Dams

Introduction

Dams are structures built across streams or rivers, usually to block water flow and cause the formation of a pond or lake upstream of the dam. The water impounded behind a dam can be used for drinking, irrigation of crops, or generating electricity. Electricity generated by dams, termed hydroelectricity, is made by allowing water to run downhill through fanlike turbines, a modern application of the ancient waterwheel principle for extracting work from falling water. The turbines turn generators that produce electricity. About 19% of the world’s electricity was hydroelectricity as of 2007.

Dams have many effects on their local environments and have long been criticized for flooding landscapes, destroying or damaging downstream ecosystems, closing off fish migration routes, and in some cases forcing the evacuation of millions of people. However, for many years it was assumed that dams are a zero-pollution, climate-neutral way to produce electricity, since they burn no chemical fuel and emit no carbon dioxide (CO₂) in daily operation. However, in the 1990s, researchers realized that some dams, especially in the tropics, are significant sources of methane, a potent greenhouse gas. Research published in 2006 reported that dams may account for more than 4% of anthropogenic (human-caused) greenhouse warming.

Historical Background and Scientific Foundations

Overview

The first known large dam was built by the Egyptians around 2800 BC. It was a massive barrier 37 ft (11 m) tall and capable of impounding 20 million cubic ft (566,000 cubic m) of water. The dam failed after a few years when water overflowed its crest, eroding the downstream side of the structure and weakening it until the dam collapsed.

Since that time, many civilizations have built dams of various sizes. However, truly large dams were not built until the 1930s, when engineers began building dams large enough to block whole river basins. Hoover Dam was completed across the Colorado River in the United States in 1936, becoming the world’s largest dam at that time. When the lake behind the dam had filled, it backed 110 mi (177 km) upstream and covered 247 square mi (639 square km). Hoover is still the second-tallest dam in the United States.

Since Hoover, thousands of dams have been built worldwide. Over half (172 out of 292) of the world’s large rivers are now dammed at one point or more. All of the world’s largest 21 rivers are dammed. Globally, there are more than 45,000 large dams, that is, dams over 49 ft (15 m) high, which altogether store more than 1,557 cubic mi (6,500 cubic km) of water, an amount equal to 15% of the world’s annual freshwater runoff. There are over 300 very large dams, namely those over 492 ft (150 m) high or impounding over 6 cubic mi (25 cubic km of water). The latest of these giant dams, and the most controversial and environmentally destructive to date, is the Three Gorges dam across the Yangtze River in China. It will impound over nine cubic mi (39 cubic km) of water after filling and is forcing the evacuation of over a million people.

As of 2004, 2.2% of the world’s primary energy was obtained from hydroelectric dams, or about 19% of the world’s electricity. Irrigation water from dams is used to grow 12-16% of the world’s food.

Dams and Climate Change

There are several connections between dams and climate change. First, hydroelectricity was long assumed to have less global-warming impact than electricity generated

WORDS TO KNOW

ANAEROBIC BACTERIA: Bacteria that grow without oxygen, also called anaerobes.

ANTHROPOGENIC: Made by humans or resulting from human activities.

CLEAN DEVELOPMENT MECHANISM: One of the three mechanisms set up by the Kyoto Protocol of 2007 to, in theory, allow reductions in greenhouse-gas emissions to be implemented where they are most economical. Under the Clean Development Mechanism, polluters in wealthy countries can obtain carbon credits (greenhouse pollution rights) by funding reductions in greenhouse emissions in developing countries.

FOSSIL FUELS: Non-renewable fuels formed by biological processes and transformed into solid or fluid minerals over geologic time. Fossil fuels include coal, petroleum, and natural gas.

GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.

HYDROELECTRICITY: Electricity generated by causing water to flow downhill through turbines, which are fan-like devices that turn when fluid flows through them. The rotary mechanical motion of each turbine is used to turn an electrical generator.

METHANE: An odorless, colorless, flammable gas, with the formula CH₄, that is the simplest hydrocarbon compound and the major constituent of natural gas.

WORLD BANK: International bank formed in 1944 to aid in reconstruction of Europe after World War II, now officially devoted to the eradication of world poverty through funding of development projects.

using fossil fuels. Although CO₂ is emitted during the manufacture of the cement, concrete, and steel that go into making a modern dam, these emissions, for a typical dam, amount to less than 10% of the emissions from generating the same amount of electricity by burning fossil fuels. However, starting in 1993, researchers began noting that some large dams might be significant sources of the greenhouse gas methane (CH₄), which has about 23 times the global-warming effect of CO₂, ton for ton. As of 2007, this issue continued to be hotly debated, with some scientists challenging the objectivity of scientists working for (or partly funded by) the hydropower industry, who have done most of the research on dam methane to date.

