Náttúrufræðingurinn - 1986, Síða 32

SUMMARY
Water power plants and
watersystems
by
Hákon Aðalsteinsson
National Energy Authority,
Grensásvegi 9,
108 Reykjavík.
The generation of energy from water
power is a clean process compared with
the use of fossil fuels or nuclear fission.
The main impact is connected with land
use, changes in the nature of the lakes
involved, and changed discharge charact-
eristics.
Every hydropower plant is composed of
one or more reservoirs, dammed water-
channels, tunnels and a dam at the power
plant.
In Iceland 4 TWh/a have been harness-
ed, 12% of the most economical hydro-
power (33 TWh/a), and 6% of the technic-
ally reasonable hydropower (64 TWh/a).
Silted glacial rivers will provide most of
the economical water power planned to be
developed in the near future. In most cases
the hydropower plants will be at the marg-
in or just below the central highland
plateau, and thus affect the inhabited are-
as only to a small extent. The most severe
impact is the inundation of vegetated land.
The sparse vegetation on the highland
plateau is often in valleys or areas where
the groundwater table is relatively high,
and those sites often make ideal sites for
reservoirs. Much effort is put into easing
conflicts between hydropower plant needs
and conservation of the most valuable
vegetated areas. For example, recently the
National Water Power Company and the
Nature Conservation Council negotiated
the maximum allowed elevation of a reser-
voir in Þjórsárver.
The situation concerning power plants
in glacial rivers is rather peculiar and diff-
erent from the usual problems connected
with power plants. The glacial rivers are
characterised by a high annual as well as
diurnal fluctuation in discharge. The silt
content is rather high during summer,
oft'en 300—1000 mg/1. The transparency is
very low in such water. Often 80-95% of
the silt is sedimented in lakes with a retent-
ion time of at least 100 days. The transpar-
ency is still low in reservoirs with the silt
content reduced to 10-30 mg/1.
The invertebrates living in glacial rivers
have not yet been investigated to any ex-
tent, but as can be seen during low flow
periods the fauna is poor because of high
silt content (low transparency), bed load
and fluctuating water level. The damming
and retention of water will reduce the silt
content and mostly eliminate the bed load
and reduce the water level fluctuation of
the rivers downstream from the power
plants. These factors are all positive for
the possibilities for algae and invertebrates
to settle and thrive. On the other hand
increased channel erosion, at least in the
beginning, may reduce the positive effects
of the above mentioned factors. The tem-
perature situation is less clear. Usually
temperature decreases downstream from
dams, except in winter, because the water
is usually tapped from the hypolimnion,
which during summer is colder than the
epilimnion. In winters the hypolimnion is
warmer than the surface water. In Ice-
landic lakes a thermocline is not developed
in lakes up to at least 100 m deep because
of strong wind-induced upmixing of the
water. Most dams are only 10-20 m deep.
Thus the temperature regime is thought to
follow different lines than usually reported
from rivers downstream from the dams.
The temperature most likely does not
increase as fast as in unregulated streams
and does not reach the same high summer
maximum, but keeps warm longer during
the autumn. In winter it is thought to be
mostly unaffected by the dam.
On the whole the temperature changes
are not supposed to have a serious impact
on life downstream in the rivers.
The most severe impact of the fluctuat-
ing water level in reservoirs is usually the
radical reduction of life in the strand zone
within the water level amplitude. In the
glacial river reservoirs this effect is enhanc-
130