Rock avalanche and resulting debris flow in Estero Parraguirre and Río Colorado, Regóon Metropolitana, Chile
Published:January 01, 2002
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A. Hauser, 2002. "Rock avalanche and resulting debris flow in Estero Parraguirre and Río Colorado, Regóon Metropolitana, Chile", Catastrophic Landslides, Stephen G. Evans, Jerome V. Degraff
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A rockslide took place at 10.33 a.m on November 29, 1987, starting from a location 4350 m above sea level on the western slope of Cerro Rabicano in the headwaters of Estero Parraguirre, a tributary of the Río Colorado, Regíon Metropolitana, Chile. It consisted mostly of limestone with gypsum and andesites, and was ~6.0 × 106 m3 in volume. The slide quickly became a rock avalanche, and later a hyperconcentrated debris flow with a volume of at least 15 × 106 m3 due to the incorporation of snow, ice, and sediments from the floor of Estero Parraguirre. Frontal waves were 20–30 m high; the high-energy flow moved with enormous destructive power, first through the channel of Estero Parraguirre, and later through the Río Colorado valley, and finally emptied into the Río Maipo after traveling ~57 km and descending a vertical distance of ~3400 m. During its rapid movement down the Río Colorado, the debris flow killed at least 37 people, and caused serious damage to the Maitenes hydroelectric plant (24 MW) and another hydroelectric plant (Alfalfal, 160 MW) that was under construction. The identification of several diamicton units deposited by prehistoric mud and debris flows in the valley of the Río Colorado, together with the enormous energy and great magnitude of destruction of the November 29, 1987, landslide event, confirm that this type of process is one of the major geological hazards in the Andean valleys of central Chile.
On Sunday, November 29, 1987, a remarkable landslide occurred in the Colorado River basin in the Andes of central Chile. The landslide was initiated by a rockslide near the Chile-Argentina border that became transformed into a high-velocity debris flow that flowed down Estero Parraguirre, a tributary of the Río Colorado, and down the Río Colorado before passing into the Río Maipo, 57 km from the landslide source.
The objective of this chapter is to review published and unpublished descriptions of the landslide and to present observations I made shortly after the November 29 event. In addition, I document the impact of the landslide on energy infrastructure facilities, drinking-water supplies, and agriculture in the region.
In addition to several unpublished reports by Chilectra S.A. and Directión General de Aguas on the events in the Río Colorado, a number of publications have reported aspects of the November 29 landslide. A brief statement of its damage was first reported by González-Ferrán (1987), who attributed the initial landslide to a regional summer thaw and the instability of the area as a result of a 1985 earthquake. This report was followed by a report by Velasco et al. (1988), who reported that the landslide began as a rockslide with an estimated volume of 3–4 × 106 m3, and attributed its cause to a combination of geological and hydrological factors. A second report by González-Ferrán (1988) presented additional information and included a detailed sketch map of the path of the landslide. He attributed the initial rockslide to earthquake shaking at 10:30 a.m. on November 29. Eisenberg and Pardo (1988) reported briefly on the nature of the seismograph records associated with the initial rockslide in Estero Parraguirre and suggested that seismic event associated with the landslide was generated by the landslide. Peña and Klohn (1988) briefly reported the event in the context of nonmeteorological flood disasters in Chile, but the first detailed published account was by Valenzuela and Varela (1991). Hauser (1993) presented a detailed summary of the Río Colorado event in his review of mass movements in Chile, and Casassa and Marangunic (1993) presented a description of the landslide and an examination of its causes, with particular emphasis on the role of incorporated snow in transforming the initial rock avalanche into a mobile debris flow.
