El Salvador - GFDRR

I. DESCRIPTION OF THE EVENT | 17
C. HOW UNUSUAL IS THIS EVENT? HISTORICAL EVIDENCE OF SIMILAR EVENTS
With regard to the extreme rainfall, according to historical records and recorded landslides, and with
respect to climate events that affect the country, this event is neither unusual nor infrequent. There is
information on previous events and there are records from the last 100 years about various lahars with
disastrous consequences in the San Vicente area. Most of them were the result of intense rains (lasting 5
to 7 days straight, according to witnesses’ testimonies).
BOX 1. THE SEVEREST LAHARS IN EL SALVADOR
1774: Lahar on the northeastern slope of the volcano, affecting the city of San Vicente.
1912: Lahar totally destroys Verapaz, causing numerous deaths both in Verapaz and in Agua Agria, a
village higher up on the slope of the volcano.
1934: Total destruction of Tepetitán by a lahar that originated in El Blanco creek, with numerous victims
(for this reason, Nueva Tepetitán was founded). Likewise, Guadalupe was struck in 1934 by a
lahar that originated in El Derrumbo creek, destroying various houses and causing victims.
1996: On the southern slope of the volcano, a landslide blocked the main road between Tecoluca and
Zacatecoluca.
2001: Landslides caused by the February 13 th earthquake on the northern and northwestern slopes
flowed toward the valley although they did not become lahars. However, the landslides deposited
over 200,000 cubic meters of sediments in the channels that drain the volcano, and thus the
instability of these sediments represents an increased risk for floods and lahars downstream
toward Guadalupe, Verapaz and Tepetitán, as demonstrated in 2009.
2001: On September 15 th , Guadalupe was struck by a lahar from El Derrumbo creek, triggered by rains.
There was one victim and numerous houses were destroyed in approximately the same zone that
was affected by the 1934 event.
2005: Tropical Storm Stan produced several landslides on the eastern shore of Lake Ilopango.
The period of return (recurrence) of situations with intensities similar to the current one is difficult to
estimate due to the lack of continuous series of precipitation by hour per day. However, it may be stated
that a critical factor was the 7-hour period of rain. The major damages caused by landslides appear to be
associated with the presence of tropical storms lasting 3 to 4 days. If rains occur for periods of 5 to 10
hours, with per-hour intensities over 30 to 50 mm in the final phase of a storm or depression, when the
soil is saturated it is almost certain that a catastrophic event will occur.