COMPARISON OF TIDAL INLET HYDRAULIC GEOMETRY: TIDAL MARSHES AND COASTAL LAGOONS

Tidal inlets govern the exchanges of water, sediment, and solutes between tidal marshes or lagoons and coastal waterways. In this study, we compared the regional hydraulic geometry of tidal marsh inlets with lagoon inlets in the Chesapeake Bay Region. Lagoon inlets respond to tidal currents, stream discharge from the watershed, and wave action, whereas tidal marsh inlet morphology are not connected to watersheds and are formed by tidal currents. We measured inlet width from air photos for all of the Chesapeake Bay lagoon inlets and the 267 tidal inlets of the Patuxent Freshwater Marsh Ecosystem. Inlet width probability distributions were used to identify representative sites for field measurements in both systems. Field measurements of inlet width, cross sectional area, and velocity were made for spring (high) tides in both systems. These data were used to calculate discharge and to establish hydraulic geometry equations. These equations represent regional hydraulic geometry equations, which are similar to the downstream hydraulic geometry equations used for fluvial systems. The regional hydraulic geometry relationships for the tidal marsh inlets suggests that these channels behave as an integrated hydraulic system during channel forming events, which appear to be ebb tides during non-vegetated periods when flow resistance is at a minimum. Although the bankful lagoon inlet regional hydraulic equations also represent a homogeneous hydraulic system, tidal current velocities are not sufficient to be the major lagoon inlet forming events. We estimated the bankful velocity and discharge for the lagoon inlets, based sediment transport constraints from field grain size data. These discharge estimates for the lagoon inlets are similar to the 2-year peak discharges estimated from regional flood frequency relationships, which are based on drainage basin area, topography, and land use. These data suggest that the lagoon inlet hydraulic geometry is maintained by frequent flood events, similar to fluvial systems. Although the lagoon inlet and freshwater tidal marsh inlets are maintained by different processes, they have similar regional hydraulic geometry relationships; both systems have higher exponents for width than found in most fluvial systems.