Evaluation of flushing efficiency at a sand trap: A case study of Pengasih Weir, Special Region of Yogyakarta

Sand trap is a part of the main building of irrigation system. Its function is to settle the sediment that is carried away from a river stream. Sand trap needs to be flushed periodically with an optimal and efficient mechanism. This research aimed to evaluate the flushing efficiency of the sand trap of the Pengasih Weir. The data consisted the bedload samples along the channel and the sand trap's existing flushing mechanism data. The results showed that the most dominant grain of the bedload trap in Pengasih Weir was fine sand with a diameter between 0.075 and 0.425 mm of 77.2%. The bedload includes uniform and well-graded sediment because the Cu and Cc values were 2.751 (less than 5) and 1.027 (in the range 1 - 3), respectively. The flushing mechanism was divided into two stages. The first stage was the combination of 1.06 m/s flow velocity and 1.1 m flow depth, and the second stage was the combination of 1.59 m/s flow velocity and 0.35 m flow depth. Both two stages were efficient in flushing the sand trap of the Pengasih Weir, which apparent from all the bedload grains that were moved based on the concept of Shields.