Etude de la formation d'un lit torrentiel Dune à forte pente et pavage

Stream bed structure study: steep slopes dune and pavement

¹ CEMAGREF, Division Protection contre les Erosions Actuellement Ingénieur Conseil, 541, Route de la Rosiére, 73700 Bourg-St-Maurice
² Professeur ENSHMG-INPG
³ INPG Consultant

Abstract

Stream beds exhibit chaotic features which are hardly understandable whereas they reflect the stream behaviour. We carried out some small-scale experiments in the aim of understanding their formation. As a first step a very small-scale model allowed us to quantify locally bed load and particle grading. Two distinct phenomena appeared : The hyperconcentrated bed load transport for which bed load is governed by hydraulic conditions. ln this case, the stream bed is relatively fine-grained and regular, the flow is rapid and the solid concentration is high. The particle grading - less important - increases the motion of big boulders. The ordinary bed load transport - close to the incipient motion threshold - for which the solid transport is quite variable and the Froude number varies around unity. Fine grains are easily moved which induces an armouring process. As a second step, using a wide open channel, we showed that the bed load intensity decreases when the bed width is able to increase. Additionally, the different morphological structures tend to stop fine and coarse grain motions while intermediate grains are moved downstream. The irreversibility of the bifurcation between hyperconcentrated and ordinary bed load transport leads to a strong spatial heterogeneity between these two phenomena. This diversity can be seen by the appearance of large scale morphological structures : steep slopes dunes.