Circulation and Effluent Dilution

Modeling in Massachusetts Bay:

Model Implementation, Verification and

Results

Richard P. Signell

U. S. Geological Survey

Woods Hole, Massachusetts

Harry L. Jenter

U. S. Geological Survey 

Reston, Virginia

Alan F. Blumberg

HydroQual, Inc. 

Mahwah, New Jersey

January, 1996

U. S. Geological Survey Open File Report 96-015

This report has not been reviewed for conformity 

with U. S. Geological Survey editorial standards
Abstract

A three-dimensional hydrodynamic model was developed as part of a cooperative U.S. Geological Survey/Massachusetts Water Resources Authority program to 

study contaminated sediment accumulation and transport in Massachusetts Bay. 

This report details the development of the model and assesses how well the model 

represents observed currents and water properties in the bay. It also summarizes 

circulation and comparative effluent dilution simulations from existing and future 

Boston sewage outfalls over a three-year period from October 1, 1989 to December 

31, 1992.

The ECOM-si model, a semi-implicit version of the Blumberg and Mellor 

(1987) Estuarine, Coastal and Ocean Model, is shown to reproduce many of the 

important hydrodynamical features of Massachusetts Bay: the seasonal evolution 

of the pycnocline, the mean flow pattern, and the strength of sub-tidal current 

fluctuations. Throughout the simulation period, during both vertically well-mixed 

and stratified conditions, the seasonal statistics of observed currents are wellrepresented by the model. The model is therefore appropriate for studying the 

average dilution of sewage effluent and other continuously discharged substances 

over seasonal time scales.

The ability of the model to reproduce individual flow events varies with season 

and location within the bay. Flow events during unstratified conditions in western 

Massachusetts Bay are particularly well-represented, indicating that the model is 

appropriate for studying processes such as the transport of suspended material 

from the future outfall site due to winter storms. Individual flow events during 

stratified conditions and in the offshore Stellwagen Bank region, however, are less 

well-represented due to small length scales (caused by upwelling and river discharge 

events) coupled with insufficient data to specify open boundary forcing from the 

Gulf of Maine. Thus while the model might be used to answer issues such as 

the frequency with which Gulf of Maine river plumes visit the new outfall site, 

attempting to predict whether a particular plume would visit the outfall site could 

be problematic.

Comparative simulations of effluent discharged from the existing and future 

Boston outfalls show that the region of relatively high effluent concentrations (1 

part effluent to 200 parts sea water) is significantly smaller with the future outfall 

and is limited to Western Massachusetts Bay during both unstratified and stratified 

seasons. The region of even higher concentration (1 part effluent to 50 parts sea 

water) that covers much of Boston Harbor with the existing outfall is non-existent 

in the future outfall simulation. Additional simulations of chlorination plant failure 

predict that the offshore location of the future outfall will lead to dramatically 

lower levels of pathogens at area beaches.

1
Contents

1 Introduction 8

1.1 Rationale ............................. 8

1.2 Physical Setting .......................... 8

1.3 Motivation for Realistic Simulations ............... 10

1.4 Organization ........................... 11

2 Model Implementation 12

2.1 Numerical Scheme ........................ 12

2.2 Model Geometry and Bathymetry ................ 14

2.3 Vertical Mixing .......................... 14

2.4 Horizontal Mixing ......................... 16

2.5 Surface and Bottom Stress .................... 17

2.6 Surface Heat Flux ........................ 20

2.7 Freshwater Input ......................... 20

2.8 Open Boundary Conditions ................... 21

2.9 Adjunct Western Gulf of Maine Model ............. 24

2.10 Boundary condition modifications for 1992 run ......... 27

2.11 Time Step ............................. 27

2.12 Sigma-Coordinate Correction .................. 27

3 Model/Data Comparison 32

3.1 Temperature, Salinity and Density ............... 32

3.2 Velocity Time Series Comparison at the Mooring Locations . . 41

3.3 Statistical Comparison of Currents at the Mooring Locations . 48

4 Effluent Modeling 57
5 Simulation of chlorination breakdown at the existing and future outfall locations 65

6 Summary 68

A Model/Data Comparison of Time Series Velocity at the Boston 

Buoy 70

B Mean Currents by Season 90

C Comparative Effluent Dilution Simulations for the Existing 

and Future Outfalls 104

D References 119
List of Figures

1.1 The Massachusetts Bay region. ................. 9

2.1 Massachusetts Bay model grid. ................. 15

2.2 Data collection sites in Massachusetts Bay ........... 18

2.3 Meteorological Data ....................... 19

2.4 Freshwater input to the Western Gulf of Maine ........ 22

2.5 Adjunct Western Gulf of Maine grid. .............. 25

2.6 Model/data comparison of OBC driven flow .......... 26

2.7 Inshore Massachusetts Bay boundary condition from WGOM

model. ............................... 28

2.8 Offshore Massachusetts Bay boundary condition from WGOM

model. ............................... 29

2.9 Observed and modeled vertical temperature difference. .... 30

3.1 Model/data comparison of near-surface salinity at Stations

BB, BS, SC and U6. ....................... 33

3.2 Model/data comparison of lower layer salinity at stations BB,

BS, SC and U6. .......................... 34

3.3 Model/data comparison of lower layer salinity at stations U2,

MN, RP and U7. ......................... 35

3.4 Model/data comparison of near-surface temperature at stations BB, BS, SC and U6. .................... 36

3.5 Model/data comparison of near-surface temperature at U2,

RP, MN, and U7. ......................... 37

3.6 Model/data comparison of lower layer temperature at BB, BS,

SC and U6. ............................ 38

3.7 Model/data comparison of lower layer temperature at U2, MN,

RP, and U7. ............................ 39
3.8 Model/data comparison of vertical density differences at stations BB, BS, SC and U6. .................... 40

3.9 Coherence between near-surface salinity time series. ...... 42

3.10 Coherence between near-surface temperature time series. ... 43

3.11 Boston Buoy velocity comparison: Jan 28, 1991 - Mar 25, 1991. 44

3.12 Boston Buoy velocity comparison: Jun 18, 1990 - Aug 13, 1990. 45

3.13 Boston Buoy Model/Data velocity statistics at 5 m depth. . . 46

3.14 Boston Buoy Model/Data velocity statistics at 23 m depth. . . 47

3.15 Model/Data velocity correlations at selected regional stations. 49

3.16 Model/Data comparison of the 1st EOF structure of the velocity time series ......................... 50

3.17 Mean flow and low-frequency variability ellipses for Winter

1990-1991 ............................. 51

3.18 Mean flow and low-frequency variability ellipses for Spring 1991 53

3.19 Mean flow and low-frequency variability ellipses for Summer

1990 ................................ 54

3.20 Mean flow and low-frequency variability ellipses for Fall 1990 . 55

4.1 Effluent comparison at 2 m depth during Winter 1990. .... 59

4.2 Effluent comparison at 2 m depth during Winter 1991. .... 60

4.3 Effluent comparison at 2 m depth during Summer 1990. .... 61

4.4 Effluent comparison at 16 m depth during Summer 1990. ... 62

4.5 Residence time for effluent in Massachusetts Bay and Boston

Harbor ............................... 63

A.I Boston Buoy velocity comparison: Jan 1, 1990 - Feb 26, 1990. 71

A.2 Boston Buoy velocity comparison: Feb 26, 1990 - Apr 23, 1990. 72

A.3 Boston Buoy velocity comparison: Apr 23, 1990 - Jun 18, 1990. 73

A.4 Boston Buoy velocity comparison: Jun 18, 1990 - Aug 13, 1990. 74

A.5 Boston Buoy velocity comparison: Aug 13, 1990 - Oct 8, 1990. 75

A.6 Boston Buoy velocity comparison: Oct 8, 1990 - Dec 3, 1991. . 76

A.7 Boston Buoy velocity comparison: Dec 3, 1990 - Jan 28, 1990. 77

A.8 Boston Buoy velocity comparison: Jan 28, 1991 - Mar 25, 1991. 78

A.9 Boston Buoy velocity comparison: Mar 25, 1991 - May 20, 1991. 79

A.10 Boston Buoy velocity comparison: May 20, 1991 - Jul 15, 1991. 80

A.ll Boston Buoy velocity comparison: Jul 15, 1991 - Sep 9, 1991. 81

A.12 Boston Buoy velocity comparison: Sep 9, 1991 - Nov 4, 1991. 82
A. 13 Boston Buoy velocity comparison: Nov 4, 1991 - Dec 30, 1991. 83 

A.14 Boston Buoy velocity comparison: Dec 30, 1991 - Feb 24, 1992. 84 

A.15 Boston Buoy velocity comparison: Feb 24, 1992 - Apr 20, 1992. 85 

A.16 Boston Buoy velocity comparison: Apr 20, 1992 - Jun 15, 1992. 86 

A. 17 Boston Buoy velocity comparison: Jun 15, 1992 - Aug 10, 1991. 87 

A.18 Boston Buoy velocity comparison: Aug 10, 1992 - Oct 5, 1992. 88 

A.19 Boston Buoy velocity comparison: Oct 5, 1992 - Nov 30, 1992. 89

B.I Modeled mean flow at 2 and 20 m depth for Winter 1989-1990. 91 

B.2 Modeled mean flow at 2 and 20 m depth for Spring 1990. ... 92 

B.3 Modeled mean flow at 2 and 20 m depth for Summer 1990. . . 93 

B.4 Modeled mean flow at 2 and 20 m depth for Fall 1990. .... 94 

B.5 Modeled mean flow at 2 and 20 m depth for Winter 1990-1991. 95 

B.6 Modeled mean flow at 2 and 20 m depth for Spring 1991. ... 96 

B.7 Modeled mean flow at 2 and 20 m depth for Summer 1991. . . 97 

B.8 Modeled mean flow at 2 and 20 m depth for Fall 1991. .... 98 

B.9 Modeled mean flow at 2 and 20 m depth for Winter 1991-1992. 99 

B.10 Modeled mean flow at 2 and 20 m depth for Spring 1992. ... 100 

B.ll Modeled mean flow at 2 and 20 m depth for Summer 1992. . . 101 

B.12 Modeled mean flow at 2 and 20 m depth for Fall 1992. .... 102 

B.13 Modeled mean flow at 2 and 20 m depth for Winter 1992. ... 103

C.I Effluent comparison during Winter 1989-90 at 2 m depth . . .105 

C.2 Effluent comparison during Winter 1989-90 at 16 m depth. . . 106 

C.3 Effluent comparison during Spring 1990 at 2 m depth. ..... 107

C.4 Effluent comparison during Spring 1990 at 16 m depth. .... 108

C.5 Effluent comparison during Summer 1990 at 2 m depth. .... 109

C.6 Effluent comparison during Summer 1990 at 16 m depth. . . .110

C.7 Effluent comparison during Fall 1990 at 2 m depth. ...... Ill

C.8 Effluent comparison during Fall 1990 at 16 m depth. ...... 112

C.9 Effluent comparison during Winter 1990-91 at 2 m depth. . . .113

C.10 Effluent comparison during Winter 1990-1991 at 16 m depth. . 114 

C.ll Effluent comparison during Spring 1991 at 2 m depth. ..... 115

C.12 Effluent comparison during Spring 1991 at 16 m depth. .... 116

C.13 Effluent comparison during Summer 1991 at 2 m depth. .... 117

C.14 Effluent comparison during Summer 1991 at 16 m depth. . . . 118
List of Tables

