Distribution of dredged material at the Rockland Disposal Site, May 1985

LOT puartiCy 1 OY

Jan (Ie

Distribution Of Dredged Material
At The Rockland Disposal Site
May 1985

Disposal Area
Monitoring System
DAMOS

ine
[84

DIF
ho, 2

Contribution 50
March 1988

DOCUMENT
LIBRARY
Woods Hole Oceanagraphic
Institution

US Army Corps
of Engineers
New England Division

MBL/WHOI

AIA

0 0301 0069485 7

DISTRIBUTION OF DREDGED MATERIAL
AT THE ROCKLAND DISPOSAL SITE

MAY 1985

CONTRIBUTION #50

17 JANUARY 1988

Report No.
SAIC - 85/7533&C50

Contract No. DACW-85-D-0008
Work Order No. 2

Submitted to:

Regulatory Branch
New England Division
U.S. Army Corps of Engineers
424 Trapelo Road
Waltham, MA 02254-9149

Submitted by:

Science Applications International Corporation
Admiral's Gate
221 Third Street
Newport, RI 02840
(401) 847-4210

US Army Corps
of Engineers

New England Division

TABLE OF CONTENTS

Page

INTRODUCTION Al.
SITE CHARACTERIZATION aL
Bo dk Bathymetry and Side Scan Surveys aL
Dene Sediment Characterization 2
2703 Vertical Profiles of Temperature and

Salinity 3
DHA Current Regime 3
25 Discussion 4
SEDIMENT PLUME TRANSPORT STUDY 5
Sioa Introduction 5
Bye Methods and Analysis 7
Bio é Results )
3.4 Discussion 10
CONCLUSIONS 12
FIGURES

TABLES

LIST OF FIGURES

Fagure)1—1! Rockland Disposal Area.

Raguse 2 —1 Bathymetric survey grid, Rockland Disposal
Site.

Figure 2-2 Bathymetric contour plot of Rockland

Disposal Site, 20 May 1985.

Figure 2-3 Results of the side scan survey conducted
at Rockland, ME on 20 May 1985.

Figure 2-4 Side scan record showing high reflectance
at center of disposal area.

Figure 2-5 Side scan record showing circular area of
reflectance indicating a single dump.

Figure 2-6 Side scan record showing result of
disposal occurring underway in a turn.

Figure 2-7 Sediment sampling locations during the
May 1985 survey.

Figure z-8 Sediment station sampling locations
during the September 1984 survey.

Figure 2-9 Vertical temperature and salinity pro-
files at Rockland Disposal Site, 21 May
1985.

Figure 2-10a Vertical temperature profiles at Rockland
Disposal Site, 22 May 1985.

Figure 2-10b Salinity profiles at Rockland Disposal
Site, 22 May 1985.

Figure 2-lla Vertical temperature profiles at Rockland
Disposal Site, 23 May 1985.

Figure 2-11b Vertical salinity profiles at Rockland
Disposal Site, 23 May 1985.

Figure 2-12 Vertical temperature and salinity pro-
files at Rockland Disposal Site, 24 May
TO 8 5ye

LIST OF FIGURES (cont. )

Figure 2-13 Three-hour low pass (3-HLP) time series
of temperature, current speed, and
direction at Rockland Disposal Site at
a depth of 20m, 22 May col Wuner os Sr

Figure 2-14 Forty-hour low pass (40-HLP) temperature
time series and current stick plots at
Rockland Disposal Site at a depth of 10m,
21 May to 11 June 1985.

Figure 3-1 Actual ship's track during plume study.

Figure 3-2 Results of the plume study conducted at
Rockland Disposal Site on 21 May 1985.

Figure 3-3 Photographs of the acoustic record for
the plume study of 21 Mey 1985.

Figure 3-4 Results of plume study conducted at
Rockland Disposal Site on 22 May i985.

Figure 3-5 Photographs of the acoustic record for
the plume study of 22 May 1985.

Figure 3-6 Results of the plume study conducted at
Rockland Disposal Site on 24 May 1985.

Figure 3-7 Photographs of the acoustic record for
the plume study of 24 May 1985.

Table

Table

Table

Table

LIST OF TABLES

Qualitative Sediment Characterization at
Rockland Disposal Site, May 1985

Results of Sediment Chemical Analysis at
Rockland Disposal Site, October 1984

Results of the Bivariate Analysis of
3-HLP Current Meter Data Collected at
Rockland at a Depth of 10m, 21 May to
Ines Se >

Estimates of Material in Water Column
After Disposal

1.0 INTRODUCTION

The Rockland Disposal Site is located in the center of
West Penobscot Bay (Fig. 1-1), 3.3 NM northeast of the Rockland

Breakwater. This site was first used during October 1973-
February 1974 for disposal of approximately 69,000 m2 (90,200
yds?) of material from Rockland Harbor. The disposal site is a

0.5 nautical mile square centered at 44°07.01'N, 69°00.3'W.
Water depths within the disposal area range from 65 to 80 meters.
The disposal site is marked with a buoy deployed and maintained
by the US Coast Guard.

An earlier baseline survey of the disposal site was

conducted during the period 24 September - 2 October 1984 to
determine existing conditions of the bottom before the start of
dredging projects from the Searsport area. This survey included

precision bathymetry, sediment characterization (chemical and
physical), a side scan survey, and REMOTS® sediment profiling.

