César Álvarez

Flushing time as a descriptor for heavily modified water bodies classification and management: application to the Huelva Harbour.

Since the flushing time is a physical descriptor used to distinguish between different types of heavily modified water bodies (HMWB), the establishment of a methodology for its calculation becomes important. In order to achieve this task, a methodological procedure involving the tide mean value and variable river flow values is proposed. The hydrodynamics were assessed using a two-dimensional model which integrates the depth-averaged mass and momentum equations in the time and space domains and includes a wet-dry point treatment method. The hydrodynamic model calibration and validation were performed on the basis of tidal gauge and velocity current measurements. A reasonable agreement with the field measurements of water elevation and velocity were achieved. On the other hand, a two-dimensional mathematical model, which solves the depth-averaged advection-diffusion equation, was properly calibrated and used to evaluate the behaviour of a conservative tracer within a water body. The transport model calibration was developed according to the field survey data carried out during late spring when the rivers flows are low. This study allowed the flushing time estimation under four scenarios showing that only the estuarine mouth presents a high renewal rate because the current velocities are higher. For heavy rain periods, a flushing time decrease was observed as river flows modify the circulation in the main channel. Neglecting the river forcing was found to be valid for the dry period.

A model for predicting the temporal evolution of dissolved oxygen concentration in shallow estuaries

The design of sewage discharge systems in estuaries needs to consider the dissolved oxygen concentration among other water quality indicators. Due to the great number of factors affecting the dissolved oxygen, the prediction of the temporal evolution of this element requires the use of mathematical tools. In the case of shallow estuaries with extensive intertidal zones, the complexity of this task increases since the water domain varies continuously. This work describes a numerical model which solves the vertical integrated transport equation including the effect of extensive tidal flats. The model makes use of the tidal velocities computed by a previous run of a hydrodynamic model. This procedure allows the dissolved oxygen modelling to be performed using a larger time step than that used for the velocity field calculation. The numerical scheme developed for the model guarantees that even in the drying and wetting of intertidal areas the water column dissolved oxygen concentration is not modified by numerical errors. The representations of the processes included in the model (advection, dispersion and reaction) have been validated successfully in several theoretical cases. An application to the Urdaibai Estuary, a shallow estuary in the Basque Country, Northern Spain, is also presented.

The Prestige Oil Spill in Cantabria (Bay of Biscay). Part II. Environmental Assessment and Monitoring of Coastal Ecosystems

Abstract Assessment and monitoring activities constituted two main tasks of the emergency response system implemented by the regional government of Cantabria (north Spain) after the Prestige oil spill. The assessment covered four types of environmental units: estuaries, rocky shores, sand beaches, and subtidal areas, up to a 300-m depth. Monitoring procedures included the chemical quantification of total hydrocarbons and polycyclic aromatic hydrocarbons in water, sediments, and benthic organisms (clams and goose barnacles), and the analysis of structural changes in intertidal communities. Disturbance of oil cover was significantly more extensive in coastal areas, rather than in estuarine areas, where 50,000 m2 were directly covered by fuel. Otherwise, the presence of oil in subtidal areas was a rare event. Results from the bioeffects analyses were in agreement with the overall impact assessments, pointing to the coastal habitats as the areas where the bioavailability of toxic components from the Prestige spill presented a significant level of risk.

Baseline study of soft bottom benthic assemblages in the Bay of Santander (Gulf of Biscay)

The Bay of Santander constitutes one of the most important estuarine areas in northern Spain (Gulf of Biscay) which has been significantly stressed by sewage discharges. However, an improvement in the environmental quality of the bay is expected once the new sewer system comes into operation. The evaluation of such an evolution needs to be contrasted with the current situation, established in specific baseline studies. For this purpose, a field study was carried out on the soft bottom assemblages of the estuary in spring of 1998. At each station, the benthic fauna abundance, grain size, organic matter, carbon, nitrogen, phosphorus and heavy metals concentrations were analysed. The community structure and the physical and chemical characteristics of the sediment reflected the higher impacts both near the points of discharges and in the low water exchange zones, whereas those areas showing higher tidal flows presented an acceptable quality. High heavy metal concentrations for Cd, Hg, Pb and Zn indicated contamination of industrial sources in several areas of the Bay. This information allows to establish a reference situation in order to improve the design of a specific monitoring program for this area.

