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Mary Baker Wiley.

Synthesis of monitoring surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992 online

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On June 23, 1991, the A disposal buoy was deployed at 41° 12.69' N, 72°21.54' W.
The near-bottom mooring (BTM- A) was positioned approximately 500 m northwest of this
buoy on July 18. The water depth at BTM-A was approximately 53 m, and the instrument
package was located one meter above the seafloor. A mid- water mooring (MWM), with a
S4 current meter 11m below the surface, was deployed southwest of BTM-A on August 1
(Table 2-1, Figure 2-1). Buoy A was moved north to its final location (41° 12.75' N,
72 °2 1.10' W) on August 8, prior to the start of disposal to further separate the disposal
location from a previously used location to the south (Table 2-1, Figure 2-1). On August 13,
the optical backscatter sensor and transmissometer package (DAISY) was added to the bottom
mooring (BTM-A). After the Connecticut River bars dredging was completed, the disposal
buoy was relocated on October 18 to location B (Table 2-1, Figure 2-1). The near-bottom
current meter mooring (BTM-B) was moved approximately 500 m to the west of B on
October 21. The water depth at BTM-B was approximately 53 m. Again, the instrument
package was housed one meter above the bottom. The DAISY instrumentation was removed
from BTM-B on November 27th. A mid-water mooring, with a current meter positioned
11m below the water surface, remained at the same location for the duration of the study.
The last data retrieval for BTM-B and MWM was December 12, 1991. The moorings were
removed from the water on January 9, 1992.



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



2.3 REMOTS®

A REMOTS® survey was conducted at CSDS on May 12 and 13, 1992. The
REMOTS® sampling grid was designed to define the limits of dredged material distribution at
CSDS. The grid encompassed disposal locations A and B and consisted of 22 stations spaced
200 m apart. Seven additional stations spaced 100 m apart were concentrated around the
dredged material deposit at B after it was detected during the bathymetric survey. After
reviewing the REMOTS® photographs from these 29 stations for the presence of dredged
material, an additional 16 REMOTS® stations were sampled to define further the boundary
between dredged material and ambient sediment (Figure 2-1). Three replicate photographs
were taken at all REMOTS® stations.



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



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3.0 RESULTS

3.1 Bathymetry

All four bathymetric surveys at CSDS (August 1991, October 1991, December 1991,
and May 1992) show a seafloor that slopes from north to south, and a northeast/southwest
trending trough in the southern half of the survey area. All disposal locations were located
on the northern slope.

The August 1991 bathymetric survey (Figure 3-1) was conducted prior to the
1991/1992 disposal season. There was no evidence in the bathymetric plot of any previous
disposal mound at the site. The next bathymetric survey was conducted on October 18 and
19, 1991 after about 53,800 m 3 of sandy/silty dredged material was released at buoy A
(Figure 3-2). Again there was no distinct mound on the contour plots, but the depth at the
disposal location decreased from approximately 54 m to 53.5 m.

A depth-difference plot, showing relative changes between the August 1991 and the
October 1991 surveys showed up to 1 m of material accumulated over wide areas northwest
and south of A. Between these areas of accumulation there were losses of up to 1 m of
material (Figure 3-3).

After the October 1991 bathymetric survey the disposal buoy was relocated to position
B. Fine-grained dredged material was removed from North Cove and released at buoy B
starting November 13. Between November 13 and December 10, 1991, 13,891 m 3 of
material was released at CSDS. When the survey was conducted (December 10), the
disposal buoy was located west of its initial location. However, positions noted on the barge
logs indicated that the barges continued to release material at the original (intended) B
location (Figure 3-4). The December 1991 bathymetric survey did not indicate any well-
defined accumulation of material at buoy location B (Figure 3-5).

A depth-difference comparison of the December and October, 1991 bathymetric
surveys did not indicate any accumulation at buoy B (Figure 3-6). Instead, material
accumulated to the west of B, and approximately 100 m south of buoy A. A loss of material
appeared further south, the same area where material accumulated between August and
October (Figure 3-3).

Between December and May, about 91,588 m 3 of fine-grained material was released
at buoy B. The May 1992 bathymetric survey shows the same north-to-south sloping
seafloor with a northeast/ southwest trending trough in the southern half of the study area
(Figure 3-7). In contrast to the 1991 surveys, the May data show a decrease in depth just
north of the B buoy where the contours bend to the south. A depth-difference comparison



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



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Figure 3-3. Accumulation (A) and erosion (B) at CSDS, August 8, 1991 to October 18 and
19, 1991 (depth in meters)



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



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Figure 3-6. Accumulation (A) and erosion (B) at CSDS, October 18 and 19, 1991 to
December 10, 1991 (depth in meters)



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992






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Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



18

between the December 1991 and May 1992 bathymetric surveys showed that 0.5 m of
material accumulated at the B buoy location (Figure 3-8).

