536 



Currents in a Strait 



The disturbances are obviously due to this since the equation (XVI. 17) is approxi- 

 mately satisfied. Putting p^ =l-028, Pa — Pi as approximately 15 x 10"^ h-^ = 25 m 

 and h^ = 45 m, and since by observation c„:C}. = 2 then equation (XVI. 17) gives the 

 critical velocity of the upper current as c„ = 1-77 m/sec, while the observed values lie 

 between 1 and 2 m/sec. 



The upward bulging of the boundary layer in the Strait of Gibraltar and the Strait 

 of Bab el Mandeb is undoubtedly due to the passage of the current over the rise in the 

 middle of the strait. Bulges such as these do not occur in a plane channel. 



{b) Tidal Effects 



Since tidal currents entering a sea strait affect the whole water mass from the sea 

 surface down to the bottom, the ebb and flood currents will be superimposed on both, 

 upper and lower currents, either reducing or accentuating them. 



Since these currents flow in opposite directions the current profile will show rapid 

 changes over a tidal period. An example can be taken of a strait 300 m deep with 

 current reversal at 200 m in which the upper current flows east and the lower current 

 flows west; the upper current is assumed with a surface velocity of 100 cm/sec decreas- 

 ing parabolically with depth, while the lower current is supposed to increase below 

 the boundary surface. The amplitude of the tidal current may be 86 cm/sec and the 

 phase 3 moon hours (ebb towards the east at 3 h and flood towards the west at 9 h). 

 The current structure over a total tidal period is then shown schematically in Fig. 

 248. At 3 h there is a current directed to the east through the entire water mass with a 

 maximum at the surface; 6 h later conditions are almost reversed and the current is 

 directed towards west with a maximum at the bottom. 



In addition to this direct influence there is also a second one affecting the boundary 

 surface. This will perform periodic internal vertical displacements initiated by the tidal 



Fig. 248. Isopleths of the current velocity (cm/sec) in a water column during a total moon 

 period with a superposition of the basic and tidal current. (Type of currents in the Gibraltar 



Strait.) 



