Testing water pollution in a two layer aquifer

Water bodies around urban areas may be polluted with chemical elements from urban or industrial activities. We study the case of underground water pollution. This is a serious problem, since under- ground water is high qualified drinkable water in a world where this natural resource is increasingly reduced. This study is focused on a two-layer aquifer. If the superficial layer is contaminated, the deeper layer could be spoiled as well. This contribution checks the equality of the mean or centered composition of the two layers, with the aim of inferring their possible hydraulic conectivity. The data to be examined are different hydro-chemical components of water, such as nitrates and nitrites (related to nitrate/nitrite poisoning of animal stock), tensoactives (toxic to the ecosystem) or potassium (it promotes eutrophization of the water), represented in mg/l. As the data are compositional, we can group the pertinent elements and compare them applying ilr transformation. The ilr transformation is used for simplicity when comparing compositional vectors. MANOVA (Multivariate Analysis of Variance) is applied on the transformed data from the two layers. This provides a hypothesis test to discern whether the two aquifer layers can be considered a homogeneous continuum or, on the contrary, they should be considered as isolated layers. An illustrative example is presented. Used data sets, being synthetic, are inspired by a real case. These analyses suggest that the two aquifers are connected.Postprint (published version

Testing water pollution in a two layer aquifer

Water bodies around urban areas may be polluted with chemical elements from urban or industrial activities. We study the case of underground water pollution. This is a serious problem, since underground water is high qualified drinkable water in a world where this natural resource is increasingly reduced. This study is focused on a two-layer aquifer. If the superficial layer is contaminated, the deeper layer could be spoiled as well. This contribution checks the equality of the mean or centered composition of the two layers, with the aim of inferring their possible hydraulic conectivity. The data to be examined are different hydro-chemical components of water, such as nitrates and nitrites (related to nitrate/nitrite poisoning of animal stock), tensoactives (toxic to the ecosystem) or potassium (it promotes eutrophization of the water), represented in mg/l. As the data are compositional, we can group the pertinent elements and compare them applying ilr transformation. The ilr transformation is used for simplicity when comparing compositional vectors. MANOVA (Multivariate Analysis of Variance) is applied on the transformed data from the two layers. This provides a hypothesis test to discern whether the two aquifer layers can be considered a homogeneous continuum or, on the contrary, they should be considered as isolated layers. An illustrative example is presented. Used data sets, being synthetic, are inspired by a real case. These analyses suggest that the two aquifers are connected

