The gross method just employed should not cause as great an 

 error in the flushing time as might at first be expected. For, when 

 A is taken too large or too small, a compensating effect results 

 from B being too small or too large. A serious deviation from a 

 reasonable estimate of the flushing time would occur when the net 

 salt transport velocity differed radically from one-fifth the veilue 

 of the maximum current velocity. 



Example: Determine the flushing time of the James River estuary. 



The navigation chart used for this example was U. S. Coast and 

 Geodetic Survey Chart No. 78. A generalized chart showing the locations 

 of the observing stations is shown in Figure 4. The area flushed 

 extended from a line connecting Pike Point and Newport News to 

 Shirley near the head of the estuary. Velocity-depth curves for 

 current station J- 17 were used in place of river flow data and are 

 shown in Figure 5. The depth of the surface of no net motion at station 

 J- 17 is about 9.5 feet and roughly corresponds to the inflection points 

 of the salinity- depth curves of Figure 6. The surface of no net motion 

 rises gradually from the head towards the mouth of the estuary, 

 and is estimated to be about 9 feet below the mean water level near 

 the boundary shown in Figure 4. The surface of no net motion will 

 be considered to be horizontal in this example. 



In Figure 4, the net landward transport of sea water through the 

 boundary ab into volume B is estimated by multiplying the average 

 velocity found at station J- 17 by the sectional area of B. 



The dead water volumes involved in these considerations are 

 neglected when computing volume B (see Fig. 1). The average net 

 velocity for sections A and B at station J- 17 can be obtained from 

 Figure 5 by means of a planimeter (i.e., by finding the centroid of 

 the area between the mean flood and ebb velocity curves). The velocities 

 thus obtained were: 



Average net ebb velocity in A . . .0.30 knot 

 Average net flood velocity in B , .0.27 knot 



The effective areas corresponding to the volume sections A and B at 

 the boundary ab were computed to be: 



Cross section for A 181,065 sq. ft. 



Cross section for B 180,087 sq. ft., 



and the corresponding volumes, A and, B. were: 



