HAL (Le Centre pour la Communication Scientifique Directe), Jun 28, 2015
Alpine gravel-bed rivers are characterized by very poorly sorted sediments and significant grain ... more Alpine gravel-bed rivers are characterized by very poorly sorted sediments and significant grain sorting is generally observed on gravel bars. The grain size variability may significantly influence the inception of movement for each of the classes represented over a gravel bar. The purpose of this paper is to present some field measurements achieved on the Arc en Maurienne River, France. The main objective is to characterize the dynamics of fine and coarse sediments for conditions close to the incipient motion of gravels and for different degrees of clogging of the river bed. The field campaign was carried out on a gravel bar located 10 km downstream of St Jean de Maurienne during the dam flushing event on June 17th, 2014. Six patches including tagged pebbles (using PIT-tag) were built up with three different characteristics: clean patches formed with gravels and pebbles only, patches with a natural mixture where tagged pebbles were substituted to naturally arranged ones, and patches artificially clogged with fine sediments. These patches were located on the side of a secondary channel on the gravel bar. Surface grain size analyses of the gravel (using the Woolman method and photo analyses using BaseGrain) as well as topographic measurements were achieved the day before the event. During the flushing event, surface velocities over the gravel bar were measured thanks to video analysis (LSPIV). Water levels and mean slope in the secondary channel were measured thanks to pressure gauges positioned on the side of the secondary channel. An estimation of the bed shear stresses over the patches was thus possible all along the event. Intense water sampling was also carried out both upstream and downstream the secondary channel. During the postevent survey the day after, the PIT-tag search showed that the coarse particles did not move. On the other hand, a significant amount of fine sediments were deposited on the patches and infiltrated on the clean patches. Grain size analysis showed that deposited sediments were much coarser than sediments in suspension. An estimation of the fine sediment deposition rate and a discussion on the sediment dynamics is proposed from these measurements.
A generic framework is proposed to evaluate the relative discharge error made when ignoring stage... more A generic framework is proposed to evaluate the relative discharge error made when ignoring stage-discharge hysteresis due to transient flow over large gauging station networks. The diagnosis is conducted using the Jones equation, based on a simple hydraulic concept relating discharge to stage and its time-gradient. The main input data used for the method are the flow resistance coefficients, the temporal stage gradients, and the bed slopes. The hysteresis effect is quantified for each gauging station and mapped using the relative discharge error. The method was applied to 2618 gauging stations of France's national hydrometry network using observational data extracted from the national hydrological archive and from Digital Terrain Models. The diagnostic results highly depend on slope estimates used as inputs. Substantial hysteresis effects were found at stations with low bed slope combined with a fast flood regime. The France application shows the difficulty to provide a firm conclusion about stations prone to hysteresis due to the slope data uncertainty. This issue is not specific to France; slope estimates at a country-level is difficult to obtain in many countries. The use of local bed slope estimates is recommended to approach the slopes of the reaches controlling the station flow dynamics.
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
The stage-discharge relations at hydrometric stations with aquatic vegetation are non-unique as t... more The stage-discharge relations at hydrometric stations with aquatic vegetation are non-unique as they vary continuously in time due to the vegetation evolution. Maintaining rating curves for establishing discharge time series at such stations is therefore difficult. To our knowledge, no rating curve model but only manual adjustment methods (e.g. shift corrections of the base rating curve or of the stage time series, use of temporary rating curves that are built based on recent gaugings) exist to deal with such stations. To fill this gap, we have developed a dynamic rating curve model accounting for transient changes due to vegetation.
Transport of moderately sorted gravels at low bed shear stress : impact of bed arrangement and fine sediment infiltration
Le but de cette these est de comprendre la dynamique des graviers au sein des rivieres alpines a ... more Le but de cette these est de comprendre la dynamique des graviers au sein des rivieres alpines a faible contrainte en utilisant des experiences en laboratoire. Ces rivieres sont souvent composees d’une large gamme de sediments, allant des argiles aux galets. Ces differentes classes sedimentaires peuvent interagir entre elles, ce qui peut rendre difficile l’estimation du transport solide. Des experiences en laboratoire ont ete conduites en ecoulements instationnaires dans un canal de 18m de long et 1m de large. Deux types de lits ont ete etudies : lits unimodaux et bimodaux. Une attention particuliere a ete portee sur la realisation des lits de graviers dans notre canal. Ils ont ete crees dans le but d’approcher au mieux la configuration des lits de rivieres alpines, c’est-a-dire avec differents arrangements et degres de colmatage du lit par des sediments fins. Les lits unimodaux sont composes de graviers peu tries avec divers arrangements de surface. Les lits bimodaux sont composes ...
Hysteresis effects can be observed on rating curves during flood events. During the propagation o... more Hysteresis effects can be observed on rating curves during flood events. During the propagation of transient flows, the celerity of the pressure wave is smaller than the celerity of the velocity wave, hence smaller than the celerity of the discharge wave. In such case, for the same given stage the discharge is higher during the rising limb than during the falling limb of the event, leading to a non-unique stage-discharge relation. The hysteresis effect is generally greater on low-slope channels and during fast floods (strong water depth gradient). In practice, this phenomenon is often neglected. Yet, substantial biases in flood prediction can arise if ignored (underestimation of the peak discharge, a time lag of the flood hydrograph and an increase of uncertainties of the discharge).
Transport de graviers à faible contrainte : impact de l'arrangement et de l'infiltration du lit par des sédiments fins
This PhD thesis aims to understand gravel dynamics in Alpine rivers at low bed shear stress using... more This PhD thesis aims to understand gravel dynamics in Alpine rivers at low bed shear stress using laboratory experiments. Alpine river beds are often poorly sorted and composed of sediments ranging from clay to pebble. To understand interactions between these classes is an issue for predicting bedload rate. Laboratory experiments were performed in a 18m long and 1m wide flume, under unsteady flows. Two types of bed were investigated: unimodal and bimodal beds. A particular attention was paid to the bed construction, which was conducted in order to obtain a nature-like bed 12with different bed arrangements and degrees of clogging. Unimodal beds were made of moderately sorted gravels with different bed surface arrangements. Bimodal beds were made of moderately sorted gravels in which fine sediments (sand or silt) were infiltrated. Gravel rate was found to be impacted by the bed arrangement degree, the fine sediment concentration within the bedload layer and the changes in bed properti...
Water management requires a good understanding of sediment transport and associated river morphod... more Water management requires a good understanding of sediment transport and associated river morphodynamics. In mountain and piedmont hydro-systems, gravel-bed rivers often transport large amounts of fine (silt-sized) sediment. The bed material can be regarded as a mixture of coarse (gravels) and fine particles (silt, clay, very fine sand). Whether they are transported separately or not cannot be answered simply, because complex interactions between these two classes of sediment exist. As a consequence, the riverbed is usually made of heterogeneous layers of gravel particles clogged by fine sediment infiltrated in the pores of the matrix. The hydraulic conditions for the deposition and erosion of such sediment mixtures are poorly known, although they are highly related to ecological (affection of river's habitat) and socio-economic issues (dam management). In this paper, new laboratory experiments are presented to characterize the influence of fine particles on sediment transport. ...
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Papers by Emeline Perret