Methodology of seasonal morphological modelisation for nourishment strategies on a Mediterranean beach

A modified 2DH morphodynamical model was employed to simulate the evolution of large-scale features with major implications for beach nourishment. The study was focused on modelling the evolution of material artificially placed in different parts of the profile, extracting or adding material to the natural bars, and quantifying how the profile responds to different wave climates and nourishment placements. The simulated results were compared with field data from a Mediterranean beach.     

Pb,Hf and Nd isotope compositions of the two Réunion volcanoes (Indian Ocean): A tale of two small-scale mantle “blobs”?

Pb, Hf, Nd and Sr isotopes of basaltic lavas from the two Réunion Island volcanoes are reported in order to examine the origin of the sources feeding these volcanoes and to detect possible changes through time. Samples, chosen to cover the whole lifetime of the two volcanoes (from 2 Ma to present), yield a chemically restricted (compared to OIB lavas) but complex distribution. Réunion plume isotopic characteristics have been defined on the basis of the composition of uncontaminated shield-building lavas from the Piton de la Fournaise volcano. The average ?_Nd, ?_Hf, ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb isotope ratios calculated for this component are + 4.4, + 9.1, 0.70411, 18.97, 15.59 and 39.03, respectively. In Pb–Pb isotope space, each volcano defines a distinct linear trend but slight variations are also detected within the various volcanic sequences. The Piton des Neiges volcano yields a distinct and significantly more scattered isotopic distribution than Piton de la Fournaise for both Pb, Hf and Nd isotope tracers. A principal component analysis of the Pb isotope data from Piton de la Fournaise reveals a major contribution of the C and EM-1 components (with a clear Dupal flavor) as main components for the modern Réunion plume. The same components have been identified for Piton des Neiges but with a stronger participation of a depleted mantle component and a weaker EM-1 contribution. The compositional change of the lavas erupted by the Piton des Neiges and Piton de la Fournaise volcanoes is attributed to the impingement of two small-scale blobs of plume material at the base of the Réunion lithosphere. Compared to other hot-spots worldwide, in particular Hawaii and Kerguelen, magmas beneath Réunion are generated from a considerably more homogeneous, compositionally more primitive plume higher in ²⁰⁶Pb. Although shallow-level contamination processes have been locally detected they did not alter significantly the composition of the plume magmas. This is tentatively attributed to mantle dynamics producing small, high-velocity blobs that ascend rapidly through the lithosphere, and to the lack of a well-developed magma chamber at depth in the lithosphere.     

Sediment flux-driven channel geometry adjustment of bedrock and mixed gravel–bedrock rivers

Sediment supply (Q_s) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse-grained, hard sediment on the geometry of bedrock channels from the Rangitikei River, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l min⁻¹) under different Q_s (between 0 and 20 g l⁻¹) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope and width-to-depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Q_s are characterized by simple in-channel morphologies with a uniform distribution of shear stress within the channel while channels with high Q_s are characterized by dynamic channels with multiple active threads and a non-uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single-thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Q_s is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Q_s. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

Coastal mesoscale eddies were evidenced during a high-frequency radar campaign in the Gulf of Lions (GoL), northwestern Mediterranean Sea, from June 2005 to January 2007. These anticyclonic eddies are characterized by repeated and intermittent occurrences as well as variable lifetime. This paper aims at studying the link between these new surface observations with similar structures suggested at depth by traditional acoustic Doppler current profiler measurements and investigates the eddy generation and driving mechanisms by means of an academic numerical study. The influence of the wind forcing on the GoL circulation and the eddy generation is analyzed, using a number of idealized configurations in order to investigate the interaction with river discharge, buoyancy, and bathymetric effects. The wind forcing is shown to be crucial for two different generation mechanisms: A strong northerly offshore wind (Mistral) generates a vortex column due to the bathymetric constraint of a geostrophic barotropic current, which can surface after the wind relaxes; a southerly onshore wind generates a freshwater bulge from the Rhône river discharge, which detaches from the coast and forms a well-defined surface anticyclonic eddy based on buoyancy gradients. These structures are expected to have important consequences in terms of dispersion or retention of biogeochemical material at local scales.     

himu-em: The French Polynesian connection

himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.     

Etude des Déformations et Inclinaisons du Sous-sol: Développements Instrumentaux et Applications dans la Région Sismique d’Arette,France

We present a modified version of a tilt and strain meter conceived to have great sensibility and good stability. These instruments were deployed in a cave at the seismic Pyrenean region of Arette (France). We set in evidence several types of phenomena, of which deformations brought about by water recharge filling up fissures on the massif are of importance. Owing to the redundance in the data, the study of the co-seismic deformations allows one to separate instrumental displacements and local readjustments from regional motions.     

A preliminary investigation of strong-motion data from the French Antilles

Strong-motion networks have been operating in the Caribbean region since the 1970s, however, until the mid-1990s only a few analogue stations were operational and the quantity of data recorded was very low. Since the mid-1990s, digital accelerometric networks have been established on islands within the region. At present there are thought to be about 160 stations operating in this region with a handful on Cuba, 65 on the French Antilles (mainly Guadeloupe and Martinique), eight on Jamaica, 78 on Puerto Rico (plus others on adjacent islands) and four on Trinidad.After briefly summarising the available data from the Caribbean islands, this article is mainly concerned with analysing the data that has been recorded by the networks operating on the French Antilles in terms of their distribution with respect to magnitude, source-to-site distance, focal depth and event type; site effects at certain stations; and also with respect to their predictability by ground motion estimation equations developed using data from different regions of the world. More than 300 good quality triaxial acceleration time-histories have been recorded on Guadeloupe and Martinique at a large number of stations from earthquakes with magnitudes larger than 4.8, however, most of the records are from considerable source-to-site distances. From the data available it is found that many of the commonly-used ground motion estimation equations for shallow crustal earthquakes poorly estimate the observed ground motions on the two islands; ground motions on Guadeloupe and Martinique have smaller amplitudes and are more variable than expected. This difference could be due to regional dependence of ground motions because of, for example, differing tectonics or crustal structures or because the ground motions so far recorded are, in general, from smaller earthquakes and greater distances than the range of applicability of the investigated equations.     

Steady- and transient-state inversion in hydrogeology by successive flux estimation

A calibration method to solve the groundwater inverse problem under steady- and transient-state conditions is presented. The method compares kriged and numerical head field gradients to modify hydraulic conductivity without the use of non-linear optimization techniques. The process is repeated iteratively until a close match with piezometric data is reached. The approach includes a damping factor to avoid divergence and oscillation of the solution in areas of low hydraulic gradient and a weighting factor to account for temporal head variation in transient simulations. The efficiency of the method in terms of computing time and calibration results is demonstrated with a synthetic field. It is shown that the proposed method provides parameter fields that reproduce both hydraulic conductivity and piezometric data in few forward model solutions. Stochastic numerical experiments are conducted to evaluate the sensitivity of the method to the damping function and to the head field estimation errors.