here, the data corresponds to the cumulative flushing time. That is, the 

 segment nearest the sea flushes in, for example, five tides; the next segment 

 flushes in five tides also, but to flush to the ocean it must flush through 

 the lower segment, taking an additional five tides. Hence, this segment 

 flushes completely to the ocean in 10 tides. The data are presented in this 

 way on the assumption that reader interest will focus on questions of the type 

 "If something is spilled here how long will it take to be diluted out of the 

 estuary?" Two points must be made about flushing time: (1) the flushing 

 times reported here probably are low, because the complete mixing assumed in 

 the model does not occur and because only the surface layer is treated in the 

 model and (2) since the flushing time is the average time water spends in the 

 estuary, it represents the time required for at least half, but not all, of a 

 pollutant to be flushed out of the estuary. 



The terms "tidal and nontidal flows" are simply a convenient way of separating 

 the two components of the estuarine water flow. In the absence of tides, 

 estuarine waters would flow steadily seawardas rivers do, and would vary as 

 fresh water flow varies. This current is nontidal flow. Superimposed on this 

 flow is a tidal flow, which flows in opposite directions between high an-:" low 

 tides. The average velocity of this flow can be computed. Obviously, tidal 

 currents vary from zero at low or high tide to a maximum at half tide, which 

 is approximately 150% of the mean flow. Local topography can cause extreme 

 changes in the currents. 



The flushing model has been applied to provide some basis for comparison. The 

 reader is cautioned that the model output is only a reasonable facsimile of 

 average values under mean flow and is intended only for comparative purposes. 

 These are not field data and should not be viewed as such. In the discussions 

 below on the hydrography of individual Maine estuaries, information derived 

 from the Ketchum model will be integrated with data from published reports. 

 Thus, the amount of information reflects the amount of study the estuary has 

 received. 



Casco Bay/Portland Harbor/Fore River . According to Hulbert (1968 and 

 1970) a cold bottom water inflow and upwelling are the major nontidal flows in 

 Casco Bay. Strong tidal mixing around shoals and around the island of the bay 

 prevents the establishment of a summer thermocline, which has formed elsewhere 

 in the Bay. At a station in the mouth of Portland Harbor, Normandeau 

 Associates (1974) observed spring (March) and neap (April) currents. In both 

 instances, flood currents were uniform regardless of depth. This is typical 

 of a partially mixed estuary. More data (including data on a greater number 

 of years) are necessary to support a general conclusion. The flushing model 

 was not applied to this estuary. 



Presumpscot and Royal estuaries . The Ketchum model was applied to these 

 estuaries, which are highly atypical (figures 5-10 and 5-11). Both are 

 bounded by barriers (Presumpscot Falls and Yarmouth Dam, respectively) at 

 their upper end within a few kilometers of the ocean. Each comprises only a 

 single segment and according to the model would be flushed by a single tide. 

 The model predicts a mean salinity for each, but this is misleading, as 

 complete flushing would result in salinities ranging from zero to full oceanic 

 levels during each tide. No data exist for these two estuaries. 



5-22 



