through the entire estuary of Figure 1 at mean high water. Consider 

 now the motion of a water particle from position 1 through position 18 

 in Figure 2. Obviously, it is only the net movement of the particle 

 which contributes to its passage through the estuary. It is, therefore, 

 required to find this net movement in order to solve the problem at hand. 



One method for accomplishing this would be to determine the average 

 flood and ebb velocities through the seaward boundaries of A and B. 

 Then 



Q^ = hd(U^t^-Uptp) (2) 



where, hd is the mean area at the seaward boundary of A, U£; and 

 Uf refer to mean ebb and flood, respectively, and t is the duration of 

 flood and ebb flow. Again, if the river flow R is computed inde- 

 pendently, 



Q3 = ed(Uptp-UEtE) (3) 



from which (2) can be determined since Q^ = R + Qg. 



The case for which only river flow data is available for the solution 

 of the problem will be discussed later on. 



When Qa and Qg can be determined, as above, the following state- 

 ments can be made: 



(a) Except for the first several tidal cycles following the initial 

 contamination of the estuary, the contaminant in B must leave the 

 estuary via cd and the opening hd (Fig. 3). 



(b) Any contaminant leaving the estuary in a tidal cycle will 

 be contained in a volume Qa- 



The following assumptions are made to supplement (a) and (b): 

 The contaminant remaining in the estuary at any time becomes uni- 

 formly distributed at high tide; and if the removal of the contaminant 

 is excessive or deficient during a time interval, the rate of removal 

 will compensate during a following time interval. 



We can now write exchange ratios for sections A and B, as 



, Qa 



(4) 



