Remote determination of the velocity index and mean streamwise velocity profiles

Abstract

AbstractWhen determining volumetric discharge from surface measurements of currents in a river or open channel, the velocity index is typically used to convert surface velocities to depth‐averaged velocities. The velocity index is given by, , where Ub is the depth‐averaged velocity and Usurf is the local surface velocity. The USGS (United States Geological Survey) standard value for this coefficient, k = 0.85, was determined from a series of laboratory experiments and has been widely used in the field and in laboratory measurements of volumetric discharge despite evidence that the velocity index is site‐specific. Numerous studies have documented that the velocity index varies with Reynolds number, flow depth, and relative bed roughness and with the presence of secondary flows. A remote method of determining depth‐averaged velocity and hence the velocity index is developed here. The technique leverages the findings of Johnson and Cowen (2017) and permits remote determination of the velocity power‐law exponent thereby, enabling remote prediction of the vertical structure of the mean streamwise velocity, the depth‐averaged velocity, and the velocity index.