Failure Mechanisms and Local Scour at Coastal Structures induced by Tsunamis

On March 11 2011, an exceptionally large tsunami event was triggered by a massive earthquake offshore, the northeast coast of Japan, which affected coastal infrastructure such as seawalls, coastal dikes and breakwaters in the Tohoku region. Such infrastructure was built to protect against the Level 1 tsunamis that previously hit the region, but not for events as significant as the 2011 Tohoku tsunami, which was categorized as a Level 2 tsunami [Shibayama et al. 2013]. The failure mechanisms of concrete-armoured dikes, breakwaters and seawalls due to Level 2 tsunamis are still not fully understood by researchers and engineers. This paper investigates the failure modes and mechanisms of damaged coastal structures in Miyagi and Fukushima Prefectures, following the authors' post-disaster field surveys carried out between 2011 and 2013. Six significant failure mechanisms were identified for the coastal dikes and seawalls affected by this tsunami: 1) Leeward toe scour failure, 2) Crown armour failure, 3) Leeward slope armour failure, 4) Seaward toe and armour failure, 5) Overturning failure, and 6) Parapet wall failure, in which leeward toe scour being recognized as the major failure mechanism in most surveyed locations. The authors also propose a simple practical mathematical model for predicting the scour depth at the leeward toe of the coastal dikes, by considering the effects of the tsunami hydrodynamics, the soil properties and the type of structure. The key advantage of this model is that it depends entirely on quantities that are measurable in the field. Furthermore this model was further refined by conducting a series of hydraulic model experiments aimed to understand the governing factors of the leeward toe scour failure. Finally, based on the results obtained, key recommendations are given for the design of resilient coastal defence structures that can survive a level 2 tsunami event.

Two fluid numerical model for coastal applications

Wave breaking is an important coastal process, influencing hydro-morphodynamic processes such as turbulence generation and wave energy dissipation, run-up on the beach and overtopping of coastal defence structures. During breaking, waves are complex mixtures of air and water (“white water”) whose properties affect velocity and pressure fields in the vicinity of the free surface and, depending on the breaker characteristics, different mechanisms for air entrainment are usually observed. Several laboratory experiments have been performed to investigate the role of air bubbles in the wave breaking process (Chanson & Cummings, 1994, among others) and in wave loading on vertical wall (Oumeraci et al., 2001; Peregrine et al., 2006, among others), showing that the air phase is not negligible since the turbulent energy dissipation involves air-water mixture. The recent advancement of numerical models has given valuable insights in the knowledge of wave transformation and interaction with coastal structures. Among these models, some solve the RANS equations coupled with a free-surface tracking algorithm and describe velocity, pressure, turbulence and vorticity fields (Lara et al. 2006 a-b, Clementi et al., 2007). The single-phase numerical model, in which the constitutive equations are solved only for the liquid phase, neglects effects induced by air movement and trapped air bubbles in water. Numerical approximations at the free surface may induce errors in predicting breaking point and wave height and moreover, entrapped air bubbles and water splash in air are not properly represented. The aim of the present thesis is to develop a new two-phase model called COBRAS2 (stands for Cornell Breaking waves And Structures 2 phases), that is the enhancement of the single-phase code COBRAS0, originally developed at Cornell University (Lin & Liu, 1998). In the first part of the work, both fluids are considered as incompressible, while the second part will treat air compressibility modelling. The mathematical formulation and the numerical resolution of the governing equations of COBRAS2 are derived and some model-experiment comparisons are shown. In particular, validation tests are performed in order to prove model stability and accuracy. The simulation of the rising of a large air bubble in an otherwise quiescent water pool reveals the model capability to reproduce the process physics in a realistic way. Analytical solutions for stationary and internal waves are compared with corresponding numerical results, in order to test processes involving wide range of density difference. Waves induced by dam-break in different scenarios (on dry and wet beds, as well as on a ramp) are studied, focusing on the role of air as the medium in which the water wave propagates and on the numerical representation of bubble dynamics. Simulations of solitary and regular waves, characterized by both spilling and plunging breakers, are analyzed with comparisons with experimental data and other numerical model in order to investigate air influence on wave breaking mechanisms and underline model capability and accuracy. Finally, modelling of air compressibility is included in the new developed model and is validated, revealing an accurate reproduction of processes. Some preliminary tests on wave impact on vertical walls are performed: since air flow modelling allows to have a more realistic reproduction of breaking wave propagation, the dependence of wave breaker shapes and aeration characteristics on impact pressure values is studied and, on the basis of a qualitative comparison with experimental observations, the numerical simulations achieve good results.