2.1 Freshwater inputs for the model. ................. 21

2.2 Modeled and observed mean flow speed on the eastern flank

of Stellwagen Bank ........................ 26

5.1 Locations monitored in chlorination failure simulations. .... 66

5.2 Bacteria concentrations for breakdown of existing plant. . ... 67

5.3 Bacteria concentrations for breakdown of future plant. .... 67
Chapter 1 

Introduction

1.1 RATIONALE

A broad theme of the U.S. Geological Survey (USGS) National Marine 

and Coastal Geology Program is understanding geologic factors which influence the long-term quality and preservation of environments in their natural 

state. Toward this goal, the USGS is developing an integrated package of 

wave, circulation and sediment transport models that can be used to study 

the movement of polluted material in coastal waters. As a first step of this development, we have implemented and tested a three-dimensional circulation 

model in Massachusetts Bay, a prototypical region of interest.

1.2 PHYSICAL SETTING

Massachusetts Bay is a semi-enclosed embayment that opens into the 

Gulf of Maine at its eastern boundary (Figure 1.1). It is roughly 100 km 

long, 50 km wide and has an average water depth of 35 m. The bay is 

bounded on the east by Stellwagen Bank, which rises to within 20 m of sea 

surface. Boston Harbor empties into western Massachusetts Bay, providing 

a significant source of contaminants to the region.

Circulation in Massachusetts Bay is driven by a combination of local and 

remote processes that vary with season (Geyer et al, 1992). Throughout 

the year water flows southward in the western Gulf of Maine, and although 

most of this current continues flowing southward over the eastern flank of 

Stellwagen Bank (largely bypassing Massachusetts Bay), a small branch flows 

into the bay and drives a weak counter-clockwise flow that enters at Cape
71W 70°30' 70°00'W

OUTFALL' \

'STELLWAGEN-.

MASSACHUSETTS

MARINE 

SANCTUARY

4300'N

42°30'

42°00'

Figure 1.1: Bathymetric map showing Massachusetts and Cape Cod Bays, 

present sewage outfalls in Boston Harbor (diamonds), location of future ocean 

outfall for treated Boston sewage in western Massachusetts Bay, and the 

approximate boundary of the Stellwagen Bank Marine Sanctuary.
Ann and exits at Race Point. The magnitude of this flow varies from less 

than 1 cm s"1 off Boston to about 3-4 cm s"1 along the western shore of 

Massachusetts Bay.

The remotely driven mean circulation pattern is modified by seasonal 

current regimes. During the winter, the bay is well-mixed vertically, and 

the wind is principally from the northwest, reinforcing the counterclockwise 

circulation. In the spring, surface warming and freshwater intrusions from 

Gulf of Maine rivers cause the bay to become stratified, and the surface 

currents in the bay are dominated by the strong and variable density driven 

flow associated with these intrusions. By summer, the stratification has 

intensified due to further surface warming. The dominant wind direction in 

summer is from the southwest, which drives strong upwelling events along 

the western and northern shores of the bay. In the fall, rapid cooling on the 

shallow western side of the bay results in a density field that temporarily 

reverses the surface mean flow.

Tidal currents in Massachusetts Bay are dominated by the semi-diurnal 

M2 constituent (period = 12.42 hours). The currents are largely bidirectional and the magnitude ranges from about 10 cm s"1 in the interior of 

Massachusetts Bay to more than 50 cm s" 1 off the tip of Cape Cod and in 

the entrances to Boston Harbor and Plymouth Harbor (Blumberg et al, 1993, 

Irish and Signell, 1992). Although the transport due to tides is generally oscillatory and therefore doesn't give rise to net transport of material, they are 

an important source of bottom-generated turbulent mixing.

1.3 MOTIVATION FOR REALISTIC SIMULATIONS

A major sewage treatment project is currently underway in Boston, principal components of which include upgrade to secondary treatment and relocation of sewage outfall pipes from Boston Harbor to a site 9 miles offshore 

(hereafter referred to as the future outfall site). Discharge at the future 

outfall site is currently estimated to begin in 1997. The impact of the new 

treatment project on Massachusetts Bay is of critical concern to the Massachusetts Water Resources Authority (MWRA), which is responsible for 

the project, and to the citizens of Massachusetts. Selection of the future outfall location was based in part on steady, two-dimensional (depth-averaged) 

modeling results that showed improved dilution of effluent at the offshore 

location. Lacking in this model, however, was the ability to simulate flow

10
under stratified and time-variable conditions (although attempts were made 

to parameterize these effects). With the pressing need to better understand 

and simulate transport processes in Massachusetts Bay, the region was chosen 

by the USGS as a testbed for developing a prototype transport model.

To address water-column effects of the future outfall, the MWRA contracted HydroQual, Inc in the Spring of 1992 to develop a three-dimensional 

water quality model of the bay. To avoid duplication of effort, since threedimensional circulation is required by both sediment transport and water 

quality models, the USGS agreed (as part of a cooperatively-funded set of 

studies with MWRA) to supply HydroQual with the necessary hydrodynamic 

calculations for their water quality model.

October 1, 1989 - July 1, 1991 was selected for testing and calibration of 

the circulation and water quality models due to a large data collection effort 

during this period by the Massachusetts Bays Program (Geyer, 1992). By 

the fall of 1992, the USGS had made significant progress on hydrodynamic 

modeling. As water quality results were not yet available, comparative dilution simulations were made to assess the difference between the existing 

and future outfall locations. Using a conservative tracer, the results suggested that the future outfall would result in a significantly smaller region 

affected by high concentrations of effluent. These results played a significant 

role in the EPA environmental assessment of the impact of the future outfall 

on endangered species on Stellwagen Bank (EPA, 1993). Although the impact of the outfall is now more realistically represented by the water quality 

model which includes biochemical processes (HydroQual and Battelle, 1995), 

the comparative dilution simulations still provide important insight into the 

transport and dispersion of material in Western Massachusetts Bay, and are 

thus presented in this report.

1.4 ORGANIZATION

This report is organized as follows: The model and its configuration are 

described in chapter 2. The model is then compared to data in chapter 3 to 

assess its capability to represent temperature, salinity, density and circulation 

in Massachusetts Bay. The modeled effluent fields from the existing and 

future outfall are presented in chapter 4. Simulated chlorination plant failures 

at the existing and future outfall sites are shown in chapter 5.

11
Chapter 2

Model Implementation

2.1 NUMERICAL SCHEME

The model used in this study is called ECOM-si, a semi-implicit variant 

of the three-dimensional Estuary, Coastal and Ocean Model (ECOM) described by Blumberg and Mellor (1987). ECOM-si was selected because it 

includes a free surface, nonlinear advective terms, coupled density and velocity fields, river runoff, heating and cooling of the sea surface, a 2.5 level 

turbulence closure scheme to represent vertical mixing (Mellor and Yamada, 

1982), and is designed to easily allow "realistic" simulations. In addition, the 

combination of orthogonal curvilinear coordinates in the horizontal plane and 

sigma-coordinates in the vertical dimension allows grid refinement in regions 

of interest without sacrificing the well-known characteristics of Cartesian grid 

schemes.

The basic equations are expressed in a sigma coordinate system

where H(x,y) is the bottom topography and 7j(x,y] is the surface elevation. 

The basic governing equations are presented here in Cartesian coordinates to 

facilitate discussion. The equations as expressed in curvilinear coordinates 

may be found in Blumberg and Mellor (1987). 

The continuity equation is

dri duD dvD du n

-^ + -a + -a" + 7T = °' 

ot ox oy ocr

12
the x momentum equation is

duD du2D duvD duu

a a dt ox dy da

d (KM du\ gD2 d /° , gD dD /o dp , _ ,_ _, TT- 4 -fTTr- > - «- / /><fc- + -5- / *7r<fr + ^ (2.3)

00- D Cfcr 90 C7X 7a o OX Ja Off

«

/90 C7X

and the y momentum equation is

duD duvD dv2D dvw ~5T + ~5 + ~^~ + "^~ 

ot ox dy Off oy

d (KM dv\ gD2 d /o oZ}^ /o ^

^~1^T^rr~ a" / ^ + -«~ / <7 'a~ 

5<r [ D 5<r J pQ dy J<r po dy Ja Off

where 77 is the surface elevation, u and v are the x and ?/ components of 

velocity, D is the total water depth D = /f + 77, w is the transformed vertical 

velocity (normal to sigma surfaces), KM is the vertical eddy viscosity, p is 

the water density, PQ is a reference water density, and Fx and Fy are the 

horizontal viscous terms defined by

' (2.5)

ax \ axy dy 

and

.

dy \ dyj

where AM is the horizontal viscosity. The parameterizations of KM and 

AM are discussed in chapters 2.3 and 2.4, respectively. The model also 

solves prognostically for temperature, salt, concentration (used here to model 

sewage effluent), turbulence kinetic energy and turbulence macroscale.

ECOM-si differs from the Blumberg and Mellor (1987) ECOM model in 

that it uses a semi-implicit scheme for calculating the free surface, therefore avoiding the gravity wave CFL condition required by explicit schemes 

(eg Casulli, 1990). This has the advantage that larger time steps may be 

taken (on the order of minutes, rather than tens of seconds). A potential 

disadvantage of implicit schemes is that they more readily damp free wave 

motions, but in strongly forced and damped shallow regions such as Massachusetts Bay, the effect is small. This was determined by halving the time

13
step and observing negligible differences in simulation results. Another disadvantage is that because the calculation of surface elevation requires solving 

a large matrix equation at each time step and efficient solution of this equation requires positive definiteness; boundary conditions for elevation must be 

formulated in matrix form and must not destroy the positive definiteness of 

the matrix. We use a combination of clamped and gravity wave radiation 

conditions on the open boundary, made possible by the implementation of 

the partially-clamped formulation of Blumberg and Kantha (1985) discussed 

in chapter 2.8.