Multivariate characterization of wave storms in coastal areas

Tesi per compendi de publicacions, amb diferents seccions retallades pels drets d'editorPremi extraordinari doctorat UPC curs 2017-2018. Àmbit d’Enginyeria Civil i AmbientalWave-storms are the responsible of the main changes in the Coast. Their detailed characterization results in a better design of any marine structure. The most common approach to describe wave-storms is to simplify the event by taking the significant wave-height (Hp), peak period (Tp) and direction (0p) at the peak of the storm and treating these variables independently. However, it is well accepted that some relationship should exist between them. What is more, the development of sophisticated numerical models in Coastal Engineering are demanding additional variables such as the duration of the wave-storm (D), the amount of associated energy (E), temporary evolution of the variables and their relationship to atmospheric climate-indices, to better reproduce the simulated processes. The main objectives and results of this thesis are as follows. First, wave-storms in the present wave climate of the Catalan Coast are characterized, assuming stationarity. The wave-storm variables modelled are: the energy at the peak of the wave-storm (Eu ), Tp , E, D, 0p and the proportions of time from the start to the storm peak and from the storm peak to the end (growth-decay rates). E, Eu , Tp and D are fit by generalized Pareto distributions (GPD). Their joint probability structure is characterized by a hierarchical Archimedean copula (HAC). 0p is characterized through a mixture of von Mises-Fisher probability distribution functions and related to E, T p and D through a multinomial logistic model. The growth-decay rates are related to D through third degree polynomials. A triangle and an irregular-trapezoid are proposed to model the wave-storm shape. In the present climate of the Catalan Coast, the constructed statistical model can serve to generate synthetic wave-storms. The most predominant 0p are north and east. The most appropriate geometric figure to describe the evolution of the wave-height is a irregular-trapezoid. For D over 100h, the peak of the wave-storm is generally closer to the end of the wave-storm than to the start. After establishing a stationary model, non-stationarity is incorporated into the characterization of wave-storms in the Catalan Coast. E, Hp , T p and D are characterized through non-stationary GPDs. The wave-storm threshold, the wave-storminess and the parameters of the GPDs are related to North Atlantic Oscillation (NAO), East Atlantic pattern (EA) and Scandinavian pattern (SC) and their first two time-derivatives, through Vectorial generalized additive models. The joint probability structure is characterized by a pseudo-time-dependent HAC. A severe greenhouse gas emission scenario is considered. The mean values of all wave-storm variables decrease in the 21st century, except for D in the northern part of the coast. A negative NAO may cause an increase in wave-storminess; the wave-storm threshold and the GPD parameters are most influenced by the dynamics of the climate-patterns, rather than by the climate-patterns themselves. The non-stationary methodology is repeated in the northwestern Black Sea, considering both a mild and a severe emission scenarios. Here, wave-storminess is not affected by the proposed climate-patterns, whereas the wave-storm threshold is strongly influenced by SC and EA. The average value of the wave-storm variables seem to have a more positive trend than in the Catalan Coast, and it is observed that an increase in mean values is related to an increase in variance. SC and EA also strongly influence the parameters of the GPDs. In the two study areas, the dependence between E and D is high, while the general dependence among the wave-storm variables is medium. In the Catalan Coast, it is expected that the dependence between E and D should increase with time. In the northwestern Black Sea, it is the dependence among all the wave-storm variables that increases with time, in both emission scenarios; the severe emission scenario presents less dependence among wave-storm variables.El enfoque más común para describir los temporales de mar es simplificar este suceso tomando la altura de ola significante (Hp), el periodo pico (Tp) y la dirección (0p) en el pico de la tormenta y tratándolas de forma independiente. Sin embargo, está ampliamente aceptado que exista al menos alguna relación entre ellas. Es más, el desarrollo de sofisticados modelos numéricos en la ingeniería de costas pide variables adicionales como la duración de tormenta (D), la cantidad de energía asociada (E), la evolución temporal de las variables y su relación con índices climáticos atmosféricos, todo para una mejor reproducción de los procesos simulados. Los objetivos y resultados principales de esta tesis son los siguientes: Primero, se caracteriza tormentas de mar en el clima de oleaje presente, de la costa catalana, suponiendo estacionalidad. Las variables modeladas son: la energía unitaria en el pico del temporal (Eu), Tp, E, D, 0p y la proporción de tiempo desde el inicio hasta el pico y desde el pico al final del temporal (ratios de crecimiento-decrecimiento). Se caracteriza E, E u, T p y D con distribuciones generalizadas de Pareto (GPD), y se caracteriza la estructura de probabilidades conjunta de estas variables vía una cópula jerárquica arquimedeana (HAC). Se caracteriza 0p con una combinación de distribución de probabilidad de von Mises-Fisher y se le relaciona con E, T p y D a través de un modelo logístico multinomial. Se propone una forma triangular o trapezoide-irregular para modelar la forma del temporal. En el clima presente de la costa catalana, el modelo estadístico construido puede generar temporales sintéticos. Las 0p principales son el norte y el este. La figura geométrica que mejor describe la evolución de la altura de ola es un trapezoide irregular. Para D mayor que 100h, el pico del temporal está generalmente más cerca del final que del principio. La media de cada variable decrece en el siglo XXI, excepto la de D, en el norte de la costa. Una NAO negativa puede causar una subida de la tormentosidad. Además, el umbral de tormenta y los parámetros de GPD están influenciados principalmente por la dinámica de los patrones climáticos, en vez de serlo por los propios patrones climáticos. Después de establecer un modelo estacionario, se incorpora la no estacionalidad a la caracterización de temporales de mar en la costa catalana. Se caracteriza E, Hp, Tp y D con GPDs no estacionarios. El umbral de temporal, la tormentosidad y los parámetros de los GPDs están relacionados con la Oscilación de Atlántico norte (NAO), el Patrón de Atántico oriental (EA) y el Patrón escandinavo (SC) y sus primeras dos derivadas temporales, a través de Modelos aditivos generalizados vectoriales. Se caracteriza la estructura de probabilidades conjunta con un HAC pseudo-dependiente del tiempo. Se considera un escenario grave de cambio climático. Se repite la metodología no estacionaria en el noroeste del Mar Negro, considerando tanto un escenario suave de cambio climático como otro grave. En el noroeste del Mar Negro, la tormentosidad de mar no está afectada por los patrones climáticos propuestos, todo y que el umbral de temporal está fuertemente influenciado por SC y EA. Los valores medios de las variables de temporal parecen tener una tendencia más positiva que en la costa catalana, y se observa que una subida de los valores medios se relaciona con otra subida de las varianzas. SC y EA afectan fuertemente a los parámetros de los GPDs. En las dos zonas de estudio, la dependencia entre E y D es alta, mientras que la dependencia general entre las variables de temporal es media. En la costa catalana, se espera que la dependencia entre E y D crezca con el tiempo. En el noroeste del Mar Negro, es la dependencia entre todas las variables de temporal la que crece con el tiempo, en ambos escenarios de cambio climático: el escenario grave presenta menos dependencia entre las variables.Award-winningPostprint (published version

