Author(s): Michio Sanjou; Wataru Aizawa; Takaaki Okamoto

Linked Author(s): Michio Sanjou, Takaaki OKAMOTO

Keywords: Velocity measurement; Automated system; USV; River flow

Abstract: In general, accurately measuring discharge and related surface velocity in natural rivers is still challenging. Furthermore, utilizing manned vehicle for observation with a commercial velocimetry in a natural stream can be dangerous. The floating method has been conventionally used, but unfortunately accuracy depends on the skill of the operator, and hence, the accuracy is uneven. The float does not always follow the mainstream and observers measure the float’s flow motion based on visual inspections. In the 1980s, developments of imaging techniques as well as microwave and ultrasonic wave devices were improved. Moreover, video imaging technique is widely being used for the measurement of surface velocity in natural rivers. Acoustic Doppler velocimetry (ADV) and acoustic Doppler current profiler (ADCP) provide not only three-dimensional velocity information but also bottom configuration. Also, ultrasonic velocity profiler (UVP) was used for flow monitoring in a small river to obtain a velocity profile to reasonably evaluate the friction velocity. However, they require large-scaled systems that have the disadvantage of portability and an observation site that is restricted to a specific place, such as on the bridge or a place with good visibility. Although ADCP-integrated moving boat methods are expected to be used for measuring the velocity and discharge in rivers, manual controls are needed to operate the moving vessel. Hence, we developed an automatic measurement system for flow velocity and direction in natural rivers using an autonomously controlled unmanned surface vehicle (USV). Oncoming mainstream velocity was measured by the propulsion force required for the USV in order to preserve the position at a measurement point. To conduct such a field mission, the system runs by changing four characteristic control stages: 1) calculation of the tentative propulsion force, 2) navigation to the target point, 3) velocity measurement by staying at the target, and 4) detection of flow direction by flowing downstream. More than 20 indoor tests were conducted under several hydraulic conditions by varying streamwise velocity, and the calibration formula was obtained by interrelating the oncoming velocity magnitude with the propulsion force required to remain at the target. The attitude control was provided with side thrusters to improve the yaw stability of the USV in the oncoming current. The adjunctive work of the side thrusters was very effective. Field tests were conducted to examine the reliability and accuracy of the present automatic flow measurements in a river. Both local velocity and direction in the river flow were measured well by the USV. Error analysis was conducted by comparing with the existing velocimetry results, and the USV was found to possess a sufficient ability to meet practical performance for the flow measurement in a natural river.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022716

Year: 2022