2.2 MODEL GEOMETRY AND BATHYMETRY

The model was configured on a 68 x 68 horizontal curvilinear orthogonal 

grid (Figure 2.1). The grid spacing ranges from approximately 600 m in 

Boston Harbor to about 6000 m along the open boundary with the Gulf of 

Maine. The grid spacing in the vicinity of the future outfall site is approximately 1000 m. The grid extends well offshore of Massachusetts Bay to allow 

exchange with the Gulf of Maine inside the model domain, and extends far 

enough north to include the Merrimack River, a large source of fresh water 

to the region. Initially 10 evenly spaced sigma levels were used in the vertical 

dimension, but final runs utilized 2 additional near-surface layers at 1 and 

4% of the water depth to better resolve river plumes and the surface mixed 

layer.

The bathymetry for the model was obtained by interpolating NOAA 

sounding data on to the model grid using an inverse distance method. The 

resulting grid was Shapiro filtered (Shapiro, 1975) to remove 2 grid length 

variability, and a maximum depth of 140 m was set to eliminate the deep 

complex topography offshore of Massachusetts Bay. The minimum depth 

was set to 3 m to avoid flooding and drying.

2.3 VERTICAL MIXING

The vertical mixing parameterization was the level 2.5 turbulence closure 

scheme of Mellor and Yamada (1982) with the extensions by Galperin et 

al (1988). An important feature of these extensions include a length-scale 

limitation that prevents the mixing length from becoming too large during 

strongly stratified conditions. The minimum "background mixing" value, 

below which the eddy viscosity never falls, was set to 5 x 10~6 m2 s" 1 . This

14
43°00'

42°30'-

Maximum grid size ~= 6000 m 

Minimum grid size ~= 600 m

42°00'

71°00' 70°30' 70°00' 69°30'

Figure 2.1: Model grid for the three-dimensional circulation model of Massachusetts and Cape Cod Bays. The curvilinear orthogonal grid allows the 

mesh resolution to vary spatially, having a minimum grid spacing of 600 m 

and a maximum spacing of 6000 m. The grid spacing in the vicinity of the 

future outfall is roughly 1000 m. The 40 m isobath is represented by the 

thick grey line.

15
value is comparable to the value of vertical mixing observed by Geyer and 

Ledwell (1994) from a dye experiment near the future outfall site.

2.4 HORIZONTAL MIXING

The horizontal mixing parameterization used the Smagorinsky (1963) formulation in which the magnitude of the horizontal mixing is proportional to 

horizontal current shear. For the water quality runs, the Smagorinsky coefficient was set to 0.1, which results in typical horizontal eddy viscosities and 

diffusivities between 5-20 m2 s-1 over much of Massachusetts Bay. Ideally, 

the magnitude of this mixing would represent processes occurring at scales 

smaller than the mesh can resolve. In western Massachusetts Bay the grid 

cell spacing is 1-2 km, and using the rule of thumb that 6-8 grid cells motions 

are the minimum length that are realistically represented, the model should 

well resolve mixing processes occurring at scales of 10 km or so. From Okubo 

(1971), we expect that observed horizontal mixing at a scale of 10 km should 

be about 10 m2 s" 1 , and at a scale of 1 km should be about 1 m2 s" 1 . Since the 

model partially resolves shear (and thus produces mixing) at scales between 

1 and 10 km, the ideal mixing level should probably be somewhere between 

1 and 10 m2 s" 1 . To test the sensitivity to the mixing level in this range, 

the model was subsequently rerun for a 3 month period using a Smagorinsky coefficient of 0.02 (five times smaller than the water quality run), and 

only small quantitative differences were observed in the resulting velocity, 

temperature, salinity and effluent concentration fields. This indicates that 

the modeled fields are dominated by resolved mixing and transport processes 

rather than the parameterized mixing of the Smagorinsky formulation. This 

is consistent with observations made during the outfall siting process, where 

it was concluded that a value of 45 m2 s"1 best represented the dispersion of 

material over the 20 km scale of western Massachusetts Bay (Adams et al, 

1990), suggesting that the effective mixing in this region may be a bit higher 

than Okubo's empirical curve. This would not be surprising considering the 

strong current shears in this region. The level of 45 m2 s" 1 was also used in 

the outfall siting model studies described by Walton et al (1990).

In addition to the specified horizontal viscosity, the velocity field was 

Shapiro filtered every 2 hours to remove 2 grid length energy. This was 

necessary to prevent 2 grid length energy along the open boundary from 

growing large enough to violate the advective CFL condition. By applying a

16
4th order Shapiro filter at 2 hour intervals, the longer wavelength energy is 

largely unaffected. For example, it can be shown over 20 filter applications 

(two days of simulation), the amplitude of 6 grid length structure is reduced 

less than 5% (see appendix of Signell, 1989).

2.5 SURFACE AND BOTTOM STRESS

The surface wind stress was assumed to be uniform over the model domain 

and was determined from wind measurements at the Boston Buoy (Station 

BB in Figure 2.2) using the Large and Pond (1981) formulation.

Although the scale of the bay is relatively small compared to most synoptic scale wind features, there is certainly some variability over the bay, 

caused in part by local effects such as the seabreeze. Some of the discrepancies between model and data, especially during the summer, are undoubtably 

the result of the idealized wind stress field. Observations were available at 1 

hourly intervals from the Boston Buoy (anemometer height 13.6 m) except 

for the period 8/31/90 - 10/4/90, when wind data from Logan Airport was 

used instead. A comparison between Boston Buoy and Logan Airport winds 

(Figure 2.1-1, Geyer et al, 1992) suggests that the winds are comparable during this time period, although the wind at Boston Buoy is weaker during the 

summer and stronger during the winter than winds at Logan. The hourly 

averaged winds were converted to wind stress, then averaged over 4 hourly 

intervals before input to the model to reduce the size of the input file.

The bottom stress is determined from the quadratic drag law applied at 

the point closest to the bed where there is a velocity estimate. This is the 

middle of the lowest grid cell. Because the distance between the the lowest 

velocity point and the bed varies with location, the bottom roughness length 

z0 is specified at each location, and then a constant stress layer is assumed 

so that an effective drag coefficient can be computed at the velocity point. 

For this study, z0 was assumed to be uniform throughout the domain with a 

value of 0.003 m, which is equivalent to a drag coefficient of 2.5xlO~3 at 10 m 

above the bottom (which is in turn a typical value used for depth-averaged 

models of coastal seas). In these runs, the bottom sigma layer thickness is 

10% of the water depth, so the lowest velocity point is 5% of the water depth 

above the bed. Within Massachusetts Bay, the maximum distance between 

the lowest velocity point and the bed is 4.8 m in Stellwagen Basin, where the 

model water depth reaches 96 m.

17
71°00' 70°30' 70WW

4300'N

4230'N

42°00'N

Figure 2.2: Data collection sites in Massachusetts Bay. Time series of currents, salinity, and temperature were collected at the 10 locations indicated. 

Meteorlogical information was obtained from the Boston Buoy, Station BB.

18
Surface Meteorological Boundary Conditions

CM

I

<D

1000

Barometric Pressure

20

O 10 

0

-10 

20

Precipation - Evaporation

Oct Feb 

1990

Jun Oct Feb 

1991

Jun Oct Feb 

1992

Jun Oct

Figure 2.3: Meteorological data used to specify wind stress, buoyancy flux 

and heat flux boundary conditions at the sea surface.

19
2.6 SURFACE HEAT FLUX

The surface heat flux was calculated using techniques described by Weller 

et al (1995), and the flux was introduced into the top sigma layer at each 

grid location. These techniques produce bulk estimates of latent, sensible 

and longwave radiation, given sea surface temperature, air temperature, insolation, relative humidity, barometric pressure and wind. The limited availability of these data led us to use air temperature, barometric pressure and 

wind data at the Boston Buoy, relative humidity at Logan Airport and insolation at Woods Hole to represent bay-wide conditions (Figure 2.3). The sea 

surface temperature used in the heat flux estimates was initially determined 

from observations at the Boston Buoy. This proved problematic, because 

Cape Cod Bay is significantly warmer than the Boston Buoy site during 

most of the summer, which should result in reduced heat flux (all else being 

equal), but there was no feedback mechanism to allow this. The result was 

that model temperatures in Cape Cod Bay became too warm. To address 

this problem, later runs used model-generated sea surface temperature fields 

to calculate surface heat flux. Using the modeled sea surface temperature 

field allows spatial variability in sea-surface temperatures to feed back into 

the heat flux routines. Under up welling conditions, for example, there can be 

10° or more difference between surface water temperatures in the Bay, which 

can in turn result in heat flux differences of 200 W m~2 or more. Allowing 

for spatial differences in the surface heat flux due to varying water temperatures led to a significantly improved correspondence between modeled and 

observed water temperatures in Cape Cod Bay. Hourly estimates of heat 

flux obtained from the bulk formulae were averaged over 4 hours to reduce 

the size of the model input file.

2.7 FRESHWATER INPUT

Fresh water was introduced directly into the model at grid cell locations 

representing the mouths of the Merrimack, Charles, Neponset and Mystic 

rivers as well as at the existing sewage effluent discharges at Nut and Deer 

Islands. In addition, input from the Penobscot, Androscoggin, Kennebec, 

and Saco Rivers were used to drive an adjunct model of the western Gulf of 

Maine that was used to help improve salinity open boundary conditions for 

the Massachusetts Bay model described here. (The western Gulf of Maine 

adjunct model is described in the open boundary condition chapter 2.9.)

20
Merrimack

Charles

Deer Island

Nut Island

Neponset

Mystic

Penobscot

Andro/Kenn

Saco

1990 Discharge 

(m's-1 )

321.8

15.2

11.8

6.0

6.5

3.0

548.5

611.6

119.2

Agency

uses

uses

MWRA

MWRA

-

-

uses

uses

uses

Location

Lowell, MA

Waltham, MA

Deer Island

Nut Island

Inferred from Charles

Inferred from Charles

Eddington, ME

N. Sidney/ Aubern, ME

Cornish, ME

Scale 

Factor

1.08

1.37

1.00

1.00

1.00

1.00

1.10

1.15

1.31

Table 2.1: Average freshwater inputs for the model during 1990. The scale 

factor is the amount that the gauged discharge was multiplied by to account 

for drainage area downstream of the gauge.

To indicate the relative importance of these sources, the mean flow during 

1990 is shown in Table 2.1 and indicates that the Penobscot, Androscoggin/Kennebec and Merrimack River systems dominate the freshwater input 

to this region. The Androscoggin and Kennebec Rivers are combined because 

they merge before discharge into the Gulf of Maine. The daily flows from 

the Neponset and Mystic Rivers were inferred by multiplying the daily flow 

from the Charles River by 0.431 and 0.195 respectively, factors determined 

by the ratio of the annual average.