Comparison of Gridded and Measured Rainfall Data for Basin-scale Hydrological Studies

Global gridded climatological (GGC) datasets, including precipitation and temperature, are becoming more and more precise, accessible, and common, but the utility of these datasets and their limits for hydrological research are still not well determined. In this paper, we compare the performance of two hydrological models that are identical in structure but built with two different inputs: rainfall from rain gauge stations and from a GGC dataset. The objective is to evaluate the utility of gridded datasets in water resource availability studies mainly for hydroelectric and agricultural purposes. The Andean basin of the Laja River, located in south-central Chile, was chosen for this study. It was based on an 18-yr simulation, and it was concluded that i) with gridded climatological datasets in a monthly water balance model, it is possible to reproduce the behavior of an Andean basin with good goodness-of-fit, but with worse results than when using inputs from rain gauges; ii) the amount of rainfall in gridded datasets in the Andean area of the Laja basin is underestimated and damped, an effect which is transferred to the simulated flows; and iii) regarding the main activities in the Laja basin, global gridded datasets are useful for hydrological studies with agricultural purposes prior to a treatment that considers the orographic effect. On the other hand, these datasets are useless for hydroelectric purposes due to the large underestimation of peak flows obtained during the rainy season.

Selecting model scenarios of real hydrodynamic forcings on mesotidal and macrotidal estuaries influenced by river discharges using K-means clustering

The long-term (>30?years) simulation of 3D estuarine hydrodynamics with high-resolution meshgrids is still a challenge in numerical modeling because of the large data set of results and the computational cost requirements. Meso and macrotidal estuaries are governed by tidal action and could be influenced by river. The complexity of their behavior, suggest data mining methods may be particularly effective in selecting short-term series from a long-term series to identify the major modes of forcing variability. This study uses K-means clustering for two aims: explaining the variability of astronomical tides and river flows, and selecting scenarios of real forcings to obtain the mean behavior with a dimensional reduction. The application to the Suances estuary has highlighted the ability to classify long-term series in small number of groups. Before conducting any simulation, the proposal also determines the minimum and optimal number of groups to consider the combined effect of both forcings. KMA has the ability to classify long-term series in small number of groups.KMA clustering approach reduces effectively the forcing dimension of the system.The method explains the variability of astronomical tides and river flows.The method selects scenarios of real forcings for modelling the mean behavior.The method helps to reduce the computational cost of estuarine numerical modelling.

ENVIRONMENTAL DESIGN AND MONITORING OF LARGE SUBMARINE OUTFALLS: AN INTEGRATED APPROACH FOR COASTAL PROTECTION

In order to address the environmental impact produced for decades to the water bodies, the authorities of the major coastal municipalities and counties in North Spain (Gulf of Biskay) initiated, during the 90‘s, the design and construction of new sanitation systems, according to the legal requirements imposed by the 91/271/EC Directive. Site planning, environmental risk assessment of discharges and objective-oriented monitoring were the three main tasks supporting the decision-making process related to the validation of the new sewerage projects. This process consists of four consecutive stages: 1) Design and selection of alternatives (strategic assessment), 2) Risk evaluation for different scenarios (impact assessment), 3) Verification of adverse effects (objective-oriented monitoring) and 4) Action plan for mitigation of registered impacts (management of pollution events).

