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Mountain River Confluence Morphodynamics in Response to Different Tributary Channel Gradients

Author(s): Theo St. Pierre Ostrander; Johannes Holzner; Bruno Mazzorana; Andrea Andreoli; Francesco Comiti; Bernhard Gems

Linked Author(s): Theo St Pierre Ostrander, Andrea Andreoli, Bernhard Gems, Johannes Holzner

Keywords: River Confluence; Fluvial Hazard; Steep Tributary; Bedload; Physical Scale Model

Abstract: Intense bedload transport from tributaries and insufficient transport capacity of the main channel can cause flooding at mountain river confluences. This can lead to overbank flooding and sedimentation on the depositional fan and into adjacent settlement areas. The hydrodynamic and morphologic processes at lowland river confluences have been studied extensively. The confluence angle and discharge ratio have been identified as the most important parameters influencing channel morphology and hydraulics. However, little information is available on confluences with steep mountain channels. This study presents results from large-scale laboratory experiments using a standardized river confluence geometry. The main channel had a gradient of 0.5% and a mobile bed that allowed 0.2 m of erosion. The tributary channel bed was fixed and had gradients of 5% and 10%. The confluence angle was 90° and the sediment concentration in the tributary channel was set at 5%, 7.5% and 10%. The discharge ratio was 0.1. Holding the discharge ratio and confluence angle constant, the results showed different confluence morphologies for both gradients, suggesting that sediment concentration and discharge significantly influence hydraulic and morphological zones in mountain stream confluences. Comparing morphological patterns across gradients, reduced velocity, and transport capacity due to channel gradient reduction resulted in intense deposition in the tributary channel, limiting the amount of sediment able to enter the confluence zone, but still resulting in similar morphological development.


Year: 2023

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