The characteristics of the waves generated in the model are determined 

 by trial and error, although prototype wave characteristics are used to 

 guide the development of model waves. 



It should be noted that no sedimentation scales are actually used 

 in estuary model studies. Time and volume scales are developed during 

 the shoaling verification, but these are only empirical estimates of the 

 scales and, therefore, should be used only for determining the relative 

 merits of various proposed plans. Fixed-bed shoaling tests should not 

 be conducted for longer durations than the verification period. 



Since it is impractical to reproduce local wind effects during 

 hurricane-surge tests or the time decay of pollutants during disper- 

 sion tests, the results of these tests must be analytically adjusted to 

 account for these effects. It is also infeasible to control the surface 

 heat exchange coefficient of the model during heat-dispersion tests, and 

 the model results must be adjusted accordingly. 



The effect of the Earth's rotation is to generate a deflecting force 

 on flowing, water particles which is referred to as the Coriolis force. 

 This force is given by Defant (1961) as 



C=2Vcosin0 C3-23J 



where V is the horizontal velocity, to the angular velocity of Earth's 

 rotation, and (j) the latitude. The effect of the Coriolis force is to 

 cause flows to veer to the right in the Northern Hemisphere and to the 

 left in the Southern Hemisphere. For this force to be correctly modeled, 

 the model should rotate faster than the Earth by a factor equal to the 

 model time scale. To rotate an even moderate-sized estuary model would 

 obviously be infeasible since the model would have to be constructed on a 

 special platform. However, the Delft Hydraulics Laboratory (1968) found 

 that the Magnus Force on a rotating cylinder in a parallel flow is anal- 

 ogous to the Coriolis force, and developed a rotating cylinder (Coriolis 

 top) to simulate the desired effect. The Coriolis effect can also be 

 simulated in the model by adjustment of the model roughness to reproduce 

 prototype lateral velocity and salinity gradients, which are the result 

 of Coriolis forces. 



d. Model Limits . The primary considerations in determining the 

 model limits or boundaries of an estuary model are the upper reaches of 

 the estuary and the ocean and their effects on the study. Therefore, the 

 upstream (river) and downstream (ocean) limits will actually be boundary 

 condition control points. Since it is impossible to predict changes in 

 tides, currents, salinities, etc., at these boundary points which might 

 result from the study, the model limits must be established beyond the 

 zone of influence of the area to be studied. 



The upstream model limit of a fixed-bed estuary model should usually 

 be located upstream from the extent of saltwater intrusion. If this is 

 not possible, a boundary control system must be constructed in which the 



60 



