J. S. Antunes do Carmo

Near-shore sediment dynamics computation under the combined effects of waves and currents

Abstract An integrated computational structure for non-cohesive sediment-transport and bed-level changes in near-shore regions has been developed. It is basically composed of: (1) three hydrodynamic sub-models; (2) a dynamic equation for the sediment transport (of the Bailard-type); and (3) an extended sediment balance equation. A shallow-water approximation, or Saint-Venant-type model, is utilized for the computation and up-to-date field currents, initially and after each characteristic computational period. A Berkhoff-type wave model allows us to determine the wave characteristics in deep water and intermediate water conditions. These computations make it possible to define a smaller modeling area for a non-linear wave–current model of the Boussinesq-type, including breaking waves, friction effects and improved dispersion wave characteristics. Bed topography is updated after each wave period, or a multiple of this, called computational sedimentary period. Applicability of the computational structure is confirmed through laboratory experiments. Practical results of a real-world application obtained around the S. Lourenco fortification, Tagus estuary (Portugal), with the intention of preventing the destruction of the Bugio lighthouse, are shown.

Numerical Modelling Of Oil Spills In Coastal Zones. A Case Study

A computational structure has been developed to forecast the time-space evolution of oil spills in marine environments. This structure was developed taking into account widely used mathematical formulations for oil spreading and weathering processes, A Eulerian transport model, that uses hydrodynamic results obtained with a two-dimensional and a quasi three-dimensional hydrodynamic model, was used to predict the oil slick transport and spread. This paper presents the general characteristics of the computational structure and the results of its application to a real case study: the Cereal accident in October 1994,

Settlement of vertical piles exposed to waves

Abstract In recent years, several authors have conducted experimental studies on the scour around piles fixed in the wave flume. In addition, the settlement of structures has been studied for the case of pipelines, due either to steady currents [International Journal of Offshore and Polar Engineering 4 (1) (1994) 30] or to wave forcing [Coastal Engineering 42(2001)313]. However, the settlement of a pile due to scour in waves has so far not been investigated. This paper reports an experimental study on the settlement of vertical cylindrical piles, either surface-piercing or entirely submerged, exposed to waves. The primary objective of the study was to investigate: (i) the scour-settlement process with piles of differing heights and densities with their bases initially lying on the seabed and (ii) the influence of the piles height and weight on the hydrodynamics vertical settlement. A series of experiments was carried out using isolated vertical cylinders of a given diameter (30 mm), different materials (PVC, aluminium, brass, copper and bronze) and different heights (30, 6 and 3 cm). The characteristics of the waves remained constant in all tests (water height=15 cm, wave period=2.1 s, local wave height=4.0 cm, Keulegan–Carpenter number=12). The cylinders were set up over a sand bed, so that they could move vertically. Both the initial settlement of each cylinder (when it occurs), due to its weight, and the evolution of additional settlement caused by flow-induced scour were measured. The tests were conducted over a period of 4000 wave cycles.

Sudden bed changes and wave-current interactions in coastal regions

Mean bottom evolutions due to currents and wind waves, even due to wave-current interactions, are easily computed by averaging mean quantities over one or more wave cycles. However, dealing with fine processes, like breaking waves and bar formations in coastal regions, great quantities of sediment are transported and, as a consequence, considerable erosion and deposition can occur quite rapidly. Other phenomena normally associated with earthquakes, like volcanic eruptions, landslides, etc. occur frequently in various coastal regions of the terrestrial globe. Those problems can only be tackled by using a more complete set of equations with improved wave dispersion characteristics, and taking into account time-bed-level changes. Other characteristic non-linearity phenomena in shallow water regions and wave-current interaction become important factors that have to be considered. From the sedimentary point of view, particularly in terms of the wave and current fields, it is not known whether the existing sand transport models are generally valid.The applicability of a computational structure, based on extended Boussinesq-type equations, and two existing sediment transport models are discussed and confirmed through published data. Numerical results obtained at Ria Formosa, Algarve, in the ambit of the European Union INDIA Project are shown.

Contribution to the preservation of healthy coastal ecosystems

The release of pollutants into coastal zones from municipal and/or industrial drainage systems is a problem with a considerable environmental impact. In order to minimize the contamination of coastal waters in certain places (e.g., along beaches and in aquacultures), solutions based on the construction of submarine sewer outfalls have been proposed. In order to ascertain the optimal conditions for the release of an emissary, in particular the place of release, it is necessary to characterize the hydrodynamic transport processes that govern the evolution and mixture of pollutants in the area of interest. Circulation in the coastal zone is complicated, given that it is determined by a set of forcing mechanisms of diverse origins (meteorological, astronomical, large-scale ocean circulation, etc.) that endow it with a considerable space-time variation. Wastewater plume behaviour and characteristics depend both on the receiving water conditions and on the discharge characteristics. Accordingly, the implementation of a submarine outfall system requires a prior study of the site where the outfall is to be constructed, in order to achieve optimal rates of dilution of the pollutants released to the environment. Mathematical modelling appears to be a very useful tool for coastal zone environmental management either for continuous monitoring analysis or in accidental ecosystem rupture. This work presents some results and conclusions of two case studies. The first is a study of the pollutant cloud released at the Sao Jacinto submarine sewer outfall, located about 3 km offshore and 3 km north of an important coastal lagoon (NW Portugal). The main focus of this study was to establish the hydrodynamic conditions in which the pollutant release might affect the coastline (principally the existing beaches and lagoon). The second case presents some hydrodynamic results and water quality aspects of the Ria de Arosa (NW Spain), and also includes an assessment of the environmental impact of wastewater discharges from nine submarine outfalls considered in the sanitation plan of this bay. Results of faecal matter concentration distribution were used in the discussion on the outfall discharges’ impact on the natural water system for different hydrodynamic scenarios. In both cases, 3D numerical models were used.

Hydrodynamics around coastal structures

The numerical model MECCA (Model for Estuarine and Coastal Circulation Assessment) – which was developed to simulate tidal currents, density currents or those generated by wind action, in bays or coastal zones (Hess, 1989) – is a “three-dimensional model”, or a quasi-3D model, which is able to satisfactorily simulate complex three-dimensional flows. The MECCA model algorithm is based on the continuity equation (under the non-compressibility hypothesis), on the Navier-Stokes equations (considering hydrostatic equilibrium and the Boussinesq hypothesis) and on conservation equations for temperature and salinity. It is structured in two modes: the external-mode, obtained by vertical integration of the Navier-Stokes equations, and the internal-mode, obtained by subtracting the equations for the external-mode velocity from the equations for the total velocity. The flow within the bottom boundary layer is alternatively described by a Prandtl type model or by a lDV model of the K-L type. We conducted a rigorous study of its characteristics and hydrodynamic capacities with the aim of extending the MECCA model to sedimentary dynamics applications (suspended and bedload transport). Numerical results were validated by comparisons with laboratory data and other numerical results and theoretical solutions. This work presents various simulations of the flow around structures commonly occurring in the fluvial and coastal environment, like structure pillars, breakwaters and piers. Particular attention has been given to the twodimensional and three-dimensional structures of the flow characteristics. We also present sensitivity analyses of the model, using different grids, as well as the two implemented boundary layer models.