Methane is produced after the dam is constructed. When a typical tropical dam is built, large areas of upstream forest are drowned. Wood and other types of organic (carbon-containing) matter that decay under water are mostly broken down by anaerobic bacteria—bacteria that thrive in the absence of oxygen—using digestive chemistry that produces CH₄ as a waste product; in contrast, microorganisms that digest organic matter where oxygen is present produce CO₂. The anaerobic rotting of wood and other organic matter under water produces methane.

An initial pulse of methane is released as the trees, soil carbon, and forest litter covered by the filling of a new dam lake rot. However, this is not the major source of dam methane, which continues to be produced as long as the dam stands. In most tropical areas, rainy seassons alternate with dry seasons. During the dry season, water continues to flow through the dam and the lake water level lowers. This creates large mud flats and shallow zones, because tropical river valleys are generally wide and gently sloping rather than narrow and steep. Water weeds and other soft, quick-growing vegetation flourish in these flats and shallows.

When the rainy season comes, the water level rises again, drowning these plants, which quickly decay underwater, releasing methane. In effect, the rising and falling lake is a vast, solar-powered machine for harvesting carbon from CO₂ in the air and re-releasing it as CH₄ with greatly increased greenhouse warming effect.

In 2005 Philip Fearnside, one of the most-cited researchers on methane production by dams, found that in 1990, 13 years after its lake filled, the Curua-Una Dam in Brazil emitted 3.6 times more greenhouse gas than oil-burning plants generating the same amount of electricity would have emitted. That is, the dam released enough methane to cause 3.5 times as much global warming as the CO₂ released by the oil plants would have.

Ivan Lima and colleagues, among other scientists, have suggested that at least some of this methane could be collected and burned to produce electricity. This would both extract useful energy and convert the methane (CH₄) to CO₂, greatly reducing its greenhouse potential. Most dams allow deep, high-pressure, methane-rich water to run through their turbines. This causes the release of much of the methane in the water and may account for up to 70% of methane emissions from dam lakes (the rest is emitted from the lake surface). Some or most of this methane might be collected for burning. As of 2007, however, such technologies had not yet been demonstrated on a practical scale.

There are other relationships between dams and climate change. Dams in high latitudes (far from the equator) do not produce large amounts of methane, and so can indeed produce electricity with less greenhouse impact than comparable fuel-burning plants would. This does not reduce their other environmental impacts, however. Counting all dams worldwide, the greenhouse impact of dams is less than half of the least-polluting fuel-burning alternatives.

Dams are also linked to climate change because they may serve as storage reservoirs in places where precipitation patterns and flood risk may be altered by climate change. Thus, some dams may help with mitigation of and adaptation to the effects of climate change. Finally, some dams may have to contend with decreased rainfall and water flow from changing climate, which might reduce water and energy benefits from those dams.

Impacts and Issues

Reports in the 1990s of significant methane emissions from dams led to intense debate on the climate-change mitigation merits of large dams. In 1997 a workshop was convened by the International Union for the Conservation of Nature and Natural Resources (an 83-state organization headquartered in Geneva, Switzerland) and the World Bank to discuss the question. The two groups established the World Commissionon Dams to investigate the effectiveness and costs of dams, including their relationship to climate change.

In November 2000 the commission released its report. Although stating that “dams have made an important and significant contribution to human devel-

opment, and benefits derived from them have been considerable,” the report also said that “in too many cases an unacceptable and often unnecessary price has been paid to secure those benefits, especially in social and environmental terms, by people displaced, by communities downstream, by taxpayers and by the natural environment.” The commission estimated that 40 to 80 million people had been displaced by dams and many species driven to extinction; it also stated that most of the benefits from 1% to 28% of anthropogenic (human-caused) greenhouse emissions might come from dams. A study by Ivan Lima and colleagues published in 2006 stated that 4% of global emissions are from dams.

The World Commission on Dams made a number of recommendations for how proposed dam projects should be judged, including whether people who would be displaced consent to the project. Its findings have been controversial because one of the commission’s founders, the World Bank, is the world’s largest funder of dam projects, having provided a total of $75 billion (1998 dollars) for 538 large dams in 92 countries since its inception in 1945. The World Bank has not accepted the commission’s recommended international standards for dam-building. Europe’s two largest public banks, the European Investment Bank and the European Bank for Reconstruction and Development, announced in 2005 that they would abide by the commission’s standards. A directive of the European Union mandates that carbon-trading hydropower projects under the CleanDevelopment Mechanism must abide by the commission’s standards.

Primary Source Connection

The following news article recognizes that in the United States, more than 87,000 dams need to be repaired. Many of these dams have already proved to be hazardous, such as a dam in Hawaii that collapsed and killed 7 people in 2006. Such obstacles as the high number of aging dams starting to deteriorate, the low number of dam inspectors, and a lack of funding for the dam safety program has interfered with the upkeep of the country’s dams.

