93 research outputs found
Empirical and analytical methods to characterize the efficiency of floods to move sediment in a small semi-arid basin
Over a long multi-year period, flood events can be classified according to
their effectiveness in moving sediments. Efficiency depends both on the
magnitude and frequency with which events occur. The effective (or dominant)
discharge is the water discharge which corresponds to the maximum sediment
supply. If its calculation is well documented in temperate or humid climates
and large basins, it is much more difficult in small and semi-arid basins
in which short floods with high sediment supplies occur. Using the example of
31Â years of measurements in the Wadi Sebdou (north-west Algeria), this paper
compares the two main statistical approaches to calculate the effective
discharge (the empirical method based on histograms of sediment supply by
discharge classes and an analytical calculation based on a hydrological
probability distribution and on a sediment rating curve) to a very simple
proxy: the half-load discharge, i.e. the flow rate corresponding to 50 % of
the cumulative sediment yield. Three types of discharge subdivisions were
tested. In the empirical approach, two subdivisions provided effective
discharge close to the half-load discharge. Analytical solutions based on
log-normal and log-Gumbel probability distributions were assessed but they
highly underestimated the effective discharge, whatever the subdivision used
to adjust the flow frequency distribution. Furthermore, annual series of
maximum discharge and half-load discharge enabled the return period
of hydrological years with discharge higher than the effective discharge
(around 2Â years) to be inferred and showed that more than half of the yearly sediment
supply is carried by flows higher than the effective discharge only every
7Â hydrological years. This study was the first to adapt the analytical
approach in a semi-arid basin and to show the potentiality and limits of each
method in a such climate.</p
Dragon-kings: mechanisms, statistical methods and empirical evidence
This introductory article presents the special Discussion and Debate volume
"From black swans to dragon-kings, is there life beyond power laws?" published
in Eur. Phys. J. Special Topics in May 2012. We summarize and put in
perspective the contributions into three main themes: (i) mechanisms for
dragon-kings, (ii) detection of dragon-kings and statistical tests and (iii)
empirical evidence in a large variety of natural and social systems. Overall,
we are pleased to witness significant advances both in the introduction and
clarification of underlying mechanisms and in the development of novel
efficient tests that demonstrate clear evidence for the presence of
dragon-kings in many systems. However, this positive view should be balanced by
the fact that this remains a very delicate and difficult field, if only due to
the scarcity of data as well as the extraordinary important implications with
respect to hazard assessment, risk control and predictability.Comment: 20 page
Origine, transport et devenir des apports naturels et anthropiques dans le lagon sud-ouest de Nouvelle-Calédonie
L’ird (Institut de recherche pour le développement) conduit depuis dix ans un programme pluridisciplinaire sur les effets des apports naturels et anthropiques sur le fonctionnement du lagon de Nouvelle-Calédonie. Celle-ci connaît actuellement une croissance de sa population et un développement de son industrie minière. Trois grands types d’apports sont pris en compte : les particules entraînées par l’érosion des sols et des sous-sols et dont l'accumulation brutale peut être responsable du dépôt de quantités considérables de sédiments, les métaux potentiellement toxiques pour les organismes vivant dans le lagon et les éléments nutritifs susceptibles de conduire les eaux jusqu'à l’eutrophisation (confinement chimique conduisant à l'élimination de nombreuses espèces vivantes et à la dominance d'un petit nombre d'autres). Le présent article offre trois exemples de problématiques environnementales qui n'ont pu être résolues qu’au moyen de la fédération de différentes approches de recherche complémentaires : l’influence des courants sur la dispersion des particules détritiques en provenance de l'île, le devenir des métaux dans le lagon et leur accumulation dans les organismes, les effets des apports en nutriments sur les communautés planctoniques et les risques d'eutrophisation. Les résultats de ces recherches sont présentés tout en conservant à l’esprit les objectifs nécessairement appliqués d’un programme de recherche pour le développement. Il s’agit principalement de définir les bases scientifiques permettant d’identifier