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Summary
Surveys
of the HARS conducted in 1996, 1998, and 2000 using a multibeam seafloor mapping
system provide a detailed view of the geology, topography and sedimentary features
of the sea floor. One of the most striking aspects of the sea floor shown within the HARS is the variability in backscatter intensity and bottom morphology over scales of a few kilometers
or less caused by both natural and anthropogenic processes. The topography, surface features, and the surficial sediments have been heavily influenced by the disposal of dredged and other material in this region
over the last century.
Major changes in the sea floor between the 1996 and 1998 include
the appearance of mounds of material, some as high as 5 meters, resulting from placement
of dredged material prior to closing of the Mud Dump Site, a circular feature
approximately 1 km in diameter and 3 meters thick resulting from a 1997 disposal
and capping project, and a circular feature also approximately 1 km in diameter
and about 0.5 meters thick resulting from capping within PRA#1. Major changes in the sea floor between 1998 and 2000 include the formation
of numerous craters caused by placement of capping material in the soft sediments
in PRA#2, and an increase in backscatter intensity in PRA#1 resulting from new
placement of capping material and modification of previously placed sediments.
The difference in backscatter intensity shows decreased as well as increased
backscatter in areas where dredged material has been placed on the sea floor.
The resolution of the multibeam system precludes utilizing differences in depth
between repeated surveys to estimate the amount of material placed in the HARS,
because modest amounts of material were placed over a large area. In addition,
more accurate data defining the amounts of placed material and the compaction
of material on the sea floor are needed to develop an accurate mass balance.
The principal use of the multibeam data is to reveal the regional surficial
geology, and to document the location of placed material and changes in sediment
properties over time through comparison of backscatter intensity.
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