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Flow structure and coarse sediment flux at tributary junctions

Titel Englisch Flow structure and coarse sediment flux at tributary junctions
Gesuchsteller/in Lane Stuart
Nummer 160020
Förderungsinstrument Projektförderung (Abt. I-III)
Forschungseinrichtung Institut des dynamiques de la surface terrestre Université de Lausanne
Hochschule Universität Lausanne - LA
Hauptdisziplin Geomorphologie
Beginn/Ende 01.10.2015 - 30.09.2019
Bewilligter Betrag 262'872.00
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Alle Disziplinen (2)

Disziplin
Geomorphologie
Fluiddynamik

Keywords (4)

Tributary Junctions; Rivers; Sediment Transport; Flow Structures

Lay Summary (Französisch)

Lead
Quelle est la dynamique des sédiments au sein de la jonction des rivières? Sites où les rivières se joignent (appelés jonctions ou confluences) sont souvent des sites où le risque d'inondation peut être élevée, et également d'une importance écologique. Ce projet porte sur la façon dont se déplace les sédiments au sein des jonctions, quelque chose que nous comprenons étonnamment mal.
Lay summary

Les tributaires des rivières se joignent à ce qu'on appelle les «jonctions» ou «confluences». On sait que lorsque les rivières se joignent, il existe une variation de la géométrie à l'aval. Nous savons aussi que la jonction des rivières provoque la formation de processus en trois dimensions qui contribuent à expliquer cette géométrie. Mais, il n'est que l'écoulement et le mélange qui est important aux jonctions. Les sédiments sont également livrés. Parfois, le taux de livraison des sédiments dans la petite rivière peut être beaucoup plus grande que ce qui peut être transporté dans la rivière principale, surtout si ce dernière est moins raide. Cela peut conduire à des problèmes de sédimentation. De même, nous savons que les jonctions des rivières peuvent créer une mosaïque complexe de l'habitat, de l'importance écologique. Pour comprendre ces processus, nous avons besoin d'une bien meilleure compréhension de la façon dont se déplace les sédiments au sein des jonctions. Dans cette étude, nous allons mesurer un grand nombre de jonctions dans le système du Rhône. Nous prévoyons d'appliquer une nouvelle technique qui nous permet de mesurer, simultanément, les sédiments fins et les sédiments grossiers (se déplaçant à la lit) afin que nous puissions décrire comment se déplace sédiments au sein de la jonction. Nous allons expliquer les observations, compte tenu des changements dans la livraison des sédiments avec les saisons. Nous allons appliquer des modèles numériques afin de mieux quantifier le mouvement des sédiments au sein des jonctions et d'établir les conditions lorsque cela devient moins efficace. Nos résultats seront utiles pour la gestion des risques d'inondation et aussi pour la restauration des rivières.

Direktlink auf Lay Summary Letzte Aktualisierung: 01.05.2015

Lay Summary (Englisch)

Lead
How does sediment move through the junctions of rivers? Sites where rivers join (known as junctions or confluences) are often sites where flood risk can be elevated, and also of ecological importance. This project addresses how sediment moves through them, something that we understand surprisingly poorly.
Lay summary

Rivers join at what are known as "junctions" or "confluences". We know that when rivers join, there is a change in the geometry of the channel downstream. We also know that the junction of rivers causes the formation of three dimensional flow processes that help to explain this geometry. But, it is not just river flows that mix at junctions. Sediment is also delivered. Sometimes, the delivery rate in the smaller river may be much bigger than that which can be transported in the main river, especially if the latter is less steep. This can lead to sedimentation problems. Similarly, we know that the junctions of rivers may create a complex mosaic of habitat, of ecological importance. To understand these processes, we need a much better understanding of how sediment moves through junctions. In this study, we will be studying a large number of river junctions with the Rhône system. We plan to apply a novel technique that allows us to measure, simultaneously, fine sediment and coarse sediment (moving at the bed) so that we can describe how sediment moves through the junction. We will explain our observations, considering how sediment delivery changes with the seasons. We will apply sophisticated numerical-mathematical models to further quantify sediment movement through the junctions and to establish those conditions when this becomes less effective. Our results will be of value for managing flood risk and also for river restoration.

Direktlink auf Lay Summary Letzte Aktualisierung: 01.05.2015

Verantw. Gesuchsteller/in und weitere Gesuchstellende

Mitarbeitende

Name Institut

Verbundene Projekte

Nummer Titel Start Förderungsinstrument
147689 SEDFATE:Sediment fate in a changing watershed during the Anthropocene 01.02.2014 Sinergia

Abstract

The junctions of tributaries and rivers are now recognised as fundamental controls on sediment flux through river basins, as biodiversity hotspots and as critical controls on river basin connectivity. Strahler (1957) stream ordering draws attention to a fundamental observation: at the vast majority of tributary junctions in a drainage basin network, one of the tributaries is smaller (width, depth, discharge), sometimes, much smaller, than the main stem. Such differences can be expressed in terms of a discharge (Q) ratio (Qt/Qm, where t = tributary; m = main stem); or a momentum ratio** (with equal densities, conventionally described as Mr = QtUt/QmUm, where U = section averaged-velocity). Survey of the literature shows: in general (1) that there have been very few studies of sediment flux at tributary junctions, notwithstanding the importance of this flux for continuity in sediment transport; and, specifically, (2) that the majority of studies (36 out of 57) of tributary junctions have been concerned with momentum ratios greater than 1, despite the likely dominance of junctions with tributary momentum ratios less than one, and perhaps substantially less than 1. Laboratory experiments published by the LCH at the EPFL suggest that the flow structure and sediment flux dynamics in low Mr tributary junctions may be substantially different to those described by classical models of tributary junctions. Yet, such junctions may be crucial for moderating the flux of sediment from tributaries to the main stem, and implicated in a range of management problems, such as problems of rapid sedimentation in tributaries and associated flooding, during extreme rainfall events. This project has the general aim of understanding the dynamics of sediment flux at medium-sized tributary junctions and will address the following objectives: (1) to provide a systematic quantification and analysis at the drainage basin scale of the prevalence and characteristics of tributary junctions to identify the relative importance and characteristics of those with low momentum ratios; (2) using the database used in (1), to select a subset of tributary junctions for intensive and repeat measurement of sediment flux dynamics using state-of-the-art measurement technologies; and (3) to develop a mathematical representation of these dynamics to quantify and to simulate tributary response associated with these interaction, building on our recent work using discrete particle modelling. Through these three steps we will aim to assess two general sets of hypotheses: (1) that coarse sediment movement through tributary junctions, even in low Mr cases, is impacted upon by flow structure topographic forcing; and (2) that the classic Best (1988) model of tributary junctions can be modified and extended to represent a wider range of forcing conditions. As an additional benefit, this work will also evaluate the transferability of existing, published, laboratory-acquired findings to the field case.
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