This paper investigates the interaction between flow patterns and geometry of in-ground stilling basin (ISB) and its influence on flushing efficiency from ISB. The main governing parameters in our study were the normalized ISB length, drop number, normalized end-sill geometry (height and width), and Froude number. Using large scale particle velcocimetry, we classified the flow patterns into four groups of B-jump, U-jump, Periodic and Steady submerged jump. We found the favorable flow pattern has a positive correlation with drop number, while a negative correlation with relative ISB length. Moreover, the lateral free spaces of the end-sill do not increase the chances for sediment to be flushed out from ISB.