change from flood tide current to ebb tide occurs, the currents go apart and the deep layers rise 

 toward the surface and the ice consequently thins out. At point b, on the other hand, the currents 

 come together, the deep layers drop down and the ice presses together. 



)>>>> 



Figure 122. Diagram of movement of particles along a vertical section for a tidal wave. 



If we closely examine figure 122, which is typical of normal tide, we see that at the point of 

 high water (point c) the flood tide current attains its greatest speed, at point d its speed is less, at 

 point h it is equal to zero, at point e the ebb current has already started, reaching its maximum 

 speed at point /, etc. From this it follows that the ice between points c and / will lag behind the 

 ice which is located at the same moment at point c . Consequently, on this whole section of the 

 profile of the flood-tide wave, a thinning of ice is observed, which reaches its maximum at point a 

 where the currents move apart. Analogously, on the section of the profile cbf, the ice is carried 

 along towards point h by the tidal currents from both sides, and at point b the compression is most 

 intense. The simple diagram shown in figure 122 is greatly distorted by the shores. 



Long ago I have shown that a line which joins points of the sea in which changes of currents 

 occur simultaneously will be parallel to the line of the front of the tidal wave, or in other words, 

 the cotidal lines. * 



It is also obvious that the forces which drive the floes together or apart are especially sharply 

 defined at the point where the floating ice meets the shore or the fast ice. 



If a tidal wave enters a strait whose depth gradually decreases towards the shore (which is the 

 most usual case), the actual appearance and location of clear spaces of open water becomes compli- 

 cated. Actually, the speed of the tidal wave is directly proportional to the square root of the depth 

 of the region. It follows that the position of the crest of the tidal wave in such a strait will be 

 slightly curved, with the convex side in the direction of movement of the tidal wave. It thus happens 



*In December 1942 during the drift of the trawler 65 along the Zinni shore of the White Sea it 

 was noticed that the open water spaces, during the tidal thinning of ice, extended approximately 

 perpendicular to the tidal currents. 



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