Current effects on scattering of surface gravity waves by bottom topography

Scattering of random surface gravity waves by small amplitude topography in the presence of a uniform current is investigated theoretically. This problem is relevant to ocean waves propagation on shallow continental shelves where tidal currents are often significant. A perturbation expansion of the wave action to second order in powers of the bottom amplitude yields an evolution equation for the wave action spectrum. A scattering source term gives the rate of exchange of the wave action spectrum between wave components, with conservation of the total action at each absolute frequency. With and without current, the scattering term yields reflection coefficients for the amplitudes of waves that converge, to the results of previous theories for monochromatic waves propagating in one dimension over sinusoidal bars. Over sandy continental shelves, tidal currents are known to generate sandwaves with scales comparable to those of surface waves. Application of the theory to such a real topography suggests that scattering mainly results in a broadening of the directional wave spectrum, due to forward scattering, while the back-scattering is generally weaker. The current may strongly influence surface gravity wave scattering by selecting different bottom scales with widely different spectral densities due the sharp bottom spectrum roll-off.Comment: submitted to Journal of Fluid Mechanics 7 Oct 200

Aspects météo-marins de la tempête du 10 mars 2008 en Atlantique et en Manche

Le 10 mars 2008, une tempête a touché la pointe nord-ouest de l’Europe pendant une marée de vive-eau, générant de nombreux dégâts et des cas de submersion sur les côtes nord-ouest de la France. Les niveaux d’eau exceptionnels atteints ce jour-là sont dus à la combinaison de deux phénomènes indépendants (surcote importante accompagnée d’une mer énorme et pleine mer de vive-eau). Les niveaux d’eau maximums à l’origine de la submersion des côtes basses ont été à chaque fois précédés par le passage d’un front froid. Les vitesses de vent et les hauteurs de houles atteintes pendant cette tempête ont une période de retour de deux ans. Les niveaux d’eau mesurés par les marégraphes ont une occurrence de deux à cent ans. Il est en revanche impossible d’estimer pour le moment la période de retour de la combinaison houle – surcote – pleine mer de vive-eau, mais elle est probablement de l’ordre de 10 ans ou plus.The 10th of March 2008, a storm reached north-western of Europe during a spring tide, generating many damages and coastal flooding on the north-western french coast. The extrem water levels recorded on that day are caused by a combinaison of independant phenomena (important storm surge, phenomenal waves, spring high tide). The maximum water levels and most coastal flooding were every time linked with a cold front. Wind speed and wave height reached during this storm have a return period of only two years. The water levels mesured by tidal gauges have a return period from two to hundred years. Nevertheless it is impossible for the moment to estimate the return period of the combinaison wave – surge – spring high tide, but it is likely of the order of ten years or more

Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation

New parameterizations for the spectra dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is non-zero only when a non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced to represent the dissipation of short waves due to longer breaking waves. Several degrees of freedom are introduced in the wave breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from gentle swells to major hurricanes, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations yield the best overall results to date. Perspectives for further improvement are also given.Comment: revised version for Journal of Physical Oceanograph

Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis

BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London

Surgical site infection after gastrointestinal surgery in high-income, middle-income, and low-income countries: a prospective, international, multicentre cohort study

Background: Surgical site infection (SSI) is one of the most common infections associated with health care, but its importance as a global health priority is not fully understood. We quantified the burden of SSI after gastrointestinal surgery in countries in all parts of the world. Methods: This international, prospective, multicentre cohort study included consecutive patients undergoing elective or emergency gastrointestinal resection within 2-week time periods at any health-care facility in any country. Countries with participating centres were stratified into high-income, middle-income, and low-income groups according to the UN's Human Development Index (HDI). Data variables from the GlobalSurg 1 study and other studies that have been found to affect the likelihood of SSI were entered into risk adjustment models. The primary outcome measure was the 30-day SSI incidence (defined by US Centers for Disease Control and Prevention criteria for superficial and deep incisional SSI). Relationships with explanatory variables were examined using Bayesian multilevel logistic regression models. This trial is registered with ClinicalTrials.gov, number NCT02662231. Findings: Between Jan 4, 2016, and July 31, 2016, 13 265 records were submitted for analysis. 12 539 patients from 343 hospitals in 66 countries were included. 7339 (58·5%) patient were from high-HDI countries (193 hospitals in 30 countries), 3918 (31·2%) patients were from middle-HDI countries (82 hospitals in 18 countries), and 1282 (10·2%) patients were from low-HDI countries (68 hospitals in 18 countries). In total, 1538 (12·3%) patients had SSI within 30 days of surgery. The incidence of SSI varied between countries with high (691 [9·4%] of 7339 patients), middle (549 [14·0%] of 3918 patients), and low (298 [23·2%] of 1282) HDI (p < 0·001). The highest SSI incidence in each HDI group was after dirty surgery (102 [17·8%] of 574 patients in high-HDI countries; 74 [31·4%] of 236 patients in middle-HDI countries; 72 [39·8%] of 181 patients in low-HDI countries). Following risk factor adjustment, patients in low-HDI countries were at greatest risk of SSI (adjusted odds ratio 1·60, 95% credible interval 1·05–2·37; p=0·030). 132 (21·6%) of 610 patients with an SSI and a microbiology culture result had an infection that was resistant to the prophylactic antibiotic used. Resistant infections were detected in 49 (16·6%) of 295 patients in high-HDI countries, in 37 (19·8%) of 187 patients in middle-HDI countries, and in 46 (35·9%) of 128 patients in low-HDI countries (p < 0·001). Interpretation: Countries with a low HDI carry a disproportionately greater burden of SSI than countries with a middle or high HDI and might have higher rates of antibiotic resistance. In view of WHO recommendations on SSI prevention that highlight the absence of high-quality interventional research, urgent, pragmatic, randomised trials based in LMICs are needed to assess measures aiming to reduce this preventable complication