To illustrate the full extent of any erosion of the material dredged from the
Connecticut River bars, and any deposition during the disposal of fine-grained material, a
comparison was done of depths between the October survey (following completion of the
sand disposal) and the May survey (following completion of the fine-grained disposal).
Losses of material between October and May are in the same area and the same amount as
between October and December. Accumulations during that time are just south of
41° 12.75' N, and at the B disposal location (Figure 3-9). The accumulation south of
41° 12.75' N is the same amount of material that had accumulated between October and
December.

3.2 Current Meter Data

The mid-water and near-bottom current meters deployed at CSDS measured current
direction and velocity from August 1 to December 12, 1991. A data report by Bohlen et al.
(1992) contains a detailed discussion of these data. In both the mid- water and the near-
bottom data, the tidal/current direction was dominated by the semi-diurnal east- west
component. For the mid- water data, the vectors were 270° on the flood tide and 95° on the
ebb. Near-bottom current directions were similar to the mid- water ebb current but shifted
slightly to the northwest on the flood at BTM-B. There was more variability in the current
direction at near-bottom (Figure 3-10). The east- west directions for both mid- water and
bottom currents were parallel to the axis of Long Sand Shoal to the north (Figure 1-1).

The current velocities at CSDS were maximum during the flood portion of the tidal
cycle. Over the long term (monthly time scale), maximum mid- water velocities were about
120 cm-s" 1 on the spring tide and 60 cm-s" 1 on the neap. At the near-bottom station,
maximum velocities were about 80 cm-s" 1 on the spring tide and 40 cm-s" 1 on the neap. In
all cases, these current velocities would be sufficient to erode fine to medium sands. For the
near-bottom current, the flood tide velocities were highest. This resulted in a net westward
trajectory for particles as they approached the bottom.

With the tidal variability removed from the current meter data at the mid-water
station, current vectors trended north and west. The combined net drift for the mid- water
current was 305° true at 4.65 cm-s" 1 . For the near-bottom meters, removal of the tidal
variability resulted in a south and west component with a combined net drift of 256° true at
8.33 cm-s' 1 .

During the current meter deployment at CSDS, two major storms passed over the
area. Hurricane Bob passed over Long Island Sound on August 19, 1991 and produced
maximum wind speeds of 45 knots. During the hurricane, the near-bottom current meter

Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



19



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Figure 3-8. Accumulation (A) and erosion (B) at CSDS, December 10, 1991 to May 12,
1992 (depth in meters)



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



20



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CSDS OCTOBER 1991 TO MAY 1992




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Figure 3-9. Accumulation (A) and erosion (B) at CSDS, October 18 and 19, 1991 to May
12, 1992 (depth in meters)



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



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Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



22

was on its side, and data were not obtained; the data from the mid-water meter showed that
the mid-day flood velocity was reduced by more than half. The succeeding flood tide current
was normal. The near bottom current meter was operational between August 28th and
September 26th, and again between October 21st and December 12th. At the end of October
1991, a major storm occurred that lasted 114 hours with sustained winds of 40 knots over
October 30 and 31. The National Weather Service determined this to be a 100-year storm;
therefore, the potential for erosion could have been high. During this "Halloween" storm,
the current meters showed no change in current strength, yet change did occur in the net
drift. The mid- water drift, normally 305° true, shifted to the west and then south for three
days. The near-bottom current net drift, normally 256° true, shifted to the west. The
combined effects of the October storm were to produce an offshore displacement in the mid-
depth waters, and shoreward displacement of bottom waters, intensifying the rate of
upwelling in the area.

3.3 Suspended Sediment Concentrations

Background levels of suspended sediment concentrations were measured at
approximately 6.5 mg-1 1 prior to the release of dredged material at CSDS on September 12,
1991. There was a weak tidal dependence in the background suspended sediment
concentrations. With the release of dredged material at buoy A, the background levels of
suspended sediment, measured approximately 1 m above the seafloor, increased by an order
of magnitude at BTM-A. The initially weak tidal dependence was increased with the advent
of dredged material disposal. In addition to the general increase in water column turbidity
during dredged material disposal, there were definite peaks during the discrete disposal
events (Figure 3-11).

