Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands   总被引：1，自引：1，他引：0  

Brian F. Atwater  Uri S. ten Brink  Mark Buckley  Robert S. Halley  Bruce E. Jaffe  Alberto M. López-Venegas  Eduard G. Reinhardt  Maritia P. Tuttle  Steve Watt  Yong Wei 《Natural Hazards》2012,63(1):51-84

Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5?km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada??s windward shore. Remnants of the breached ridges stand 3?m above modern sea level, and ridges seaward of the breaches rise 2.2?C3.0?m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5?km to the south of the breached ridges. It tapers southward from a maximum thickness of 40?cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks??cerithid gastropods and the bivalve Anomalocardia??and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada??s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8?km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island??s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650?C1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15?km to Anegada??s south in 1960.  相似文献   

On the third-body perturbations of high-altitude orbits     

Martin Lara  Juan F. San-Juan  Luis M. López  Paul J. Cefola 《Celestial Mechanics and Dynamical Astronomy》2012,113(4):435-452

The long-term effects of a distant third-body on a massless satellite that is orbiting an oblate body are studied for a high order expansion of the third-body disturbing function. This high order may be required, for instance, for Earth artificial satellites in the so-called MEO region. After filtering analytically the short-period angles via averaging, the evolution of the orbital elements is efficiently integrated numerically with very long step-sizes. The necessity of retaining higher orders in the expansion of the third-body disturbing function becomes apparent when recovering the short-periodic effects required in the computation of reliable osculating elements.  相似文献   

The role of ejecta in the small crater populations on the mid-sized saturnian satellites   总被引：1，自引：0，他引：1  

Edward B. Bierhaus  Luke Dones  José Luis Alvarellos  Kevin Zahnle 《Icarus》2012,218(1):602-621

We find evidence, by both observation and analysis, that primary crater ejecta play an important role in the small crater (less than a few km) populations on the saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500 m/pixel. We use recent updates to crater scaling laws and their constants (Housen, K.R., Holsapple, K.A. [2011]. Icarus 211, 856–875) to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (v_min), and (iii) velocities less than v_min. Although the vast majority of mass on each satellite is ejected at speeds less than v_min, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. In particular, Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface, see Dobrovolskis, A.R., Lissauer, J.J. [2004]. Icarus 169, 462–473; Alvarellos, J.L., Zahnle, K.J., Dobrovolskis, A.R., Hamill, P. [2005]. Icarus 178, 104–123; Zahnle, K., Alvarellos, J.L., Dobrovolskis, A.R., Hamill, P. [2008]. Icarus 194, 660–674) is not yet well understood. Finally, our work provides further evidence for a “shallow” size–frequency distribution (slope index of ～2 for a differential power-law) for comets a few kilometers diameter and smaller.  相似文献   

Modelling the cross-shore profiles of sand beaches using artificial neural networks     

Isabel López  Luis Aragonés  Yolanda Villacampa 《Marine Georesources & Geotechnology》2013,31(6):683-694

Abstract

Artificial neural networks (ANN) have been widely used successfully to solve coastal engineering problems. In this article, they are used to model the cross-shore profile of sandy beaches taking into account the possible effect of marine vegetation (Posidonia oceanica). Sixty ANNs were generated by modifying both the inputs and the number of neurons in the hidden layer. The best results were obtained with the following inputs: wave height perpendicular to the coast and the associated period and probability of occurrence, median sediment size, profile slope, and energy reduction factor due to P. oceanica. With these inputs and 10 neurons in the hidden layer, a mean absolute error of 0.22?m during training and 0.21?m during the test was obtained, which represents an improvement of 81.2% and 55.5% compared to models without and with P. oceanica.  相似文献   

Morphological characteristics and sand volumes of different coastal dune types in Essaouira Province, Atlantic Morocco     

Germán Flor-Blanco  Germán Flor  Saadia Lharti  Luis Pando 《Geo-Marine Letters》2013,33(2-3):101-115