The present study was conducted during the period 19-24
May 1985 to assess the transport of dredged material during
disposal operations and to perform bathymetric and side scan
surveys of the area after approximately 360,000 ya3 (275,400 m?)
of dredged material from the Searsport project had been

deposited. Vertical profiles of temperature and salinity were
conducted, and current meters were deployed for approximately one
month at the disposal site. The results of the data analysis

were used to estimate the percent of material expected to reach
the bottom during disposal operations and to delineate the extent
of dredged material throughout the disposal site.

2.0 SITE CHARACTERIZATION
Pali Bathymetry and Side Scan Surveys

A bathymetric survey was performed over an area 1200 m
by 1200 m surrounding the disposal site. The survey, comprised
Of 50) Lanes; IANO) aul, aLohores,, spaced) 25 meters “apart, was

accomplished using a 24 kHz fathometer system operating in
conjunction with the SAIC Navigation and Data Acquisition System
which is based on an HP 9920 microcomputer system.

Figure 2-1 shows the bathymetric survey area in
relation to the disposal site, the present location of the
disposal buoy and the location of the Reference Site established
for the sediment sampling program.

A contour chart of depths at the disposal site is shown
in Figure 2-2a. The site is characterized by a depression which
is well-defined in the northern portion of the site, but widens
and shoals toward the south, completely losing its identity over

1

the southern half of the site. Depths range from about 65 meters
to 80 meters within the surveyed area. Comparison with the
bathymetric survey conducted in September 1984 (Figure 2-2b)
reveals no significant development of a disposal mound near the
buoy location.

Figure 2-3 shows the results of the side scan survey at
the Rockland site conducted in May 1985. The outlined area
indicates the presence of intermediate acoustic reflectance that
could be caused by soft natural bottom or old dredged material.
The small dark areas show the location of higher acoustic
reflectance characteristic of dredged material. The central area
of the site is well covered by dredged material and is surrounded
by individual disposal events. Long narrow areas probably
indicate disposal that occurred underway or during a turn.

Figure 2-4 is a photograph of the side scan record at
the center of the area showing this high acoustic reflectance.
Figure 2-5 shows a circular area, probably the result of a single
dump. Figure 2-6 shows the result of disposal occurring during a
UG mM. The ‘side’ Scan “records! "did ~/not) sdeteces anys Vance
accumulation in the form of a mound.

22 Sediment Characterization

Figure 2-7 shows the location of sediment samples taken
at the Rockland Disposal Site to visually characterize the bottom
and detect the extent of dredged material. Table 2-1 describes
the sediment collected at each station with a 0.1 m2 Smith-
MacIntyre grab sampler. The pattern of stations sampled was
determined by visually identifying dredged material in the grab
and proceeding until natural bottom was encountered. The spatial
delineation of dredged material from the grab samples compared
well with the areas of high reflectance measured with side scan
sonar. Minor discrepancies resulted from the patchy nature of
Single dump loads. The areas of intermediate reflectance could
indicate the presence of old dredged material.

During the September 1984 survey, sediment samples were
collected at stations on North-South, East-West, Northwest-
Southeast, and Northeast-Southwest transects (Fig. 2-8), as well
as at the Reference Site (2000m east). Table 2-2 contains the
results of the chemical analyses of the sediment samples.

The results of the chemical and physical analyses
indicate little variation throughout the site. The physical
tests indicate the similarity between locations with a pattern
common to most of the samples; olive in color, high percentage of
fines (average about 90% and predominantly clays), low fine sand
percentages (around 10%) and very little medium and coarse
material (usually less than 1%). These values also occur at the

reference site. The only exception of note was at 200S where the
amount of fine sand was 18%.

The chemical data also reflected this uniformity along
the transects by showing rather small differences between
individual location replicates. The concentrations for the trace
metals were low throughout the area. COD and organics were
typical for natural bottom silt/clay. An exception was at 200SW,
where relatively high concentrations of oil and grease were
found, most likely from the presence of dredged material. The
results of the chemical analyses at a majority of the stations in
the disposal area are similar with those from natural bottom.
C:N ratios were mostly between 7 and 9 (typical of offshore
natural sediment). Mg:Ca ratios averaged between 4-6, indicating
relatively few shells. Dredged material may have influence at
400W; the Mg:Ca ratios here averaged about 1 (relatively high in
shells) and the C:N ratio averaged about 11 (typical of inshore
harbors and estuaries). The variation in Mg:Ca and Fe results at
400NE may be caused by local natural bottom conditions and not
dredged material.

os} Vertical Profiles of Temperature and Salinity

On* seach day” “Orn the) perlodini2iy May, Y= 24 May alos 5.
vertical profiles for temperature and salinity were measured at
the disposal buoy to determine the structure of the water column.
Figures 2-9 to 2-12 show the temperature and salinity data

collected each day. The temperature ranged from approximately
Geer acy Che jbottom to!) Plc fat the surface wwalth” a thermocline
present at about 10 meters by 23 May. Salinity ranged from

approximately 29.6 ppt at the surface to 32.2 ppt at the bottom.
Throughout the tidal cycle, little effect was seen on the shape
of the vertical profiles. Wind-driven water movement could
account for changes in salinity and temperature at the surface.
Certain vertical profiles are seen to contain fewer data points
than others. This was due to intermittent data signals being
received by the computer. Alternatively, data were recorded
manually at specified depths.

2.4 Current Regime

During the ;period) 21 “May to.11 June’ 1985, current
measurements were made at Rockland in order to determine the

overall current regime of the area. A string of two General
Oceanics current meters at depths of 10 meters and 60 meters was
deployed about 300m SE of the disposal buoy. Due to a tape

malfunction, no data were collected by the 60 meter instrument.
Figure 2-13 presents the current speed and direction and
temperature data for the observation period. These data have
been 3-hour low pass filtered (3-HLP) to emphasize the tidal
components.

