Final Report on Current Meter Measurements at the 106-Mile Site in Support of Municipal Waste Disposal

United States
Environmental Protection
Agency
Office of Water
(WH-555F)
EPA 842-S-i2-012
June 1992
Final Report on Current
Meter Measurements  at
the  106-Mile Site in
Support of Municipal
Waste Disposal

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             FINAL REPORT
    CURRENT METER MEASUREMENTS
          AT THE 106-MILE SITE
             IN SUPPORT OF
       MUNICIPAL WASTE DISPOSAL
              April 25, 1988
U.S. ENVIRONMENTAL PROTECTION AGENCY
    Office of Marine and Estuarine Protection
              Washington, DC
     Prepared Under Contract No. 68-03-3319

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                              TABLE OF CONTENTS

EXECUTIVE SUMMARY	      }

1.   INTRODUCTION	      1

     1.1  Background	.	      1
     1.2  Physical Oceanography of the 106-Mile Site	      1

          1.2*1    Overview.**»». .*..»..»*,».»	....	»*.*...».      1
          1.2.2    Field Programs Applicable to 106-Mile Site	      6
          1.2.3    Summary of Findings.*	*	      8

     1.3  Obj ecti ves	     23

2.   FIELD PROGRAM	     24

     2.1  Moor i ngs	     24
     2.2  Data Processing	.	     28

3.   DATA INTERPRETATION AND ANALYSIS	*	**	*	     29

     3.1  Introduction	     29
     3.2  Time Series	     30
     3.3  Basic Statistics	     43
     3.4  Inertial and Tidal Motions	     52
     3.5  Spectra and Coherence	     63

4.   SUMMARY	     67

5.   REFERENCES	     71


                               LIST OF  TABLES


Table 2-1     EPA 106-Mile Site Moorings	     26
Table 3-1     Events Occurring at the 106-Mile Site,
              September 1986 to April 1987	     35
Table 3-2     Basic Statistics for 3-HLP and 40-HLP
              records from moorings X and Y	     49
Table 3-3     Basic statistics for 3-HLP and 40-HLP
              records from MASAR moorings C and 0 for
              two six-months periods in 1984 and 1985	     51
Table 3-4     Frequency distribution of speeds and
              directions and summary statistics for the
              3-HLP current records from  (a) X1Q61,
               (b) X1063, (c) X1067, (d) X1068, and
               (e) Y1061	     53

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LIST OF FIGURES
Figure 1-1 Locations of slopewater between the edge of
the continental shelf and the Gulf Stream,
from Cape Hatteras to the Grand Banks 3
Figure 1-2 Mid-AtUntie Slope and Continental Rise
showing moorings in place during 1984-1985 7
Figure 1-3 Empirical scheme of slopewater circulation
containing Coastal Labrador Sea Hater (CLSW)
inflow from the Grand Banks partly retro-
fleeting, partly flowing southwestward along
the continental margin; a western Slope Sea
gyre; and inflow from the Gulf Stream
th ermoc line 9
Figure 1-4 Upper-level currents averaged over the period of
displaced Gulf Stream position, 10 October 1984-
January 1986 11
Figure l-5a MASAR hydrographic section for the northern transect
on February 21, 1985 14
Figure l-5b MASAR hydrographic section for the northern transect
on May 14, 1985 15
Figure l-5c MASAR hydrographic section for the northern transect
for September 9, 1985 16
Figure 1-6 The mean position, standard deviation and extreme
positions of the GS front taken from all
available digitized, satellite derived sea-
surface temperature maps for the month of
October 1985 19
Figure l-7a Temperature, salinity, and density distributions
in a vertical section across the continental shelf
on 19 July 1975 for a section off the eastern
shores of Virgina 21
Figure l-7b Temperature, salinity, and density distributions
in a vertical section across the continental
shelf on 21 July 1975 for the same transect as
Figure l-7a 22
Figure 2-1 A chart of the 106-Mile Site with EPA mooring
positions X and Y, and MASAR mooring positions
C and D, indicated 25
Figure 2-2 The configurations of EPA moorings X and Y deployed
southwest and northeast of the 106-Mile Site,
respectively* 27
Figure 3-1 40-HLP stick plot of the first three months of