Altogether three coastal dune fields, one located north and two south of the city of Essaouira, Atlantic Morocco, have been investigated to establish the distribution and overall sand volumes of various dune types. The purpose of the study was to characterize and classify the aeolian landforms of the coastal dune belt, to estimate their sand volumes and to assess the effectiveness of coastal dune stabilization measures. The northern dune field is 9 km long and lined by a wide artificial foredune complex fixed by vegetation, fences and branches forming a rectangular grid. Active and ephemeral aklé dunes border the inner backshore, while some intrusive dunes have crossed the foredune belt and are migrating farther inland. The total sand volume of the northern dune belt amounts 13,910,255 m³. The central coastal sector comprises a much smaller dune field located just south of the city. It is only 1.2 km long and, with the exception of intrusive dunes, shows all other dune types. The overall sand volume of the central dune field amounts to about 172,463 m³. The southern dune field is characterized by a narrower foredune belt and overall lower dunes that, in addition, become progressively smaller towards the south. In this sector, embryonic dunes (coppice, shadow dunes), tongue-like and tabular dunes, and sand sheets intrude from the beach, the profile of which has a stepped appearance controlled by irregular outcrops of old aeolianite and beach rock. The total volume of the southern dune field amounts 1,446,389 m³. For the whole study area, i.e. for all three dune fields combined, a sand volume of about 15,529,389 m³ has been estimated. The sand of the dune fields is derived from coastal erosion and especially the Tensift River, which enters the sea at Souira Qedima some 70 km north of Essaouira. After entering the sea, the sand is transported southwards by littoral drift driven by the mainly north-westerly swell climate and the Trade Winds blowing from the NNE. This sand feeds the beaches along the coast, from where it is blown obliquely onshore to generate the dune fields. The maximum sand input occurs in the north, from where it gradually decreases southwards, this being also reflected in the grain size and dune typologies. The study shows that dune stabilization measures have been reasonably effective along most of the coast, with the exception of a large area immediately north of Essaouira where the almost complete destruction of the plant cover has reactivated sand mobility—this may in the future threaten the city.  相似文献   

Evolution of the Salinas-El Espartal and Xagó beach/dune systems in north-western Spain over recent decades: evidence for responses to natural processes and anthropogenic interventions   总被引：1，自引：0，他引：1  

Germán Flor-Blanco  Germán Flor  Luis Pando 《Geo-Marine Letters》2013,33(2-3):143-157

The confining barrier comprising the Salinas-El Espartal beach/dune system forms part of the mouth complex of the Avilés estuary on the central coast of Asturias (NW Spain). In this study the evolution of the beach and its dune field, as well as the estuary, is established based on appraisal of both natural and anthropogenic processes. In particular, dredging in the estuary mouth has had a strong negative impact on the system, including the recession of the dune front by between 20 and 30 m, and degradation of the seafront, first built at the edge of the beach dunes in 1965 and rebuilt in 1994. By contrast, the dumping of dredged material at a nearby beach, Xagó, has caused a remarkable dune progradation of 45 m on average, creating aeolian tabular sheets. The future dredging management of the mouth of the Avilés estuary should be directly related to the evolution of the El Espartal and Xagó dune fields.  相似文献   

Insights into the composition of household vulnerability from multicriteria decision analysis   总被引：5，自引：0，他引：5  

Hallie Eakin  Luis A. Bojrquez-Tapia 《Global Environmental Change》2008,18(1):112-127

Vulnerability is a multidimensional concept associated with high uncertainty in measurement and classification. Developing a vulnerability index from the diverse and often incommensurate data that form the basis of vulnerability assessments is often a core challenge of vulnerability research. Problematically, many vulnerability indices are based on the implicit or explicit assumption that each indicator of vulnerability is of equal importance. In this paper we propose a procedure to engage constructively with the inherent subjectivity and uncertainty of assigning weights to disparate indicators used in vulnerability assessments, using common tools of multicriteria decision analysis (MCDA) and fuzzy logic. To illustrate our proposed methodology, we present a case study of rural livelihood vulnerability in the state of Tamaulipas, México. In our case study, we combine a livelihoods framework with MCDA to weigh household attributes according to their relative importance in driving household vulnerability. This approach requires the explicit articulation of the relationship of each indicator to the umbrella concept (vulnerability) as well as of each indicator to every other indicator. In recognition of the inherent uncertainties involved in assigning any particular unit of analysis to a specific vulnerability class, we use fuzzy logic to create the final categories of household livelihood vulnerability to climatic risk. Our analysis reveals how different structures of livelihood assets and activities contributes to household sensitivity and capacities in a region characterized by variable climatic conditions, stagnant incomes, increasing market stress and declining farm productivity.  相似文献   

