5.9.1 Physical effects
A hydroelectric scheme is not basically a consumer of water, but the installation does ‘rearrange’ the resource. Diverting a river into a canal, or a mountain stream into a pipe, may not greatly change the total flow, but it can have a marked effect on the environment. Furthermore, evaporation from the exposed surface of a large reservoir may appreciably reduce the available water supply.
Any structure on the scale of a major hydroelectric dam will affect its environment in many ways in addition to the hydrological changes. The construction process itself can cause widespread disturbance, and the effect on a fragile eco-system can be long-lasting. In the longer term, a large reservoir is bound to bring other significant environmental changes. Whether these are seen as catastrophic, beneficial or neutral will depend on the geographical and biological situation – and on the points of view and interests of those concerned.
Around 35 major dam failures have occurred since 1960 (defined as those resulting in serious material damage and/or deaths (Wikipedia, 2011a; 2011b)), although many dams are for purposes other than hydroelectricity: flood control, water supply, irrigation or recreation. A US study found that only 2400 of the country’s 80 000 dams had hydroelectricity plants (U.S. Dept of Energy, 2005), so it is not therefore surprising to find only five or six hydroelectric plants in the world list of major dam failures.
Silt accumulation behind dams has been a known problem for many years. Its build-up reduces the volume of stored water and consequently the hydro potential of a site. The Hoover Dam for instance, lost about one sixth of its useful storage volume in its first thirty years, although the loss rate was reduced when the Glen Canyon dam was built 370 miles upstream.
France has many dams constructed during the early twentieth century on rivers previously used by Atlantic fish, and as licences became due for renewal in the 1990s, stringent requirements were introduced for the construction of fish ladders or similar passages. One consequence was the decommissioning of dams deemed unsuitable for renewal, on environmental or economic grounds (ERN, 2000).
It has long been known that vegetable matter that would normally decay in the air to produce carbon dioxide (CO2) could decay anaerobically under water to produce methane (CH4). When this was identified as a much more potent greenhouse gas than CO2, the question arose as to whether hydro schemes that flooded land previously covered in vegetation should join the fossil fuels as significant contributors to global warming.
Detailed studies of individual reservoirs were carried out, the study of a hydroelectric plant on the river Aare in northern Switzerland estimated that each m2 of the lake surface was releasing about 0.15 grams of methane per day – much more than would be expected for its temperate location, and equivalent to a total annual methane release of about 150 tonnes (EAWAG, 2010), probably due to anaerobic digestion of the particularly large annual quantity of vegetable matter brought down by the river. However, the report also observed that a coal-fired power plant producing the same electrical output would release approximately 40 times more greenhouse gases, expressed in CO2 equivalent terms.
We now need to look at hydroelectricity’s social effects.