The Eruption of Nevado Del Ruiz Volcano Colombia, South America, November 13, 1985 (1991) / Chapter Skim
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2 The November 13, 1985 Eruption and Subsequent Lahars
2 The November 13, 1985 Eruption and Subsequent Lahars
Pages 12-42
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From page 12... ...
Light ash fall was reported in Manizales and Chinchina, 25-30 km northwest and west of the summit. An avalanche, initiated when the eruption mobilized snow, ice, and volcanic debris, descended 27 km down the upper Azufrado River valley (McClelland and Simkin, 1986~.
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From page 13... ...
Herd (1986) notes that they scoured and melted substantial amounts of the ice cap surface, and that these meltwaters were channelled into the Las Nereidas, Molinos, Guali, Azufrado, and Lagunillas valleys.
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From page 14... ...
The rapid accumulation of hot pyroclastic material on the ice cap during the eruption accelerated the ice-melting process and added large quantities of water to the headwaters of valleys draining the ice
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From page 15... ...
This strength reduction is due to the excess pore water pressure buildup that develops in loose granular soils during the shearing process and which ultimately can become so large that the effective stress between grains approaches zero. Before the eruption, a large percentage of the available shear strength of the soil was required to maintain stability of the sediments on the steep 10°12° slopes (Point A in Figure 2.1~.
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From page 16... ...
The study team believes that the earthquake-generated ground motions recorded by Acosta (1845) could have played a major factor in "liquefying" large amounts of loose, saturated sediments deposited by previous eruptions near the summit, resulting in the debris flows.
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From page 17... ...
The study team's field efforts were concentrated on the three eastern and northeastern flows, which can be summarized as follows: The northeast lahar descended the Guali River valley in a northeasterly direction and flowed approximately 58 km to the foothills at the western edge of the Magdalena plain. The lahar flowed out from the narrow canyon channel 1.5-2.0 km onto the plain, and then turned almost 90° to the north, continuing down the Guali River valley.
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From page 18... ...
The upper Azufrado River valley is aligned with the 3km-wide lahar-filled trench produced by previous volcanic eruptions. From the vicinity of the summit ice cap, the Azufrado River flows northeast for approximately 26 km to an elevation of 1,800 m, and then turns almost 150° south for another 10-12 km, at which point it joins the Lagunillas River.
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From page 19... ...
The central part of Armero, built on higher deposits from previous lahars, was not swept away by the 1985 mudflows (Figure 2.6~. The hills of Tertiary sedimentary rocks immediately east of Armero, with elevations up to 500 m above sea level, further prevented the lahars from flowing directly east into the Magdalena River.
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From page 20... ...
Downstream from Armero the main flows followed the southeast-oriented Lagunillas River channel for 2-3 km and then shifted into a more easterly direction north of the present Lagunillas River channel, which may have been partially filled by previous lahars (Acosta, 1845~. The lahar continued to flow east for approximately 6 km on a relatively flat surface with an average inclination of 0.3-0.4 degrees (8 m/km)
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From page 21... ...
Aerial photographs taken within a few days after the eruption show high flow marks several meters above the present lahar surface that were left by these reservoirs of mud. The northern branch of the lahar that spilled out of the Lagunillas River canyon into the floodplain swept away the northern section of Armero and abutted against sedimentary rock hills immediately northeast of the city.
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From page 22... ...
The study team ahmin~.rl the. arrival time Ot th'^^ 1~;~^ ~ —- ~ ~ ~ ~ ~ ~~ ~— ~~ ~~ ~ ~— ~V~~Ll~ll O By ~nrerv~ew~ng eyewitnesses during the course of the team's field work.
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From page 23... ...
Interpolations of the average flow velocities downstream from the foothills toward the Magdalena River, suggest a mean flow velocity of approximately 10-15 km/hr on the plain. This estimate corresponds well with eyewitness accounts of persons who were saved by sprinting ahead of the flow for short distances to reach safety in the hills.
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From page 24... ...
Honda Containment of flood advance by sedimentary hills east of Armero Near junction of Guali Flood damage to building and Magdalena rivers foundations Floodplain—1.5 River plain of Guali Size of boulder transported by km east of Guali River mudflow River canyon
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From page 25... ...
As the flows descended, they eroded valley walls and floors, stripping away a wide range of materials ranging in size from the clays derived from altered bedrock and the organic layer to large alluvial boulders weighing several tons. The particle size distribution, extent, and morphology of the lahar deposits at various locations beyond the mouths of canyons have similarities to typical alluvial fan deposits.
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From page 26... ...
This general pattern of distribution of the coarse lahar deposits was observed in the floodplain areas near the edge of the foothills, within a zone 5-7 km from the mouth of the river canyons. Beyond this zone, the large boulders and cobbles decrease in maximum size and abundance and gradually disappear.
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From page 27... ...
approximately 2.3 km downstream from the mouth of the Guali River canyon (Site 13, Table 2.2) , the study team encountered a large, angular boulder (approximately 8 m long, 6.5 m wide, and 4 m tall)
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From page 28... ...
The boulders were found approximately 6 km downstream from the mouth of the Lagunillas River canyon. THICKNESS OF LAMAR DEPOSITS A simple method for determining the thickness of the viscous mudflows at various locations along their flow paths was to make estimations by
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From page 29... ...