The land–sea coastal border: a quantitative definition by considering the wind and wave conditions in a wave-dominated, micro-tidal environment

A quantitative definition for the land–sea (coastal) transitional area is proposed here for wave-driven areas, based on the variability and isotropy of met-ocean processes. Wind velocity and significant wave height fields are examined for geostatistical anisotropy along four cross-shore transects on the Catalan coast (north-western Mediterranean), illustrating a case of significant changes along the shelf. The variation in the geostatistical anisotropy as a function of distance from the coast and water depth has been analysed through heat maps and scatter plots. The results show how the anisotropy of wind velocity and significant wave height decrease towards the offshore region, suggesting an objective definition for the coastal fringe width. The more viable estimator turns out to be the distance at which the significant wave height anisotropy is equal to the 90th percentile of variance in the anisotropies within a 100 km distance from the coast. Such a definition, when applied to the Spanish Mediterranean coast, determines a fringe width of 2–4 km. Regarding the probabilistic characterization, the inverse of wind velocity anisotropy can be fitted to a log-normal distribution function, while the significant wave height anisotropy can be fitted to a log-logistic distribution function. The joint probability structure of the two anisotropies can be best described by a Gaussian copula, where the dependence parameter denotes a mild to moderate dependence between both anisotropies, reflecting a certain decoupling between wind velocity and significant wave height near the coast. This wind–wave dependence remains stronger in the central baylike part of the study area, where the wave field is being more actively generated by the overlaying wind. Such a pattern controls the spatial variation in the coastal fringe width.Peer ReviewedPostprint (published version

Multivariate statistical modelling of future marine storms

Extreme events, such as wave-storms, need to be characterized for coastal infrastructure design purposes. Such description should contain information on both the univariate behaviour and the joint-dependence of storm-variables. These two aspects have been here addressed through generalized Pareto distributions and hierarchical Archimedean copulas. A non-stationary model has been used to highlight the relationship between these extreme events and non-stationary climate. It has been applied to a Representative Concentration Pathway 8.5 Climate-Change scenario, for a fetch-limited environment (Catalan Coast). In the non-stationary model, all considered variables decrease in time, except for storm-duration at the northern part of the Catalan Coast. The joint distribution of storm variables presents cyclical fluctuations, with a stronger influence of climate dynamics than of climate itself.Peer ReviewedPostprint (author's final draft

A multivariate statistical model of extreme events: an application to the Catalan coast

