Journal of Coastal Research, Special Issue No. 65, 2013 1188 Portela, et al. Effect of salinity on the settling velocity of fine sediments of a harbour basin Lus Ivens Portela, Susana Ramos, Antnio Trigo Teixeira †National Laboratory of Civil Engineering, LNEC, Av. do Brasil 101, 1700-066 Lisboa, Portugal lportela@lnec.pt ‡Dept. of Civil Engineering and Architecture, Instituto Superior TØcnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal susana.c.ramos@ist.utl.pt trigo.teixeira@civil.ist.utl.pt INTRODUCTION Fine-grained sediments commonly found in many estuaries are mixtures of fine sediment fractions (in the clay and silt grain size ranges, i.e. finer than 63 m) and organic matter of diverse nature (Costa, 1995; Whitehouse et al., 2000). Fine-grained sediments in suspension are subject to flocculation. Therefore, settling velocities may not be directly related to grain size through Stokes’ law (Berlamont et al., 1993; Portela, 1997). Salinity is known to increase the cohesion of clay minerals, and thus flocculation, according to the double-layer theory (Maggi, 2005). However, in the estuarine environment, this mechanism is complicated by organic coatings on the particles and by organic and biological aggregation (Van Leussen, 1988). As a result, the influence of salinity on the settling velocity of natural particles is often unclear. Several classic studies have examined the influence of salinity on the settling velocity (Migniot, 1968; Burt, 1986), but the topic is rarely addressed in the recent literature (Mietta et al., 2009; Mikeš and Manning, 2010). In the past 20-30 years, many studies have focused instead on other controlling factors, such as suspended sediment concentration and turbulence (Shi and Zhou, 2004; Pejrup and Mikkelsen, 2010). Due to the fragile nature of flocs (i.e. aggregates of discrete particles) it has been argued that their properties and settling velocities should be determined in situ (Whitehouse et al., 2000; Mikkelsen et al., 2004). Field studies have the advantage of minimizing changes in the size distribution of the flocculated sediment, but the natural variability of field conditions makes it difficult to study the regulating factors of settling velocity in a systematic manner. This can be better achieved through controlled laboratory experiments (Manning et al., 2007). The purpose of the present paper is to examine the effect of salinity on the settling velocity of fine sediments, through laboratory experiments. The sediments were collected from a marina basin in the Tagus estuary (Portugal). STUDY AREA With an area of about 320 km 2 , the Tagus estuary is a large mesotidal system (Figure 1). In the middle-upper estuary, the maximum tidal range is 4.5 m, the mean spring range 3.3 m and the mean neap range 1.6 m. Due to the large tidal prism, the flow is driven mainly by the tide. Tidal currents in the middle-upper estuary reach 1 m s -1 during spring tides. The main source of freshwater is the Tagus river with a mean flow of 300 m 3 s -1 . Salinity at the location varies with the tide and the river flow, with typical values between 25 and 30. Fine-grained sediments are the dominant surface material in the intertidal areas of the upper estuary, where suspended sediment concentrations range between 50 and 250 mg l -1 (Portela, 1996). ABSTRACT Portela, L.I., Ramos, S. and Trigo-Teixeira, A., 2013. Effect of salinity on the settling velocity of fine sediments of a harbour basin. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T. J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 1188-1193, ISSN 0749-0208. Salinity is known to increase the cohesion of clay minerals, and thus the flocculation of fine-grained sediments in suspension. However, the influence of salinity on the settling velocity of natural particles is often unclear, due to organic and biological aggregation and other controlling factors. This paper examines the effect of salinity on the settling velocity of fine sediments collected in a harbour basin in the Tagus estuary. The sediment sample consists of silt- and clay-sized particles (D10 = 2 m; D50 = 9 m; D90 = 37 m). Quartz is the main constituent and illite the main clay mineral. The experiments were conducted in a 2.25-m high settling column, for a constant initial concentration of fine sediment (1.5 g l -1 ) and different salinity values (0, 5, 10, 15 and 30). During each experiment, samples were collected at 10 vertical levels and at 10 time instants. Settling velocities were calculated on the basis of the time- evolution of suspended sediment concentrations. At the start of the experiments, after cessation of turbulence, settling velocities increased in direct relation with the increase in salinity, the maximum values ranging between 0.3 mm s -1 under freshwater conditions and 0.9 mm s -1 for a salinity of 30. After 5 hours, the proportion of the initial sediment remaining in suspension was 45% under freshwater conditions and only about 10% for salinities between 10 and 30. Using the mass-weighted mean settling velocity to describe the deposition fluxes, it is concluded that settling increases by a factor of 6.5 between freshwater and marine conditions. ADDITIONAL INDEX WORDS: Cohesive sediment, harbour siltation, settling column. www.JCRonline.org ____________________ DOI: 10.2112/SI65-201.1 received 07 December 2012; accepted 06 March 2013. ' Coastal Education & Research Foundation 2013 www.cerf-jcr.org