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DAMOS Contribution #110 - Executive Summary |
Over
100,000 m3 of mechanically dredged fine-grained material was released at a
taut-wire moored buoy at the Cornfield Shoals Disposal Site (CSDS) during
disposal operations in 1991 and 1992. Despite the fact that this is a high
energy dispersive disposal site, the fine-grained dredged material did form a
mound that was detected during bathymetric and REMOTS® surveys in 1992. Over
time, active bed transport of ambient sediment at the site has produced areas
of accumulation and areas of loss. Active bed transport also may have caused
the fine-grained deposit to be covered by sand. If sand has been transported
over the fine-grained material, the usefulness of REMOTS® sediment-profile
photographs in mapping the dredged material deposit is limited. The sand makes
it difficult for the camera prism to penetrate to the depth of the dredged
material. Due to the limitation of the REMOTS® method, a swept frequency
subbottom profile survey was conducted at CSDS in 1994 to map the extent of
this fine-grained deposit. The
1994 bathymetric survey detected minimal accumulation (25 cm) over the
disposal mound since 1992. The 1994 bathymetry also detected accumulations
over a broad area southeast of the disposal mound. At the disposal location,
the subbottom profile survey detected an acoustic horizon at 1 m below the
seafloor. Southeast of the disposal location, the subbottom record identified
a sand wave field. Taken together, the results of the bathymetry and
subbottom surveys mapped evidence of fine-grained dredged material
accumulation at CSDS as well as evidence of active bed transport of ambient
sand. Fine-grained
material was released at CSDS to examine whether this dispersive site was
suitable for the disposal of cohesive, mechanically dredged silts and clays.
The survey results suggest two approaches to the management of fine-grained
dredged material at CSDS. Fine-grained material is likely to accumulate at
CSDS if it is disposed at a taut-wire moored buoy. This management approach
would allow close management of the material, but it may reduce site
capacity. Fine-grained material may not accumulate if it is disposed in
relatively small volumes at numerous discrete locations (e.g., LORAN-C
coordinates). If the release of fine-grained dredged material is managed this
way, resuspension and dispersal may prevent mound formation and retain site
capacity. |
Dated May 28, 1997