72 



and numerical techniques can be found in Blumberg and Mellor (1987). 



In the USGS studies, the ECOM-si model is configured to encompass all of 

 Massachusetts and Cape Cod Bays, which combine to form a roughly 100x50 km 

 semi-enclosed basin with an average depth of 35 m in the western Gulf of Maine (figures 1 

 and 2). The area also includes Stellwagen Bank. The modeling was done over an 18 month 

 period, based on data from January 1990 to July 1991. 



To solve the equations of the model, the model is driven with observed data at the 

 lateral model boundaries and at the sea surface: 



(a) Tidal elevations. The model is driven with tidal elevation at the offshore 

 boundaries using data from a well-calibrated tidal model of the Gulf of Maine 

 (Naimie and Lynch, 1991). Tidal data are specified for each five minute interval 

 over the modeled time span. 



(b) Wind. Wind direction and speed measurements are introduced into the model as 

 surface conditions based upon meteorological data from the National Oceanic and 

 Atmospheric Administration Boston Buoy, which is located very near the location of 

 the proposed outfall. Values are given for measurements every four hours and are 

 applied uniformly over the modeled area. 



(c) Heat flux. Heat flux is also applied uniformly over the modeled area, and is 

 calculated using insolation data from the Woods Hole CX;eanographic Institution, 

 relative humidity from Logan Airport, and wind speed, air temperature and sea 

 surface temperature values from the Boston Buoy. As in the case of wind, heat flux 

 values are introduced in four hour intervals. 



(d) Fresh water inflow. Data for river runoff from the Charles and Merrimack 

 Rivers are introduced into the model using daily USGS gauged measurements 

 interpolated into four hour values. Freshwater inflow due to the current MWRA 

 effluent discharges at Nut and Deer Island are also included. When modeling the 

 effect of the proposed outfall, the fresh water associated with the effluent is 

 discharged at the new location. 



Data for calibrating and verifying the model were obtained through an intensive field 

 program during the period 1990-1991 (Geyer et al, 1992). Verification was based primarily 

 on comparing the subtidal circulation statistics at nine current observation sites located in 

 Massachusetts and Cape Cod Bays. The model results compare favorably with observed 

 conditions, indicating valid predictions of dispersion and transport within western 

 Massachusetts Bay. 



The model and observed data indicate that the major horizontal transport mechanisms 

 in Massachusetts Bay are wind and river runoff. Tides play an important role in vertical 



