|Sean P. Leathem, U.S. Geological Survey, St. Petersburg, FL
Guy Gelfenbaum, U.S. Geological Survey, St. Petersburg, FL
Gregg R. Brooks, Department of Marine Science, Eckerd College, St. Petersburg, FL.
A series of submerged, shore-normal sand bars with wavelengths of 30-250 meters and heights of 0.5-2.5 meters (Figs. 1 & 2) have been identified offshore from the northern tip of Anna Maria Island, Florida. These bars extend alongshore approximately 3.0 kilometers and offshore nearly 4 kilometers. Evidence suggests that the bars have been present since at least the late 1800s.
Historical aerial photographs from the early 1940s through the mid 1990s provide an excellent means of qualitative comparison of the bars for this time period. The period of time that separates the photographs ranges from three to fifteen years, with a four or five year separation being the most common. Orthorectification of the historical photographs is being used to quantitatively assess historical trends of bar movement. The orthorectification process entails referencing known points on land to their correct geographic position. In this way, all of the points in the photographs are referenced to the same geographic datum, thereby removing any spatial discrepancies caused by differences in the altitude, tilt, azimuth, and focal length of the camera and differences in scale of the photographs.
Errors in geographic positions after the orthorectification process are 1-3 m, allowing quantitative substantiation of historical movement and migration patterns of the sand bars. Results of analyses of the orthorectified photos clearly show movement or migration taking place in the bar field. Specifically, two types of analyses were done to show the movement that is taking place and to try to quantify the magnitude of that movement. One method which allows quantification is the tracing of bar crests for different years and the comparison of the position of these crests from photograph to photograph. The positions of the southern most bar crest, for all of the years represented with aerial photographs, were traced and then projected onto one image (Fig.3). Rates of movement were calculated for the time periods between photos. In the forty year period, from 1951 to 1991, the southern edge of the bar field moved 200-400 m to the south, with rates of movement varying with distance offshore and from year to year (Fig.4). Rates of movement range from 5-12 m/yr and have an average of 7.9 m/yr. The same method was attempted for all of the bar crests in the bar field, but it proved impossible to positively identify bar crests from photo to photo, mainly due to the long time span between available aerial photos.
An alternative method that was used to determine movement throughout the bar field involved representing two photos, from different years, with different segments of the color spectrum. One image was represented with red (1991 photo), while the other image was represented with green and blue (1980 photo). By then overlaying the two images (adding the colors), overall bar field movement was determined by the combinations of colors represented. Results clearly show bar movement throughout the entire field, as well as showing the changes in bar field morphology that have occurred in the amount of time between the two photos being compared. Most bars appear to have moved between 1/2 and 1 wavelength to the south between 1980 and 1991. While there were some changes in bar morphology, such as curving and bifurcation, the bars remained nearly linear and overall bar field morphology remains nearly the same. While this method provided a good overall picture of the bar field and how it changed, it is difficult to quantify the movement that took place.
Southward movement of the bar field is also suggested by the asymmetrical form of the bar crests. Bathymetric data collected over the bar field were used to calculate the slope of the bottom on each side of the bar crests. Figure 5 shows a definite southward asymmetry in the bar crests. While the north facing sides of the bars have a fairly constant slope of just under two degrees, the south facing sides exhibit much more variable slopes with averages around three degrees. The slope of the south facing sides gradually decrease with increased distance from shore, until, at a distance of 2.5-3 km from shore, the bars become symmetrical.
Efforts are currently underway which will provide information on the short term movement within the entire bar field, as well as providing information on the seasonal and weather based variations in movement, and the forces which control or are involved in this movement. The information gained will not only provide a better understanding of the bars themselves, but also provide insight into the nearshore processes in the vicinity of Anna Maria Island. Results may also provide clues to historical processes that are no longer taking place or that have been modified. This information adds to a greater overall understanding of the nearshore processes of the west Florida coast.