Short term geomorphic observations in Nepal Himalayas: Contribution to the knowledge of geo-disasters along a road corridor

International audienceIn the Himalayas, geo-disasters are recurrent features that generally result from the combination of several hazards (gullying, landsliding, flooding),affecting vulnerable, poorly prepared, human communities

Geomorphic changes induced by the April-May 2015 earthquake sequence in the Pharak-Khumbu area (Nepal)

International audienceLandsliding is a common process shaping mountain slopes. There are various potential landslide triggers (rainfall, bank erosion, earthquakes) and their effectiveness depends on their distribution, frequency and magnitude. In a Himalayan context, the effects of monsoon rainfall can be assessed every year whereas the unpredictability and low frequency of large earthquakes make their role in triggering slope instability more obscure.Here we present field observations carried out in the Pharak-Khumbu area (East Nepal, Dudh Kosi catchment)before and after the April-May 2015 earthquakes. The limited size and shallow depth of the newly generated slope failures are noteworthy. More generally, these geomorphic changes and their characteristics maybe explained by the nature of the 2015 climate: while the 2014-2015 winter was unusually dry, snowfalls and rainfalls were abundant during March and April, hence increasing the pore pressure and the potential instabilityof slopes and/or snow cover. Conversely, the rather weak monsoon rainfall of the 2015 summer did not takeadvantage of the many cracks opened by seismic shaking. Yet this may leave only a short delay for large landslidedevelopment during the next monsoon season

Erosion assessment in the middle Kali Gandaki (Nepal ) : A sediment budget approach

International audienceActive mountains supply the largest sediment fl uxes experienced on earth. At mountain range scale, remote sensingapproaches, sediments provenance or stream power law analyses, collectively provide rough long-term estimatesof total erosion. Erosion is indeed controlled by rock uplift and climate, hence by a wide range of processes(detachment, transport and deposition), all operating within drainage basin units, yet with time and spatial patternsthat are quite complex at local scale. We focus on the Kali Gandaki valley, along the gorge section across theHigher Himalaya (e.g. from Kagbeni down to Tatopani). Along this reach, we identify sediment sources, storesand sinks, and consider hillslope interactions with valley floor, in particular valley damming at short and longertime scales, and their impact on sediment budgets and fluxes. A detailed sediment budget is presented, constrainedby available dates and/or relative chronology, ranging from several 10 kyr to a few decades. Obtained resultsspan over two orders of magnitude that can best be explained by the type and magnitude of erosional processesinvolved. We show that if large landslides contribute signifi cantly to the denudation history of active mountainrange, more frequent, medium to small scales landslides are in fact of primary concern for Himalayan population

La difficile gestion des risques naturels en Himalaya : une question d’échelle ? Le cas du Népal

En Himalaya, les risques naturels sont sous le contrôle de facteurs physiques difficilement maîtrisables. Au Népal en particulier, ils se développent dans un contexte socio-économique dominé par une grande pauvreté et marqué par une forte urbanisation. À travers plusieurs exemples (inondations dans différents contextes topo-climatiques, menaces sismiques), nous montrons comment la prise en compte du passé et du présent d’une part, du local et du global d’autre part, est plus que jamais indispensable à une bonne appréhension des risques. Les structures de gestion gouvernementales sont très centralisées et concernent surtout les villes. Leurs actions sont complétées par celles plus souples de structures financées par l’aide étrangère (ONGs, ICIMOD), qui agissent souvent à l’échelle des communautés villageoises tout en raisonnant à l’échelle de la chaîne himalayenne.In the Himalayas, natural hazards and risks are under the control of physical factors, hence are difficult to mitigate. More specifically in Nepal, they occur in a socio‑economic context of poverty and growing urbanization. Through several examples (floods in different topo-climatic contexts, seismic threats), we show how taking into account the past and present on the one hand, the local to global scale on the other hand, is more essential than ever for a good risk understanding. Governmental management structures are highly centralized and they mainly concern cities; their actions are complemented by those of more flexible structures supported by foreign aid (NGOs, ICIMOD), which often act at the level of village communities, while reasoning at the scale of entire Himalayan chain

The paraglacial, giant landslide dam of Phoksumdo Lake (Dolpo District,Western Nepal Himalaya)

Abstract HKT-ISTP 2013 B

SAMCO: Adaptation de la société aux risques en montagne dans un contexte de changement global

National audienceLe projet SAMCO (Society Adaptation for coping with Mountain risks in a global change COntext, 2013-2016) a pour but de développer un ensemble d'outils méthodologiques permettant d'améliorer la résilience des sociétés soumises aux impacts des risques en milieux montagneux. Ces outils visent à mesu-rer la capacité d'adaptation des écosystèmes et des sociétés à partir de l'analyse de cas réels provenant de 3 sites pilotes représentatifs

