Sediment and stable carbon isotope record of the Helgoland mud area

The Helgoland mud area in the German Bight is one of the very few sediment depocenters in the North Sea. Despite the shallowness of the setting (<30 m water depth), its topmost sediments provide a continuous and high-resolution record allowing the reconstruction of regional paleoenvironmental conditions for the time since ~400 a.d. The record reveals a marked shift in sedimentation around 1250 a.d., when average sedimentation rates drop from >13 to ~1.6 mm/year. Among a number of major environmental changes in this region during the Middle Ages, the disintegration of the island of Helgoland appears to be the most likely factor which caused the very high sedimentation rates prior to 1250 a.d. According to historical maps, Helgoland used to be substantially bigger at around 800 a.d. than today. After the shift in sedimentation, a continuous and highly resolved paleoenvironmental record reflects natural events, such as regional storm-flood activity, as well as human impacts at work at local to global scales, on sedimentation in the Helgoland mud area.

Debris flow magnitudes: a global catalog

The episodic occurrence of debris flow events in response to stochastic precipitation and wildfire events makes hazard prediction challenging. Previous work has shown that frequency-magnitude distributions of non-fire-related debris flows follow a power law, but less is known about the distribution of post-fire debris flows. As a first step in parameterizing hazard models, we use frequency-magnitude distributions and cumulative distribution functions to compare volumes of post-fire debris flows to non-fire-related debris flows. Due to the large number of events required to parameterize frequency-magnitude distributions, and the relatively small number of post-fire event magnitudes recorded in the literature, we collected data on 73 recent post-fire events in the field. The resulting catalog of 988 debris flow events is presented as an appendix to this article. We found that the empirical cumulative distribution function of post-fire debris flow volumes is composed of smaller events than that of non-fire-related debris flows. In addition, the slope of the frequency-magnitude distribution of post-fire debris flows is steeper than that of non-fire-related debris flows, evidence that differences in the post-fire environment tend to produce a higher proportion of small events. We propose two possible explanations: 1) post-fire events occur on shorter return intervals than debris flows in similar basins that do not experience fire, causing their distribution to shift toward smaller events due to limitations in sediment supply, or 2) fire causes changes in resisting and driving forces on a package of sediment, such that a smaller perturbation of the system is required in order for a debris flow to occur, resulting in smaller event volumes.

Diatom and hydrocarbon analysis of sediment core NBP00-03-JPC38

In this study, we present a unique high-resolution Holocene record of oceanographic and climatic change based on analyses of diatom assemblages combined with biomarker data from a sediment core collected from the Vega Drift, eastern Antarctic Peninsula (EAP). These data add to the climate framework already established by high-resolution marine sedimentary records from the Palmer Deep, western Antarctic Peninsula (WAP). Heavy sea ice conditions and reduced primary productivity were observed prior to 7.4 ka B.P. in relation with the proximity of the glacial ice melt and calving. Subsequent Holocene oceanographic conditions were controlled by the interactions between the Westerlies-Antarctic Circumpolar Current (ACC)-Weddell Gyre dynamics. A warm period characterized by short seasonal sea ice duration associated with a southern shift of both ACC and Westerlies field persisted until 5 ka B.P. This warm episode was then followed by climate deterioration during the middle-to-late Holocene (5 to 1.9 ka B.P.) with a gradual increase in annual sea ice duration triggered by the expansion of the Weddell Gyre and a strong oceanic connection from the EAP to the WAP. Increase of benthic diatom species during this period was indicative of more summer/autumn storms, which was consistent with changes in synoptic atmospheric circulation and the establishment of low- to high-latitude teleconnections. Finally, the multicentennial scale variability of the Weddell Gyre intensity and storm frequency during the late Holocene appeared to be associated with the increased El Niño-Southern Oscillation frequency.