The temperature trace reveals initial readings of
approximately 6.4°C warming to an average value of 8.3°C with a
maximum value of about 9.8°C. Higher temperatures are detected
on the ebb tide as warmer waters from the shallower coastal areas
flow by the disposal area.

The current direction varies in the N-S axis over the
tidal cycle. The peak current velocities occur on the flood tide
in the northerly direction with maximum values of approximately
AOwmeMn/iSee (Lable wy 23). Table 2-3 presents the results of
bivariate analysis, of the current data §(3-HLP) © revealing “the
average current speed to be approximately 13 cm/sec. The current
a@irection is northerly for approximately 503 (sum) (or @29%9) Vand
18.6%, see boxes in Table 2-3) of the period and southerly only
22%-(sum Of VoL? and 5.22) .0f) theiperiod. a horn s6> om (SUMmOtaire,
23.8 and 138%) of the period, the current velocitresswene anieche
range of 4-16 cm/sec. Onily 913% / (sum iof 5-0), 93:56), W275), ele camamcl
0.4%) of the period saw velocities greater than 24 cm/sec (0.5
knots).

Figure 2-14 presents the temperature data and a current

stick plot for 40-hour low-pass filtered data (40-HLP). The
temperature time series depicts the average values after the
tidal component is removed. The net, non-tidal flow in the

GQisposal area is consistently to the north-northeast quadrant at
a maximum of approximately 15 cm/sec.

DEaS Discussion

The results of the bathymetric survey did not reveal
the development of a disposal mound near the location of the
disposal buoy. Rough estimates from scow logs of the volume of
dredged material deposited between the September 1984 survey and
the present one are approximately 360,000 ya3 (275,400 m?). This
large volume of material would be expected to create a mound if
controlled point disposal was conducted. A combination of
factors including, the -depth aati the dasposal: pone ai(7 0m m)sn cae
wide scope (3 times the water depth) of buoys normally
established by the US Coast Guard, and, apparently, a practice of
depositing dredged material while the scow is underway has caused
the material to be spread over a large area. Disposal of dredged
material in a water depth of 70 m allows a significant amount of
water to be entrained during the convective descent phase of
disposal. A large percentage of the dredged material will then
have a lower density due to the increased water content causing a

slower descent and increased spreading from the initial disposal
location.

Figure 2-3 shows the wide distribution of dredged

material detected by the side scan sonar. The outline in the
figure indicates the area of intermediate reflectance that
usually signifies thin layers of dredged material. The pattern

4

of distribution suggests that some scows began depositing the
material before and/or after reaching the disposal buoy.
However, a large percentage of the material has apparently
accumulated southeast of the buoy.

A rough estimate of the amount and thickness of dredged
material can be calculated using the approximate area of seafloor
indicated by the side scan sonar survey to be covered with
dredged material. Examination of Figure 2-3 shows an area
roughly 700 by 700m square of intermediate acoustic reflectance,
usually indicating dredged material. Assuming that approximately
275,400 m? of material (estimated from scow logs) was evenly
spread over this area, a layer of dredged material of
approximately 0.5 m thickness could be expected. When
considering that both the loss of interstitial water during
descent and compaction after impact with the bottom would reduce
the actual volume of material expected to be seen on the bottom
by as much as 40% (Tavalaro, 1983), the thickness of the dredged
material may actually be approximately 0.3 m. It would be
difficult to confirm the presence of this layer with bathymetry
in a depth of 70 m due to the limitations of available fathometer
systems and a combination of errors associated with the speed of
sound, tide, and navigation. More detailed study of this area
with precision bathymetric surveys at a smaller lane spacing (25
m) would detect any significant topographic features caused by
dredged material disposal while REMOTS® sediment profiling would
be needed to accurately measure the thickness of the dredged
material layer and determine its areal limits.

3.0 SEDIMENT PLUME TRANSPORT STUDY
8}.5aL Introduction

In order to assess the potential impact of dredged
material disposal on the surrounding environment, a plume study
was conducted to track suspended material in the water column
after a disposal event. Three studies were performed during the
period of 21 May to 24 May 1985. Although attempts were made to
track plumes on both the flood and ebb tides, coordination of
ScOw arrival and dumping, weather, and tide prevented studies
occurring during ebb tide. However, because the dominant current
feature is the flood tide (with maximum peak velocities and long
durations), emphasis on the flood tide was warranted due to its
greater potential for transport.

Multi-frequency acoustic profiling has been under
investigation since 1975 as a means of measuring concentrations

of suspended matter in the water column. Much of the work has
been carried out by the NOAA Atlantic Oceanographic and
Meteorological Laboratory in Miami. The work has included study

of diffusion properties and acoustic measurements, as well as
development of a model to describe the relationship between

5

concentrations of total suspended matter in the water column and
acoustic profile observations. A diffusion model allowing
prediction of diffusion velocities and spatial variation has been
postulated and tested.

The following relevant observations are taken from a
technical paper "SOME OBSERVATIONS ON DREDGED MATERIAL DUMPING IN
THE NEW YOR! BIGHT", by Dr. John Proni and Dr. John Tsai at the
NOAA-AOML facility.

ales Spatial» and .temporal, «variation: | dusing
diffusion following a dump predicts a two-
process diffusion with different diffusion

velocities and spatial variation. During the
active phase, heavy materials settle through
Gravitation, “~seaching, the, “boron: The

downward momentum may generate vertical
mixing of the water column inside the plume,
and resuspension from the bottom. During the
passive diffusion phase, the sharp edges of
the plume disappear and dissipate into the
surrounding water. Rate of diffusion slows
down as the particle size distribution
changes from coarse heavy material to
smaller, lighter particulate matter. This
dual diffusion process was clearly observed
during the dredged material dump in
Massachusetts Bay in February 1983.

