Surface-water quality and flow Modeling Interest Group
1 U.S. Geological Survey, Woods Hole, MA, U.S.A.
2 U.S. Geological Survey, Reston, VA, U.S.A.
3 HydroQual, Inc, Mahwah, NJ, U.S.A.
Please direct correspondence to:
Richard P. Signell
USGS, Woods Hole Field Center
384 Woods Hole Road
Woods Hole, MA 02543-1598
Internet: rsignell@usgs.gov
Phone: (508) 457-2229
FAX: (508) 457-2309
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Citation:
Signell, R.P., Jenter, H.L., and Blumberg, A.F., 2000, Predicting the physical
effects of relocating Boston's sewage outfall, J. Estuarine, Coastal and
Shelf Sci., 50, 59-72.
Abstract
Introduction
Environmental Setting
Boston Harbor
Massachusetts Bay
Numerical Model
Model Results
Model-Data comparison
Tidal Currents
Subtidal Currents
Seasonal Stratification
Effluent Simulations
Winter Effluent Simulations
Summer Effluent Simulations
Effect of Outfall Relocation on Salinity and Velocity
Fields
Discussion
Conclusions
Acknowledgements
ReferencesThe simulations predict that the new bay outfall will greatly decrease effluent concentrations in Boston Harbor (relative to the harbor outfall) and will not significantly change mean effluent concentrations over most of Massachusetts Bay. With the harbor outfall, previous observations and these simulations show that effluent concentrations exceed 0.5% throughout the harbor, with a harbor wide average of 1-2%. With the bay outfall, effluent concentrations exceed 0.5% only within a few km of the new outfall, and harbor concentrations drop to 0.1-0.2%, a ten-fold reduction. During unstratified winter conditions, the local increase in effluent concentration at the bay outfall site is predicted to exist throughout the water column. During stratified summer conditions, however, effluent released at the seabed rises and is trapped beneath the pycnocline. The local increase in effluent concentration is limited to the lower layer, and as a result, surface layer effluent concentrations in the vicinity of the new outfall site are predicted to decrease (relative to the harbor outfall) during the summer.
Slight changes are predicted for the salinity and circulation fields. Removing the freshwater associated with the effluent discharge in Boston Harbor is predicted to increase the mean salinity of the harbor by 0.5 psu and decrease the mean salinity by 0.10-0.15 psu within 2-3 km of the outfall. Relative to the existing mean flow, the buoyant discharge at the new outfall is predicted to generate density-driven mean currents of 2-4 cm/s that spiral out in a clockwise motion at the surface during winter and at the pycnocline (15-20 m depth) during summer. Compensating counterclockwise currents are predicted to spiral in toward the source at the bottom. Because the scale of the residual current structure induced by the new discharge is comparable to or smaller than typical subtidal water parcel excursions, Lagrangian trajectories will not follow the Eulerian residual flow. Thus, mean currents measured from moorings within 5 km of the bay outfall site will be more useful for model comparison than to indicate net transport pathways.
The Bay Outfall location was selected primarily on the basis of 2D depth-averaged modeling studies (Walton et al, 1990). These studies determined that the site would result in dramatically lower effluent levels in Boston Harbor and would lead to greater dilution of effluent compared to other potential inshore sites. Because the receiving waters of Massachusetts Bay are strongly stratified during the summer months and the new outfall diffuser was designed to trap effluent beneath the seasonal pycnocline, there was a clear need for further studies that could test and refine the predictions of the 2D models.
The U.S. Geological Survey (USGS), as part of the National Coastal and Marine Geology Program, has implemented and refined a 3-D hydrodynamic model for Massachusetts Bay to be used in polluted sediment transport studies. Through a cooperative agreement with the Massachusetts Water Resources Authority (MWRA), the USGS has provided hydrodynamic model results that were used for input to a 22 component water quality model (HydroQual and Normandeau, 1995). Model assessment was performed primarily from January 1, 1990 to July 1, 1991, a period when intensive moored and shipboard information were obtained under funding from the Massachusetts Bays Program (Geyer et al, 1992). Simulations with existing and future outfall locations were run and compared over a three-year period (January 1, 1990 to January 1, 1993). This work predicts the influence of the new outfall location on effluent concentrations, salinity and circulation in Boston Harbor and Massachusetts Bay.
