ACCESS NUMBER: 30660-19936 

STUDY TITLE: Analysis and Synthesis of Louisiana Coastal Boundary Current Data

REPORT TITLE: Aspects of the Louisiana Coastal Current

CONTRACT NUMBER: 14-35-0001-30660-19936 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1997-2005 

COMPLETION DATE OF REPORT: July, 2005 

CUMULATIVE PROJECT COST: $133,993.91 

PROJECT MANAGER: Lawrence J. Rouse 

AFFILIATION: Louisiana State University 

ADDRESS: Department of Oceanography & Coastal Sciences, Louisiana State 

University, Baton Rouge, LA 70803 

PRINCIPAL INVESTIGATORS: Lawrence J. Rouse, Jr., William J. Wiseman, Jr. 

KEY WORDS: Physical Oceanography, Gulf of Mexico, coastal currents, LouisianaTexas Shelf, modeling, model data comparison, continental shelf 

BACKGROUND: Many existing, reduced dimensionality models of the dynamics of the 

Louisiana Coastal Current are successful at reproducing the qualitative features of the 

circulation at specific time scales and periods of the year. Local, wind-driven models 

reproduce credibly the flow variability over the shallow regions of the west 

Louisiana/east Texas inner shelf, particularly during the winter season, at sub-tidal 

frequencies within the weather band (Crout et al, 1984; Lewis and Reid, 1985; Walker et 

al., 2001). Results are noticeably improved when care is taken to accurately account 

for variability of the bottom dissipation and far-field forcing (Current, 1996). The earlier 

models were unable to account for alongshore pressure gradients, while that of Current 

(1996) did. We were, thus, encouraged to assess the importance of such pressure 

gradients from observations. 

The success of such wind-driven models is surprising, since they ignore the riverine 

buoyancy flux and consequent strong stratification that is characteristic of this region of 

the Gulf of Mexico. A fully three-dimensional, time-dependent model of the region 

(Herring et al., 1999) did not perform significantly better at reproducing observations, 

although it did a good job of capturing the long term mean flows and very low frequency 
ACCESS NUMBER: 30660-19936 

variability. It was also able to reproduce observed stratification moderately well 

(Wiseman et al., 2000). 

OBJECTIVES: This study presents some new physical oceanographic modeling 

results obtained for the Texas-Louisiana shelf, as well as a review and assessment of 

past modeling efforts for this region. Extensive data sets are available from the 

Louisiana-Texas inner shelf region, and provide a vast source of information for model 

development, model forcing, and model verification. 

DESCRIPTIONS: Several simple local models were successfully run for hindcasting 

observations, and results are presented in this study. Existing reduced dimensionality 

models, local wind-driven models, and a fully three-dimensional, time-dependant model 

of the region were employed in hindcast mode. Model output obtained for this study was 

compared with observational data, and information from independent data was used in 

model development and forcing. The value of including such factors as bottom 

dissipation, far field forcing, and alongshore pressure gradients in modeling the 

Louisiana Coastal Current was examined. These models have had some limited 

success in hindcasting observations, but all have issues that remain and these are 

discussed. 

SIGNIFICANT CONCLUSIONS: In an attempt to discern the plausibility of treating the 

Louisiana Coastal Current as an arrested topographic wave (Csanady, 1978), we 

applied an existing vertically-integrated, non-linear numerical model forced by monthly 

mean wind fields. The results were intriguing but inconclusive. While the momentum 

balance generally appeared to be in quasi-steady state, i.e. the local accelerations were 

negligible compared to other forces, the advective accelerations in the model were, at 

specific sites, non-negligible. More interesting was the fact that the dominant balance of 

forces in the model was between the Coriolis forces, the surface wind stress, and the 

pressure gradients. 

A qualitatively acceptable comparison of output from the Yankovsky and Chapman 

model with observations suggests that further development of a robust, time-dependent 

theory would be beneficial. 

Comparisons of full water column velocity measurements from a bottom-mounted 

ADCP and alongshore bottom pressure gradients are far less satisfying. The bottom 

layer alongshore currents are significantly coherent with the observed alongshore 

pressure gradients in bands between 25 to 33 hours and near 50 and 100 hours. The 

source of the pressure gradient is unclear. 

STUDY RESULTS: A summary and critique of model results is presented, as well as 

suggestions for fruitful areas of future progress and appropriate uses for these various 

types of models on the Louisiana-Texas inner shelf region. 

There appear to exist situations in time and space along the Louisiana-Texas inner shelf 

where the bathymetry is sufficiently simple and a single forcing function sufficiently 
ACCESS NUMBER: 30660-19936 

strong that a simplified balance of forces realistically describes the first order dynamics 

of the coastal current. Such conclusions, though, cannot be generalized to the entire 

coast or to all time periods. The Louisiana Coastal Current is a fully three-dimensional, 

time-dependent system. True understanding of the system will only result from field, 

theoretical, and numerical simulation studies that account for all four dimensions.

STUDY PRODUCTS: Rouse, L. J., Jr., W.J. Wiseman, Jr., and M. Inoue. 2005. 

Aspects of the Louisiana Coastal Current. U.S. Dept. of the Interior, Minerals 

Management Service, Gulf o Mexico OCS Region, New Orleans, LA. OCS Study MMS 

2005-039. 50 pp.