The study area is located in the Regíon Metropolitana and is within a segment of the Río Colorado basin in the Andean region of central Chile, 50–60 km east of the capital of Santiago (Figs. 1 and 2). The area is accessible by a paved road up to the Maitenes hydroelectric plant. From there, secondary roads lead to the installations of the Alfalfal hydroelectric plant. The tributary valleys of the Río Maipo attract many tourists because of their pleasant climate and proximity to Santiago. In addition to the use of its local water resources for hydroelectric power, drinking water, and irrigation in the Santiago basin, this area is also the site of important limestone and gypsum mines, and livestock farming (ovine, bovine, and caprine).
Climate and hydrology
The area around the Maitenes hydroelectric plant (elevation 1200 m above sea level (a.s.1.; Fig. 2) has a warm temperate climate with mean annual temperatures in the range of 12–14 °C. At >1700 m above sea level, the mean annual temperatures are low (10.5 °C), characteristic of Andean environments in central Chile. In winter, at elevations of more than 3200–3500 m a.s.l., the prevailing temperatures for most of the day are in the range 20–25 °C, characteristic of periglacial conditions. For the period 1931–1978, the weather station at the Maitines plant recorded a mean annual precipitation of 522.3 mm, in the form of rainfall concentrated in the winter months (May-August) (Noguera, 1990). In areas located >1800 m a.s.l., precipitation occurs as snow, concentrated during the winter season. This type of climate is suited to the intense degradational activity as a result of the development of freeze-thaw processes.
The Río Colorado drainage basin, to where it discharges into de Río Maipo (Fig. 2), has a surface area of ~2770 km2 (Directión General de Aguas, 1987). Figure 2 shows the mean annual flow discharge expressed in cubic meters per second at various points of interest in the basin. Its flow pattern is of the rain-snow type with two annual peaks: one in winter (June–August) and one in the spring (October-December). During both peaks, the river usually undergoes a substantial increase in the volume of solid debris, acquiring a muddy appearance due to the high concentrations of debris incorporated through the erosion of valley fills and the foot of active debris cones, formed by the thermoclastic disintegration of the ochre-colored mud and sand-bed deposits of the Colimapu formation.
Due to its complex and prolonged tectonic history, the sedimentary sequences of the Andes of central Chile are intensively fractured and deformed. The regional structure incorporates many synclines, overturned folds, and unconformities. In the higher parts of the basin of the Río Colorado, the sedimentary sequences strike predominantly north-south and dip steeply, 70°–75°, both to the east and to the west. This geological structure particularly favors the occurrence of landslides from valley slopes with high relief. The dominant fault pattern is north-south. Igneous activity has given rise to the presence of hydrothermal alteration zones, and the development of geomechanically weak rocks. Dikes, sills, and diapiric injections cause similar effects in the local sedimentary sequences.
As is typical of the Andean region of central Chile, the study area consists of a complex sequence of sedimentary (both terrigenous and marine) and igneous (intrusive, hypabyssal and extrusive) rocks that date from the upper Tertiary-Jurassic (Thiele, 1981). Figure 3 summarizes the geological units identified in the headwaters of Estera Parraguirre, where the initial rockslide occurred.
During the Quaternary, glacial activity was particularly intense in this region and played a major role in shaping the local landscape. Large volumes of erosional products from morainic deposits were later reworked and mobilized by fluvial processes and deposited to form sedimentary fills (fluvial terraces of debris, gravel, and sand) of the main Andean valleys. Aerial photographs taken in 1955 show that in the upper reaches of Estero Parraguirre, the valley floor was covered by large accumulations of moraine produced by Holocene deglaciation that left them outside the current glacier ice boundary, as well as fluvial, colluvial, and debris materials. These developed prominent “corded” structures similar to lava flows, as a result of successive solifluction-type slides originating in debris cones and produced by the mechanical disintegration of the ochre-colored mud-sand layers of the Colimapu Formation (Fig. 3). Circular depressions, or kettle holes, are also present and were created through gradual subsidence in response to the thawing of the blocks of ice and snow incorporated in the debris deposits.