Examination of the daily discharge over the three year simulation period 

shows that although the maximum discharge occurs during the spring of 

each year, there are secondary maxima in the fall (Figure 2.4). Fall 1990 in 

particular has unusually high discharge, almost as large as the preceding and 

following spring discharges.

2.8 OPEN BOUNDARY CONDITIONS

Open boundary conditions are required for elevation, temperature, salinity and tracer concentration (used here to represent effluent). Because the 

elevation field at each time step is solved for with a fully implicit scheme, the 

boundary condition must be specified as part of the matrix solution for the

21
6000

5000

4000

3000

2000

1000

0

3000

2500

2000

1500

1000

500

0

80

60

<g40

20

Freshwater Input

Gulf of Maine & Boston inputs

Gulf of Maine inputs 

Boston inputs

Major Rivers

Boston Sources

Rivers: Charles+Neponset+Mystk 

Effluent: Deer + Nut

Oct Apr 

1990

Oct Apr 

1991

Oct Apr 

1992

Oct

Figure 2.4: Freshwater input to the Western Gulf of Maine. The top panel 

shows the combined discharge due to the Kennebec, Androscoggin, Penobscot 

and Merrimack Rivers compared to the Boston discharge. The middle panel 

shows the individual contributions of the major rivers, and the lower panel 

show the relative contributions of rivers and effluent to discharge in Boston 

Harbor.

22
elevation field. In addition, the matrix solver used in this code requires the 

matrix to be symmetric and positive definite, which puts further restraints 

on the type of boundary condition imposed. The partially clamped boundary condition of Blumberg and Kantha (1985) can be shown to satify these 

conditions, and is expressed as

where rjdata is specified from data. If Tiag is very large a pure radiation 

condition is approached, and if Tiag is very small a purely clamped (specified) condition is approached. In the Massachusetts Bay model, the offshore 

boundary elevation was clamped while the southern boundary radiated gravity waves. The values for elevation used along the offshore boundary were a 

combination of MI, A^, and 82 tidal elevations derived from the tidal model 

of Lynch and Naimie (1993) and low-frequency fluctuations obtained from 

the adjunct Gulf of Maine model described in chapter 2.9. The MI tidal 

elevations obtained from Lynch (personal communication, 1993) were used 

to synthesize 5*2 and N% constituents by scaling the M% amplitudes by the 

observed S^/M^, and N^/M? ratios at the Boston Buoy and shifting the 62 

and N-2 phases by the amount observed at the Boston Buoy. This is a reasonable approximation since the M2 , .$2 and NZ amplitude and phase vary 

in much the same manner in the Gulf of Maine (e.g. Moody et al, 1984).

Temperature, salinity and effluent are also specified along the open boundaries. If the flow is directed out of the domain, the interior values are simply 

advected out of the domain. When outflow turns to inflow, the water property values slowly move toward specified values over a defined relaxation time 

to avoid artificial fronts from developing. In the Massachusetts Bay model, 

the relaxation time was specified to vary linearly from 3 days at the most 

northern boundary cell to 30 days at the southern boundary cell.

Initially the temperature and salinity values along the boundary were 

specified by climatology interpolated from the Bedford Institute of Oceanography AFAP database (Drinkwater, 1992), while the effluent concentration 

field was set to zero. Although it was hoped that most local river effects 

would be due to the Merrimack and the Charles, early runs showed that the 

gross salinity picture for Massachusetts Bay could not be adequately specified 

by parameterizing the other Gulf of Maine rivers by seasonal climatology. To

23
address this problem, an adjunct model of the western Gulf of Maine was developed to provide better salinity boundary conditions for the Massachusetts 

Bay model.

2.9 ADJUNCT WESTERN GULF OF MAINE MODEL

The western Gulf of Maine model grid is 90 x 45 cells and has a typical grid 

spacing of 2-4 km (Figure 2.5). The bathymetry was generated in identical 

fashion to the Massachusetts Bay model and the model was driven by the 

same wind and heat flux forcing. The elevation boundary conditions were 

specified as A/2, 52 and N2 tides derived from Lynch and Naimie (1993). 

There were no subtidal fluctuations specified, so the low-frequency elevation 

was effectively clamped at zero along the offshore boundary. At the northern 

boundary, a mean sea surface slope is imposed to drive a coastal current. 

Along this boundary the low-frequency elevation varied linearly from 0.06 

m at the coast to 0.00 m at the open boundary 45 km offshore. This was 

picked to drive a mean alongshore flow of 5-10 cm s"1 at locations where 

mean currents were observed by Vermersch et al (1979). The mean flow in 

the Gulf of Maine model is held constant throughout the year. Recent data 

collected on the eastern flank of Stellwagen Bank at a location determined to 

be a good measure of the large-scale circulation in the Gulf of Maine (Bogden 

et al, 1995) suggests, encouragingly, that there might be little variability in 

the seasonal mean flow. A comparison of the mean flow observed in spring, 

summer and fall of 1994 at the Stellwagen Bank mooring with seasonallyaveraged mean flow obtained from the model at this location over 1990-1992 

shows remarkable agreement (Figure 2.6). Outside of the buoyant surface 

layer, the observed mean flow due to large scale circulation in the Gulf of 

Maine changes very little with season (Table 2.2). The mean flow at 27 m, 

for example, appears to be about 10 cm s"1 throughout the year, and the 

mean flow at 50 m is nearly constant at 4 cm s"1 .

The temperature and salinity along the open boundary were specified by 

monthly climatology determined from the Bedford Institute of Oceanography 

AFAP database. Model output averaged over the tidal cycle was output 

along the cells that constituted the open boundary for the Massachusetts 

Bay model, the converted into low-frequency elevation boundary conditions 

and temperature and salinity boundary conditions at standard levels (1, 2.5, 

5.0, 7.5, 10, 15, 20, 30, 40, 50, 75 and 140 m). The resulting boundary

24
44°30'

44°00'

42°00

V

< Western Gulf of Maine Grid

< Massachusetts Bay Grid

i i i i i 

71°00' 70°30' 70°00' 69°30' 69°00' 68°30'

Figure 2.5: Western Gulf of Maine grid, showing major rivers. Grid spacing 

is 2-4 km. The isobaths shown are 60, 100 and 200 m.

25
Win Spr Sum Fa

5m

27m \ \

50m

Figure 2.6: Model/data comparison of mean flow on the east flank of Stellwagen Bank. The thick grey arrows are observed means from 1994, while 

the thin black arrows are modeled means from 1990, 1991 and 1992. The 

flow at this location is strongly controlled by large-scale circulation in the 

Gulf of Maine that is parameterized in the model through the open boundary 

conditions.

Winter 

Spring 

Summer 

Fall

MODEL (1990-1992) 

cm s" 1

5m 27 m 50 m

10.1 7.2 3.3 

10.2 8.4 2.6 

12.5 10.4 3.1 

14.3 11.6 3.7

DATA (1994) 

cm s" 1

5m 27 m 50 m

nd nd nd 

16.1 10.0 4.3 

9.7 10.2 4.1 

19.5 10.8 4.3

Table 2.2: Modeled and observed mean flow speed on the eastern flank of 

Stellwagen Bank at 5, 27 and 50 m depths. Below the river plume influenced 

surface layer, the mean flow at 27 m is relatively constant at 10 cm s" 1 .

26
conditions for Massachusetts Bay, as intended, represent more variability 

and Gulf of Maine discharge-driven events than would have been possible 

with open boundary conditions based on climatology (Figures 2.7, 2.8).

Inter-annual variability at frequencies less than the synoptic scale weather 

band (3-10 days) is not dramatic, although certain events stand out, such as 

the sustained decrease in surface salinities near the coast during the fall of 

1990, consistent with the abnormally large runoff during this time period.

2.10 BOUNDARY CONDITION MODIFICATIONS FOR 1992 RUN

Although the temperature and salinity boundary conditions obtained 

from the climatology and the western Gulf of Maine adjunct model result 

in a quite realistic representation of the observed temperature and salinity 

structure in Massachusetts Bay, actual data obtained near the boundary during the simulation time period can further improve the representation. To 

improve the 1992 water quality "verification run", several sets of CTD data 

were obtained from the National Marine Fisheries Service (Holzwarth-Davis 

and Taylor, 1993) and used to make very low-frequency (weeks to months) 

corrections to the temperature and salinity conditions.

2.11 TIME STEP

To achieve optimal efficiency for these multi-year runs, the time step was 

varied with season and forcing conditions as determined by trial and error. 

Although ECOM-si is not restricted by the gravity wave CFL condition, it 

is limited by the advective CFL condition. During the spring and summer 

months in particular, the time step needed to be decreased. During the 

winter, the time step was usually 414 s (108 steps per MI tidal cycle), except 

for a few short periods generally coinciding with winter storms. During the 

summer, the time step was usually 207 s (216 steps per MI tidal cycle).

2.12 SIGMA-COORDINATE CORRECTION

In regions of steep topography, sigma-coordinate models are prone to errors in the calculation of pressure gradient and horizontal diffusion terms 

(Beckman and Haidvogel, 1993). In the modeling here, initial stratification 

results were not encouraging in that the modeled temperature stratification 

during the summer was 50% weaker than observed at the future outfall site 

(Figure 2.9) and in Stellwagen Basin. Although vertical mixing was initially

27
25

20

Open Boundary Condition 10 km off New Hampshire Coast (i=67,j=59)

15

O

10

O 1-1

Temperature

34

32

30

3 

W 

Q.

28

26

24

Oct Apr 

1990

Oct Apr 

1991

Oct Apr 

1992

Oct

Figure 2.7: Inshore Massachusetts Bay boundary condition from WGOM 

model.

28
Open Boundary Condition 50 km off Cape Ann (i=67,j=39)

25.-T

20

15

O

10

Temperature

34

32

30

COCL

28

26

24 Li

Salinity

Oct Apr 

1990

Oct Apr 

1991

Oct Apr 

1992

Oct

Figure 2.8: Offshore Massachusetts Bay boundary condition from WGOM 

model.

29
10

(0

D

|o

(D

O

Vertical temperature 

difference (5 m - 23 m) 

at the new outfall 

location

observed data

" 'S >, model with 

J \f\ ;-.'} sigma correction

?i.

model without 

sigma correction n

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

1990

Figure 2.9: Observed and modeled temperature difference (5 m - 23 m) at 

Station BB (see Figure 2.2). Before the a correction, the model greatly underpredicted the degree of temperature stratification in the summer months. 

Observed data is missing from late August to November.

