Section 127. Significance of Tidal Pfienomena in Navigation in Ice 



Several examples have already been cited of the possibility or even necessity of using the 

 periodic tidal phenomena for navigation among ice floes. 



The greatest need for this is experienced in a sea like the White Sea. Here the tidal phenom- 

 ena are sharply defined and navigation in certain years continues throughout the entire winter, thus 

 encompassing all the phases of the ice cover (its formation, period of existence, and destruction) 

 and where navigation is carried on under diverse ice conditions (in fast ice and among floating ice). 



A very typical example of navigation in fast ice was the navigation in the channel made by ice- 

 breaker in the mouth of the Sevemaya Dvina, along the Maimaksa River and along the Sevemaya 

 Dvina up to Bakaritsa. 



It must be considered that although the floating ice is here divided from the ice frozen to the 

 shore by a tidal crack, the channel made by the icebreaker divides the floating ice into two parts to 

 a considerably greater degree than the tidal cracks divide floating ice from the shore. Therefore, 

 with every rise of the river level due to high tide to effect of wind the channel expands slightly and, 

 contrarily, with every fall of the level it contracts slightly. From this it follows that it is easier 

 for the icebreakers to break the channel and conduct ships along it when the level is up than when it 

 is down. 



The situation on the border of the fast ice and floating ice is no less typical. Here much de- 

 pends on the direction of the edge of the fast ice relative to the front of the tidal wave, or in other 

 words, relative to the direction of cotidal lines in the given region. 



In the case of the mouth of the Severnaya Dvina and the Letini shore of the Dvinski Gulf, the 

 cotidal lines, extending from east to west, approach these points from the north. Thus, at the 

 mouth of the river with the start of ebb-tide there begins a gradual receding from the edge of the 

 fast ice and a thinning of the floating ice reaching its maximum at the time of change from ebb to 

 flood-tide and sometimes making a sharply defined polynya bordering the fast ice. For example, 

 such polynyas 50 to 80 m in width, were found by air reconnaissance on 22 January 1942 at the 

 mouth of the Sevemaya Dvina. It is by no means possible to explain their presence by winds since 

 for several days before this the prevailing winds, although weak, had been of northerly components. 



As the flood tide develops, the thinning of ice is gradually eliminated and by the end of flood 

 tide the floating ice approaches close to the fast ice creating the flood-tide compression and some- 

 times causing hummock formation. 



Along shores which extend in the direction of propagation of the tidal wave, as for example 

 along the Zimni shore of the White Sea, the tidal compressions and thinnings occur according to the 

 general rules; that is, the thinning begins with the start of slowing of the flood-tide current, reaches 

 its maximum at the change from flood-tide current to ebb-tide and then the floes gradually begin to 

 come together. At the point of change from ebb-tide current to flood-tide the maximum compres- 

 sion occurs . All these changes in the entire expanse occur with the speed of propagation of the 

 tidal wave. 



Keeping in mind the recorded phenomena and the fact that the tidal currents are stronger 

 when the amplitude of the tide is greater, we may, for example, compare the navigational condi- 

 tions among the ice floes in the neck of the White Sea along the Terski and Zimni shores. 



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