DESCRIPTION OF LANDSLIDE ON NOVEMBER 29, 1987
Initial rockslide and rock avalanche
At 10:33 a.m. on November 29, 1987, a rockslide occurred on the western side of Cerro Rabicano, in the headwaters of Estero Parraguirre (Fig. 3). As noted in the following, the impact of the rockslide on the valley bottom was measured on nearby seismographs in the Maipo valley. In addition, at about this time people on the ground and a commercial airline pilot flying over the area noticed a huge dust cloud rising out of the headwaters of Estero Parraguirre (Valenzuela and Varela, 1991; Casassa and Marangunic, 1993). The slide originated at ~4350 m a.s.l. on a very steep slope consisting of highly fractured rocks (Figs. 4 and 5). The rocks in the source area are not mapped in detail, but consist mainly of limestone with intercalations of gypsum and andesite. They strike roughly north-south, parallel to the axis of the valley, and dip 70°–75° very steeply to the west (Fig. 3). Valenzuela and Varela (1991) noted that in parts of the slope the dip of the beds is almost vertical and some strata may be overturned. Subvertical joints are also present (Fig. 3).
The rockslide consisted of a long thin slab measuring ~1000 m in length, extending from ~4350 to 3400 m a.s.l., with a mean width of 500 m and an average thickness of 20 m (Fig. 4), yielding an estimated volume of 6.0 × 106 m3, a little larger than previous estimates (cf. Valenzuela and Varela, 1991; Hauser, 1993; Casassa and Marangunic, 1993). Very well preserved ridges suggest that the main sliding surface consisted of a curviplanar, smooth bedding plane in limestone, and the upper rupture surface around the top of the slide corresponded to a preexisting penetrative fracture, perpendicular to bedding (Fig. 5).
Only for brief moments following the initial slide did the rock mass behave like a rigid body before disintegrating into a rock avalanche. During its rapid descent, the material underwent further disintegration and may have incorporated colluvial debris from the valley sides toward the base of the source slope. The mass of debris hit the bottom of Estero Parraguirre (elevation 3400 m a.s.l.) valley after traveling a slope distance of ~1.3–1.5 km and a maximum vertical descent of ~950 m.
After reaching the bottom of the valley the debris extended across the toe of a debris-covered glacier (Fig. 3), up and over a 50-m-high interfluve, suggesting a minimum velocity of 31 m/s, calculated using the energy-head formula. The debris extended down the other side of the ridge into the toe of another debris-covered glacier (or rock glacier) before turning 90° to the south and down Estero Parraguirre, 3.3 km from the scarp (Fig. 3). Based on the superelevation of the debris line, Casassa and Marangunic (1993) calculated a speed of 24 m/s in this sharp bend 2.0 km from the slide's point of origin.
Debris flow in Estero Parraguirre
The natural condition of saturation of the deposits that formed the valley fill at the time of the failure, together with the rapid melting of the snow and ice accumulated there, led to the rock avalanche becoming transformed into a huge debris flow in the upper part of Estero Parraguirre, probably within 5 km of its source. During its travel down Estero Parraguirre, the debris flow increased considerably in volume by incorporating part of the fluvial terraces in the valley and debris from the foot of extensive colluvial fans (debris consisting of gravel and sand and fragmented rock materials, respectively) located along the valley. In addition, the debris flow must have incorporated a significant volume of snow in this section of its travel. As noted by Valenzuela and Varela (1991), a snow cover of 5–10 m existed in the Estero Parraguirre at the time. Casassa and Marangunic (1993) estimated that the equivalent of 7 × 106 m3 of water was incorporated into the debris flow from snow along its path and glacier (and/or segregated ice) near its source. This produced a debris flow in Estera Parraguirre with a total volume of at least 15 × 106 m3.
