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DAMOS Contribution #105 - Executive Summary |
Preliminary
evidence of active bed transport at the Cornfield Shoals Disposal Site (CSDS)
prompted the National Marine Fisheries Service and Connecticut Department of
Environmental Protection to express concern about possible sediment transport
towards, and impact on, shellfish beds located north of the site, north of
Long Sand Shoal. This preliminary study was designed to determine if the
field-observed transport at the disposal site is consistent with the
predicted east-west transport pattern found in the historical record. This
investigation at CSDS between July 1991 and May 1992 provided strong
circumstantial evidence for active bed transport. This study included a
series of bathymetric surveys, current meter suspended sediment data
collection, and REMOTS photography of the sediment-water interface. The
circumstantial evidence for active bed transport included shifts in areas of
sediment accumulation and erosion, increased suspended sediment deposits at
the sediment-water interface. In general, active bed transport appears to
follow the historical east-west current direction, and the extent of the
transport depends on the type of material. The actual direction and magnitude
of active bed transport apparently depend on local topography and the nature
of materials being transported. Sediment
accumulated over a broad area near the disposal points of sand hydraulically
dredged from the Connecticut River in September 1991. Subsequent bathymetric
surveys showed apparent movement of this material to the west. A clearly
defined deposit formed at the disposal point for fine-grained material
mechanically dredged from North Cove, Connecticut. This deposit of
fine-grained dredged material apparently did not move, but was partially
covered by bedload transport of adjacent coarse-grained sediments. The present study concluded
that: ·
The predominant transport direction at the site
appears to be east-west . This is supported by current meter deployments and
observed erosion and deposition patterns. ·
Sediment disposed at the side was not immediately
dispersed and was defined within discrete deposits. This was observed for
both fine-grained and sandy sediments. ·
The dispersion process appears to occur over a
period of weeks to months. The dispersion rate for fine-grained materials may
be markedly slowed by sand armor that migrates over the more cohesive, less
erosive silt-clay. ·
The above findings, taken over the ten month time
period, reduce concerns about far field transport of material over oyster
beds to the north. The dispersion of material over a time scale of years, and
the effectiveness of the sand armor over the silt-clays, are unknown. |
Dated March 6, 1997