Flávio Augusto Bastos da Cruz Martins

3D modelling in the Sado estuary using a new generic vertical discretization approach

Abstract Sado estuary (Portugal) is a mesotidal well-mixed estuary. The flow however, displays strong three-dimensional features associated with bathymetry variations. The flow is simulated using a three-dimensional primitive equation model based on the finite-volume method and incorporating a new concept for the vertical discretization. The innovative features of the model are analysed and the advantages of the finite-volume method to implement the generic vertical discretization are put into evidence. The model was validated using water level, velocity and salinity measurements in several stations along the estuary. Results show the influence of main channels strong curvature on the generation of secondary flows inside the estuary. The steep bathymetry of the outer platform gives rise to a recirculation flow in the vertical plane that lasts for most of the tidal cycle. This structure is considered important for the sediment transport in that region. The above mentioned non-linear effects leave their print in both the transient and residual velocity fields. The results obtained help to understand the role of three-dimensional structures on the water exchange and sediment transport in the estuary. The results also confirm the suitability of the approach used in this model to simulate estuarine and coastal flows with strong three-dimensional effects.

Modelling the influence of nutrient loads on Portuguese estuaries

The effects of implementing Directive 91/271/EEC of 21 May 1991 (Waste Water Treatment Plan Directive) and Directive 91/676/EEC of 12 December (Nitrates Directive) are analysed in 7 Portuguese estuaries (Minho, Lima, Douro, Mondego, Tagus, Sado and Guadiana) and two coastal lagoons (Ria de Aveiro and Ria Formosa), with a modelling approach. MOHID Water Modelling System was used to perform simulations with three nitrogen load scenarios for each system: a reference scenario, a 50% nitrate removal by agriculture scenario and another with a 100% nutrients removal by waste water treatment plants (WWTP). It is shown that the interaction between hydrodynamic and ecological processes is an important feature to study trophic problems in estuaries. Ecological processes such as primary production only occur inside the system if the residence time of water is high enough to enable organismal activity and if the adequate conditions are found (e.g. light, nutrients, temperature). From the model results it is possible to conclude: (i) in systems with short residence time a reduction in nutrient load will only produce a decrease in nutrient transit and will not affect the system’s global ecological status (e.g. Douro Estuary); (ii) in systems with long residence time the effects will range from significant, when primary production is mostly limited by nutrients (e.g. Ria de Aveiro), to non-significant, when primary production in the system is light-limited (e.g. Tagus Estuary).

MOHID 2000 - A coastal integrated object oriented model

Mathematical modeling, although a recent science, can be considered old when the time scale used to measure its age is the information technology evolution. FORTRAN 77 has been the most popular programming language among hydrodynamic modelers over the last two decades. This language was adequate to the (low) complexity of the models, limited by the exiting computers. The fast increase of the computing capacity allowed the development of more complex and multidisciplinary models. The necessity of new programming techniques became clear. It was started the era of the Object Oriented programming. Object Oriented programming isolates different parts of the code and allows communication among them using simple and robust interfaces. MOHID 2000 is an integrated modeling tool programmed in FORTRAN 95, using an Object Oriented strategy. It is the evolution of a set of models originally programmed in FORTRAN 77. Taking advantage of the new language possibilities, with a single code it is possible to perform different simulations (1D, 2D or 3D) and use a variety of vertical discretizations. This technique has also turned easy to integrate hydrodynamic, water quality, sediment transport and pollutants dispersion simulations using eulerian or lagrangian formulations. Having in mind the development of a tool accessible to the final user, the modeler must think about on the algorithms and code, but also on pre and post processing of the model data. MOHID 2000 input data files can be constructed by a graphical user interface. Output files use the standard HDF format making results exploring simple and allows the use of different graphic packages.

Towards a common oil spill risk assessment framework – Adapting ISO 31000 and addressing uncertainties

Oil spills are a transnational problem, and establishing a common standard methodology for Oil Spill Risk Assessments (OSRAs) is thus paramount in order to protect marine environments and coastal communities. In this study we firstly identified the strengths and weaknesses of the OSRAs carried out in various parts of the globe. We then searched for a generic and recognized standard, i.e. ISO 31000, in order to design a method to perform OSRAs in a scientific and standard way. The new framework was tested for the Lebanon oil spill that occurred in 2006 employing ensemble oil spill modeling to quantify the risks and uncertainties due to unknown spill characteristics. The application of the framework generated valuable visual instruments for the transparent communication of the risks, replacing the use of risk tolerance levels, and thus highlighting the priority areas to protect in case of an oil spill.

