Investigating the Sediments Evacuation

Dam construction alters the natural balance between sediment inflow and outflow by increasing water depth and decreasing velocity. As a result, the sediment transporting capacity of the flow is reduced and sedimentation occurs. Reservoirs around the world have lost major part of their storage capacity due to sedimentation. Since, constructing new reservoirs is difficult due to lack of suitable dam sites and high costs of construction; reclaiming lost storage capacity has recently received an increased attention. An attractive technique for removing deposited sediments from reservoir is flushing. The phenomenon of flushing was studied in this research by performing experiments on a one-dimensional reservoir model. A total of six experiments with pressurized and drawdown flushing using circular, triangular and rectangular bottom outlets were conducted. Sand with a bulk density of 1420 kg/m3 and a D50 of 0.28 mm was used as sediment material. It was found that pressurized flushing has a very local effect. A flushing cone is formed in a short time but only a small amount of sediments in the vicinity of the outlet are removed. Retrogressive erosion takes place during drawdown flushing and a flushing channel is formed, resulting in removal of significant quantities of sediments. Results indicated that the amount of sediments flushed is a function of outflow water discharge, water level in the reservoir, water surface slope and shape of the low level outlet.