ACCESS NUMBER: 30660-19953 

STUDY TITLE: Coastal Marine Environmental Modeling II

REPORT TITLE: Dispersion in Broad, Shallow Estuaries: A Model Study 

CONTRACT NUMBER: 14-35-0001-30660-19953 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1997-1998 

COMPLETION DATE OF REPORT: December 2001 

COSTS: FY 1997: $84,961; CUMULATIVE PROJECT COST: $84,961 

PROJECT MANAGER: Masamichi Inoue 

AFFILIATION: Coastal Studies Institute, Louisiana State University 

ADDRESS: Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 

70803 

PRINCIPAL INVESTIGATORS*: Masamichi Inoue, William J. Wiseman, Jr., Dongho 

Park, Dubravko Justic and Gregg Stone 

KEY WORDS: Louisiana estuaries; Gulf of Mexico; numerical simulation; dispersion; 

flushing; salinity distribution; Barataria Basin; Breton Sound 

BACKGROUND: Estuaries are important coastal features, worldwide. Uses of these 

coastal systems present potentially conflicting demands, often requiring management 

based on extensive monitoring. One cost-effective alternative to extensive monitoring is 

modeling. Some monitoring is still required to calibrate the models, define forcing 

functions and boundary conditions and skill assess the model's usefulness, but the 

resulting savings over the cost of a complete monitoring program should be significant. 

Louisiana's bar-built estuaries are broad and shallow with mean depths of only a meter 

or two. Tidal currents and local wind have significant impacts on these estuaries. 

Management decisions concerning these environments concern the dominant physical 

processes, sediment erosion, transport and deposition, eutrophication, and recruitment 

to the estuarine nursery grounds, among others. Arguably the most accurately 

modeled, at this time, are the physical processes. Previously, a depth-integrated twodimensional numerical hydrodynamic model was developed to study circulation, 

transport and flushing characteristics in two significantly different estuaries 

Terrebonne/Timbalier Basin (a sediment-starved, low-runoff environment) and Barataria 

Basin (an estuary with an extensive network of small creeks and channels in the upper 

basin). The hydrodynamic model has been carefully calibrated, skill-assessed, and 

applied. 

OBJECTIVES: This study extends the previous modeling efforts. First, the depthintegrated two-dimensional hydrodynamic model is used to identify the impact of local 
ACCESS NUMBER: 30660-19953 

wind forcing on dispersion and flushing characteristics in Barataria Basin. The second 

objective is to incorporate baroclinic pressure gradient due to variable salinity and/or 

temperature distributions into the hydrodynamic model. The third objective is to study 

the role of waves and currents on suspended sediments by actually measuring currents 

and suspended sediments in the field. 

DESCRIPTION: There are three components covered in this report; an application of a 

previously developed two-dimensional depth-integrated hydrodynamic model, an 

enhancement of the depth-integrated hydrodynamic model, and a field program to 

measure currents and suspended sediments. Specifically, this report covers the 

following subjects; 1) a comparative model study of Barataria Basin, Louisiana, to 

delineate the role of wind forcing on flushing and dispersion characteristics, 2) an 

enhancement of the two-dimensional depth-integrated hydrodynamic model to include 

baroclinic pressure gradient; 3) the preliminary results of the first field deployment in 

Terrebonne Bay to measure currents and suspended sediments. The purpose of the 

field deployment is to document the role of waves and currents on sediment 

resuspension and eventually allow development and calibration of a suspended 

sediment module to be coupled to the hydrodynamic model. An accurate advection 

scheme is employed for advective transport of temperature and salinity. The 

hydrodynamic model that includes baroclinic pressure gradient has been successfully 

applied to Breton Sound. From the isolated measurements conducted so far, it is clear 

that wave-induced sediment resuspension is an important and poorly understood 

process within the shallow coastal bays of Louisiana. 

SIGNIFICANT CONCLUSIONS: Usefulness of a depth-integrated two-dimensional 

hydrodynamic model is demonstrated by examining the local wind impact on dispersion 

and flushing characteristics of Barataria Basin. Wind enhances flushing as well as 

dispersion characteristics of the estuaries. The impact of wind forcing becomes more 

prominent where the tidal energy is higher and the local geomorphology is complex. 

STUDY RESULTS: Using a depth-integrated two-dimensional hydrodynamic model 

forced by observed tides and winds, significant role of local wind forcing on flushing and 

dispersion characteristics of Barataria Basin is clearly demonstrated for the typical 

summer condition investigated. Flushing times for various subbasins were shorter by 

20-40% under wind forced condition compared to without wind. Diffusivity values 

without wind forcing fall slightly below Okubo’s range. Wind forcing enhances diffusivity 

values. In particular, wind forcing in combination with coastal and island trapping gives 

rise to the highest diffusivity values. The hydrodynamic model that includes baroclinic 

pressure gradient has been successfully applied to Breton Sound. 

 

STUDY PRODUCTS: Park, D., M. Inoue, and W. J. Wiseman, Jr., 1999: A verticallyintegrated model of Barataria bay, Louisiana: Transport, diffusion, and flushing 

characteristics under summer condition. American Geophysical Union Spring Meeting. 

Boston, MA. 
ACCESS NUMBER: 30660-19953 

Inoue, M. and W. J. Wiseman, Jr., 2000: Transport, stirring and mixing processes in a 

Louisiana estuary: A model study. Estuarine, Coastal and Shelf Science, 50, 449-466. 

Park, D., M. Inoue, and W. J. Wiseman, Jr., 2000: Wind, tide, and buoyancy-driven flow 

in Barataria Bay, LA. 10th International Biennial Conference on Physics of Estuaries 

and Coastal Seas. Norfolk, Virginia. 

Inoue, M., W. J. Wiseman, Jr., D. Park, D. Justic, and G. Stone, 2001: Dispersion in 

Broad, Shallow Estuaries: A Model Study. OCS Study MMS 2001-054 U. S. Dept. of 

the Interior Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

La. 72 pp. 

*P.I.’s affiliation may be different than listed for Project Manager(s).