Author(s): M. Welber; W. Bertoldi

Linked Author(s):

Keywords: Glacier-fed rivers; Physical modeling; Sediment transport; Bedload hysteresis; Morphodynamics

Abstract: In this work, a large physical model was used to investigate the sediment transport regime and morphodynamics of a river system subject to cyclic fluctuations of flow and sediment supply. Glacier-fed rivers exhibit a distinctive flow regime characterized by daily and seasonal fluctuations of discharge and sediment flux. Past research has shown that the response of a river channel to unsteady inputs depends on many factors, including hydrograph shape and magnitude, grain size distribution, and channel morphology. In this work, we used a physical model of a gravel-bed river to answer the following questions: 1. What is the typical sediment flux signature of a glacier-fed river? 2. How do water and sediment supply unsteadiness affect channel morphology? 3. How does channel morphology modulate sediment waves? The physical model consisted of a 25-m long rectangular, straight channel with fixed banks. Slope was set to 0.01 and three values of channel width were selected (0.15, 0.3 and 0.6 m) in order to obtain a range of channel configurations (plane bed, alternate bars and wandering, respectively). Two sediment mixtures were used as bed and supply material: a well-sorted sand (d50=1.0 mm, d90=1.1 mm) and a poorly-sorted mixture (d50=1.3 mm, d90=2.5 mm). The flume was equipped with a software-controlled feeding system which enabled discharge and sediment supply to be assigned as a function of time at 1-min resolution. Bedload output was recorded every 10 seconds. Topographic information was acquired using a laser profiler or derived from high-resolution images using Structure-from-Motion. For each channel configuration, the flume was first run under steady flow conditions to estimate transport capacity and to reconstruct equilibrium bed morphology for a range of discharges. Afterwards, daily discharge fluctuations were simulated as sequences of 30 symmetrical triangular hydrographs. Minimum and maximum discharge and hydrograph duration were defined in order to simulate the typical daily discharge regime of an Alpine gravel bed, glacier-fed river. Sediment supply at each hydrograph step was set to transport capacity; in addition, runs were repeated with shifted sedigraphs, in order to simulate real scale cases where discharge peaks lag behind bedload peaks. Bed topography was acquired at four flow stages (minimum and maximum discharge and halfway through the rising and falling limb). Within each set of hydrographs, bedload output shows significant differences between cycles, which can be linked to the inherent variability of sediment transport. The average response of the channel to cyclic input was defined by computing the ensemble mean of bedload output signals over the 30 hydrographs. Average bedload output is higher during the rising phase, resulting in a clockwise hysteresis cycle with respect to water discharge. Hysteresis is stronger for short hydrographs and independent from sediment supply lag. This suggests that sediment input timing may have a substantial effect on bedload flux in the uppermost sections of a glacier-fed river, while further downstream hydraulic regime and channel characteristics are the dominant factors governing sediment waves. This finding is confirmed by the analysis of longitudinal bed profiles acquired at different hydrograph stages. During model runs with shifted sedigraphs, the channel undergoes a sequence of underfeeding and overfeeding phases that cause alternate bed degradation and aggradation, but these slope variations are limited to a few channel widths downstream of channel inlet. Changes in elevation are largest for the smallest channel width and in the case of alternate bars.

DOI: https://doi.org/10.3850/978-981-11-2731-1_220-cd

Year: 2018