Determining discharge-coefficient ratings for selected coastal control structures in Broward and Palm Beach counties, Florida /

Shipping list no.: 99-0001-P.

Do man-made habitats affect spatial patterns of mollusk abundance?

XVIII Simposio Ibérico de Estudios de Biología Marina (SIEBM), Gijón (Asturias), 2 al 5 de septiembre de 2014.

Crustaceans and fish abundances and species at and around artificially introduced tetrapod fields in the southern North Sea, 2010-2011

In the coming decades, artificial defence structures will increase in importance worldwide for the protection of coasts against the impacts of global warming. However, the ecological effects of such structures on the natural surroundings remain unclear. We investigated the impact of experimentally introduced tetrapod fields on the demersal fish community in a hard-bottom area in the southern North Sea. The results indicated a significant decrease in fish abundance in the surrounding area caused by migration effects towards the artificial structures. Diversity (HB) and evenness (E) values exhibited greater variation after the introduction of the tetrapods. Additionally, a distinct increase in young-of-the-year (YOY) fish was observed near the structures within the second year after introduction. We suggest that the availability of adequate refuges in combination with additional food resources provided by the artificial structures has a highly species-specific attraction effect. However, these findings also demonstrate that our knowledge regarding the impact of artificial structures on temperate fish communities is still too limited to truly understand the ecological processes that are initiated by the introduction of artificial structures. Long-term investigations and additional experimental in situ work worldwide will be indispensable for a full understanding of the mechanisms by which coastal defence structures interact with the coastal environment.

Analysis and development of ecologically based approaches to coastal defense

Climate-change related impacts, notably coastal erosion, inundation and flooding from sea level rise and storms, will increase in the coming decades enhancing the risks for coastal populations. Further recourse to coastal armoring and other engineered defenses to address risk reduction will exacerbate threats to coastal ecosystems. Alternatively, protection services provided by healthy ecosystems is emerging as a key element in climate adaptation and disaster risk management. I examined two distinct approaches to coastal defense on the base of their ecological and ecosystem conservation values. First, I analyzed the role of coastal ecosystems in providing services for hazard risk reduction. The value in wave attenuation of coral reefs was quantitatively demonstrated using a meta-analysis approach. Results indicate that coral reefs can provide wave attenuation comparable to hard engineering artificial defenses and at lower costs. Conservation and restoration of existing coral reefs are cost-effective management options for disaster risk reduction. Second, I evaluated the possibility to enhance the ecological value of artificial coastal defense structures (CDS) as habitats for marine communities. I documented the suitability of CDS to support native, ecologically relevant, habitat-forming canopy algae exploring the feasibility of enhancing CDS ecological value by promoting the growth of desired species. Juveniles of Cystoseira barbata can be successfully transplanted at both natural and artificial habitats and not affected by lack of surrounding adult algal individuals nor by substratum orientation. Transplantation success was limited by biotic disturbance from macrograzers on CDS compared to natural habitats. Future work should explore the reasons behind the different ecological functioning of artificial and natural habitats unraveling the factors and mechanisms that cause it. The comprehension of the functioning of systems associated with artificial habitats is the key to allow environmental managers to identify proper mitigation options and to forecast the impact of alternative coastal development plans.

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.

