Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

The purpose of this study is to consider sediment deposition in large reservoirs with high suspended sediment inflows. A characterising feature of this kind of reservoir is the existence of turbidity currents due to density differences between inflow and ambient waters of the reservoir. Therefore, two major parts can be found in this study: firstly, experiments and analysis of the experiments on gravity currents, and secondly the development of a computer model for reservoir sedimentation modelling.

Some experiments were conducted using a laboratory flume to consider different aspects of gravity currents including: the development of the head of gravity currents, the body of subcritical gravity currents, and deposition due to the head and the body of turbidity currents. The analyses of the measured velocities, concentrations and the sizes of sediment particles have been presented. By using the data collected from this study and other available data a n e w coefficient for the equation of the head of gravity currents was proposed with the help of a statistical package. Sediment transport by the head of gravity currents is discussed. Based on the calculated water entrainment and using other available water entrainment data, an equation for the water entrainment coefficient was proposed. A new equation for sediment entrainment over an erodible bed was presented by using the available data from other investigators.

A new procedure for the prediction of sediment processes in reservoirs was developed recognising the fact that turbidity currents affect long-term sedimentation, particularly when suspended sediment concentration is relatively high. Based on this a new computer program, DEPO , for the prediction of sediment processes in reservoirs was developed by incorporating the effects of turbidity currents on long term sedimentation. Although the model is theoretically one-dimensional, some options exist for the distribution of sediment deposited on the bed or for sediment scoured from the bed. This makes the model a pseudo two dimensional model.

To verify the proposed model, four different turbidity currents were run in the laboratory flume. The computations performed by DEPO for: the water elevation, the height of the turbidity current and the amount of the deposited material on the bed, showed excellent agreement with the measured values. The proposed model was also tested by application to a prototype situation, Dez Reservoir (a large reservoir in the south-west of Iran). Test results showed the capabilities of the model as a practical tool for the prediction of long term reservoir sedimentation. The DEPO model was tested using Dez Reservoir to consider the effects of alternative bottom gates on deposited sediment. By using the alternative bottom gates the amount and the pattern of sediment deposited in the reservoir were affected significantly. The height of the sediment deposited in the reservoir was reduced, particularly in the region from the d a m wall to 20 km upstream of the dam. The estimated volume of deposited sediment was reduced by about 55 percent and the trap efficiency was reduced to 0.46. The model was also utilised to predict the future volume and bed elevation of Dez Reservoir after 60 years of operation.

The results showed the capabilities of the model for predicting long term reservoir sedimentation, for the management of reservoirs, for considering the effects of the bottom gate on reservoir life, and for controlling turbidity currents in reservoirs.

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