Journal of Coastal Research, Special Issue 56, 2009 Journal of Coastal Research SI 56 213 - 217 ICS2009 (Proceedings) Portugal ISSN 0749-0258 Origin and Evolution of Tagus Estuarine Beaches R. Taborda†, P. Freire‡, A. Silva†, C. Andrade∞ and M. C. Freitas∞ †LATTEX, IDL Lisbon University, Lisbon 1749-016, Portugal {rtaborda, amasilva}@fc.ul.pt ‡National Laboratory of Civil Engineering, Lisbon 1700-066, Portugal pfreire@lnec.pt ∞CEGUL Lisbon University, Lisbon 1749-016, Portugal {candrade, cfreitas}@fc.ul.pt ABSTRACT TABORDA, R., FREIRE, P., SILVA, A., ANDRADE, C. and FREITAS, M. C., 2009. Origin and evolution of Tagus estuarine beaches. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 213 – 217. Lisbon, Portugal, ISSN 0749-0258 Estuaries are unique systems with an unquestionable economical, ecological and recreational value which has motivated a large amount of research work. Out of the research focus emerge the estuarine margins and more particularly the estuarine beaches, especially in what concerns their macro-scale evolution. This work aims to understand the origin and evolution of Tagus estuary beaches. To meet this objective, a detailed geomorphological characterization of peri-estuarine margins, complemented with geological and sedimentological data, was performed. This study was coupled with the analysis of meteorological and hydrodynamic processes that control the estuarine sedimentary dynamics; particular attention was given to local waves through the application of a wind-wave generated model calibrated with field data. The origin of Tagus estuarine beaches is related with the Holocene transgression, which promoted the drowning of the present-day upper estuarine depression and generated a wide and shallow fetch-limited basin. From the transgressive maximum onwards the estuary initiated an infilling process dominated by fluvial fine-grained deposition within the estuarine basin, whereas sand input was retained in a bay-head delta. At the same time, fetch-limited waves promoted the erosion of estuarine margins, initiating beach development, related to the growth of sand spits anchored in Plio-Pleistocene reliefs. This process depended on wave energy and also on mean wave power direction, which controlled the spit growing pattern. At present, beach evolution reflects the disturbance of sediment budget due to the increase of human intervention in estuarine margins. ADITIONAL INDEX WORDS: wave generation, sediment transport, Holocene transgression. INTRODUCTION Estuaries are unique systems with an unquestionable economical, ecological and recreational value. This has motivated a huge amount of research work, though, as expected, this effort has been strongly biased towards the environments with either larger ecological relevance (mudflats and salt marshes) or greater economical importance (subtidal areas, mostly related with industrial and port activities). Out of the research focus emerge the estuarine margins and more particularly the estuarine beaches. This is mainly due to the fact that estuarine beaches generally have an unattractive appearance, the water quality is usually poorer than in ocean beaches, reducing their recreational value, and support less diversified and productive habitats, in comparison to other portions of the estuary. However, the dynamics and evolution of estuarine beaches are the basis of some estuarine management problems, for example, the loss of marsh areas by erosion or by sedimentary burial under transgressing bay barriers, the loss of buildings, roads, and upland habitat as a result of beach erosion (NORDSTROM, 1992). Moreover, estuarine beaches can also have large recreational and ecological value; for instance, in cases where water quality is suitable for bathing, estuarine beaches can be considered an alternative to ocean beaches, near urban areas. Furthermore, while beaches are generally regarded as “deserts” in what concerns life support, for ecologists they are key ecosystems (SCAPINI, 2003). Only recently these issues were properly recognized and, since the pioneer work of NORDSTROM (1992), the number of publications on estuarine beaches has increased exponentially. Research work on estuarine beaches has, generally, focused on short term dynamics (e.g. JACKSON et al. 2002; NORDSTROM et al., 2003), geomorphic-biotic interactions (JACKSON et al., 2005) and management problems (NORDSTROM, 1992; JACKSON, 1996). However, process-based studies dealing with larger timescales, that can give valuable insights on estuarine beach origin and evolution, are still relatively scarce. Accordingly, the main objective of the present work is to contribute to understand the dynamics of beaches in the Tagus, inner estuary, in what concerns their origin and long-term evolution, focusing on the forcing mechanisms and sediment budget. This analysis was supported by the study of sediment supply and key distribution processes, and linked with the Late Pleistocene and Holocene paleogeography of the lower estuary. STUDY AREA The Tagus estuary, located on the Portuguese western coast near Lisbon, is one of the largest estuaries in Europe covering an area of nearly 320 km 2 , extending about 40 km from the estuary mouth up to Vila Franca de Xira (Figure 1), the upward limit of the saline intrusion in normal meteorological conditions. The Tagus estuary presents a peculiar morphology, with a large and shallow inner domain, maximum margin separation along the NNE-SSW direction of 15 km and a narrow fault-controlled deep inlet channel trending ENE-WSW. The estuary is a semi-diurnal 213