High elevation of low-relief surfaces in mountain belts: does it equate to post-orogenic surface uplift?

International audienceWe present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level (sea level). After active uplift, the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography. Smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins

Impact of transient groundwater storage on the discharge of Himalayan rivers

International audienceIn the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times such as soils, groundwater, snow and glaciers. In the central Himalaya, the water budget is thought to be primarily controlled by monsoon rainfall, snow and glacier melt, and secondarily by evapotranspiration. An additional contribution from deep groundwater has been deduced from the chemistry of Himalayan rivers, but its importance in the annual water budget remains to be evaluated. Here we analyse records of daily precipitation and discharge within twelve catchments in Nepal over about 30 years. We observe annual hysteresis loops--that is, a time lag between precipitation and discharge--in both glaciated and unglaciated catchments and independent of the geological setting. We infer that water is stored temporarily in a reservoir with characteristic response time of about 45 days, suggesting a diffusivity typical of fractured basement aquifers. We estimate this transient storage capacity at about 28km3 for the three main Nepal catchments; snow and glacier melt contribute around 14km3yr-1, about 10% of the annual river discharge. We conclude that groundwater storage in a fractured basement influences significantly the Himalayan river discharge cycle

Effects of large dams on the aquatic food web along a coastal stream with high sediment loads

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a small coastal river with high sediment loads, and make it possible to anticipate the effects of dam removal on the future river ecosystem

Origin of the high elevated Pyrenean peneplain

Peneplanation of mountain ranges is generally considered the result of long-term erosional processes that smooth relief and lower elevation near sea level. Therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift. Such an interpretation has been proposed for the Pyrenees where high-elevation, low-relief erosional surfaces rose up to more than 2000 m. Because the Pyrenean foreland basins are filled with very thick continental deposits, which have buried the early jagged landscape, we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering. We propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range, therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief. Such a process allows a high-elevation, low-relief erosional surface to develop at the scale of the range. In the Pyrenees, occurrence of high-elevation, low-relief erosional surface remnants does not imply a posttectonic uplift, but is instead due to the dissection of the initial Miocene high-elevation, low-relief surface by the recent drainage system, the erosive activity of which has been enhanced by global climate change from the late Pliocene onward

Analyse de la relation concentration vs débit d'eau : méthodologie

International audienceRating curve technique between suspended particle matter concentration C and water discharge Q is often used to estimate sediment concentrations and fluxes when in-situ measurements are lacking. This requires significant datasets with appropriate fitting procedures. However, a large dispersion of plotted data induces large uncertainties in the determination of the rating model parameters. In addition to errors from measurement sources and non-univocal trends, a large part of the dispersion of the plots results from a non respect of the underlying hypothesis of causality between C and Q. A correct rating curve methodology should consider only the part of Q that leads to erosion. First, this requires the definition of the source, and the magnitude, of discharge that will generate erosion, and second, the implementation of a mixing model to fix the dilution rate at a specific location in the channel network. Both conditions require a good knowledge of the hydrological context of the studied watershed. Once these steps have been carried out, the fitting procedure can be applied and should be performed carefully, with the identification of the likely threshold of discharge between the erosion processes. This methodology should be applied at least to improve our knowledge of the physical significance of the rating parameters.La corrélation empirique entre la concentration en matière en suspension, C, et le débit d'eau, Q, est souvent utilisée pour pallier le manque de données in-situ. Cette approche nécessite cependant une base de données conséquente et une méthodologie d'analyse appropriée. Bien souvent, la dispersion des points sur les graphiques est importante et, de ce fait, le calage des paramètres des modèles de régression imprécis. Afin de diminuer la dispersion des nuages de points, il est fondamental de respecter au mieux la relation supposée causale entre C et Q. Ceci nécessite de connaître l'hydrologie du bassin versant étudié et d'identifier les parts respectives de Q qui a priori engendrent ou non de l'érosion, notamment la prise en compte ou non des débits issus du sous-sol. Une fois cette phase de l'analyse faite, l'approche empirique prend son sens et se focalise sur les débits impliqués dans l'érosion sur le bassin versant. Il est dès lors possible d'appliquer des formalismes simples pour modéliser des fonctions de transfert au sein du réseau hydrographique et de définir : les seuils de mobilité des sources de sédiments, les déphasages temporels entre les réponses des différentes sources, la dynamique des stocks de sédiment disponibles et la fonction de mélange qui en résulte. Si cette approche s'écarte de l'approche purement empirique, elle permet néanmoins de poser des hypothèses explicatives pour mieux comprendre les données (C, Q)

