MONITORING NEAR AND OFF SHORE SEDIMENT FORMATIONS BY SATELLITE
Charles C. Watson, Jr
GIS Administrator, Town of Hilton Head Island
1 Town Center Court, Hilton Head Island, SC 29928 USA
(803) 842-8900
ABSTRACT
In 1990, a portion the Atlantic Ocean beach of Hilton Head Island was renourished using sand taken
from nearshore borrow sites. The Town's Geographic Information Systems (GIS) staff has been studying the
effects of the renourishment project on offshore areas through the use of satellite images and data. The study
consisted of acquiring SPOT images and historical photography of the Hilton Head area from various times
before and after the renourishment project. The images were processed and specific features converted to GIS
coverages for analysis. Bathymetric data from nautical charts was updated using bathymetry derived from the
satellite data. Results of the study consist of data showing the geometry and motion of shoals both before and
after the renourishment. In addition, the configuration and extent of a large area of underwater 'sand waves ,
located from the shore to 5 kilometers offshore, were described and monitored. The full extent and
configuration of this area were previously unknown. The use of the satellite derived data in various computer
models is also discussed. This study revealed several points in the process of coastal planning where satellite
derived data could be profitably used, such as borrow site selection and ongoing monitoring.
KEY WORDS: GIS, MAPPING, CHANGE-DETECTION, COASTAL-APPLICATIONS
INTRODUCTION
Hilton Head Island is a resort community located on the southern
end of the Atlantic coast of the state of South Carolina. Affluent
part-time residents and tourists are the driving force of the local
economy, visiting the Island primarily for its sandy beaches and
controlled development. As with many barrier islands along the
United States' southeastern coastline, Hilton Head is slightly to
moderately erosional along most of its shoreline. Unfortunately,
developers have chosen to build on the primary dune line in many
areas, and have left very little buffer between their buildings and
the ocean. Since the economics involved would not permit
allowing these multi-million dollar investments to fall in to the sea,
the municipal government has made a major commitment to
returning the beach to its configuration of the early 1980's and
maintaining it in that state. The technique chosen by the Town's
consulting engineer was to 'renourish' the beach by pumping sand
from offshore borrow sites on to the eroding beach (Figure 1).
After nearly 5 years of planning, primarily focused on funding the
7.92 million dollar (US) project, the renourishment was undertaken
in the summer of 1990. Soon after the completion of the project
several groups of citizens complained that the project had resulted
in erosion in their areas of the beach. At the request of the Town
Manager, the Geographic Information System (GIS) administrator
began an independent study of possible adverse effects of the
renouishment project. This paper describes how the GIS, aerial
photography, and images from the SPOT satellite were used to
create a data base to analyze the possible effects of the
renourishment project.
Sound
Mainland
Joiner Bank
Site
Renourishment
Project Area
[=] Gaskin Bank Site
Figure 1: Study Area
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DESCRIPTION OF THE STUDY
The study of the renourishment project centered on mapping the
offshore areas. The three major tasks undertaken during this
study are shoal location and migration monitoring, updating
bathymetric data, and monitoring underwater features. Also noted
are actual wave refraction patterns seen on some SPOT images.
Shoal Location and Migration Monitoring
Fill material for the beach nourishment project was obtained from
two borrow sites. The first site was located near a hooked spit
adjacent to the north end of Hilton Head Island. Joiner bank is
part of the dynamic shoal system that borders Port Royal sound.
As originally designed, part of the spit was to be excavated to
provide the fill material. The project design was finalized in 1986,
using National Ocean Survey (NOS) nautical charts. These charts
were based on data obtained in the 1970's. Between that time
and the time the project was designed, the part of the spit which
was proposed to be excavated had become submerged, and the
hooked end of the spit had migrated over 700 meters. By the time
work began on the project, the spit had moved an additional 220
meters. Despite the changes to the location, work proceeded and
nearly 900,000 cubic meters of material was removed from the
site, increasing the water depth by 3.3 meters. Soon after project
completion, severe erosion was noted along the shoreline directly
landward of Joiner bank, prompting citizen complaints.
The first stage of the GIS study of the renourishment project was
to determine the location and movement of Joiner bank before and
after the project. The Town of Hilton Head GIS is implemented
using ESRI's ARC/INFO package on Sun Microsystems
workstations. Much additional work was done on IBM PC
compatible systems using Clark University's IDRISI package and
software developed by the GIS Administrator. Several sources of
material were used to create five coverages ranging in time from
1974 to 1991. NOS charts, created from 1974/75 field data, were
digitized manually using a Calcomp 9100 digitizer. These were
the same charts used by the Town's consultant for the project
design. An aerial photograph from January 1984 was scanned
and stored as a raster file. Three SPOT satellite images were
obtained: multispectral images from February 86 and May 89, and
a panchromatic image from March 1991. For comparison
purposes, aerial photography from January 1986 was compared
with the February 86 satellite image. Most of the analysis was
concentrated on the three satellite images.
The basic method for creating a line coverage from the raster
image data was quite simple. A 'radiance contour' map was
created for each image near a cutoff value. This cutoff value
represented the 'low tide line' for sand in the image, and was
selected based on the radiance of sand on the island beach front
at pixels known from surveys to be near the low tide mark. For the
multispectral images, band 3 (0.79 to 0.89 micrometers) was used,
as this band has the steepest water absorption. As a check, the
