Abstract: Historical Beach Changes, Florida "Big Bend" Coast

W. F. Tanner

Historical erosion and deposition rates have been compiled for a stretch of coast extending from a point due south of Tallahassee, almost as far west as Mexico Beach, in the Florida Panhandle. The open Gulf of Mexico beaches within this stretch total 135 km, from Mashe's Island to the extreme tip of St. Joseph Peninsula. Passes and unprotected marshlands occur here and there, especially toward the eastern end of the study reach, and have not been included.

Of the 135 km of open Gulf beaches, 52 percent have been eroding within historical times, 34 percent have been more or less stable, and 14 percent have been aggrading. By "stable" is meant: changes of not more than 10-20 cm/yr. Much of this "stable" coast has been retreating at a modest rate, and parts which were truly stable, up until about 5 years ago, are now undergoing erosion. Outside of the "stable" category, the ratio between eroding and aggrading beaches (km/km) is about 3.7-to-1.0. This ratio may also apply within the "stable" class.

Maximum erosion rate has been recorded at Cape San Blas, where measured beach retreat was close to 11 m/year between 1875 and 1942. Roughly the same rate held from the late 1700s to 1875, although 18th century charting was not precise.

Weighted mean recession rates have been 0.93 m/year for St. Joseph Peninsula, and 1.0 m/year for Dog Island. Very little of the open beach front has grown seaward during the study interval. Most of the deposition has taken place as lateral extension of spits, creating new beach front; even part of this new beach has been undergoing erosion in the short term, although counted here as accretion over the total period of study. Rates (m/year) are more precise when obtained over longer study periods. Hence comparison of two sets of air photos, only (for example) 5 years apart, does not yield results as reliable as comparison of two favorable-scale charts made 100 years apart. In general, long-term rates in the area tend to be greater than short-term rates, by factors of as much as 3.0 to 6.0. This could be due to a long-term change in wave climate, perhaps by a reduction in hurricane incidence and intensity. However, various observations support the concept that much, or most, of this discrepancy depends (inversely) on the duration of the study period. Therefore the larger erosion values are taken to be more nearly correct than the smaller (short-term) values.

The late Holocene history of the area (latest 5,000 years) includes, within the last few centuries, a change away from general deposition to general erosion. This change was effected in a substantial part prior to the publication of reasonably accurate maps, charts, and air photos, and therefore rates of shoreline change over the last 25-100 years cannot be extrapolated very far backward into time.

The coast, from south of Tallahassee for 300 km to a point near Port Richey, Florida, has "zero" energy characteristics, and therefore--with a few small exceptions--has not shown either erosion or deposition.

AAPG Search and Discovery Article #90978©1975 GCAGS-GC Section SEPM Annual Meeting, Jackson, Mississippi