Environmental Science in the Coastal Zone Issues for Further Research (1994) / Chapter Skim
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3 Modeling Transport Processes in the Coastal Ocean by Alan F. Blumberg...
3 Modeling Transport Processes in the Coastal Ocean by Alan F. Blumberg...
Pages 20-43
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From page 20... ...
The eventual impact of these discharges is the result of complex interactions among the pollutant inputs from all sources; the various chemical forms of the constituents present in the water column and their associated chemical reactions with each other; and the complex marine food chains, which can exchange nutrients and other chemicals between He water column and the underlying sediment. To understand all of these interactions, it is necessary to define the hydrodynamic transport processes governing the movement and mixing of the constituents as Hey are forced by various hydrographical (runoff, estuarine circulation)
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From page 21... ...
Before the presentation of the Massachusetts Bay case study, a short discussion of numerical models themselves will be provided, briefly reviewing where coastal-ocean circulation modeling is today and describing the physically comprehensive circulation model developed by Blumberg and Mellor (1980) which will be used to elucidate the various transport processes in Massachusetts Bay.
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From page 22... ...
Currently, there is considerable controversy concerning the extension of the Boston sewage outfall pipe from the mouth of Boston Harbor to a site 9 miles offshore. The public living around the coast of Massachusetts Bay and Cape Cod Bay is concerned that Boston is improving its pollution problem at the expense of the bays and that swimming beaches, shellfish beds, fishing resources, and the endangered right whale population that feeds in the bays may be jeopardized.
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From page 23... ...
~ ,CO~ l:SY | 23 43°00 N 42°00' FIGURE 3.1 Bathymetric mapshowing Massachusetts and Cape Cod Bays, present sewage outfalls in Boston Harbor (solid triangles) , location of new ocean outfall for treated Boston sewage in western Massachusetts Bay (average flow about 20 m3/s)
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From page 24... ...
and may be important locally at the tip of Cape Cod and along the western , but they have little impact on transporting material over distances comparable side of Stellwagen Bank to the size of the bays. Subtidal Currents Observations suggest that horizontal transport of material is accomplished by advection due to the mean flow through the bays and the dispersive effect of subtidal wind-driven and river-runoff events.
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From page 25... ...
The grid spacing in Me vicinity of the proposed outfall is roughly 1000 m. There are currently 10 vertical o-levels in the model, evenly spaced throughout Me water column.
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From page 26... ...
Shown are tidal ellipses, which indicate the observed velocities over the tidal cycle. They also represent the excursions water parcels would make if they moved with the tidal currents observed at the mooring.
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From page 27... ...
27 60 cm/s 54 48 42 36 30 24 18 12 6 cm/s C a ~ :3 FIGURE 3.4 Maximum tidal-current speed at the water surface off the tip of Cape Cod and in the entrances of smaller embayments such as Boston and Plymouth Harbors.
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From page 28... ...
Adding the mean wind stress of 1 dyne/cm2 to the coastal current forcing dramatically changes the simulation of the mean flow. The mean wind is from the west and drives a realistic, simulated southeastward current along the coast from Boston to Cape Cod, which exits the bays at Race Point (Figure 3.7)
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From page 29... ...
..'...,' ~51.15:,:.\ ,.... U2 it, All Data 4-8 m it, -71 .0 -70.8 -70.6 -70.4 -70.2 -70.0 Longitude 29 FIGURE 3.5 Map showing the mean flow (solid arrow)
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From page 30... ...
girl Environmental Science in the Coastal Zone Memmack River \d Boston 10 km 1 1 10 cm/s PIvmouth C a ~ Sea-surface elevation (cm) 3.~5 2.61 2.06 1.52 0.98 0.44 -0.11 -0.65 -1.19 - 1.73 FIGURE 3.6 Modeled surface current and elevation response to the Gulf of Maine coastal current.
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From page 31... ...
Modeling Transport Processes in the Coastal Ocean ;~r Merrimack ~ River `_' Sea-surface elevation (cm) 3.60 10 km I_ 10 cm/s C a ~ 31 3.04 1.93 0.81 0.26 -0.86 - 1.4Z .~ FIGURE 3.7 Modeled surface current and elevation response to the Gulf of Maine coastal current and Me mean wind stress.
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From page 32... ...
This along-bay wind drives strong flow downwind in the shallow water near the coast. The convergence of surface water along the northern shore of Cape Cod indicates the presence of strong downwelling.
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From page 33... ...
33 Cal~ ·.
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From page 34... ...
in well-mixed conditions. The cross-bay wind drives downwind surface flow of nearly the same magnitude over most of the Massachusetts Bays.
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From page 35... ...
Wind and river effects often interact nonlinearly, generating extremely complex flow patterns, even for fairly simple forcing functions. As an example, turning on the Merrimack River under the influence of a mean wind from the west gives rise to lateral and vertical salinity gradients that act with the wind and local topography to yield small-scale lateral eddying structures in western Massachusetts Bay that change markedly with time and position in the water column (Figure 3.14)
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From page 36... ...
b) Mean daily averaged wind stress amplitude at the Large Navigational Buoy and temperature difference between 5-m and 23-m water depth.
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From page 37... ...
] l~7~7B 1 ~ .~ 16.67 16.11 15.56 15.00 FIGURE 3.12 Modeled surface current end sea-surface temperature respond to a wind stress from the southwest of 1 dyne/cm2 (about 7 m/s)
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From page 38... ...
The observed surface-temperature patterns are quite similar to the modeled upwelling event, especially along the coast as shown in Figure 12.
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From page 39... ...
The runoff event creates strong and complex currents in the vicinity of the proposed outfall site, which would be difficult to resolve with moored instruments.
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From page 40... ...
It must be mentioned that the most critical factor limiting the development of truly predictive models is an understanding of the complex interaction between the coastal waters and those of the offshore ocean. Typically, coastal ocean models cover a limited region along the coast with their offshore extent ending at the continental shelf break.
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From page 41... ...
was applied from the west, and shown are dilution isosurfaces of 250 and 500 after 9 days of simulation. As the plume rises to the surface, it is pulled toward Boston by Me bottom currents, but when it reaches the surface it moves with the surface currents and is transported along the coast toward Cape Cod.
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From page 42... ...
The data assimilation, that is, the process of this blending, is undoubtedly the most powerful tool presently available for extracting information and insight from the sparse coastal-ocean data sets and the imperfect model results. ACKNOWLEDGEMENTS This work has been funded by the Massachusetts Water Resources Authority through contracts with both HydroOual (Marine Technical Environmental Services Contract #37 to Normandeau Associates)
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From page 43... ...
1992. Physical Oceanographic Investigation of Massachusetts and Cape Cod Bays, Technical Report WHOl-92-x (in press)
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