Debris-flow velocities and superelevation in a curved laboratory channel

Abstract

Abstract:The vortex equation is often used to estimate the front velocity of debris flows using the lateral slope of the flow surface through a channel bend of a given radius. Here we report on laboratory experiments evaluating the application of the vortex equation to channelized debris flows. Systematic laboratory experiments were conducted in a 8 m long laboratory flume with a roughened bed, semi-circular cross section (top width 17 cm), and two different bend radii (1.0 and 1.5 m) with a common bend angle of 60°, and two channel inclinations (15°and 20°). Four sediment mixtures were used with systematic variations in the amount of fine sediment. In the experiments, 12 kg of water-saturated debris were released in a dam-break fashion, and multiple experiments were conducted to verify the repeatability for a given sediment mixture. Data are available for 69 experimental releases at a channel inclination of 20°and 16 releases at an inclination of 15°. Flow velocity was determined with high-speed video, and flow depth and the lateral inclination of the flow surface (superelevation) were measured using laser sensors. In general, the results from an individual sediment mixture are repeatable. We found that the channel slope as well as centerline radius have a significant influence on the correction factor k used in the vortex equation. Relatively coarse-grained sediment mixtures have larger superelevation angles than finer-grained mixtures. We found a statistically significant relation between the correction factor and Froude number. Correction factors of 1 < k < 5 were found for supercritical flow conditions. However, for subcritical flow conditions the correction factor shows a larger value as a function of the Froude number, which leads to an adaption of the forced vortex formula considering active and passive earth pressures. Finally, based on our experimental results, we present a forced vortex equation for debris-flow velocity estimation without a correction factor. Mots-clés : coulée de débris, vitesse de front de coulée, inclinaison latérale, modèle physique, équation de vortex force.

DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters – Part 2: Model validation with experiments

“…We use this case study to illustrate to what extent the calibrated model can predict flow behavior with different water contents without recalibration. The second experimental case used for validation was designed to study the sensitivity of debris flows to channel curvature (Scheidl et al, 2015). The channel had a semicircular cross section and was composed of two curves with different radii.…”

“…With β as the average surface inclination transverse to the flow direction, a correction factor k * can be defined as the ratio between the gradient tan(β) of super-elevation of debrisflow material and the corresponding gradient of clear water with the same average flow velocity. Based on the forced vortex approach with the assumption of a constant radius of the channel bend, Scheidl et al (2015) investigated the effect of flow velocity and super-elevation for several debrisflow mixtures. However, for a mathematical derivation of the slope induced super-elevation, we refer to Pudasaini et al (2005).…”

“…The experiments were performed by releasing 0.0067 m 3 of material from a reservoir, through a transitional "box-tochannel" reach, into a channel of half-circular cross section with 0.17 m diameter and a constant downslope channel inclination of 20 • (Scheidl et al, 2015). The channel was arranged in an S shape, with a first 60 • curve to the left with 1.5 m curve radius followed by a second curve to the right with 1 m curve radius (Fig.…”

“…Here, we consider the mixture with the largest clay content (mixture A of Scheidl et al, 2015), where less demixing and phase separation were observed, and focus on the first curve of 1.5 m radius. The flow height in a cross section, two-thirds of the way through the curve, was measured by three lasers across the channel (Fig.…”

Exploiting LSPIV to assess debris-flow velocities in the field

“…Hungr et al, 1984;Chen, 1987;Prochaska et al, 2008;Scheidl et al, 2014). The measured parameters (flow heights, velocity, and slope) from post-event surveys for this equation can also be used to estimate flow resistance coefficients to understand the viscosity and sediment concentrations of the debris flows (e.g.…”