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Documents published in Scipedia

• Physical modeling of a stepped spillway without sidewalls

  S. Estrella, M. Sánchez-Juny, E. Bladé, J. Dolz

  Canadian journal of civil engineering (2015). Vol. 42 (5), pp. 311-318

  
  

  Abstract

  The interest of a consulting company in designing stepped spillways in roller compacted concrete (RCC) dams led us to propose the possibility of building this type of spillway without sidewalls. Previous research on stepped spillways has focused on characterizing the complex hydraulic behavior of flow on these structures, as well as design criteria. Such studies have usually been conducted on stepped spillways with a constant width along the spillway, that is, with sidewalls. In the present work, we report the results of the physical modeling of a generic stepped spillway without sidewalls (slope 1v:0.8h). In general terms, the lack of sidewalls produces a lateral expansion of water and therefore a non-uniform longitudinal and transversal discharge distribution. Consequently, the flow type, characteristic water depth, velocity, air concentration, and pressure fields change along and across the spillway. The resulting data demonstrate that the distribution of the different variables studied depend on the specific discharge at the entrance and the spillway height.

  Abstract
  The interest of a consulting company in designing stepped spillways in roller compacted concrete (RCC) dams led us to propose the possibility of building this type of spillway [...]

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• Local isotropy indicator for SAR image filtering: application to Envisat/ASAR images of the Doñana Wetland

  B. Martí-Cardona, C. Lopez, J. Dolz

  IEEE journal of selected topics in applied earth observations and remote sensing (2015). Vol. 8 (4), pp. 1614-1622

  
  

  Abstract

  This paper explores a geometrical and computationally simple operator, named Ds, for local isotropy assessment on SAR images. It is assumed that isotropic intensity distributions in natural areas, either textured or nontextured, correspond to a single cover class. Ds is used to measure isotropy in processing neighborhoods and decide if they can be considered as belonging to a unique cover class. The speckle statistical properties are used to determine suitable Ds thresholds for discriminating heterogeneous targets from isotropic cover types at different window sizes. An assessment of Ds as an edge detector showed sensitivities similar to those of the ratio edge operator for straight, sharp boundaries, centered in the processing window, but significantly better sensitivity for detecting heterogeneities during the window expansion in multiresolution filtering. Furthermore, Ds presents the advantage versus the ratio edge coefficient of being rotationally invariant, and its computation indicates the direction of the main intensity gradient in the processing window. The Ds operator is used in a multiresolution fashion for filtering ASAR scenes of the Doñana wetland. The intensities in isotropic areas are averaged in order to flatten fluctuations within cover types and facilitate a subsequent land cover classification. The results show high degree of smoothing within textured cover classes, plus effective spatial adaptation to gradients and irregular boundaries, substantiating the usefulness of this operator for filtering SAR data of natural areas with the purpose of classification.

  Abstract
  This paper explores a geometrical and computationally simple operator, named Ds, for local isotropy assessment on SAR images. It is assumed that isotropic intensity distributions [...]

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• Mixing dynamics at the confluence of two large rivers undergoing weak density variations

  C. Ramón, J. Armengol, J. Dolz, J. Prats, F. Rueda

  Journal of geophysical research: oceans (2014). Vol. 119 (4), pp. 2386-2402

  
  

  Abstract

  Simulations of tracer experiments conducted with a three‐dimensional primitive‐equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers (Ebro and Segre), with distinct physical and chemical properties, at their confluence, upstream of a meandering reservoir (Ribarroja reservoir). Mixing rates downstream of the confluence are subject to hourly scale oscillations, driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence. Even though density contrasts are low—at most O(10−1) kg m−3 difference among sources—and almost negligible from a dynamic point of view—compared with inertial forces—they are important for mixing. Mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40% larger than those occurring under neutrally buoyant conditions. The buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing (distortion of the mixing layer). For strong density contrasts, though, when the contact area between the streams becomes nearly horizontal, larger density differences between streams will lead to weaker mixing rates, as a result of the stabilizing effect of vertical density gradients.

  Abstract
  Simulations of tracer experiments conducted with a three‐dimensional primitive‐equation hydrodynamic and transport model are used to understand the processes controlling [...]

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• Envisat/ASAR Images for the Calibration of Wind Drag Action in the Donana Wetlands 2D Hydrodynamic Model

  A. Ramos, B. Martí-Cardona, E. Bladé, J. Dolz

  Remote Sensing of Environment (2014). Vol. 6 (1), pp. 379-406

  
  