Towards the development of an operational tool for oil spills management in the Algarve coast

Portugal is strongly vulnerable to sea hazards due to intense vessel traffic and sea conditions. The southwest region off the Iberian Peninsula lies in the main route from the Mediterranean and Southern Hemisphere to the Northern Europe, causing a ship concentration in a narrow band off Cape São Vicente. Tankers represent a significant part of the vessel traffic and the occurrence of oil spills cannot be disregarded. Cape São Vicente region is part of a Natural Park with 110 Km of coastline, integrated in the European Natura 2000 network and its socio-economic context is closely related with sea resources exploitation, particularly fishing and tourism. Recognizing the importance of accurate information systems for the decision making process in an oil spill situation, this work presents the development of an integrated tool to support the process in the Algarve coast. The system relies in a regional operational mathematical model based on the MOHID modelling system. The system core is composed by three models (3D hydrodynamics, wave and Lagrangian transport) all linked in the same system and exchanging information in real time. Oil advection and weathering processes are coupled to the Lagragian transport model. The overall operational system includes external operational data products as inputs, to ensure a successful validation of the results. The system is linked to stakeholders and response authorities using a geographic referenced database based on Mapserver technology that will include relevant information for oil spill management.

Wind and freshwater influence over hydrocarbon dispersal on Patos Lagoon, Brazil

The two-dimensional barotropic, hydrodynamic and transport model MOHID is applied to the Patos Lagoon system using a nested modelling approach to reproduce both the lagoon and estuary hydrodynamics. A new Lagrangian oil spill model is presented and used to simulate a hypothetical oil spill in the estuary. Hydrodynamic fields are validated and used to force the oil model. Results show that the hydrodynamics of this system is mainly controlled by the wind and freshwater discharge. The dispersion, concentration and thickness evolution of the oil in the first day after the spill is determined by the equilibrium between these two factors. The freshwater discharge is the major factor controlling the oil dispersion for discharges greater than 5000 m3 while the wind assumes control for lower discharge amounts. The results presented are a first step toward a coastal management tool for the Patos Lagoon.

Enhancing the management response to oil spills in the Tuscany Archipelago through operational modelling.

A new approach towards the management of oil pollution accidents in marine sensitive areas is presented in this work. A set of nested models in a downscaling philosophy was implemented, externally forced by existing regional operational products. The 3D hydrodynamics, turbulence and the oil transport/weathering models are all linked in the same system, sharing the same code, exchanging information in real time and improving its ability to correctly reproduce the spill. A wind-generated wave model is also implemented using the same downscaling philosophy. Observations from several sources validated the numerical components of the system. The results obtained highlight the good performance of the system and its ability to be applied for oil spill forecasts in the region. The success of the methodology described in this paper was underline during the Costa Concordia accident, where a high resolution domain was rapidly created and deployed inside the system covering the accident site.

A relocatable ocean model in support of environmental emergencies

During the Costa Concordia emergency case, regional, subregional, and relocatable ocean models have been used together with the oil spill model, MEDSLIK-II, to provide ocean currents forecasts, possible oil spill scenarios, and drifters trajectories simulations. The models results together with the evaluation of their performances are presented in this paper. In particular, we focused this work on the implementation of the Interactive Relocatable Nested Ocean Model (IRENOM), based on the Harvard Ocean Prediction System (HOPS), for the Costa Concordia emergency and on its validation using drifters released in the area of the accident. It is shown that thanks to the capability of improving easily and quickly its configuration, the IRENOM results are of greater accuracy than the results achieved using regional or subregional model products. The model topography, and to the initialization procedures, and the horizontal resolution are the key model settings to be configured. Furthermore, the IRENOM currents and the MEDSLIK-II simulated trajectories showed to be sensitive to the spatial resolution of the meteorological fields used, providing higher prediction skills with higher resolution wind forcing.

Simulating vertical water mixing in homogeneous estuaries: the SADO Estuary case

In this article, a three-dimensional primitive equation model is applied to Sado Estuary, which is a meso-tidal homogeneous estuary. Despite the homogeneity of its density field, the flow presents important vertical structures. This article shows that the common practice of using two-dimensional depth-integrated models in homogeneous estuaries can filter important vertical hydrodynamic features, such as secondary and circulating flows. The physical processes associated with these flows are analysed. The strong curvature of the channels and the strong bathymetry gradients were identified as the principal morphologic features of the estuary responsible for the generation of the flows. The model possesses a generic vertical mesh geometry approach, which is used to test the sensibility of the model to the vertical discretization when simulating these flows.

Integrated monitoring of South Portugal water bodies: a methodology towards WFD.

The challenge on implementation of the EU Water Framework Directive (WFD) (2000/60/EC) fosters the development of new monitoring approaches. The directive promotes the use of modelling techniques to assist all phases of the process, from characterization and establishment of reference conditions to identification of pressures and assessment of impact. This work is based on the above principles. A classical monitoring of the water status of the main transitional water bodies of Algarve (South of Portugal) is combined with advanced in situ water profiling and hydrodynamic, water quality and ecological modelling to build a complete description of the system. The aim is to demonstrate a methodology where traditional monitoring and modelling tools can be joined together to draw a holistic picture. The results show that globally the water bodies present a good trophic status. Eutrophication symptoms are not generalized. Due to physical transport and dispersion, nutrient enrichment is not the only factor limiting growth; residence time is also an important factor. Some confined regions with high residence times are at present endangered by point and diffuse sources of pollution. The microbiologic impact of waste water treatment plants (WWTP) is confined to regions between 500 m and 1 km from the discharges.