Lugares adecuados para la utilización de nuevos materiales y formas con geotextiles en la gestión integral de la zona litoral como protección ambiental

La erosión costera es un fenómeno que ha cobrado importancia durante los últimos años, debido a sus repercusiones tanto en el turismo, como en el incremento de riesgo para las infraestructuras y para la población allí ubicada. Se define como el retroceso progresivo de la línea de costa cuyas causas pueden ser de origen natural o antropogénicas. La costa merece la máxima protección, y su gestión debe asegurar su integridad física y su libre acceso, al igual que un uso público por parte de todos. En este horizonte, la reducción de costes, la facilidad de construcción y velocidad de realización, las obras de geotextil se han mostrado como una alternativa muy seria a las obras duras u obras clásicas, tradicionalmente realizadas a lo largo de las costas. Pero esta alternativa de uso conlleva un mayor conocimiento en el comportamiento de éstas, su diseño y durabilidad. Las obras de geotextil, como su nombre indica, se realizan con elementos tridimensionales de geotextil (cosidos o no cosidos) rellenos de arena. Estos elementos se pueden considerar innovadores, económicos, ecológicos y alternativos frente a los tradicionales. Se distinguen tres tipos denominados: sacos, tubos y contenedores. El principal objetivo de esta tesis se centra en la determinación de las zonas de la costa apropiados para la utilización de estructuras compuestas de elementos de geotextil rellenos de arena, como obras de defensa de costas, con el fin de facilitar su uso en la ingeniería de costas. Para ello se ha clasificado tanto las distintas zonas del perfil longitudinal de una playa, como los tipos de costa en función de la variable altura de ola significante. Se han determinado las limitaciones de esta variable en las distintas fórmulas de estabilidad de estas estructuras. Y finalmente, en función de la máxima limitación de altura de ola significante y de sus posibles valores en las zonas y tipos de costa, se han determinado las zonas más apropiadas para el uso de las estructuras compuestas por elementos de geotextil. Finalmente se han redactado las conclusiones de la investigación y se han propuesto nuevas líneas de investigación relacionadas con esta Tesis Doctoral. Coastal erosion is a phenomenon that has become more important in recent years because of its impact on both on tourism, and increased risk to infrastructure and the population located there. It is defined as the gradual decline in the coastline whose causes may be natural or anthropogenic origin. The coast deserves maximum protection, and management should ensure their physical integrity and their free access, as well as public use by all. In this line, cost reduction, ease of construction and completion rate, geotextile works have been shown as a very serious to hard works or classics alternative, traditionally performed along the coasts. But this alternative use entails greater insight into the behaviour of these, design and durability. Works of geotextile, as its name suggests, are made with three-dimensional elements of geotextile (sewn or stitched) filled with sand. These items can be considered innovative, economic, and ecological alternative compared to traditional. Bags, tubes and containers, three known types are distinguished. The main objective of this thesis focuses on determining the appropriate areas of the coast for the use of composite structures of elements filled geotextile sand as coastal defence works, in order to facilitate their use, in coastal engineering. For it, has been classified by both the different areas of the longitudinal profile of a beach as the shoreline types depending on the variable significant wave height. We have determined the limitations of this variable in the different formulas stability of these structures. And finally, depending on the maximum limitation of significant wave height and its possible values in the areas and types of coastline, have determined the most appropriate for use in structures composed of elements of geotextile areas. Finally, the conclusions of the research have been addressed and the proposal of new lines of work related to the topic has been made.

Ocean sprawl: challenges and opportunities for biodiversity management in a changing world.

The last few decades have seen rapid proliferation of hard artificial structures (e.g., energy infra-structure, aquaculture, coastal defences) in the marine environment: ocean sprawl. The replacement of natural, often sedimentary, substrata with hard substrata has altered the distribution of species, particularly non-indigenous species, and can facilitate the assisted migration of native species at risk from climate change. This has been likened to urbanization as a driver of global biotic homogenization in the marine environment—the process by which species invasions and extinctions increase the genetic, taxonomic, or functional similarity of communities at local, regional, and global scales. Ecological engineering research showed that small-scale engineering interventions can have a significant positive effect on the biodiversity of artificial structures, promoting more diverse and resilient communities on local scales. This knowledge can be applied to the design of multifunctional structures that provide a range of ecosystem services. In coastal regions, hybrid designs can work with nature to combine hard and soft approaches to coastal defence in a more environmentally sensitive manner. The challenge now is to manage ocean sprawl with the dual goal of supporting human populations and activities, simultaneously strengthening ecosystem resilience using an ecosystem- based approach.

