Analysis of Open-channel Velocity Measurements Collected with an Acoustic Doppler Current Profiler

Acoustic Doppler Current Profilers (ADCP’s) are easier to apply and more rapidly collect velocity data than traditional current-meter instruments. However, flow discharge estimates in natural rivers and canals based on ADCP measurements rely on extrapolations of the measured velocity profiles into the unmeasured zones of the flow. Furthermore, the accuracy and reliability of ADCP’s have not been rigorously tested and formal protocols for discharge measurements with ADCP’s have not been developed. Therefore, a comparison of the velocity profiles measured with an ADCP with theoretical velocity distributions is necessary. The analysis of two sets of approximate two-dimensional open-channel velocity distributions collected with a fixed ADCP at the Chicago Sanitary and Ship Canal (CSSC) at Romeoville, Illinois, is presented here. These measured velocity distributions are compared with the logarithmicand power-law velocity distributions. The shear velocity and Nikuradse’s equivalent sand roughness were estimated by fitting the data to the logarithmic law. The shear velocity and Nikuradse’s equivalent sand roughness estimated from the first and second data sets are 0.23 m and 0.046 m/s, and 0.27 m and 0.020 m/s, respectively. Results indicate that both the logarithmicand power-law fit the measured data well. The exponent of the power law was estimated to be very close to 1/6, which links the power law with Manning’s equation. Furthermore, the values of Nikuradse’s sand roughness of both data sets are equivalent to a Manning’s n ≈ 0.030, which seems consistent with the channel surface and bed conditions of the CSSC at Romeoville.