Debris-flow susceptibility of upland catchments

Over the last three decades, many regional studies in mountain ranges under temperate climate revealed that it is possible to discriminate debris-flow and fluvial fans from morphometric indicators measured at the scale of the catchment and the fan itself. The most commonly used indicators are the Melton index (R), a normalized index of the gravitational energy of the catchment, and the fan slope (S). A wide range of thresholds have been proposed for discriminating purpose, but these are generally based on a small population of catchments and may be highly influenced by ambiguous fans included in the data set. A database of 620 upland catchments from several mountain ranges under temperate climate was compiled from the literature to propose robust discriminant morphometric thresholds for debris-flow versus fluvial responses. Linear discriminant analysis (LDA) and logistic regression (LR) were performed using the whole data set, and a leave-one-out cross-validation was used to evaluate performances of the models. Sensitivity and specificity scores obtained for LDA and LR were 0.96 and 0.73, and 0.95 and 0.75, respectively. It is also shown that the channel slope above which debris-flow is observed decreases with the gravitational energy of the catchment. Limitations of the morphometric discrimination are discussed.     

Tectonic heritage in drainage pattern and dynamics: the case of the French South Alpine Foreland Basin (ca. 45–20 Ma)

The spatial and temporal organization of depositional environments in drainage networks of foreland basins reflect the tectonic and erosional dynamics associated with the development of mountain belts. We provide field evidences for the initiation and evolution of a complex drainage system in the French South Alpine Foreland Basin related to Western Alps exhumation. Sedimentological and structural analyses of the Eocene–Early Miocene succession were investigated in the (1) Argens/Peyresq, (2) Barrême/Blieux/Taulanne and (3) Montmaur/St‐Disdier sectors. Combined with the existing structural data set, we propose a new model that integrates the regional tectonic activity, the palaeovalley orientation and their dynamics through time. The Eocene–Miocene deposits clearly show the existence of N–S‐oriented palaeovalleys. The systematic presence of early NE–SW‐ to N–S‐oriented strike‐slip and extensional faults in the palaeovalleys suggests that these tectonic structures were responsible for the formation of the initial N–S‐oriented basin‐floor topographies. The vertical offset of the strike‐slip faults induced sufficient accommodation space for the Cenozoic sedimentation since the Middle Eocene. It implies the creation of N–S‐oriented palaeovalleys during the northward Pyrenean‐Provençal phase, pre‐dating westward Alpine compression. Later, the Oligocene Alpine tectonic phase induced drainage expansion toward the orogenic wedge and the erosion of the exhumed internal massifs by transverse streams. The establishment of new connections between the old topographic lows formed a longitudinal drainage pattern that remains the locus of deposition in a regional sedimentary routing system. In this model, former strike‐slip faults correspond to weakness zones overprinted by the westward Alpine shortening that allowed the formation of the modern piggyback basin structure of the foreland and the long‐time preservation of the palaeovalley geometry.     

Paleoclimatic record of a long deep sea core from the eastern Mediterranean

A deep-sea core over 16 m long from the crestal area of the Mediterranean Ridge has been investigated with different techniques, including quantitative micropaleontology, stable isotopes (measured on the epipelagic species Globigerinoides ruber and on the mesopelagic species Globorotalia inflata), and clay mineralogy. The resulting record of climatic fluctuations can be cross correlated to other Mediterranean cores by means of isochronous lithologies (tephra layers and sapropels). The climatic record of the Mediterranean is similar in character, phase, and chronology to the records investigated in the equatorial Pacific and in the Caribbean. Isotope stages 1 to 17 have been recognized. Cyclically repeated stagnant cycles resulting in sapropel deposition complicate both the isotopic and the faunal signal. The isotopic investigations reveal that the temperature change in the surface layers of the eastern Mediterranean was no greater than 8°C in the late “glacial” Pleistocene. The chronostratigraphic and biostratigraphic interpretation of Core KS09 indicate that the mean sedimentation rate was 2.4 cm/1000 years, a value very close to the 2.5 cm/1000 years calculated for the entire Quaternary section at DSDP Site 125, also located in the crestal area of the Mediterranean Ridge in the Ionian Basin. The base of KS09 is likely to be very close to the Brunhes/Matuyama boundary dated at 0.7 my.     

