Characteristics of Surface-to-Depth-Averaged Velocity Conversion Factors Based on ADCP Measurements

This study investigates surface-to-depth-averaged velocity relationships for discharge estimation across a wide range of river scales and flow conditions. The analysis focuses on conversion factors used in surface-velocity-based discharge estimation and their hydraulic controls. A total of 172 ADCP datasets collected from Korean rivers between 2016 and 2018 were reprocessed using an updated version of QRev to obtain consistent and reliable velocity and discharge estimates. Surface velocities were estimated using QRev-based extrapolation and power-law velocity profile methods, and conversion factors were calculated as the ratio of depth-averaged velocity to surface velocity.

The mean conversion factor was 0.88 for the extrapolation method and 0.85 for the power-law method, with most values ranging between 0.80 and 0.90. The estimated uncertainty was approximately 7–8%. Analysis of hydraulic variables showed that conversion factors increased with water surface width and mean depth, whereas weak negative trends were observed with mean velocity and shape factor. Correlation coefficients were generally below 0.5, indicating substantial scatter and limitations in generalizing conversion factors based on single hydraulic parameters.

Acknowledgements
This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Research and Development on the Technology for Securing the Water Resources Stability in Response to Future Change program, funded by Korea Ministry of Climate, Energy and Environment (MCEE) (RS-2024-00336020)

Keywords : Surface velocity, Depth-averaged velocity, Conversion factor, ADCP, Natural rivers