The presence of mud splashes and the marked superelevation of the debris line along sections of the valley side confirms the spectacular speed and mobility of the flow, largely resulting from the confinement of the debris in the narrow Parraguirre valley (Fig. 6). Peña and Klohn (1988) reported that the discharge of the debris flow in Estero Parraguirre could have been as much as 10000 m3/s and suggested that the flow traveled at speeds of as much as 15 m/s on an average slope or 4.5°. Witnesses in Estero Parraguirre reported that the debris flow showed an incredible capacity for moving and incorporating large, heavy boulders. As confirmed by eyewitnesses, while advancing, the slide's water-saturated front developed a series of waves. The boulders practically floated on the surface as if they were very light.
Two shepherds and a number of sheep were killed by the debris flow in the lower reaches of Estero Parraguirre (González-Ferrán, 1988).
Debris flow in the Río Colorado
Based on an average velocity of 15 m/s in Estero Parraguirre, it is estimated that the debris flow reached the Río Colorado (17 km from source) at elevation 2050 m a.s.l. in ~19 min, i.e., at ~10:52 a.m. On reaching the confluence most of the debris flow spilled over the surface of interfluvial terraces, discharging directly into the Río Colorado (Figs. 2 and 7), which at this time of the year has a mean discharge of 13 m3/s. Some of the debris followed the Parraguirre channel and discharged into the Colorado some distance down stream. Most of the debris, however, created an instant dam (Fig. 8) and temporarily blocked the Río Colorado. When the dam collapsed minutes later, the debris flow resumed its travel in a series of kinematic waves down the Río Colorado, a product of the collapse of temporary dams developed along its path and ponding due to constrictions in its path. An eyewitness estimated the velocity of the debris flow to be ~10 m/s just downstream from the Parraguirre-Colorado confluence (17 km from the source; Cassas and Marangunic, 1993).
The debris flow overran the La Paloma Camp (Figs. 2 and 9: elevation 1630 m a.s.l., 25 km from source) 8 km downstream from the Parraguirre-Colorado confluence, at ~11:20 a.m. (Valenzuela and Varela, 1991), 47 min after the initial rockslide; this suggests an overall velocity for the landslide at this point of 9 m/s. The camp that housed construction crews involved in the Alfalfal project suffered the highest number of deaths in the landslide (at least 30 deaths), and the debris flow caused much damage to machinery (Fig. 9). If it is assumed that the average velocity below the Parraguirre-Colorado confluence was 10 m/s, the debris flow should have arrived at La Paloma at ~11:20 a.m. The delay in the arrival of the debris flow at La Paloma may indicate that the major part of the debris flow stopped at the confluence damming the Río Colorado for ~15–20 min, before being remobilized by the collapse of the debris dam.
An indication of the energy of the debris flow is found in the fact that it transported a giant boulder that had been located at the Parraguirre-Colorado confluence, upstream of Paloma Camp (Figs. 2 and 10). The boulder measured ~1000 m3 in volume, and weighed an estimated ~2700 t; it was transported 11 km by the flow along the river bed (Hauser, 1993), a dramatic illustration of a transport mechanism involving dispersive pressures in a mix of flowing granular material. Through the loss of transport energy and deceleration, the giant boulder was deposited downstream of Paloma Camp, at a location where the Colorado valley changes its hydraulic profile and widens considerably (Figs. 2, 10, and 11).
Farther downstream, the first wave of debris was noted at the water intake of the Maitenes plant (point 11 in Fig. 2; elevation 1200 ma.s.l.; 41 km from source) at ~12:41 a.m., ~100 min after initial rockslide in the headwaters of Estero Parraguirre, suggesting an overall velocity for the landslide at this point of ~7 m/s. It is possible that the first wave was the first arrival of the debris flow that bypassed the damming at the Parraguirre-Colorado confluence. The second and biggest wave, probably corresponding to the breaching of the dam at the confluence, reached the Maitenes Plant 23 min later at 12:37 p.m. (average velocity of ~10 m/s from the La Paloma Camp) and was the most destructive. Note that the interval between the first and second waves corresponds roughly to the estimate of the duration of damming at the confluence, 24 km upstream. At the front of the debris flow, the frontal waves reached maximum heights of 30–35 m above the level of the river channel. A much smaller third wave was recorded at the Maitenes power plant at 4:20 p.m.
