Carolina Marine Resources Center. The SCMRC divers noted "a 
layer of loosely consolidated mud that varied from less than one 
foot to an unknown depth beyond the divers' reach" during a dive 
conducted approximately two weeks before the satellite image 
acquisition. (Van Dolah, et al, 1992) Samples obtained at the 
borrow site showed a significant increase in silt and organic 
material (from 596 pre-project to 3696 after). This change in 
composition is probably the source of the change in reflectance of 
the material in the borrow site. 
One unexpected benefit of this study was the ability to map a 
complex system of underwater sand dunes located from 1 to 5 
kilometers offshore. Although the presence of these dunes has 
been known for some time, the full extent and configuration of 
these fields were not well established. Previous studies (such as 
Henry and Harding, 1985) were based on data derived from 
soundings, and did not provide a complete picture of these 
features. Figure 3 shows the location and geometry of the largest 
of these dune fields. The bathymetric GIS coverage was 
classified in order to find crests and troughs. The crest/valley 
vector coverage was superimposed over the satellite image for 
visual inspection. Spacing was determined by measuring the 
distances from crest to crests both visually and using GIS 
functions. The waves range up to 2 meters in height, and in some 
cases are over two kilometers in length. The waves are relatively 
evenly spaced, ranging from 140 to 150 meters apart. The dune 
fields appear relatively stable, with very little change detected 
between the 1986 and 1989 images. A slight migration to the 
SW, on the order of 40 meters, was possible, perhaps in 
response to the slow landward migration of Gaskin Bank. In 
several instances, the waves appear to extend from Gaskin Bank 
nearly to the shoreline. Further study of the dunes and their 
relation to indicent waves and shoreline changes are underway. 
RESULTS AND EVALUATION OF THE STUDY 
Summary of Results of the Study 
Excavation of the Joiner bank site definitely resulted in changes to 
the environment. This conclusion is backed up by on site studies 
by SCMRC. Direct changes as a result of the dredging of Gaskin 
bank, other than the actual excavation of the site itself, were not 
detected. The wave refraction study indicated some energy 
dispersal as a result of the deeper site. The Barret and Grenadier 
shoals at the south end of the island were relatively stable. The 
system of sand waves was accurately mapped, and a baseline 
established for their continued monitoring. 
Implications for Coastal Planning 
The study of effects of the beach renourishment project on Hilton 
Head Island was completed in less than four months using mainly 
"off the shelf" hardware and software. Most of this study was 
conducted on an IBM PC compatible system (Intel 386 processor 
with math coprocessor, Trident SVGA graphics, 150MB Hard 
Drive, Laser Printer) using IDRISI, a grid based GIS available at 
low cost from Clark University. IMDISP, a public domain program 
available from NASA's Jet Propulsion Laboratory, was used for 
some image manipulation and processing functions. The wave 
refraction models were originally written in C on a Sun 
SPAROSstation, but were refined and rewritten in C++ on the PC. 
Presentation maps were generated using UNIX workstation based 
ARC/INFO, but could have been done equally well on the PC. If 
the object were to update NOS charts for pre-project planning, the 
study could have been performed for about $14,000 (US), 
including software, hardware, and the SPOT image. This would 
have amounted to 0.1896 of the project cost. Because the original 
project design used older NOS charts, changes to the geometry of 
Joiner Bank detected by either aerial photography or a satellite 
image analysis may have resulted in a different configuration for 
the borrow site for the northern section of the project. In addition, 
the political benefits of having used the best available information 
should not be underestimated. 
324 
SUMMARY 
Engineering projects that alter the natural environment should be 
planned carefully, using the most current data possible. The 
availability of satellite imagery in the 20m/pixel or better range 
presents an excellent opportunity for current maps and models to 
be used in planning these projects at the local level. In the 
coastal environment, this data can be effectively used for updating 
the geometry and movements of shoals and underwater sediment 
formations. Although suspended sediment loads may interfere 
with accurate determination of bathymetry, with care remotely 
sensed data may be used for updating water depths for use in a 
variety of models. The costs involved are minimal compared to 
the overall costs of coastal engineering projects. 
REFERENCES: 
Henry, V., and Harding, J., 1985. Results of A Study to Locate 
Suitable Beach Nourishment Material in the Nearshore Area Off 
North Forest Beach and Palmetto Dunes Resort, Hilton Head 
Island, SC. Report for the Town of Hilton Head Island. 
Olsen Associates, Inc., 1987. Wave Refraction Analysis of the 
Existing Bathymetry and Potential Impact of Offshore Borrowing at 
Hilton Head Island, SC. Report for the Town of Hilton Head 
Island. 
Olsen Associates, Inc. 1992. Hilton Head Island Beach 
Restoration Project Monitoring Report, Year 1. Report for the 
Town of Hilton Head Island. 
US Army Corps of Engineers, 1973. Shore Protection Manual. 
Van Dolah, R., Wendt, P. Martore, R. Levisen, M, and Roumillat, 
W. 1992. Final Report for A Physical and Biological Monitoring 
Study of the Hilton Head Beach Nourishment Project. 
  
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