  Abstract

  Donana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every summer and progressively flood again throughout the fall and winter seasons. Given the flatness of Donana's topography, the wind drag action can induce the flooding or emergence of extensive areas, detectable in remote sensing images. Envisat/ASAR scenes acquired before and during strong and persistent wind episodes enabled the spatial delineation of the wind-induced water displacement. A two-dimensional hydrodynamic model of Donana wetlands was built in 2006 with the aim to predict the effect of proposed hydrologic restoration actions within Donana's basin. In this work, on-site wind records and concurrent ASAR scenes are used for the calibration of the wind-drag modeling by assessing different formulations. Results show a good adjustment between the modeled and observed wind drag effect. Displacements of up to 2 km in the wind direction are satisfactorily reproduced by the hydrodynamic model, while including an atmospheric stability parameter led to no significant improvement of the results. Such evidence will contribute to a more accurate simulation of hypothetic or design scenarios, when no information is available for the atmospheric stability assessment. Doñana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every summer and progressively flood again throughout the fall and winter seasons. Given the flatness of Doñana’s topography, the wind drag action can induce the flooding or emergence of extensive areas, detectable in remote sensing images. Envisat/ASAR scenes acquired before and during strong and persistent wind episodes enabled the spatial delineation of the wind-induced water displacement. A two-dimensional hydrodynamic model of Doñana wetlands was built in 2006 with the aim to predict the effect of proposed hydrologic restoration actions within Doñana’s basin. In this work, on-site wind records and concurrent ASAR scenes are used for the calibration of the wind-drag modeling by assessing different formulations. Results show a good adjustment between the modeled and observed wind drag effect. Displacements of up to 2 km in the wind direction are satisfactorily reproduced by the hydrodynamic model, while including an atmospheric stability parameter led to no significant improvement of the results. Such evidence will contribute to a more accurate simulation of hypothetic or design scenarios, when no information is available for the atmospheric stability assessment

  Abstract
  Donana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every [...]

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• Mixing and circulation at the confluence of two rivers entering a meandering reservoir

  C. Ramón, A. Hoyer, J. Armengol, J. Dolz, F. Rueda

  Water Resources Research (2013). Vol. 49 (3), pp. 1429-1445

  
  

  Abstract

   A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical‐chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities—Δρ/ρ 0 ≈ O (10−5)—and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Δρ/ρ 0 of O (10−3), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.

  Abstract
   A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical‐chemical [...]

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• Wetland inundation monitoring by the synergistic use of ENVISAT/ASAR imagery and ancilliary spatial data

  B. Martí-Cardona, J. Dolz, C. López

  Remote Sensing of Environment (2013). Vol. 139, pp. 171-184

  
  

  Abstract

  Wetlands are among the most ecologically important ecosystems on Earth and their sustainability depends critically on the water resources. In a scenario of climate change and increased anthropogenic pressure, detailed monitoring of the water resources provides a fundamental tool to assess the ecosystem health and identify potential threats. Doñana wetlands, in Southwest Spain, dry out every summer and progressively flood in fall and winter to a maximum extent of 30,000 ha. The wetland filling up process was monitored in detail during the 2006–2007 hydrologic cycle by means of twenty-one Envisat/ASAR scenes, acquired at different incidence angles in order to maximize the observation frequency. Flood mapping from the two uncorrelated ASAR channel data alone was proved unfeasible due to the complex casuistic of Doñana cover backscattering. This study addresses the synergistic utilization of the ASAR data together with Doñana's digital elevation model and vegetation map in order to achieve flood mapping. Filtering and clustering algorithms were developed for the automated generation of Doñana flood maps from the ASAR images. The use of irregular filtering neighborhoods adapted to the elevation contours drastically improved the ASAR image filtering. Edge preservation was excellent, since natural edges closely follow terrain contours. Isotropic neighborhoods were assumed of a single class and their intensities were averaged. As a result, intensity fluctuations due to speckle and texture over areas of the same cover type were smoothed remarkably. The clustering and classification algorithm operate on individual sub-basins, as the pixel elevation is more accurately related to the cover classes within them. Vegetation and elevation maps plus knowledge of Doñana backscattering characteristics from preceding studies were initially used to select seed pixels with high confidence on their class membership. Next, a region growing algorithm extends the seed regions with new pixels based on their planimmetric adjacency and backscattering Mahalanobis distance to the seeds. During the seed region growth, new pixels' possible classes are not constrained to their cover type according to the vegetation map, so the algorithm is able to capture temporal changes in the vegetation spatial distribution. Comparison of the resultant classification and concurrent ground truth yielded 92% of flood mapping accuracy. The flood mapping method is applicable to the available ASAR images of Doñana from six other hydrologic cycles.

  Abstract
  Wetlands are among the most ecologically important ecosystems on Earth and their sustainability depends critically on the water resources. In a scenario of climate change [...]

