Effects of turbulence on the transport of individual particles as bedload in a gravel‐bed river

This study investigates the association between mean and turbulent flow variables and the movement of individual particles in a gravel‐bed river. The experimental design implemented in the Eaton‐North River (Québec, Canada) is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical flow velocity components using a vertical array of three electromagnetic current meters (ECMs). The frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long film. Mean and turbulent flow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling. When particles are sliding, weak differences are present for the mean streamwise velocity and normal vertical stresses. Instantaneous Reynolds shear stresses are significantly lower for sliding events which was not expected but could be explained by the important dominance of Quadrant 3 events (inward interactions). When particles are rolling, only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when Quadrant 2 (ejections) dominate the flow field. For both sliding and rolling particles, vertical and/or streamwise fluid accelerations show high magnitude values when compared to periods without transport. For sliding particles, streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration. For rolling particles, streamwise and vertical acceleration are predominantly of opposite sign. These results suggest that fluid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure field around particles. In this study, fluid acceleration seems to play a more important role than Reynolds shear stress or normal stresses for bedload movements. Copyright © 2010 John Wiley & Sons, Ltd.     

Extension tardi-orogénique et formation des bassins intracontinentaux: le bassin stéphanien des Cévennes

Résumé

Le bassin carbonifère des Cévennes, voisin du décrochement sénestre de Villefort, a été étudié en intégrant les données sédimentologiques, les données structurales du socle et du remplissage, ainsi que les données de la pétrologie métamorphique du socle. Le remplissage du bassin est contrôlé par des failles subméridiennes induisant une subsidence localisée en début d’ouverture. Cette subsidence devient plus régionale en fin d’histoire. Un grenat mangane- sifère est associé aux minéraux du faciès schistes verts, des blastes d’andalousite recoupent l’ensemble. Cette association minéralogique indique un gradient de température de 50°C/km. Ce gradient est semblable au gradient estimé dans le bassin sur la matière organique. La schistosité porte trois familles de linéation : 1- une famille à biotite-chlorite-quartz, orientée N45 ± 20° et associée à la mise en place des nappes cévenoles; 2- une famille à minéraux phylliteux dont l’orientation est située autour de N90 et associée à l’ouverture du bassin; 3- une famille à minéraux phylliteux orientée NO horizontale sur des plans de schistosité redressés près de la faille de Villefort. La schistosité régionale est affectée par des plis asymétriques, des bandes de cisaillement et des zones cata- clasées, le tout ayant une cinématique en faille normale vers l’Est. Les relations entre ces différentes structures suggèrent un continuum de déformation depuis des niveaux ductiles jusqu’à des niveaux fragiles. Le remplissage du bassin est affecté par des décrochements parallèles à la faille de Ville- fort. Le bassin des Cévennes s’est donc ouvert dans un contexte extensif est-ouest sur une croûte épaissie. La région a ensuite été reprise par des décrochements nord-sud dont celui de Villefort.     

Retention behaviour of humic substances onto mineral surfaces and consequences upon thorium (IV) mobility: case of iron oxides

The influence of humic acids (HA) on the retention of Th(IV) onto the surface of Fe oxides has been investigated. Thorium (IV) is here considered as an analogue of tetravalent radionuclides (Zr, Hf, U, Np, Pu). Hematite (α–Fe₂O₃) and gœthite (α–FeOOH) have been chosen as model phases because of their widespread presence in the earth's crust. Retention experiments were performed by batch procedure at constant ionic strength and varying pH in the range 2–10; the HA concentration varied in the range 1–100 mg l⁻¹. Compared with the former data obtained on the retention of Th(IV) with the same hematite sample free of HA, the study of a ternary system (i.e. hematite–HA–Th), clearly shows the influence of HA on the retention mechanisms of Th(IV) even in neutral to slightly alkaline medium. In particular, it is illustrated that the retention of Th(IV) onto Fe oxides closely depends upon the ratio between humic and surface (Fe oxide) sites. This ratio has to be greater than a critical value in order to impede the sorption of Th(IV).     

Receiver function method in reflection seismology

The receiver function method was originally developed to analyse earthquake data recorded by multicomponent (3C) sensors and consists in deconvolving the horizontal component by the vertical component. The deconvolution process removes travel path effects from the source to the base of the target as well as the earthquake source signature. In addition, it provides the possibility of separating the emergent P and PS waves based on adaptive subtraction between recorded components if plane waves of constant ray parameters are considered. The resulting receiver function signal is the local PS wave's impulse response generated at impedance contrasts below the 3C receiver.We propose to adapt this technique to the wide‐angle multi‐component reflection acquisition geometry. We focus on the simplest case of land data reflection acquisition. Our adapted version of the receiver function approach consists in a multi‐step procedure that first removes the P wavefield recorded on the horizontal component and next removes the source signature. The separation step is performed in the τ?p domain while the source designature can be achieved in either the τ?p or the t?x domain. Our technique does not require any a priori knowledge of the subsurface. The resulting receiver function is a pure PS‐wave reflectivity response, which can be used for amplitude versus slowness or offset analysis. Stack of the receiver function leads to a high‐quality S wave image.     

