Geologic structure and hydrodynamics of Egmont Channel: an anomalous inlet at the mouth of Tampa Bay, Florida

High-resolution bathymetry surveys of Egmont Channel were conducted in 1999 and 2001 using a Kongsberg Simrad EM 3000 multibeam bathymetric system. These data were supplemented with other bathymetry data, seismic profiles, underwater scuba observations, and current velocity data, in order to investigate the geologic and hydrodynamic characteristics of Egmont Channel, which is the main shipping channel for Tampa Bay. The cross sectional area (17,964 m2) and the tidal prism (6x108 m3) for Egmont Channel derived in this study are larger than theoretically predicted. The tidal prism is two orders of magnitude greater than previously calculated. This result indicates the current velocities near the deepest part of the channel, referred to herein as Egmont Deep (~30 m), are faster than the rest of the Tampa Bay and the large cross sectional area is most likely due to the large tidal prisms associated with storm events. Currents measured at Egmont Deep and the Sunshine Skyway Bridge (~11 km away) with Acoustic Doppler Current Profilers, have a high correlation (97%) indicating the current velocities at Sunshine Skyway Bridge can be used as a proxy for current velocities at Egmont Deep. Seismic profile data indicate that both the mouth of Tampa Bay and the bay proper contain many stratigraphic depressions. Egmont Deep is located at one of these depressions. Bathymetry and seismic data indicate that the main ebb jet for Tampa Bay is deflected northward by a local stratigraphic high underlying Egmont Key. This deflection appears to cause the asymmetry of Egmont Channel. The repeated high-resolution multibeam bathymetric surveys document sediment bedform migration. Large subaqueous dunes in the north and western portions of Egmont Channel have moved at least 13 m in a WNW direction over 24 months. Medium-sized dunes are superimposed on the large dunes. Smaller sandwaves appear intermittently in Egmont Deep, which is bounded to the north by a linear steep scarp (~38°) and by a more gradual slope (>10°) to the south. The area of the deep fluctuates in size due to the erosion and deposition of gravelwaves. Analysis of seismic data and SCUBA observations suggest that the most likely origin for Egmont Deep is a combination of dissolution of lower strata limestone during a lower sea level, causing partial collapse, followed by tidal current scour that maintains the deep. A three-dimensional seismic survey and drilling is needed to further test this proposed origin.