PROBLEM DAMS ON THE RISE IN US

The Kaloko dam in Hawaii stood 116 years—until last year when it collapsed after heavy rains, killing seven.

Potential disaster was averted in April in Hollis, N.H., when a dozen families were evacuated and engineers made a controlled breach of an old pond dam to keep it from failing.

Such incidents are warning signs that many of the nation’s more than 87,000 dams are in need of repair. Last month’s high-profile collapse of the 1-35 bridge in Minneapolis focused America’s attention on bridge problems. The nation’s dams are worse off.

In 2005, the last time the American Society of Civil Engineers rated America’s infrastructure, bridges received a “C” grade; dams earned a “D.”

Even that rating may be generous, a Monitor analysis of dam-inspection data shows. Since 1999, the number of “high-hazard” dams rated “deficient” has more than doubled, according to data from the Association of State Dam Safety Officials (ASDSO) in Lexington, Ky. High-hazard dams are those whose failures could cause fatalities. In 1999, the US had 546 such dams rated deficient. By last year, it had 1,333.

A second category of “significant-hazard” dams (so-called because they threaten substantial property loss) saw a rise from 339 to 949 deficient dams over the same period. In all, 2.6 percent of the nation’s dams are deficient, according to the ASDSO.

“The growth of deficient high-hazard dams in this country is a major issue,” says Brad Larossi, legislative chairman for the ASDSO, which represents dam-safety inspectors in all states. “The trend is rising at such a steep slope, much faster than states can do [dam] rehabilitation. Without question the overall trends are clear.”

Several factors are behind the rise. Old dams continue to deteriorate or may fail suddenly because of inadequate spillways and trees growing on dams. Many states don’t have enough dam engineers to keep up proper maintenance, causing the repair backlog to grow. And as more homes and businesses are built closer to dams, the hazards increase, a phenomenon dam-safety experts call “hazard creep.”

Some experts claim that some of the rise is due to better reporting, an encouraging sign. “To be frank, there’s been in the past a reluctance in some quarters to identify too many dams as deficient,” says Mark Ogden, administrator for dam-safety engineering at the Ohio Department of Natural Resources in Columbus. “But there’s also been a strong effort by our association to increase awareness of this problem. We all are realizing we need an honest assessment.”

Some states are seeing a faster rise in deficient dams than others. Pennsylvania leads the pack with 215 deficient high-hazard dams, 172 more than in 1999. Not far behind is Ohio, with an increase of 158. Other states, such as Colorado, New Jersey, and California have seen declines. Some of that is due to better funding, experts say. All three have boosted dam budgets by a third or more since 1999.

Those increases are in contrast to federal dam spending. The nation’s dam-safety program, which helps fund safety inspector and engineer training, has not been fully funded in at least five years, Mr. Larossi says. Actual funding is about $5.9 million, well below the $9 million budgeted, he says.

As a result, the number of full-time inspectors has not increased since 1997 (excluding Florida, which claims to have hired 45 inspectors). That leaves each inspector responsible for about 195 dams on average; the ASDSO recommends no more than 50.

“We have seen increased awareness over the importance of adequate funding for state inspectors, but these offices are still understaffed,” says Stephanie Lindloff, of American Rivers, an environmental group.

Mark Clayton

CLAYTON, MARK. “PROBLEM DAMS ON THE RISE IN US” CHRISTIAN SCIENCE MONITOR (SEPTEMBER 13, 2007).

See Also Clean Development Mechanism

BIBLIOGRAPHY

Periodicals

Fearnside, Philip M. “Do Hydroelectric Dams Mitigate Global Warming? The Case of Brazil’s Curua-Una Dam.” Mitigation and Adaptation Strategies for Global Change 10 (2005): 675-691.

Lima, Ivan B. T., et al. “Methane Emissions from Large Dams as Renewable Energy Resources: A Developing Nation Perspective.”Mitigation and Adaptation Strategies for Global Change (February 2007).

Yardley, William. “Climate Change Adds Twist to Debate Over Dams.” New York Times (April 23, 2007).

Web Sites

International Rivers Network. “Citizen’s Guide to the World Commission on Dams.” http://www.irn.org/wcd/wcdguide.pdf (accessed March 29, 2008).

New Scientist. “Hydroelectric Power’s Dirty Secret Revealed.” http://www.newscientist.com/article/dn7046.html (accessed March 29, 2008).

United Nations Environment Programme. “Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.” http://www.dams.org/docs/kbase/contrib/env253.pdf (accessed March 29, 2008).

World Commission on Dams. “Dams and Development: A New Framework for Decision-Making.” http://www.dams.org//docs/report/wcdreport.pdf (accessed March 29, 2008).

Larry Gilman