2s Acoustic backscatter measurements can be
correlated with measurements of total
suspended matter in the water column as:

eo or N|

where I is acoustic intensity, N is the
number of particles per unit volume, anda is
assumed to depend on particle shape, size,
density, compressibility and frequency.
Direct and linear relationships are found
between observed acoustic intensity and
measured total suspended matter.

In the Rockland Disposal Area study, the following
tasks were attempted:

ive Track and observe the extent of plume
dissipation and movement over a period of
time following the dump.

Dis Measure ambient and in-plume concentrations
of total suspended matter by analyzing water
samples taken in selected locations where
acoustic intensity could also be observed.

5 Using the measured values of total suspended
matter as "ground truth", make estimates of
total suspended matter concentration based on
acoustic backscatter measurement alone.

SZ Methods and Analysis

The Datasonics Model DFS-2100 Acoustic Remote Sensing
System was used at 200 kHz for performing the acoustic plume
tracking. High power output, low receiver noise levels and
calibrated control of signal level allows monitoring of extremely
HOW concentrations - Of » material invecthe s waters =column “and
acquisition of suspended sediment concentration levels when
correlated with ground truth sampling.

in satcempting ste. relate = acoustic “backscatter
measurements in a plume of suspended particulate matter to
Guantitative “concentration “levels, ‘one “must “measure” the
reflection, or backscattering characteristics of the material in
the scattering volume of interest.

The echo or reverberation level received back at the
towed vehicle transducer from particulate scatterers in the
dredged material plume may be expressed as part of a standard
sonar equation as follows: (See definition of terms below).

RL = SL - 40 Log R - 2eR + Sy, + 10 Log V (1)
Equation 1 summarizes signal losses within the water

column. The acoustic receiver voltage output measured and
recorded during a survey can be defined as follows:

out rms = RL + RS + GAIN
Ors
RL = out rms - RS - GAIN @)

Equating equations 1 and 2.
out rms = SL-(40 Log R + 2aR) + (Sy, + 10 Log V) + RS + GAIN (3)
Definition of Terms
RL: Reverberation Level, or backscattered acoustic intensity

from a random, homogeneous distribution of scatterers throughout
a defined volume of water.

SL: Source Level, a measure of on-axis acoustic intensity of the
transmitted signal.

AO) CYC IR ae Aese Two way transmission loss from the transducer
down to the scattering volume of interest and back to the
transducer. The 40 Log R component is due to spherical spreading
loss, while the 2eR loss is due to absorption (primarily a
function of frequency).

Sy tneO Loge: A measure of the backscattering strength due to
volume reverberation. Tt a5 thelyS, term) which! \wadslecom-elate
with concentration of !scatterers, (jon totalaesuspended matters i
is a measure of the backscattering volume which will increase
with depth as the volume of water encompassed by the acoustic
signal increases due to the transducer beam pattern.

RSE Receive Sensitivity or transfer function of the receiving
transducer .in conversion (of) the secho, epressunesswaveR aeoggan
electrical voltage.

GAIN: Overall receiver gain, including Time Varying Gain (TVG).

Examination of Equation 3, when using the Datasonics
Model DFS-2100 Acoustic Remote Sensing System, reveals the
following:

a The receiver gain incorporates TVG which
compensates precisely for the 40 Log R + 2aR
transmission loss term. These two terms then
fall out of the equation. If overall gain is
changed, however, the change must be factored
into the calculations.

Die Receive Sensitivity is a constant, determined
by laboratory measurement. This term can be
ignored in making concentration calculations
from acoustic observations because relative
concentration measurements, with respect to
measured total suspended matter samples, are
being made.

Bo Source Level may be ignored so long as it
remains constant. It must be accounted for
whenever a change is made.

4. Vo rms, the measured receiver output, is then
proportional gto (Se 04 510) eg Vy. Velsmva
function of pulse width, equivalent beamwidth
and range (R) Erom the jetrnansducersy sito
scattering volume. V is a measure of the
reverberating volume at any given depth.

8

BOTAN ye EVEN a paint hekhen wSaZey uaduisitrn buitcsom\) | sathe
concentration of total suspended matter will be proportional to
the measured rms voltage output of the receiver, when corrected
for depth.

Ses Results

The plume studies were conducted at disposal events
when the towed scow was unloaded by opening the bottom doors. As
soon as disposal was complete and the scow was underway, the
research vessel, with navigational control and the Acoustic
Remote Sensing System aboard, began executing patterns of
parallel tracks to determine the boundaries of the plume. After
the motion of the plume was determined, the ship's track was
modified to continually encompass the boundaries. Figure 3-1
presents an actual ship's track followed during the first two
hours of the plume study on 24 May 1985.

During each study, water samples were collected with
Niskin water samplers as the acoustic sensing transducer passed
through the plume. Sampling depths were determined from the
acoustic results of a pass through the area immediately prior to
sampling. The samples were analyzed for total suspended sediment
@ng/Ah)== and “used = to callilbrate’ the’ “output voltages!” from “the
acoustic record.

PA MAverL OS Sia ienP lL umMenSitudy.