Wave height (regular and irregular waves) reduction in 2m water depth over a 40-m-long salt marsh test section in a large scale laboratory flume

Coastal communities around the world face increasing risk from flooding as a result of rising sea level, increasing storminess, and land subsidence. Salt marshes can act as natural buffer zones, providing protection from waves during storms. However, the effectiveness of marshes in protecting the coastline during extreme events when water levels and waves are highest is poorly understood. Here, we experimentally assess wave dissipation under storm surge conditions in a 300-m-long wave flume that contains a transplanted section of natural salt marsh. We find that the presence of marsh vegetation causes considerable wave attenuation, even when water levels and waves are high. From a comparison with experiments without vegetation, we estimate that up to 60% of observed wave reduction is attributed to vegetation. We also find that although waves progressively flatten and break vegetation stems and thereby reduce dissipation, the marsh substrate remained remarkably stable and resistant to surface erosion under all conditions.The effectiveness of storm wave dissipation and the resilience of tidal marshes even at extreme conditions suggest that salt marsh ecosystems can be a valuable component of coastal protection schemes.

Water level time series and sediment grain size model

The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 µm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.

Hummingbird: Privacy at the time of Twitter

In the last several years, micro-blogging Online Social Networks (OSNs), such as Twitter, have taken the world by storm, now boasting over 100 million subscribers. As an unparalleled stage for an enormous audience, they offer fast and reliable centralized diffusion of pithy tweets to great multitudes of information-hungry and always-connected followers. At the same time, this information gathering and dissemination paradigm prompts some important privacy concerns about relationships between tweeters, followers and interests of the latter. In this paper, we assess privacy in today?s Twitter-like OSNs and describe an architecture and a trial implementation of a privacy-preserving service called Hummingbird. It is essentially a variant of Twitter that protects tweet contents, hashtags and follower interests from the (potentially) prying eyes of the centralized server. We argue that, although inherently limited by Twitter?s mission of scalable information-sharing, this degree of privacy is valuable. We demonstrate, via a working prototype, that Hummingbird?s additional costs are tolerably low. We also sketch out some viable enhancements that might offer better privacy in the long term.

Coastal floods and decadal changes: the climate factor

Observation has widely shown for nearly all last century that the Spanish (Dynamic) Maritime Climate was following around 10 to 11 year cycles in its most significant figure, wind wave, despite it being better to register cycles of 20 to 22 years, in analogical way with the semi-diurnal and diurnal cycles of Cantabrian tides. Those cycles were soon linked to sun activity and, at the end of the century, the latter was related to the Solar System evolution. We know now that waves and storm surges are coupled and that (Dynamic) Maritime Climate forms part of a more complex “Thermal Machine” including Hydrological cycle. The analysis of coastal floods could so facilitate the extension of that experience. According to their immediate cause, simple flood are usually sorted out into flash, pluvial, fluvial, groundwater and coastal types, considering the last as caused by sea waters. But the fact is that most of coastal floods are the result of the concomitance of several former simple types. Actually, the several Southeastern Mediterranean coastal flood events show to be the result of the superposition within the coastal zone of flash, fluvial, pluvial and groundwater flood types under boundary condition imposed by the concomitant storm sea level rise. This work shall be regarded as an attempt to clarify that cyclic experience, through an in-depth review of a past flood events in Valencia (Turia and Júcar basins), as in Murcia (Segura’s) as well.

Understanding extreme Spanish coastal flood events

The Santa Irene flood event, at the end of October 1982, is one of the most dramatically widely reported flood events in Spain. Its renown is mainly due to the collapse of the Tous dam, but its main message is to be the paradigm of the incidence of the maritime/littoral weather and its temporal sea level rise by storm surge accompanying rain process on the coastal plains inland floods. Looking at damages the presentation analyzes the adapted measures from the point of view of the aims of the FP7 SMARTeST Project related to the Flood Resilience improvement in urban areas through looking for Technologies, Systems and Tools an appropriate "road to de market".