Farther east, in central Armero, at an approximate distance of 2.5 km from the canyon mouth, the accounts of survivors who spent the night on top of the hospital indicate that the mudflow almost reached the roof of the building. The study team observed that the lahar deposits at this location completely filled the first floor of the hospital building and partially filled : ~~ :: ~ ::: : ~~::~::~:~:~ ~~:~ ~~-~ ~~ ~~ ~~ - : :~ i: : ~ i: ~ :~ Ace: ~ ~~ :~ ::~ : : i: ~ ~~ ~ ~ ~~ i: ~ ~ ~ : FIGURE 2.12 A granary complex located within Armero.
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From page 30... ...
Along the northern branch of the Lagunillas lahar, at a location 0.5 km north of Arm ero (2.5 km from the canyon mouth) , the height of the mud traces left on a large standing tree reached approximately 4-4.5 m above the ground surface; debris was entangled in tree limbs at this height.
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From page 31... ...
A view toward the downstream side of the warehouse (Figure 2.16) shows that the long axis of the warehouse is in an east-west direction perpendicular to the course of the Santo Domingo Creek, which nans between the warehouse and the hills shown in the background.
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From page 32... ...
Approximately 1.5 km north of warehouse II, the study team examined the lahar deposits near a former ranch. Although the buildings were still standing, moderate structural damage had occurred.
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From page 33... ...
The fact that the lahar overtopped the culvert indicates that it was up to 4 m thick at this point. The thickness of the northern branch of the Lagunillas lahar was measured near its terminus at the National Serpentarium Administration Building, located approximately 5.5 km north of Armero (Site 1A, Table 2.2~.
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From page 34... ...
Additional measurements of the thickness of the lahar deposits in the floodplain near Arm ero were carried out in 1986 by INGEOMINAS using electrical resistivity methods. Vertical profiling was conducted at seven locations on the lahar using a Schlumberger array.
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From page 35... ...
THE NOVEMBER 13, 1985 ERUPTION ID SUBSEQUENT HIS 35 FIGURE 2.18 The administration building at a serpentarium 5.5 km north (downstream) from Armero.
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From page 36... ...
36 TABLE 2.3 Estimated Thickness of Lagunillas Lahar TlIE NEVADO DEL RUIZ VOLCANO Site Approximate Distance from No. Location Canyon Mouth Depth of Flow Notes 9 Lagunillas River 1.0 km upstream 30 m above Estimated valley: 1 km river bed upstream canyon's mouth 6 Armero warehouse I: 1.0 km in front 7-8 m Measured on west side of town of canyon's mouth Armero: tall trees 2.5 km in front 4-5 m at mudline Measured 300 m north of town of canyon's mouth + 1.5 m of sediments = 5.5-6.5 m total depth 3 Warehouse II: 1.5 3 km north of 2.5 m at mudline Measured km north of Armero canyon's mouth + 1.5 m of sediment = 4.0 m 1A Serpentarium: 5.5 6.0 km north of km north of Armero canyon's mouth total depth 0.9 m at mudline Measured + 0.3 m of sediment= 1.2 m total depth TABLE 2.4 Measured Thickness of Lahar Deposits in the Vicinity of Armero Measurement Approximate Distance No.
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From page 37... ...
During its inspection three months after the eruption, the study team found that in some low-lying areas the surface of the deposits was still moist and that small amounts of surface dewatering could occasionally be observed. Accounts by eyewitnesses reaching Armero in the early hours of the morning after the eruption indicate that the surface water coming out of the lahar was hot enough to burn the flesh of survivors and that associated vapors had a strong sulfuric odor.
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From page 38... ...
not, at first, safely reach the victims and, when they could, found the people severely burned and encased in hardened mud. At the time of the field inspection, the primary consolidation was nearly complete, and shrinkage and desiccation of the lahar deposits were well under way.
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From page 39... ...
A quantitative evaluation of the reduction in the thickness of the initial mudflow was carried out based on measurement of the uniformly horizontal mudflow tracks left on the wall of the Administration Building. The surface of the mudflow reached a height of approximately 90 cm above the initial ground surface.
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From page 40... ...
If an assumption is made that most of the lahar's thickness reduction took place during the primary consolidation process, an estimated value of the unit strain induced can be obtained using Equation 2.1 as follows: e S= H 1 + e (2.1) - o Where S = settlement of the layer e = change in void during primary consolidation process e0 = initial void ratio of the deposits before primary consolidation started H = initial thickness of the layer Replacing values (Equation 2.2)
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From page 41... ...
Several eyewitness accounts indicated that a substantial portion of the primary consolidation occurred within a few days to a few weeks after deposition. The fluidity of the Lagunillas lahar was noted in a partially excavated pit around a jeep buried in the lahar deposits.
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From page 42... ...
This observation indicates that a substantial thickness of the lahar had densified and strengthened in a relatively short period of time. Other phenomena besides desiccation and shrinkage could have contributed to the thorough and rapid hardening of the lahar deposits, for example, the development of a cementation between particles as a result of the combination of the acidic fluids and elevated temperatures within the voids, depositing chemical precipitates in intergranular voids during cooling.
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