IMPROVED TIDAL FLUSHING CAPACITY BY HARBOR PLANNING: THE STUDY CASE OF THE CEUTA HARBOR

摘要

Water quality in harbor areas is a topic of increasing interest because of the need of making sustainable both their economic and environmental services. Besides the common sources of point and diffuse pollution related to navigation and port activities, the physical restriction to the renovation capacity induced by breakwaters, dikes and docs is a relevant factor to define the quality and ecological potential of these water masses (Directive, 200/60). Transport time scales, such as the Flushing Time and Residence Time, are sensitive descriptors to these hydromorphological alterations. In this work we show that the flushing capacity of the harbor water masses can be significantly improved by appropriate design of multiple harbor entrances. For that, we have analyzed the water dynamics of the Ceuta harbor (North Africa). This harbor differs from conventional harbors since it has two opposing entrances connecting two bays and it has been reported as a high biodiversity water mass (Guerra-Garcia & Garcia-Gomez, 2004). Using a 2DH eco-hydraulic model and data sets from local monitoring surveys, we show that the coupled dynamics of the two opposing entrances, under local climate conditions, increases significantly the renewal capacity of the Ceuta harbor. Model results indicate that the tidal flushing of the harbor is increased up to 20% for all the tidal ranges in presence of the two open entrances. Water level differences induced by astronomical and meteorological tides at the North and South bays, and the local head losses induced by the morphology of the South entrance are found to be the underlying causes for that increment. Moreover, the seasonal variability and the residence time distribution are found to be significantly influenced by the two entrance dynamics. This conclusion is consistent with the distribution of pollution levels measured in the sediments of the Ceuta harbor. Our findings highlight that multiple harbor entrances should be taken into consideration in the design, construction and management of harbor areas to improve their renovation capacity.