Transport, Mixing and Stirring Processes in a Louisiana Estuary: A Model Study

Abstract

Transport and mixing processes in a broad and shallow estuary in Louisiana, Terrebonne/Timbalier Basin, are examined using a depth-integrated two-dimensional numerical model. Using current meter records previously obtained, the model calibration yielded correlation coefficients between simulated and observed current components of 0·89–0·95. It appears that the bottom friction in the bay is relatively large with a Manning's coefficient of 0·07 producing the best results. The large bottom friction appears to be due to a combined effect of currents and the surface wave field which is not explicitly accounted for in the numerical model. Despite the small tidal range, tidal forcing dominates circulation in the bay. During equatorial tides, tidal currents on the order of 20cms−1could develop in a broad area of the bay while in tidal passes currents could reach 50–60cms−1. During tropic tides, strengths of the currents in the bay could easily be double those during equatorial tides. Local wind forcing is also important in controlling general flow direction inside the bay, in particular during equatorial tides. Flushing time, estimated by a particle tracking technique, was 27 days, that appears to be in agreement with observations. Horizontal diffusivities computed using tracer particles are comparable to the previous estimates of horizontal diffusion coefficients compiled by Okubo (1974). The larger values appear to be due to coastal trapping. Mixing of water masses, based on particle tracking, is found to consist of continuous stretching, folding and break-up of material lines due to interaction of wind-driven and tidal currents with bottom and coastal topography. Time evolution of the boundary between the two water masses depends on the initial tidal phase. However, this dependence lasts only until coastal trapping becomes dominant in controlling the time evolution of the boundary. Coastal trapping appears to be an important process by which stirring and mixing processes are enhanced, thus making stirring and mixing ‘more efficient’ and ‘chaotic’. Those observations point to the need to resolve small-scale shear flow patterns, in both space and time, and detailed bottom and coastal topography in order to understand transport, stirring and mixing processes in the broad, shallow estuaries typical of the northern Gulf of Mexico.