Document Type : Research Article
Authors
Tarbiat Modares University
Abstract
Introduction: Currently, large dams in the world, due to the high amount of sediments in the reservoir, especially around the intake, have operational problems. One of the solutions for this problem is pressure flushing. In this type of flushing, a mixture of water and sediment is removed from bottom outlets form dam reservoir and a funnel shaped crater is created in the vicinity of the outlet opening. In laboratory experiments carried out in this study, pressure flushing with the expansion of bottom outlet within the reservoir and its statistical analysis of bursting events were investigated. The structure of the turbulent flow is not fully understood due to their complexity and random nature. Klein et al. Introduced the turbulence bursting in this kind of flow and Nezo and Nakagora suggested that the events resulting from turbulence bursting has a significant effect of transferring the sediment particles.
Materials and Methods: For the purposes of this study, the experiments were conducted with a physical model 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. The main reservoir of the model was 5m long and the sediments were placed within this part of the model. The sediment particles were non-cohesive silica with uniform size and with median diameter (d50) 1.15mm and geometrics standard deviation (σg) 1.37. 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 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, after a while sedimentwere 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. After the run of each experiment, the bed level of scouring was measured using laser meters, and the volume of flushing cone was calculated by Surfer software. For investigation of turbulence parameters, the measurement of flow velocity in 0.5cm from the bed of flushing cone in the central axis of the outlet channel in the flow rate of 3 liters per second and water level of 47.5cm for three expansion sizes of the outlet channel (10, 20, and 30cm) was performed. The flow velocity measurement was done using an Acoustic Doppler Velocimeter. This device is capable of measuring instantaneous velocity in three directions.
Results and Discussion: The results indicated that 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 an average of 48, 83 and 113% and flushing cone volume for the average amount of 50, 74 and 96% compared to the case when the outlet channel is not developed. Also the analysis of the turbulence parameters showed that in the nearest axis to the inlet of the bottom outlet channel in which the maximum depth of flushing cone, the occurrence probability of sweep and ejection are maximum and impact angle of moment force due to these events is minimized. However the dominant event here is ejected which was also observed in laboratory experiments the particles were transferred into the channel as suspended load. By increasing the distance from the inlet opening of the channel the occurrence probability of sweep and ejection are decreased and impact angle of moment force due to these events is increased, but again, these two events are the dominant events in this regions and sweep is more important than ejection, that the observations also verify the particles transferred as bed load in these region. Ultimately, it comes to a region where the probabilities of all four events are the same and where the sediment flushing cone reaches the primary sedimentation level that scouring and sedimentation don’t take place there. By increasing the expansion size of the bottom outlet channel, the occurrence probability of sweep and ejections are increased and impact angle of moment force due to these events is decreased .So that at the place of the maximum depth of flushing cone, the probability of ejection in 10cm outlet channel is 0.39 and for 20 and 30cm outlet channels corresponds to 0.44 and 0.47, respectively .
Conclusions: In this study, the effect of expansion of bottom outlet channel within reservoir and its statistical analysis of bursting events was investigated. Results showed that, expansion of bottom outlet channel within the reservoir has positive and tangible effects on the size of the flushing cone and quadrant analysis of bursting events showed that the occurrence probability of sweep and ejection are greater than other events in the bed of flushing cone. Also with increasing in the expanding size of outlet channel, occurrence probability of dominant events is increasing and impact angle of turbulent force is decreasing. In fact it can be said that, the factors that cause increased dimensions of the flushing cone with the expansion of the bottom outlet channel within the reservoir are the increase of the occurrence probability of sweep and ejection events and decrease of impact angle of turbulent force to these events.
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