In September 1986, two current meter moorings were deployed for a period
of seven months on the 2500-m isobath, respectively, northeast and southwest
of the 106-Mile Deepwater Municipal Sludge Disposal Site. The moorings were
designed to monitor the current and temperature structure of the upper layers
of the ocean in the vicinity of the 106-Mile Site in order to assess the
effects of various current regimes on the disposal of sludge. The southwest
mooring was instrumented with current meters at 50-, 100-, 250-, and 1000-in
nominal water depths, and with temperature recorders at approximately 25-m
spacing from the current meters from 50m to 250m. The northeast mooring had
data from the 250- and 1000-m levels.
The 106-Mile Site is situated, with water depths ranging from
approximately 1000m to 2700m, in a distinct part of the coastal ocean between
the Gulf Stream and the shelf break known as the Slope Sea. Mem flow, for
the deployment period, was 6-7 cm s-1 southwest along the isobaths and was
uniform with depth over the upper 250m of the water column. This
characteristic southwest flow at the 106-Mile Site is considered to be the
northern part of the anticlockwise western Slope Sea gyre. During the
seven-month deployment, an energetic warm-core ring (86-E) moved through the
site in November followed by a smaller, clockwise rotating, warm eddy that
was apparently a remnant of warm-core ring (86-F). Another small warm eddy,
mostly confined to the continental slope, was observed in October trailing a
large warm-core ring (86-A) that had passed through the 106-Mile Site in
early September. This small, energetic, October warm eddy drew out a 75- to
100-m thick filament of cool shelf water into the Slope Sea.
The January to March period was not affected by eddies and was
characterized by strong surface cooling that produced a homogeneous 250-m
deep layer of 12°C water by convective overturning in early February. After
the formation of the 12°C water, known as the slopewater pycnostad, the
upper 50 to 100m of the water column experienced a number of warm outbreaks.
One was due to a large Gulf Stream filament trailing a meander crest, and
one was due to the interaction of the Gylf Stream and a ring (86-1) that was
stalled near the Hudson Canyon in March and April.
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In February, at the 1000-m level, a cool, small ("12 km long)
Labrador Sea Water (LSW) anomaly was observed advecting along the 2500-ra
isobath with the mean flow. Many of the observed phenomena in these data,
including shelfwater intrusions, small warm eddies, warm outbreaks, and LSW
anomalies, have rarely or never been observed in time series records. Major
features such as warm-core rings, the mean southwest flow, and the formation
of the 12°C slope water have been previously studied in recent Slope Sea
observational programs; however, the databases from these studies are
limited.
11
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1. INTRODUCTION

1.1 BACKGROUND

The 106-Mile Deepwater Municipal Sludge Site was designated in 1986
for the disposal of municipal sludges. As part of a monitoring program for
the disposal of sludge at the site, two current meter moorings were deployed
to monitor the temperature and current structure of the upper ocean over a
seven-month period from September 1986 to April 1987, Background material
on the dumping of sludge at the 106-Mile Site is given in EPA (1987).
Both the near- and farfield fate of sludge dumped In the deep ocean
should be evaluated, including the analysis of the transport and mixing
processes at the site and 1n adjacent waters, for a wide range of space and
time scales. The physical disposal processes involved in the dispersal of
sludge are advection by the current field, mixing by turbulence and
large-scale shear flows, and sinking of sludge particles through the water
column. Transport and mixing are profoundly affected by the types of
circulations encountertd by the sludge particles on their way to the
seafloor. The Slope Sea, the region between the Gulf Stream and the shelf
break in the Mid-Atlantic Bight where the 106-Mile Site is situated, is a
very complex region of the coastal ocean. Circulation features affecting the
106-Mile Site include the Gulf Stream, warm-core anticyelonic rings, the
Slope Sea gyre, and shelf-slope front exchange processes. These processes
are briefly discussed in terms of the transport and mixing of sludge waste in
the Monitoring Plan ( EPA , 1987). The physical oceanography of the
southern part of the Mid-Atlantic Bight slope is summarized in the following
section.