Wave extreme events can be understood as the combination of Storm-intensity, Directionality and Intra-time distribution. However, the dependence structure among these factors is still unclear. A methodology has been developed to model wave-storms whose components are linked together. The model is composed by three parts: an intensity module, a wave directionality module, and an intra-time distribution module. In the Storm-intensity sub-model, generalized Pareto distributions and hierarchical Archimedean copulas have been used to characterize the storm energy, unitary energy, peak wave-period and duration. In the Directionality and Intra-time sub-models, the wave direction (at the peak of the storm) and the storm growth–decay rates are linked to the variables from the intensity model, respectively. The model is applied to the Catalan coast (NW Mediterranean). The outcomes denote spatial patterns that coincide with the state of knowledge. The proposed methodology is able to provide boundary conditions for wave and near-shore studies, saving computational time and establishing the dependence of the proposed variables. Such synthetic storms reproduce the inter-variable co-dependence of the original data.Peer ReviewedPostprint (published version

First considerations on environmental friendly solutions to protect the southern Romanian coast

The aim of this work is to assess the effect of a nature-based solution for reducing wave heights on the Southern Romanian coast. Apart from investigating the presence of seagrass from the environmental point of view, there is also a need to assess its impact on the coastal hydrodynamics. The impact on the wave heights of a seagrass meadow located on the Southern Romanian coast, has been analyzed by means of a wave model. In this purpose, several numerical simulations have been performed, both for low and average offshore wave conditions, available from a previous wave climate study, which used a 30 years climate data set. A first set of simulations have been performed in the absence of seagrass. Then a seagrass meadow has been added to our grid and the wave model has been run in the same offshore wave conditions. The differences in computed nearshore wave heights reach around 4% for moderate energy waves. These results show that, on the Southern Romanian coast, seagrass could be regarded as an additional measure for nearshore wave attenuation.Peer ReviewedPostprint (published version

Coastal flooding and erosion under a changing climate: implications at a low-lying coast (Ebro delta)

Episodic coastal hazards associated to sea storms are responsible for sudden and intense changes in coastal morphology. Climate change and local anthropogenic activities such as river regulation and urban growth are raising risk levels in coastal hotspots, like low-lying areas of river deltas. This urges to revise present management strategies to guarantee their future sustainability, demanding a detailed diagnostic of the hazard evolution. In this paper, flooding and erosion under current and future conditions have been assessed at local scale at the urban area of Riumar, a touristic enclave placed at the Ebro Delta (Spain). Process-based models have been used to address the interaction between beach morphology and storm waves, as well as the influence of coastal environment complexity. Storm waves have been propagated with SWAN wave model and have provided the forcings for XBeach, a 2DH hydro-morphodynamic model. Results show that future trends in sea level rise and wave forcing produce non-linear variations of the flooded area and the volume of mobilized sediment resulting from marine storms. In particular, the balance between flooding and sediment transport will shift depending on the relative sea level. Wave induced flooding and long-shore sand transport seem to be diminished in the future, whereas static sea level flooding and cross-shore sediment transport are exacerbated. Therefore, the characterization of tipping points in the coastal response can help to develop robust and adaptive plans to manage climate change impact in sandy wave dominated coasts with a low-lying hinterland and a complex shoreline morphology.Peer ReviewedPostprint (published version

Impact of climate change on nearshore waves at a beach protected by a barrier reef

Barrier reefs dissipate most incoming wind-generated waves and, as a consequence, regulate the morphodynamics of its inbounded shorelines. The coastal protective capacity of reefs may nevertheless be compromised by climate change effects, such as reef degradation and sea-level rise. To assess the magnitude of these climate change effects, an analysis of the waves propagating across the barrier reef is carried out in Flic-en-Flac beach, Mauritius, based on scenarios of future sea levels and predicted coral reef condition. In the study, both the mean wave climate and extreme event conditions are considered. The results show that lower coral structure complexity jointly with higher water levels allow for higher waves to pass over the reef and, therefore, to reach the shoreline. In addition, modeling for cyclonic conditions showed that nearshore waves would also increase in height, which could lead to major coastal morphodynamic changes. Measures aimed at preserving the coral reef may allow the system to accommodate for the gradual climatic changes forecasted while keeping its coastal protective function.Peer ReviewedPostprint (published version