Flow Observation of the Tonle Sap River by ADCP and Cause of Decrease in Observation Accuracy

Abstract

  This paper analyzes the results of observation of the flow in the Tonle Sap River in Cambodia from March 2017 to March 2018, which were obtained using an acoustic Doppler current profiler (ADCP) with the goal of gaining a better understanding of the complicated flow characteristics of the river's seasonally repeating normal and reverse flows and obtaining reliable calibration and validation data for a hydraulic model. The objective of this study is accuracy evaluation of ADCP and clarify the factors to reduce accuracy of moving boat technique based on the obtained data equipped with a real-time kinetic (RTK) -GPS. The data were gathered at the Prek Kdam water level observation station, at which the river width is approximately 600 m. ADCP observation was carried out using the moving boat technique, which involved the towing of a HydroBoard-II equipped with a Xylem M-9 ADCP system and RTK-GPS module. The findings obtained were as follows: 1) It was possible to observe the seasonally repeated normal and reverse flow rates of the Tonle Sap River accurately by using ADCP. In 17 out of 22 times observation sessions, the CV of the flow discharge during round-trip observation was less than 5%. 2) Low data accuracy could arise from several factors if an inappropriate observational method was applied. To avoid this, care must be taken to understand the particulars of ADCP observation using moving boat technology. 3) When the transducer depth was shallow and the boat speed and pitch angle were large, Signal Noise Ratio (SNR) attenuation occurred as a result of air entrapment. To reduce this factor, the depth of transducer is very important.