Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 359

lack of aesthetics of unsubmerged inflatable dams may have to be considered.
Both variations can be run without problems and can handle very high discharges.
Ice, driftwood and bed load can be conveyed without causing any damage.
Fig. 8.7 Cross-section through a dam installation with fitted fish-belly flap gate (left) and
inflatable dam (right) (see /8-7/)
For plants where the headwater level does not have to be kept constant, dams
without moveable parts are also applicable. This is mainly the case for diversion-
type power stations with a higher head. With higher heads the slightly varying
headwater level has only little influence on the usable head and can thus be ac-
cepted in this case. Using such a fixed dam designed hydraulically to convey the
design flood while maintaining a certain upstream water level, reduces the in-
vestment costs significantly. For fixed dams and weirs, very natural designs
should be used (e.g. rough ramps), that might be used as fish passes at the same
time.
For steeper streams and higher heads Tyrolean weirs are used for the diversion
of water via the bottom without impoundment. A grid or a perforated plate built
into the bottom of the riverbed conveys the water into a channel below. Tyrolean
weirs have no moveable parts and can be overloaded very well. Bed load exceed-
ing the diameter of the space between screen bars is conveyed downstream. There
are ecological benefits if Tyrolean weirs do not stretch across the entire width of
the stream and therefore part of the water remains in the river.
Reservoir. Mountainous regions (e.g. the European Alps) create the natural con-
ditions for water storage. Such natural or artificial lakes - as daily, weekly,
monthly or annual reservoirs or in some cases as interannual reservoirs - can cre-
ate a balance between the fluctuating natural water supply and the equally fluctu-
ating demand for electrical energy over time. Pumped storage power stations can
also temporarily store a surplus supply of base-load power from thermal power
stations or run-of-river power stations for its later use as peak-load power.
The delayed outflow from the reservoirs during low water periods in winter
also contributes to an increased flow in the lower rivers and thus to an increase in
power generation in the power stations installed there.