Movie 1 [1.7 Mbytes, using Indeo 5 compression]
Circulation in Massachusetts Bay is controlled by the large-scale circulation in the Gulf of Maine, local wind forcing, and intrusions of low salinity water associated with Gulf of Maine rivers. The observations of Geyer et al. (1992) reveal a mean annual flow in the surface waters of Massachusetts Bay (Figure 2) that is consistent with earlier drift bottle measurements of Bigelow (1927) and Bumpus & Lauzier (1965). The Maine coastal current flows south with typical speeds of 5-15 cm/s along the Maine and New Hampshire coasts (Normandeau Associates, 1975; Vermersch et al., 1979). When it reaches Cape Ann there is a branch point (Blumberg et al., 1993; Lynch et al., 1997) and much of the flow follows the topography southward past Stellwagen Bank and to the east of Cape Cod. A weaker branch of this current (2-5 cm/s) flows into Massachusetts Bay around Cape Ann, southward along the western shore of Massachusetts Bay, and flows out of the Bay at Race Point (Bumpus, 1973; Geyer et al., 1993). The mean circulation pattern is often altered by wind and runoff events and, as evident by the variability ellipses shown in Figure 2, the sub-tidal fluctuations are typically stronger than the mean, except at stations U2 and RP. The Bay Outfall (near station BB) is in a region of very weak mean flow west of the persistent southward current.
There is seasonal variation in stratification in Massachusetts Bay, with well-mixed conditions during winter and strong stratification during summer (Geyer et al., 1992). The pycnocline is generally found at 15-20 m depth during the summer, with typical density differences of 3-6 kg/m3. The stratification strongly influences both water properties and dynamics, greatly reducing the vertical exchange between surface and bottom waters and isolating the bottom layer from the direct influence of wind stress and river runoff.
The seasonal variations in stratification, wind stress, and river discharge change the nature of transport and dispersion processes in the bay over the course of the year. During the winter, generally northerly wind events drive basin-scale flows that enhance the counter clockwise circulation, southward flow along the western shore of Massachusetts Bay and northward flow against the wind in the deeper central regions of the bay (Butman, 1975; Brickley, 1994). In the spring, shallow (5-15 m) fresh plumes associated with river runoff in the Gulf of Maine intrude into the bay, commonly generating strong currents of 20-30 cm/s and flow structures with 10-30 km spatial scales (Butman, 1976; Lee, 1992). As the summer progresses, stratification builds, and the wind response of the surface layer is more efficient due to reduced friction. The surface currents at the new outfall location, for example, are strongest during summer, when the wind stress is weakest (Blumberg et al, 1993). The southwesterly winds that frequently occur during the summer result in upwelling along the western and northern coast of the bay, adding additional variability to the density field and density-driven circulation. During September and October, the mean currents along the western shore reverse and flow northward as the result of strong cooling that occurs near the coast (Geyer et al, 1992).
There are twelve vertical layers in the model, with three layers in the top 10% of the water column and the remaining nine layers evenly distributed over the remaining 90% of the water column (runs with increased vertical resolution did not qualitatively change results). The model domain extends well offshore of Massachusetts Bay to facilitate exchange with the Gulf of Maine, and extends to the north to include the Merrimack River, a major source of fresh water to Massachusetts Bay.
Elevation on the open boundaries is specified by a radiation condition with a restorative term that allows the boundary to relax back to specified elevation conditions (Blumberg and Kantha, 1985). In the model runs described here, a very small relaxation time is chosen along the eastern boundary, resulting in essentially a "clamped" condition. The elevation is specified along this boundary as a combination of M2 tides obtained from a Gulf of Maine tidal model (Lynch & Naimie, 1993) and subtidal sea level fluctuations obtained from a western Gulf of Maine model (Signell et al, 1996). Along the southern open boundary off Cape Cod, a gravity wave radiation condition is applied to allow traveling wave energy to exit from the domain.
Salinity and temperature on the open boundary are specified by advecting interior values out on outflow, and relaxing over a period of several days toward specified values on inflow. The values on inflow are specified by the western Gulf of Maine model along the northern part of the eastern boundary, and by climatology along the rest of the open boundary.
