334 



Tides and Tidal Currents in the Proximity of Land 



pointing to great resistance: The maximum velocity is not at the surface, 

 but below it and is reached 2 h before high water; the velocity of propagation 

 of the wave is only 10 m/sec, whereas ygh gives 14-7 m/sec. Since the 

 currents are practically alternating, we can assume s = and fx = vq 

 = 690 cm -1 g sec -1 , but then the agreement with the observations is not good. 

 The observed currents in the upper layers change more slowly with depth 

 than those computed, but more rapidly close to the bottom. The computed 

 currents reach a maximum velocity about IT h before high water, but the 

 observed difference is 2 h. Fjeldstad has also taken into account the re- 

 sistance offered by the ice and has thus obtained a better agreement. For 

 the entire layer 0— 12 m the observations give in the vector diagram for 

 the currents V — 38 cm/sec, aV = — 5 cm/sec and a priming of the phase 

 of t = — 2 h, whereas the theory gives V = 38 cm/sec, aV — — T56 and 

 / = -l-2h. 



The discussion made by Thorade of the current and tide observations in the 

 Deutsche Bucht shows that the agreement with the theory is only partially 

 satisfactory. Figure 138 gives a comparison and it shows that the observed 



Oeptn 



*- 3tm 



50 lOO 



^ 



cm /sec 



Fig. 138. Tide currents in the Deutsche Bucht of the North Sea. <p = 54°. 0'N., I = 7°30' 

 18-21 June, 1924, (Thorade). Left: according to theory; right: according to the observations. 



current ellipses are not in bad agreement with the theory. It is quite apparent 

 that, with an alternating tidal current at the surface (frictionless current), 

 there will be a rotation to the left approaching the bottom, and the ellipses 

 will grow wider and wider. Observations made at the Wester Sill show also that, 

 with a surface current rotating to the right, there is an alternating current 