certains outils de diagnostic environnemental et de développer des approches de simulation mathématique susceptibles d'offrir une vision synthétique et prévisionnelle de l’état et du devenir des environnements lagonaires sous influence anthropique.For the past ten years, the ird (Institut de recherche pour le développement) has developed a multidisciplinary programme dealing with the effects of natural and anthropogenic terrigenous inputs on the New Caledonia coral reef lagoon which is currently subjected to environmental pressure due to population increase and development of the mining industry. The ongoing study focused on three main categories of inputs: (i) particles generated by erosion processes and responsible for excessive sediment inputs in the lagoon, (ii) metals exhibiting a potentially lethal effect on the lagoon biota, (iii) nutrients responsible for eutrophication. This article presents three examples of environmental issues that could be addressed through complementary research approaches: (i) the dispersion of terrigenous inputs as commended by currents, (ii) the fate of metals in the lagoon and their accumulation in the biota, (iii) the effects of nutrient enrichment on pelagic communities. Results are presented while keeping in mind the necessary applied outcomes requested from a research programme devoted to development issues. Such outcomes are mainly related to the identification of suited environmental diagnostic tools and to the development of modelling approaches yielding synthetic and predictive information on the status and fate of coral reef lagoons subject to anthropogenic stress
Impact of the spatial resolution of satellite remote sensing sensors in the quantification of total suspended sediment concentration: A case study in turbid waters of Northern Western Australia
The impact of anthropogenic activities on coastal waters is a cause of concern because such activities add to the total suspended sediment (TSS) budget of the coastal waters, which have negative impacts on the coastal ecosystem. Satellite remote sensing provides a powerful tool in monitoring TSS concentration at high spatiotemporal resolution, but coastal managers should be mindful that the satellite-derived TSS concentrations are dependent on the satellite sensor's radiometric properties, atmospheric correction approaches, the spatial resolution and the limitations of specific TSS algorithms. In this study, we investigated the impact of different spatial resolutions of satellite sensor on the quantification of TSS concentration in coastal waters of northern Western Australia. We quantified the TSS product derived from MODerate resolution Imaging Spectroradiometer (MODIS)-Aqua, Landsat-8 Operational Land Image (OLI), and WorldView-2 (WV2) at native spatial resolutions of 250 m, 30 m and 2 m respectively and coarser spatial resolution (resampled up to 5 km) to quantify the impact of spatial resolution on the derived TSS product in different turbidity conditions. The results from the study show that in the waters of high turbidity and high spatial variability, the high spatial resolution WV2 sensor reported TSS concentration as high as 160 mg L-1 while the low spatial resolution MODIS-Aqua reported a maximum TSS concentration of 23.6 mg L-1. Degrading the spatial resolution of each satellite sensor for highly spatially variable turbid waters led to variability in the TSS concentrations of 114.46%, 304.68% and 38.2% for WV2, Landsat-8 OLI and MODIS-Aqua respectively. The implications of this work are particularly relevant in the situation of compliance monitoring where operations may be required to restrict TSS concentrations to a pre-defined limit
Beam Charge Asymmetries for Deeply Virtual Compton Scattering on the Proton at CLAS12
The parameterization of the nucleon structure through Generalized Parton
Distributions (GPDs) shed a new light on the nucleon internal dynamics. For its
direct interpretation, Deeply Virtual Compton Scattering (DVCS) is the golden
channel for GPDs investigation. The DVCS process interferes with the
Bethe-Heitler (BH) mechanism to constitute the leading order amplitude of the
process. The study of the reaction with polarized
positron and electron beams gives a complete set of unique observables to
unravel the different contributions to the cross section. This
separates the different reaction amplitudes, providing a direct access to their
real and imaginary parts which procures crucial constraints on the model
dependences and associated systematic uncertainties on GPDs extraction. The
real part of the BH-DVCS interference amplitude is particularly sensitive to
the -term which parameterizes the Gravitational Form Factors of the nucleon.
