Continuous measurement of the flow velocity and flow rate in a river during a flood is required, but there is no effective measurement technique yet. An ultrasonic flow velocimeter is useful for evaluating the flow rate of a river in the normal velocity range, but it is known that the measurement of the flow velocity is difficult during a flood. In this paper, we present the quantitative estimation results of ultrasound transmission characteristics for river flow measurement. From the field and laboratory experiments, the increase of background noise and attenuation of the sound due to bubbles are the largest problem for the velocity measurement during a rainfall flood. Each bubble of about 1/3 radius of a falling water drop is generated during rainfall, and seems to generate microbubbles of up to about 1/30 radius of a water drop, simultaneously.
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