Caracterización multivariada de los temporales para su aplicación en el dimensionamiento del manto principal

En esta tesis se presenta una metodología para la caracterización del oleaje, dentro del marco de las nuevas Recomendaciones para Obras Marítimas (ROM 0.0.-00 y ROM 1.0-09), por ser una de las principales acciones que afectan a la estabilidad de las estructuras marítimas. Debido al carácter aleatorio intrínsecamente multivariado de la acción considerada, las tormentas, su caracterización paramétrica se realiza en términos de funciones cópula uniparamétricas. Las variables consideradas son altura de ola significante del pico de la tormenta, el periodo medio asociado y la magnitud, o número de olas, de todo el ciclo de solicitación. Para establecer un patrón teórico de evolución de la tormenta que permita extrapolar las muestras fuera de la región con datos se analizan los modelos teóricos existentes, comprobándose que no reproducen adecuadamente las tormentas constituidas por estados de mar con un peso importante de oleaje swell. Para evitar esta limitación se proponen cuatro modelos teóricos de evolución de tormentas con distintas formas geométricas. El análisis de los modelos existentes y los propuestos pone de relieve que el Modelo Magnitud Equivalente de Tormenta (EMS= Equivalent Magnitude Storm) con la forma triangular es el que mejor adapta las tormentas constituidas por estados de mar típicos del viento. Para tormentas con un mayor grado de desarrollo, el modelo teórico de tormenta EMS con la forma trapezoidal es el adecuado. De las aproximaciones propuestas para establecer el periodo medio de los sucesivos estados de mar del ciclo de solicitación. la propuesta por Martín Soldevilla et al., (2009) es la más versátil y , en general , mejor reproduce la evolución de todo tipo de tormentas. La caracterización de las tormentas se complementa con la altura de ola máxima. Debido a la mayor disponibilidad y longitud temporal de los datos sintéticos frente a las registros, la práctica totalidad de los análisis de extremos se realizan con tormentas sintéticas en las que la distribución de olas individuales es desconocida. Para evitar esta limitación se utilizan modelos teóricos de distribución de olas acordes a las características de cada uno de los estados de mar que conforman la tormenta sintética. Para establecer dichas características se utiliza la curtosis y en función de su valor la altura de ola máxima se determina asumiendo una determinada distribución de olas. Para estados de mar lineales la distribución de olas individuales de Rayleigh es la considerada. Para condiciones no lineales de gran ancho de banda el modelo de distribución de olas propuesto por Dawson, (2004) es el utilizado y si es de banda estrecha las predicciones de (Boccotti, (1989), Boccotti et al., (2013)) se compara con las resultantes del modelo de Dawson. La caracterización de la evolución de las tormentas en términos multivariados es aplicada al estudio de la progresión del daño del manto principal de diques en talud, y al rebase de las olas. Ambos aspectos cubren el segundo objetivo de la tesis en el que se propone una nueva formulación para el dimensionamiento de mantos constituidos por bloques cúbicos de hormigón. Para el desarrollo de esta nueva formulación se han utilizado los resultados recogidos en los estudios de estabilidad del manto principal de diques talud realizados en modelo físico a escala reducida en el Centro de Estudios de Puertos y Costas (CEDEX) desde la década de los 80 empleando, en su mayoría, bloques paralelepípedos cúbicos de hormigón. Por este motivo y porque los últimos diques construidos en la costa Española utilizan este tipo de pieza, es por lo que la formulación planteada se centra en este tipo de pieza. Después de un