Journal of Coastal Research, Special Issue 57, 2011 Journal of Coastal Research SI 64 pg - pg ICS2011 (Proceedings) Poland ISSN 0749-0208 Three dimensional circulation modeling in the Dee Estuary R. Bolaños†, J.M. Brown† and A. Souza† †National Oceanography Centre NERC Liverpool L3 6EL, UK rbol@pol.ac.uk ABSTRACT BOLAÑOS, R., BROWN J.M. and SOUZA, A., 2011. Three dimensional circulation modeling in the Dee Estuary. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), pg – pg. Szczecin, Poland, ISSN 0749-0208 In estuarine areas nonlinear wave-current-stratification interaction is important in determining the 3D current circulation. A 3D ocean model has been implemented in the Dee estuary, Liverpool Bay (UK), to evaluate its performance and describe the coastal oceanography of a shallow estuarine region. The model includes both barotropic and baroclinic processes arising from tides, rivers and full atmospheric forcing, and is coupled to a spectral wave model. The model is evaluated by means of comparison with in situ salinity, current and wave data. The model accurately predicts sea level elevations finding errors in the order of a few cm. Velocity however is less accurate and very sensitive to geographical location. The timing of the model is correct, however, the magnitudes are underestimated when comparing with the ADCP data, this occurs mainly at flood tide. The consideration of 3D stratification effects improved the estimations of the residual velocities when compared with ADCP data, this was more evident in the cross-channel component which is less tidal dominant. Density distribution is mainly controlled by salinity which presents a tidal signal. Wave effects are relatively small; taking into account their associated processes did not improve the model when simulating currents or salinity distribution. ADITIONAL INDEX WORDS: Coastal modeling, wave-current interaction, stratification, POLCOMS, WAM, GOTM INTRODUCTION Coastal waves and currents are highly variable and can have a significant impact on human activities and structures. Coastal environmental processes do not take place in isolation but interact to form a complex system. Theoretical work on wave-current interaction has been taking place over several decades (Andrews and McIntyre, 1978; Mellor, 2008; Ardhuin et al., 2008). The main effects of waves on the mean flow commonly considered are due to the radiation stress and Stokes drift, although interaction with turbulence can also be an important process (Babanin et al., 2009). A coupled 2D wave-current model using an unstructured grid applied to a hurricane in the Gulf of Mexico and a storm in the Adriatic Sea (Roland et al., 2009) shows the importance of considering radiation stress when modeling water levels. Tang et al., (2007) implemented a wave-current interaction formulation and evaluated the model by comparison with surface drifters. They showed that the Stokes drift was the dominant effect with a contribution of about 35%. They also show a reduction of momentum transfer from wind to currents if waves are taken into account. Improved surge simulations have been found when accounting for a wave dependent surface roughness (Moon et al., 2009) and a wave-current bottom roughness (Jones and Davis, 1998). Estuarine circulation is characterized by strong tidal currents, rough bathymetry, strong turbulence, and steep density gradients due to the interaction between ocean and river waters. Even though tidal current speed can be one order of magnitude greater than the residual current, the latter plays an important role in net exchange of particles in suspension. Estuarine residual currents have been attributed to gravitational circulation, where a density gradient drives a seaward flow in the surface layer and landward flow near the bed (e.g., Pritchard, 1952; Hansen and Rattray, 1965). Recently, tidal pumping due to tidal asymmetry in turbulent mixing, which results from the strain-induced periodic stratification (Simpson, 1990) has also been suggested to cause residual currents (Stacey, 2001). The relative importance of baroclinic (density gradient) and barotropic (tidal pumping) components on residual circulation depends on the particular characteristics of the estuary such as tidal range, freshwater input, and morphology. The main objective of the present work is to assess the main processes to be taken into account in an optimized 3D model (POLCOMS, Proudman Oceanographic Laboratory Coastal-Ocean Modeling System) of the estuary. Special attention will be paid to the effect of stratification and wave-current interaction. POLCOMS has been implemented for different areas such as the Irish Sea (Osuna and Wolf, 2005), the North Sea (Holt and James, 1999), the Mediterranean Sea (Bolaños et al., 2010), but it has not been validated in an estuary at the resolutions considered in the present work. The Dee Estuary The Dee is a macrotidal, funnel-shaped estuary situated in the eastern Irish Sea (Figure 1). It has a length of about 30 km, with a maximum width of 8.5 km at the estuary mouth. The average tidal prism in the Dee is 4x108 m 3 , annual mean river discharge is only 31 m 3 /s making the Dee a tidally dominated estuary. The tidal range during mean spring at Hilbre Island is approximately 10 m.