The plume study began at 0920 EST as the scow began the
disposal operation. One scow of 1575 cu yds (1205 m?) was dumped
approximately 200 m east of the disposal buoy. The study was
continued until 1051 EST. Low tide occurred at 0547 EST and high
tide at 1157 EST. Flood tide was in progress during the survey,
providing a N-NE flow. Figure 3-2 presents the results of the
plume survey.

The area of heavy concentrations (averaging about 1000
mg/l, Fig. 3-3) occurred below a depth of 50m and did not extend
far beyond the area directly beneath the disposal location.
Within 15 minutes of the completion of the disposal operation,
concentrations averaging approximately 40 mg/l (Fig. 3-3)
occurred from a depth of 20 m to the bottom and extended
approximately 300 m from the initial location in the N-NE
quadrant. Within forty minutes of disposal, suspended sediment
concentrations were averaging less than 10 mg/l (Fig. 3-3) at
depths below 35 m and extended less than 500 m from the disposal
point, still within the boundaries of the designated disposal
site. Further surveying detected no significant levels of
suspended sediment above background concentrations of 3-5 mg/l.

2:2) MayanliGisiobuaai=r a Plumem SituGiy:

The second plume study began at 0645 as one scow of
1900 cu yds (1450 m?) completed disposal operations approximately
150 meters south of the disposal buoy (Fig. 3-4). With low tide
occurring at 0623 EST, flood tide was just beginning to develop.
Again, the heavy concentrations of suspended material, averaging
Greater than’ 1000 mg/l “(Rig. 93-—5)),) cccumned (directly Soeneatchysene
disposal location throughout the water column. Within forty
minutes of disposal, concentrations averaging greater than 25
mg/l (Fig. 3-5) occurred below 30 m to the bottom and extended
approximately 350 meters to the north. One hundred minutes after
disposal, concentrations averaging only 12 mg/l (Fig. 3-5)
occurred at depths below 50 m and extended less than 500 meters
north-northeast of the disposal point, still within the disposal
site.

ZA MayanlO8 >in umMem Stuy

The third plume study began at 1025 EST as two scows
(tandem: oad) containing “a > total wor 364 On vecumiydsEs (27/80 m?)
completed disposal operations within 50m SE of the disposal buoy

(Fig. 3-6). .With low tide: occurring at.0744 ESTwand hugh trdemat
1355 EST, the flood tide was fully in progress, producing maximun
tidal current velocities to the N-NE. Heavy concentrations

averaging about 1400 mg/l of suspended material (Fig. 3-7) were
detected directly beneath the disposal location throughout the
water column. Within one hour of disposal, concentrations of
suspended material of 110 mg/l (Fig. 3-7) occurred from 20 m to
the bottom and were centered at a point approximately 700 meters

north of the disposal location. After more than two hours, low
concentrations of suspended material, averaging about 13 mg/l
(Fig. 3-7), occurred below 50 m and were measured within a 400 m

radius around a point 1700 meters N-NE of the disposal location,
or as much as 1000 meters beyond the northern boundary of the
disposal site. Background levels of 3-5 mg/l were measured at
the disposal buoy just before this survey was conducted and just
outside of the disposal site when no disposal operations were
being conducted.

3.4 Discussion

Of the three plume studies conducted at the Rockland
disposal area during the period 21 to 24 May 1985, the first two,
although occurring on the flood tide, did not detect any elevated
concentrations of suspended material above background levels (3-5
mg/l) outside of the designated disposal area. The survey
conducted on 24 May 1985 resulted in measurable concentrations
(averaging 13 mg/l) of suspended material as far north as 1000
meters beyond the disposal site boundary.

10

In an attempt to estimate the amount of material that
could be transported out of the disposal area, preliminary
calculations were made from the results of the plume studies. A
representative range of values for bulk density of the dredged
Maeerlale Of 1 S4 yO 16 g/cm? was used to calculate the dry mass

of material in the scow. The estimated scow volumes were
obtained from NED. From each plume study, estimates of the
depths where the suspended sediment concentration occurred were
used in the calculations. Table 3-1 presents the estimates for

the percent of material present in the water column after the
disposal event. These calculations included determining the mass
of material (mass,) in the suspended sediment cloud as:

maSSo = A X D XGES

where A = the area of the sediment cloud (m2),
D= the height of the water column (m) with suspended
sediment, and
S= eae suspended sediment concentration (mg/l or
g/m-),

and determining the mass of material (mass,) in the scow as:
mMaSSg = scow volume (m3) X bulk density (g/cm?) x 10°
aneiy famalliy.:

% material = (mass, / maSsg) X 100.

The estimates for the percent of disposed material
still in the water column vary widely for the three plume

studies. This is due to the estimated values for area and depth
of the suspended sediment concentrations determined in Figures 3-
Bn (BOG ehavel iSO Although the acoustic measuring system can

detect suspended sediment vertically from the surface to the
bottom and along the ship's track (horizontally), another pass of
the ship through the suspended material is needed to delineate
the spatial area. The interval of time required for this allows
the suspended material cloud to settle or spread. Qualitative
judgments were made in order to graphically illustrate the best
approximation of the suspended sediment clouds. Despite the
variation in estimates of material in the water column, an
important feature common to all three plume studies is that
within two hours, 93% or more of the material was on the bottom
and suspended sediment concentrations were similar to background
levels. For the plume studies conducted on 24 May, tandem scow
loads were deposited at the disposal site. Doubling the volume
of disposed dredged material increased the initial sediment
concentrations (1420 mg/l versus 950 or 1100 mg/l on 21 or 22
May, Table 3-1) due to more material being available for
suspension and increased the distance from the disposal point
that the suspended material was tracked (1700 m versus 500 m on

alta

De andy 22 eeMavin nalolehy aSi—5) irs The distribution of suspended
sediment immediately after disposal would probably be less for a
single scow load of the same volume because more material would
make up the single descending plume and a smaller percentage of
material would experience the entrainment of water at the water
column/plume interface.

