The mechanisms and characteristics of a complex rock-debris avalanche at the Nigeria–Cameroon border, West Africa
Introduction
Slope failures are common in the highlands on the Nigeria–Cameroon border (Fig. 1). On 27th October 2010, several landslides with varying run-out distances occurred simultaneously a few days after a rainfall event had ceased. Two of the rapid slope movements coalesced (Fig. 2) forming a rock-debris avalanche whose volume and movement shook the immediate and remote towns in Nigeria and Cameroon. The landslide occurred on the westernmost part of an extensive upland area, stretching from eastern Nigeria and covering most of the part of Cameroon known as the Cameroon Volcanic Line. This relatively high elevation plateau complex, with elevations greater than 1400 m on average and known as the Cameroon line (Wilson and Guiraud, 1992), is the result of Oligocene to recent volcanism, which led to the formation of anorogenic ring complexes and smaller volcanoes as well as a general uplift of the whole region.
Landslides on such landscapes can lead to the creation of steep hillslope units (Oguchi, 1996, Katsube and Oguchi, 1999). Eye witnesses reportedly felt tremor and observed fire on the hills. An estimated 5 M m3 of rocks and debris was moved more than 2 km from the crown of the slide at 850 m above sea level to the toe of the valley within just a few minutes. The materials ranged from mud and soil debris to blocks up to 20 m in diameter. Three people were killed, and resources including farmland, crops and forest were destroyed. The number of human casualties was small because the landslide occurred in a mountain range far from residential zones. Because of the potential for landslide events to re-occur in this vulnerable environment, it is crucial that we develop a good understanding of the failure mechanism that caused the landslide on this site.
Slope failures have a wide range of causative and triggering factors, which may be geological, hydrological or structural (Selby, 1993, Cruden and Varnes, 1996, Wieczorek, 1996, Takahashi, 2001). Critical rainfall events are known to initiate movements (Guzzetti et al., 2008, Tsai and Chen, 2010, Tsai and Wang, 2011), yet the factors that induce slope failures are often site-specific, as factors that are relevant in one area may not be relevant in another. For instance, Ngecu et al. (2004) showed that some landslides in East Africa were associated with factors including earthquakes and deforestation, but this may not have been the case here. The objectives of this study are to identify the mechanisms and the geomorphologic processes that are important in failures in this region, with a view to obtaining data for future hazard zonation and susceptibility maps. This may be important in developing warning, evacuation and loss-reduction strategies.
Section snippets
Climatic setting
The study area lies within 6°30′–6°45′N and 9°30′–9°45′E, and covers the ridges that form the western part of the extensive Adamawa highlands, which dominates the easternmost parts of Nigeria and the western part of Cameroon. The area has a tropical climate, with a dry season (monthly minimum or zero rainfall from November to March) and a wet season (widespread monthly high rainfall from late April to October). The mean monthly temperatures vary from 22 °C to 28 °C in the wet season and from 28 °C
Method
Long-term residents were first interviewed to evaluate the location, distribution and frequency of landslide events in the area, as well as the intensity of the events, the origin of the rock-debris avalanche and weather conditions at the time of the events. Following Dai et al. (2002) and Fell et al. (2008), information was gathered on the presence of past landslide scars, precursors of slope failures, slope geometry, type, morphology and casualties of landslide events. On the strength of the
Geomorphologic and geologic analyses
The landslide occurred on one of several NNE–SSW trending, roughly symmetrical ridges (Fig. 5). These ridges are residual features of erosion that took advantage of what seem to be fracture-controlled lineaments. The ridge has an elevation of 1050–1450 m and rises 800 m above the valley floor.
