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International audience; Debris-flows are known to have important volume growth from channel erosion on steep slopes. However, it is still difficult to quantify the relationship of debris-flow scouring and slope. High densities of multi-date cross-sections in both the Manival and Real debris-flow catchments (French Alps) have clearly shown that debris-flows have significant scouring with large spatial variability. Bedload transport was observed to be at equilibrium with little variability. Field observations of channel deformations show that debris-flow scouring is strongly controlled by upstream slope and storage conditions. A logarithmic relationship is proposed as an empirical fit for the prediction of channel erosion. Since the important volumes of debris-flows are composed of scoured material, this empirical model provides a basis for estimating potential magnitudes using simple channel profile and active channel width.

Airborne Laser Scanner (ALS) surveys are widely used to obtain high-resolution DTMs to be used in natural hazards preventive analysis, e.g. flood and debris-flow modelling, as well as to reconstruct sediment budget through DoD (DEM of Difference) analysis, in particular in mountain basins after large events . In contrast, the use of ALS to capture the morphometric features of stream channels is less favorable as the infrared signal, characteristic of the most common LiDAR instruments, do not penetrate deep water bodies and thus do no permit to analyze the wet portion of channels. Yet, surveys carried out by these instruments can provide sufficient data to characterize and analyze morphometrically stream channels featuring either very shallow flows relative to bed roughness(e.g. steep mountain channels) or relatively limited wet areas (e.g. braided rivers). So far, very few investigations have deployed bathymetric LiDAR surveys, also known as green LiDAR, to map river systems. Bathymetric LiDAR has the potential to offer high-resolution DTMs for morphometric analysis in river systems complementary to those mentioned above (i.e. channels with perennial and substantial wet areas) and where bathymetric surveys using boats are not easily carried out due to fast and turbulent flows. These conditions are typical of mild-sloping, single-thread or wandering mountain rivers, widely distributed worldwide, in which detailed morphometric analysis are thus very challenging when carried out through traditional approaches. We will present a state of the art of morphometric analyses used to characterize channel morphology, including published and unpublished examples from step-pool channels as well as from sinuous-meandering and braided rivers, based on both infrared and green LiDAR data.

International audience

International audience

International audience; Debris-flows mobilize high sediment loads and are often responsible for most of the sediment yield from upland headwaters and may endanger the safety of life and infrastructure in the vicinity of torrent channels. Two very active debris-flow torrents with different physiographic settings have been equipped in the French Alps, the Manival Torrent (3.6 km²) and the Réal Torrent (2.3 km²). At these two sites, debris-flow monitoring systems are based on a combination of different techniques involving rain gauges, soil vibrations and flow elevation measurements and high-resolution/frequency imagery acquisition. Only flash floods with bed load transport have been observed on the Manival Torrent. Conversely, in spring/summer 2011 four meteorological events triggered debris flows on the Réal Torrent. Most of the debris fronts deposited immediately in the upstream part of the channel. Only one debris-flow propagated from the sediment source area to the basin’s outlet. A 1.8 m high debris front propagated with a mean velocity of about 3 m s-1. Its volume was 4,400 m3 upstream and it increased because of bed erosion. The debris flow reached 6,100 m3 in the middle part of the basin and then decreased slightly further downstream (8,600 m3). These measurements highlight the interaction between the debris flow and the torrent channel in the first part of the basin. These results are in accordance with previous sediment budgets estimated for the debris flows which occurred in 2009 and 2010.

International audience; Historical investigation of small alpine catchments throughout the Alps have shown that periods of high debris flow magnitude and frequency result from intense landslides events. Hence, the supply of sediment and debris recharging channels prone to debris flow can be closely related to rock slope instabilities. Mass wasting production through hillslope erosion processes seems therefore to be implicitly linked to the geomorphic and geostructural conditions prevailing at the source areas (mostly composed of cliffs). Since 2009, a monitoring of the cliffs and gullies connected to the Manival torrent (Chartreuse Massif, northern french Alps) has been performed periodically with terrestrial laser scanner (LiDAR). Time series comparison of the LiDAR data has enabled to observe erosion activity and sediment flux contributing to accumulate debris into gullies. The acquired high resolution point clouds enabled to perform a local scale structural analysis of the slopes supplying the torrent and tributary gullies with sediment. The main discontinuity sets and their related structural domains were extracted using the software Coltop 3D that enables an analysis of the topography by representing the dip and dip direction of the slopes of a DEM with a unique colour code ranging in value of a Schmidt Lambert projection. The spatial variability of the structural domains was compared quantitatively with the zones of debris production response in terms of morphological features (geometry of the slopes) and structural setting (dip and dip direction of the different joint sets, bedding planes, joints spacing, etc.). Kinematic analyses were performed across the entire topography to highlight failure mechanism and their correlation with zones of sediment production. The maximum discontinuity frequency and its directions were compared with the orientation of active gullies and scree deposits. Such analyses were finally correlated to the regional structure (lineaments, folds and faults, etc.) and provide an assessment of the contribution of the structural setting to the destruction/erosion of the catchment area supplying sediments to the channel. This ongoing study documents the mode of sediment production on cliffs and the temporal storage/discharge in adjacent subcatchment by analysing the morphostructural control over the debris flow catchment based on high-resolution DEM and geological field study. This study shows that the process-related morphology and slope failure susceptibility can be used as proxies for evaluating terrain implication in the process of debris supply or for preliminary assessing potential erosion zones leading to debris flow activity.

