Summary

The exploitation of superficial minerals deposits from the continental shelf as a supply of sediments for different activities (e.g., for beach nourishment, industrial and construction purposes) represents an interesting alternative to the usual extractions performed in rivers and estuaries, namely due to its quality, availability and transport. The pits left after dredging are not immutable in time. They tend to infill at a rate that is dictated by the dredged volume, the depth at which the extraction was made (distance to shore) and by the hydrodynamic conditions (mean and wave induced currents) prevailing at those sites. Under certain conditions, the infill process and modifications of the nearshore wave conditions around the pit might have a negative impact in the coastal sediment budget, promoting erosion at the shore. Other local disturbances are likely to occur that will produce impacts on the fauna, flora and water quality. This project will assess the physical impacts of sand mining on the shoreface and shoreline evolution. For describing these processes knowledge on the hydrodynamic and sediment transport processes is required. This study will be performed with a two dimensional horizontal morphodynamic model, MORSYS2D, and with a one line model (LITLINE) that will address the shoreline changes associated with the modification of the wave field due to the modified bathymetry. Two offshore areas in the mainland Portuguese continental shelf located in the western and southern coasts, characterized by different energetic wave regimes, will be chosen as possible areas where extraction sites will take place. A sedimentology characterization of the study areas will be performed allowing the mapping of the geological structure and the sediments distribution at the seabed. This information will be used to establish possible configuration scenarios of the extraction sites, in terms of the dimensions, volumes extracted and distance to shore, as well as giving realistic field conditions necessary for the computational simulations. Short-medium and long term simulations with the numerical models will be performed allowing the evaluation of the time scale of regeneration of the extraction sites and the impacts of mining on shore line evolution for each of the configurations. The influence of different wave regimes representative for each study area and extreme wave height events will also be assessed. Appropriate guide lines concerning the extractions sites will be recommended as those that minimize the impact on the coastal sediment budget together with fast regenerating times. A third area located offshore the Faial island in Azores will also be studied. The aim is to monitor the evolution of sandpits within a time span of one year, to evaluate the impact of the dredging on the seabed and establish the sediment dynamics.