Ice-Covered Lakes Environment - Freezing of Lakes and the Evolution of their Ice Cover - page 259

Fig. 8.11 The road of life was
the transport route across the
frozen Lake Ladoga, which
provided the only access to the
besieged city of Leningrad in
the winter months during the
World War II. The road forms
part of the World Heritage Site
Saint Petersburg and Related
Groups of Monuments. Source
Wikimapia

more than one car loading the same site; second, the speed must be below the shallow
water wave speed in the basin to avoid resonance between bending wave in the ice sheet
and shallow water wave. In the of cial lake ice roads in Finland, ice roads are opened after

the congelation ice thickness has exceeded 20 cm.
In more details, it has also been recommended to stick to the accepted speed, trying not
to change it. If the ice strength is not high, then movement of caravans is unacceptable,
and the speed of an individual car must be greater than the speed of the wave propagation.
Also when the ice strength is not high, then the next car can start its movement only after a
certain time necessary to provide a decay of ice cover vibrations. Near the coast, counter
movements or cars are unacceptable, since due to re

fl ection of waves superposition of

amplitudes may follow.
There are techniques to strengthen or weaken the ice. Strengthening can be done by
pumping water on ice or into snow on ice, and let the ice form naturally (Palosuo 1982).
Weakening of ice by forced vertical convection by pumping air to lake bottom was made
to small degree in the Second World War in Finland, and thereafter the method was
utilized to keep ferry routes and log pools of sawmills ice-free (Vakkilainen 2001).
People in islands in cold regions had severe problems in the past when lake ice
prevented both boat traf c and on-ice traf c. These periods are called kelirikko in Finnish

language. Literally . The length of this

period depends on the autumn or spring weather conditions and is therefore difficult to
predict both for its timing and its length. Typically the length is about 1 month each year.
Hovercrafts have been constructed for traf

' kelirikko ' means ' traf c conditions broken down '

c on lake ice almost 100 years ago but they
have not grown into any major industry for help in kelirikko period (Fig. 8.12 ). They are
used by pilots in winter shipping in the Baltic Sea and in rescue operations.

8.3.4 Navigation

The main negative impact of a lake ice cover is to prevent boat traf c (Fig. 8.13 ). Only in

large lakes, such as in the Great Lakes of North America, Lake Saimaa in Finland and
Lake V

ä nern in Sweden, shipping can be continued in ice conditions. The advantage of