National audience; The Manival near Grenoble (French Prealps) is a very active debris-flow torrent equipped with a large sediment trap (25 000 m3) protecting an urbanized alluvial fan from debris-flow. We began monitoring the semdiment budget of the catchment controlled bu the trap in Spring 2009. Terrestrial laser scanner is used for monitoring topographic changes in a small gully, the main channel, and the sediment trap. In the bedload transport after the events in 2009 and 2010 are presented revealing the dependency of the sediment supply from the production zone of the torrent catchment. The past two years the torrent storage has been identified as to have a decreasing trend despite seasonal recharge.

National audience; Ce papier présente les enjeux scientifiques, opérationnels et techniques d'un observatoire de mesure des crues et laves torrentielles installé en 2010 sur cinq torrents des Alpes françaises (Isère et Alpes- Maritimes). Couplée à des relevés topographiques à haute-résolution et des données pluviométriques spatialisées, ce type d'installation, encore inédite en France, permettra d'observer, mesurer et analyser plus précisément les mécanismes de déclenchement et de transfert des écoulements torrentiels en milieu montagneux.

International audience; The Manival near Grenoble (French Prealps) is a very active debris-flow torrent equipped with a large sediment trap (25000 m3) protecting an urbanized alluvial fan from debris-flows. We began monitoring the sediment budget of the catchment controlled by the trap in Spring 2009. Terrestrial laser scanner is used for monitoring topographic changes in a small gully, the main channel, and the sediment trap. In the main channel, 39 cross-sections are surveyed after every event. Three periods of intense geomorphic activity are documented here. The first was induced by a convective storm in August 2009 which triggered a debris-flow that deposited ~18000 m3 of sediment in the trap. The debris-flow originated in the upper reach of the main channel and our observations showed that sediment outputs were entirely supplied by channel scouring. Hillslope debris-flows were initiated on talus slopes, as revealed by terrestrial LiDAR resurveys; however they were disconnected to the main channel. The second and third periods of geomorphic activity were induced by long duration and low intensity rainfall events in September and October 2009 which generate small flow events with intense bedload transport. These events contribute to recharge the debris-flow channel with sediments by depositing important gravel dune propagating from headwaters. The total recharge in the torrent subsequent to bedload transport events was estimated at 34% of the sediment erosion induced by the August debris-flow.

International audience; The general aim of the study is to improve the understanding of sediment responses during flash floods and debris flows in small French Alpine torrent catchments (2.3 48.7 km2) by means of high-frequency monitoring of water and sediment fluxes (recording time step of 200 ms. to 5 min.). This study is part of two European research programs (RiskNat-Interreg-Alcotra and Paramount-Interreg-Alpine Space Program).

International audience; Rock slope instabilities are implicitly linked to the supply of sediment and debris recharging channels prone to debris flow. Hence, the incorporation of bed-rock structure and terrain morphology can be relevant in the analysis of sediment budget and debris flow hazard assessment. Here, the mode of debris production of the Manival catchment (nothern French Alps) is documented by the study of its morphostructural aspects extracted from high resolution DEM. Terrain implication in the process of debris supply is evaluated by : a) A systematic classification of the major morphological units based on the slope gradient that enables a spatial analysis of zones of debris production and deposition. b) A detailed structural analysis performed on DEM in order to identify potential unstable slopes. c) An analysis of the gullies orientation that informs in term of structural control of the sources zones. d) Localisation of high density joints sets that document about whether sources of continuous debris production are controlled by the structural setting of the catchment. These DEM-based indicators can be used as proxies for assessing the influences of the current topography and enable to quantify a degree of susceptibility to mass wasting and hillslope erosion activity. This present contribution suggets some directions for characterizing sediment flux dynamic in small alpine catchment.

International audience; The monitoring of sediment transport in the Manival Torrent and its catchment (Chartreuse mountain range, France) began in spring 2009 with a sediment budget approach. The objective is to explore topographic, spatial, and morphologic correlations with sediment transport activities throughout the catchment and its torrent channel.

