Elementary excitations of the BCS model in the canonical ensemble

6 págs.; 5 figs.; 1 tab. ; PACS number(s): 74.20.Fg, 71.10.Li, 73.21.La, 75.10.JmWe have found the elementary excitations of the exactly solvable BCS model for a fixed number of particles. These turn out to have a peculiar dispersion relation, some of them with no counterpart in the Bogoliubov picture, and unusual counting properties related to an old conjecture made by Gaudin. We give an algorithm to count the number of excitations for each excited state and a graphical interpretation in terms of paths and Young diagrams. For large systems the excitations are described by an effective Gaudin model, which accounts for the finite-size corrections to BCS. ©2003 American Physical SocietyThis work has been supported by Grant No. BFM2000-1320- C02-01/02.Peer Reviewe

Beach nourishment effects on sand porosity variability

Article history: Received 7 June 2013 Received in revised form 7 October 2013 Accepted 10 October 2013 Available online 14 November 2013A standard assumption in coastal engineering is that the porosity of natural beach sand (non-cohesive) is 40%. However, is this assumption correct for all beach sand? This paper proposes an accurate and simplified method to assess changes in sand porosity after beach nourishment bymeans of in-situ density surveys through a nuclear densimeter. This novel application has been applied to different beaches in the southwest of Spain according to the tidal range, grain size and beachmorphology in several terms. General results showthat sand porosities range from 25.6% to 43.4% after beach nourishment works. This research can be considered a support tool in coastal engineering to find shifting sand volumes as a result of sand porosity variability after beach nourishment and later marine influence

Mobility evolution of a highly active coastal dune (SW Spain) in several terms

Extreme wind conditions generated near the Strait of Gibraltar (SW Spain) have given rise to the transgressive Valdevaqueros dunefield (Tarifa). The dune advance has resulted in gradual invasion of the adjacent road and ecosystem. Despite restoration and control activities performed within the last decades, strong east winds have favoured dune destabilisation. The current work focuses on analyzing the dune profile evolution in the long term (years), medium term (months), short term (days) and very short term (hours) from topographic data, which were collected with a total station and a differential GPS from 1995 to 2009. Sand transport rates of 140 m3 m-1 year-1, 25.5–36.5 m3 m−1 month−1, 22.52 m3 m−1 day−1 and 0.93 m3 m−1 h−1 were measured for the long-term, medium-term, short-term and very short-term periods, respectively. During an eleven-year period, the average migration rate was found to be of 17.5 m year-1, although the very short term monitoring under a severe easterly sandstorm demonstrated that the dune brink was able to experience an advance of 1.75 m in only 24 h. These values were compared with the theoretical sand transport rate for Valdevaqueros dune, based on the classic Bagnold’s equation as well as other more recent formulae, to obtain a ratio between the real and the theoretical rates for each study period. These results together with the sand drift potential following the Fryberger’s method (up to 10,000 vector units) demonstrate that Valdevaqueros is a dunefield with one of the highest sand transport capacities in Europe

Evidencias del incremento en la movilidad de dunas costeras en el último medio siglo como respuesta a la intervención humana

Given the risk of two roads being buried by the sand of two highly mobile coastal dunes (Valdevaqueros and Bolonia, SW Spain), several measures have been taken over the last half century in order to stabilize them or at least slow their progress: installation of several rows of 1-m-high concrete structures, planting of species such as Pinus pinea, Retama monosperma and Ammophila arenaria, wicker and wooden fencing, and sand mining (only performed on the Valdevaqueros dunefield). The evolution of the two dunes was investigated through the interpretation of intensive topographical monitoring and aerial photography. Average migration rates of approximately 10 m yr–1 were detected and the burial of anthropic structures has eventually occurred. This process has caused a serious regional problem, making dune management of these areas a difficult challenge. The comparison of the evolution of these two dunes has been of great importance in assessing the effect of historical human intervention and has provided a new perspective for future dune management strategies. The results obtained show that management measures based on sand removal have been proven to be unsustainable. Moreover, the monitoring methodology presented herein has proven very useful in predicting dune advance rates. For instance, the Bolonia dune could reach the nearest road in approximately 12 years.Se han tomado diferentes medidas a lo largo del último medio siglo tratando de estabilizar o al menos ralentizar el progreso de dos dunas costeras de alta movilidad (Valdevaqueros y Bolonia, SW España) dado el riesgo de enterramiento de sendas carreteras: Entre las medidas tomadas cabe destacar la instalación de filas de estructuras de hormigón de 1 m de altura, la plantación de diferentes especies (como el Pinus pinea, la Retama monosperma o la Ammophila arenaria), vallas de madera y brezo, y la extracción de arena (sólo en la duna de Valdevaqueros). La evolución de ambas dunas se ha investigado a través de la comparación de levantamientos topográficos y fotografías aéreas. Se han detectado tasas medias de avance de aproximadamente 10 m/año hasta que, finalmente, se han enterrado estructuras antrópicas. Este proceso ha causado graves problemas convirtiendo en un difícil reto la gestión de dunas de estas áreas. La comparación de la evolución de estas dos dunas ha resultado de gran importancia a la hora de evaluar el efecto de la intervención humana, y ha proporcionado una nueva perspectiva para las futuras estrategias de gestión. Entre los resultados obtenidos, debería destacarse el que la gestión basada en la retirada de arena ha resultado insostenible. Además, la metodología de seguimiento aquí presentada ha probado ser de gran utilidad al predecir las tasas de avance de las dunas. Por ejemplo, la duna de Bolonia podría alcanzar la carretera próxima en unos 12 años

