Research papers Dispersal of Mississippi and Atchafalaya sediment on the Texas–Louisiana shelf: Model estimates for the year 1993 Kehui Xu a,b,n , Courtney K. Harris c , Robert D. Hetland d , James M. Kaihatu e a Department of Marine Science, Coastal Carolina University, Conway, SC, USA b Burroughs & Chapin Center for Marine & Wetland Studies, Coastal Carolina University, Conway, SC, USA c Virginia Institute of Marine Science, Gloucester Point, VA, USA d Department of Oceanography, Texas A&M University, College Station, TX, USA e Department of Civil Engineering, Texas A&M University, College Station, TX, USA article info Article history: Received 24 May 2010 Received in revised form 19 April 2011 Accepted 11 May 2011 Available online 26 May 2011 Keywords: Texas–Louisiana shelf Sediment transport Mississippi Delta Atchafalaya Bay Numerical modeling abstract A three-dimensional coupled hydrodynamic-sediment transport model for the Texas–Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow- water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed. At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20–40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner- most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters ( o10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the ‘‘Dead Zone’’ (low-oxygen area) within a seasonal–annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment- transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and ‘‘stepwise-like’’ compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave–current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf. Published by Elsevier Ltd. 1. Introduction 1.1. Background Large rivers play a key role in delivering freshwater, sediment, and nutrients to the ocean (Milliman and Meade, 1983; Milliman and Syvitski, 1992). Large-river deltas and associated coastlines are especially important interfaces between continents and oceans for material fluxes that globally impact oceanographic processes (Bianchi and Allison, 2009). The Mississippi River, the largest in North America, drains 41% of the continental United States before entering the northern Gulf of Mexico (Fig. 1A). The State of Louisiana contains about 40% of the nation’s coastal and estuarine wetlands which are vital to recreational and agricultural interests, and is home to the state’s $1 billion per year seafood industry (Stone and McBride, 1998). The Mississippi Delta, its associated wetlands and barrier islands developed over geological timescales in response to continuous accumulation of fluvial sediment and reworking by physical oceanographic processes Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.csr.2011.05.008 n Corresponding author at: Department of Marine Science, Coastal Carolina University, P.O. Box 261954, Conway, SC 29528-6054, USA. Tel.: þ1 843 349 6494; fax: þ1 843 349 4042. E-mail address: kxu@coastal.edu (K. Xu). Continental Shelf Research 31 (2011) 1558–1575