SEDIMENT TRANSPORT AND DISPERSAL IN THE NEARSHORE OF “FLASH-FLOOD” RIVERS Manel Grifoll 1 , Vicenç Gracia 2 , Jorge Guillén 3 , Manuel Espino 4 and Agustín Sánchez-Arcilla 5 River sediment dispersal on the near-shore of “flash-flood” rivers is investigated using a coupled wave-current- sediment transport model. Besòs and Llobregat rivers (short and mountainous rivers in NW Mediterranean Sea, near to Barcelona City) are used as examples to study the sediment transport under “flash-flood” regime. The modeling system COWAST which includes the coupling between the water circulation model ROMS and the wave model SWAN, is applied to assess the sediment dispersal mechanisms and deposition in the coastal area off the two river mouths. Preferential depositional areas such as mud-belts were identified from the simulations. The sediment dispersal pattern obtained by the model agrees with observational measurments. Complementary numerical simulations revealed sorting of sediment grain size in the cross-shelf direction. Keywords: sediment transport; river dispersal; mud-belt; coupling INTRODUCTION The river-borne sediment dispersal and the interface between the river and the shelf involve physical and geological mechanisms acting at different temporal and spatial scales (Wright and Nittrouer 1995). Observational studies have revealed that, in many coastal environments, episodic storms dominate sediment fluxes and subsequent dispersal patterns (Sherwood et al. 1994, Warner et al. 2008a, Bever et al. 2011, Grifoll et al. 2013a). The sediment flux during storm events can be several orders of magnitude larger than during calm conditions (Guillén et al. 2006, Ulses et al. 2008, Bever et al. 2011). In consequence, preferential depositional patterns during river floods can partially explain long-term fine deposits observed on the sea floor (Ogston et al. 2000). On longer time-scales, advection of sediment by shelf currents can redistribute sediment and determine final deposition patterns (Sherwood et al. 1994, Harris et al. 2008, Bever et al. 2009, Xu et al., 2011, Xue et al. 2013). River sediment dispersal on the shallow continental shelf is investigated using a coupled wave- current-sediment transport model. Besòs and Llobregat rivers (short and mountainous rivers in NW Mediterranean Sea, near to Barcelona City) are used as examples to study the sediment transport under “flash-flood” regime. River “flash-flood” regime is characterized by sudden river freshwater peaks after rainy events and is typical for the Mediterranean Climate. The modeling system COWAST (Warner et al. 2010) which includes the coupling between the circulation model ROMS and the wave model SWAN, is applied to assess the sediment dispersal mechanisms in the coastal area off the two river mouths (see Figure 1). The sediment balance calculations consider river source, bed-load and suspended transport, and sedimentation/re-suspension processes due to waves and currents. Preliminary computations have been performed in order to identify the cross-shelf sorting of sediment grain-size observed in the D50 measurements (Grifoll et al. 2014). In this case the model was initialized with 4 homogenous sediment classes (D50 in mm): 0.125, 0.063, 0.06 and 0.015mm. Characteristic hydrodynamic conditions were applied monitoring the time-evolution of the class percentages. In a second step, the COAWST code has been used to simulate the hydrodynamics and sediment dispersal patterns for a one-year period. Open boundary conditions for currents were obtained from a nested system fed by the MyOcean system (see details in Grifoll et al. 2013b). Wave information at numerical boundaries was obtained from buoy measurements in the study area (see Figure 1.b). Hydrodynamics (currents, waves and sea-level) and sediment (suspended sediment concentrations) variables were recorded during two specific field campaigns and used to assess the skill of the code (see details in Grifoll et al. 2013b, Grifoll et al. 2014). Numerical computations agree reasonably well 1 Laboratory of Maritime Engineering, Technical University of Catalonia, Jordi Girona, 1-3, Barcelona, Catalonia, 08028, Spain. 2 Laboratory of Maritime Engineering, Technical University of Catalonia, Jordi Girona, 1-3, Barcelona, Catalonia, 08028, Spain. 3 Marine Science Institute - CSIC, Passeig Marítim de la Barceloneta, 37 - 49, Barcelona, Catalonia, 08028, Spain. 4 Laboratory of Maritime Engineering, Technical University of Catalonia, Jordi Girona, 1-3, Barcelona, Catalonia, 08028, Spain. 5 Laboratory of Maritime Engineering, Technical University of Catalonia, Jordi Girona, 1-3, Barcelona, Catalonia, 08003, Spain. 1