Author(s): G. M. Smart; H. J. Biggs
Linked Author(s): Graeme M. Smart
Keywords: No Keywords
Abstract: Advances in sensors and drone technologies now permit remote collection of high temporal and spatial resolution data from rivers. Uses of such data can range from flow gauging to spatial distributions of aquatic macrophytes. Usual practice in remote gauging is to measure the fluctuating free-surface velocity us or mean surface velocity us and convert this into a depth-averaged velocity U, using an ‘α’ coefficient, where α = U/us. A standard α value assumed for field measurements is 0.85. However, appropriate α values depend on the shape of the streamwise velocity profile u (z) and, since there are significant variations in streamwise velocity profiles described in the literature, we should expect corresponding variations in the U/us ratio. The main culprits causing deviations from conventional velocity profiles are surface wind, streamwise coherent structures and secondary flows. To determine which U/us ratio is appropriate for a given site and flow, this paper investigates the effect of shear velocity, flow depth, boundary roughness and turbulence parameters. We develop a range of predictive equations to convert surface velocity to depth averaged velocity and test these with high resolution flume data. For logarithmic velocity profiles α = 1 - u*/ (κus). Assuming u* = (gHS) 0.5 this provides a practical method for remote sensing α at a range of flow conditions. For power law velocity profiles with exponent m, α = 1/ (m + 1), with α = 0.857 for Manning’s equation which is a specific case of a 1/6th power law. Measurements of turbulence at the water’s surface offer another approach with U ≈ us - 3σus and a simple “rule-of-thumb” that suggests U ≈ us, min. Analytic considerations indicate that common U/us ratios may strongly overestimate depth-averaged velocity with permeable boundaries such as vegetation. Initial results are promising, however further testing of the equations with field data and refinement to account for variations in profile shape due to secondary currents or drag effects at the surface are required.
Year: 2020