Reassessment of the development and hazard of the Rampac Grande landslide, Cordillera Negra, Peru
© Vilímek et al. 2016
Received: 16 November 2015
Accepted: 8 April 2016
Published: 18 April 2016
The Erratum to this article has been published in Geoenvironmental Disasters 2016 3:16
The initial investigation analysed the complex Rampac Grande slope deformation from April 2009 (Landslides 8(3):309-320, 2011). The primary research in 2009 also identified an unrealistic explanation (raw mineral exploration) of the triggering of the landslide and the intention of the local authorities (from the administration centre of Carhuaz) to take measures to minimize the possible future risk to the local population. We also examined the adaptation measures introduced by the local authorities to reduce the risk for the local community.
Unstable landslide material has been left after the 2009 event in the sources and transportation zone and several blocks were described as being only in a temporarily stable state. Landslide propagation could also follow the already existing lateral tension cracks identified in 2009. Areas of reactivation from 2012 were localized and triggering precipitation was evaluated.
This study concluded that there is still a hazard of remobilization of specific parts of the landslide in Rampac Grande with potentially damaging effects on the buildings located close to the accumulation area.
The area of Rampac Grande landslide is located within sediments of Middle/Upper Cretaceous age (INGEMMET 1995). The rocks contain argillite and schist interlaid with sandstones, limestone and small amount of gypsum. Sediments are highly permeable due to fracturing and their weathering (Gutierrez et al. 2004). The Rampac Grande has been classified as a complex landslide with rotational character in the most upper part and significant translational character transforming into flows in the middle and lower sections (Klimeš and Vilímek 2011). The complex landslide exaggeration is 383 m and maximum length and width are 863 and 450 m.
Water infiltrates the landslide area through superficial sediments and deep gullies, which in some places cut up to 20 m into the massif enabling the water to reach deeper layers of the bed rock. The topographic infiltration area of the complex landslide is only about 3 Km2, but based on our field observations and morphology of the surrounding slopes, it may be more than twice as large as the topographic infiltration area. Observation of the fissure spring at the NW border of landslide with measured outflow of 0.07 l/s supports this opinion (Klimeš and Vilímek 2011). This water infiltration may occur deeper into the massif.
All natural hazards in the department Ancash were analysed (Vilímek et al. 2013) in terms of frequency of recurrence (in the period 1971-2009). The analyses revealed that the most frequent type of natural hazard was “alluvion” (a local term for debris flow), followed by floods and extreme rainfall. If we group various forms of mass movement together, they comprise a dominant portion among natural hazards. A major portion of natural hazards are generated by the direct impacts of extreme hydrometeorological events.
The research carried out on landslides which maintains a state of marginal instability (e.g. Carey et al. 2015) revealed the importance of the relationship between the level of groundwater and pore water pressure. With respect to the fact, that our area of study does not allow us to equip the landslide like in less remote areas, we are looking at the influence of precipitation on slope movement reactivation. For instance Longoni et al. (2016) combined geological, geomorphological and geophysical data for the Ronco landslide study (Northern Italy). They also stressed the necessity of diverse sources in study of complex landslides because of the possible data uncertainty what is also true for our locality in the Cordillera Negra.
There are several reasons why we returned to this area and by using field research and a comparative study of photos we tried to reassess the landslide hazard. In general we consider the area as being unstable because some material has been left in the transportation zone; moreover some blocks are still in only a temporarily stable state. The rainy seasons following 2009 were analysed and compared with the triggering conditions in 2008/2009. Landslide propagation could follow the already existing lateral tension cracks identified in 2009.
The rainy seasons following 2009 were analysed and compared with landslide triggering conditions in 2008/2009. The precipitation record was analysed from the point of view of cumulative data as well as short term precipitation. Unfortunately, no station exists directly around the landslide. We had to use the Yungay (22 km to the NW, 2 530 m a.s.l.) and Huaráz (27 km to the SE, 3 050 m a.s.l.) stations, which are both available on www.senamhi.gob.pe.
