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OCR for page 351
PHYSICAL TRANSPORT INVESTIGATIONS AT NEW BEDFORD, MASSACHUSETTS
Allen M. Teeter
U.S. Army Engineer Waterways Experiment Station
ABSTRACT
Migrations of sediment, sediment-associated contami-
nant, and dissolved materials released by proposed dredging
and disposal operations were predicted as part of an engi-
neering feasibility study of dredging cleanup. Highly
contaminated sediments blanket most of upper New Bedford
Harbor (the Acushnet River estuary), and were found to be
escaping from the upper harbor toward Buzzards Bay. Field
measurements, laboratory tests, and computer models, each
necessary to support the others, comprised the study. Field
measurements characterized hydraulic conditions and trans-
port mechanism for salt, sediment, and contaminants for a
series of surveys. Suspended material was found to migrate
from Buzzards Bay upstream in the estuary at concentrations
generally below 10 ppm, and settle in the upper harbor at
about 2,200 kg per tidal cycle. The flux of PCB-Aroclors
was found to be seaward and averaged 1. 55 kg per tidal
cycle . Laboratory tests for settling, depos ition, and era -
sion of sediment material were carried out. The most mobile
sediment fraction was found to make up 28 percent of the
sediment. Models for hydrodynamics and sediment transport
were applied to the upper New Bedford Harbor, and used to
predict sediment and contaminant migration for dredging and
disposal scenarios. Results indicated that the flux of
sediment materials from the upper harbor would be 15 to 20
percent of the rate of sediment resuspension.
INTRODUCTION
An engineering feasibility study (EFS) of a possible Superfund
dredging cleanup for upper New Bedford Harbor was conducted by the U.S.
Army Corps of Engineers (COE). The COE's Waterways Experiment Station
(WES) Hydraulics Laboratory evaluated hydraulic conditions and sediment
migration as part of the WES Environmental Laboratory dredging and dis-
posal EFS conducted for the U.S. Environmental Protection Agency (EPA),
Region I, under the direction of COE's Missouri River Division, and in
cooperation with its New England Division. Highly contaminated sedi-
ments blanket most of the upper New Bedford Harbor (Acushnet River
Estuary), threatening to spread to other harbor areas and adjoining
351
OCR for page 352
352
Buzzards Bay, and adversely impacting fisheries resources. The EFS is
one component of EPA studies that will lead to a Superfund cleanup of
the harbor and upper harbor.
The Acushnet River estuary EFS had components addressing physical
and chemical testing of sediments to determine appropriate dredging
limits, acquisition of bathymetric and geotechnical information, and
study of contaminant behavior under simulated field conditions. An EFS
report series is in preparation. Averett (1988) gives an overview of
the study. The WES Hydraulics Laboratory investigated hydraulic condi-
tions and transport mechanisms for salt, sediments, and contaminants as
part of the EFS (Teeter, 1988~. The approach of the study was to inte-
grate prototype measurements, laboratory data, and model results to
quantify present conditions and predict dredging and disposal effects.
The objectives were to evaluate
1. contaminant and sediment migrations away from resuspension
points, and out of the upper harbor,
2. the hydraulics of the present and dredged upper harbor, and
3. concentrations of sediment and contaminant in the upper harbor
during dredging and disposal releases.
Confined disposal facilities (CDF) are diked settling basins that
can include treatment methods to enhance contaminant settling. Con-
fined aquatic disposal (CAD) is a controlled operation using excavated
cells or chambers in the estuary bed, which are capped after filling.
Suspended sediments (total suspended material [TSM]) will be discharged
with CDF effluent, and released during CAD filling. The rate of sedi-
ment release, sediment characteristics, and ambient conditions will
control the amount of sediment that will escape from the proximity of
the dredge and disposal facilities, and from the upper harbor during
dredging. This paper describes methods used to predict sediment and
contaminant escape.