Results from the current meter data showed that the
dominant flow was to the N-NE and that the maximum current
velocities occurred on the flood tide. For the Rockland area,
NOAA tidal current tablles estimate that slack water before flood
tide occurs approximately 1 hour before low tide and that maximum
flood tide current) velocities occur about 3 hours vatter Vow jeder
This suggests that the sediment transport out of the disposal
site during the 24 May plume study was caused by the peak tidal
Velocities, occurring ulat: gchatiiatimer This period of maximum
transport appears to be relatively short. The survey of 21 May
occurred one hour later in relation to the stage of the tide, and
no material was detected outside the disposal site. Current
velocities greater than 24 cm/sec occurred only 13% of the time,
or approximately 1.6 hours during each tidal cycle.

Examination of Figures 2-9 to 2-12 indicates that the
thermocline usually occurred at or near the 10 m depth where the
current meter was deployed. Therefore, the results of the
current meter analysis would reflect the potential for movement
of sediment that may be suspended at that depth. However, a
closer look at the results of the acoustic records obtained
during the plume tracking. (Figures 3=3', 3-5, and) 3-7) sadudamnoc
reveal a significant accumulation of suspended sediment at the
thermocline.

In order to estimate the long-term effect of disposal
at this site based on conditions described above, a calculation
was made to determine the percent of material that could be
expected to leave the disposal site during a prolonged disposal
project. If we assume that after disposal approximately 6% of
the material will be in the water column and available for
transport by current velocities greater than 24 cm/sec, then only
0.8% (or 6% times 13%) of the material disposed during the entire
dredging project is available for transport outside the site
boundaries. As mentioned earlier, this material would be so
widely dispersed that detection of any accumulation would he
almost impossible.

4.0 CONCLUSIONS

The results of the precision bathymetric survey
conducted on 20 May 1985 did not detect any significant
development of a mound of dredged material from disposal
operations occurring since September 1984. The side scan survey
revealed the presence of dredged material near the disposal buoy,

12

as well as isolated patches surrounding the center of the area.
Visual identification of dredged material from sediment grab
samples correlated well with the results of the side scan survey.
The distribution of intermediate acoustic reflectance (usually
signifying thin layers of dredged material) suggests that
disposal operations ranged out to more than 300 meters from the
bpueyv (which had a Scope of up to 150 m) and did’ not create a
disposal mound detectable with bathymetric survey procedures.
The depth of the disposal location (70 m) also contributes to
wider distribution of disposed material by the entrainment of
water during descent and the subsequent reduction in the density
of the material.

Results of chemical analysis of sediment samples
collected in September 1984 did not reveal any significant
elevations in chemical concentrations that could indicate the
presence of contaminated dredged material. In general, no trends
could be seen throughout the sampling area. Slightly higher
concentrations of oil and grease at some stations indicate the
potential for isolated patches of recently deposited dredged
material existing.

The Rockland Disposal Area experiences its maximum (40
em/s)) ptidal current) velocities only | at maximum 9 flood) tide
conditions in a N-NE direction. Because the flood tide is the
dominant feature in the current regime, it yields the greatest
potential for transport of suspended sediment introduced by
dredged material disposal operations. Although the data from the
bottom current meter was lost, bottom current velocities
sufficient to resuspend and transport large amounts of dredged
material out of the disposal site are not expected, based on data
collected by the meter at the 10 m depth.

If disposal- occurred only on maximum flood tide (a
worst case), an estimate of the material transported out of the
disposal site would be approximately 6%. However, if disposal
occurred evenly at all stages of the tide, this estimate reduces
ieOmme lor. Once this small percentage of material has settled
outside the disposal site, it would be so widely distributed as
to be undetectable. The fact that no significant accumulations
of sediment were detected in the N-NE direction from the disposal
buoy by either bathymetry or side scan sonar supports this
conclusion. Alternatively, if adverse levels of sediment
accumulation were detected outside the disposal site, disposal
operations could be scheduled to avoid peak flood tide.

In summary, although the potential for the movement of
suspended sediment produced by disposal operations exists at the
Rockland Disposal Site, results of the plume tracking experiments
indicate that this potential is small. Analysis of current meter
data suggests that any transport would occur to the N-NE,
although no evidence of accumulation of dredged material in that

Als}

direction “was detected’ by bathymetry “orn ) siden iscanmysonaz:
Finally, transport of suspended material out of the disposal site
would only be expected to occur when disposal operations take
place at maximum flood tide.

14

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Figure 2-4. Side scan record showing high reflectance at
center of disposal area (see Figure 2-3).
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Se es

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Figure 2-5.

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reflectance indicating a single dump (see

Figure 2-3). Scale lines (vertical) are
7.5 meters apart.

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Figure 2-6. Side scan record showing result of disposal
occurring underway in a turn (see Figure 2-3).
Scale lines (vertical) are 7.5 meters apart.

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Figure 2-9.
21 May 1985.

Rockland Disposal Site,

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Vertical temperature and salinity profiles
Rockland Disposal Site, 24 May 1985.

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Figure 3-1.

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Actual ship's track during plume study.

ae

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3508

NORTH

950 mg/l at 09330 EST

Figure 3-2. Results of the plume study conducted at Rockland

Disposal Site on 21 May 985. (Axes tanrescentered
at disposal location. Distances in meters:

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500 500

Figure 3-4. Results of plume study conducted at Rockland
Disposal Site On 22 May 1985. "Axes: ane centered
at disposal location. Distance in meters.

