Don’t freak out: Dams generally do well in earthquakes

View from Lookout Point Dam (Photo by Sarah Rask, Hydroelectric Design Center)

View from Lookout Point Dam (Photo by Sarah Rask, Hydroelectric Design Center)

 

By Scott Clemans, Public Affairs Office

Scientists tell us that the Pacific Northwest is due for a very large earthquake – possibly as large as magnitude 9.0 – from the Cascadia Subduction Zone off the Pacific coast. (U.S. Geological Survey graphic)

Scientists tell us that the Pacific Northwest is due for a very large earthquake – possibly as large as magnitude 9.0 – from the Cascadia Subduction Zone off the Pacific coast. (U.S. Geological Survey graphic)

The Pacific Northwest experienced a 9.0 magnitude earthquake on Jan. 26, 1700.  316 years later, scientists tell us the Pacific Northwest is due for another very large earthquake a – from the Cascadia Subduction Zone off the Pacific coast. Such a large earthquake could potentially impact our dams.

The historical performance of dams in seismic events has been exceptionally good. Only one concrete dam in modern history has ever failed as the result of a seismic event, mainly due to the fault running directly beneath it. Generally, concrete dams have sustained only minor damage.

The data on the performance of embankment dams is more limited, but the number that have failed as the result of a seismic event is still extremely small.

For example, in the March 11, 2011, 9.0 earthquake off Japan – basically the western Pacific version of a Cascadia event – only one small irrigation dam (of older and possibly inadequate design and construction) completely failed. Of 252 dams inspected the next day, six other embankment dams had shallow cracks on their crests, but were functioning with no problems.

In another recent example, no embankment dams failed and only a few suffered more than minor damage in the Feb. 27, 2010, 8.8 earthquake off Chile – basically the South American version of a Cascadia event.

The likelihood of a complete dam failure as the result of a seismic event depends on the size and location of earthquake, the reservoir level, the dam’s current operational status and a host of other factors. But in general, the risk of a dam failure in the U.S. contributes only a very small portion of the overall risk to even the most exposed individuals living below a dam.

U.S. Army Corps of Engineers dams were built to the seismic standards of the day; the agency is now conducting studies of all its dams to better understand their vulnerability to a Cascadia-level event. Experts expect that at least some Corps dams would sustain deformation and other damage – including damage to spillway gates, regulating outlets and/or powerhouses – that might impact the Corps’ ability to manage downstream flows, but may not necessarily lead to a catastrophic dam failure.

Regardless of likelihood, though, one of the most effective risk reduction measures for such an event – or any other lesser dam safety emergency – is an effective evacuation plan. The Corps encourages residents downriver from its dams to work with their local emergency management agencies to understand their risk, learn to how get emergency alerts, and develop a plan to respond.

The 2011 earthquake off Japan – basically the western Pacific version of a Cascadia event – caused the failure of only one small irrigation dam and damage to six others out of over 250 in the area. (Japan Meteorological Agency graphic)

The 2011 earthquake off Japan – basically the western Pacific version of a Cascadia event – caused the failure of only one small irrigation dam and damage to six others out of over 250 in the area. (Japan Meteorological Agency graphic)

If any earthquake were to occur, the Corps’ emergency action plan is clear: Dam operators would make immediate inspections, searching for evidence of damage or disturbance. Depending on the magnitude and distance of the earthquake and the intensity of the effects in the immediate vicinity, a team of dam safety experts might also be dispatched to conduct more in-depth technical inspections and make recommendations for remedial action, if necessary.

Obviously, if the operators’ inspection finds the dam’s ability to manage downstream flows is compromised, the Corps would immediately alert city and county emergency managers downriver, who would in turn issue alerts and evacuation notices as needed.

The Corps regularly conducts complex drills with county and city emergency managers to plan how everyone would jointly work through such emergency scenarios.