30
suspected, the problem turned out to be caused by spurious up-slope mixing caused by <j-coordinate errors. These errors were significantly reduced 

by removing the domain-averaged vertical salinity, temperature, and density 

profiles before calculating horizontal differences (e.g. Beckmann and Haidvogel, 1993). The effectiveness of this technique depends upon the degree 

to which the vertical structure varies over the domain. In domains such 

as Massachusetts Bay, in which the stratification is nearly uniform over the 

region, this technique greatly improves the model prediction in comparison 

to observed data (Figure 2.9). The more the vertical structure varies over 

the domain, however, the less useful the technique in reducing <j-coordinate 

errors. In domains where both stratified and well-mixed regions exist, errors would actually increase in the well-mixed region since subtracting the 

domain-averaged vertical profile would result in more vertical structure locally.

31
Chapter 3

Model/Data Comparison

3.1 TEMPERATURE, SALINITY AND DENSITY

The comparison between modeled and observed temperature, salinity and 

density at most sites shows that the model generally does a good job at reproducing seasonal cycles and top to bottom density differences (Figures 3.1- 

3.8). There are some clear differences however. While most of surface salinity 

variations during the spring and summer are well represented (Figure 3.1), 

the salinity drop associated with the large fall 1990 discharge appears in the 

model about three weeks too late, and during spring of 1991 the surface 

salinities in the model are 0.5-1.0 psu too salty. The lower layer salinities 

(Figures 3.2 and 3.3) during this time period are also 0.5-1.0 psu too salty, 

so the vertical density structure is minimally affected. It does suggest, however, that if a better match of the salinity magnitude was desired it would be 

necessary to improve upon the climatological salinity boundary conditions 

for the western Gulf of Maine model. These boundary conditions specified 

a fixed seasonal cycle, assuming that all interannual variability was due to 

local river discharge in the western Gulf of Maine. It is reasonable to expect that interannual variability in discharge from the St. Lawrence river 

and variations in the Gulf of Maine circulation would also be responsible for 

interannual salinity fluctuations in the water entering Massachusetts Bay.

The modeled temperatures and salinities appear to track some of the 

observed shorter period fluctuations in addition to the general seasonal cycle. 

The coherence between modeled and observed water properties is generally 

significant at 95% confidence over periods of 2-20 days (Figures 3.9 and 3.10).

32
Near-Surface Salinity (4-5 m)

32

31

30

32

Boston Buoy (BB)

31

30-

32

31

30-

32

31

30-

Jun Aug 

1990

Oct Dec Feb Apr 

1991

Jun

Figure 3.1: Model/data comparison of near-surface salinity at Stations BB, 

BS, SC and U6.

33
Lower-Layer Salinity

33

32

31

33

Boston Buoy (BB) at 23 m

32

31 -

Broad Sound (BS) at 19 m

33

32

31 -

Scituate (SC) at 24 m

33

32

31 -

Stellwagen Basin (U6) at 45 m

Jun 

1990

Aug Oct Dec Feb 

1991

Apr Jun

Figure 3.2: Model/data comparison of lower layer salinity at stations BB, 

BS, SC and U6.

34
Lower-Layer Salinity

33

32

31

Cape Ann (U2) at 60 m

33

32

31 -

Manomet (MN) at 25 m

33

32

31 -

Race Point (RP) at 55 m

33

32

31 -

Cape Cod Bay (U7) at 25 m

Jun 

1990

Aug Oct Dec Feb 

1991

Apr Jun

Figure 3.3: Model/data comparison of lower layer salinity at stations U2, 

MN, RP and U7.

35
Near-Surface Temperature (4-5 m)

O

O

o

o

20

15

10

5

20

15

10

5

20

15

10

5

20

15

10

5

Boston Buoy (BB)

Broad Sound (BS

Scituate (SC)

Stell Basin (U6

Jun Aug Oct Dec Feb Apr Jun 

1990 1991

Figure 3.4: Model/data comparison of near-surface temperature at stations 

BB, BS, SC and U6.

36
Near-Surface Temperature (4-5 m)

O

o

o

o

20

15

10

5

20

15

10

5

20

15

10

5

20

15

10

5

Cape Ann (U2)

Manomet (MN)

Jun 

1990

Aug Oct Dec Feb 

1991

Apr Jun

Figure 3.5: Model/data comparison of near-surface temperature at U2, RP, 

MN, and U7.

37
o

o

o

o

20

15

10

5

20

15

10

5

20

15

10

5

20

15

10

5

Lower-Layer Temperature

Boston Buoy (BB) at 23 m

Broad Sound (BS) at 18 m

Scituate (SC) at 23 m

Stellwagen Basin (U6) at 45 m

Jun 

1990

Aug Oct Dec Feb 

1991

Apr Jun

Figure 3.6: Model/data comparison of lower layer temperature at BB, BS, 

SC and U6.

38
Lower-Layer Temperature

O

o

o

o

20

15

10

5

20

15

10

5

20

15

10

5

20

15

10

5

Cape Ann (U2) at 60 m

Manomet (MN) at 25 m

Race Point (RP) at 55 m

Cape Cod Bay (U7) at 25 m

Jun Aug Oct Dec Feb Apr 

1990 1991

Jun

Figure 3.7: Model/data comparison of lower layer temperature at U2, MN, 

RP, and U7.

39
Vertical Density Difference

73 

'Wg> 2 

1

Boston Buoy (BB) 

[23 m - 5 m]

3

0)

Broad Sound (BS) 

[18m-5m]

3

O>

Scituate (SC) 

[23 m - 5 m]

73

D)

*(/> 2

Stell Basin (U6)

[45 m - 4 m] -

Jun 

1990

Aug Oct Dec Feb 

1991

Apr Jun

Figure 3.8: Model/data comparison of vertical density differences at stations 

BB, BS, SC and U6.

40
The temperature coherence decreases somewhat with increasing frequency, 

but the salinity coherence doesn't show any obvious dependence on frequency.

Model fluctuations in salinity are comparable in strength to observations 

(Figure 3.9) except over Stellwagen Basin (U6), where the modeled salinity 

fluctuations are less that half the strength of the observations. The strong 

observed fluctuations of near-surface salinity at U6 (Figure 3.1) are associated 

with intrusions of low-salinity water from the Gulf of Maine (Geyer et al. 

1992), and it appears that these intrusions are either under-represented or 

are somewhat smeared out by the model.

Model fluctuations in temperature are comparable in strength to observations (Figure 3.10) except at Scituate (SC) and Broad Sound (BS), where the 

modeled temperature fluctuations are about 50% stronger than the observations. The strong modeled fluctuations of near-surface temperature at these 

near-shore locations are associated with upwelling events, so one explanation 

is that the model surface layer responds a bit too strongly to upwelling winds.

3.2 VELOCITY TIME SERIES COMPARISON AT THE MOORING

LOCATIONS

The quality of the comparison between modeled and observed currents 

varies substantially with space and time. At the Boston Buoy, for example, 

events are frequently well-represented during the winter (Figure 3.11), but 

are only occasionally well-represented during the summer (Figure 3.12). As 

explained in Signell et al (1994), this is because the flow regime in winter is 

largely wind-driven with large spatial scales. Thus small deviations in the 

structure or timing of the circulation between between the model and reality 

will not result in large errors at the mooring locations. During the stratified 

seasons, however, the spatial scales are considerably smaller due to upwelling, 

intrusions of Gulf of Maine river water, etc. Small deviations in the structure 

or timing during the stratified seasons will therefore result in larger errors 

at the mooring locations. For example, if the model representation of an 

intrusion is off by 5 km, the whole signal could be missed at a particular site 

like the future outfall location.

The degree of agreement between model and data can be quantified by 

the complex correlation coefficient, which is high during the winter and considerably lower during the summer (Figures 3.13 and 3.14). Although the

41
Coherence: Modeled/Observed Near-Surface Salinity (4-5 m)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Frequency (cycles per day)

Amplitude Ratio: Modeled/Observed Near-Surface Salinity (4-5 m)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Frequency (cycles per day)

Figure 3.9: Top Panel: Coherence between modeled and observed nearsurface salinities at Broad Sound (BS), Boston Buoy (BB), Scituate (SC) 

and Stellwagen Basin (U6). The 95% confidence level is 0.3. Lower Panel: 

The amplitude ratio of model salinity fluctuations to observed salinity fluctuations.

42
Coherence: Modeled/Observed Near-Surface Temperature (4-5 m)

0.8

0.6

0.4

0.2

1.5

0.5

SC

I_________I__________I__________!_________I

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Frequency (cycles per day)

Amplitude Ratio: Modeled/Observed Near-Surface Temperature (4-5 m)

U6

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Frequency (cycles per day)

Figure 3.10: Top Panel: Coherence between modeled and observed nearsurface temperatures at Broad Sound (BS), Boston Buoy (BB), Scituate 

(SC) and Stellwagen Basin (U6). The 95% confidence level is 0.3. Lower 

Panel: The amplitude ratio of model temperature fluctuations to observed 

temperature fluctuations.

43
25

Figure 3.11: Boston Buoy velocity comparison: Jan 28, 1991 - Mar 25, 1991.

44
10

E 0

o

-10

10

V)

E 0 

o

-10

DATA: 23 m

MODEL: 23 m

SSSM//^ \\\w<S

18 

Jun

1990

25 2 

Jul

16 23 30 6 

Aug

13

Figure 3.12: Boston Buoy velocity comparison: Jun 18, 1990 - Aug 13, 1990.

45
Statistical Comparison of Model/Data Velocity at BB_05

0.8 

0.6 

0.4 

0.2

0

15

10

Complex Correlation

o

Magnitude of mean current

15

10

o

Magnitude of standard deviation

Dec Jun 

1990

Dec Jun 

1991

Dec Jun 

1992

Dec

Figure 3.13: Boston Buoy Model/Data velocity statistics at 5 m depth.

46
Statistical Comparison of Model/Data Velocity at BB_23

0.8

0.6

0.4

0.2

15

Complex Correlation

10

I

o

O 1-1

Magnitude of mean current

15

10

i

o

Magnitude of standard deviation

Dec Jun 

1990

Dec Jun 

1991

Dec Jun 

1992

Dec

Figure 3.14: Boston Buoy Model/Data velocity statistics at 23 m depth.

47
correlations are considerably lower in the summer, the modeled mean and 

standard deviation of the low-frequency currents are comparable to observed 

values and do not show any clear bias.

The relatively good correlation between observed and modeled currents at 

the Boston Buoy is also seen at the other western Massachusetts Bay stations 

Scituate and Manomet (Figure 3.15). The correlations are significantly lower 

in Cape Cod Bay and eastern Massachusetts Bay.

