RIVER ICE: (IV.) — Any ice formed in or carried by rivers. 



Shallows and shoals in rivers of fresh or only slightly saline water, 

 swept over twice a day by very powerful tides, are the most favorable 

 places for the rapid formation of ice. If it is assumed that each tide 

 sweeps away any ice formed during the ebb of the preceding tide, forma- 

 tion of new ice at the rate of perhaps 6 inches per day is a possibility, 

 as in the winter climate of southern Quebec. 



Ice expands and contracts with temperature changes in the same way 

 as rocks, although the presence of water in contact with the under side 

 of river ice reduces its range of temperature. Rising temperatures cause 

 expansion that exerts outward pressure toward the banks. Normally 

 there are enough weak points in the ice cover to permit the release of 

 this pressure by buckling of the cover or crumbling of its edges. Oc- 

 casionally the temperature rise is so rapid that enormous expansive 

 pressures are developed at the edges of the ice where exposed piers, bulk- 

 heads, or sea walls are overturned or shoved bodily. When the expansion 

 is less rapid the ice has time to adjust itself at weak points by crumbling 

 and internal shear, so that little damage is done. Sometimes the entire 

 frozen surface of the river bank, including rocks, earth, and sand, is 

 sheared free from the unfrozen subsoil and piled up in characteristic long 

 ridges parallel to the stream channel. This action on mid-channel sand 

 bars piles them up above water level. 



Similarly, falling temperatures cause contractions of the ice cover. 

 As ice is very weak in tension, the stress is easily released by numerous 

 strain cracks. The ice already shoved up on the bank is not retracted 

 but remains perched on the bank. Contraction of ice is usually harmless 

 to engineering structures. However, the combination of alternate expan- 

 sion and contraction causes a ratchet action that is more severe than 

 either action separately. As contraction takes place at colder tempera- 

 tures, the water that rises in the strain cracks freezes at once, filling 

 and sealing the cracks. When temperatures rise again the filled cracks 

 cannot close, so that the entire expansion must take place at the edges 

 of the cover. Similarly, compression cracks formed by buckling freeze 

 solid and are stronger than before. Each cycle of contraction and ex- 

 pansion shoves the edges of the sheet farther up the bank. ' 



Piers or walls exposed to the full force of ice expansion are usually 

 protected by maintaining a belt of open water in front of them. As 

 new ice freezes in the open lane of water, or as ice expansion closes it, 

 this protective belt must be reopened and the ice cakes removed. A 

 clever use can be made of the residual heat of the water under the ice. 

 A perforated air pipe is placed along the bottom of the river in front of 

 the wall or gate. The rising air bubbles drag warm bottom water up- 

 ward and cause active circulation that prevents surface freezing and 



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