Ce projet a pour objectif principal d'améliorer la connaissance du transport solide des torrents pour une meilleure prise en compte des sédiments dans la gestion des risques liés aux crues torrentielles. Le fil conducteur est de s'appuyer sur les plages de dépôt alpines pour le recueil de données détritiques en contexte torrentiel. Cette démarche a été entreprise selon plusieurs échelles spatiales, avec un premier volet régional destiné à établir des liens statistiques entre les caractéristiques physiques des bassins versants et leur production sédimentaire, et un deuxième volet local dont l'objectif est d'analyser la complexité des transferts sédimentaires internes d'un petit bassin versant très actif (Torrent du Manival). Une étude dendrogéomorphologique est également proposée sur ce site à titre exploratoire.

International audience; Manival Torrent, a small mountain stream located in the Northern French Prealps, experiences debris flows annually. The torrents catchment (3.6 km2, 1,200 m relief) is composed of marl and limestone beds with increasing limestone towards the ridge. A field monitoring program started in 2009 to study the geomorphic responses to flow events. The aim of this study is to implement an event-based sediment budget approach for monitoring coarse sediment transport and characterizing the spatial variability of the sediment response in the main channel. The monitoring of 38 cross-sections (within a 1.8 km length) provides total volumes of erosion and deposition throughout the main channel. At the downstream end of the surveyed reach, a sediment trap (capacity: 25,000 m3) is used for measuring sediment output. Terrestrial LiDAR is used for monitoring erosion and deposition wherever accessible in the catchment area which includes the sediment trap, reaches of the torrent, talus slopes, gullies, and cliffs. Precipitation is monitored by a rain gage in the upper catchment. An airborne LiDAR survey was done in spring 2009 providing a high-resolution DTM of the catchment. This intense field monitoring allows us to reconstruct morphological changes induced by a debris flow event that occurred on August 24, 2009. A sediment budget analysis was performed to determine the main sediment sources of the torrent and to evaluate the respective contribution of channels and hillslopes to the sediment yield.

International audience; Quantification of volumes for coarse sediment transport in small upland catchments is of great concern for the protection and prevention against flood hazards in mountainous terrains. This contribution presents a starting research project dedicated to the characterization and prediction of the sediment responses of small alpine catchments. The main objectives are to (1) explore the potential of sediment traps for the characterization of sediment yields at the regional scale and to (2) quantify event-based sediment budgets at the catchment scale. The regional approach is based on the exhaustive inventory of sediment traps, the characterization of watershed physical features from GIS analysis of conventional spatial databases and the reconstruction of sediment yields from archives analysis. This dataset will allow us to develop empirical relationships between basin characteristics and sediment yields and to characterize the main factors controlling erosion in the Northern French Alps. The Manival Torrent (drainage area: 7 km2) is one catchment in the Northern Prealps which channel cross-sections and a sediment trap will be frequently surveyed providing total coarse sediment yields, change of temporary sediment storage in channel, and location and characterization of sediment supply from hillslopes. Multidate high-resolution DEM from airborne LiDAR surveys is also planned to be used for the quantification of sediment responses. Intended results for this study will contribute to the need for quantifiable approaches for the prediction of catchment sediment responses and the understanding of the channel-hillslope coupling processes

In mountainous catchments, the quantification of sediment yield is of paramount relevance for land-use planning and design of sediment control structures. However, deciphering the contribution of the different sediment transport processes (debris flows, debris floods and bedload transport) is often challenging as they are strongly controlled by basin morphometry, hydrological regime, and sediment supply. Therefore, long-term instrumental monitoring through catchment-scale sensor networks can provide precious information, especially if coupled with high-resolution topographical surveys. The Gadria catchment, located in the eastern Italian Alps, offers the possibility to perform a systematic monitoring of sediment transport processes. This catchment typically features several low-magnitude flood episodes and a few debris-flow events per year, from late spring to early fall. Starting from 2011, various instruments mainly devoted to debris-flow detection (geophones, video cameras, flow stage sensors) have been installed along the main channel, just upstream of a retention basin. High-resolution topographical surveys of the retention basin are carried out each year, at the beginning and at the end of the summer season and after debris-flow events. Rainfall is measured in the intermediate part of the catchment and in the headwaters, while PIT-tracing of bedload was performed in the main channel. In this work, we present the reconstruction of the sediment dynamics at the catchment scale during the 2014 and 2015 monitoring seasons. Instrumental monitoring was used to estimate the contribution of the different flow processes, and data from topographical surveys to quantify the transported volumes. Results show that (i) coarse sediment yield is driven by sporadic debris flows while flood events allow the continuous fine-sediment migration along the channel network; (ii) volume estimations may be significantly different - up to 30% lower - if performed through a DEM of Difference (DoD) analysis of the retention basin or by analyzing monitoring data; (iii) a multi-parametric monitoring is needed to decipher sediment dynamics at catchment scale.