The March 5, 1987, Ecuador Earthquakes Mass Wasting and Socioeconomic Effects (1991) / Chapter Skim
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5 Mass Wasting and Flooding
5 Mass Wasting and Flooding
Pages 51-82
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From page 51... ...
that are progressively closer to the epicenters of the earthquakes, located about 25 km NEW of Reventador Volcano: QuitoBaeza, Baeza-Salado, and Reventador Volcano (Figure 5.1~. Field observations for the first two areas were made mostly by land along the TransEcuadorian highway from Quito to the mouth of the Salado River; the observations in the Reventador area were made by car, helicopter, and airplane.
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From page 52... ...
The types of failures falls and topples—occurring the farthest from the epicenter confirm observations by Keefer (1984~. We also observed fresh slumps and soil flows in sandy morainal deposits at the headwaters of the Papallacta River (Figure 5.1)
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From page 53... ...
Some shallow debris flows in residual soils could be seen on the crests of slopes around the town. In addition, a small number of rock slides and rock glides, as well as a few slips and slumps in old and recent alluvial terraces (Figure 5.2)
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From page 54... ...
Slope failure began as slips at the highest points of the steep slopes; the slip surfaces coincide with the top of weathered rock and the slips moved rapidly downslope, becoming debris flows (Figure 5.3~. This type of landsliding endangered the highway and pipeline, but the slips were generally very thin (less than 1 m)
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From page 55... ...
Reventador Volcano Geological Background This area includes the greatest intensity of landsliding triggered by the March 5 earthquakes. The Reventador Volcano zone, as defined here, is located in the sub-Andean region; it centers on Reventador Volcano and is bounded by the valleys of the Salado River on the W
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From page 56... ...
headwalls of the slopes, and were transformed into debris avalanches arid debris flows that cascaded down the gullies onto the low terrace that forms the left bark of the river. River to 3,560 m at the top of Reventador Volcano.
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From page 57... ...
The main valley walls of the FIGURE 5.5 Landslides along the Trans-Ecuadorian highway 3 km W of its crossing of the Salado River. These rock falls, slides, and avalanches, which were triggered by the March 5, 1987, earthquakes, had blocked the highway almost continuously in die sketch shown in this photograph.
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From page 58... ...
58 _ ~ ~ of of of ~ ~ · ~—, ~ ° E C _ O 8 so 0 ° ~ , ~ of e_ e 9 e ~ ~ ' ~ _ ' ~ ~ , ~ ' , - ' ~ ' ~ As, _ ; c)
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From page 59... ...
Salado River have overall slopes that usually are between 25 and 35° in their middle reaches. Tributaries of the Salado have variable overall slope angles, about 40° in their midcourses but lower values in their lower courses and uppermost reaches.
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From page 60... ...
General Characteristics of Landslides in the Reventador Zone More than 90 percent of the observed landslides began as shallow slips or slides of residual soils and highly weathered rock on the uppermost parts of the slopes of the main valleys or on the slopes of the lower-order tributaries (Figure 5.8~. Average thicknesses of these slips were from 1.5 to 2.0 m, with a thickness range of a few decimeters to 5 m.
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From page 61... ...
Whether they began on a main valley slope or the valley slope of a lower-order stream, flows moved to the channels of the lowerorder tributaries and entrained the colluvial/alluvial fills along the channels, eroding the material to the bedrock surface. The debris flows then proceeded to the higher-order tributaries or to the main river valleys.
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From page 62... ...
The same threshold values were obtained for the valley walls of the Salado River and of the Coca River at the confluence of the Salado and Quijos rivers (Figures 5.9 A,B,C)
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Note reconstructed Trans-Ecuadorian highway bridge across the Salado River.
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v~4T=~ {a.) A: <25 A: 25- 75 C: 75-/00 o lo 20 30 40 ~ FIGURE 5.10 Map showing percentages of denudation of vegetation due to landsliding in the vicinity of Reventador Volcano, blarch 5, 1987.
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From page 65... ...
Coca River between the mouths of the Salado River and the Reventador River, denudation is greatest on the left wall of the Coca River valley; downstream from the Reventador River, denudation is about the same on both sides of the Coca River for comparable elevations. Opposite Reventador Volcano, denudation appears greater on the left side of the Coca valley because the ground surface continues to rise toward the crater, whereas the topography flattens at an elevation of 1,500 and 1,700 m on the right side.
