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Laboratory Experiments for Channel Aggradation in Supercritical Flow

Author(s): Alessio Radice; Barbara Zanchi; Ehsan Heydari

Linked Author(s): Alessio Radice, Ehsan Heydari

Keywords: Sediment aggradation; Supercritical flow; Overloading ratio; Aggradation front; Flood hazard

Abstract: A flash-flood event may involve a large volume of sediment material transported along the channel. Aggradation of sediment at a critical location may reduce the conveyance of the river channel and, consequently, increase the maximum water level; therefore, banks may be much more prone to overflow than in the pre-aggradation condition. Knowledge of the aggradation processes may be thus needed to appropriately quantify a flood hazard, and requires numerical or experimental work. This study presents a laboratory investigation of the aggradation phenomena in supercritical flows. These flow conditions have been much less studied than the subcritical ones in the scientific literature. As a matter of fact, the present campaign follows an earlier one, performed in the same facility, for aggradation fronts in subcritical flow; in the previous campaign we characterized the shape of the aggradation fronts, their height and celerity under a range of hydro-dynamic and sediment-overloading conditions. In the present work, experiments have been performed at the Mountain Hydraulics Lab of the Politecnico di Milano (Lecco campus), using a tilting flume with rectangular cross section (width = 0.3 m), filled with lightweight particles (Polyvinyl Chloride grains with a size of 3.6 mm). An experimental series is presented where all the runs had the same initial bed slope of 1.2% and the same flow rate of 5 l/s, while different overloading ratios were imposed (the overloading ratio being the ratio between a sediment feeding rate at the inlet and the transport capacity at initial flow) in a range between 1.1 and 1.9. Many movies have been acquired during the experiments; by utilizing various image processing methods, experimental data have been obtained for the water and bed profiles, sediment feeding rate, and sediment transport capacity of the flows. After the beginning of an experiment, the aggradation height is larger close to the sediment feeding point and decreasing downstream. The channel thus tends to a new slope, higher than the initial one, as it tends to achieve a new transport capacity in equilibrium with the sediment feeding rate imposed at the inlet. Temporal and spatial scales of propagation need to be defined and measured. A dimensionless comparison of aggradation profiles at different times seems to reveal some similarity between the profiles for the different runs, but the results are not conclusive. The recognition of an aggradation front and the quantification of a related celerity of propagation are hindered by a markedly dispersive shape of the aggradation profiles, where a neat separation between an undisturbed and an aggraded reach is not detected. The present results, while providing novel phenomenological insight, call for analysis tools specifically devoted to the supercritical conditions, as in the latter the aggradation process differs from that in subcritical flows.


Year: 2022

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