Tides and Tidal Currents in the Proximity of Land 



335 



in the layers above the bottom and then a rotation of the current to the left 

 at the bottom as required by the theory. However, when going into details, 

 deviations become noticeable which render the interpretation difficult. Figure 

 139 shows a section through stream filament according to the observations 



Lunar hours (Greenwich) 



Fig. 139. Section through stream filament in tide current in the Deutsche Bucht, 18 21 June, 

 1924, (Thorade). and — , observed; , computed. 



on which Fig. 138 is based. In the bottom layers, the increase in velocity is much 

 less than required by the theoretical curve. The reason for this might be 

 found in the presence of a thin boundary layer at the bottom, over which the 

 upper watermasses glide. Disturbances appear also at the surface; the max- 

 imum velocity is found at 5 m depth, below this there is a rapid decrease down 

 to 10 m, although there were no singularities in the disturbance of the density. 

 Thorade has endeavoured to compute also the coefficient of friction for single 

 cases ; however, the results were unsatisfactory. Acceleration, pressure force, 

 Coriolis force and friction, being connected to each other in the equations of 

 motion, he tries, for a case with particularly good observations to determine 

 the friction, when the first three quantities are given. Apparently, the frictional 

 resistance is not proportional to the momentary differences in velocity, but, 

 depending upon depth, to those velocity differences which prevailed more or 

 less long before. Such an inertia of the turbulence would not be improbable, 

 but the differences in phase which were found were indeed too great as compared 

 with the differences in velocity. It is possible to gain an idea of the order of 

 magnitude of the coefficients of friction by forming mean values over an 

 entire tidal period. Thorade finds the following values: 



