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Introduction
Accurate and quantitative data on beach changes induced by coastal
storms is critical to understanding coastal morphodynamics
and the mitigation of coastal erosion hazards. Collection of pre-
and post-storm coastal topographic data is difficult using
traditional
surveying methods. In most cases, only a few profile lines
are available and this limits detailed understanding of how beaches
change in response to storm impact. Recent advances in Airborne
Laser Terrain Mapping (ALTM) technology allow rapid collection
of beach topography, making it possible to do large scale quantitative
mapping of erosion, dune scarping,
and overwash deposition caused
by large storms. Hurricane Floyd (category 4 on Saffir-Simpson
scale) moved northward along the Atlantic Coast of Florida
during September 14-15, 1999 (Figure 1). Floyd was an abnormally
large
hurricane (diameter > 950 km, Figure 2) resulting in considerable
damage on hundreds of kilometers of beaches and dunes.
Figure 1. Map showing study area and hurricanes involved with
our study. Hurricane Dennis (August 24 - September 5) is in purple
and Hurricane Floyd (September 7 - 17) is in green. Study area
is outlined in red. Note that Hurricane Floyd was a Category 4
when passing our study area. Figure
2. False color
image of Hurricane Floyd approaching the Florida peninsula. Hurricane
Floyd was a Category 4 and an abnormally large system. Data Acquisition
We collected data on September 11 and October 3 over Indian River
and St. Lucie counties, flying at a speed of 180 kilometers per
hour, altitude of 550 meters, with scan angle of 13 degrees,
and laser repetition rate of 10,000 pulses per second. Three
overlapping lines were flown on each day to accommodate for missing
the shoreline during one of the passes. Beach elevations accurate
to better than 15 cm were determined at surface points with a
nominal spacing of about 2 meters. The survey measured the topography
of a 35 km long portion of beach. Data
Processing
The data was split into 1 km tiles and we
produced a 1 meter resolution digital elevation model (DEM) for
each date and tile. Volume
changes were calculated defined by a user specified digitized
polygon from the dunes to the water surface. Beach elevations
accurate to better than 15 cm. were determined at surface points
with a nominal spacing of about 2 meters. This data was used
to produce a 0.5 meter resolution Digital Elevation Model (DEM)
for each date.
Figure 3. Part A B C D E. Three identical sample areas for September,
October, and October minus September data sets. All heights are
in meters
NAVD88. October minus September shows the polygon that was used
to calculate the volume change. Also, a north and south transect
show the profiles for their respective data sets. September is
in blue, October is in purple and the difference is in green. Notice
the waves to the right of the analyzed area. Conclusions
We found a net beach volume loss for the 35 km shoreline is 230,000
m©¯ by comparing the beach topography of pre- and post-Hurricane
Floyd. However, the beach erosion caused by Hurricane Floyd may
be underestimated because the post storm survey was flown two
weeks after Floyd passed the study area. The beach may partially
recover during the two weeks. In addition, Hurricane Dennis passed
by the study area before the pre-storm survey. Thus beaches were
already eroded before Floyd, making it difficult for Floyd to
remove more sand. The storm wave direction and shoreline orientation
play important roles in beach erosion. The protruded areas facing
hurricane onshore wind suffer more erosion than other areas.
This results in remarkable spatial variations in net beach volume
change per unit shoreline. The beach volume loss per unit shoreline
varies from 2 to 18 m©˜ (Figure 4). All beaches show
net erosion. Furthermore, the erosion magnitude display considerable
spatial changes along and across the shore. In some cases the
entire beach profile exhibits erosion. Yet in other cases the
backshore erosion is coincident with foreshore accretion due
to the movement of beach cusps or vice versa. The ALTM technology
allows us to describe storm-induced beach changes in three dimensions
with great detail. This technology demonstrates the ability to
measure hurricane impact on coastal areas at spatial scales of
tens of kilometers in less than one hour. With predictions of
increased hurricane activity in future decades, this technology
will revolutionize our ability to study beach response to storm
impact.
Figure 4. DOQs (Digital Orthoquads) showing the study area and
35 polygons or tiles that were used for volume change analysis.
Volume changes shown to the right of the DOQs. Notice increased
erosion downdrift of Port St. Lucie inlet.
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