The core of the ridge is made up of Pan-African granites surrounded by the relatively easily weathered Precambrian migmatitic gneisses of the basement. The migmatitic gneiss regions form gentle footslope and
Discussion
The study area has a shallow water table in the dry season (2 to 3 m, Fig. 9) and during the rainy season a minimal amount of rain readily decreases the depth to groundwater. The underlying rocks, especially weathered granites, are susceptible to landslides (Chigira, 2001, Sasaki et al., 2001, Dahal et al., 2009). The rock-debris avalanche that occurred here was similar to the Ontake debris avalanche described by Sassa (1987), in that it occurred on a deposit of granitic soil overlying
Conclusions
In this case study, the steepness of slopes, geometry of the valleys, drainage pattern, geology, rock fracturing and increased pore pressure due to rain infiltration were the major factors in causing the rock-debris avalanche. The study area is underlain by basement rocks made up of coarse porphyritic, biotite and biotite hornblende granite, migmatites and undifferentiated schist. The basal rock units were migmatites and gneisses, while the upper section of the basement at higher elevations
Acknowledgements
This research was carried out under the auspices of the International Program on Landslides (IPL Project 150) and World Centre of Excellence Project, initiated by the International Consortium on Landslides, UNESCO, ISDR and UNISDR. We are grateful to Professors Sassa Kyoji and Fukuoka Hiroshi of the Research Centre on Landslides, Disaster Prevention Research Institute, Kyoto University, Japan, who facilitated the field and laboratory analyses. We acknowledge Professor Fawu Wang of the
References (52)
- et al.
Geology, geomorphology and dynamics of the 15 February 2010 Maierato landslide (Calabria, Italy)
Geomorphology
(2014) Micro-sheeting of granite and its relationship with landsliding specifically after the heavy rainstorm in June 1999, Hiroshima Prefecture, Japan
Eng. Geol.
(2001)- et al.
Topographic precursors and geological structures of deep-seated catastrophic landslides caused by Typhoon Talas
Geomorphology
(2013) - et al.
Representative rainfall thresholds for landslides in the Nepal Himalaya
Geomorphology
(2008) - et al.
Landslide risk assessment and management: an overview
Eng. Geol.
(2002) - et al.
Obliquely convergent tectonics and granite emplacement in the Trans-Saharan belt of Eastern Nigeria; a synthesis
Precambrian Res.
(2002) Factors affecting the magnitude of post-glacial hillslope incision in Japanese mountains
Catena
(1996)- et al.
Fluvial geomorphology and paleohydrology in Japan
Geomorphology
(2001) - et al.
Pore-pressure generation and movement of rainfall-induced landslides: effects of grain size and fine particle content
Eng. Geol.
(2003) - et al.
Downslope volume enlargement of a debris slide-debris flow in the 1999 Hiroshima, Japan, rainstorm
Eng. Geol.
(2003)
Some fluidized landslides triggered by the 2011 Tohoku Earthquake (Mw 9.0), Japan
Geomorphology
Magmatism and rifting in Western and Central Africa, from the Jurassic Recent times
Tectnophysics
Stability of spoil heap
Q. J. Eng. Geol.
A new ring shear apparatus and its application to the measurement of residual strength
Geotechnique
Characteristics of cohesionless soils affecting the stability of slopes and earth fills
J. Boston Soc. Civ. Eng.
Factors affecting liquefaction and cyclic mobility
J. Geotech. Eng. Div.
Landslide types and processes
Failure characteristics of rainfall-induced shallow landslides in granitic terrains of Shikoku Island of Japan
Environ. Geol.
Rapid intrusion of magma into wet rock: groundwater flow due to pore pressure increases
J. Geophys. Res.
The influence of pore pressure on the mechanical properties of Kayenta sandstone
J. Geophys. Res.
Granitoids of the Older Granite suites in southeastern Nigeria
Adv. Appl. Res.
Guidelines for landslide susceptibility, hazard and risk zoning for land-use planning
Eng. Geol.
The rainfall intensity–duration control of shallow landslides and debris flows: an update
Landslides
Review of the classification of landslides of the flow type
Environ. Eng. Geosci.
The effect of water-saturation on the stability of problematic slopes at the Iva Valley area, Southeast Nigeria
Arab. J. Geosci.
The effect of relative density and confining stress on shear properties of sands with varying grading
Geotech. Geol. Eng.
Cited by (17)
Mechanical Failure Mechanisms of Unstable Rock Masses with Sandstone and Mudstone Interbeds as the Base
2022, Geotechnical and Geological EngineeringRapid Characterization of Landslide-Debris Flow Chains of Geologic Hazards Using Multi-method Investigation: Case Study of the Tiejiangwan LDC
2022, Rock Mechanics and Rock EngineeringAnalysis on Damaging Mechanism of Sand-Mud Interbedded Strata Type Perilous Rock
2021, Research Square