Environmental Engineering Reference
In-Depth Information
closed, but on a host of other factors as well, over which the power company can-
not have any influence: For instance, forests and swamps throughout the water-
shed keep changing their characteristics as they are deforested and the ground
ploughed for reforestation, some areas are ditched to enhance wood or hay har-
vests, or abandoned ditches are eroding and the landscape reverts to a state as it
might have been many decades ago. All this influences how much water is - and
other substances are - in the river.
Another factor largely out of reach of the control room managers is the
weather: How much it rains or snows, how quickly the snow melts, how much wa-
ter evaporates or seeps into the ground is something the power company can only
react to - not steer or control. Even the intricate forecasting technology that meas-
ures - among other things - snow depth, temperature and discharge across the wa-
tershed and combines these data with meteorological information to calculate sce-
narios for the state of the river, is insufficient for actual decision-making.
Particularly concerning the marked seasonal variations of the river and its envi-
ronment over the year, these climatic factors influence the river much beyond the
reach of hydroelectricity engineers. Even those members of staff who fully trust in
the flow-manipulation machinery are aware that the forming of an ice cover on the
river in late autumn and the breaking up of the ice in spring are very sensitive pe-
riods, when hydroelectricity production must be subordinated to more pressing is-
sues on the river. In autumn, it is important that a stable ice cover forms on the
river as quickly as possible. On the one hand, this has to do with the demands of
the riverside population, who are interested in using the river ice for ice-skating,
skiing, snowmobiling and even playing 'ice golf', alongside the ice bridges that
enable transport across the river in some places. On the other hand however, a re-
liable ice cover is essential for the very functioning of the hydropower stations. A
continuous ice cover, preferably with some snow on top, acts as an insulator be-
tween the water in the river and the air above. The water will then never quite
reach the freezing point, even if air temperature reaches minus 30 degrees Celsius
or colder, which is not infrequent in a Lappish winter. If this ice cover is not con-
tinuous, however, the air will cool the river water below the freezing point, where
it forms so-called 'frazil ice', small ice crystals in the water that readily stick to
any surface the water touches. Thus, not only can the flow of the river be ham-
pered when this frazil ice accumulates around rocks in the river bed, but it can also
seriously damage hydropower facilities, like the turbine's rotor blades, should it
enter a power station. Frazil ice forms particularly in river stretches with fast water
movement, such as rapids, but also in calmer stretches, if no ice cover has formed
due to continuous and pronounced changes in the water level - as created by hy-
dropower generation. Therefore, in order to preserve infrastructure and good rela-
tions with the neighbours, manipulation of the river in late autumn is a very deli-
cate matter for the hydropower company.
Similar observations apply in spring: When the snow melts and the river swells
to a tenfold volume of its winter flow, the ice crust breaks up and heavy ice floes
float down the river. The amount of water starkly exceeds the capacity of the tur-
bines, so that floodgates have to be opened at the dams, to channel a large portion
of the river water around the hydropower stations. On the one hand, because water
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