MORPHOLOGIC CHANGES OVER THE PAST 50 YEARS
Analysis of geomorphic change and of shoreline adjustments over the past 50 years indicates that
Sebastian Inlet has responded to both natural factors and engineering activities (Fig. 6). To quantify
the inlet changes in the long-term data were documented through historical aerial surveys and
shorter-term field surveys of inlet morphology. Annual to semi-annual beach profiles and
topographic surveys of the study area were collected from the summer of 1989 to the summer of
2000. These consisted of high-resolution topographic surveys of the ebb shoal, flood shoal, sand
trap, and beach profiles tied in with submerged survey transects every kilometer along the shoreface.
Shoreline change
An extensive search for historical aerial photography was performed for the study area, returning
36 aerial surveys between February 1943 and July 2000. These photosets are variable with respect
to spatial resolution, study area coverage, size, and quality. In general, only photographs having a
spatial resolution of 1.5 m or better were retained for the study. The 23 dates selected for analysis
had a minimum approximate scale of 1:25000 and covered at least one-third of the study area. Each
photo was exported to a Tagged Image Format (TIF) and rectified to a Florida State Plane, East
Zone projection with an NAD 1927 Datum. This projection was selected primarily because it is
compatible with most historic survey data, a well-developed flood shoal that is inter-tidal, and a
prominent submerged ebb shoal.
Two sources of ground control points (GCP's) served as projection references to rectify imagery,
including 24 sets of DGP coordinates collected at visible landmarks and U.S. Geologic Survey
Digital Ortho-Quarter-Quads (DOQQs). The USGS DOQQs were obtained in a UTM Zone 17
projection and re-projected to the study coordinate system. An image-to-image rectification process
was used to collect reference points for the unrectified imagery from the DOQQs. A minimum of
seven combined control points was used for each image, and these points were adjusted until the
RMS error of point scatter was less than 3 m. GIS software ArcView 3.2 ImageAnalyst was
The beach areas of the geo-referenced images, from the wet/dry line to the vegetation line, were
identified by means of supervised classification/isodata clustering. This results in the generation of
a detailed polygon that is an estimation of the beach between the low-tide terrace (the wet/dry line)
and the toe of the dune or the vegetation line. The photo polygons and the corresponding fly date
were then merged, resulting in a continuous (except where intersected by the inlet) polygon
representing the entire backshore on a given date. A baseline running the length of the study area
and roughly parallel to State Road A1A, the main highway on the barrier island, was then used as a
common reference for transects spaced at 7.6-m (25 ft) intervals. This customized method of
determining shoreline positions using an ArcView extension termed BEACHTOOLS is described
by Hoeke and Zarillo (2001). Transects serve to determine the positions of the vegetation line and
the wet/dry line and continue the same shore-parallel position throughout the time series. This
procedure is applicable to a number of analyses, including changes in beach widths, quantitative
erosion accretion studies, and spectral analysis for rhythmic topographies, etc. (Hoeke and Zarillo
2001).
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Zarillo et al.