FIELD DATA COLLECTION
Figure 1 shows the layout and approximate dimensions of New Bedford
Harbor, which is located on the north shore of Buzzards Bay and is the
estuary of the Acushnet River. The mean tide range at New Bedford Har-
bor is 1.1 m, and the spring range is 1.4 m. Surface-to-bottom sali-
nity differences are generally less than 0.5 ppt. The Acushnet River
has a mean annual freshwater discharge of about 0.85 cubic meters per
second. The upper harbor is shallow, with an average depth of only
about 1 m at mean low water.
Survey dates, tides, freshwater flows, and winds were as follows:
OCR for page 353
353
Survey Freshwater in- Tidal range at Wind direction,
date flow m /see tide gauge 3, m speed, km/hr
Mar 6 1.17 1.04 S. 24-32
Apr 24 1.50 1.65 NE, 8-12
then 32-48
SW, 16-24
Jun 5 0.25 1.04
Water
temperature
4°C
11°C
17°C
Nine stations were sampled repeatedly over tidal cycles (Figure 1~.
Current speed and direction, salinity, and TSM were sampled 0.6 m up
from the bed and below the surface, and at mid-depth. Station 9 was
sampled only at mid-depth.
Bridge Flux of TSM
The Coggeshall Street Bridge is a key point at which transport
measurements and predictions were made. Bridge fluxes of TSM were
estimated by integrating half-hour measurements of velocity (u) and TSM
FIGURE 1 Sampling and gauging WOOD STREE7
locations for New Bedford Harbor.
L
UPPER
HARBOR
- TIDE GAGE &
A UTOMA TIC
AUTOMA TIC `~ f LUX RANGE
SAMPLER . · }` . Am,. ' 4 5 6
COGGESHALL STREETS——-a I NSET AT
COGGESHALL STREET BRIDGE
\ ~ POPES ISLAND
· ODE GAGE =2
~= ~~ ~ it. - FAI R HAVEN
NEW BEDFORD-. `~\ 7~,,,:
APPROXIMATE SCALES
500 0 500 1000 YD
50.0 0 500 1000 M
LEGEND
O BOAT STATION
~ Tl DE GAGE
HURRICANE -x
BA PRIER . .~ \
. ' , '. ~ \~\
· 'N \\
BUZZA RDS
BAY
".?~2 \\
·-'\ \\
.- · - ~ \\
\\
\\
W. .'--N
CLAR K'S -\
Hi- POINT .}
it,
~ TIDE GAGE =1
OCR for page 354
354
concentration over the tide-corrected, cross-sectional area, and
integrating in time. Results are shown in Table 1.
The net flux of TSM was always found to be upstream, although
fluxes in either direction were at least twice the net flux values.
Average flux of TSM into the upper harbor, corrected for tidal asym-
metry, was about 2,200 kg per tidal cycle. The freshwater inflow added
some additional sediment, on the order of a several hundred kg per
tidal cycle. Shoaling resulting from the deposition of 2,500 kg per
tidal cycle amounts to 3 mm per year when spread over the entire sur-
face area of the upper harbor at a bulk wet density of 1.5 g/cm (775
dry-g/liter). The average sedimentation rate for the harbor has been
estimated to be 7 mm per year (Summerhayes et al., 1977~.
Estuarine TSM Flux Components
Figure 2 shows a plot of the longitudinal distribution of depth-
and tidal-averaged TSM concentration. TSM concentrations were lowest
at the most seaward stations, and increase upstream. A turbidity maxi-
mum occurred in the upper harbor. Differences in TSM concentration
between spring- and reap-tide surveys were small, indicating that redis-
persion of near-bed suspended material rather than erosion of bed sedi-
ments contributed to tidal variations in TSM.
The most important flux components for TSM were tidal pumping. TSM
transport by steady vertical shear closely associated with transport by
gravitational circulation, was small for New Bedford Harbor. Mechan-
isms for upstream tidal pumping were evaluated. Maximum TSM resuspen-
sion produced by the highest tidal currents, usually occurring near low
water, were transported in the flood direction, producing upstream tid-
al pumping. Water column redispersion time scales were much shorter
than settling time scales, and produced phase differences between tidal
velocities and TSM concentrations (tidal pumping).