Electric resistivity tomography (ERT) was used to investigate subsurface conditions within and outside the 2009 landslide, searching for potential sliding planes susceptible to reactivation. We constructed one profile across the main scarp (Fig. 1) in a place where we suspected possible future sliding activity due to the location of a spring just below the main scarp. We used a 4point light hp 10w (Grinat et al. 2010) instrument with 27 electrodes on three chains with a 3 m electrode spacing and maximum depth penetration of 12 m. Different arrangements of electrodes (e.g. Schlumberger, Wenner and dipole-dipole) were used and the obtained apparent resistivity values were processed in RES2DINV software using the least-squares inversion method in order to represent the true resistivity of the subsurface. We performed a maximum five iterations, keeping the root mean square (RMS) error below 10 %.
Precipitation data analysis
The deep-seated movement at the end of the 2008/09 rainy season was due to significant cumulative precipitation, reaching 950 mm, with the precipitation reaching a total of 229.6 mm during March 2009 (Klimeš and Vilímek 2011). During the periods 2011/12, 2012/13 and 2013/14 the monthly totals during the rainy season were much lower, with the highest being between 100 and 200 mm at the Yungay station (January to March). The level of precipitation in March 2013 reached only 205.7 mm and January 2013 was rather dry.
Some examples of variation in daily precipitation between the 2 nearest stations to the landslide locality in Rampac Grande
Areas of reactivation
Some areas of the Rampac Grande landslide were reactivated but the activity could not be compared with the 2009 event. We identified remobilization of the landslide material in the scarp area—‘a’ (Fig. 1) which was considered as being highly unstable by Klimeš and Vilímek (2011). In addition to Mejía and Meza (2012), local people confirmed the sliding of this material, which probably occurred during the 2013/14 rainy season. The reactivation also involved the head scarp, which retreated by at least 1 m. We were not able to distinguish the accumulation of the slide material from the original one.
The main reactivation in Rampac Grande happened during the 19th of April 2012 (Mejía and Meza, 2012) and is probably based on the intensive, short term precipitation which activated only the near surface layers in zone ‘c’ (Fig. 1). Remobilisation of accumulation produced two smaller debris flows which we identified in the eastern as well as western part of the landslide area—‘k’ and ‘g’ (Fig. 1).
A comparison of historical photographs (from 2009) revealed a large remobilization of the landslide material to the left of the transportation zone (Fig. 2a,b). Processes responsible for this remobilization are mainly water erosion starting as sheet erosion and propagating to rill and gully forming erosion. The most prominent gully formed along the east side of the landslide (‘e’, ‘k’ in Fig. 1) reaching a depth of up to 6 m in the accumulation area. This gully re-established the main drainage which existed before the landslide formation (Klimeš and Vilímek 2011) and now drains the whole landslide body. A similar process is responsible for considerable removal of the material from the western part of the landslide (‘b’ in Fig. 1) and in the place where the irrigation channel crosses the landslide. A freshly looking debris flow (200 m long) with a source area outside the landslide probably appeared during the 2013/14 rainy season. It deposited a thin layer (0.4 m) of material reaching the main landslide accumulation area (‘l’ in Fig. 1).
Along the incision of the ‘traditional’ stream, an accumulation of new material was deposited in the accumulation part of the landslide (Fig. 4a,b). We assume it was deposited by reactivation in 2012. The major process involved the removal of material from the landslide area is water erosion—flows with high sediment loads.
The main changes in the landscape evolution of the landslide area are connected with fluvial erosion. The un-vegetated surface began to be modulated by rill erosion and the former water drainage created just after the landslide event is specified by deep-ward erosion (up to 5 m). The identification of types and intensity of fluvial erosion is important because it works as a destabilising process for possible future mass movements. Many open cracks were identified on the eastern margin of the complex landslide but not in the uppermost part. Future propagation of cracks will destabilise the side areas (Fig. 5).
Removal of the landslide material which occurs mainly by water erosion results in steepening of the slopes within the scarp and upper transportation area of the landslide and also creates longer slopes below landslide parts that were described as unstable in 2009 (numbers 1, 2, 3 and 4 in Fig. 1). We consider this process to be especially dangerous in the case of three individual landslides (numbers 1, 2, 3 and 4 in Fig. 1) as landslides 1 and 2 are exposed to continuous and quite intensive unloading at their toes. They have clear scarps with evidence of vertical movements of 5 m (landslide 1) and 7 m (landslide 2), which occurred during 2009. There are also two temporarily stable landslides (1a, 2a on Fig. 1) just next to them. Along with the toe unloading, we assume that weathering of the loosened landslide material mainly along the shear planes further diminishes their stability. Considering these recent processes and the long history of landsliding in this area, we consider it highly probable that landslide 1 or 2 (Fig. 1) will undergo major movement in the next 50 years.