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From page 66... ...
and involved residual soil that became very fluid during failure. Stability thresholds as a function of slope angle are 35 to 40° for the main valley walls of the Coca and Salado rivers and 30 to 35° for the upper portions of the ancient Reventador Volcano cone.
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From page 67... ...
The normal stress levels for 2-m-deep slides are not in excess of 0.5 kg/cm2; thus, these soils were tested at stress levels several times greater than field conditions at Reventador. A review of the literature on structured soils (loess, residual soils, sensitive clays)
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From page 68... ...
k=0.25 . 0.8 0.6 L 1 1 1 1 1 20° 30° /J~ k=0.35 40° 50° 60° 70° Slope Angle FIGURE 5.12 Pseudostatic and static safety factors vs.
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From page 69... ...
can explain the S-m-deep slope failure at the Salado pumping station with a value of k = 0.35, but cannot explain the frequent shallower failures (commonly 1 to 2 m deep) observed nearby.
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From page 70... ...
headwaters of the Malo River and 10 to 15 km from the upper Salado River, presumably on the basis of denudation intensity. Whereas the position of the epicenters may be open to question, we suggest that the increase in denudation near Reventador Volcano may also
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From page 71... ...
Degree of saturation greatly influFIGURE 5.15 Aerial view of the NE valley wall of the Malo River showing extreme denudation of slopes due to slips/avalanches/flows and of the valley bottom due to debris flows and flooding. Note the vegetation trimline that indicates the maximum height of the debris flow/flood, about 25 m above the current river level.
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From page 72... ...
Last, but not least, the materials near Reventador Volcano, being mostly residual soils formed on pyroclastic materials, may simply be more susceptible to failure. EVOLUTION OF MASS-WASTING PROCESSES One of the more striking characteristics of the mass wasting caused by the March 5, 1987?
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From page 73... ...
Whichever of these estimates is the more accurate, a large percentage of this huge mass of material combined with water in the Coca and Aguarico Rivers and their tributaries to form thick debris flows that descended these tributaries of the upper Amazon. Because these debris flows occurred at night, and thus were not easily observed, we are not sure of their character.
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From page 74... ...
"trimlines" on the lower valley walls, which indicated the heights to which the rivers rose when charged with the debris flows, and (3) damage to the Trans-Ecuadorian pipeline and highway due to deposition and erosion, we know that considerable flooding occurred (Figures 5.16A,B, 5.17A,B, and 5.18~.
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From page 75... ...
Note landslide (single arrow near center of photo) that badly damaged the Salado pumping plant of the Trans-Ecuadorian oil pipeline.
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From page 76... ...
The addition of this timber debris undoubtedly affected the physical character of the debris flows, and as discussed below, probably impeded their passage through narrow bedrock constrictions in the narrow stream valleys. As noted by Hakuno et al.
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From page 77... ...
"hydraulic" damming, in which stream flow, highly charged with debris, was impeded in passing through narrow bedrock constrictions in the stream channels, and (2) blockage of streams by debris flows issuing into the main stream from its tributaries.
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From page 78... ...
noted that the Upper Rhine River in Graubunden Canton, Switzerland, was briefly dammed in 1585, 1807, and 1868 by debris flows issuing from the Nolla Torrent. The Xiao River in northern Yunan Province, China, has been dammed briefly seven times in this century by large debris flows that issued from Jiangiia Gully, a major tributary of the Xiao.
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From page 79... ...
that were deposited in the channel of the Coca River immediately upstream from the mouth of the Malo River (Figure 5.21~. Possibly, another such debris-flow blockage occurred where the Salado River enters the Quijos-Coca River, because a large amount of sediment was deposited at this point (Figures 5.16 and 5.17~.
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From page 80... ...
Note trimline in jungle cover along lower left valley wall upstream of the river constriction. This trimline indicates the maximum height (about 10 m above current river level)
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1989. Transformation of dilative and contractive landslide debris into debris flows an example from Marin county, California.
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From page 82... ...
American Society of Civil Engineers, Geotechnical Engineering Division. Proceedings of the Specialty Conference on Engineering and Construction in Tropical and Residual Soils, Honolulu, January 1982, 30-57.
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