Modelling the influence of currents on wave propagation at the entrance of the Tagus estuary L. Rusu, M. Bernardino, C. Guedes Soares n Centre for Marine Technology and Engineering (CENTEC), Technical University of Lisbon, Instituto Superior Te´cnico, Portugal article info Article history: Received 11 July 2010 Accepted 15 May 2011 Editor-in-chief: A.I. Inceik Available online 8 June 2011 Keywords: Waves Currents Interactions Estuary Numerical models abstract A study of the wave propagation and of the consequences of the influence of currents on waves in the Tagus estuary is performed in the present work. For this purpose a high-resolution SWAN domain was coupled to a wave prediction system based on the two state of the art phase averaged wave models, WAM for wave generation and SWAN for nearshore wave transformation. The most important factors affecting the incoming waves are the local currents and the wind. These influences were evaluated by performing SWAN simulations in the target area with and respectively without considering the tide level and tide induced currents. The model results were compared with wave measurements, validating in this way the results of the wave prediction system developed herewith. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The Tagus estuary is a roughly a pear-shaped estuary, approxi- mately 30 km long and varying in width from approximately 15 km at the northern upstream section to 2 km at the down- stream entrance, just south of Lisbon, covering an area of approximately 350 km 2 . Morphologically the estuary can be divided into three parts: upstream, middle and downstream sections. The upstream area of the estuary is very shallow with an average depth of 2 m and with extensive inter-tidal flats. The middle part is deeper (average of 7 m), and the downstream part reaches depths of 46 m and is the main navigation channel of the estuary. This channel is 12 km long and 2–4 km wide. The bathymetry of the Tagus estuary, the target area of the present work, is illustrated in Fig. 1. This estuary is meso-tidal with spring and neap tide ampli- tudes of 1.5 and 0.6 m, and its circulation is mainly tidally driven. The wind is of secondary importance for estuarine circulation. In the entrance channel the current velocity may reach 2.5 m/s in spring tide. The circulation in the mouth of the estuary is also influenced by the wave field. In the study developed by Teles et al. (2003), the increasing of the wave penetration in the area at the entrance of the channel is discussed. This advanced penetration is verified for various incident directions producing the so called ‘sea beatings’, phenomenon which the pilots in the channel are encountering until about the middle of the channel. In fact, the southern part of the channel is often exposed to waves. Most of the time, in the highest waves, the swell is the dominant component. However, strong winds are also characteristic in that area and they can induce locally generated wind seas. The final conclusion resulting from the study of Teles et al. (2003) is that waves represent a determinant factor in the morpho-dynamic equilibrium in the downstream area. Various other studies such as, Oliveira (2000), Vieira and Bernardino (2005), Santos et al. (2007), were also performed. However, the influence of currents on waves in the estuary was not included in those studies. As regards the processes associated with the wave–current interactions in coastal areas, an important role is played by the water level (either induced by storm surge, wave induced set up or tide). The water depth variation due to such causes has a direct impact on the wave modelling. There are several studies showing such importance in wave–current interactions as for example that of Liu and Xie (2009). The importance of a better understanding of the wave condi- tions in the channel is enhanced by the fact that this channel represents the principal connection between the Port of Lisbon and the ocean. Hence this sector is subjected to high navigation traffic and high current conditions can occur making the interac- tions between waves and currents quite relevant. In particular, at the entrance in the navigation channel of the Tagus estuary strong interactions occur between the incoming ocean waves and the tide induced currents because the narrow geometry of the channel makes the angles between wave directions and current directions to be either close to 01 for following currents or close Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/oceaneng Ocean Engineering 0029-8018/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2011.05.016 n Corresponding author. E-mail address: guedess@mar.ist.utl.pt (C. Guedes Soares). Ocean Engineering 38 (2011) 1174–1183