FIGURE 2 Longitudinal dis- 15
tribution of TSM concentration.
~
/
O _
1 2 3 5 7 8 9
STATION NUMBER
OCR for page 355
355
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OCR for page 356
356
Vertical mixing was found to be more intense during flood tidal
phases and was less damped by density effects on flood tidal phases.
PCB Fluxes
Bridge fluxes of PCBs are shown in Table 1. Flow-proportioned ebb
and flood PCB Aroclor concentrations were multiplied by the tidal
volumes to obtain ebb and flood PCB fluxes. The difference between ebb
and flood fluxes is the tidal net flux. Observed net fluxes were
always seaward (negative) with a mean net flux of -1.25 kg per tidal
cycle.
Tidal biases were removed from the raw tidal net fluxes by summing
net-f~ow fluxes (freshwater volume times mean concentration) and tidal
pumping fluxes (the difference between ebb- and flood-mean concentra-
tions times the mean tidal volume). Corrected flux values are also
shown in Table 1, and were also seaward with a mean flux of -1.55 kg
per tidal cycle.
Floatable material samples at the bridge were low in PCBs. Accur-
ate transport rates could not be estimated for floatable material, but
were at least several orders of manitude less than that for suspension.
PCBs attached to sediment particles at the surface of the bed could
be exchanged into the overlying sediment suspension by a physical par-
ticle exchange mechanism, and thus be mobilized for transport. Such an
exchange could take place without a significant mass flux of sediment.
A particle exchange theory, based on aggregation and disaggregation of
cohesive particles at the sediment/bed interface was developed during
this study. That analysis used laboratory data on another estuarine
sediment. The results indicated that particle exchange could be an
important transport mechanism.
LABORATORY TESTING
Depositional and erosional characteristics of fine-grained sedi-
ments vary greatly, and are critical to the prediction of sediment and
contaminant migration. Direct laboratory testing on sediments less
than 74 Em from the study area was therefore undertaken.
Deposition (D), or flux of sediment material to the bed, is the sum
over a number of fractions of settling flux multiplied by deposition
probability:
n
D ~ ~ PiWs Ci
. ~ .
1~1 ~
(2)
where Ws is the settling velocity, P is the probability that an aggre-
gate reaching the bed will remain there, C is the concentration just
above the bed, the subscript i indicates a sediment fraction, and n is
the number of fractions. P varies linearly from O at a critical shear
stress for deposition, ~cd, to 1 at zero bed shear stress, rb ~ 0
OCR for page 357
357
The functional form 1 ~ rb/rcd where rb < Ad is used for P.A sus-
pension of uniform material in a steady, uniform flow will either
deposit completely or remain entirely suspended depending on whether
the bed shear stress is below or above ~cd, according to Eq. 2. The
objective of the deposition testing was to determine ~cd, and the
magnitude of the product P Ws for each sediment fraction identified.
The mode of resuspension (used synonymously with erosion)
considered important to potential contaminant migration at New Bedford
Harbor is particle erosion. At rb above a critical value, particles
individually dislodged from the sediment bed as interaggregate bonds
are broken. Particle resuspension (E) is related to the shear stress
in excess of a critical value, and to an erosion rate constant (M),
thus:
E = M ( b 1) , rb > arc
tic },
where arc is the critical erosion shear stress (Ariathurai et al.,
1977~. Observed erosion does not follow Eq. 3 indefinitely. Sus-
pension concentrations above experimental eroding beds often reach
equilibrium values that depend on the bed shear stress. Equilibrium
suspensions form as erosion rates decrease with time to zero, while the
flow remains constant. Eqilibrium suspensions have been related to
vertical inhomogeneity in the bed (either particle characteristics or
bed density) or to armoring by selective erosion at the bed surface.
See Figure 3 for the configuration of the sediment water tunnel.
This testing device was developed for this study to safely test contami-
nated sediments. It was a closed-conduit sediment water tunnel, open
to the air only at a small expansion chamber. The water tunnel had a
uniform cross-section area, which changed from rectangular in the hori-
zontal, deposi tion/resuspension sections to circular in the vertical
settling and pumping sections. The water tunnel was calibrated so that
propeller speed could be related to average velocity and bed shear
stress.