Cliff-top storm deposits on Banneg Island, Brittany, France: effects of giant waves in the Eastern Atlantic Ocean

International audienceThis study is based on the morphosedimentary analysis of the cliff-top storm deposits accumulated on the coast of Banneg Island located in the archipelago of Molène (Brittany, France). These CTSDs comprise large, tabular clasts quarried from the upper part of the cliff and the backing scoured platform by giant oceanic storm waves impacting directly the western coast of the island. An analysis of the distribution and the geomorphology of these accumulations were carried out using the DGPS topographic surveys. Most of the clasts are organised into imbricate boulder clusters or ridges deposited between 7.5 and 14.5 masl. 52 accumulations were inventoried from the north to the south of the island, representing a global volume of 1000 m3. The median size of the clasts calculated is equivalent to 0.8 × 0.6 × 0.4 m and a weight of 0.6 t. The largest one measuring 5.3 × 3.9 × 0.5 m (≈ 32 t) is located in the centre of the island (ridge #28). It has been deposited 14 m inland from the edge of the cliff at the elevation of 9 m. Sediment analysis shows that clast sizes become smaller with increasing distance from the shoreline, but there is no relationship between the sorting and the distance inland. A study of the hydrodynamic conditions inducing clast transport was undertaken by an analysis of the wave data from the 1989 to the 1990 winter storms. Models of wave runup indicate that their highest water levels may have reached up to 19 masl, 5 to 10 m higher than the top of the cliff. Submersion by giant storm waves has been more important and frequent in the centre and the south of the island. Wave data over the 1979 to the 2007 period shows that no events as powerful as those of the 1989 to the 1990 winter were recorded during the last 30 years. Yet, it appears that the 1979–1990 decade was characterized by important morphogeneous events while the following period (1990–2007) has experienced a sharp decrease in storm events. These variations could be attributed to the inversion from a negative towards a positive phase in the North Atlantic Oscillation index

Sédimentation supratidale de haute énergie générée par des vagues géantes. Le cas des blocs cyclopéens de l'île de Banneg (archipel de Molène, Finistère)

Cette étude porte sur l'analyse morphosédimentaire et dynamique des blocs cyclopéens de l'île de Banneg, située à la limite nord ouest de l'archipel de Molène (Finistère). Ces dépôts grossiers sont constitués de dalles rocheuses arrachées au substrat par les houles de tempêtes océaniques attaquant de plein fouet la façade occidentale de l'île. Un relevé topo-morphologique au DGPS de la zone d'étude a permis leur analyse. Ils prennent le plus souvent l'allure de remparts culminants entre 7 et 14,5 m (IGN 69) d'altitude. 52 amas de blocs ont été recensés du nord au sud de l'île, représentant un volume total d'environ 1000 m3. Ils se répartissent en 6 groupes principalement localisés du nord au sud de l'île, au droit des rentrants de la côte. L'analyse granulométrique effectuée sur 908 échantillons montre que la taille médiane des blocs avoisine 0,8 x 0,6 x 0,4 m, pour un poids équivalant à 0,6 tonne. Le plus gros bloc, 5,30 x 3,90 x 0,50 m, situé au centre l'île, pèse 32 tonnes et a été projeté à 14 mètres en retrait de la falaise à une altitude de 9 m. Si la taille des blocs diminue avec la distance par rapport au rivage, aucune relation clairement marquée n'est notée entre ce dernier paramètre et le tri granulométrique. Les mesures du tuilage indiquent que la direction de la houle incidente à l'origine de leur mise en place est de secteur ouest (270°) à nord-ouest (330°) du centre au sud de l'île. L'étude des conditions hydrodynamiques responsables de la mise en place de ces dépôts a été réalisée sur les tempêtes de l'hiver 1989-1990. Les données de houle utilisées correspondent à des simulations numériques du LNHE-EDF calculées au large de Ouessant qui ont été validées à partir des mesures in situ de la bouée BEAIII. Ces données au large ont ensuite été propagées à la côte à l'aide d'un modèle de réfraction du SHOM (CRESTP). Il s'avère que les caractéristiques de houle proche du rivage restent pratiquement identiques que celles du large, avec une atténuation négligeable dans la majorité des évènements. L'étude des conditions hydrodynamiques montre que les niveaux marins extrêmes de débordement (marée observée + run-up) ont pu atteindre près de 19 m (IGN 69) de hauteur (soit 5 à 10 m au dessus des falaises). Toutefois, la submersion a été beaucoup plus importante et fréquente au centre et au sud, qu'au nord et au centre nord de l'île. Un rétro-calcul sur la période 1979-2002 indique qu'aucun épisode aussi morphogène que ceux précédemment cités n'a été enregistré durant ces 24 années. Il apparaît toutefois que la décennie 1979-1990 a été marquée par un nombre important d'épisodes morphogènes, alors que celle qui suit (1990-2002) enregistre une nette diminution de la fréquence des tempêtes

