Part 1 : Model Description and Validation

,IIIha COBB, M.; KEEN, T.R., and WALKER, N.D., 2008. Modeling the circulation of the Atchafalaya Bay system during eef eeeeeeee winter cold front events. Part 1: model description and validation. Journal of Coastal Research, 24(4), 1036-1047. West Palm Beach (Florida), ISSN 0749-0208. The Atchafalaya Bay system consists of a series of five shallow bays in southern Louisiana (U.S.A.) that are dominated by the circulation of the Atchafalaya River plume. Winter cold fronts have a significant impact on the resuspension and transport of sediments in this region, and a better understanding of the circulation during these events is absolutely necessary for determining the sediment transport patterns of the Atchafalaya Bay system and the adjacent shelf area. Understanding the circulation of this region is also crucial for environmental studies as well. This work describes the implementation of the Navy Coastal Ocean Model (NCOM), a three-dimensional numerical circulation model for tide, river, and wind-forced circulation in the Atchafalaya Bay system. The model has a cell size (Ax) of -800 m and is nested to a northern Gulf of Mexico model (Ax 5000 m), which is itself nested to the global NCOM (Ax = 1/80). Atmospheric forcing is supplied by the Navy Operational Global Atmospheric Prediction System (NOGAPS) (Ax = 10). These models are used to simulate the hydrodynamics of the Atchafalaya Bay system and Atchafalaya river plume between December 1997 and January 1998 during the passage of three winter cold fronts. The water levels, salinity, and currents predicted by NCOM are in reasonable agreement with available measurements and tide-gauge elevation data. Errors in ebb tides and wind-driven circulation are attributable to uncertainties in the bathymetry and the low spatial and temporal resolution of the NOGAPS wind fields. ADDITIONAL INDEX WORDS: Coastal processes, Atchafalaya Bay system, river plume, salinity front, sediment transport, remote sensing, NCOM model, cold front. INTRODUCTION tive tool for coastal management. Perhaps more important, This paper describes and validates a realistic numerical these models can also provide long-term hindcasts, enabling hydrodynamic simulation of the Atchafalaya Bay region, coastal scientists to determine how a particular region has which is an important component of Louisiana's coastal ecobeen affected in the past by changes in river discharge, sedsystem. The shallow, muddy, and nutrient-rich environments iment transport, and energetic events such as winter cold of coastal Louisiana contribute substantially to the seafood fronts (or tropical storms) over a period of several years. To and recreational economies of the state, but these sensitive apply these models to river-dominated bays and estuaries, it areas are threatened by rising sea level and coastal erosion, is imperative that they first be validated with in situ field with short-term rates of shoreline retreat as high as 10 Myy data as well as remotely sensed observations (i.e., data ob(PENLAND et al., 2005). The extensive coastal wetlands also tained from satellites and flyovers). play an important role as buffers to protect the city of New Orleans from storm surges during hurricanes. Ultimately, Background and Study Area the long-term survival of this fragile environment is dependent on water circulation and sediment transport. A better The 1500 km Atchafalaya Bay system (Figure 1), which understanding of hydrodynamic and sediment transport procomprises five contiguous bays: Vermilion Bay, West and cesses within these coastal areas is, therefore, essential from East Cote Blanche Bays, Atchafalaya Bay, and Fourleague economic, environmental, and societal perspectives. Bay, represents one of Louisiana's most dynamic coastal enNumerical models for circulation and sediment transport vironments. This region was an abandoned delta complex of have the potential to provide accurate forecasts over large the Mississippi River until the 1950s when a new episode of coastal regions, thus providing an important and cost-effecdelta building began (ROBERTS, ADAms, and CUNNINGHAM, 1980; SCHLEMON, 1975). Since the 1970s, the Atchafalaya DOI: 10.2112/07-0877.1 received 23 April 2007; accepted in revision and Wax Lake subaerial deltas have grown where Atchafa18 July 2007. laya River water is discharged into Atchafalaya Bay via the