Three sediment fractions were identified, and designated 1, 2, 3.
The 'developed from the analysis of the data were 0.42, 0.33, and
0.043 N/m for fractions 1, 2, and 3, respectively. Considering all
results, the approximate composition of the sieved composite sample was 30,
30, and 40 percent for fractions 1, 2, and 3, respectively. Ws's were
about 2, 1, and O.006 mm/see for fractions 1, 2, and 3, respectively.
Median Ws values are shown In Figure 4. For the most erodible fraction,
Sac was found to be 0.06 N/m . The most easily eroded fraction is the
same fraction identified as the slowest to deposit (fraction 3~. Only
about an additional 15 percent of the total bed material, or half of
fraction 2, eroded between 0.06 and 0.9 N/m2, and the remainder of the
material had Acts greater than 0.6 N/m .
(3)
MATHEMATICAL MODELING
Near-field Plume and CAD Models
Suspended-sediment plume calculations were performed to evaluate
OCR for page 358
358
3°o - \
,~ am x em x 213.4cm DUCT
`~ SAMPL ING TUBE
`` ~ AND 3 SAMPLING
\5o. ''by PORTS ~ VARIABLE SPEED
\\ hi\ At/ DRI VE MO TOR
\\ ~~< ~
F W \~'° W:
t ~ ) ME ER ~ 'I
~ D 20.3cm ID
~\ ~
~ it\ ~ ~ ACCESSPOflT \\ ;
nF.5rF~nn`lG TURf \:
NOTE NOT TO SCALE
CONSTRUCTED OF 1.25cm
THICK CLEAR ACRYLIC.
_ 20.3ctn ID
ASCENDING TUBE
- DRI VE SEA F T
~ AND PROPEL L ENS
\ / ~ TRANSI T/ON
~ EL BOW
FIGURE 3 Isometric view of sediment water tunnel.
the escape of sediments and contaminants from proposed dredging and dis-
posal site outfalls in upper New Bedford Harbor. Near-field analyses
of the escape of sediments from a CAD during the filling phase were
also performed.
The near-field plume and CAD models assumes an infinitely small,
vertically well-mixed, suspended sediment source. Model suspended sed-
iment plumes are advected away from the source in the X direction,
spread or diffused in the Y direction, and allowed to settle. A diffu-
sion velocity formulation was used to introduce a length scale-depen-
dence into model plume spreading, similar to those observed in field
experiments.
The required data for the plume model were Qs, H. U. and Ws. A
site depth of 1 m was assumed, the average depth of the upper harbor.
The remaining two variables, U and Us, were assigned distributions.
Plume calculations were made for a test matrix of 16 conditions
formed by four values of current speed, and four values of settling
velocity. Plume predictions for dredging in upper New Bedford Harbor
indicate that on average about 35 and 29 percent of the material
released at the dredge head will escape from 50 and 100 m of the site,
respectively. The remainder settled within this radius. Sediment that
escaped did so at the highest current speed, and had the lowest set-
tling rate. However, escape totals were highest for the moderate
OCR for page 359
1 .O
~ 0.1
-
C:
o
LD
z
-
z
C)
0.01
0.00
J-8
I ' Arks- -l - I-r I I I I L
_
~ J-8
COMPOSlTEm y
my. COtPOSlTEl~
WS = 0.000113 C-4/3, MM/SEC
y~—COMPOSI TE ~
I I I I I,l I I
I ~ I I
To 100 no logo
CONCENTRATION, MG/9
FIGURE 4 Median settling velocity/concentration relationship for
settling test phases and field samples at grid cell J-8.
current speeds, and for the lowest settling rat"
speeds had the greatest frequency of occurrences.
example plot of plume concentration contours.
Average CAD release ratios were calculated for _ __
test matrix in U. Ws, and H. Four depositional classes and current
speed ranges were identical to those used in the plume calculations.
Results indicated that only the finest or slowest settling fraction
escaped from the CAD, and that the escape of this fraction was almost
complete.
