Effects of Lateral and Vertical Constrictions on Flow in Rough Steep Channels with Bedload

Abstract

Two-phase flows occurring at flow constrictions such as bridges or open sediment check dams are complex, especially for steep rivers with bedload transport. Dangerous bedload deposition and backwater effects may occur in steep mountain rivers at bridges. In contrast, sediment deposition is desirable at open check dams combined with sediment traps. For design purposes, the discharge and bedload capacity across these flow constrictions must be known. The energy losses, discharge capacity, and bedload transport capacity of vertical and lateral flow constrictions are experimentally studied in a rough, 2% inclined, trapezoidal channel. Both free surface and pressurized flow conditions, as caused by lateral and vertical flow constrictions, respectively, were analyzed because both may occur at bridges and check dams. The experiments demonstrate that the vertical flow constrictions cause a faster increase in the backwater depth, with increasing discharge, than lateral constrictions. The resulting upstream flow conditions can be described by the upstream Froude number, defined as a function of the constriction dimensions (height and width). The bedload transport capacity through the flow constriction decreases with the upstream Froude number. The practical relevance of the findings is illustrated by a design example of flow constrictions at open sediment check dams.

Experimental study of sediment traps permeable for frequent floods

“…Sci. Previous studies have shown that the retention of bed load is hydraulically initiated as soon as the barrier causes a hydraulic jump upstream underlying generally supercritical flow conditions (Schwindt et al, 2017a(Schwindt et al, , 2017b. Supercritical flow conditions are typical in mountain rivers but not omnipresent (e.g., Heyman et al, 2016).…”

“…Therefore, the experimental set-up (as used by the authors in Schwindt et al, 2017a;2017b) included a widened deposition area, according to typical sediment traps in a mountain-river-like environment.…”

“…25 (dimensionless) and a bottom channel width of w = 0.11 m (Schwindt et al, 2017a;2017b). The channel had a roughness according to a Mannings'n of n ≈ 0.02.…”

“…3 and Schwindt et al, 2017b). The barrier incorporated a flow constriction for the hydraulic control and a bar screen for the mechanical control of 15 bed load retention.…”

“…This choice has been made to minimize the effects of the barrier on the flow when the guiding channel was not overtopped. An opening height of 0.040 m is required to hydraulically trigger sediment retention for a discharge of 5.5 l/s (Schwindt et al, 2017a;Schwindt et al, 2017b). 10…”