El Salvador - GFDRR

12 | El Salvador: Damage, Loss, and Needs Assesment
2. Triggers
The main triggers of landslides were the rains that took place from November 7 to 8. The rain had two severe
consequences: an increase in water flow in natural drainage channels on the lower slopes of the San
Vicente (Chinchontepec) volcano, and an increase in the pressure of water runoff on deposits of loosely
compacted materials and on bedrock.
Due to the wind from the north that accompanied the precipitation, the slopes facing this direction
experienced rain gusts that generated major environmental humidity which cooled when it encountered
an obstacle. The increased runoff resulted in erosion on the floor of river channels and on riverbanks.
Cuts on steeper slopes destabilized adjacent slopes and caused an increase in the materials that
flowed toward streams. The slope also gave rise to the accumulation of runoff pressure which reduced
the slope’s stability. When a large part of the slope totally collapsed, river streams were blocked and the
rupture of these obstructions, due to pressure, caused the lahars. All of these processes caused a flow of
water with a high concentration of sediments. When the discharge of this mixture exceeds a critical value,
the flow becomes a lahar. Under these circumstances, the mass does not behave according to classic laws
of hydraulics and what scientists call “non-Newtonian flows” occur, since the ratio of water to solids is
less than 1 and sometimes reaches values of 0.50 or even less. Next, sedimentation of materials occurs
when the lahars reach less steep slopes, at the foot of the volcano, clogging channels and reducing the
rivers’ carrying capacity (drainage or load). This caused new river courses to open, the most notable case
being that of Verapaz. Another element in this process is the pulsating character of the flow: the frontal
part of the lahar contains thicker materials that reduce its speed, causing dams which, when they break
up, make a new acceleration possible. Eyewitnesses in Verapaz observed several surges prior to the main
landslide that devastated the city.
Smaller lahars continued along the channels of rivers without entering towns, although the volumes
of materials filled these channels and caused them to overflow in several directions, affecting nearby lands
and vast populated zones. Measurements by D-SNET and MARN make it possible to estimate the magnitude
of the principal lahars that surged from the San Vicente volcano.
Stream
(population affected)
TABLE 3. CHARACTERISTICS OF LAHARS ON SAN VICENTE VOLCANO
Sedimentation area
Thickness of
sedimentation
Volume of
sedimentation
Runoff distance
El Derrumbo (Guadalupe) 250,000 m 2 0.5 – 2.5 m 360,000 m 3 6 km
La Quebradona (Verapaz) 150,000 m 2 0.5 – 2.0 m 250,000 m 3 6 km
El Amate Blanco (Tepetitán) 250,000 m 2 0.5 – 2.0 m 300,000 m 3 6 km
Source: D-SNET/MARN.