Modeling heavy metal concentration distributions in estuarine waters: an application to Suances Estuary (Northern Spain)

Abstract Using numerical modeling as a tool to simplify management tasks is a less expensive complementary method to indirectly monitor estuarine zones. Nevertheless, information regarding the sedimentary dynamics and the pollutants present in the area, specifically in the sediment, must be known before a model can be used successfully. The two-dimensional transport model “SOLTOX” allowed the study of the fate and transport of heavy metals in estuarine waters, with particular application to the zinc evolution along the Suances Estuary (Northern Spain) which, historically, has suffered the effects of industrial pollution. As many pollutants can be adsorbed on suspended solids, advection–diffusion transport analyses were tackled to model not only the zinc but also the suspended solids concentration distributions. The importance of specific parameters related with the suspended solids transport modeling was also explored. Using the collected data during a whole tidal cycle for suspended solids and current velocities, a graphical method was applied to obtain the initial values for the calibration and validation of the model main parameters. The model was first calibrated against the field data for suspended solids. The obtained results exhibited the critical shear stress for deposition as the overriding parameter. The model was then used to investigate the zinc transport processes in the Suances Estuary. A reasonable agreement between modeled and measured data was achieved for the suspended solids and zinc. Moreover, the results demonstrated a dependence on the background concentration of the pollutant in the sediment. However, the background concentration in the water column had little effect on the model results.Graphical Abstract.

Effects of sewage effluents and seasonal changes on the metabolism of three Atlantic rivers

Sewage inputs on fluvial ecosystems affect benthic communities and alter trophic networks resulting in changes on river functioning. Functional indicators (e.g. river metabolism) have been proposed as a valuable tool to evaluate ecosystem impairment. In the present study we monitored river metabolism in spring (few days after a major flood) and in summer (after 35days of low flow conditions) using both single-station and two-stations methods over a 24h period up and downstream of wastewater treatment plant (WWTP) effluents on three Atlantic river reaches located in northern Spain (Europe). Concurrently with river metabolism, we characterized environmental characteristics (flow, velocity, depth, pH, water temperature, nutrients, etc.), benthic macroinvertebrate communities and biofilm (algae and epilithic biomass). Ecosystem Respiration (ER24) was similar at the different periods and locations, but Gross Primary Productivity (GPP) tended to decrease in impacted reaches (downstream WWTPs) and in summer (except in the Saja River). However, the balance of the metabolic processes showed a trend towards autotrophy in the largest river, while WWTP effluents increased its autotrophy. Chlorophyll a concentration was >4 times larger in spring than in summer in all river reaches, while epilithic biomass followed a similar but less obvious pattern. Increase of invertebrate scraper densities (mainly, Potamopyrgus antipodarum) seems to be a plausible explanation for biofilm biomass temporal patterns in all sites (higher in spring than in summer), altering GPP and ER24 patterns. Thus, metabolism rates show different responses to WWTP effluents depending on season and on the relationships among functional and structural components, with special focus on the composition and structure of macroinvertebrate communities. Increasing our understanding of cause-effect relationships on the impairment of aquatic ecosystems needs to account for both structural and functional components and their interactions.

Modeling of coastal water contamination in Fortaleza (Northeastern Brazil)

An important tool in environmental management projects and studies due to the complexity of environmental systems, environmental modeling makes it possible to integrate many variables and processes, thereby providing a dynamic view of systems. In this study the bacteriological quality of the coastal waters of Fortaleza (a state capital in Northeastern Brazil) was modeled considering multiple contamination sources. Using the software SisBaHiA, the dispersion of thermotolerant coliforms and Escherichia coli from three sources of contamination (local rivers, storm drains and submarine outfall) was analyzed. The models took into account variations in bacterial decay due to solar radiation and other environmental factors. Fecal pollution discharged from rivers and storm drains is transported westward by coastal currents, contaminating strips of beach water to the left of each storm drain or river. Exception to this condition only occurs on beaches protected by the breakwater of the harbor, where counterclockwise vortexes reverse this behavior. The results of the models were consistent with field measurements taken during the dry and the rainy season. Our results show that the submarine outfall plume was over 2 km from the nearest beach. The storm drains and the Maceió stream are the main factors responsible for the poor water quality on the waterfront of Fortaleza. The depollution of these sources would generate considerable social, health and economic gains for the region.