1.2 PHYSICAL OCEANOGRAPHY OF THE 106-MILE SITE

1.2.1 Overview

The physical environment of the 106-Mile Site determines how dumped
waste will be transported away from the site and mixed with the surrounding
waters, and eventually determines its fate. A thorough knowledge of the
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physical environment is needed in order to assess where waste goes, what the
rates and mechanisms of mixing with the receiving waters are, and where on
the seabed it ends up. Because sewage sludge has essentially the same
density as sea water and the majority of particles are silt sized, the sludge
constituents will take several weeks to several months to reach the seabed in
water depths, typical of the site, of 200 to 2700m. Because the current
flows are fairly strong (~0-50 cm s-1), the particles may be transported a
considerable distance in that time. Therefore, a large scale view of the
circulation (i.e., patterns of water movement) is required to assess the fate
of the sludge due to currents and mixing. The physical environment needs to
be characterized on time scales ranging from days to years and length scales
of a few km to ~1000 km. The circulation at the 106-Mile Site should be
viewed in the context of the circulation of the Mid-Atlantic Bight
encompassing the continental shelf, slope, and rise, and the Gulf Stream (GS)
from New England to Cape Hatteras. The oceanographic regime occurring at the
time and place of a release is, of course, important for Initial mixing and
dispersion,* however long-term farfield transport and fate are a more complex
issue because of the variability of the receiving waters over a wide range of
length and time scales, and the close proximity of radically different
current systems in the Mid-Atlantic Bight.
The 106-Mile Site is situated within a distinct part of the ocean
known as the Slope Sea (Csanady and Hamilton, 1987). It Is probably one of
the most complex bodies of water in the world, and though the Slope Sea and
the Gulf Stream have been studied since the 1930s (Rossby, 1936), it is only
recently that a modern synthesis of the circulation has been put together
(Csanady and Hamilton, 1987; SAIC, 1987) and some of its features confirmed
by the recent Mid-Atlantic Slope and Rise (MASAR) and Shelf Edge Exchange
Program (SEEP-I) experiments. Figure 1-1 shows a sketch of the Slope Sea
bounded by the Gulf Stream in the south and shelf water in the north. The
upper 50 to 75m of the water column in the Slope Sea is often a mixture of
shelf water, slopewater and Gulf-Stream-derived watif, producing complex
salinity and temperature structures. The shelf break region between Cape
Hatteras and Georges Bank has a complex salinity and temperature front
separating the cooler, fresher shelf waters from the warmer, more saline
slopewaters. The mixing processes along this shelf-slope front, producing
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50°N-
40° -
SMLF
•S£A WATEK
SLOPtWAJtK
80°W
70C
60*
50'
Figure 1-1.
Location of alopewater between the edge of the continental shelf
and the Gulf Stream, from Cape Hatteras to the Grand Banks,
Most of the area is occupied by slopewater except In the
northeast corner where coastal Labrador Sea water intrudes.
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cross-front exchanges possibly important of introducing sludge onto the
shelf, are extremely complex and not well understood or comprehensively
measured.
The recent MASAR experiment has shown that the Gulf Stream has
profound influences on the circulation over the slope. The Gulf Stream, part
of the western boundary current system of the North Atlantic Ocean, leaves
the continental margin at Cape Hatteras and behaves like a free-meandering
jet, flowing into progressively deeper water. It is warmer than surrounding
waters and separates the Sargasso Sea on the south side from the much cooler
slopewater on the north. There is a sharp temperature front, easily observed
from satellites, between the shoreward side of the jet and the slopewater,
. that is often referred to as the "north wall" of the Gulf Stream (GS). The
high speed currents of the GS are generally restricted to the upper 1000m of
the water column. Maximum surface speeds may exceed 200 on/s. Gulf Stream
meanders are large shoreward (north) and seaward (south) sinuous
displacements of thi GS front that move east along the path of the jet. The