0 EST

22 eng/el ac
0730

EST

1100 mg/l at
0650

WD jinej/ ik. ete

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0830

Photographs of the acoustic record for the plume study

Of 22s Mays s9i8ior

3 ie

Figure

wa)

NORTH

LSymo/l Gt 12 sk5 Si

TAO} MOA NG it gee lelee2 ORE Si

Buo
y 1000 2000

1492-mg/l-at—+0-:3)-EST —————_-

Figure 3-6. Results of the plume study conducted at Rockland
Disposal Site on 24 May 1985. Axes are centered
at disposal location. Distances in meters.

ay
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Photographs of the acoustic record for the plume study

Of 24s May, 918 )Sie

3=7S

Figure

i
nay
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i
ye 4 My y
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Table 2-1

. . a . . *
Qualitative Sediment Characterization

Station

200W
100W

50W
CENTER
50E
100E

150E
200E
300E

400E
450E

500E

200N/150E
100N/150E
100S/150E
200S/150E
300S/150E

400S/150E
500S/150E

600S/150E

at Rockland Disposal Site
May 1985

Sediment Description

Natural bottom

Dredged material; sandy silt with wood,
clay, shale fragments

Soft sandy silt with clay clumps and
gravel, possibly old dredged material

Sandy silt over silty sand with gravel,
well-colonized

Dredged material, sandy silt with clay
clumps and stones

Similar to 50E with hard packed clay
clumps

Similar to 100E with cobble
Large stone (25cm, incomplete grab)

Sandy silt with large clay clumps and
gravel

Sandy silt with hard clay and stones

Soft natural bottom, well colonized,
Nephtys, starfish, sea cucumber

Same as 450E

Natural bottom, starfish

Large stone, incomplete grab

Dredged material with clay clumps, stones
Same as 100S/150E

Dredged material with coarse sand, gravel,
clay clumps

Silty sand, stones, wood chips, colonized,
possibly old dredged material

Dredged material with clay clumps, gravel,
shell fragments

Natural bottom, starfish, worms

* . .
Benthic organisms are mentioned when seen in grab sample.

Station

200S/300E
200S/150E
200S/50E
200S/50W
200S/150W

200S/250W
200S/350W

200S/450W
400S/150E
400S/150W

500S/250W

600S/350W

Table 2-1 (cont. )

Sediment Description

Natural bottom

Dredged material with clay clumps, stones
Sandy silt, clay clumps, gravel

Large clay clump

Natural bottom with traces of dredged
material

Possibly old dredged material colonized by
tube worms

Clay clumps on natural bottom, tube worms,
starfish

Natural bottom
Old dredged material, wood chips, colonized

Dredged material with sand, gravel, clay
clumps, wood chips

Dredged material with sand, gravel, clay
clumps

Natural bottom, soft sediment, worms

Table 2-2

Results of Sediment Chemical Analysis at Rockland Disposal Site

Station

400N-A
-B
-C

Mean

Std.Dev.

200N-A
-B
iC

Mean

Std.Dev.

CTR -A
8}
=¢

Mean

Std.Dev.

Mean
Std. Dev.

400S-A
-B
—@

Mean

Std.Dev.

2000E-A
-B
AS

Mean

Std.Dev.

Fe 2n
x104

2.66 134
2.60 160
2.88 137
Pe igfal 144
Or ai2 12
De 204
D313 1Oe)
2.52 149
Diao 158
0.08 34
2.38 139
2.59 159
3.00 137
2.66 145
0.26 10
2034 103
2.40 134
2.45 141
2.40 126
0.04 iy
2.75 123
2.318 132
252 146
2.55 134
O15 9
2.35 151
2 e5i 124
2.59 110
2.48 128
O10 ay)

October 1984

(ie

© wo ~
O}E Aw

°

ONNNO ONNNN
O1 U1}00 \/O OO oO WO}O OY)

ONINNN
PON OU

RP WIR WwW
Oo, OO O OW

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OO} Oo Ul

90 ON Oo
NNIW OO WM

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NM WI Fr WwW

oO Pf} OW

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Polk ow

Station

400W-A
-B
-C

Mean

Std.Dev.

Mean

Mean

Mean
Std.Dev.

400E-A
—i5
=<

Mean

Std.Dev.

2000E-A
-B
AG

Mean

Std.Dev.

BrOO
3.40
0.30

Drape?
Px) 0)
32.00
2.54
0.36

3)52'5
Bra27
0.65
Zo.
MG BE

2.82
Sots
3.94
BiG Sik
0.47

4.10
By dual
4.06
Bi. 116,
0.46

3.66
3.40
2.50
2) 6 cue)
O50

Table 2-2 Continued.

COD
x10?

Fe
x104

- indicates no data available.

O @jO ~) ©
NI NID NW OO

O MW]o 10
e e
b O}P op wb

ONIN NN
° oa |b
Fr OO) OV

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WNP YO

e

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PULP ow

Station

Mean
Std.Dev.

CTR -A
=B
—

Mean

Std.Dev.

200SW-A
-B
-C

Mean

Std.Dev.

400SW-A
-B
—C

Mean

Std. Dev.

2000E-A
-B
—

Mean

Std.Dev.

2.64
Syeles,
0.54

Sis dal
7h 5 Se
AES
37210)0
0.08

Srey2jD
Socal
OMG5:
Bool
eee

3738
2.28
2.82
2.83
0.45

302
= 174
4.07
3579/0
0.12

3.66
3.40
Zhc15)0)
Boake)
0.50

Table 2-2 Continued.

