Although earthquakes and tsunamis are less frequent
in the Dover Strait
than over active subduct
ion zones, a
plausible potential exists for intraplate earthquakes of magnitude Mw=6.9 generating a tsunami with damaging
consequences. In April 1580, an earthquake shook the region violently and destructions were reported as far as
London in the north and
Rouen in the south. Despite fair weather conditions, a series of abnormal sea waves was
reported in several harbours
(Calais, Boulogne and Dover)
on the same day
. A first step was to produce a range
of numerical coseismic tsun
ami simulations and to compare them with historical witness accounts. Results raise
the question of whether such earthquakes could also
trigger
chalk flow
-
generated tsunamis along cliff lines on
both sides of the Strait. Gravity
-
driven collapses affect the
chalk cliffs periodically, but local tsunami waves
caused by very large mass movements could reach heights of several met
er
s and, for example, strike Dover
.
Le projet HOMONIM phase 3 est mené par MétéoFrance et le Shom sous la maîtrise d'ouvrage de la Direction Générale de la Prévention des Risques (DGPR). Il vise à améliorer les capacités opérationnelles de modélisation des niveaux marins et des vagues à la côte pour aider la gestion du risque de submersion marine. Les principaux travaux de R&D en cours et à venir dans le projet HOMONIM3 sont abordés dans cet article : configuration de façade Manche – Atlantique du nouveau modèle de niveau marin TOLOSASW, calibration de la prévision d'ensemble (PE) de surcotes, mise en place d'une PE des vagues en côtier, modélisation couplée vagues/niveaux/courants à très haute résolution, pour le littoral Nord-Aquitain, et la modélisation non hydrostatique. Ces résultats sont susceptibles de servir aux modèles de prévision des inondations à proximité des estuaires ou des fleuves près de leur embouchure, en leur fournissant, sur leur frontière maritime, une condition limite plus réaliste, et cohérente avec les prévisions utilisées pour le dispositif de la Vigilance Vagues-Submersion (VVS), ainsi qu'une information, pour un jour donné, sur la prévisibilité des modèles ou sur l'existence de scénarios océaniques alternatifs.
Abstract. Current storm surge hazard maps in the French West Indies are essentially based on simple statistical methods using limited historical data and early low-resolution models which do not take the effect of waves into account. In this paper, we infer new 100 and 1000 year surge levels in Guadeloupe from the numerical modelling of storm surges induced by a large set of synthetic events that are in statistical agreement with features of historical hurricanes in the North Atlantic Basin between 1980 and 2011. Computations are performed using the wave-current coupled model ADCIRC-SWAN with high grid resolutions (up to 40–60 m) in the coastal and wave dissipation areas. This model is validated against observations during past events such as hurricane HUGO (1989). Results are generally found to be in reasonable agreement with past studies in areas where surge is essentially wind-driven, but to differ significantly in coastal regions where the transfer of momentum from waves to the water column constitutes a non-negligible part of the total surge. The methodology, which can be applied to other islands in the Lesser Antilles, allows to obtain storm surge level maps that can be of major interest for coastal planners and decision makers in terms of risk management.
Bengal delta shoreline, spanning Bangladesh and India, gets hit every 3 years on average by a major tropical cyclone. Although their occurrence is relatively moderate compared to other tropical regions (accounting for only 5% of global cyclones), the impact of these events is major, accounting for 50% of the victims recorded worldwide. This is due to the very low topography of the delta above sea level (less than 5 meters), high storm surge induced water level and flooding, combined with the high density of the vulnerable population. On one hand, the unavailability of long-term reliable water level data on a sparse tide-gauge network along the coastline has hindered the assessment of storm surge hazards. The application of hydrodynamic modelling to fill the data gap also suffers from the unavailability of a reliable long-term storm dataset over the region. The complex topography of the Bengal delta, with defence structures, and a dense network of rivers presents another modelling challenge. Finally, the interaction of tide, surge and wave further complicate the numerical complexity, needing a coupled modelling framework. Thanks to advancements made to acquire high-quality regional nearshore bathymetry and topography (Krien et al. 2016, Khan et al. 2019), as well as coupled storm surge modelling (Krien et al. 2017, Khan et al. 2021), the tidal and storm surge dynamics over the Bengal delta is now well captured by recent high-resolution coupled SCHISM-WWM Bay of Bengal model (Khan et al. 2021). To estimate the risk of storm surge and associated flooding across the Bengal delta, we have integrated the wave-coupled hydrodynamic model of Khan et al. (2021) for a large ensemble (~3600 cyclones, ~5000 years of storm activity) of synthetic cyclones generated through the statistical-deterministic method of Emanuel (2006). Our storm and surge ensemble covers the whole range of natural variability of storm frequency, size, intensity and track location, with a dense spatial distribution. The interactions among the tide, surge, and waves are modelled explicitly at high spatial resolution. The storm surge-induced water level at various return periods, up to 500 years, is then determined at high spatial resolution (250m at the coast) using a ranking-based technique. The dataset distributed here represents the storm surge water level estimate (e.g. total water level from the tide, surge, and wave computed dynamically through the model) at 25 to 500 year return period (25-year step). The corresponding variable in the self-describing netCDF data file is 'maxelev'. The estimated storm surge water level values are interpolated in a 30" (~1km at the equator) structured grid over the Bengal delta from the original unstructured-grid model outputs (250m resolution at the coast). This dataset is a part of a manuscript, currently being submitted to Natural Hazards and Earth System Sciences (https://nhess.copernicus.org/). Please cite the original paper, along with the dataset if used in your work as - Khan, M. J. U., Durand, F., Emanuel, K., Krien, Y., Testut, L., and Islam, A. K. M. S.: Storm surge hazard over Bengal delta: A probabilistic-deterministic modelling approach, Nat. Hazards Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/nhess-2021-329, in review, 2021.
