316 



CAMERON AND PRITCHARD 



[CHAP. 15 



constitutes the major mechanism for exchange or renewal of the waters, domi- 

 nating the processes of tidal exchange and wind-induced circulation . 



Distance - Miles From Head of Harbor 



Fig. 2. Typical longitudinal section of the salinity distribution in Baltimore Harbor. 

 The Chesapeake Bay at the mouth of the harbor is at the right end of the figure. 

 The arrows show the net flow pattern. 



5. Flushing in Estuaries 



The circulation in estuaries has important implications in the field of pollu- 

 tion study and control. For any estuary in which this problem is significant it is 

 important to be able to calculate the extent to which a pollutant may accumu- 

 late in the estuary before a "steady-state" concentration is reached. Further- 

 more, the variation in concentration of the pollutant with location in the 

 system must be predictable. 



The factors which bear on this important problem are associated with the 

 type of circulation that prevails in the estuary under study. In the salt- wedge 

 estuary, it is clear that a pollutant which is lighter than the water of the salt 

 wedge will be retained in the surface layer and pass out to sea at approxi- 

 mately the velocity of the surface flow. On the other hand, in a moderately 

 stratified estuary a portion of such a pollutant would find its way into the 

 bottom layer because of the vertical mixing which carries fresh water down- 

 ward into that layer. It would then take part in the upriver flow of the layer, to 

 be distributed more broadly than in the first case considered. The prediction 

 of pollution, therefore, depends heavily on the ability to understand and 

 evaluate the essential processes, both advective and non-advective, which are 

 responsible for the distribution of properties in the estuary. 



It was suggested by Ketchum (1951) that with certain fundamental assump- 

 tions (the most significant being that complete mixing occurs within volume 

 segments governed by the average excursion of a particle of water on the flood 