Natural Arsenic in Groundwater and Alkaline Lakes at the upper Paraguay basin, Pantanal, Brazil

A distinctive feature of Nhecolândia, a sub-region of the Pantanal wetland in Brazil, is the presence of both saline (alkaline) and freshwater lakes. Saline lakes were attributed to a past arid phase during the Pleistocene, but recent studies have shown that the geochemistry of the saline lakes arises from the current concentration process of fresh waters that is supplied to the Pantanal every year. The region is mainly used for cattle farming, more recently eco-tourism is becoming widespread and water conservation agencies worry about arsenic contents in surface and shallow ground water. The study was carried out along a 600 m-long transect that linked a freshwater lake to a saline lake. The spatial distribution of arsenic is studied, based on soil morphology, according to pH, redox conditions (Eh) and dissolved organic carbon (DOC), and also compared to distribution of major elements. Three main processes, responsible for the chemical variability, were identified: (1) the concentration of the solution under the influence of evaporation and the associated precipitations (Mg-calcite, illite, sodium carbonate); (2) oxidation of sulphides included in clay layers and subsequent development of acid conditions; (3) buffering of acid conditions by clay dissolution. Arsenic contents are not affected by the last two processes (2 and 3) but evolve in proportion to the concentration. Arsenic concentrations in water ranged from 0.11 mg/l to 3.68 mg/l, i.e. 368 times higher than the World Health Organization (WHO) guidelines for drinking water. The arsenic speciation [index word 1] as calculated from field conditions indicated that the As(V) redox state dominated in the groundwater and in the lakes, whereas some proportion of As(III) could occur on the shore and in the sediments of the saline lake. As(III) from the sediment can be remobilised and dispersed into the water following the disturbance of the sediments by cattle that come to drink water. Arsenic contents increased with most of the other dissolved species; hence competitive adsorption [index word 16] prevent dissolved arsenic regulation by solid phase reaction. Moreover, the pH-values increased above 8 with increasing arsenic, favouring As(V) desorption. As a consequence, the good correlation established between arsenic contents and electrical conductivity (r2 = 0.97) should help to indirectly evaluate the arsenic contents in shallow groundwater and lakes of the region

SAMCO: Society adaptation for coping with mountain risks in a global change context

International audienceThe SAMCO project is intended to contribute to the development of a proactive resilience framework enhancing the overall resilience of societies on the impacts of mountain risks

Les variations glaciaires en Haute Durance (Briançonnais, Hautes-Alpes) depuis la fin du XIXe siècle : mise au point d'après les documents d'archives et la lichénométrie

Les travaux reconstituant les variations glaciaires récentes se sont longtemps fondés sur des séries de mesures directes établies sur quelques glaciers sélectionnés. Nous proposons de compléter ces séries en étudiant les fluctuations, depuis la fin du XIXe siècle, du front d’une trentaine de glaciers, situés sur la retombée orientale du Massif des Ecrins. Des relevés géomorphologiques, la lichénométrie, et les documents anciens ont été utilisés conjointement, ce qui a permis d’identifier trois générations de moraines, délaissées respectivement entre 1915-1930, autour de 1950 et lors de la décennie 1970. Ces formations suggèrent l’existence de trois stades d’avancée et/ou de stationnement des glaces dans un contexte général de retrait glaciaire depuis le Petit Âge de Glace. Au cours du XXe siècle, la surface englacée a diminué dans ce secteur de 30% environ (35 à 25 km2). Cette évolution s’explique par un bilan de masse déficitaire de l’ordre de -0,2 à -0,4 mee.an-1, valeurs qui sont de même ordre de grandeur que celles établies ailleurs dans les Alpes Occidentales.Until recently, studies on glacial fluctuations were mostly relying on direct measurements of a few, selected glaciers. In order to complete these former datasets, we studied thirty glaciers located in the eastern part of the Massif des Ecrins. By using geomorphology surveys, lichenometry dating method, coupled with archives investigations, we identified three main stages of moraines deposition: between 1915-1930, around 1950, and in the 1970’s. These moraines suggest the occurrence of three stages of glacial advances and/or stationnary equilibrium in a general context of glacier recession subsequent to the Little Ice Age. The glaciated surface has decreased from 35 to 25 km2 during the 20th century, thus representing a mass-balance ranging from -0,2 to -0,4 mwe.yr-1, in agreement with what is known from other parts of western Alps