Success on increasing number of students that pass the coastal engineering subject

During the last ten years, the ratio of students that do not take the final exam in the subject of Coastal Engineering at the University of Cadiz was 1/3 of the total number of enrolled students. Furthermore, the ratio of students who did not pass the test was 50%. Hence, only a third part of the total amount of students successfully passed the exam. In order to improve this percentage, a working procedure hypothesis was implemented and the results are shown herein. This procedure implies some “homework”, the duration of which is similar to the theoretical lessons, i.e. 2 hours a week. Assignments are not mandatory and they only represent 10% of the final mark. Weekly dedication is directly related to the main aspects of the theoretical lessons and includes practical exercises as well as the search of particular cases about the topics explained at the classes. This methodology allows the students to: a) improve their knowledge of the subject; b) check their learning progress; c) introduce them to the e-learning; d) learn complementary concepts of scientific research by using e-databases; e) refresh and update the theoretical concepts provided during the live lectures; f) add new teaching material to enhance the comprehension and study of the subject; g) feel motivated by the possibility of increasing their marks. Achievement monitoring is performed just before each lesson. This way, any possible doubts derived from the exercises resolution are clarified. Despite representing only a low percentage of the final mark, a complete dedication of the students was achieved. The application of this procedure during the previous academic year resulted in 97.4 % of the students successfully passing the final test

Gestión de la limpieza de playas por derrame de hidrocarburos en la bahía de Algeciras

El mar es el principal medio de transporte por el que se mueve más mercancías del mundo. La naturaleza de la mercancía transportada es de muy diversa naturaleza, incluyendo productos peligrosos tanto para la salud humana como para el medio ambiente. Fruto de esta intensa actividad, con más frecuencia de la deseable, ocurren accidentes que conllevan el derrame de estas sustancias peligrosas que transportan, incluyendo el combustible del propio buque. Todo ello se traduce en ocasiones en un significativo deterioro ambiental, dando lugar a episodios de contaminación en las playas. En Andalucía, y en el caso de la Bahía de Algeciras, el riesgo de contaminación accidental en el litoral es destacable, si atendemos tanto a su gran extensión como a la elevada densidad en el tráfico marítimo (últimos sucesos: buque SIERRA NAVA, b/ NEW FLAME, b/ FEDRA, b/ TAWE y b/ SHENZHEN en apenas 3 años). El Estrecho de Gibraltar es uno de los puntos de mayor tráfico petrolero en todo el mundo. Cada año transitan por el Estrecho de Gibraltar entre 4.000 y 5.000 petroleros, es decir, entre 10 y 15 diarios. Más de 90.000 buques mercantes cruzan el Estrecho de Gibraltar y muchos de éstos recalan en la Bahía de Algeciras. A esto hay que sumar que el Puerto de Algeciras es el número uno de España en tráfico marítimo y que su Bahía cuenta con un polígono industrial petroquímico, incluida una refinería que procesa alrededor de 10 millones de toneladas de crudo al año (Refinería Gibraltar-San Roque). A eso se une que los Puertos del Estrecho y sus proximidades, se constituyen uno de los principales puntos de abastecimiento de combustible del Mediterráneo, a través de operaciones de “bunkering”. Consecuentemente, en esta franja costera de la provincia de Cádiz, el riesgo de contaminación accidental en el litoral es elevado. Por ello, ante la probabilidad de accidentes que puedan generar graves consecuencias económicas y ambientales, es necesario desarrollar herramientas de gestión de emergencias eficaces y adecuadas al objeto de minimizar este impacto. Para ello es importante contar con una adecuada planificación y distribución de recursos, acorde con las características, el riesgo y la vulnerabilidad de cada zona.Capítulo de Libro: XI Jornadas Españolas de Costas y Puertos, celebradas en 2011 y publicado en 2012. pp. 887. ISBN-13: 978-84-695-3707-

Morphological evolution in the migrating dune of Valdevaqueros (SW Spain) during an eleven-year period

Located in Tarifa, near the Strait of Gibraltar, the migration of Valdevaqueros dune was controlled by sand extractions for building purpose. As the Spanish Shore Act was approved in 1988, the sale of sand was forbidden and the dune started to move forward, burying the leeward pine wood and the only road to Maspalomas village. Consequently, six topographic levellings have been taken during the last eleven years. Sedimentary granulometries and meteorological features were taken into account. D50 values have been calculated for dune foot, windward and dune top. Wind speed threshold is estimated above 15 km/h. East winds are responsible for dune movement, whose advance rate has reached very high values (over 18 m/year). Slope and volumetric changes have been determined, providing a clearer explanation of the dune evolutio