The greatest losses in human life and material damage were caused by the wave that occurred at 12:37 p.m. in the La Paloma Camp(Ugarte, 1988).
In continuing its descent down the valley, the debris flow created another blockage upon reaching the mouth of the Olivares River (Fig. 2). The collapse of this dam allowed the debris flow to resume its descent, moving with particular force over the water intake of the Maitenes hydroelectric plant and the power house of the Alfalfal project (points 10 and 11 in Fig. 2). This section of the debris-flow path also involved numerous human victims and high material losses. Eyewitnesses reported that on the valley slopes, the movement of the flows caused air currents and wind strong enough to uproot trees and shrubs, incorporating particulate material from the ground, and causing active dust clouds.
By the time the debris flow reached the power house of the Maitenes hydroelectric plant (elevation 1150 m a.s.l.; point 11 in Fig. 2; Fig. 12), 8.0 km downstream from the Alfalfal construction site, the workers there had been alerted and had evacuated the plant after shutting off the turbines. The gradual loss of energy of the debris flow in this section can be attributed to the energy losses due to successive changes in the direction of the flow as it followed the meandering channel of the Río Colorado. In the section of the Colorado valley, i.e., between the Alfalfal and Maitenes power plants, the debris flow had an estimated velocity of 11 m/s (Valenzuela et al., 1989; Valenzuela and Varela, 1991).
After traveling ~6 km downstream of the Maitenes plant, the debris flow finally discharged into the channel of the Río Maipo (elevation 900 m a.s.l.), 57 km from the site of origin. At that time the discharge of the Río Maipo was ~18.0–20.0 m3/s. From this point downstream, the larger cross section of the Río Maipo channel efficiently transported the entire volume of the flow without causing significant additional damage. The level of the Río Maipo increased between 2 and 4 m (Valenzuela and Varela, 1991; Casassa and Marangunic, 1993), equivalent to an increase in volume of ~20 × 106 m3.
At the gauging station where the Río Maipo empties into the Pacific Ocean (223 km from the detachment site), two peaks in river discharge were recorded in the morning of November 30, 1987: 150 m3/s at 3:00 a.m. and 175/m3/s at 7:00 a.m. (Peña and Klohn, 1988; Fig. 13). The volume of the flood represented by these peaks is 7 × 106 m3 (Casassa and Marangunic, 1993) and the travel time to sea level from the source rockslide in the headwaters of Estera Parraguirre was, based on the second peak, 18 h, 27 min, suggesting an average velocity of 3 m/s for the overall debris flow-flood event.
According to available information, when construction activities began in the Colorado valley, there was no preparedness for great-magnitude mass movements or any indication that they could occur.
The deposits resulting from the flows may be described as granular, gravel and sandy gravel fractions predominating, with abundant interstitial clay. The deposits are poorly sorted, and exhibit poor stratification. The morphology of the debris includes levees, lobes, wave-lake surface features, and depressions. Cones of fine soil extruded during the consolidation of the debris are also found on the surface of the debris. Inverse grading, where the larger rock fragments predominate around the deposit surface, is noted around the edges of the deposits. This characteristic is attributable to the fact that the larger fragments tend to move toward the free surface of the flow, where shear strength is lower.
The resulting thickness of the debris flow deposits along the floor of the Colorado valley varies; it is 0.6 m in the La Paloma section, ~2.5–2.7 m around the power house of the Maitenes plant (Fig. 12), and 4 m at the Alfalfal power plant (Valenzuela and Varela, 1991).