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• Physical and chemical characterisation of superficial sediment of the Ribarroja Reservoir (River Ebro, NE Spain)

  P. López, J. Dolz, M. Arbat, J. Armengol

  Limnetica (2012). Vol. 31 (2), pp. 327-340

  
  

  Abstract

  Con el objetivo de determinar la heterogeneidad especial de las características físicas y químicas de los sedimentos superficiales del embalse de Ribarroja, se ha analizado la distribución de tamaños de las partículas y las concentraciones de los elementos mayoritarios (Al, Fe, Si, K, Ca, Mg, Ti, Mn, P) y minoritarios (Mo, Nb, Zr, Y, Sr, Rb, Th, Pb, Sn, Ce, Ga, Zn, W, Cu, Co, Ni y V) en dieciséis puntos distribuidos a lo largo del embalse. Globalmente, el sedimento era bastante homogéneo: el tamaño del grano estaba dominado por limo y arcilla, y los componentes de mayor tamaño se relacionan con una mezcla de carbonatos y silicatos meteorizados, con niveles moderados de nitrógeno y fósforo. Las concentraciones de los elementos minoritarios no sobrepasaron los niveles de Nivel de Toxicidad Probable, aunque el níquel y el cromo sobrepasaron el Nivel Umbral de Toxicidad. Un análisis de Componentes Principales realizado con los valores normalizados de los elementos minoritarios ha mostrado cuales son los principales procesos que determinan la variabilidad en la composición de los sedimentos, el grado de alteración/meteorización de los materiales y la asociación de algunos metales a los agregados orgánicos/arcilla.

  Abstract
  Con el objetivo de determinar la heterogeneidad especial de las características físicas y químicas de los sedimentos superficiales del embalse de Ribarroja, [...]

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• Integration of 1D and 2D finite volume schemes for computations of water flow in natural channels

  E. Bladé, M. Valentín, J. Dolz, J. Aragón-Hernández, G. Corestein, M. Juny

  Advances in Water Resources (2012). Vol. 42, pp. 17-29

  
  

  Abstract

  A wide variety of flood simulation models are available nowadays. Some of them use a 1D approach and others a 2D one, but there are also some which allow the performance of integrated 1D–2D simulations. These latter models, which have important advantages in optimizing computational costs, commonly use the 1D approach in river channels and the 2D one in floodplains. The coupling of 1D and 2D flows usually ensures mass conservation and makes use of simplified weir-type or friction slope equations, but neglects momentum transfer between the two domains. This paper presents a fully conservative method for the coupling of 1D and 2D domains to be used in numerical schemes based on finite volumes. The method, based on a discretization of the numerical fluxes which ensures the conservation of mass and momentum, is verified with simple test cases. The proposed scheme is compared with the standard method based on the source term of the equations and is applied to the hydrodynamic characterization of a river-reservoir system situated in the River Ebro in Spain.

  Abstract
  A wide variety of flood simulation models are available nowadays. Some of them use a 1D approach and others a 2D one, but there are also some which allow the performance of [...]

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• Water temperature modeling in the lower Ebro river (Spain) : heat fluxes, equilibrium temperature, and magnitude of alteration caused by reservoirs and thermal effluent

  J. Prats, J. Dolz, J. Armengou, R. Val

  Water Resources Research (2012). Vol. 48, pp. 1-15

  
  

  Abstract

  The lower Ebro River (Spain) is subject to the thermal and hydrological alterations caused by the system of reservoirs of Mequinenza, Riba‐roja, and Flix and to the thermal effluent of the nuclear power plant of Ascó, located 5 km downstream from the three reservoirs. In this paper, a modeling approach is used to determine the equilibrium temperature and recuperation distance at the study reach for different seasons and hydrological years. The intensity of the alteration caused by the reservoirs and nuclear power plant effluent is studied in reference to the equilibrium temperature and discharge rate. Mean daily water temperature downstream from the reservoirs is higher than mean daily equilibrium temperature in the fall and is lower during the rest of the year. Changes in the heat fluxes induced by thermal alteration are also addressed, showing important variations in evaporation and conduction to the atmosphere.

  Abstract
  The lower Ebro River (Spain) is subject to the thermal and hydrological alterations caused by the system of reservoirs of Mequinenza, Riba‐roja, and Flix and to the thermal [...]

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• Comparison of Models for Calculation of Diel Sediment-Water Heat Flux from Water Temperatures

  J. Prats, A. Ramos, J. Armengol, J. Dolz

  Journal of Hydrology Engineering (2011). Vol. 137 (10), pp. 1135-1147

  
  

  Abstract

  We investigated sediment-water heat flux estimation in relation to applications (hydrodynamics simulation, evaporation studies, or global change effects assessment) in which sediment temperatures are not available because of technical complications (difficulties in installing sensors, for example) or because of methodology used (remote sensing, for example). We used field sediment temperature data measured every 10 min to 1 m depth at Doñana National Park marshland to obtain sediment thermal properties and to calculate diel sediment-water heat exchange through Beck’s sequential function specification method. We compare four models for the simulation of sediment-water heat flux by using surface temperatures. Two models need initial estimated inside sediment temperatures; the other two do not. Influence of estimated initial temperature profiles depends on the temperature distribution assumption and is significant for three days or fewer at the daily timescale. A model that does not use initial sediment temperatures provides accurate estimations with low computation time.

  Abstract
  We investigated sediment-water heat flux estimation in relation to applications (hydrodynamics simulation, evaporation studies, or global change effects assessment) in which [...]

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