Abstract: The influence of sediment on the flow measurement accuracy is an important problem in hydraulics. In this study, taking time-difference ultrasonic flowmeters as examples, the potential sediment influence on their measurement accuracy in sediment-laden open channel flows was investigated based on indoor experiments at different sediment concentrations (0 ~ 27 kg/m3, natural sand from the Yellow River irrigation district in Ningxia with medium particle size of 0.015 mm was used). The flow measurement errors were quantified and compared against the benchmark devices of sediment insensitive pipe electromagnetic flowmeter and Parshall flume. The results indicate that ultrasonic flowmeters overestimate the flowrate and the deviation increases with sediment concentration. In the concentration range of our study, the multi-channel box-type ultrasonic flowmeters are insensitive to the sediment concentration and suffer a maximum deviation of 7%, while the single-channel measurements are very poor with a deviation up to 300% - 500% under the influence of sediment.

Key words: sediment-laden open channel flow, sediment concentration, time-difference ultrasonic flowmeter, measurement accuracy