Heat flux and wind stress are specified every 4 hours from meteorological data obtained at the National Weather Service Buoy near the site of the proposed outfall in western Massachusetts Bay (Figure 1). The heat flux components are calculated using techniques described by Weller et al. (1995). Observations are used for all variables except sea surface temperature, which is obtained from the model. Using the model sea surface temperature allows strong spatial gradients, such as those that occur in upwelling areas, to affect the heat flux in a realistic manner. It also provides a feedback mechanism to prevent very shallow water from becoming too warm. This formulation provides improved comparisons with time series temperature measurements, especially in Cape Cod Bay (Signell et al., 1996). Daily river discharges and effluent discharges are specified with data obtained from the USGS and the MWRA.
Sewage effluent is simulated as a conservative tracer, introduced at the grid cells that correspond to the appropriate outfall locations. For the Harbor Outfall simulations, effluent is released at two locations within the harbor corresponding to the main discharges. For the Bay Outfall simulations, the total amount of effluent discharge measured at the Deer and Nut Island plants is released at the offshore location of the new outfall. The effluent is released in the lowest grid cell, but rises due to positive buoyancy. The model grid cell into which the effluent is discharged is approximately 1 km x 1 km, while in reality the effluent will be discharged as a 2 km long line source (through a 55-riser-pipe diffuser system). The relatively coarse grid and hydrostatic formulation of the large scale model does not allow explicit simulation of the small-scale convective mixing processes that occur as less dense water ascends through the water column. Blumberg et al (1996), however, found that the trap height of the plume simulated in the model compared closely with predictions from small-scale buoyant plume models. Zhang et al (1999) attributed the success to three factors: (1) the total dilution is partly due to large scale density exchange flow that the far field model can resolve; (2) the strong pycnocline provides a natural ceiling for the plume; (3) there is beneficial feedback such that if the entrainment is overpredicted, the trap height will be underpredicted and less total dilution will occur. Obtaining the correct trap height is important, since horizontal mixing and transport processes vary greatly with depth during the stratified season.
Qualitatively, the model agrees with the observed subtidal circulation during both stratified and unstratified conditions (Figures 5a and 5b). The counterclockwise mean flow through most of the bay, the weak mean flow at the Bay Outfall site, and the structure of the observed current variability in western Massachusetts Bay are all basic features of the system that are represented by the model. Quantitatively, however, the agreement between data and model varies substantially with season and region within the bay. During winter, relatively high correlation (0.6-0.8) between data and model time series of subtidal currents is found in western Massaschusetts Bay, with significantly lower correlation (0.4 or less) at stations U7 in central Cape Cod Bay and at the stations U2, U6 and RP near Stellwagen Bank. During summer, correlation between data and model is lower, never exceeding 0.6. The correlation between data and model currents tends to track the correlation between wind stress and observed current, indicating that the model can represent the timing of locally generated wind events. Due to the semi-enclosed, shallow nature of western Massachusetts Bay and the dominance of wind-driven events during winter, the model-data correlation during winter tends to be high. In the summer and in other regions of the bay, the response is more complex, as remote wind events and density driven events associated with surface trapped river plumes and upwelling play a larger role in the subtidal current response. Small errors in the temporal and spatial phasing of these events destroy the direct correlation between data and model (Signell et al, 1993; Signell et al, 1996). This indicates that the model should not be used to predict the event scale behavior of effluent plumes, except perhaps during strong winter wind events.
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 provide a reasonable representation of the "climate" of the currents over a few weeks or months. If the "climate" of current variability is reasonable, the model should provide a useful prediction of the average plume behavior, or the mean effluent concentration field. In western Massachusetts Bay, the errors in the mean flow are typically less than 3 cm/s and the errors in current fluctuation intensity are less than 40%. More substantial errors in the current climate exist at the other locations. In particular, both the mean and variability in the summer surface flow at stations U2, SC and U7 are underrepresented by the model, suggesting that the counterclockwise flow through the bay was more active than simulated during this season. The error in the mean flow at RP is attributable to a model-data discrepancy in the location of a tide-induced residual eddy in this region.
