the florida state university college of arts and sciences evolution of ...

on the beach face as sea level rises. This increased energy could result in the erosion of ridges
instead of deposition. The topographic survey shown in Figure 4.24 suggests that a large ridge
was eroded between 1977 and 2006. This ridge was surveyed during the Stapor and Tanner
(1977) survey but is not present today. Despite the increase in storm frequency that may be seen
in the St. Vincent Island data (the removal of the ridge from set K and the increase in energy
seen in the granulometric results), the island has not been overwashed or destroyed during any
historical high magnitude storm. This is because the island is unique in terms of its size and its
elevation. It is also stabilized by mature vegetation.
Rates of beach ridge formation were estimated based on the dated sites and an estimated
count of the number of ridges between each dated site. Color aerial photographs, LIDAR
imagery and high-resolution SAR images were used to count the beach ridges. It was difficult to
recognize many of the ridges, since they are low-lying. Many of the ridges are discontinuous or
merged together, which also makes identification difficult. For this reason, the intervening
swales were counted instead of ridges. The swales were counted because the dark vegetation
showed up clearly on the images. In any case, the counts should be taken as a best estimate only.
The time interval represented by the spacing between the St. Vincent Island ridges varies from 0
to 57 years, with an average of 35 years (Table 4.8). This is similar to the estimate of 40 to 60
years per ridge made by Tanner (1991). There is no simple explanation for the periodicity of the
ridges. The mechanism for forming swales is not clearly understood. The processes responsible
for terminating the building of one ridge and initiating the building of a new one are possibly
related to storm frequency or changes in wave regime (Tanner, 1988).
The OSL ages obtained in this study have been superimposed on a Gulf of Mexico sea
level curve in Figure 4.42, with the data tabulated in Table 4.8. The oldest ridge dated in this
study was at site SVI 015, which provided an OSL age of 4,100 +/- 300 years. At that time sea
level was higher than present and following a falling trend. Sea level fell below present level
and continued to fall until approximately 3,800 years. At this point it rose slightly and remained
stable for approximately 250 years. At approximately 3,500 years sea level began to fall again.
At this time, the ridges in the vicinity of site SVI 002 formed. Sea level then started to rise until
it reached its present level at approximately 2,800 years. It rose above present level and the
ridges at site SVI 003 and SVI 004 formed. Sea level rose until 1,800 years ago, at which point
it began to fall. It reached its present level at 1,150 years ago and then fell slightly below present
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