Eight years later, the morphology of the deposits and the effects of the debris flow are still evident. Levees along the edge of the current channel of Estero Parraguirre and the Río Colorado as well as the eroded edges fluvial terraces, signs of superelevations of the debris line in bends, mud splashes, and thick mantles of granular debris-flow deposits on remnants of fluvial terraces can still be seen.
FACTORS THAT CONTRIBUTED TO THE INITIAL ROCKSLIDE
Question of a seismic trigger
González-Ferrán (1988, 1995) linked the occurrence of the initial rockslide to surface tremors generated by increased activity of the Tupungatito volcano, located 11 km east of the rockslide site (Fig. 2). González-Ferrán (1988) supported his hypothesis with reference to seismic events recorded on November 28, 29, and 30, 1987, by the seismological networks in the Andes of Chile and Argentina.
Eisenberg and Pardo (1988) established, however, that no sign of earthquake activity was recorded at the seismological stations in central Chile on November 29, or on previous days. They reported that the Argentina network had also not recorded any relevant sign of activity associated with the Tupungatito volcano. Eisenberg and Pardo reported that at 10:33 a.m. of November 29, 1987, only a small tremor with a local magnitude equivalent to 4.5 was recorded. This tremor is instead attributed to the sudden fall of a large rock mass from a point located near the headwaters of the Parraguirre Creek. The authors concluded that “this event, as far as we can see from the records, was not preceded by a triggering earthquake…” (Eisenberg and Pardo, 1988, p. 15). Based on these categorical precedents, Eisenberg and Pardo (1988) suggested that this seismic activity was an effect of, not a cause of, the slide. Having eliminated seismic activity as a triggering cause of the rock avalanche, other causative mechanisms must be identified.
Role of water
The timing of the initial landslide corresponded to the final stages of an intense ablation event a few days earlier that acted on a heavy thickness of snow which had accumulated in large quantities during the previous winter (Moreno y Asociados, 1988).
In this study, the initial rockslide may have responded to a gradual, prolonged, and persistent loss of shear strength, acting along a potential rupture surface that coincided with a continuous bedding plane in the limestone sequence. The rapid saturation of the rocks by incorporating surface water, both through open cracks and stratification planes, had effective stresses and produced instantaneous losses in the shear strength of the limestone rock mass.
Snowfall records provided by the Directión General de Aguas at stations located in the Andean region of central Chile confirm that in winter of 1987, snowfall reached levels much higher than the annual average. In the winter of 1987 at the Laguna Negra station (located in Fig. 1), for example, the water equivalent to snowfall reached 1473 mm, while during the period 1951–1990, the annual average was only 566 mm. In addition, on the days before November 29, 1987, the temperatures recorded in the Andes Mountains in the headwaters of the Río Colorado exceeded the average daily temperatures for the period (Fig. 14; adapted from Casassa and Marangunic, 1993).
The causative effect of water as a triggering agent of the initial slide is seen in the presence of the remaining or relict “roped” boulders along the current crown or discharge area of the initial rock slide. These suggest that the drained rock mass is stable and that neutral stress at the time that the slide occurred had been decisive in triggering the process.
That the limestone sequence involved in the initial slide (the product of intensive regional tectonism) shows prominent local deformations, suggests that the respective internal friction angles along the potential rupture surfaces along bedding planes may have dropped to residual values. Surface alteration caused by activity in a periglacial environment may have partly destroyed the cohesion along the potential rupture surfaces. Chemical activity, such as dissolution, acting on the limestone and gypsum in the sequence may have caused important changes in the existing roughness of the potential rupture surfaces, favoring the development of smooth or low-friction surfaces (cf. Cruden, 1985). The abundant presence of gypsum as fill in the fissures of local sequences constitutes an effective agent for mechanical disintegration, given its capacity for hydration, whether acting independently or jointly with intense local cryogenic activity (freeze-thaw).
The intense Holocene deglaciation that took place in the part of the valley where the initial slide was generated suggests that the valley slopes underwent important losses in lateral confinement and associated stress relief, creating an environment conducive to the development of unstable slopes.
