Hydraulic Investigation of Pressure Flushing with Expanding Bottom Outlet Channel within the Dam Reservoir

Sedimentation in dam reservoirs is an important issue which requires to be considered within the operation life of the dam. In order to maintain long time storage in dam reservoirs, sediment removal from reservoirs is an essential issue. There are numerous methods to perform this goal. One of the known Sediment removal methods is hydraulic flushing. This method is classified in two free and pressure classes. Pressure flushing is considered as an effective method in removing local accumulated sediments behind the dam and around the location of valves and turbines. During the flushing operation, water level in the reservoir creates pressure on the sediment, causing the removal of the sediment and after a while of flushing a funnel shaped crater is created in the vicinity of the bottom outlet opening. In this study, the effect of expansion of bottom outlet channel within the reservoir on the dimensions of the flushing cone, also its hydraulics were investigated.
The experiments were conducted with a physical model of a rectangular box with 7m length, 1.4m width, and 1.5m height, consisting of three parts namely the inlet of the model, the main reservoir, and settling basin. In the inlet of the model, the turbulence of the inflow is disappeared and a uniform flow enters the main reservoir. Since in pressure flushing the water surface level inside the reservoir is constant during the operation, the water level is adjusted by using a spillway positioned in the inlet of the model during the whole procedure. The main reservoir of the model was 5m long and the sediments were placed within this part of the model. The sediment particles used were non-cohesive silica sediments with uniform size and with median diameter(d50) 1.15mm and geometrical standard deviation(σg) 1.37. In order to perform flushing, an outlet made of Plexiglas with the diameter of 5cm aligned with the central line of flow was used. To investigate the effect of the bottom outlet channel expansion within the reservoir on the dimensions of flushing cone, the experiments carried out with different discharges and water depths above the bottom outlet in different expansion size of outlet channel in constant sediment level of 20cm above the center of the channel. Also experiments in the same hydraulic conditions without channel expansion within the reservoir were conducted as control tests. To perform the experiments, first the Plexiglas tube was positioned in its place as the outlet channel. The model was slowly filled with water until the water surface elevation reached to a desired level. The bottom outlet was manually opened until the outflow discharge, become equal to the inflow discharge. After a while the sediments were discharged with the water flow in very high concentrations through the outlet channel (sudden discharge) and a funnel shaped crater was formed in front of it. At the end of each experiment, the flushing outlet was closed in which the incoming discharge was set to zero then water was carefully and slowly drained from the main reservoir. After the run of each experiment, the bed level of scouring was measured using laser distance meters with the accuracy of 1mm, and the volume of flushing cone was calculated by Surfer 10.0 software. In order to investigate the hydraulics of the flow, the measurement of flow velocity in the flow rate of 3 liters per second and reservoir water level of 47.5cm for three expansion sizes of outlet channel (10, 20, and 30cm) was performed. The flow velocity measurement was done using an Acoustic Doppler Velocimeter with a sampling rate of 200 Hz. This device is capable of measuring moment velocities in three directions (velocity in directions x, y and z are u, v and w respectively).
Results showed that, the expansion of bottom outlet channel within the reservoir has positive effects on the dimensions of the flushing cone. Therefore, the relative amount of bottom outlet channel expansion for 0.5, 1 and 1.5 times height of the sediments in the reservoir, leads to increase in flushing cone length for average of 48, 83 and 113% and flushing cone volume for average amount of 50, 74 and 96% compared to the case when the outlet channel is not developed. Using experimental results, two equations with good estimation are presented for calculating the flushing cone volume and length. Also the results indicate that, with increasing in the Froude number, sediment discharged from reservoirs is increasing, as in the higher expansion size of the bottom outlet, effect of the Froude number on the amount of the sediment discharged is high. The analysis of the flow hydraulics indicates that, the flow velocity vectors while getting closer to the outlet opening, lose their expanded status and velocity gradient in the adjacent areas increases significantly. The analysis of results for flow velocity measurements are presented in detail in this paper.