Preprints
https://doi.org/10.5194/egusphere-2023-2432
https://doi.org/10.5194/egusphere-2023-2432
13 Dec 2023
 | 13 Dec 2023
Status: this preprint is open for discussion.

Limited effect of the confluence angle and tributary gradient on Alpine confluence morphodynamics under intense sediment loads

Theo St. Pierre Ostrander, Thomé Kraus, Bruno Mazzorana, Johannes Holzner, Andrea Andreoli, Francesco Comiti, and Bernhard Gems

Abstract. Confluences are dynamic morphological nodes in all river networks. In mountain regions, they are influenced by hydraulic and sedimentary processes occurring in steep channels during extreme events in small watersheds. Sediment transport in the tributary channel and aggradation in the confluence can be massive, potentially causing overbank flooding and sedimentation into adjacent settlement areas. Previous works dealing with confluences have been mainly focused on lowland regions, or if focused on mountain areas, the sediment concentrations and channel gradients were largely under-representative of mountain river conditions. The presented work contributes to filling this research gap with 45 experiments using a large-scale physical model. Geometric model parameters, applied grain size distribution, and the considered discharges represent the conditions at 135 confluences in South Tyrol (Italy) and Tyrol (Austria).

The experimental program allowed for a comprehensive analysis of the effects of (i) the confluence angle, (ii) the tributary gradient, (iii) the channel discharges, and (iv) the tributary sediment concentration. Results indicate, in contrast to most research dealing with confluences, that in the presence of intense tributary sediment supply and a small tributary to main channel discharge ratio (0.1), the confluence angle does not have a decisive effect on confluence morphology. Adjustments to the tributary channel gradient yielded the same results. A reoccurring range of depositional geomorphic units was observed where a deposition cone transitioned to a bank-attached bar. The confluence morphology and tributary channel gradient rapidly adjusted, tending towards an equilibrium state to accommodate both water discharges and the sediment load from the tributary. Statistical analyses demonstrated that confluence morphology was controlled by the combined channel discharge and the depositional or erosional extents by the sediment concentration. Applying the conclusions drawn from lowland confluence dynamics could misrepresent depositional and erosional patterns and the related flood hazard at mountain river confluences.

Theo St. Pierre Ostrander, Thomé Kraus, Bruno Mazzorana, Johannes Holzner, Andrea Andreoli, Francesco Comiti, and Bernhard Gems

Status: open (until 07 Mar 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2432', Anonymous Referee #1, 02 Feb 2024 reply
    • AC1: 'Reply on RC1', Theo St Pierre Ostrander, 13 Feb 2024 reply
      • RC2: 'Reply on AC1', Anonymous Referee #1, 21 Feb 2024 reply
Theo St. Pierre Ostrander, Thomé Kraus, Bruno Mazzorana, Johannes Holzner, Andrea Andreoli, Francesco Comiti, and Bernhard Gems
Theo St. Pierre Ostrander, Thomé Kraus, Bruno Mazzorana, Johannes Holzner, Andrea Andreoli, Francesco Comiti, and Bernhard Gems

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Short summary
Mountain river confluences are hazardous during localized flooding events. Results from a physical model were used to determine the dominant controls over mountain confluences. Contrary to lowland confluences, in mountain regions, the channel discharges and then the tributary sediment concentration controls morphological patterns. Applying conclusions drawn from lowland confluences could misrepresent depositional and erosional patterns and the related flood hazard at mountain river confluences.