Moderate current
Figure 5 shows an
a three-dimens tonal
Estuarine Numerical Modeling
Computer codes RMA-2V and RMA-4 of the TABS-2 numerical modeling
system (Thomas and McAnally, 1985) were used to schematically model ver-
tically averaged hydrodynamics and sediment transport' respectively.
To properly describe boundary conditions, the model domain was extended
downstream to the hurricane barrier. A numerical mesh of 219 elements
OCR for page 360
25
15
n
in,
(n
C)
_5
-tO
360
NEW BEDFORD DREDGE PLUME
ISOPLETHS AT 40 20, 10, 5, 2.5, t.25 mg/Q
-15
-20
-25
0 ~ 0 20 30
I I 1 1 . I 1 1
40 50 60
X-DISTANCE, M
70 80 90 100
FIGURE 5 Example plume concentration isopleths for Qs ~ 5 g/sec, IJ
0.03 m/see and Ws ~ 0.01 mm/sec.
was developed to cover the study area for use by both RMA- 2V and
RMA- 4 . The upper harbor portion of the mesh is shown in Figure 6 .
A mean-tide sequence was applied to the seaward boundary of the
model . At the upper end, velocities corresponding to a constant fresh-
water inflow of 0.85 m /see were specified.
lathe numerical hydrodynamics model was verified to field data by
adjusting friction coefficients and turbulent exchange coefficients.
Hydrodynamic computations were performed by "spinning up" the model
from a steady, flat, water surface condition. Results from the hydro-
dynamics model were used to construct an 8 - tidal - cycle sequence for
nput to the sediment transport model ~
Sediment transport modeling was performed to estimate escape prob-
abilities from the upper harbor for various sediment materials which
might be resuspended as a result of dredging. Transport of resuspended
material was modeled as a steady mass loading at specified points.
Only sediments released from the mass-load~ng point were included in
computations . Three mass- loading locations were employed to represent
various locations where sediment releases might occur along the axis of
the estuary. Five sediment fractions were modeled to characterize the
range of sediments that might be actually released. The effective sedi-
ment depos ition coeff icient used was
fix ~ WSP
H
The five depositional fractions tested covered the range 0.10 C ox
25.6, where c' has the units of 1/day.
An example contour plots of the concentration field with ~ ~ O
OCR for page 361
361
FIGURE 6 New Bedford upper Harbor mesh
showing CAD exclusion zone (hashed) for
5.5-ft spring tide.
/ // rr
66. \ ~
~-
_~ 5 4
U7
~S~
~ COGGESHALL STREET BRIDGE
~2N
1 1/ 17~\
for flood and ebb tide phases are shown in Figure 7. Maximum concentra-
tions were about 3 mg/liter for a release rate of 15 g/sec. However,
the estuarine model over-estimates spreading near the source, and
near-field predictions should be applied here. Concentrations were
proportional to release rates.
Sediment transport results were used to calculate the escape proba-
bilities of resuspended sediments Pleased in the upper harbor. Average
transport rates under the Coggeshall Street Bridge were computed for
flood and ebb tidal phases after the model had reached dynamic equil-
ibrium. The difference between ebb and flood transport rates, normal-
ized by the mass loading rate, represents the escape probability.
Escape probabilities were calculated for each depositional sediment
fraction at three source locations in the upper harbor, and with three
variations of geometry representing dredging changes.
A plot of escape probability versus a-infinity for three source
locations is shown in Figure 8 for the existing estuary geometry.
Source locations are shown in Figure 6.
OCR for page 362
362
I SOLINES
1 .1
2 .4
3 .7
4 1.
5 1.3
6 1.6
7 1.9
8 2.2
9 2.5
10 2.8
I ~1
~ ~—~ ~ ~~
~ As)
~ =
FIGURE 7 Sediment concentration field, ebb tide.