Mapping of tension cracks (4 on Fig. 1) suggests a deep discontinuity crossing the narrow ridge to the east of the Rampac Grande landslide. Possible future propagation of these cracks may result in mobilization of approximately 25,000 m3 of the estimated volume of the rocks causing a smaller scale disaster than the landslide in 2009. Recent knowledge does not allow us to estimate the hazard degree of such a scenario. Nevertheless, the 2009 landslide proves it is feasible; therefore, we recommend establishing a long term project of monitoring of this part of the landslide, whose main requirement is a positive approach of the local community and their active involvement in the monitoring.
Interaction of the community and natural hazards
Despite the large attention the event attracted in the days following its occurrence and the largely proper rescue action of the local authorities, all of the governmental organizations on various different levels failed to take any effective action to lower the possible future risk caused by the event. The local community still largely believes that the landslide occurred due to mineral exploration; there are no signs informing of the danger the landslide still possess (e.g. new material in the accumulation area) and only minor technical measures were taken to try to stabilize the landslide. Unfortunately, no changes have been made in the settlement and several houses are directly endangered by a similar event to 2009 (see Fig. 1). Also, there are no signs warning the local inhabitants that the landslide area still poses a danger to them and they should be ready to evacuate or should consider this hazard when using the land for agriculture or when living directly below the landslide.
The local community begun to grow small trees over the landslide accumulation where the slope inclination is lower and some small areas are covered by crops (according to the field visit in 2014); nevertheless, the density of the vegetation cover is still low. Moreover, the local community has begun to utilize the landslide area (e.g. establishing fields, planting fruits, and crossing it with cattle) for everyday use, which may strengthen their understanding of the landslide as an integral part of their environment. Such a mental image does not contribute towards vulnerability reduction.
To change these unfavourable conditions with respect to possible future landslide risk mitigation, development project of the Czech Development Agency will be conduct during 2016. It aims on landslide hazard dissemination among the local inhabitants, identification hazardous and safe areas around the landslide placing evacuation signs to show the proper evacuation rotes in case of the landslide reactivation. Simple but reliable manual tape extensometer monitoring of the most dangerous landslide parts will be also established involving also placement of rain gauge station on convenient site near the landslide. The provincial government of Carhuaz is involved and supports these activities.
The uncertainty of the precipitation analysis is not only due to the small differences of the positions between the landslide head scarp and the rain gauge stations (elevation and position inside the valley) but also the local differences in the precipitation records even within short distances because of the high mountains in the surroundings. During the time when the Yungay station registered very high short-term precipitation (see Table 1) the Huaráz station which is 48 km away from Yungay showed only low amounts per day (and also no significant rains occurred a few days before or after). This means that the local differences in precipitation in such a high mountain area are very significant, and we cannot be completely sure of the exact triggering totals of precipitation. Similar situation we already described in another part of Peruvian Andes (Vilímek et al. 2006).
Five years on from the main Rampac Grande landslide only small reactivations have been identified; nevertheless, several parts of the complex landslide were unstable in 2009 and continuing fluvial erosion enhanced this hazard. The comparison of precipitation revealed that the reactivation identified by local people as taking place on the 19th of April 2012 did not match any enormous precipitation event at the nearest stations.
The anthropogenic measures are both positive and negative. The revegetation could be considered as positive measure even if it is not sufficient. The negative influences unfortunately prevail e.g. a simple water pipeline is crossing the landslide body and in the case of any rupture the concentration of water into the landslide area will be a problem, and on the other hand the local people avoid the possibility of concentrating the water flow out from the landslide area (western margin). They have also not refused any directly endangered houses (Fig. 1) and have even not informed the local residents about the hazard. The wrong idea about the triggering factor in the village is still evident—they are afraid of mining, the effects of which have not been confirmed and are not too worried about high precipitation, which will be of greater importance in the future (especially during El Niňo). It seems that the local population are not sufficiently educated about the landslide hazard and do not reflect it in their everyday lives, which creates negative conditions for future disasters.
This work was carried out thanks to the support of the long-term conceptual development research organisation RVO: 67985891.
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