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IV. RECONCILIATION OF KINEMATIC AND DYNAMIC PROCESSES 



The considerations preceding the table of scales have not 

 included the scaling of density and vrind velocity. The scaling 

 of density influences mixing, which is essentially a dynamic proc- 

 ess, and the scaling of wind velocity involves several considera- 

 tions which are most accurately reconciled by an empirical process 

 in the present state of the model making art, 



i/ 



Density scales ;- The circulation in estuaries frequently involves 



the mixing of two or more water masses of different density. In 

 direct kinematic (Froude) scaling these density differences 

 should be maintained l/l in a model. The l/l scale of densities 

 leads to satisfactory results if the problem involves only the 

 kinematics of incursion. If, for instance, a large mass of salt 

 water flows in from the sea under fresh water as in a lagoon or 

 estuary, the velocities, volimes and times of arrival of the main 

 water masses are correct, but the mixing of the two water masses 

 is usually quite dissimilar and unrealistic. The water masses 

 tend to retain their identity too long in the model and to stratify. 

 This result is inevitable, for a fractional scale kinematically 

 similar model can rarely be made to possess a l/l ratio of both 



'2I While it is customary to consider density in model theory, it 

 is usually simpler to measure specific gravity with respect to 

 pure water at some standard temperature in experimental work. 

 The specific gravity so measured is numerically equivalent to 

 dens ity. 