Another way of assessing the degree to which the model compares with 

data is to look at the empirical orthogonal functions (EOF's) of the model and 

data. At mooring locations where at least 60% of the time period May 1990 

- June 1991 contained good data, time domain EOF's were computed from 

the east/west and north/south components of the sub-tidal velocity time 

series. The first modes of the observed and modeled velocities explain 50% 

and 69% of the variance, respectively, and have reassuringly similar structure 

(Figure 3.16). This indicates that surface currents in western Massachusetts 

Bay constitute the largest amount of the energy in both the model and the 

data.

3.3 STATISTICAL COMPARISON OF CURRENTS AT THE MOORING

LOCATIONS

Reproducing the day to day variations of ocean "weather" at specific 

locations may be required to warrant the use of the model to address some 

issues, but for other issues it may only be necessary to correctly reproduce the 

statistics or "climate" of the currents over a few weeks or months. To assess 

the capability of the model to represent the climate, the mean current and 

low-frequency ellipses from the data and model were compared at the moored 

locations for each season. The seasons are defined by distinct dynamical 

regimes instead of the conventional dates: Winter (November 1 to March 1) 

represents a wind-dominated, well-mixed regime; Spring (March 1 to June 

1) represents a runoff-influenced, transitional stratification regime; Summer 

(June 1 to September 1) represent an upwelling, strongly stratified regime; 

and Fall (Sepember 1 to November 1) represents a transitional overturning 

regime.

During the winter, as expected from the relatively high correlations at 

the moorings, the mean and low-frequency ellipses compare quite well (Figure 3.17). The ellipses are shown centered around the tip of the mean flow,

48
May

Figure 3.15: Model/Data velocity correlations at selected regional stations.

49
MODEL: 1990/04/24-1990/04/24

170

160

150

140

130

120

110

100

90

170

160

150

140

130

120

110

100

90

/

220 240 260 280 300

DATA: 1990/04/24-1991/06/15

X

220 240 260 280 300

Figure 3.16: Model/Data comparison of the 1st EOF structure of the velocity 

time series

50
Winter 1990-1991:1990/10/31-1991/02/28

180

170

160-f

150

140H

130

120

110

100

90

180

170 

160 

ISO140- 

130 

120 

110 

100 H 

90

Figure 3 

1991

220 240 260 280 300 320

Lower Layer (18-25 m)

220 240 260 280 300 320 

17: Mean flow and low-frequency variability ellipses for Winter 1990-

51
and the arrows and ellipses are scaled to correspond to the distance that particles traveling at the observed velocity would move in 1 day. This means that 

about two-thirds of the time (the percentage of time particles move less than 

one standard deviation), particles released at the moored locations would 

be found somewhere inside the ellipse after 1 day. Modeled displacements 

are very close to predicted displacements with the exception of Race Point, 

where the observed mean flow is close to northward while the modeled flow 

is close to eastward. Analysis indicates that this discrepancy is most likely 

due to a shift in the modeled pattern of tide-induced residual currents rather 

than a major error in representing flow out of Massachusetts Bay. There 

is strong (10 cm s"1 ) northward mean flow in the model just 2 km to the 

southwest, the result of a small (5 km) tide-induced residual eddy off the tip 

of the Cape.

During the spring the surface current variability increases due to intrusions of low salinity water from Gulf of Maine rivers and weaker frictional 

resistance to wind driving since the surface layer becomes somewhat decoupled from the lower layer. The model reflects the increased intensity of surface 

currents and the general pattern of the current "climate," doing particularly 

well at the future outfall site (Figure 3.18). The modeled surface currents 

are slightly stronger than the observations near Stellwagen Bank and slightly 

weaker than the observations at Scituate and Manomet, however. At Race 

Point there is a discrepancy between the modeled and observed mean flows 

very similar to winter, consistent with the interpretation that this is due to 

slight spatial error in representation of the tidal residual eddy off the tip of 

the Cape. As with the surface currents, the lower layer currents are also 

stronger than observed near Stellwagen Bank, but are close to the observed 

currents at the other locations (except Scituate, where the modeled currents 

are extremely weak).

In the summer, strong stratification results in much stronger currents 

in the upper layer than in the bottom layer. The model represents this 

behavior quite well, and again is a close match to the current "climate" at 

the future outfall site (Figure 3.19). The model currents are again stronger 

than observed near Stellwagen Bank, and Cape Cod Bay surface currents are 

also stronger than observed.

In the fall the surface mean flow reverses and flows northward at Scituate 

and Manomet in both the observations and the model while the mean flow 

near Stellwagen Bank still flows to the south (Figure 3.20). The ellipses are

52
Spring 1991:1991/02/28-1991/05/31

Lower Layer (18-25 m)

220 240 260 280 

km

300 320

Figure 3.18: Mean flow and low-frequency variability ellipses for Spring 1991

53
Summer 1990:1990/05/31-1990/08/31

Lower Layer (18-25 m)

220 240 260 280 

km

300 320

Figure 3.19: Mean flow and low-frequency variability ellipses for Summer 

1990

54
Fall 1990:1990/08/31-1990/10/31

180

Lower Layer (18-25 m)

Upper Layer (4-8 m)

61

ICcnVs

Data 

Model

220 300 320

Figure 3.20: Mean flow and low-frequency variability ellipses for Fall 1990

55
generally quite similar except at Race Point, where there appears to be some 

flow into Massachusetts Bay that is not apparent in the data.

The degree to which the model compares with data can be used to identify appropriate uses of the modeled hydrodynamics. For example, since the 

current "climate" is well represented in western Massachusetts Bay throughout the years, it is appropriate to use the model to study the near-continuous 

release of effluent in this region by the existing and future outfalls. Likewise, 

since individual events are well-represented in western Massachusetts Bay 

during unstratified conditions, the model is well suited to studying the transport of suspended material from this region during individual winter storms. 

On the other hand, predicting the detailed evolution of a low-salinity plume 

from a spring runoff event is not possible, at least partially due to inadequate 

information about the large scale forcing from the Gulf of Maine.

56
Chapter 4 

Effluent Modeling

Since the hydrodynamic model represents the stratification and statistics of 

low-frequency currents in the western part of Massachusetts Bay rather well, 

it is appropriate to use the model to investigate the transport of effluent 

from the existing and future outfalls over timescales of weeks to months. To 

model the effluent, a conservative tracer was added to the code, modeled the 

same as temperature and salinity, but with no dynamical effect. The effluent 

concentration was set to 100 at the Nut and Deer Island diffusers for the 

existing outfall run. For the future outfall run, the discharge from Nut and 

Deer Islands was added together and introduced at the future outfall, again 

with a concentration of 100. Along the open boundary, far from the sources, 

the effluent concentration was set to zero. Since the loading concentration is 

arbitrary, the effluent concentrations were converted to effluent dilutions for 

the purposes of presentation.

Since ECOM-si is a hydrostatic model with grid spacing of about 1 km in 

the vicinity of the future outfall, it cannot represent the physics of the turbulent entrainment process that occurs when the effluent is discharged from 

the diffusers. By designing the grid so that the size of the diffuser grid cell is 

comparable to the area of zone of initial dilution, however, it is possible to 

dilute the effluent by approximately the same amount as predicted by nearfield models such as EPA's ULINE. This allows ECOM-si to effectively model 

the height to which the plume rises (Blumberg et al., 1994). Ph.D work by 

Xue-Yong Zhang (in preparation) at MIT has shown that with the relatively 

strong pycnocline observed in Massachusetts Bay there is considerable latitude in choosing the size of the diffuser grid cell to produce approximately

57
the right trap height.

Effluent was discharged and tracked from the existing and future outfalls 

from October 1989 to September 1991, and the results have been presented in 

a variety of forms. Some animation clips comparing the two outfall locations 

during winter and summer periods can be seen on the World Wide Web 

at http://crusty.er.usgs.gov. The 200:1 dilution level is about the level at 

which the nutrient signal from the outfall should become lost in background 

variability, and thus is highlighted in presentations.

During the winter, highest effluent concentrations from both the existing 

and future outfalls occur at the surface, since the initially diluted effluent 

is lighter than the well-mixed, dense seawater (Figure 4.1). The existing 

outfall, due to the shallow depth and confines of Boston Harbor, results in a 

much larger region of poorly diluted effluent. There is very little difference 

in the size of the 200:1 dilution region between 1990 and 1991, although the 

size of the 800:1 dilution region changes considerably (Figures 4.1 and 4.2).

During the summer, the highest effluent concentrations from the existing 

outfall are again found at the surface, since the effluent is effectively discharge 

directly into the surface layer due to strong tidal mixing in Boston Harbor. 

The 200:1 dilution region is very similar to the winter region (Figure 4.3). 

At the future outfall, however, the effluent plume becomes trapped below 

the pynocline since the seawater near the surface has warmed and freshened 

to the point where it is lighter than the initially diluted effluent. At 16 m, 

the depth of maximum areal extent of the 200:1 effluent surface, the region 

affected by the future outfall is still considerably smaller than the surface 

extent of the 200:1 region from the existing outfall (Figure 4.4). Average 

dilution comparisons for other seasons are presented in Appendix C.

While the dispersion of effluent can be characterized by the plots of the 

average effluent dilution, a residence time for effluent in Massachusetts Bay 

and Boston Harbor can be determined by dividing the total amount of effluent 

in the bay or harbor by the rate of loading. This calculation of residence time 

assumes that both the input effluent and export from the system are relatively 

steady. The boundary of Massachusetts Bay was specified as a straight line 

between Cape Ann and Provincetown, and the boundary of Boston Harbor 

was specified as a straight line across the harbor mouth. The residence times 

for Massachusetts Bay are very similar for the existing and future outfall 

scenarios, and show a distinct seasonal cycle (Figure 4.5).

Overall, the residence time for effluent in Massachusetts Bay is from 40-

58
Mean Effluent Dilution: 1990/01/02-1990/03/05

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 240 260 280

km

300 320

Figure 4.1: Effluent comparison at 2 m depth during Winter 1990.

59
Mean Effluent Dilution: 1990/11/08-1991/03/12

EXISTING OUTFALL 

DEPTH = 2 m

220 240 260 280 300 320

220

FUTURE OUTFALL 

DEPTH = 2 m

300 320

Figure 4.2: Effluent comparison at 2 m depth during Winter 1991.

60
Mean Effluent Dilution: 1990/06/06-1990/09/07

EXISTING OUTFALL 

DEPTH = 2 m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 2 m

220 240 260 280 

km

300 320

Figure 4.3: Effluent comparison at 2 m depth during Summer 1990.

