| BRIDGES AND WATER REPLACEMENTS. |
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The Baltic is parted in three areas: The Baltic proper, the Bottenocean and the Bottenvik. The water in the Baltic is brackish water because, on a yearly basis, there is carried more fresch water than salt water into the Baltic. The water that flows into the Baltic from the the Skagerak through the danish belts has a relatively high salinity, about 30 grams of salt per thousand grams of water. Because of the fact that salt water is heavier than fresh water, the salt water, which flows into the Baltic, will lay down beneath the fresh baltic water.
Names of different parts of the Baltic:
The figure demonstrates the soundings and the salinity of the water in different parts of the Baltic. The different colours indicate the salinity.
The heavy, saline Skagerak water flows towards the deeper parts of the
Baltic proper and replaces the old, light baltic water. Only seldom a total
replacement of water happens in the deeper parts of the Baltic and the
Bottenocean. This means, that water in these areas are deficient in oxygen,
which results in the death of the sea floor. Today, 17.000 km2 of the sea
floor is deficient in oxygen all year.
Another problem concerning the baltic is that the water replacement
25 to 50 years. This means, that the poisonous substances and the nutrients
will be concentrated in the Baltic. Yhe problem with the slow water replacement
in the Baltic has led to extensive considerations while building the Great
Belt Bridge and planning the Sound Bridge, because the bridges might narrow
the water replacement. Among other considerations, it has been decided
that the bridges may not hinder the flow of water in the Baltic. This is
called the zero solution. When digging away some of the sea floor, it should
compensate for the negative effect of the piers of the bridges on the water
flow. However, it is not certain how digging away some of the sea floor
will affect the aquatic milieu. The Baltic is also affected by the water
polution, but that is another matter.
The top of the figure illustrates the effect of digging away some of
the sea floor beneath the bridge in contrast to the bottom of the figure
illustrating a normal sea floor.