EFFECT ON HYDROELECTRIC FACILITIES, DRINKING-WATER SUPPLIES, AND AGRICULTURAL ACTIVITIES
The fortunate coincidence that the avalanche happened on a Sunday, when most of the people who usually worked in the area were enjoying a day of rest, resulted in a comparatively low loss of life, even though the number of fatalities reached at least 37. At the time, there were ~1800 people employed in the construction of the Alfalfal hydroelectric plant.
Despite enormous efforts by the rescue teams, only a few bodies were recovered. Large segments of the access roads to the plant and construction sites of the Maitenes and Alfalfal hydroelectric plants owned by Chilectra S.A. (the largest distributor of electricity to Santiago) were destroyed, including 14 metal bridges. Four construction company camps were razed by the flows, as were series of works for the Alfalfal project, which at the time were either at an advanced stage of construction and/or complete. Damage to the power house of the Maitenes plant (24 MW) was ~$12 million U.S. It is estimated that the structural damages were not as high, given the low velocity of impact of the flows in the section and the fact that they moved in a homogeneous manner. The cleaning and rehabilitation operations lasted ~8 months. The total losses for Chilectra S.A. were estimated as $40 million U.S.
As result of the flows, the present Alfalfal hydroelectric plant has encountered operational problems that were not anticipated in its design. These are tied to losses in turbine time efficiency, caused by the dragging of larger quantities of solids in the waters of the Río Colorado that has resulted in high wear and tear on some of the turbine parts. In addition, cleaning operations using de-sanders and settling basins have had to be increased.
The Metropolitan Sanitary Waterworks Company, which at the time provided 85% of the drinking water for the city of Santiago (population 4.7 million) by capturing ~15.0 m3/s from a facility located in the La Obra section of the Río Maipo (point 12 in Fig. 2), faced serious problems in water supply after the November 29, landslide because of severe damage to its plants. Problems included a considerable increase in the content of dissolved solids in the water, and the collapse of settling tanks and filtration facilities. For 15 days after the landslide, the company was forced to reduce the water supply to Santiago to 30% of the normal average. During the water shortage in Santiago, cases of extreme urgency were served using 108 portable reservoirs with a 305 m3 capacity and 33 tank trucks with a 343 m3 capacity (F. Pérez, Metropolitan Sanitary Waterworks Company, 1995, oral commun.).
In the valleys of the Estero Parraguirre and the Río Colorado, the debris flow caused the death of ~400 heads of cattle, goats, donkeys, and sheep that were grazing there. The event totally changed the morphology around the site of Baños de Salinillas (Fig. 2), which many people visit in the summer for the therapeutic properties of the local hot spring.
Eight years after the debris flows, vast extents of the flat lands around the channels of the Parraguirre Creek and the Colorado River, which historically had been used for animal pasture, are still not being used. The cover of grasses and shrubs is slowly beginning to be restored.
Currently, the slope where the initial catastrophic slide took place on November 29, 1987, is on occasion visually inspected by helicopter. The purpose of the inspections is to detect signs (e.g., the presence of morphological or structural depressions, cracks, and/or deformations in the ground) related to either possible reactivating mechanisms or the beginning of new rock avalanches, in order to plan preventive measures and procedures. Under the supervision of Chilectra S.A., these activities aim to guarantee the safety and efficient operation of the Maitenes and Alfalfal hydroelectric plants.
