1%, and 29.5% at the slope of 2%, made the biggest decrease in the current head velocity compared to the control. Also, with the concentration of 20g/l at the slopes of 1 and 2%, the position 5 made the biggest decrease in the velocity with 33.9 and 38.3%. Slope investigations showed that with the concentration of 20 and 40 g/l, the slope increase will reduce the effect of the plates on the velocity.