meanders have wavelengths ranging from 100 km to 1000 km, periods of 4 to 100
days, and downstream propagation speeds dependent upon wave periods such that
4- to S-day period waves, with wavelengths of about 180 km, propagate at
about 40 km/day and 40-day period waves, with wavelengths of 600 km,
propagate at about 20 km/day (Tracey and Watts, 1986). The meander
displacements are constrained near Cape Hatteras, but show an almost linear
increase in amplitude as they move east. Near the New England Seamounts, the
path of the GS has an envelope that is about 500 km wide (Cornillon, 1986).
The larger meanders can pinch off to the north or south of the GS
to produce detached clockwise rotating warm-core rings or anticlockwise
rotating cold-core rings, respectively. A warm-core ring is essentially a
rotating, bowl-shaped parcel of Gulf Stream and Sargasso Sea water about 100
to 150 km in diameter, 1000-m deep in the center, with maximum speeds of
about ISO cm/s. Once it has formed in the Slope Sea, it moves west at
approximately 6 km/day (Joyce, 1984). These rings have complex life
histories before they coalesce with the Gulf Stream, often near Cape
Hatteras, Warm-core rings can interact with the GS and shelf waters,
extruding warm and cold streamers from these waters that can wrap around the
outer edges of the ring. The passage of a ring along the slope can
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apparently trigger instabilities and waves on the shelf-slope front that may
also form smaller warm- and cold-core eddies, about 20 to 50 km in diameter,
In the upper slope aiding the exchange of shelf and slopewaters. About five
to eight rings per year move into the western Slope Sea (Fitzgerald and
Chamberlin, 1983; Brown et al«, 1986), Thus, sludge dumped into the strongly
sheared currents of a ring will be transported and mixed in a different
physical and biological environment than when a warm-core ring is not present
over the dumpsite.
The previous discussion has concentrated on the upper-layer
circulation processes of the Slope Sea. Because sludge particles settle
through the water column, the deep circulation over the slope and rise also
needs to be considered. Strong fluctuating currents (20 to 60 cm/s) are
found near the bottom on the continental rise of the Mid-Atlantic Bight.
They are due to planetary wave motions, having periods of 10 to 100 days.
thought to be generated by the meandering Gulf Stream. These currents are
known as topographic Rossby waves (TRWs) (Rhines, 1971; Luyten, 1977;
Hamilton, 1984) and are characterized by fluctuating currents that increase
in magnitude from about 1000-m depth towards the seabed. These TRWs
propagate away from the deep 6S region towards the slope in a generally west
or southwest direction. The near-bottom TRW currents may be strong enough to
move fine sludge particles that reach the bottom, thus helping to disperse
them.
The near-bottom mean flow in water depths shallower than about
4000m on the continental rise and lower part of the slope is part of the
Western Boundary Undercurrent (WBUC). The WBUC is formed by the sinking of
cold arctic water in the region of the Denmark Strait and has been traced as
far south as the Grand Bahama Bank (27 R), On the Mid-Atlantic Bight slope
and rise, this mean current (~2 to 3 cm/s) follows the general trend of the
isobaths and passes under the Gulf Stream at Cape Hatteras. Thus, sludge
particles in the lower water column tend to be transported southwest by this
deep current system. Over the deeper part of the steep slope (1000- to
2000-m depth), fluctuating and mean currents seem to be weaker with a
different vertical structure than on the rise. This lower slope region way
be a sink for suspended sediment derived from land and the continental shelf.
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The few deep measurements, made by the SEEP-I and MASAR programs, over the
lower slope tend to confirm the existence of this low energy region.
This brief review of the various regimes and phenomena that affect
the farfield fate of sludge material dumped at the 106-Mile Site is followed
by a discussion of recent measurement programs (primarily MASAR and SEEP-I)
and their implications for transport and mixing. The discussion will
concentrate on the upper-layer circulation of the Slope Sea, including the
effects of warm-core rings and Gulf Stream incursions, and the possibilities
of exchanges with the outer shelf across the shelf-slope front.