SUMMARY AND CONCLUSIONS
Assessments of sediment and contaminant migration out of the upper
New Bedford Harbor for proposed dredging and disposal were made from
information and analyses developed by field, laboratory, and various
model studies. Upper New Bedford Harbor is a sheltered area with low
current speeds, typical of many areas where contaminated sediments
reside. Paradoxical tidal fluxes were found for suspended sediments
and sediment-associated contaminants, implying that depositional sites
do not retain all particle-associated contaminants.
Average PCB flux
from the upper estuary was seaward about 1.55 kg Aroclor per tidal
cycle. However, the Acushnet estuary was found to be depositional, and
a reasonably efficient sediment trap. TSM was imported from the
coastal areas of Buzzards Bay, pumped upstream by tidal action, and
formed turbidity maximums in the upper harbor.
The sediment fraction slowest to settle and deposit comprised 28
percent of the composite sediment and represents by far the greatest
potential for sediment and contaminant migration. Other fractions will
OCR for page 363
363
100
80
60
co
m
O 40
LL
Q
An
lo
~ ~s
0 0.1
LEGEND
\3 ~ \
1) LOADING AT NODE 66
2) LOADING AT NODE 54
3) LOADING AT NODE 19
a, day~1
FIGURE 8 Escape probabilities for sediments released at three points
along the upper harbor (see Figure 6 for locations).
not be highly mobile in the upper harbor.
Experimentally determined erosion thresholds were used with numer-
ical hydrodynamics results to identify areas where CAD cells should not
be sited. Based on these results, the area of the channel upstream
from the bridge were not recommended for CAD sitting. See Figure 6.
Results from the dredge plume model indicated that an average,
weighted by occurrence frequencies, of about 33 percent of the resus-
pended material will escape from a 100-m radius of the dredging site.
Results from the CAD cell model indicated that all of the fine fraction
sediment expelled with slurry pore water will escape from CAD cells.
Modeling results indicated that the CAD may cause the greatest
release of sediments and contaminant materials. The escape probability
for bulk sediments resuspended at the point of dredging will average 27
percent but will depend on the highly variable bed sediment composi-
tion. General information on dredge resuspension rates and suspended
sediment releases during CAD filling is scarce. Pilot dredging and
disposal, planned for the fall of 1988, will provide direct measure-
ments for this site.
The methods employed in this study could be applied to similar
sites. However, studies of high-current systems will have to give
greater attention to the difficult experimental and theoretical aspects
of erosion processes.
ACKNOWLEDGMENTS
This work was sponsored by EPA Region I, and monitored by Mr. Frank
OCR for page 364
364
Ciavettieri. The EPA has not reviewed this paper, and the views
expressed herein are those of the author and not necessarily those of
the EPA. Permission to publish was granted by the Office, Chief of
Engineers.
REFERENCES
Ariathuria, R., R. C. MacArthur, and R. B. Krone. 1977. Mathematical
Model of Estuarine Sediment Transport. Technical Report D-77-12.
Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station.
Averett, D. E. 1988. Study Overview. Report 1 of 12, New Bedford
Superfund Project: Acushnet River Estuary Engineering Feasibility
Study Series. Technical Report EL-88-15. Vicksburg, Miss.: U.S.
Army Engineer Waterways Experiment Station. In preparation.
Summerhayes, C. P., et al. 1977. Fine-Grained
Sediment and Industrial Waste Distribution and Dispersal in New
Bedford Harbor and Western Buzzards Bay, MA. WHOI-76-115. Woods
Hole, Mass.: Woods Hole Oceanographic Institution.
Teeter, A. M. 1988. Sediment and Contaminant Hydraulic Transport Inves-
tigations. Report 2 of 12, New Bedford Superfund Project: Acushnet
River Estuary Engineering Feasibility Study Series, Technical
Report EL-88-15. Vicksburg, Miss.: U.S. Army Engineer Waterways
Experiment Station. In preparation.
Thomas, W. A. and W. H. McAnally. 1985. User's Manual for the General-
ized Computer Program System: Open-Channel Flow and Sedimentation:
TABS-2. Instr. Rpt HL-85-1. U. S. Army Engineer Waterways Experi-
ment Station, Vicksburg, Miss.
Representative terms from entire chapter:
bedford harbor
