school energy&environment

Relation with the ecosystems

The relation with the ecosystems is fundamental when designing a hydroelectric power plant. Two aspects are strictly linked to the collection of superficial water and can provoke two different impacts:

• impact related to the variation (reduction) of the water quantity, with possible consequences for the users, that could argue about the use of water and impact on aquatic fauna;
• impact related to the change in the water quality as a consequence of quantity variation (i.e. higher concentration of pollutants) and as a consequence of vegetation change on river banks.

If a dam for a basin power plant is built, the consequences will be the following ones: above the barrier a reservoir will form and therefore there will be running water (lothic water) moving in still water (lenthic water), with a longer time needed to exchange water and a possible impact on the ecosystem. Underneath the barrier, until the area where the water used by the plant is released, the watercourse may be dry for some periods of time unless a continuous release is guaranteed so that the river has a suitable minimum flow rate. The minimum flow rate (to be guaranteed according to the law), that ensures the natural development of all biological and physical processes, is called “minimum vital outflow”. All these aspects have to be taken into consideration during the impact assessment. This is why some choices are made during the design phase and precise precautions are taken to avoid any type of damage to the ecosystem.  The reduction in water flow rate does not have to be excessive, and it is necessary to respect the minimum vital outflow value, since otherwise it is possible to damage the deposit, incubation, growth and transit of fish. With regard to the latter aspect, it is necessary to take into consideration the movement of fish that go up the current and the fish that go down the current, by building the adequate passages, installing the most suitable nets to prevent the fish from entering into the intake areas and get into the turbine (some types of turbines can kill the fish).  When a dam is built to supply a hydroelectric plant, it is necessary to think about the different ways water can be used: drinking water, agricultural water or industrial water. The size and management of the dam must be compatible with all these needs, by optimising the use of water as a resource, since in some regions water is not sufficient to satisfy all these needs.