Exchange Processes In Shallow Estuaries

ABSTRACT A modular approach to the analysis of mixing and flow characteristics in shallow tidal estuaries is presented using South Florida's Biscayne Bay as an example. The method depends on isolating relatively simple characteristic flow regimes in different parts of an estuary. These can be considered as building blocks, which when recombined in different configurations are capable of yielding a qualitative model for a specific estuary. Such models are of immediate value in preliminary assessments of estuarine water quality and interaction problems. This method can provide an effective base for further studies where more precise information is needed. INTRODUCTION Historically, studies of estuarine circulation have been conducted in deep openings to the ocean with large river outflows. Exchange processes were considered to be governed by the relative volumes of fresh water outflow vs. tidal prism. Solutions for surface elevation and flow are usually obtained either analytically with simplified versions of the linear hydrodynamic equations of motion, or numerically by fitting a grid to the estuarine geometry and finding stepwise solutions within each grid mesh. Unfortunately, these approaches have led to the belief that solutions obtained for a particular estuary are not applicable to other areas, thus requiring a complete study of each estuary of interest. Much of this complexity of estuarine circulation models arises from the necessity to cope with different dynamic regimes in different parts of any specific estuary. It is often possible, however, to divide an estuary into subregions, each with different but simpler dynamic characteristics. A set of dynamic models for such homogeneous subregions can be looked upon as a kit of building blocks. Suitably combined, these building blocks will yield a qualitative model for any specific estuary. The practical advantage of this method is that identification of flow regimes can be accomplished with a minimal observational program. Preliminary estimates of exchange rates and assessments of water quality can thus be made with modest efforts and expenditures. Where more precise information is needed, the method supplies an effective base for detailed studies. METHOD OF ANALYSIS Estuarine exchange processes are typically controlled by the forces of tides, winds, river run-off, evaporation, and precipitation. These forces display large variations in time and space between different estuaries and within any given estuary due to the geographic locations and complexities of shapes. The modular approach separates an estuary into characteristic regions as shown in FIGURE 1. The building blocks consist of regions of direct exchange between an estuary and the coastal waters (A), interior basins (B), the regions of exchange between basins (C), and the region of river influence (D). There is a large amount of literature on inlet hydraulics and effects of river run-off; therefore, the emphasis of this paper will be on mixing processes within the interior of estuaries. Tidal Exchange in the Interior of Estuaries Tidal currents in the interior regions of estuaries will normally show a simple longitudinal reversing pattern aligned with the major axis of the basins.