primer análisis de las fórmulas de cálculo y de evolución existentes, se llega a la conclusión de que es necesario realizar un esfuerzo de investigación en este campo, así como ensayos en laboratorio y recogida de datos in-situ con base a desarrollar fórmulas de evolución de daño para mantos constituidos por piezas diferentes a la escollera, que tenga en cuenta las principales variables que condiciona su estabilidad. En esta parte de la tesis se propone un método de análisis de evolución de daño, que incluye el criterio de inicio de avería, adecuada para diques en talud constituidos por bloque cúbicos de hormigón y que considera la incidencia oblicua, el daño acumulado y el rebase. This thesis proposes a methodology to estimate sea waves, one of the main actions affecting the maritime structures stability, complying with (ROM 0.0.-00 & ROM 1.0-09.Due to the multivariate behavior of sea storms, the characterization of the structures of sea storms is done using copula function. The analyzed variables are the significant height wave, mean period and magnitude or number of waves during the storm history. The storm evolution in terms of the significant height wave and the mean period is also studied in other to analyze the progressive failure modes. The existing models of evolution are studied, verifying that these approximations do not adjust accurately for developed waves. To overcome this disadvantage, four evolution models are proposed, with some geometrical shapes associated to fit any development degree. The proposed Equivalent Magnitude Storm model, EMS, generally obtains the best results for any kind of storm (predominant sea, swell or both). The triangle is recommended for typical sea storms whereas the trapezoid shape is much more appropriate for more developed storm conditions.The Martín Soldevilla et al., (2009) approach to estimate the mean period is better than others approaches used.The storm characterization is completed with the maximum wave height of the whole storm history. Due to synthetic historical waves databases are more accessible and longer than recorded database, the extreme analyses are done with synthetic data. For this reason the individual waves’ distribution is not known. For that limitation to be avoided, and depending on the characteristics of every sea states, one theoretical model of waves is choose and used. The kurtosis parameter is used to distinguish between linear and nonlinear sea states. The Rayleigh model is used for the linear sea states. For the nonlinear sea states, Dawson, (2004) approach is used for non-narrow bandwidth storms, comparing the results with the Boccotti, (1989), Boccotti et al., (2013) approach, with is used for narrow bandwidth storms. The multivariate and storm evolution characterization is used to analyze of stone armour damage progression and wave overtopping discharge. Both aspects are included in the second part of the thesis, with a new formula is proposed to design cubes armour layer. The results the stability studies of armour layer, done in the Centre for Harbours and Coastal Studies (CEDEX) laboratory are used for defining a new stability formula. For this reason and because the last biggest breakwater built in Spain using the cube, the damage progression is analyze for this kind of concrete block. Before to analyze the existing formulae, it is concluded that it is necessary more investigation, more tests in laboratory and data gathering in situ to define damage evolution formulae to armour of other kind of pieces and that takes to account the principal variables. This thesis proposed a method to calculate the damage progression including oblique waves, accumulated damage, and overtopping effect. The method also takes account the beginning of the movement of the blocks.

