The State of Dams and Levees following the M8.3 Illapel, Chile Earthquake

October 5, 2015 by  
Filed under Dams and Levees

Following the September 16, 2015 M8.3 earthquake near Illapel, Chile, there is little reported damage to dams.  The Chilean President noted on September 17 that dams in the country were in good condition.  The GDACS Earthquake Impact report lists only one dam, the Cogoti dam, as at risk.  The Cogoti dam is 92 km from the earthquake epicenter.  Peak acceleration contour maps from the USGS following the earthquake suggest a peak ground acceleration between 0.2 and 0.3g at the Cogoti Dam (approximately located at the red dot).

map_dam

The Cogoti Dam is a concrete face rockfill dam completed in the late 1930s.  Dams usually fail by floods and not earthquakes.  Due to the large mass of the rockfill, earthquakes generally do not induce the high pore water pressure that is necessary for instability and liquefaction.  Reports from July 2015 indicate that the reservoir behind the dam is dry due to a drought, thus the risk of liquefaction is low to nonexistent.  The dam has previous been subjected to several large earthquakes, including in 1943 when it experienced peak ground acceleration of 0.19g, and in 1997 when it experienced a peak ground acceleration of approximately 0.27g.  The 1943 earthquake induced significant dam embankment settlement estimates of between 400 and 600mm, though investigators have concluded that the settlement was inevitable.  They also concluded that further earthquakes would cause only minor embankment settlement.

Damage to levees as a result of the M8.3 earthquake on September 16, 2015 has not yet been reported as of the date this posting. After the M8.8 Chile Earthquake on February 27, 2010, there was also limited reported damage to levees and other earthen structures.

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