Characteristics of exchange flow in a multiple inlet diurnal estuary: Mobile Bay, Alabama

Abstract

Abstract Flow interactions between two inlets of Mobile Bay, Alabama, and the Gulf of Mexico are described with ship-mounted ADCP surveys obtained on consecutive days. These two inlets, Main Pass (MP) and Pass aux Herons (PaH), remain interdependent throughout the diurnal tidal cycle. A phase lag of 31.6° (~2.1 h) affects the diurnal tidal currents between the two openings in the lower Mobile Bay. Tidal discharge through each inlet exhibits a time lag, that results in a phase-dependent discharge ratio between the two inlets. Consequently, the magnitude and direction of exchange flows between the two inlets are time-dependent. Early in the rising tide, Mobile Bay receives seawater only through MP while simultaneously discharging estuarine water through PaH. During the beginning of falling tide, Mobile Bay receives seawater through PaH while simultaneously discharging estuarine water through MP. Net transports through MP and PaH during the study period are positive and negative, respectively. Thus, Mobile Bay receives seawater from the Gulf of Mexico through MP and discharges a portion of this seawater, along with estuarine water, to eastern Mississippi Sound through PaH. The characterization of magnitudes and spatial structures of exchange flows in the lower bay determine how buoyant suspended matter connects between the Gulf, Bay, and Sound.