A thesis submitted in partial fulfilment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1989

When a dam is constructed across a stream to form a reservoir, the velocity of the flow is reduced and a major part, or all, of the sediment is deposited in the reservoir. On the other hand, the downstream portion of the dam is eroded due to release of low sediment content water. The extent of the sedimentation in the reservoir and the downstream degradation in the long run of the dam is of utmost importance. The engineering feasibility of the dam depends on the forecasting of that sediment behavior. The sedimentation aspect of the proposal of Low Pa Mong Dam of 210 m MSL is investigated here. A one-dimensional mathematical model is developed using the water continuity and momentum equation and sediment continuity equation. The backwater solution is derived for unsteady flow condition and then the morphological computations is done by the modified Lax scheme. The calculations are carried out by developing a powerful computer program in FORTRAN language. The model is calibrated for the year 1979 and 1980. Then it is executed for the flood frequencies of 10 year and 50 year flow and the change of the bed is predicted for 1 to 4 and 10, 30 and 50 years period. The average river bed level change on the 200 km upstream as well as downstream of the dam are plotted for that period and the morphological changes of the average bed level have been obtained. It is found from the study that the average rise of the bed in the reservoir will be 2 m and 6 m and the downstream scour depth will be 1 m and 3 m for a 10 year and 50-year lifetime of the darn which amounts sedimentation of 160 million and 480 million cubic meter and erosion of 80 million and 240 million respectively.