State of the art in the conceptual design of flotation circuits     

David A. Mendez  Edelmira D. Gálvez  Luis A. Cisternas 《International Journal of Mineral Processing》2009,90(1-4):1-15

The present study summarizes reports on optimal flotation circuits design over the period from 1989 to present. A review is made on the resolution characteristics of each study, leading to a classification of the approaches into four different groups, in which conclusions are based on either (A) mathematical solutions without binary variable, (B) mathematical solutions with binary variable, (C) heuristic solutions, or (D) genetic algorithms. These groups aim at a common scheme of resolution, varying in both flotation and milling models employed, characteristics of their superstructures, and functions which each study proposes to optimize, as well as the effects of the application of each strategy on the form of the solution of the circuit (selection of stages, selection of equipment, and dimensions of equipment).Within these either implicit or explicit multi-objective optimization problems there are challenges strongly related to the obtaining of better flotation and milling/classification models, as well as the treatment of uncertainty related to important parameters (kinetics, composition, external economic factors) and changes in previously established configurations using methods in which the designer's criteria influence the application of models and restrictions on the problem. Also, the incorporation of environmental issues needs more attention.  相似文献   

The typology of ancient quarries within the Paleocene limestone of Álava in northern Spain     

Luis M. Martínez‐Torres 《Geoarchaeology》2009,24(1):42-58

Paleocene limestone and dolomitized limestone of Álava, Spain, were used as building materials from the Neolithic age until the Neoclassical, predominantly in the production of headstones, sculpture, and stonework during the Roman era (1st to 4th centuries) and Romanic and Gothic periods (10th to 15th centuries). The quarrying methods employed are related to topographic relief. Five types of ancient quarry have been identified: escarpment, dip‐slope (ascending and descending), edge, and trench. The oldest quarries (1st to 4th centuries) are the trench‐cut stone pits of “Raso de Cezama.” Later, from the Romanic period (10th to 15th centuries), descending dip‐slope, escarpment, and edge‐based quarries were worked, such as those at Achuri, Ajarte, and Santa Elena. Finally, an ascending dip‐slope quarry was developed at Marquínez during the Neoclassical period (18th century). © 2009 Wiley Periodicals, Inc.  相似文献   

Soil slip susceptibility assessment using mechanical–hydrological approach and GIS techniques: an application in the Apuan Alps (Italy)     

Giacomo D’Amato Avanzi  Francesco Falaschi  Roberto Giannecchini  Alberto Puccinelli 《Natural Hazards》2009,50(3):591-603

This study aims at contributing to the soil slip susceptibility assessment in a typical basin of the southern Apuan Alps, Italy. On June 1996, this basin (Cardoso Torrent, 13 km² large) was hit by an extremely heavy rainstorm (maximum intensity of about 160 mm/h), which caused many landslides (debris slide–debris flows) and valley bottom flows (hyperconcentrated flows), destruction and deaths. Detailed surveys provided the characterization of the main factors (geological, geomorphologic, hydrological, hydrogeological and geotechnical) which contributed in triggering landslides. In order to evaluate the soil slip susceptibility in this area, a physically based model was applied and a GIS analysis of digital elevation model was performed. This approach couples a mechanical model based on an infinite slope form of the Mohr–Coulomb failure criterion, and a steady-state hydrological one (a modified version of Shalstab, which considers the cohesion of the debris material potentially involved in landsliding). GIS techniques allowed evaluating the effects of topographic convergence and drainage area on slope failure. In this way, based on the infiltration rate, the triggering of the June 1996 landslides was simulated and the critical rainfall thresholds assessed at about 200–250 mm/24 h.  相似文献