CoD
x10°

Fe
x104

- indicates no data available.

154

(eye

ONIN ON
OnION Fr

oN aA
PON ®8 wo

© wlo ~~ o
b O}S iB tw

oO O|N © Oo
WNIO NM U1

ONN~ Oo
WoW OO

ONINNN
PON OU

OO & oO

NNW Oo U1

OW} NM &
Ov Nir ON

oO fj Ww
\O 0VjO U1 OV

POL ao»
Oo wlw nw

Table 2-2 Continued.

% COD Fe

Vol. X10? x104

Station NED (ppm) (ppm)
4O0ONW-A S76 0.63 AG AUS)
3} 3.64 0.40 2.49

AG BL. 4 ORISw/i ZO)

Mean B15 15 0.47 2.39
Std. Dev. O22 O)5.ALZ Opaka
20O0NW-A PX (Oak 0.18 BEA
-B PENS 0.64 Py Sh)

-C PRESS) 0.56 ene)

Mean PRINT) 0.46 PRAT
Std.Dev. O35 O20 ORi2
CER =A Ba 0.40 PBS Sie}
-B Bis2a) 0.38 BESS)

=¢C OS (SS) ORAS 31010
Mean Dice Sal: Onsi2 2.66
Std.Dev. 122 (OVS AL) ONZE
200SE-A By CAT 0.28 2.44
-B BEES Ons 2.48

-C Ba NS) 0.28 2.415

Mean Br 45 0233 2.46
Std. Dev. O23 OR 1O)7/ 0.02
400SE-A SrO2 0.38 Dedi2
-B Phe AE 0.38 22516

—C BLS Onan 253

Mean See (0)ib 0.39 2.54
Std. Dev. 0.64 OOM: 0.02
2000E-A 3.66 O35 7B S35)
-B 3.40 Oe S7/ 7d Sy al

=C PAO) ORS7/ 2.59

Mean Sh ALG) 0.36 2.48
Std.Dev. 0.50 OO 0.10

- indicates no data available.

Gr

Pr OlF Oo
POW

O wWla ~ 0
bO}, Bt

ONINNN
NW 0V{O7) &

ONO ON
O1 P| 0%

ONINNN
RP ANA UW

oO AO}|O NO
OR PN

ORION FP
SI OJoO WW

OM & oO
MNIW OW

oO 8]R Bw
OPI|O wow

OF}, PO
ON VAIO OF

Pol ow
owlyo yuo

Results of the Bivariate Analysis of 3-HLP Current Meter Data

FREQUENCY DISTRIBUTION

Table 2-3

Collected at Rockland at a Depth of 10m
21 May to 11 June 1985

1.00 HOURLY DATA STATION: RCK11 © SHRLP SPANNING 5/21/85 TO 6/11/85 521 DATA POINTS
DIRECTION PERCENT
DEGREES

Oe ometes, #8 5:4) 520/042. 3:6 119 1.2) 1-2) 1.0) 22 wd]

SOICOmemame 20420) Ie Deesby-2.) 0) 00) 0) 20> 920i «20 6.0

BUesvOMy lsh 4. Aasls2 24) 10) 720 50!) 1.0, ".0) 01" 20 9

POc(2OMmmte2n2s5 aise 8%. 2 0/0 20. Oe 60 <0 5.8
20EISOMMN NSO) 56 (2kS 9161.2 «0 20 1.0.0) 0. <0 7.9
150-180] 42.7 3 29 167 B10 8 2 OO 18.7
180-210 OMe 2eeloz il Sims D ste tereAS ec tee Qin a0) ay50 3.2
MOeTsOen Meaty 0) 72 1.2) 6052.0 60 0 9.0 60) 20 my
=D OM eds 4 a2 20) 160) 1200050 | 20) 020. 20 1.0
Di0-S00Mme 50) .6) eeeOle01 4.0 0) 0.0 40) 60.0 6
SDOEISOMene 7 Ts) eA, 0.2 <s0ura0 1.0) 20) 1-020 1.0
FESO=S6O]p teed 2 7) 40000 2.5002.9 1.2 Hof L1,Si 1.2 542 [ee]

SPEED 0 16 20 32 «36 WO

; eens

cH/s dlsaest2_16)) 20-24
PERCENT dla mera tne 4.0 100.00
MEAN DIR 119 120 Isl 197 154 107 me Tey TOT Te
STD DEV 9«:100 110-120 140 135 145 140 164 162 230

SUMMARY STATISTICS
MEAN SPEED = 13.16 2A/S MAXIMUM 3 40.71 CH/S MINIMUM = 1.31 CH/S RANGE = 39.40 CH/S
STANDARD DEVIATION = 8.37 CH/S SKEWNESS = 1,09

IN A COORDINATE SYSTEM WHOSE Y AXIS 13 POSITIONED

MEAN X COMPONENT = 2.19 CM/S
MEAN Y COMPONENT = 4.68 CH/S

STANDARD DEVIATION = 3
STANDARD DEVIATION 2 14

00 DEGREES CLOCKWISE FROM TRUE NORTH

85 CH/S SKEWNESS = 13
13 CA/S SKEWNESS = 11

MEAN

SPEED

16.01

7,38

6.42

KIN
SPEED

1.31

MAX
SPEED

40.27

16.81

14.55

19.16

18.36

32.83

29.12

15.26

9.77

5.88

17.49

40.71

STD. DEV

9.24

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