One-dimensional velocity profiles in open-channel flow with intense transport of coarse sediment

Abstract

Abstract. The paper deals with laboratory experiments in open-channel flows with intense transport of model sediment (coarse plastic particles) in our new tilting flume. The major objectives of the paper are: 1. to discuss applied measuring methods, 2. to analyze measured velocity profiles. Ad 1. A profile of the longitudinal component of local velocity was measured across the vertical axis of symmetry of a flume cross section using three independent measuring methods (Prandtl tube, Ultrasonic Velocity Profiler, Acoustic Doppler Velocity Profiler). Due to strong stratification of the flow in the flume, parts of the profile are measured in regions of very different local concentrations of sediment (from virtually zero concentration to the maximum concentration of bed packing). This makes measurements complicated, particularly for ultrasonic measuring techniques. Profiles measured using the different techniques are evaluated and mutually compared. Ad 2. The layered character of the flow causes that shapes of velocity profiles tend to be different in the transport layer (rich on transported particles) above the bed and in the solids-free region between the top of the transport layer and the water surface. Shapes of the profiles are analyzed. Particular attention is paid to the logarithmic profile in the solids-free region of the flow cross section. The profile can be handled using the law of the hydraulically-rough wall. In the law, the eroded top of the bed with the transport layer is supposed to be the rough boundary and appropriate values are sought for its variables.

Experimental investigation of internal structure of open-channel flow with intense transport of sediment

“…This sub-layer of sliding grains is negligible at low θ but tends to grow to almost 40% of the transport-layer thickness (i.e. 4 to 5 grain layers) at the highest values of θ for HSF30-tests (Zrostlík et al, 2015). A very similar result has been obtained for TLT25 (e.g.…”

“…The profiles may be considered approximately linear in the transport layer. In a case of absent transport layer (the lower plane-bed regime), a velocity profile is logarithmic above the plane bed as confirmed by one of our earlier tests (Zrostlík et al, 2015). A question arises as to whether the log-profile is maintained in the water layer if the transport layer develops (the upper plane-bed regime).…”

“…In the transport layer itself, the velocity distribution differs from the logarithmic distribution. Based on our results as in Figure 6 and in the earlier publications Zrostlík et al, 2015) and on observations of others (Armanini et al, 2005;Capart and Fraccarollo, 2011), we assume that the velocity profile is linear below the logarithmic profile, A schematic composition of the velocity profile is shown on the left-hand side of Figure 7. On the right-hand side of Figure 7, the shape of a schematic profile of sediment concentration is given.…”

“…(a) (b) (Zrostlík et al, 2015). As with the PT, our Ultrasonic Velocity Profiler (UVP, MetFlow SA) is set to measure only the local longitudinal velocity component of the flow.…”

“…More details about the experimental set-up and the measuring techniques can be found in Zrostlík et al (2015).…”