Movie 2 [2.6 Mbytes, using Indeo 5 compression]
To more fully assess the predicted change in effluent concentration caused by relocation of the outfall from the harbor to the bay, we compute the mean concentration field over all the winter months of the three-year simulation the Harbor and Bay Outfall scenarios. Comparing horizontal sections of mean winter effluent concentration, we see that when effluent is discharged from the Harbor Outfalls, surface concentrations are relatively high in Boston Harbor, decrease rapidly with distance from the Harbor entrance, and are low throughout most of Massachusetts Bay (Movie 3, upper left panel). The mean effluent concentration has a maximum of about 2% (1 part effluent to 50 parts seawater) in Boston Harbor, and decreases rapidly outside the harbor mouth to a level of about 0.15% at the location of the Bay Outfall. The effluent is further diluted and transported toward the southeast with the mean flow, resulting in effluent concentrations of about 0.125% in Cape Cod Bay (1 part effluent to 800 parts seawater). For the Harbor Outfall simulation, the 0.5% level is exceeded throughout Boston Harbor and extends 10 km to the southeast along the shore. A vertical section from Boston Harbor to Cape Cod shows that the effluent is mostly well mixed from top to bottom, with some slight stratification in western Massachusetts Bay (Figure 7a).
When effluent is discharged from the Bay Outfall, the buoyant effluent rises to the surface through the nearly uniform density water column and is then dispersed, vertically mixed, and eventually transported toward the southeast (Movie 3, upper right panel; Figure 7b). Away from western Massachusetts Bay, the levels are similar to the Harbor Outfall simulation. The effluent concentration exceeds 0.5% only within 2-3 km from the outfall, while the effluent concentration is less than 0.25% throughout Boston Harbor. Concentrations in Cape Cod Bay are about 0.125%, similar to the Harbor Outfall simulation.
Movie 4 [2.5 Mbytes, using Indeo 5 compression]
Examining the mean effluent concentrations over the summer months, we find that for effluent released from the Harbor Outfalls, concentrations are high in the harbor, decrease rapidly offshore and are low throughout most of Massachusetts Bay (Movie 3, lower left panel). The maximum concentration in the Harbor is about 1.6%, slightly less than the 2% levels found during winter. When the effluent leaves the harbor during the summer, however, it remains in the surface layer, and therefore spreads more effectively across isobaths toward the east. As a result, effluent levels in excess of 0.25% are found in the surface waters directly above the location of the Bay Outfall (Figure 8a).
When effluent is discharged from the Bay Outfall during summer, the buoyant effluent rises to the base of the pycnocline (about 15 m from the surface) and then spreads laterally in the lower layer (Movie 3, lower right panel). Effluent levels at the surface are small, slightly exceeding 0.125% in Boston Harbor. The peak effluent level is about 1.4% at the outfall site, greater than the maximum winter level because stratification confines the effluent to the lower layer. Concentrations in the lower layer decrease rapidly away from the outfall site, with levels exceeding 0.5% again found only within 2-3 km of the outfall. In Cape Cod Bay, the Bay Outfall results in slightly higher effluent levels below the pycnocline, as the Harbor Outfall effluent is mostly trapped in the upper layer (Figure 8b). These effluent levels are still relatively low (<0.125%), however, comparable to winter levels in the lower layer.
Summarizing the winter and summer effluent results, the simulations indicate that the bay outfall location will dramatically reduce effluent concentrations in Boston Harbor and have a small impact on effluent concentrations in most of Massachusetts Bay. During each season, there is a greater than 10-fold reduction in the harbor averaged effluent concentration (Table 1).
During the summer, effluent from the Bay Outfall induces a circulation at the pycnocline that is similar to the induced near-surface winter circulation (Figure 10b). The influence of this outfall-induced circulation is even more noticeable in the mean flow at the pycnocline (Figure 10a), since the lower layer flow is less strongly driven by surface forcing.
During the summer stratified months, there is an additional benefit to the Bay Outfall location if the goal is to reduce the impact of anthropogenic nutrient loading. The effluent from the present Harbor Outfalls exits Boston Harbor in the lighter surface layer, discharging nutrients into the photic zone in western Massachusetts Bay. Effluent from the Bay Outfall released at approximately 30 m depth displays dramatically different behavior: it rises to the base of the pycnocline (about 15 m from the surface) and is trapped, spreading out below the pycnocline. This was a design consideration of the new outfall: the waters below the pycnocline have relatively high nutrient levels and are largely aphotic, thus it was determined that the nutrients in the discharged effluent would have less impact on the ecosystem if the plume received enough initial dilution to be trapped in the lower layer. This anticipated benefit is supported by the water quality modeling studies by HydroQual & Normandeau (1995) who used the hydrodynamic results presented herein as input. As the effluent from the new outfall is trapped below the pycnocline, surface layer values are very small. The level of nutrients in the surface layer in western Massachusetts Bay actually decreases with the new outfall location, even directly over the new outfall site.
