Journal of Coastal Research, Special Issue No. 70, 2014 Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) 604 Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) Sandra Plecha†, Ana Picado†, Pedro Chambel-Leitão‡, João M. Dias†, Nuno Vaz† †NMEC, CESAM University of Aveiro Aveiro, Portugal {sandraplecha, ana.picado, joao.dias, nuno.vaz}@ua.pt ‡ MARETEC Technical University of Lisbon Lisbon, Portugal chambelpc@ist.utl.pt INTRODUCTION Tidal dominated coastal systems, such as estuaries and lagoons, have experienced over the time, changes in geomorphology (especially erosion), which may be attributed to sea level rise (SLR), variations in the amount of sediment deposited on the coast and to anthropogenic degradation (Santos and Miranda, 2006). These changes could increase marginal inundation risk, salinity intrusion on adjacent land, marginal and bottom erosion, as well as restricting the navigation in shallow channels. Moreover, SLR may also contribute to changes in estuarine patterns of dissolved and non-dissolved matter, such as fine-grained cohesive sediments. The importance of cohesive sediments in coastal regions has been widely recognized, playing an important role in the functioning of healthy ecosystems, since their transport, propagation and concentration influence the development of flora and fauna in estuaries and lagoons. Additionally, changes in sea level could induce changes in the spatial distribution of cohesive sediments and influence ecosystem biogeochemistry. The variation in composition and spatial variability in suspended sediment dynamics render them difficult to study in the field. This is further hindered by complex interactions between tide and freshwater discharge. One way to overcome difficulties and study spatial and temporal variability of suspended sediments is to use high resolution numerical models. In this study a numerical model is used to evaluate cohesive sediment dynamics in Ria de Aveiro (Figure 1), under both present mean sea level and a future sea level rise (SLR) scenario (+0.42 m). Several studies have been undertaken on the effect of SLR in Ria de Aveiro. For example, Lopes et al. (2001) analyzed numerically the influence of tides and river inputs on suspended sediment concentration (SSC) in Ria de Aveiro lagoon, emphasizing Laranjo Bay from a bio-geophysical point of view. Also, Dias et al. (2003), Lopes et al. (2006) and Abrantes et al. (2006) used numerical models to investigate the spatial and temporal variability of SSC in this lagoon. Those authors concluded that tidal, wind- and river-induced residual currents contribute to the net export of water and sediments seaward, which is consistent with the ebb dominant nature of the system. In this study, a high resolution numerical model is used to investigate the impact of SLR in the spatial and temporal patterns of suspended sediment in the Ria de Aveiro. STUDY AREA The Ria de Aveiro lagoon (Figure 1), is located in a temperate climate area on the northwest coast of Portugal. Between the Vouga river basin and the Atlantic Ocean, this lagoon is highly influenced by marine and fluvial interaction. The lagoon is characterized by a complex geometry comprising several channels and intertidal areas, and is connected to the Atlantic Ocean through a narrow channel of 1.3 km length, 350 m wide and more than 30 m deep. The Ria de Aveiro is a mesotidal system (with an average range at the inlet of 2.0 m) with a tidal semidiurnal pattern (Dias et al., 2000). The tidal prism of the lagoon is approximately 139.7×10 6 m 3 for maximum spring tide and 65.8×10 6 m 3 for minimum neap tide (Picado et al., 2010), with the water flux through the inlet ranging between 6.26×10 3 m 3 s -1 for spring tide and 2.95×10 3 m 3 s -1 for neap tide (Lopes et al., 2013). ABSTRACT Plecha, S., Picado, A., Chambel-Leitão, P., Dias, J.M., Vaz, N., 2014. Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13 th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 604- 609, ISSN 0749-0208. Suspended sediment concentrations are simulated at Ria de Aveiro, a lagoon located in a temperate climate area in the northwest of Portugal. The fine-grained suspended sediment concentration is analyzed using the numerical model MOHID (www.mohid.com) and spatial maps of instantaneous and maximum concentration and also temporal variability at specified locations are analyzed in order to characterize the influence of the tide and sea level conditions in the suspended sediment concentrations within the lagoon. The highest suspended sediment concentrations were found in upstream areas during ebb conditions due to the river’s proximity, while the minimum concentrations were observed near the inlet due to the presence of marine water poor in suspended sediments. When a 0.42 m sea-level rise (an estimate of conditions in 2100) is modelled, a decrease in suspended sediment concentration is observed for the overall study area, as result of an increase in the tidal prism. ADDITIONAL INDEX WORDS: Suspended sediment, sea level rise, sediments modeling, MOHID. www.JCRonline.org ____________________ DOI: 10.2112/SI70-102.1 received 29 November 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2014 www.cerf-jcr.org