The separation of the imaginary parts of the interference and DVCS amplitudes
provides insights on possible higher-twist effects. We propose to measure the
unpolarized and polarized Beam Charge Asymmetries (BCAs) of the process on an unpolarized hydrogen target with {\tt
CLAS12}, using polarized positron and electron beams at 10.6~GeV. The azimuthal
and -dependences of the unpolarized and polarized BCAs will be measured over
a large phase space using a 100 day run with a luminosity of
0.66cms.Comment: Proposal to the Jefferson Lab Program Advisory Committee (PAC51
Evaluation of thin film adhesion to a compliant substrate by the analysis of progressive buckling in the fragmentation test
DELINEAMENTO AMOSTRAL EM RESERVATÓRIOS UTILIZANDO IMAGENS LANDSAT-8/OLI: UM ESTUDO DE CASO NO RESERVATÓRIO DE NOVA AVANHANDAVA (ESTADO DE SÃO PAULO, BRASIL)
GLORIA - A globally representative hyperspectral in situ dataset for optical sensing of water quality
The development of algorithms for remote sensing of water quality (RSWQ) requires a large amount of in situ data to account for the bio-geo-optical diversity of inland and coastal waters. The GLObal Reflectance community dataset for Imaging and optical sensing of Aquatic environments (GLORIA) includes 7,572 curated hyperspectral remote sensing reflectance measurements at 1 nm intervals within the 350 to 900 nm wavelength range. In addition, at least one co-located water quality measurement of chlorophyll a, total suspended solids, absorption by dissolved substances, and Secchi depth, is provided. The data were contributed by researchers affiliated with 59 institutions worldwide and come from 450 different water bodies, making GLORIA the de-facto state of knowledge of in situ coastal and inland aquatic optical diversity. Each measurement is documented with comprehensive methodological details, allowing users to evaluate fitness-for-purpose, and providing a reference for practitioners planning similar measurements. We provide open and free access to this dataset with the goal of enabling scientific and technological advancement towards operational regional and global RSWQ monitoring
Beam spin asymmetry measurements of deeply virtual π0 production with CLAS12
The new experimental measurements of beam spin asymmetry were performed for the deeply virtual exclusive pi0 production in a wide kinematic region with the photon virtualities Q2 up to 6.6 GeV2 and the Bjorken scaling variable xB in the valence regime. The data were collected by the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab with longitudinally polarized 10.6 GeV electrons scattered on an unpolarized liquid-hydrogen target. Sizable asymmetry values indicate a substantial contribution from transverse virtual photon amplitudes to the polarized structure functions. The interpretation of these measurements in terms of the Generalized Parton Distributions (GPDs) demonstrates their sensitivity to the chiral-odd GPD ET, which contains information on quark transverse spin densities in unpolarized and polarized nucleons and provides access to the nucleon's transverse anomalous magnetic moment. Additionally, the data were compared to a theoretical model based on a Regge formalism that was extended to the high photon virtualities
Recent changes in climate, hydrology and sediment load in the Wadi Abd, Algeria (1970–2010)
Here we investigate the changes of temperature, precipitation, river runoff
and sediment transport in the Wadi Abd in northwest Algeria over a time series of
40Â hydrological years (1970–2010). Temperature increased and precipitation
decreased with the reduction in rainfall being relatively higher during the
rainy season. A shift towards an earlier onset of first rains during summer
was also found with cascading effects on hydrology (hydrological regimes,
vegetation, etc.) and thus on erosion and sediment yield. During the 1980s,
the flow regime shifted from perennial to intermittent with an amplification
of the variations of discharge and a modification of the sediment regime
with higher and more irregular suspended particulate flux. Sediment flux was
shown to almost double every decade from the 1970s to the 2000s. The sediment regime
shifted from two equivalent seasons of sediment yield (spring and fall) to
a single major season regime. In the 2000s, autumn produced over 4Â times more
sediment than spring. The enhanced scatter of the <i>C</i>–<i>Q</i> pairs denotes an
increase of hysteresis phenomena in the Wadi Abd that is probably related to
the change in the hydrologic regime. At the end of the period, due to
irregularity of the discharge, the ability of a rating curve to derive
suspended sediment concentration from river discharge was poor
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