Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng Effect of wind, river discharge, and outer-shelf phenomena on circulation dynamics of the Atchafalaya Bay and shelf Mohammad Nabi Allahdadi b , Felix Jose a, ⁎ , Eurico J. D'Sa b , Dong S. Ko c a Department of Marine & Ecological Sciences, Florida Gulf Coast University, Fort Myers, FL, USA b Louisiana State University, Department of Oceanography and Coastal Sciences, Coastal Studies Institute, Baton Rouge, LA, USA c Oceanography Division, Naval Research Laboratory, Stennis Space Center, MS, USA ARTICLE INFO Keywords: Atchafalaya Bay Mike 3 model Coastal currents Lagrangian model Vertical eddy viscosity Cold fronts ABSTRACT The influence of wind, river discharge, and outer-shelf variations on the circulation of the Atchafalaya Bay and the adjoining inner shelf were examined using a 3-dimensional circulation model. Current and water level data from three stations along a transect off the Marsh Island were used for model calibration and skill assessment. Coastal current and its spatial distribution were significantly affected by open boundary conditions. Model sensitivity analysis suggested that the vertical eddy viscosity has a substantial impact on the energy and momentum transfer across the water column in this shallow bay-shelf environment. It was also shown that westward to northwestward currents dominated in the study area during the non-summer months and that would transport westward large volume of sediments discharged from rivers during the spring flood season. This sediment load is contributing to the progradation of the Chenier Plain along the southwestern Louisiana coast. A particle tracking Lagrangian model validates the westward migration of suspended sediments originating from the river mouth area during the spring season. 1. Introduction The Atchafalaya Bay, located on the western flank of Louisiana inner shelf, forms part of the greater Mississippi River drainage system. The Bay and the adjoining shelf are strongly influenced by the sheer volume of fresh water and sediment plume discharged from rivers, particularly during the spring flood season with circulation in the shallow shelf mainly driven by the wind (Allahdadi et al., 2011). About 19–29% of the river water and 30–40% of sediment load from the Mississippi River is diverted through the Atchafalaya River to the Atchafalaya-Vermillion Bay and then to the Gulf of Mexico (Mossa and Roberts, 1990; Allison et al., 2000; Walker and Hammack, 2000). Fresh water and sediment load are discharged through two main outlets, viz., Wax Lake outlet and Morgan City channel (Roberts and Sneider, 2000). For the inner Atchafalaya shelf, water quality is highly affected by seasonal hydrodynamics and morphology of the shelf, which modulates the salinity in the shelf and along the shoreline (Cobb et al., 2008a; Allahdadi et al., 2011). In this context, understanding the hydrodynamics within the Atchafalaya Bay and the adjoining inner shelf is essential for studies aiming to determine the fate and dispersal of fresh water and sediment load from the Atchafalaya River. For instance, strong southward currents associated with passage of cold fronts during winter/spring season have been identified for their significant effect on sediment transport inside and outside of the Atchafalaya Bay (Feng and Li, 2010). Currents in the Atchafalaya shelf follow the general circulation pattern of Louisiana coast (Cochrane and Kelley, 1986) and is influenced by seasonal wind, tides, river discharge, and outer-shelf variations induced by the Loop Current Eddies (Oey, 1995; Allahdadi et al., 2011). However, depending on the location, relative contribution of each individual forcing on the circulation is different and difficult to differentiate. A modeling study for the Louisiana shelf by Oey (1995) concluded that wind forcing accounts for up to 50% of the transport over the inner shelf with river discharge and outer shelf eddies contributing to the rest. Tide-generated currents are very weak due to the small tidal range over the Louisiana shelf (average of 0.4 m) and are of mixed-diurnal in nature (Wright et al., 1997). The dominant wind effect with varying direction for different seasons results in different circulation patterns. Easterly to south- easterly winds during most of the year (September to May) produce mostly westward (down-coast) currents over the inner-shelf (Cochrane and Kelly, 1986; Li et al., 1997; Allahdadi et al., 2013). A simultaneous eastward current produced by anticyclonic Loop Current eddies along the shelf break can generate a cyclonic gyre http://dx.doi.org/10.1016/j.oceaneng.2016.10.035 Received 2 March 2016; Received in revised form 23 September 2016; Accepted 21 October 2016 ⁎ Corresponding author. Ocean Engineering 129 (2017) 567–580 Available online 05 November 2016 0029-8018/ © 2016 Elsevier Ltd. All rights reserved. MARK