The tsunami caused by the 2007 Peru earthquake (Mw 8.0) provoked less damage than by the seismic shaking itself (numerous casualties due to the earthquake in the vicinity of Pisco). However, it propagated across the Pacific Ocean and small waves were observed on one tide gauge in Taiohae Bay (Nuku Hiva, Marquesas, French Polynesia). We invert seismological data to recover the rupture pattern in two steps. The first step uses surface waves to find a solution for the moment tensor, and the second step uses body waves to compute the slip distribution in the source area. We find the slip distribution to consist of two main slip patches in the source area. The inversion of surface waves yields a scalar moment of 8.9 1020 Nm, and body-wave inversion gives 1.4 1021 Nm. The inversion of tsunami data recorded on a single deep ocean sensor also can be used to compute a fault slip pattern (yielding a scalar moment of 1.1 1021 Nm). We then use these different sources to model the tsunami propagation across the Pacific Ocean, especially towards Nuku Hiva. While the source model taken from the body-wave inversion yields computed tsunami waves systematically too low with respect to observations (on the central Pacific Ocean DART buoy as on the Polynesian tide gauge), the source model established from the surface-wave inversion is more efficient to fit the observations, confirming that the tsunami is sensitive to the low frequency component of the source. Finally we also discuss the modeling of the late tsunami arrivals in Taiohae Bay using several friction coefficients for the sea bottom.
Cette etude a pour principal objectif d'apporter des contraintes sur la structure! mecanique des zones de subduction et sur l'intensite des forces qui s'opposent a la tectonique des plaques. A l'aide de modeles dynamiques 2D de zone de subduction, cartesiens et instantanes, avec des rheologies Newtoniennes ou en loi de puissance, nous montrons dans une premiere partie que les donnees gravimetriques ne peuvent etre reproduites que si le couplage entre les plaques est limite, et que la resistance de la lithosphere subductante lors de la flexure est relativement faible. La gravite et les anomalies de geoide (de longueurs d'ondes comprises entre 100 km et 4000 km environ) sont en general correctement reproduites dans le cas ou les contraintes deviatoriques sont moderees. Nos resultats indiquent que seule une petite portion du poids de la plaque plongeante est transmise a la lithosphere en surface. Pres de 10% de l'energie est dissipee dans la zone de contact entre les plaques, 10-20% dans la region de flexure, et plus de 70% dans le manteau sub-lithospherique. Les tractions en base de plaques induisent un deplacement net de la lithosphere, qui se traduit par une asymetrie des vitesses en surface. Dans le cas d'une circulation mantellique globale, les anomalies de geoide determinees numeriquement presentent une bosse a des longueurs d'onde intermediaires (λ≈2000-4000km), qui n'est pas observee. Nous montrons qu'il est possible de reconcilier les observations avec les predictions du modele si l'on tient compte de forces de resistance au flux mantellique associees aux transitions de phases du manteau profond. Afin de mieux comprendre l'origine de ces forces, nous nous sommes interesses par la suite aux variations volumiques accompagnant les changements de phase. A l'aide de modeles analytiques et numeriques simples, nous montrons que les changements de volume macroscopiques (a l'echelle du manteau) peuvent alterer de maniere significative le flux mantellique et les observables de surface comme la gravimetrie, dans le cas d'une discontinuite fine et visqueuse. En partant du modele de croissance de grain de Morris [2002], nous montrons par ailleurs que les changements de volume a l'echelle microscopique sont susceptibles d'engendrer des deflections des zones de transitions de plusieurs kilometres, y compris dans le cas de rheologies non-Newtoniennes pour lesquelles la viscosite effective est relativement faible.
Abstract. In view of the high vulnerability of the Lesser Antilles small islands to cyclonic hazards, realistic very fine scale numerical simulation of hurricane-induced winds is essential to prevent and manage risks. The present innovative modeling aims at combining the most realistic simulated strongest gusts driven by tornado-scale vortices within the eyewall and the most realistic complex terrain effects. The Weather Research and Forecasting (WRF) model with the Nonlinear Backscatter and Anisotropy (NBA) Large Eddy Simulation (LES) configuration was used to reconstruct the devastating landfall of category 5 Hurricane Irma (2017) on Saint Barthélemy and Saint Martin islands. The results pointed out that the 30-m scale seems necessary to simulate intense 400-m scale vortices leading to extreme peak gusts like 132 m s−1 over sea. Risk areas associated with terrain gust speed-up factors greater than one have been identified for the two islands. The comparison between the simulated gusts and the remote sensing building damages highlighted the major role of structure strength linked with the socio-economic development of the territory. The present modeling method could be easily extended to other small mountainous islands to improve the understanding of observed past damages and to develop safer urban management and appropriate building standards.