Mesures in situ et modélisation du transport des sédiments en rivière (application au bassin versant de la Vilaine)

L'objet de ce mémoire est l'étude du transport des sédiments dans les rivières. Une analyse de la corrélation entre les flux d'eau et de sédiments est réalisée par une approche statistique, sur la base de chroniques de données journalières moyennes établies pour des bassins versants de superficies et lithologies différentes. A des échelles d'espace et de temps courtes, les processus physiques mis en jeu dans les phénomènes de transport sont abordés par la mesure en rivière de profils de vitesse et de concentration et par une approche rhéologique complémentaire. Les modèles théoriques sont confrontés aux mesures de terrain. In situ, la variabilité des paramètres mécaniques montre la nécessité d'utiliser des traceurs environnementaux pour intégrer les modes de transport des sédiments. Les travaux sont centrés sur les radioéléments Béryllium-7 et Iode-131. La méthodologie de traçage est développée et les résultats portent sur la cinétique de transport des sédiments dans la Vilaine.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Highly seasonal suspended sediment and bed load transport dynamic in tropical mountain catchments

ISBN : 978-1-315-62320-7; 978-1-138-02945-3International audienc

Understanding the effect of reservoirs on suspended sediments and biogeochemical fluxes over seasonal and storm scales

International audienceReservoirs such as dams modify the transport, the retention and the remobilization of sediments and nutrientsin rivers. Such modifications can lead to strong degradation of downstream ecosystems as consequence ofdisturbances of biogeochemical cycles or habitat. When such negative impacts or cost of upgrading safety systemsoutweigh the benefits of dams, removal operation may be considered, e.g. Service (2011) inventoried more than500 removals in USA in 10 years. The modification on River fluxes induced by the presence of a reservoir as wellas the changes likely to occur in the case of its removal will depend on the fluxes from the catchment, on the functioningof the reservoir itself, and on the linkage between both systems. Therefore, we argue that characterizing themodification of hydrological, sediment and dissolved fluxes induced by dams should be achieved (i) on a varietyof time scales (storms, seasons, years), and (ii) within an integrative approach that combine sediments, associatedelements, and biological variables to enable understanding the biogeochemical processes impacted by the reservoir.We conducted such an approach on the Sélune River, a 91 km river long, flowing to the Mont Saint-Michel Bay(France), draining a catchment of 1 083 km2, and regulated by two hydroelectric dams:, La-Roche-qui-Boit andVézin (16 and 36 m respectively). A set of station has been equipped to monitor water, sediment and dissolvedelements fluxes from upstream to downstream the 2 dams since 2015. Sensors recorded every hour or less streamflow and physical and chemical parameters such as turbidity, temperature and conductivity on 4 stations. Watersamples were collected every week and at higher frequency during storms on 2 stations to determine anions,dissolved nitrogen and silica, dissolved and total phosphorus concentrations and Carbone and nitrogen content ofsuspended solids. Results over 2 contrasted water years and more than 30 sampled storm events, showed that thetwo dams have 2 main effects:(i) They act as buffer regarding suspended sediment and phosphorus annual and storm fluxes by trappingsuspended solids. They also buffer concentration variations during storm events by mixing all dissolved elementswith the water they store.(ii) They act as biogeochemical reactors seasonally: in summer when light and temperature conditions areenhancing phytoplankton production into the reservoirs, the dissolved phosphorus and silica nutrients areconsumed in the reservoirs whereas lower C/N content of downstream suspended material suggest exportation ofautochthonous products from the reservoir.No effect on stream flow dynamics is observed due to specific regulation strategy. Annual nitrate loads upstreamand downstream to the dams do not show significant difference.Service, R.F. (2011) Will Busting Dams Boost Salmon? Science 334, 888–892

Control of sediment concentration in major rivers of the Nepal Himalayas

1 p.International audienc