Simulating morphodynamics with unstructured grids: Description and validation of a modeling system for coastal applications Xavier Bertin * , Anabela Oliveira, André B. Fortunato Estuaries and Coastal Zones Division, National Laboratory of Civil Engineering, Av. do Brasil, 101, 1700-066 Lisbon, Portugal article info Article history: Received 18 March 2008 Received in revised form 8 September 2008 Accepted 5 November 2008 Available online 14 November 2008 Keywords: Morphodynamic model Model assessment Wave–current interactions St. Trojan Beach Óbidos Lagoon abstract Morphodynamic modeling systems are being subjected to a growing development over the last decade and increasingly appear as valuable tools for understanding and predicting coastal dynamics and mor- phological changes. The recent improvements of a 2DH unstructured grid morphodynamic modeling sys- tem are presented in this paper and include the implementation of an adaptive morphodynamic time step, the integration and full coupling of a wave model and the forcing by large scale wave and tide mod- els. This modeling system was first applied to a dissipative wave-dominated beach located on the French coast, where the availability of field data allowed for a fine calibration and validation of wave-induced flows and longshore transport, and an assessment of the various sediment transport formulae. The mod- eling system was then applied to a very dynamic Portuguese tidal inlet where numerical tests show the computational efficiency of using an adaptive time step. Morphodynamic simulations of this inlet with real wave and tidal forcings resulted in realistic morphological predictions. The two applications show that the improved modeling system is able to predict hydrodynamics, transport and morphological evo- lutions in complex coastal environments. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Morphodynamic modeling systems consist of a set of modules to simulate shallow water flows, wave propagation, sediment transport and bottom evolution. These systems have been in exten- sive development over the past 15 years in Europe (de Vriend et al., 1993; Wang et al., 1995; Johnson et al., 1994; de Vriend, 1996; Nicholson et al., 1997; Lesser et al., 2004) and in the US (Holliday et al., 2002; Kubatko et al., 2006; Long et al., 2008). The increasing volume of literature on coastal morphodynamic modeling over the past five years highlights the growing interest of coastal research- ers and engineers in these techniques. Among these recent publica- tions, several were focused on the numerical methods used to solve the Exner equation (Johnson and Zyserman, 2002; Callaghan et al., 2006; Fortunato and Oliveira, 2007a; Long et al., 2008), others presented and validated new modeling systems (Lesser et al., 2004; Fortunato and Oliveira, 2004; Kubatko et al., 2006; Saied and Tsanis, 2008), while others described applications of pre-existing modeling systems to complex coastal environments (Sutherland et al., 2004; Grunnet et al., 2004; Jones et al., 2007). Since the developments of these modeling systems have been fuelled, to a large extent, by the need to address coastal engineer- ing problems, one would expect successful applications to grow rapidly, as new and better tools became available. Yet, these appli- cations remain scarce in the literature and do not seem to be increasing (Cayocca, 2001; Work et al., 2001; Grunnet et al., 2004). This observation suggests that simulating coastal morpho- dynamics remains a challenging task and that increasing the sophistication of the models does not necessarily improve the quality of morphological predictions. Indeed, models comparisons do not always recommend the most sophisticated approaches. For instance, Grunnet et al. (2004) suggest that the improved represen- tation of physics brought by a fully 3D approach may not outweigh its higher computational cost, and that using a 2DH approach may perform equally well if the final goal is bathymetric evolution solely. This study presents recent developments of the unstructured grid morphodynamic modeling system MORSYS2D (Fortunato and Oliveira, 2004, 2007a). This modeling system aims at simulat- ing hydrodynamics, transport of non-cohesive sediments and mor- phological evolutions in real coastal systems driven by tides, waves, wind and river flows, such as tidal inlets. MORSYS2D was improved in the spirit of integrating less sophisticated physics than other modeling systems, which often implies excessive model tun- ing and large computational resources, but placing greater empha- sis on the forcings and on the representation of the main processes, to perform long-term simulations with a reasonable computational cost. These recent developments are described in Section 2 and in- clude the implementation of a time-adaptive morphodynamic time 1463-5003/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ocemod.2008.11.001 * Corresponding author. Tel.: +351 218443758; fax: +351 218443016. E-mail address: xbertin@lnec.pt (X. Bertin). Ocean Modelling 28 (2009) 75–87 Contents lists available at ScienceDirect Ocean Modelling journal homepage: www.elsevier.com/locate/ocemod