The disadvantages of
hydroelectric power plant (hydroelectricity):
Failure Hazard
Dam failures have been some
of the largest man-made disasters in history. Also, good design and
construction are not an adequate guarantee of safety. Dams are tempting
industrial targets for wartime attack, sabotage and terrorism.
For example, the Banqiao
Dam hydroelectric power plantfailure in Southern China directly
resulted in the deaths of 26,000 people, and another 145,000 from epidemics.
Millions were left homeless. Also, the creation of a dam in a geologically
inappropriate location may cause disasters like the one of the Vajont Dam in
Italy, where almost 2000 people died, in 1963.
Smaller dams and micro hydroelectric
power plant facilities create less risk, but can form continuing
hazards even after they have been decommissioned. For example, the small Kelly
Barnes Dam failed in 1967, causing 39 deaths with the Toccoa Flood, ten years
after its power plant was decommissioned in 1957.
Large Power Outages caused
by dam failures
Large dams, whilst generally
reliable can suffer catastrophic failure to the dam itself, or the connections
and substations, leading to extremely large and sudden loss of output, which
can plunge an entire network off line, for hours or even months depending on
the damage. Hence whilst these are regarded as "firm" or "despatchable"
sources, in reality duplication or back up has to be provided. Examples are:
● the November 2009 Brazil and Paraguay
blackout dam failure: 14 GW
● the 2009 Sayano-Shushenskaya hydro
accident: 6.4 GW
● the Banqiao Dam failure: 18 GW
These are very large losses
of power; for comparison, the average UK power demand is around 37 GW.
Limited Service Life
Almost all rivers convey
silt. Dams on those rivers will retain silt in their catchments, because by slowing
the water, and reducing turbulence, the silt will fall to the bottom. Siltation
reduces a dam's water storage so that water from a wet season cannot be stored
for use in a dry season. Often at or slightly after that point, the dam becomes
uneconomic. Near the end of the siltation, the basins of dams fill to the top
of the lowest spillway, and may cause the dam to fail during any season. Some
especially poor dams can fail from siltation in as little as 20 years. Larger
dams are not immune. For example, the Three Gorges Dam in China has an
estimated life that may be as short as 70 years.
Dams' useful lives can be
extended with sediment bypassing, special weirs, and forestation projects to
reduce a watershed's silt production, but at some point most dams become
uneconomic to operate.
Environmental damage
Large reservoirs required for
the operation of hydroelectric power plants result in submersion of
extensive areas upstream of the dams, destroying biologically rich and
productive lowland and riverine valley forests, marshland and grasslands. The
loss of land is often exacerbated by the fact that reservoirs cause habitat fragmentation
of surrounding areas.
Hydroelectric power plant projects can be disruptive to
surrounding aquatic ecosystems both upstream and downstream of the plant site.
For instance, studies have shown that dams along the Atlantic and Pacific
coasts of North America have reduced salmon populations by preventing access to
spawning grounds upstream, even though most dams in salmon habitat have fish
ladders installed. Salmon spawn are also harmed on their migration to sea when
they must pass through turbines. This has led to some areas transporting smolt
downstream by barge during parts of the year. In some cases dams have been
demolished (for example the Marmot Dam demolished in 2007) because of impact on
fish. Turbine and hydroelectric power-plant designs that are
easier on aquatic life are an active area of research. Mitigation measures such
as fish ladders may be required at new projects or as a condition of
re-licensing of existing projects.
Generation of hydroelectric
power plant changes the downstream river environment. Water exiting a
turbine usually contains very little suspended sediment, which can lead to
scouring of river beds and loss of riverbanks. Since turbine gates are often
opened intermittently, rapid or even daily fluctuations in river flow are
observed. For example, in the Grand Canyon, the daily cyclic flow variation
caused by Glen Canyon Dam was found to be contributing to erosion of sand bars.
Dissolved oxygen content of the water may change from pre-construction
conditions. Depending on the location, water exiting from turbines is typically
much warmer than the pre-dam water, which can change aquatic faunal
populations, including endangered species, and prevent natural freezing
processes from occurring. Some hydroelectric projects also use canals to divert
a river at a shallower gradient to increase the head of the scheme. In some
cases, the entire river may be diverted leaving a dry riverbed. Examples
include the Tekapo and Pukaki Rivers in New Zealand.
Greenhouse gas emissions
Lower positive impacts are
found in the tropical regions, as it has been noted that the reservoirs of
power plants in tropical regions may produce substantial amounts of methane and
carbon dioxide. This is due to plant material in flooded areas decaying in an
anaerobic environment, and forming methane, a very potent greenhouse gas.
According to the World Commission on Dams report, where the reservoir is large
compared to the generating capacity (less than 100 watts per square metre of
surface area) and no clearing of the forests in the area was undertaken prior
to impoundment of the reservoir, greenhouse gas emissions from the reservoir
may be higher than those of a conventional oil-fired thermal generation plant.
Although these emissions represent carbon already in the biosphere, not fossil
deposits that had been sequestered from the carbon cycle, there is a greater
amount of methane due to anaerobic decay, causing greater damage than would
otherwise have occurred had the forest decayed naturally.
In boreal reservoirs of
Canada and Northern Europe, however, greenhouse gas emissions are typically
only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new
class of underwater logging operation that targets drowned forests can mitigate
the effect of forest decay.
In 2007, International Rivers
accused hydropower firms for cheating with fake carbon credits under the Clean
Development Mechanism (CDM), for hydropower projects already finished or under
construction at the moment they applied to join the CDM. These carbon credits –
of hydropower projects under the CDM in developing countries – can be sold to
companies and governments in rich countries, in order to comply with the Kyoto
protocol.
Population relocation
Another disadvantage
of hydroelectric power plant is the need to relocate the people living
where the reservoirs are planned. In February 2008, it was estimated that 40-80
million people worldwide had been physically displaced as a direct result of
dam construction. In many cases, no amount of compensation can replace
ancestral and cultural attachments to places that have spiritual value to the
displaced population. Additionally, historically and culturally important sites
can be flooded and lost. Such problems have arisen at the Three Gorges Dam
project in China, the Clyde Dam in New Zealand and the Ilısu Dam in
Southeastern Turkey.
Affected by flow shortage
Changes in the amount of
river flow will correlate with the amount of energy produced by a dam. Lower
river flows because of drought, climate change or upstream dams and diversions
will reduce the amount of live storage in a reservoir therefore reducing the
amount of water that can be used for hydroelectricity. The result of diminished
river flow can be power shortages in areas that depend heavily on hydroelectric
power plant.