Second is shifting environmental conditions. Third is changing land uses. New housing and business development upstream is perhaps a greater near-term worry than climate change, Tschantz said. Meanwhile, development downstream puts more people in the path of potential destruction. Fourth is oversight.
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Federal dams have attentive managers, sophisticated monitoring, and cash for repairs. But more than three out of four dams are regulated by states, where the commitment to dam safety varies considerably. Roughly one-third of the most dangerous dams do not have emergency plans to guide a downstream evacuation, according to state data filed with the U. Nearly two-thirds of dams have private owners, such as homeowners associations that want lakefront property, who may have neither the desire nor the money for repairs. Added up, there are two conclusions: Dam safety has never been stronger.
And dam owners and regulators still have much work to do.
Then boom! A tropical storm comes zipping up and you realize that the dam is not so good. The universe of American dams is expansive. There are tailings dams that hold back a slurry of mine wastes, stock ponds for irrigation or watering cattle, and artificial lakes for sailing and speedboats. There are dams to detain flood waters and dams to filter debris. Then there are the hydropower behemoths such as Grand Coulee and Hoover, symbols of 20th-century engineering might. Though iconic, these canyon-bridging concrete plugs are the minority. Most dams are small structures less than 25 feet tall made of packed dirt and rock and built more than 50 years ago.
US Dam Infrastructure Earns a "D" in American Society of Civil Engineers Report Card | frESH
Surprisingly little is known about why individual dams fail. Few states do autopsies to learn precisely what went wrong. McCann has found that the U. Not every dam failure is judged by the same criteria. The United States has a three-tier rating system that classifies a dam based on the destruction resulting from failure. The rating system is used to set design standards; the greater the risk the stricter the codes. Low-hazard dams are expected to cause minimal property damage. Richland County, South Carolina notes that several of its dams will fail in a year flood.
Significant-hazard dams are a step up. They might destroy infrastructure or cause severe property damage if breached. The most worrisome category is high-hazard potential. A rupture at one of these dams could kill people. Eric Halpin, deputy dam safety officer for the U. It gets harder in the future: back to back wettest years followed by five years of drought. All this has an impact on dams. Dams should have an updated emergency action plan EAP. These plans coordinate actions between the dam owner, regulators, emergency responders, police, and the local community.
Plans organize the chain of command and establish triggers for evacuation orders. They include inundation maps that show the land that would be flooded if the dam released all its water.enter
In an Era of Extreme Weather, Concerns Grow Over Dam Safety
The Ka Loko Dam had no emergency plan. The number of high-hazard dams with an EAP has more than doubled since EAPs need to be reconsidered consistently. In , California tore down the foot-high San Clemente Dam on the Carmel River because the dam was at risk of collapsing in an earthquake. Whether a dam requires an EAP depends on its classification and state standards.
Some states require a plan only for high-hazard dams. That relates to a second problem: incorrect classification. Conditions above and below dams change. The number of high hazard dams has increased by 67 percent since , according to data in the National Inventory of Dams.
The need for a better accounting of how upstream development magnifies floods was one of the major conclusions of a Northeastern University assessment of the South Carolina dam failures. Counties in the center of the state saw 17 to 24 inches of rain in a hour period. Congressional Research Service.
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