The background turbidity levels at BTM-A did not increase significantly until three
days into the Saybrook Outer Bar disposal. Since the Saybrook Outer Bar contained medium
to fine sands, the initial increases in suspended sediment concentrations were due only to
discrete disposal events. As material accumulated on the seafloor, background levels
increased, presumably due to tidal resuspension and winnowing. Once the material eroded
or formed a lag deposit, the background turbidity levels began to decrease. A gradual, but
lower magnitude increase in background levels occurred during disposal from the Calves
Island Bar and Essex Island Bar projects. However, the amount of the resuspended material
in the bottom water decreased, from the Saybrook Outer Bar to the Calves Island Bar and
Essex Island Bar projects, as the material became coarser (Bohlen et al. 1992).

3.4 REMOTS®

The REMOTS® sediment-profile survey was conducted in May, after the completion
of dredged material disposal at CSDS. The REMOTS® photographs detected dredged
material at 15 out of 45 stations. Most of these stations were within 400 m of disposal

Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



23



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Figure 3-11. Suspended sediment concentrations at CSDS during disposal operations,
September and October 1991



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



24_

buoy B (Figure 3-12). The dredged material released at buoy location B, and detected in the
REMOTS® survey, was fine-grained material from the North Cove project. At the disposal
buoy location, very little sand covered the fresh dredged material (Plate 3-1). At
progressively greater distances from the center of the disposal mound, a layer of sand
covered the dredged material (Plate 3-2). This sand layer appeared to thicken with distance
from the mound center (Figure 3-13). An exception was REMOTS® Station D3 where fresh
dredged material was found with little or no sand cover. The presence of sand at a
REMOTS® station limited the penetration depth of the camera prism. If there were areas in
this REMOTS® survey where more than 4 to 5 cm of sand covered finer grained material,
the camera did not penetrate the sediment far enough to detect it.

The ambient sediment detected in the REMOTS® photographs was either fine sand of
a uniform grain size, or coarser sand with shell fragments and pebbles. The photographs of
the fine sand sediment showed bedforms, or sand ripples, but there were no visible
macrofauna (Plate 3-3). The sand ripples, approximately 1 to 3 cm in height, were
concentrated south of 41° 12. 75' N. REMOTS® photographs of the coarser grained sands .
contained shell fragments and pebbles with hydroids (Plate 3-4). The presence of hydroids
on lag surfaces of shell and gravel was noted for an area approximately 2 nmi southwest of
the disposal site in May 1987 (Fenster et al. 1990). Except for Station C13, sand ripples and
shell fragment were mutually exclusive.



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992



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4.0 DISCUSSION

The series of bathymetric surveys conducted at CSDS between August 1991 and May
1992, the current and suspended sediment information from bottom and mid- water moorings,
and the REMOTS® survey, all provided circumstantial evidence for active bed transport at
the site. The depth-difference comparisons between consecutive bathymetric surveys
provided some indication of shifts in sediment accumulation due to both the release of
dredged material at different locations as well as transport by bottom currents.

Between the August and October surveys, barges released dredged material around
buoy A (Figure 3-4). A depth difference plot of these two surveys showed some
accumulation up to 200 m east and south of the disposal buoy (Figure 3-3). This is
consistent with the barge disposal pattern. Large areas of accumulation were also seen more
than 200 m south of A and north and west of A. Each of these latter areas extended 400 m
or more east/ west. The source of this accumulation could be the material released by the
barges. Background levels of suspended sediments increased during dredged material
disposal as the material accumulated on the seafloor and was subjected to tidal resuspension
and winnowing. The increased background levels of suspended sediment during September,
if they indicated that the material has been resuspended and is then being moved (active bed
transport), could explain the accumulation pattern seen in Figure 3-3. Variations in the
predominant current flow between BTM-A and BTM-B (Figure 3-10) may be due to
variations in local topography (Bohlen et al. 1992). Similar modifications to the bottom
current by local topography may cause the slight southwest/northeast trend in the
accumulation pattern south of buoy A.

From October to December, only 13,891 m 3 of fine-grained material from the
dredging of North Cove was released at CSDS. The depth-difference between the October
and December surveys shows no accumulation at the disposal location (Figure 3-6). It does,
however, show a loss of material at the southern edge of the survey, and an accumulation
running east/ west just south of A. The apparent displacement of this material along the east-
west axis is again consistent with the dominance of westerly currents.

Despite evidence for active sediment transport at CSDS, material did accumulate at
the B disposal location between October and May (Figure 3-9). The accumulation of
dredged material at buoy B may be due to the amount and type of dredged material released
at CSDS. More than twice as much material was disposed at location B (103,375 m 3 ) than at
location A (50,803 m 3 ). The dredged materials released at B were fine-grained, cohesive
sediments that had been mechanically dredged. Cohesive sediments dredged in this fashion
are likely to be deposited at the disposal site as blocks or clumps of material that will reach


2

Online LibraryMary Baker WileySynthesis of monitoring surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992 → online text (page 2 of 3)