Timing ótimo para investir na proteção costeira na zona centro de Portugal

A presente dissertação insere-se no âmbito da gestão das zonas costeiras, que na atualidade enfrentam um grave problema de erosão. Este torna-se um problema social com especial importância devido à concentração populacional e atividade económica que se verifica nas regiões litorais. Neste contexto muitos têm sido os estudos físicos e económicos levados a cabo no sentido de encontrar soluções para lidar com este problema. O principal objetivo desta dissertação foi avaliar qual a importância que o timing em que é efetuado o investimento na proteção pode ter na defesa das zonas costeiras. Por outras palavras, perceber se existem ou não vantagens em adiar o investimento na proteção por um determinado período de tempo. Foi definida como área de estudo o trecho costeiro de 20 km compreendido entre a Praia da Barra (Aveiro) e a Praia de Mira (Coimbra). Através da utilização do modelo numérico Long-Term Configuration (LTC) foi simulada a evolução da linha de costa por um período de 80 anos, seguida de uma análise custo-benefício ambiental em que foi considerada a possibilidade de investimentos em estruturas de defesa (no caso desta dissertação esporões com 100m, 200m e 300m de comprimento) a serem efetuados após 0, 10, 20 ou 30 anos. Foram também contempladas restrições orçamentais, e ainda, efetuada uma análise de sensibilidade à taxa de desconto utilizada. Os resultados mostram que quanto mais cedo se iniciar proteção das zonas costeiras, mais benefícios económicos serão retirados desse investimento. Os cenários que se apresentaram como os mais vantajosos são aqueles em que o investimento é levado a cabo no Ano 0 e onde se opta por uma proteção integral ou quase integral da linha de costa. Ainda assim, em todos os timings de investimento analisados foi possível encontrar cenários economicamente viáveis e dos quais resultariam benefícios económicos. A análise com inclusão de restrições orçamentais mostra que a utilização destas deixa de fazer sentido à medida que o investimento é adiado, funcionando então o próprio retardar do investimento como uma medida de contenção orçamental. A análise de sensibilidade à taxa de desconto revelou que a opção por uma taxa de desconto de 4% acaba por não influenciar de maneira significativa os resultados obtidos.

A national coastal erosion risk assessment for Scotland

The geography of Scotland, with a highly undulating hinterland, long and indented coastline, together with a large number of islands, means that much social and economic activity is largely located at the coast. The importance of the coast is further highlighted by the large number of ecosystem services derived from the coast. The threat posed by climate change, particularly current and future sea level rise, is of considerable concern and the associated coastal erosion and coastal flooding has the potential to have a substantial effect on the socioeconomic activity of the whole country. Currently, the knowledge base of coastal erosion is poor, which serves to hinder the current and future management of the coast. This research reported here aimed to establish four key aspects of coastal erosion within Scotland: the physical susceptibility of the coast to erosion; the assets exposed to coastal erosion; the vulnerability of communities to coastal erosion; and the coastal erosion risk to those communities. Coastal erosion susceptibility was modelled here within a GIS, using data for ground elevation, rockhead elevation, wave exposure and proximity to the open coast. Combining these data produced the Underlying Physical Susceptibility Model (UPSM), in the form of a 50 m2 raster of national coverage. The Coastal Erosion Susceptibility Model (CESM) was produced with the addition of sediment supply and coastal defence data, which then moderates the outputs of the UPSM. Asset data for dwellings, key assets, transport infrastructure, historic assets, and natural assets were used along with the UPSM and CESM to assess their degree of exposure to coastal erosion. A Coastal Erosion Vulnerability Model (CEVM) was produced using Experian Mosaic Scotland (a geodemographic classification which identifies 44 different social groups within Scotland) to classify populations based upon 11 vulnerability variables. Dwellings were assigned a CESM and CEVM score in order to establish their coastal erosion risk. This research demonstrated that the issue of coastal erosion will impact on a relatively low number of properties compared to those impacted by flooding (both coastal and fluvial) as many dwellings are already protected by coastal defences. There is therefore, a considerable future liability, and great pressure for coastal defences to be maintained and upgraded in their current form. The use of the CEVM is a novel inclusion within a coastal erosion assessment for Scotland. Use of the CEVM established that coastal erosion risk is not distributed equally amongst the Scottish coastal population and highlighted that risk can be reduced by either reducing exposure or reducing vulnerability. Thus far in Scotland, reducing exposure has been the primary management approach, which has a number of implications with regards social justice. This research identified the existing data gaps that should be addressed by future research in order to further improve coastal management in Scotland. Future research should focus on assessing historical coastal change rates on a national scale, improve modelling of national scale wave exposure, enhance the information held about current coastal defences and, determine the direct and indirect economic cost associated with the loss of different asset types. It is also necessary to clarify the social justice implications of using adaptation approaches to manage coastal erosion as well as establishing a method to communicate the susceptibility, exposure, vulnerability and risk aspects whilst minimising the potential negative impacts (e.g. property blight) of releasing such information.