PREHISTORIC DEBRIS-FLOW DEPOSITS IN THE COLORADO VALLEY
An inspection of exposures in the fluvial terraces along the current channel of the Río Colorado around the power house of the Maitenes hydroelectric plant makes it possible to identify the presence of different layers of deposits, the lithological nature of which has marked affinities with those resulting from the flows reported herein. In the section, the terrace consists of a 8.65-m-thick sequence in which three layers can clearly be seen corresponding to paleodebris flows (B, D, and E in Fig. 15) interbedded with obviously fluvial layers (A and C in Fig. 15). In the interface between layers C–D and D–E, two clear soil horizons can be identified, each 0.55 m thick, which suggests that both torrential events originated as a result of flows with recurrences of at least 100 yr. No absolute dating is available to determine the moment when these episodes took place during the geological evolution of the Colorado valley. In any case, the presence of the three paleodebris-flow deposits makes it possible to establish that at times during its postglacial geological history, the geological, tectonic, geomorphological, and climatic conditions closely resembled those of today, and created environments favorable to the occurrence of large-scale mass movements. In the area studied, the flows that took place on November 29, 1987, left a deposit only 0.45 m deep caused by a process having a large volume and enormous torrential and highly destructive power. Therefore, having left a deposit the depth of which is 7.3 times greater, paleodebris-flow D is the product of a slide of incalculable volume and energy.
The chapter presents a detailed description and analysis of the high-magnitude Estero Parraguirre-Río Colorado rockslide, which occurred November 29, 1987, when a spur of sedimentary rocks (~ 6.0 × 106 m3), due to the combination of various factors, became detached from the flank of a high peak in the Andean region of central Chile. The slide quickly evolved to a voluminous and thundering rock avalanche, and later into a hypercon-centrated debris and mud flow; traveling with high mobility in steep confined valleys, it caused at least 37 human deaths, and important material damage to the operation of a hydroelectric plant, and to the works of another hydroelectric plant that was under construction at the time.
The combination of hydrological, morphological, geological, and structural conditions that triggered the high-magnitude event is common to Chile's Andean region. Because of this, the region is particularly prone to mass-movement events; further events can be expected, highlighting the need for caution in the future developments of this region. The design and construction of any future facility in this zone must involve a hazard evaluation. It should be feasible to evaluate the stability of the area by continuous terrestrial or aerial visual control to detect unusual activity that would probably precede major events. On a reconnaissance scale, vertical aerial photos can provide accurate geomorphological characteristics related to specific sites, in order to outline earlier major mass movements. Further engineering issues arise from the type of the event and in predicting the likelihood of further events in the area.
The mass movement described herein is a good example of the enormous potential damage that can be caused by waves related to debris and mudflow events generated from rock avalanches in Chile's Andean region, where rock masses are able to detach from steep pinnacles that make up the main mountain ranges. The occurrence or dominance of a particular process is mainly related to source of water and the availability of soil and weathered or fractured rocks.
The phenomenon described herein could by considered as the most reasonable approximation to the maximum destructive mass movement that can occur in Chile's central Andean region.
I thank Chilectra S.A. engineers Patricia Alvarado and Armando Lolas for providing technical data, necessary field-work facilities, and for reading earlier version of the text; S.G. Evans (Geological Survey of Canada), R.H. Brady III (California State University), and S. Mora (Instituto Costarricense de Electricidad, Costa Rica), who prepared the English version of this paper, made very, useful, careful, and constructive comments and criticisms, and provided detailed and thoughtful reviews of the original manuscript, helping to focus many ideas and concepts; and O. González, F. Morales, and C. Morales of the Cartographic Section of SERNAGEOMIN, who kindly prepared all the figures in the paper. I also thank M.T. Cortés, P. Hofer, S. Villar, and M.P. Salgado for untiring help.
Figures & Tables
>This volume documents further advances in our knowledge of catastrophic landslides since the pioneering compilations of the late 1970s by Barry Voight. It provides a worldwide survey of catastrophic landslide events written by leading authorities. Catastrophic Landslides begins by drawing upon South America to dramatically illustrate the impact of these phenomena on human populations. The occurrence of catastrophic landslides, including site-specific insights, is shown through six events of the past 20 years. Several other chapters focus on the mechanisms involved with catastrophic landsides both in relation to geologic factors in a particular geographic area as well as to specific geologic processes.