The control of reservoir sedimentation through hydraulic flushing has been employed in several parts of the world to sustain the useful storage capacity of the reservoirs. However, no comprehensive analysis has been conducted to understand the sediment flushing processes. In this study, laboratory experiments were performed to investigate the flushing processes during drawdown flushing, including outflow sediment discharge, characteristics of the flushing channel and flushing effectiveness. From both laboratory and field data, it is found that outflow sediment discharge can be well related to a hydrauic parameter which is a function of outlet discharge, water-surface gradient and flushing channel width. It is also found that flushing efficiency increases dramatically when retrogressive erosion emerges. A geomorphic relationship is used to estimate the equilibrium width of the flushing channel. A 1-D diffusion model presented herein is employed only to simulate the general trend of bed profile evolution and the amount of reservoir sediment removal during flushing in order to evaluate the applicabilities and limitations of this model. Simulated results of this diffusion model agree well with laboratory data in a narrow flume (with essentially 1-D flow) with nearly uniform flow condition after rapid drawdown operation. However, there is a 15% average error if this diffusion model is applied to a wide reservoir (relative to the width of the flushing outlet) as performed in our laboratory. A two-dimensional or three-dimensional model is needed to study flushing processes involving strong lateral erosion in a wide reservoir.