Impacts of greenhouse gases and aerosol direct and indirect effects on clouds and radiation in atmospheric GCM simulations of the 1930–1989 period

Among anthropogenic perturbations of the Earths atmosphere, greenhouse gases and aerosols are considered to have a major impact on the energy budget through their impact on radiative fluxes. We use three ensembles of simulations with the LMDZ general circulation model to investigate the radiative impacts of five species of greenhouse gases (CO₂, CH₄, N₂O, CFC-11 and CFC-12) and sulfate aerosols for the period 1930–1989. Since our focus is on the atmospheric changes in clouds and radiation from greenhouse gases and aerosols, we prescribed sea-surface temperatures in these simulations. Besides the direct impact on radiation through the greenhouse effect and scattering of sunlight by aerosols, strong radiative impacts of both perturbations through changes in cloudiness are analysed. The increase in greenhouse gas concentration leads to a reduction of clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the longwave spectrum and increasing absorption of solar radiation by reduction of cloud albedo. Increasing anthropogenic aerosol burden results in a decrease in high-level cloud cover through a cooling of the atmosphere, and an increase in the low-level cloud cover through the second aerosol indirect effect. The trend in low-level cloud lifetime due to aerosols is quantified to 0.5 min day^–1 decade^–1 for the simulation period. The different changes in high (decrease) and low-level (increase) cloudiness due to the response of cloud processes to aerosols impact shortwave radiation in a contrariwise manner, and the net effect is slightly positive. The total aerosol effect including the aerosol direct and first indirect effects remains strongly negative.     

New combined log and tracer test interpretation method for identifying transfers in fissured aquifers

Tracer tests represent the most appropriate approach for assessing hydrodispersive parameters such as transversal and longitudinal dispersivities or kinematic porosity on an aquifer scale. They are generally carried out by injecting a tracer in a borehole and measuring its concentration over time in neighboring boreholes by extracted volume sampling or downhole measurements. Logging is one of the most suitable methods for evaluating fissured reservoirs. But short circuits between fractures with different hydraulic potential through boreholes induce mixing phenomena that cannot be avoided without packers. This mixing can shift the breakthrough curves deduced from the logs for each producing fracture and distort determination of their location.
The method proposed in this paper aims at measuring the flow rate and the solute breakthrough for hydraulically active fractures, in open boreholes. It involves estimating a velocity profile along the borehole column by the analysis of two successive logs: a shift function according to depth is thus determined by comparison between log portions on each successive one. The velocity gradients reflect the inward or outward flow rates produced by each fracture. On the basis of these flow rates, it is possible to determine the mixing effects inside the borehole and then to plot unbiased breakthrough curves for each producing fracture.
This method was applied at a granitic site in the eastern Pyrenees. In spite of some questionable limitations, the results showed that the method seems adapted to situations with many fractures. The precise hydraulic pattern which is obtained at the borehole scale is discussed in terms of a dual porosity model. Furthermore, interpretation of the breakthrough curves for fractures corrected for mixing effects revealed that Peclet numbers are strongly underestimated if this phenomenon is not considered.     

Geochemical modelling of the tonalitic and trondhjemitic granulites from the Itabuna-Salvador-Curaçá Block,Bahia, Brazil

The studied tonalitic and trondhjemitic granulites are located in the SSE granulitic domain of the São Francisco craton, Bahia, Brazil, where they represent most of the southern part of the Archean and Paleoproterozoic Itabuna-Salvador-Curaçá Block (ISCB). Chemically, the tonalitic and trondhjemitic granulites belong to a low-K calc-alkaline suite; their REE patterns are steep with strong LREE/HREE fractionation and no significant Eu anomaly. Garnet-bearing mafic granulites that occur as enclaves in the tonalitic and trondhjemitic granulites were derived from basalts and/or gabbros of tholeiitic affinity. Geochemical modelling showed that the tonalitic and trondhjemitic granulites were produced by moderate fractional crystallization of an assemblage of hornblende and plagioclase, with subordinate amounts of magnetite, apatite, allanite and zircon. The garnet-bearing mafic granulites would be the source of the magmas that generated these rocks. Partial melting left a residue made up of plagioclase, garnet, orthopyroxene and hornblende.     

9. Life On Earth... And Elsewhere?

This concluding chapter is divided into two main parts. The first part is a summary of the main facts and events which constitute the present body of knowledge of the chronology of life in the solar system, in the form of “highlights” in astronomy, geology, chemistry and biology. The second part raises the interrogation “Is life universal?”, and tries to provide answers based on these facts and events. These answers turn out to differ widely among the various disciplines, depending on how far they feel able to extrapolate their current knowledge.     