Contrasting regional discharge evolutions in the Amazon basin (1974–2004)

Former hydrological studies in the Amazon Basin generally describe annual discharge variability on the main stem. However, the downstream Amazon River only represents the mean state of the Amazonian hydrological system. This study therefore uses a new data set including daily discharge in 18 sub-basins to analyze the variability of regional extremes in the Amazon basin, after recalling the diversity of the hydrological annual cycles within the Amazon basin. Several statistical tests are applied in order to detect trends and breaks in the time series. We show that during the 1974–2004 period, the stability of the mean discharge on the main stem in Óbidos is explained by opposite regional features that principally involve Andean rivers: a decrease in the low stage runoff, particularly important in the southern regions, and an increase in the high stage runoff in the northwestern region. Both features are observed from the beginning of the nineties. These features are also observed in smaller meridian sub-basins in Peru and Bolivia. Moreover we show that the changes in discharge extremes are related to the regional pluriannual rainfall variability and the associated atmospheric circulation as well as to tropical large-scale climatic indicators.     

An external–internal mode coupling for a 3D hydrodynamical model for applications at regional scale (MARS)

This paper presents a 3D model in sigma coordinates. Although the principles it is based on have been established for some time, some original aspects for this type of 3D mode splitting model are presented here. The model was designed to simulate flows in various coastal areas from the regional scale down to the inshore scale of small bays or estuaries where circulation is generally driven by a mix of processes. The processes to be modeled enable simplifications of the Navier–Stokes equations on the classic Boussinesq and hydrostatic hypotheses. These equations are transformed within a sigma framework to make free surface processing easier. The main point of our demonstration focuses on the original aspect of the coupling between barotropic and baroclinic modes especially designed for ADI. It explains how full consistency of the transport calculated within the 2D and 3D equation sets was obtained. Lastly, we describe the physical processes simulated on a realistic configuration at a regional scale in the Bay of Biscay.     

Tsunami hazard and exposure on the global scale

In the aftermath of the 2004 Indian Ocean tsunami, a large increase in the activity of tsunami hazard and risk mapping is observed. Most of these are site-specific studies with detailed modelling of the run-up locally. However, fewer studies exist on the regional and global scale. Therefore, tsunamis have been omitted in previous global studies comparing different natural hazards. Here, we present a first global tsunami hazard and population exposure study. A key topic is the development of a simple and robust method for obtaining reasonable estimates of the maximum water level during tsunami inundation. This method is mainly based on plane wave linear hydrostatic transect simulations, and validation against results from a standard run-up model is given. The global hazard study is scenario based, focusing on tsunamis caused by megathrust earthquakes only, as the largest events will often contribute more to the risk than the smaller events. Tsunamis caused by non-seismic sources are omitted. Hazard maps are implemented by conducting a number of tsunami scenario simulations supplemented with findings from literature. The maps are further used to quantify the number of people exposed to tsunamis using the Landscan population data set. Because of the large geographical extents, quantifying the tsunami hazard assessment is focusing on overall trends.     

Correspondence between the distribution of hydrodynamic time parameters and the distribution of biological and chemical variables in a semi-enclosed coral reef lagoon

Hydrodynamic modeling can be used to spatially characterize water renewal rates in coastal ecosystems. Using a hydrodynamic model implemented over the semi-enclosed Southwest coral lagoon of New Caledonia, a recent study computed the flushing lag as the minimum time required for a particle coming from outside the lagoon (open ocean) to reach a specific station [Jouon, A., Douillet, P., Ouillon, S., Fraunié, P., 2006. Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model. Continental Shelf Research 26, 1395–1415]. Local e-flushing time was calculated as the time requested to reach a local grid mesh concentration of 1/e from the precedent step. Here we present an attempt to connect physical forcing to biogeochemical functioning of this coastal ecosystem. An array of stations, located in the lagoonal channel as well as in several bays under anthropogenic influence, was sampled during three cruises. We then tested the statistical relationships between the distribution of flushing indices and those of biological and chemical variables. Among the variables tested, silicate, chlorophyll a and bacterial biomass production present the highest correlations with flushing indices. Correlations are higher with local e-flushing times than with flushing lags or the sum of these two indices. In the bays, these variables often deviate from the relationships determined in the main lagoon channel. In the three bays receiving significant riverine inputs, silicate is well above the regression line, whereas data from the bay receiving almost insignificant freshwater inputs generally fit the lagoon channel regressions. Moreover, in the three bays receiving important urban and industrial effluents, chlorophyll a and bacterial production of biomass generally display values exceeding the lagoon channel regression trends whereas in the bay under moderate anthropogenic influence values follow the regressions obtained in the lagoon channel. The South West lagoon of New Caledonia can hence be viewed as a coastal mesotrophic ecosystem that is flushed by oligotrophic oceanic waters which subsequently replace the lagoonal waters with water considerably impoverished in resources for microbial growth. This flushing was high enough during the periods of study to influence the distribution of phytoplankton biomass, bacterial production of biomass and silicate concentrations in the lagoon channel as well as in some of the bay areas.