Enregistrement morpho-sédimentaires des conditions météo-océaniques en mer d'Iroise. Le cas de la plage fuyante de l'île de Trielen (archipel de Molène, Finistère)

Depuis 2002, l'île de Trielen située dans l'archipel de Molène et partie intégrante de la Réserve Naturelle d'Iroise, fait l'objet d'un suivi morpho-sédimentaire à la demande du gestionnaire, la SEPNB – Bretagne Vivante. Ce travail, financé par la DIREN Bretagne, est basé sur des levés topographiques réalisés à l'aide d'un DGPS centimétrique. L'objectif est (i) de quantifier l'évolution à long terme des formes d'accumulations constituées de deux petites plages de poche situées au sud de l'île et d'un vaste cordon de galets septentrional formant une plage fuyante étirée d'ouest en est (figure 1) ; (ii) d'analyser le rôle des variations météo-marines interannuelles en mettant l'accent sur les conditions de houles et les courants de marée. Dans le cadre de cette présentation, l'accent est mis sur le cordon nord. Les résultats obtenus sur ces cinq années d'observations (2002-2007) montrent que le bilan sédimentaire est relativement stable : +360 m3 (cette valeur rapportée à la surface 28500 m2 relève vraisemblablement des marges d'erreur inhérente à la mesure). Il semblerait donc qu'aujourd'hui, les formations littorales de l'île ne soient plus alimentées faute de matériel disponible. Leur évolution se ferait donc à partir de l'unique stock en présence dont la mobilisation serait conditionnée par l'action des forçages météo-marins. La résultante des mouvements sédimentaires montre que durant la période 2002-2005, le stock de galet s'est déplacé vers l'ouest (en sens inverse des houles dominantes), à partir de prélèvements effectués sur la face arrière du cordon. Depuis 2006, cette dynamique s'est inversée, les déplacements se sont fait vers l'est suivant le sens des houles dominantes d'ouest. La modélisation de la circulation hydrodynamique dans l'archipel de Molène inciterait à penser que l'action des courants de marée joue un rôle prépondérant dans le déplacement sédimentaire lorsque les conditions énergétiques de la houle sont faibles. Ainsi, pour les hivers de 2002 à 2005 caractérisés par de grosses tempêtes, on suppose que les courants de marée résiduels orientés nord-est sud-ouest ont favorisé les transferts de galets vers l'ouest (2320 m3 entre 2004 et 2005). A l'inverse, durant les hivers 2006 et 2007 marqués par des fortes tempêtes d'ouest, l'action de la houle a pris le dessus sur les courants de marée et a entraîné des déplacements vers l'est pouvant atteindre 3900 m3. Le mouillage de courantomètres au niveau de la partir distale du cordon est envisagé et permettrait de confirmer cette hypothèse

Evaluation de la technique STABIPLAGE® mise en place sur deux plages du Finistère : les Sables Blancs à Plobannalec-Lesconil-Loctudy, et Boutrouilles à Kerlouan

International audienceUn suivi morphosédimentaire a été réalisé sur deux sites finistériens afin d'évaluer la fonctionnalité du procédé STABIPLAGE® : la plage des Sables Blancs sur laquelle 5 structures en épi ont été mises en place afin de bloquer une partie du transit longitudinal ; la plage de Boutrouilles équipée d'une structure longitudinale en pied de dune afin de fixer le trait de côte. Dans les deux cas, les STABIPLAGE® remplissent leur rôle de défense côtière tant que les conditions hydrodynamiques sont peu énergétiques. En période de fortes tempêtes, l'action des courants de retour et/ou de la submersion marine exacerbée, induit des phénomènes d'érosion que les STABIPLAGE® ne peuvent enrayer