LARGE AMPLITUDE SEICHE IN ST. JOSEPH BAY, FL- OBSERVATIONS AND IMPLICATIONS

摘要

St Joseph Bay, FL is on the northern shore of the Gulf of Mexico. It is formed by St. Joseph Peninsula, a curving, rapidly - eroding sand spit with the prominent Cape San Blas located at its southern tip. A section of the peninsula at a site called Stump Hole will likely breach in the near future. The new inlet will bisect the only road available to peninsula residents and visitors to St Joseph Peninsula State Park. Tidal hydraulic data were collected to identify and quantify the hydrodynamic processes governing the bay system and to calibrate and verify a numerical hydrodynamic circulation model (ADCIRC) for predicting the long-term fate of a new inlet. Instruments were placed in the Gulf of Mexico offshore of Stump Hole, in the bay behind Stump Hole, and in the bay near its entrance. Data were collected for four days, capturing spring tides and, coincidentally, passage of a strong cold front. Raw pressure data were reduced to water elevation time series relative to NGVD. Local conversions to NAVD and mean seal level were calculated from analysis of regional spatial and temporal trends in MSL. The tidal circulation model predicted that an inlet, once formed, would be stable over the long term under tidal forcing alone. However, measurements showed that sub-tidal long wave energy (associated with the passage of the cold front) occurred with periods near the resonant seiche modes of the bay. This could have significant impacts on the flow regime through any new inlet over short time scales. The phase lag of one component between the outside and the inside of the bay at the potential breach site was near 180 deg, resulting in a much larger hydraulic head across Stump Hole than tidal analysis alone would predict. Potential impacts include short term scour of the inlet beyond the equilibrium size during certain meteorological events (including hurricanes), and introduction of water with much higher sediment loads, discharged from adjacent Apalachicola Bay, into the relatively clear waters of St Joseph Bay.