1.2.2 Field Programs Applicable to 106-Mile Site

The most useful data on currents and hydrography of the western
Slope Sea and the Gulf Stream were taken during two extensive moored array
studies funded by the Department of Energy (DoE): the Shelf Edge Exchange
Program (SEEP-I, Aikman et al.f 1987); and the Minerals Management Service
(MMS): the Mid-Atlantic Slope and Rise (MASAR) study (SAIC, 1987); and
associated studies. A sketch map of the arrays is given in Figure 1-2. The
complete array (SEEP, MASAR, the Gulf Stream current meter moorings off Cape
Hatteras, and the Inverse Echo Sounder Array under the GS (funded by the
Office of Naval Research (ONR) and the National Science Foundation (NSF)) was
in place during 1984. The MASAR arrays were maintained from March 1984 to
March 1986. Both the MASAR and SEEP programs had extensive hydrographic
components. The MASAR study conducted eight seasonal hydrographic surveys
over the two years, using closely spaced stations along the two mooring
transects. This study produced the first documentation of the seasonal cycle
of formation and erosion of the slopewater pycnostad, a well-mixed layer of
water found between about 50m and 200m throughout the Slope Sea with a
temperature of 12°C, salinity 35.50/00, and hence a density of 27.0 sigma-t.
Because the MASAR northern mooring transect off New Jersey is just to the
south of the 106-Mile Site, it provides the best long-term set of current
meter measurements appropriate to the site. The U.S. Environmental
Protection Agency (EPA) measurement program, discussed in this report, was
designed to extend and complement to the MASAR study.
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76 M 75 W 74 H 73 H 72 H 71 M
N
70
41 N
40 N
39 N
SB H
37 N
•Inverted Echo Sounders
©Inverted Eeho Sounders
with Current Meters
ml tear MMS MOOT ings
if 6 Month MMS Moorings
A SUP
EPA Nuclear Dump Site
Mind Measurement Buoys
SEtP-J
36 N
35 N
42 N
N
N
39 N
38 N
37 N
36 N
78 W 75 M 74 W 73 M 72 N 71 W
70
3S N
Figure 1-2. Mid-Atlantic Slope and Continental Rise showing moorings in
place during 1984-1985. The 106-Mile Site is marked.
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Site-specific studies at the 106-Mile Site were conducted by the
National Oceanic and Atmospheric Administration (NOAA) between 1974 and 1978
(Ingham et al., 1977) concentrating on warm-core ring tracking (Bisagni,
1976) and hydrography. There is also extensive historical hydrography,
mainly in the eastern part of the Slope Sea, including Gulf Stream '60
(Fuglister, 1963) and McLellan's extensive surveys (McLellan et al., 1953;
McLellan, 1956, 1957). Prior to MASAR and SEEP, Woods Hole maintained a
mooring at "Site D" in 2600m of water south of Nantucktt Island (Thompson,
1977). However, because of the use of surface floatation on the moorings,
the upper-layer measurements were contaminated by mooring motions due to
surface waves and the later (1972 to 1973) subsurface moorings concentrated
on deep currents. Near bottom currents have been measured at a number of
sites on the continental rise, including the 2800-m low-level radioactive
waste site which is adjacent and southeast of the 106-Mile Site (Hamilton,
1982; 1984).

1.2.3 Summary of Findings

1.2.3.1 Slope Sea Circulation

The underlying circulation pattern of the Slope Sea has been
derived from historical hydrography (Csanady and Hamilton, 1987) and is shown
in Figure 1-3. The prominent western gyre is probably variable in size and
strength depending on the configuration of the Gulf Stream, the strength of
the inflow from the Labrador Sea, and the strength of the large scale wind
forcing. The detailed dynamics are not established, and the evidence for the
gyre and its variability is largely empirical. Large perturbations such as
warm-core rings and upper-slope eddies are essentially superimposed on this
basic circulation. The 106-Mile Site and the northern MASAR mooring transect
are approximately situated in the strongest part of the southwest flowing arm
of the anticlockwise gyre. This fact provides an explanation for the
predominantly southwest mean currents ("10 cffl/s) that have been measured at
the 106-Mile Site (Bisagni, 1983) and the MASAR northern transect (SAIC,
1987), Evidence for a distinct northeast return flow along the north wall of
the GS comes primarily from sea surface temperature maps derived from the
8
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50°-
45°-