Estimations of surface velocity distributions from X-band radar images during floods observed at the mouth of Tenryu River, Japan

Abstract

X-band radar observations with a transmitting frequency of 9.4 GHz were conducted for over 10 h at the mouth of Tenryu River, Japan, to estimate surface velocity distributions during floods in 2010 and 2020 and analyse changes in the flow field caused by dredging in 2014. X-band radar is an imaging radar that can collect backscatter of floating objects flowing downstream with the flood flow every 2 s. The observation area extends approximately 1000 m × 1000 m. Particle image velocimetry is applied to the image sequences to guess the surface flow pattern. Flow direction in the main river channel changed smoothly, while surface flow speeds ranged from 1 to 3 m s^–1 depending on the discharge. Water levels were computed using the estimated surface flow velocities, river cross-section from survey data, and observed discharge for indirect validation: it agreed within an error of 0.3 m with the observed water level. This paper highlights radar’s potential for estimating surface velocity distribution during floods and harsh weather conditions.

Keywords:

• Flood
• particle image velocimetry (PIV)
• surface velocity measurement
• X-band radar

Acknowledgements

The authors are grateful for the support provided by Yuki Wada, a former lab member, for preliminary analyses and for providing the MATLAB code.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Japan Society for the Promotion of Science KAKENHI [grant number 19H02244 and 22H01595] and the River Fund, The Foundation of River & Watershed Environment Management. The first author is supported by the Monbukagakusho (MEXT) scholarship.