Estudio dinámico de cimentaciones en obras portuarias

Los diques de abrigo verticales son estructuras monolíticas que sirven para reflejar el oleaje creando, de este modo, una zona abrigada en el lado tierra. Son estructuras de contención de gravedad, es decir, su peso es el elemento resistente fundamental. Las solicitaciones sobre estas estructuras son de tipo dinámico (oleaje) y consisten, fundamentalmente, en un empuje frontal y una subpresión en la base que varían en el tiempo. Habitualmente, las acciones sobre los diques de abrigo se establecen mediante fórmulas empíricas, que se describen en la presente tesis si bien, para obras de especial importancia, suelen medirse en modelos reducidos de laboratorio. Cuando el cimiento en que han de apoyarse estas estructuras no presenta una resistencia al corte suficiente, el apoyo se realiza en banquetas que redistribuyen la carga y que están formadas por materiales granulares. En la práctica habitual para conocer la estabilidad de estas estructuras frente a un temporal, tras establecer las acciones de cálculo (empuje frontal y subpresión) se efectúa un cálculo pseudoestático en el que se consideran condiciones drenadas o no drenadas del cimiento en función de su permeabilidad. Se conoce que en los suelos saturados, bajo cargas cíclicas, tiende a producirse una elevación de las presiones intersticiales y una reducción de tensiones efectivas así como una degradación del módulo de deformación tangencial en función del número de ciclos de carga, pudiéndose producir el fenómeno conocido como licuefacción (arenas) o movilidad cíclica (suelos más finos). El objeto de la tesis es explorar la posibilidad, con la tecnología actual, de analizar la estabilidad de los diques de abrigo verticales en cuyo cimiento existen suelos blandos, proponiendo un procedimiento para evaluar la estabilidad dinámica en este tipo de obras. Para ello se han revisado los procedimientos actualmente utilizados para definir las acciones de cálculo, los principales modelos de comportamiento dinámicos de suelos saturados disponibles y los procedimientos de cálculo. Una vez investigado el estado del arte sobre este tema, se propone un procedimiento de cálculo en el que, utilizando el programa comercial FLAC, se establecen las acciones cíclicas sobre un dique de abrigo vertical tipo con distintas condiciones de apoyo, aplicando, para el cimiento, un modelo de comportamiento tipo hiperbólico con generación de presiones intersticiales cuyos parámetros pueden obtenerse de ensayos de campo y laboratorio. Por último, una vez descrito el procedimiento, se aplica a un caso real en el que se produjo un fallo en la cimentación que desembocó en el hundimiento de parte de un dique vertical situado en el puerto de Barcelona, presentándose los resultados obtenidos del análisis efectuado y comparándolos con los obtenidos utilizando los métodos de cálculo habituales. Vertical breakwaters are monolithic structures built to reflect sea waves, thereby providing a sheltered area on the land side. They are gravity retaining structures, that is, their own weight is their basic resisting mechanism. Loads acting on these structures are dynamic (waves) and consist essentially in a frontal thrust and an uplift pressure on the base, which both vary over time. Usually, actions in breakwater design are established by empirical formulas, which are described in this thesis. For works of particular importance, such forces are measured in small-scale laboratory tests. When there are no soils with enough shear strength under the planned vertical breakwaters, they usually rest on granular berms which redistribute the load. Nowadays, after establishing the acting forces on the breakwater (front push and uplift pressure), a pseudostatic calculation (with drained or undrained conditions depending on the foundation permeability) is normally done to analyze the stability of these structures against storm waves. It is known that pore pressures tend to rise in saturated soils under cyclic loading and, consequently, there is a reduction of effective stress. A degradation of the shear modulus also occurs depending on the number of load cycles. All of these effects can bring about the phenomenon known as liquefaction in sands or cyclic mobility in fine-grained soils. The aim of the thesis is to explore the possibility that current technology provides to analyze the stability of vertical breakwaters founded on soft soils, and to suggest a method to evaluate the dynamic stability in this type of works. To this end, a review has been made of procedures currently used to define the actions in calculations, the main models of dynamic behaviour of saturated soils available and of calculation procedures. Once the state of the art on this subject has been reviewed, a method of calculation is proposed that uses the commercial program FLAC and is applied to a typical vertical breakwater on a range of different foundation conditions. For the foundation soil, a hyperbolic constitutive model with pore pressure generation has been employed, whose parameters can be obtained from field and laboratory tests. Finally, the described procedure is applied to an actual case where a foundation failure occurred that led to the sinking of several caissons in a vertical breakwater located in the port of Barcelona. The results obtained with the proposed method are compared with those obtained using conventional methods.

Appropiate locations for geotextile bag revetments: an analysis

Social pressure exerted by urban development, the increase in erosion on many coastal stretches, and the rise in sea level due to climate change over the last few decades have led governments to increase investment in coastal protection. In turn, a reduction in costs and increases in ease of construction and rate of implementation have led to sand-filled geotextile elements, such as bags, tubes, and containers, becoming an alternative or supplement to traditional coastal defence materials, such as rubble mounds, concrete, and so on. Not all coastal zones are appropriate for sand-filled geotextile structures as coastal defences. This article analyses suitable zones for locating geotextile bag revetments to protect coasts from storm erosion and concludes that the least suitable zones are the surf zone (on an open coast and on a slightly protected coast) and deep water (on an open coast), except if a suitable reinforcement is carried out when the demand makes it necessary this build this kind of defence.