Section 128. Tidal Ice Maps 



It is clear that tidal compressions and thinnings of ice are particularly characteristic in 

 narrow straits with very fast tidal currents and jagged shores and irregular bottom relief. Due in 

 the periodicity of currents and resulting large horizontal gradients of speed, the water here moves 

 with great diversity of speed and direction in closely adjacent points so that the deep water is there- 

 by raised and lowered. Wherever the deep masses are raised upwards, a thinning of ice is observed 

 and wherever the surface layers are forced down an accumulation of floes is found. With the change 

 from flood-tide to ebb-tide the picture may change to the reverse in certain regions but in other 

 regions it remains permanent and typical for the given region. 



As an example, figure 123 shows one of the diagrams compiled by Burke for the neck of the 

 White Sea for 3 hours after high water at Sesnovets Island. The diagram was compiled on the basis 

 of the "Atlas of Tidal Currents in the neck of the White Sea" and of Burke's personal observations. 



Burke shows that a peculiarity of the ice cover in the neck of the White Sea is its turbulent 

 condition due to the strong tidal currents. 



The wind is a second factor in the movement of ice. However, despite the wind action, the 

 regular tidal shifting of ice is not completely upset but only altered. 



In the region under consideration, the ice as a general rule is thinned by the ebb-tide current 

 running north and, conversely, is concentrated or pressed together by the flood-tide which runs 

 south. The strongest compression occurs in the regions where the currents meet. Regular changes 

 and thinnings of ice do not occur in identical degree in all regions of the sea. There are regions 

 where close ice predominates. According to local terminology, such a region is called a kol ob 

 if it is located along the shore and an ostrog if it is distant from the shore. There are regions, 

 called rasdel i , where scattered ice is predominant regardless of the force and direction of the 

 wind. 



The approximate positions of kolobi, ostrogi and razdeli in the neck of the White Sea are 

 shown in figure 124 (according to Burke) . 



It is clear that such maps have great importance in navigation in ice and many delays and 

 failures can be explained by the absence of such maps . 



LITERATURE: 21, 54, 62. 



344 



