El Salvador - GFDRR

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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.
I. DESCRIPTION OF THE EVENT | 13 Specialized literature 9 offers models to determine the speed historically achieved by lahars. A “typical” lahar can reach great speed on a volcano’s steepest slope (up to 50 m/s). In the upper part of the alluvial fan, speed may vary from 15 m/s to 25 m/s, while in the lower part, in the fan’s softest zone, it may vary from 5 m/s to 15 m/s. However, it is surprising how lahars are able to maintain their movement for many kilometers, even on nearly flat lands (as occurred on the coastal plains of the Department of La Paz). The lahars moved 6 to 10 kms, carrying rocks weighing 10 to 20 tons, with slopes no steeper than 2 to 3 o . This was possible due to the high water content that reduced friction on the stream bed to nearly zero. The cut of the section of the lahar that flowed over Verapaz measured nearly 100 m 2 . With a flow speed of 10 m/s, the discharge must have been around 1,000 m 3 /s. Damage to vegetation and buildings in Verapaz shows that the lahar had a height of about 3 meters. If one assumes a speed of 10 m/s on the vertical wall, pressure would have been around 200 kPa (kiloPascals per area), which corresponds to 20 ton/m 2 , a force that even highly resistant buildings could not have withstood. 9 • Kinematic models of straight-line movement of flows: a) V = Vo + gt; where V: final speed (m/s), Vo: initial speed (m/s), a: acceleration of movement (m/s 2 ), and t: arrival time at stabilization zone (seconds). b) e = Vot + 1/2gt2; where e: the scope of material conveyed (in km). c) V2 = Vo2 + 2gh. • Darcy Law to determine the discharge of material conveyed in m3/seconds. Q = V. A; V: speed of flow (m/s), A: section of channel through which the flow moves given m 2 • Mathematical model of relationship for the calculation of geometric parameters of mudflow: V = Width (A, given in meters) x Length (l, given in meters) x Thickness (E, given in meters) = m 3 ; where E: is the scope of material conveyed in km; and A = width x l given in m 2 , being the physical area occupied by the mudflow.
Page 1: El Salvador Damage, Loss, and Needs
Page 5 and 6: | iii ACKNOWLEDGEMENTS We wish to e
Page 7 and 8: Acknowledgement | v The external su
Page 9 and 10: | 1 PRESENTATION At the request of
Page 11 and 12: | 3 SUMMARY AND CONCLUSIONS Through
Page 13 and 14: | 5 I. DESCRIPTION OF THE EVENT A.
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AUSTRALIA BELGIUM BRAZIL CANADA DEN
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El Salvador - GFDRR

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