Performance Optimization of a Floating Breakwater Model Using SPH Method, with a Practical Application

Máster en Oceanografía

Struttura dei popolamenti meiobentonici nella zona intertidale dell'Alto Adriatico a differenti gradi di antropizzazione

The present work is part of the European project THESEUS (Innovative technologies for safer Europeans coasts in a changing climate). The main goals are to provide adequate integrated methodologies for strategic planning of sustainable coastal defence. The present study investigates the structure and composition of meiobenthonic populations of the intertidal zone in four beaches along the Northern Adriatic coast of Emilia Romagna: Lido di Spina, Bellocchio, Lido di Dante e Cervia. The four sites are different for the level of human impacts and for the different management interventions against coastal erosion. The analysis of biotic and abiotic variables revealed different responses due mainly to site-specific characteristics of the investigated sites, in particular as regards the site of Bellocchio. The growing interest in ecosystems of sandy beaches has recently highlighted the importance of the ecological role of meiofauna, emphasizing the need to develop studies aimed to conservation as well as to the use of these organisms as descriptors of the environmental status. The present study showed that the response of the organisms of meiofauna was highly sensitive to the specific environmental conditions of the four sites considered. Therefore it appears to be possible to consider the response of meiofauna to environmental and anthropogenic stressors as supplementary information to the responses of macrobenthic communities, which have been, until now, widely recognized and used as syncretic indicators of the ecosystem status.

Practical case with regard to applying the functional and environmental method to low crest level, detached breakwaters

The research work as presented in this article covers the design of detached breakwaters since they constitute a type of coastal defence work with which to combat many of the erosion problems found on beaches in a stable, sustainable fashion. The main aim of this work is to formulate a functional and environmental (but not structural) design method, enabling the fundamental characteristics of a detached breakwater to be defined as a function of the effect it is wished to induce on the coast, and taking into account variables of a different nature (climate, geomorphology and geometry) influencing the changes the shoreline undergoes after its construction. With this article, it is intended to submit the final result of the investigation undertaken, applying the detached breakwater design method as developed to solving a practical case. Thus it may be shown how the method enables a detached breakwater’s geometric pre-sizing to be tackled at a place on the coast with certain climate, geomorphology and littoral dynamic characteristics, first setting the final state of equilibrium it is wanted to obtain therein after its construction.

Regeneración de las playas y rehabilitación de los canales de desagüe de Beira (Mozambique)

The “Basic Infrastructure for Development and Sustainability” Cooperation Group of the Universidad Polítecnica de Madrid has developed a project in the city of Beira, Mozambique, financed by the Spanish International Development Cooperation Agency, to mitigate the consequences associated with climate change, which together with the city’s location and the lack of suitable maintenance in the area, have left Beira more exposed to flooding and coastal erosion. In order to provide a solution to these problems, consideration has been given to the renovation of coastal defence infrastructure and the system of stormwater drainage channels.