ABSTRACT

The "Big Bend" coast of Florida includes two major subdivisions: the "zero energy" shore from about St. Marks south and east to about New Port Richy, and the low-to-moderate energy beaches from St. Marks west to (in this study) the northern tip of St. Joseph Peninsula. The "zero" energy coast has been stable through historical time, and promises to continue to be stable into the near future.

The low-to-moderate energy segment, on the other hand, has been stable-to-active. About 52% has been eroding, and only 14% has been aggrading; the remaining 34%, although apparently stable, may have been active at rates too small to have been detected in this study (rates less than 10-20 cm/yr). The excess sand (more erosion than deposition) is accounted for in various ways, one important factor being that much sand is accumulated in water deeper than the new space made available by attack on the beach front.

The late Holocene history of the area includes general deposition up until some time (not a fixed date, from place to place) within the last five centuries; severe erosion over the last century or two; and perhaps more modest erosion in the last 20-40 years. Various considerations lead to the concept that the present modest erosion is a minimum, and that more severe erosion probably should be expected in the future.

The fastest measured local erosion rate in the study area has been 11.2 m/yr for the 67-year interval from 1875 to 1942. Approximately the same rate held for the century prior to 1875. Common measurable erosion rates, over the most recent 30 years, have been between 0.20 and 1.0 m/yr, but it is necessary to believe that at least some apparently stable beaches eroded at rates between zero and 0.20 m/yr.