The new discharge location not only influences the effluent concentrations, but has an effect on the salinity and circulation fields since it represents relocation of 21 m3/s of fresh water, approximately half the quantity that currently enters Boston Harbor. During winter months, the salinity in Boston Harbor is predicted to increase by about 0.5 psu when the outfall is moved offshore, and the near-surface salinity decreases by about 0.15 psu within a few km of the Bay Outfall location. That the salinity effect on Boston Harbor is larger than at the Bay Outfall site is consistent with the effluent dilution results: there is more dilution of fresh water and effluent at the Bay Outfall site. Increasing the salinity of Boston Harbor reduces the offshore gradient in salinity, thereby reducing the small but identifiable estuarine circulation between Boston Harbor and western Massachusetts Bay. Tidal flushing and wind-driven currents, however, dominate Boston Harbor exchange (Signell & Butman, 1992). Thus the reduction in estuarine exchange has little significance. Stronger circulation changes are seen in the vicinity of the new discharge, where the buoyant source creates a local mounding of fresh water. The mean surface currents spiral outward in a clockwise fashion, while the entrained water at the bottom spirals inward in a counterclockwise fashion, showing the influence of the Earth's rotation. The scale of this mean flow is comparable to the typical daily excursions of water parcels driven by wind and tide-driven currents, thus the mean flow pattern is not expected to indicate water parcel pathways (Zimmerman, 1979; Signell & Butman, 1992). Nonetheless, the small scale predicted mean flow modification due to introduction of freshwater at the new outfall would confound efforts to use Eulerian current measurements at the site to determine the general direction of effluent transport.
Having predicted changes in the effluent concentration, salinity and velocity fields due to the new outfall, how much confidence can we have in the predictions? What are the error bars? These are difficult questions to address quantitatively. Certainly there is room for improvement in the match between modeled and observed hydrodynamics. Boundary conditions are a major source of error, and improving them through larger scale simulations, additional measurements, or data assimilation would lead to more accurate circulation in the bay. Bogden et al (1996), for example, showed that the model skill could be improved 50% during certain events by utilizing current meter data within the Bay to improve the boundary conditions. Also, comparison of the transfer function between low-frequency wind stress and 5 m currents in western Massachusetts Bay indicates that the modeled current response at 5 m is 30% stronger than observed, a condition that could possibly be improved by better wind stress determination or by better representation of near-surface mixing processes.
While a closer match to the observed hydrodynamics could plausibly be made with a better model, it is unclear how much these improvements would improve the simulated effluent concentrations. An attempt to simply relate the observed level of subtidal current variability at the Bay Outfall site with simulated plume size, for example, was not successful. The difficulty is that the processes that determine the dispersal of effluent are complex. Movie 5 shows the effluent and velocity fields from the Bay Outfall simulation at 5 m during 45 days of winter. The simulated winter flow field contains many features with spatial scales of order 10 km. It appears that the dispersion of the plume is strongly affected by horizontal shear dispersion operating at this scale. Determining the relationship between the hydrodynamics and the large-scale dispersion processes in this type of complex system remains an important research topic. If this relationship was determined, we might be able to estimate the error in our effluent concentrations predictions based on the model-data mismatch in hydrodynamics. We might also find, however, that our moored array was not sufficient to measure the dispersive characteristics.
Although a quantitative assessment of error is beyond our current level of understanding, there is reason to have qualitative confidence in the predicted results. As mentioned previously, the model does characterize the basic nature of circulation and stratification in the system and yields predictions of effluent from the harbor outfall that are consistent with observations. A quantitative assessment of the model performance will be conducted after the bay outfall comes on line, utilizing new model runs and data collected by special plume tracking studies.
While challenges remain for obtaining more realistic simulations, the model in its present state has been an extremely useful tool for both scientists and managers. Visualization of the model results, particularly in the form of effluent concentration movies, has been an effective means for communicating results to managers and the interested public. Future work will include exploring the relationship between hydrodynamics and plume dispersion and testing the model predictions once the bay outfall comes on line.
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