Bed load transport in a very steep mountain stream (Riedbach, Switzerland): Measurement and prediction. Issue 12 (23rd December 2016)

• Record Type: Journal Article
• Authors: Schneider, Johannes M.; Rickenmann, Dieter; Turowski, Jens M.; Schmid, Bastian; Kirchner, James W.
• Is Part Of: Water resources research. Volume 52:Issue 12(2016:Dec.)
• Page Start: 9522
• Page End: 9541
• Publication Date: 2016-12-23
• Subjects: bed load transport -- steep streams -- bed load traps -- geophones -- prediction -- two‐phase transport; Hydrology -- Periodicals; 333.91
• DOI: 10.1002/2016WR019308
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: Compared to lower‐gradient channels, steep mountain streams typically have rougher beds and shallower flow depths, making macro‐scale flow resistance (due to, e.g., immobile boulders and irregular bedforms) more important as controls on sediment transport. The marked differences in hydraulics, flow resistance, and grain mobility between steep and lower‐gradient streams raise the question of whether the same equations can predict bed load transport rates across wide ranges of channel gradients. We studied a steep, glacier‐fed mountain stream (Riedbach, Ct. Valais, Switzerland) that provides a natural experiment for exploring how stream gradients affect bed load transport rates. The streambed gradient increases over a 1 km stream reach by roughly one order of magnitude (from 3% to 38%), while flow discharge and width remain approximately constant. Sediment transport rates were determined in the 3% reach using Bunte bed load traps and in the 38% reach using the Swiss plate geophone system. Despite a ten‐fold increase in bed gradient, bed load transport rates did not increase substantially. Observed transport rates for these two very different bed gradients could be predicted reasonably well by using a flow resistance partitioning approach to account for increasing bed roughness ( D 84 changes from 0.17 m to 0.91 m) within a fractional bed load transport equation. This suggests that sediment transport behavior across this large range of steep slopes agrees with patterns established in previous studies for both lower‐gradient and steep reaches, and confirms the applicability of the flow resistance and bed load transport equations at very steep slopes. Key Points: Fluvial bed load transport measured at bed gradients from 3% up to 38% Bed load transport approximately constant with increasing bed gradient Bed load transport equations for flat streams also apply in steep channels