Flash flood mitigation as a positive consequence of anthropogenic forcing on the groundwater resource in a karst catchment

The Mediterranean coastal region is prone to high-intensity rainfall events that are frequently associated with devastating flash floods. This paper discusses the role of a karst aquifer system in the flash floods of a Mediterranean river, the Lez river. Most of the Lez river watershed is located on karst terrains where interactions between surface water and groundwater take place. During extreme rainfall events, the presence of fractures and well-developed karst features in carbonate terrains enhances the infiltration processes and involves the concentration of the recharge into highly organized and permeable flow paths. The groundwater, therefore, quickly moves towards the natural outlets of the karst system. The influence of the Lez karst aquifer system on the associated river floods dynamics is analysed while considering the spatially distributed rainfall, as well as the time series of the groundwater level within the aquifer and of the Lez river discharge measured at various gauging stations. Special attention is given to the relative importance of the surface and underground processes involved in flash flood genesis. It is shown that the karst groundwater contributes to flash floods under certain conditions, while high-rate pumping within the karst aquifer, which generates significant drawdown, may mitigate flash floods under other conditions.     

Multi‐scale assessment of overflow‐driven lateral connectivity in floodplain and backwater channels using LiDAR imagery

Overflow‐driven lateral connectivity significantly influences the spatial distribution and diversity of floodplain habitats and biota. Proper understanding of lateral connectivity in floodplain and backwater channels is therefore critical for assessment of river quality and for targeting management or restoration actions. In this study, we present a methodological framework for spatial and temporal assessments of overflow‐driven lateral connectivity at two spatial scales: bypass reach and backwater channel. Firstly, we compute the relative elevations, as well as overflow discharge, duration, and frequency using a simple, raster‐based method that uses a LiDAR digital elevation model (DEM), rating curves, and streamflow time series. Subsequently, we analyse the accuracy of this approach with respect to the accuracy of a DEM and evaluate its further applications. Altogether, four 10‐km‐long bypass reaches and 11 backwater channels are analysed, located along the Rhône River corridor in France. The results proved the precision of the method to be affected by the LiDAR DEM accuracy, which was on average more precise in a typically homogeneous floodplain setting rather than for backwater channel plugs with pronounced topographic complexity and usually riparian forest canopy. Amongst the four studied reaches, Brégnier Cordon proved to have the greatest flooding dynamics, followed by Belley and Chautagne. The hydrological connectivity pattern of Pierre Bénite differed significantly. Three longitudinal patterns of hydrological connectivity of backwater channels displayed stepwise advancement of the water. The presented results can be used to assess ecological potential of floodplain habitats and their historic and prospective evolution through time. Copyright © 2014 John Wiley & Sons, Ltd.     

Improved discrimination of subglacial and periglacial erosion using 10Be concentration measurements in subglacial and supraglacial sediment load of the Bossons glacier (Mont Blanc massif,France)

Deciphering the complex interplays between climate, uplift and erosion is not straightforward and estimating present‐day erosion rates can provide useful insights. Glaciers are thought to be powerful erosional agents, but most published ‘glacial’ erosion rates combine periglacial, subglacial and proglacial erosion processes. Within a glaciated catchment, sediments found in subglacial streams originate either from glacial erosion of substratum or from the rock walls above the glacier that contribute to the supraglacial load. Terrestrial cosmogenic nuclides (TCN) are produced by interactions between cosmic ray particles and element targets at the surface of the Earth, but their concentration becomes negligible under 15 m of ice. Measuring TCN concentrations in quartz sand sampled in subglacial streams and in supraglacial channels is statistically compliant with stochastic processes (e.g. rockfalls) and may be used to discriminate subglacial and periglacial erosion. Results for two subglacial streams of the Bossons glacier (Mont Blanc massif, France) show that the proportion of sediments originating from glacially eroded bedrock is not constant: it varies from 50% to 90% (n = 6). The difference between the two streams is probably linked to the presence or absence of supraglacial channels and sinkholes, which are common features of alpine glaciers. Therefore, most of the published mean catchment glacial erosion rates should not be directly interpreted as subglacial erosion rates. In the case of catchments with efficient periglacial erosion and particularly rockfalls, the proportion of sediments in the subglacial stream originating from the supraglacial load could be considerable and the subglacial erosion rate overestimated. Here, we estimate warm‐based subglacial and periglacial erosion rates to be of the same order of magnitude: 0.39 ± 0.33 and 0.29 ± 0.17 mm a^?1, respectively. Copyright © 2015 John Wiley & Sons, Ltd.