61
Mean Effluent Dilution: 1990/06/06-1990/09/07

EXISTING OUTFALL 

DEPTH = 16m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 280 300 320 

km

Figure 4.4: Effluent comparison at 16 m depth during Summer 1990.

62
120

100

o 80

>. 

a

3 60

40

20

10

9

& 8 

7

6

5

30

25

Residence Time of Effluent in Massachusetts Bay and Boston Harbor

Massachusetts Bay

Existing Outfal 

Future Outfall

Boston Harbor

15

Effluent Discharge Rate

Jan Jun 

1990

Nov Apr 

1991

Sep Feb 

1992

Jul

Figure 4.5: Residence time for effluent determined by dividing the total 

amount of effluent in a the bay or harbor by the effluent loading rate.

63
100 days, while the residence time in Boston Harbor is 5-9 days. The effluent 

has longer residence times in the winter and shorter residence times in the 

summer in both the harbor and the bay. Since the effluent discharge does not 

have a clear seasonal cycle, the change in residence time is presumably due to 

increased flushing. For the bay, the residence time calculated via this method 

changes over time scales that are comparable to or less than the residence 

time itself, indicating that an interpretation as total amount of effluent in the 

bay is more appropriate (since the loading is relatively constant). Effluent 

levels in the bay build during the fall and winter, reach a peak in March, 

then fall precipitously in April as the bay is flushed by energetic springtime 

currents. The lowest amounts of total effluent are found in summer, and as 

the water column destratifies, the effluent levels begin to climb again.

64
Chapter 5

Simulation of chlorination 

breakdown at the existing and 

future outfall locations

One of the design considerations for the future outfall site was that it should 

be more than one tidal excursion offshore so that in the event of a chlorination 

failure, untreated effluent would not be swept into Boston area beaches on 

the incoming tide. Signell et al. (1992) showed that even at spring tide, 

the future outfall location is well offshore of the region from which Boston 

Harbor draws water on flood tide. To quantify the expected benefit of the 

offshore locations, however, chlorination failures were simulated in ECOMsi and the bacteria concentrations were monitored at beaches throughout 

Massachusetts Bay. Chlorination failure was modeled as a breakdown that 

releases concentrations of 5,000,000 cells/100 ml for a period of 1 day. A 

e-folding decay timescale of 12 hours was used to simulate die-off of the 

bacteria.

Since currents are variable, chlorination failures were simulated every 10 

days over a one year period from October 1989 to October 1990, and the 

observed concentration levels were monitored at 15 different beaches and 

harbors along the coast (Table 5.1).

Two runs were conducted, one for the existing outfalls at Nut and Deer 

Islands, and one for the future outfall near the Boston Buoy. For each location, the number of events where the concentration (counts/100 ml) exceeded 

various values were calculated, and are shown in Tables 5.2 and 5.3. Events

65
Number Longitude Latitude Location

1

2

3

4

5

6

7

8

9

10

11

12

13

14

-70.6667

-70.8333

-70.9333

-70.9667

-71.0000

-70.8833

-70.7500

-70.7000

-70.6333

-70.6333

-70.5167

-70.3333

-70.0667

-70.2333

42.5833

42.4917

42.4333

42.3833

42.2750

42.3000

42.2333

42.1333

42.0167

41.9500

41.8000

41.7417

41.9000

42.0667

Glouchester Harbor

Marblehead Neck

Nahant Beach

Winthrop Beach

Wollaston Beach

Nantasket Beach

N. Scituate Beach

Humarock

Duxbury Beach

Plymouth Beach

Sagamore Beach

Sandy Neck

Great Island

Race Point

Table 5.1: Locations monitored in chlorination failure simulations.

were only counted if they occurred at least 5 days after the previous event. 

The future outfall location obviously has a large beneficial effect, as the 

the total number of events with concentrations greater than 5 counts/100 ml 

dropping from 136 to 6 and the number of events greater than 10 counts/100 

ml dropping from 130 to 0. The existing outfall caused 12 events greater 

than 1000 counts/100 ml, whereas the highest concentration for the future 

outfall was less than 10 counts/100 ml.

66
Counts/100 ml Nahant Winthrop Wollaston Nantasket N. Scituate 

__

>10

>25

>50

> 125

>250

>500

> 1000

>2500

>5000

19

15

7

4

0

0

0

0

0

0

35

33

24

20

11

6

2

0

0

0

37

37

37

37

37

30

23

12

2

0

33

31

28

19

9

7

3

0

0

0

7

4

2

1

0

0

0

0

0

0

Table 5.2: Bacteria concentrations for breakdown of existing plant. Note: 

Glouchester Harbor, Marblehead Neck, Humarock, Duxbury Beach, Plymouth Beach, Sagamore Beach, Sandy Neck, Great Island, and Race Point 

had no events that exceeded 5 counts/100 ml.

Counts/100 ml Nahant Winthrop Wollaston Nantasket N. Scituate

>5

> 10

>25

>50

> 125

>250

>500

> 1000

>2500

>5000

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

4

0

0

0

0

0

0

0

0

0

Table 5.3: Bacteria concentrations for breakdown of future plant.

67
Chapter 6 

Summary

Comparison of model results with observations indicated that ECOM-si simulations are appropriate for studying the continuous release of effluent in 

western Massachusetts Bay, as they reproduce the most important features 

of the observed stratification and current regime in this region. The seasonal 

development and breakdown of the thermocline occur within a few weeks of 

the correct time, the vertical structure has the right top-to-bottom density 

difference and about the right vertical profile (though the modeled pycnocline 

is not quite as sharp as reality), and the currents responsible for transporting 

material have about the right mean flow and level of variability. The fact 

that the best comparisons are found on the western side of Massachusetts 

Bay reflects the strong effect of local wind driving in this region, an effect 

that can be directly modeled rather than parameterized by boundary conditions. Further offshore, near Stellwagen Bank for example, the currents and 

water properties are largely determined by Gulf of Maine conditions, and 

the model does not compare as well. Although significant improvements to 

salinity boundary conditions were obtained by including an adjunct model 

to model the western Gulf of Maine, boundary conditions remain one of the 

largest roadblocks to obtaining more realistic simulations. In the future, 

improvement of boundary condition information may be obtained from a 

time-varying coarser scale model of the entire Gulf of Maine or possibly from 

data collected from sensors at strategic locations and assimilated into model 

runs.

The effluent simulations show that with either the existing or future outfall, the region of Massachusetts Bay that experiences relatively high effluent

68
concentration (1 part effluent: 200 parts sea water) is relatively small and 

confined to western Massachusetts Bay. The region of high concentration 

(1:50) that covers much of Boston Harbor with the existing outfall is completely eliminated with the future outfall scenario. This is consistent with 

the results of the two-dimensional modeling conducted as part of the outfall 

siting process (MWRA, 1988). The subsurface areal extent of the bottom 

trapped summertime plume is about the same size as the winter plume for 

the future outfall scenario. Although the initial vertical mixing of the effluent 

is confined to the lower half of column during the summer, this effect is offset by stronger currents and shears during the summer that act to increase 

dispersion. The total amount of effluent in the bay obtained from the future outfall is within 10% of that obtained from the existing outfall, and the 

time series track each other closely. Effluent levels build up over the winter, 

reaching their peak in March before being greatly reduced by the increased 

flushing of the system in April.

69
Appendix A

Model/Data Comparison of 

Time Series Velocity at the 

Boston Buoy

70
10

E 0

-10

MODEL: 23m

1 8 

Jan

1990

15 22 29 5 12 

Feb

19 26

Figure A.I: Boston Buoy velocity comparison: Jan 1, 1990 - Feb 26, 1990.

71
I 2 

1 0

! 2 U e.

Wind Stress at BB

Jfi

10

0

-10

DATA: 23 m

26 5 

Feb Mar 

1990

23

Figure A.2: Boston Buoy velocity comparison: Feb 26, 1990 - Apr 23, 1990.

72
28 4 11 

Jun

18

Figure A.3: Boston Buoy velocity comparison: Apr 23, 1990 - Jun 18, 1990.

73
10

0

10

DATA: 23 m

Figure A.4: Boston Buoy velocity comparison: Jun 18, 1990 - Aug 13, 1990.

74
Wind Stress at BB

1-2

10 TA: 5 m

o

-10

24 1 

Oct

Figure A.5: Boston Buoy velocity comparison: Aug 13, 1990 - Oct 8, 1990.

75
Figure A.6: Boston Buoy velocity comparison: Oct 8, 1990 - Dec 3, 1991,

76
10-

10 17 24 31 28

Figure A.7: Boston Buoy velocity comparison: Dec 3, 1990 - Jan 28, 1990.

77
25

Figure A.8: Boston Buoy velocity comparison: Jan 28, 1991 - Mar 25, 1991.

78
dynes/cm2 o to* 

Wind Stress at BB

.. kA/^^V i-jllmib i/. ... .... .. VJ///A A\Ut,UL. .11. A%^.\ - rvli. \l/^4 '/^» -\\UnlA& -

f/f W/\\ " ' ^ X^r'5^?7'^^ ^ "^^ nr -

25 1 

Mar Apr

1991

20

Figure A.9: Boston Buoy velocity comparison: Mar 25, 1991 - May 20, 1991.

79
CM

10

I 0

o

-10

MODEL: 23 m

20 

May 

1991

27 3 

Jun

10 17 24 1 

Jul

15

Figure A.10: Boston Buoy velocity comparison: May 20, 1991 - Jul 15, 1991

80
CM

10

-10

DATA: 23 m

\\i

Figure A.11: Boston Buoy velocity comparison: Jul 15, 1991 - Sep 9, 1991.

81
9 16 23 30

Figure A.12: Boston Buoy velocity comparison: Sep 9, 1991 - Nov 4, 1991.

82
-10-

30

Figure A. 13: Boston Buoy velocity comparison: Nov 4, 1991 - Dec 30, 1991.

83
10

I 0 

o

-10

DATA: 5 m

(0

o

-10-

10

CO

E 0

-10

MODEL: 23

30 6

Dec Jan

1991 1992

13 20 27 3 

Feb

10 17 24

Figure A.14: Boston Buoy velocity comparison: Dec 30, 1991 - Feb 24, 1992.

84
10

E 0 

o

-10

24

Feb

1992

2 

Mar

16 23 30 6 

Apr

13 20

Figure A. 15: Boston Buoy velocity comparison: Feb 24, 1992 - Apr 20, 1992.

85
10

I o

o

-10

10

I o

O

-10

DATA: 23 m

MODEL: 23 m

20 

Apr 

1992

27 4 

May

11 18 25 1 

Jun

15

Figure A.16: Boston Buoy velocity comparison: Apr 20, 1992 - Jun 15, 1992.
Wind Stress at BB

4^^

-10-

10

(I)

E 0 

o

-10

DATA: 23 m

CO

e o 0

-10

MODEL: 23 m

15 22 

Jun 

1992

29 6 

Jul

13 20 27 3 10 

Aug

Figure A. 17: Boston Buoy velocity comparison: Jun 15, 1992 - Aug 10, 1991.

