326 



Tides and Tidal Currents in the Proximity of Land 



if n , „ > - : force contra solem (small axis of the ellipse of force more 

 R-\-S a ^ 



than /: a times the large axis), frictionless tidal current contra solem, 



with very weak velocities. 



This compilation shows the extreme preponderance of the frictionless tidal 

 currents cum sole on the rotating earth. Thorade has compiled in Fig. 132 

 a few cases of diagrams of forces with corresponding current diagrams. The 



force 6 



Current 



Fig. 132. Three examples of diagrams of tidal forces and corresponding current diagrams. 

 The vectors of the force originate at the centre of the dotted ellipse and go to the points 

 indicated by the hours of the tide. The corresponding vectors of the 'frictionless' cur- 

 rent connect the centre of the full drawn ellipse and the corresponding hours of the tide 



(Thorade). 



results are in full agreement with those found by Sverdrup in the analytical 

 way. Their importance and utilization are limited by the fact that in shallow 

 seas, where current measurements are easier, the frictional influences extend 

 from the bottom to the surface layers, 

 (c) Influence of the Friction 



The influence of the friction on tidal currents has been the subject of papers 

 by Sverdrup and Thorade, and also by Fjeldstad (1929), after Lamb had 

 already treated the subject in a very elementary form in his Textbook of 

 Hydrodynamics (1932). It is important to ascertain the influence of the friction 

 caused by the bottom which, through turbulence will make itself felt in 

 a more or less extensive bottom layer. The conditions, in this case, are similar 

 to those for stationary ocean currents, except that here the tide waves are of 

 a periodic nature. The assumptions regarding the friction are the same in both 

 cases. Sverdrup and Fjeldstad have found integrals for the equations of wave 

 motion, whereas Thorade treats the problem in a more synthetical way, 

 which is more elementary, very clear and it gives a good insight in the way 

 friction works. We will follow essentially Thorade's method (1931 p. 152). 