87
10

-10

DATA: 23 m

-10-

10 17 24 31 7 

Aug Sep 

1992

14 21 28 5 

Oct

Figure A. 18: Boston Buoy velocity comparison: Aug 10, 1992 - Oct 5, 1992.
10

I 0 

o

-10

DATA: 23 m

30

Figure A.19: Boston Buoy velocity comparison: Oct 5, 1992 - Nov 30, 1992.

89
Appendix B

Mean Currents by Season

90
Winter 1989-1990:1990/01/01 -1990/03/01

200

200 220 240 260 280 300 320 340

200

180

160

80

200 220 240 260 280 300 320 340

km

Figure B.I: Modeled mean flow at 2 and 20 m depth for Winter 1989-1990.

91
Spring 1990: 1990/03/01-1990/06/01

200 220 240 260 280 300 320 340

200 220 240 260 280 300 320 340

km

Figure B.2: Modeled mean flow at 2 and 20 m depth for Spring 1990.

92
Summer 1990:1990/06/01-1990/09/01

200

200 220 240 260 280 300 320 340

200

ISO160

140

120-

100-

80-

200 220 240 260 280 300 320 340

km

Figure B.3: Modeled mean flow at 2 and 20 m depth for Summer 1990.

93
Fall 1990:1990/09/01 -1990/11 /01

200 220 240 260 280 300 320 340

80

200 220 240 260 280 300 320 340

km

Figure B.4: Modeled mean flow at 2 and 20 m depth for Fall 1990.

94
Winter 1990-1991:1990/11701 -1991/03/01

200

180

80

200 220 240 260 280 300 320 340

/ Lower Layer (25 m)

\\\ \ ^-v v-\> \ \

^->W\\

200 220 240 260 280 300 320 340

km

Figure B.5: Modeled mean flow at 2 and 20 m depth for Winter 1990-1991.

95
Spring 1991:1991703/01 -1991706/01

200

180

160

140

120

100

200 220 240 260 280 300 320 340

200 220 240 260 280 300 320 340

km

Figure B.6: Modeled mean flow at 2 and 20 m depth for Spring 1991.

96
Summer 1991:1991/06/01-1991/09/01

200 220 240 260 280 300 320 340

200

180-

80

200 220 240 260 280 300 320 340

km

Figure B.7: Modeled mean flow at 2 and 20 m depth for Summer 1991.

97
Fall 1991:1991/09/01-1991/11/01

200

180

160

140-

120

100

200 220 240 260 280 300 320 340

200

180

160-

140

/

Lower Layer (25 m)

A K\ '

120

100

200 220 240 260 280 300 320 340

km

Figure B.8: Modeled mean flow at 2 and 20 m depth for Fall 1991.

98
Winter 1991-1992:1991/11/01-1992/03/01

200

200

180 J

200 220 240 260 280 300 320 340

200 220 240 260 280 300 320 340

km

Figure B.9: Modeled mean flow at 2 and 20 m depth for Winter 1991-1992.

99
200

Spring 1992:1992/03/01-1992/06/01

200 220 240 260 280 300 320 340

80

200 220 240 260 280 300 320 340

km

Figure B.IO: Modeled mean flow at 2 and 20 m depth for Spring 1992.

100
Summer 1992:1992/06/01-1992/09/01

200

200 220 240 260 280 300 320 340

80

200 220 240 260 280 300 320 340

km

Figure B.ll: Modeled mean flow at 2 and 20 m depth for Summer 1992.

101
Fall 1992:1992/09/01 -1992/11 /01

200

180-

80-

200

200 220 240 260 280 300 320 340

100

80-

200 220 240 260 280 300 320 340

km

Figure B.12: Modeled mean flow at 2 and 20 m depth for Fall 1992.

102
Winter 1992: 1992/11/01-1992/12/31

200 220 240 260 280 300 320 340

120-

100-

80

200 220 240 260 280 300 320 340

km

Figure B.13: Modeled mean flow at 2 and 20 m depth for Winter 1992.

103
Appendix C

Comparative Effluent Dilution 

Simulations for the Existing 

and Future Outfalls

104
Mean Effluent Dilution: 1990/01/02-1990/03/05

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 240 260 280 300 320 

km

Figure C.I: Effluent comparison during Winter 1989-90 at 2 m depth. Dilution contours of 200, 400 and 800 are shown. Areas where the dilution is 

less than 200 are shaded in grey. The 40 m depth contour around Stellwagen 

Bank is indicated by the dotted line.

105
Mean Effluent Dilution: 1990/01/02-1990/03/05

320

EXISTING OUTFALL 

DEPTH = 16m

e

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 280 300 320 

Figure C.2: Effluent comparison during Winter 1989-90 at 16 m depth.

106
Mean Effluent Dilution: 1990/03/05-1990/06/06

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 300 320

Figure C.3: Effluent comparison during Spring 1990 at 2 m depth.

107
Mean Effluent Dilution: 1990/03/05-1990/06/06

EXISTING OUTFALL 

DEPTH = 16m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 280 300 320 

km

Figure C.4: Effluent comparison during Spring 1990 at 16 m depth.

108
Mean Effluent Dilution: 1990/06/06-1990/09/07

EXISTING OUTFALL 

DEPTH = 2 m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 2 m

220 300 320

Figure C.5: Effluent comparison during Summer 1990 at 2 m depth.

109
Mean Effluent Dilution: 1990/06/06-1990/09/07

EXISTING OUTFALL 

DEPTH = 16m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 280 300 320 

km

Figure C.6: Effluent comparison during Summer 1990 at 16 m depth.

110
Mean Effluent Dilution: 1990/09/07-1990/11/08

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 240 260 280 

km

300 320

Figure C.7: Effluent comparison during Fall 1990 at 2 m depth.

Ill
Mean Effluent Dilution: 1990/09/07-1990/11/08

EXISTING OUTFALL

DEPTH = 16m

FUTURE OUTFALL 

DEPTH = 16m

320

220 240 260 280

km

300 320

Figure C.8: Effluent comparison during Fall 1990 at 16 m depth.

112
Mean Effluent Dilution: 1990/11/08-1991/03/12

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 320

Figure C.9: Effluent comparison during Winter 1990-91 at 2 m depth.

113
Mean Effluent Dilution: 1990/11/08-1991/03/12

EXISTING OUTFALL 

DEPTH = 16 m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 280 

km

300 320

Figure C.IO: Effluent comparison during Winter 1990-1991 at 16 m depth.

114
Mean Effluent Dilution: 1991/03/12-1991/06/14

EXISTING OUTFALL 

DEPTH = 2 m

FUTURE OUTFALL 

DEPTH = 2 m

320

220 240 260 280

km

300 320

Figure C.ll: Effluent comparison during Spring 1991 at 2 m depth.

115
Mean Effluent Dilution: 1991/03/12-1991/06/14

EXISTING OUTFALL 

DEPTH = 16m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 16m

220 240 260 

km

280 300 320

Figure C.12: Effluent comparison during Spring 1991 at 16 m depth.

116
Mean Effluent Dilution: 1991/06/14-1991/09/15

EXISTING OUTFALL 

DEPTH = 2 m

220 240 260 280 300 320

FUTURE OUTFALL 

DEPTH = 2 m

220 240 260 280 

km

300 320

Figure C.13: Effluent comparison during Summer 1991 at 2 m depth.

117
Mean Effluent Dilution: 1991/06/14-1991/09/15

EXISTING OUTFALL 

DEPTH = 16m

FUTURE OUTFALL 

DEPTH = 16m

220

320

320

Figure C.14: Effluent comparison during Summer 1991 at 16 m depth.

118
Appendix D 

References

REFERENCES

Adams, E., R. Kossik, D. J. Cosier, J. K. MacFarlane, and P. M. Gschwend, 

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Modeling, edited by M. L. Spaulding, pp. 380-389. American Society of Civil 

Engineers, ASCE press, New York, 1990.

Agency, E. P., Assessment of Potential Impact of the MWRA Outfall on Endangered Species, Technical report, EPA Region 1, JFK Federal Building, Boston, 

Massachusetts 02203, 1993.

Authority, M. W. R., Final Secondary Treatment Facilities Plan, Technical report, 

MWRA, 1988.

Beckmann, A. and D. B. Haidvogel, Numerical Simulation of Flow around a Tall 

Isolated Seamount. Part I: Problem Formulation and Model Accuracy, J. Phys. 

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Blumberg, A., Z. Ji, and C. K. Ziegler, Confirmation of the Near Field Behavior from a far field circulation model of Massachusetts Bay, Technical report, 

Massachusetts Water Resources Authority, 1994. 25 pp.

Blumberg, A., R. Signell, and H. Jenter, Modeling Transport Processes in the 

Coastal Ocean, Journal of Environmental Engineering, 1, 31-52, 1993.

Blumberg, A. F. and L. H. Kantha, Open Boundary Condition for Circulation 

Models, J. Hydraulic Engineering, 111, 273-255, 1985.

Blumberg, A. F. and G. L. Mellor, A description of a three-dimensional coastal 

model, pp. 1-16, Coastal and Estuarine Sciences, Vol. 4. AGU, Washington, 

DC, 1987.

Bodgen, P., P. Malanotte-Rizzoli, and R. P. Signell, Open-ocean boundary conditions from interior data: Local and remote forcing of Massachusetts Bay, J.

119
Geophys. Res., C12, submitted, 1995.

Casulli, V., Semi-implicit finite difference methods for the two-dimensional shallow 

water equations, J. Computational Physics, 86, 56-74, 1990.

Drinkwater, K., AFAP climate studies in the Scotia-Fundy Region, Technical Report 92/72, N2126, NAFO SCR, 1992. 8 pp.

Galperin, B., L. H. Kantha, S. Hassid, and A. Rosati, A Quasi-equilibrium turbulent energy model for geophysical flows, J. Atmosph. Set., ^5, 55-62, 1988.

Geyer, W., G. Gardner, W. Brown, J. Irish, B. Butman, T. Loder, and R. Signell, 

Physical Oceanographic Investigation of Massachusetts and Cape Cod Bays, 

Technical Report MBP-92-03, Massachusetts Bays Program, U.S. EPA Region I/Massachusetts Coastal Zone Management Office, Boston, Massachusetts, 

1992. 497 pp.

Geyer, W. R. and J. Ledwell, Massachusetts Bay dye study, 1994. Submitted to 

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