Elsevier

Geomorphology

Volume 234, 1 April 2015, Pages 1-10
Geomorphology

The mechanisms and characteristics of a complex rock-debris avalanche at the Nigeria–Cameroon border, West Africa

https://doi.org/10.1016/j.geomorph.2014.12.040Get rights and content

Abstract

We describe a rock-debris avalanche which occurred on steep, symmetrical ridges resulting from fracture-controlled erosion in the valley. The fractures were partially filled with clayey materials, probably derived from the weathering of feldspar. Major fault lines trending N–S were located less than 7 km from the landslide location. Exposed sections revealed that the basal rock units were migmatites and gneisses, while the upper section consisted of porphyritic granites. A failure of the residual clay-rich soil, composed of visible crystals of feldspar and mica, is thought to have triggered a long chain of events that led to the development of a rock-debris avalanche, which diverted the course of the rivers in the valley. The area was characterised by a shallow water table in the dry season (2 to 3 m) and this might have facilitated the formation of a slip surface at the regolith–rock interface. Field observations and laboratory analysis showed that the regolith probably failed first because of high pore pressure build-up and rapid reduction in shear resistance; this then triggered the failure of the fractured rock units. The slope movement was perpendicular to the foliation of the gneissic rocks, which probably contributed to landslide mobility. Of interest was that changing saturation level at constant relative density of about 32% resulted in either complete or limited liquefaction, indicating that the mechanism of failure depended on the moisture content of the regolith.

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)

  • G. Wang et al.

    Some fluidized landslides triggered by the 2011 Tohoku Earthquake (Mw 9.0), Japan

    Geomorphology

    (2014)
  • M. Wilson et al.

    Magmatism and rifting in Western and Central Africa, from the Jurassic Recent times

    Tectnophysics

    (1992)
  • A.W. Bishop

    Stability of spoil heap

    Q. J. Eng. Geol.

    (1973)
  • A.W. Bishop et al.

    A new ring shear apparatus and its application to the measurement of residual strength

    Geotechnique

    (1971)
  • A. Casagrande

    Characteristics of cohesionless soils affecting the stability of slopes and earth fills

    J. Boston Soc. Civ. Eng.

    (1936)
  • G. Castro et al.

    Factors affecting liquefaction and cyclic mobility

    J. Geotech. Eng. Div.

    (1977)
  • D.M. Cruden et al.

    Landslide types and processes

  • R. Dahal et al.

    Failure characteristics of rainfall-induced shallow landslides in granitic terrains of Shikoku Island of Japan

    Environ. Geol.

    (2009)
  • P.T. Delaney

    Rapid intrusion of magma into wet rock: groundwater flow due to pore pressure increases

    J. Geophys. Res.

    (1982)
  • R.K. Dropek et al.

    The influence of pore pressure on the mechanical properties of Kayenta sandstone

    J. Geophys. Res.

    (1978)
  • B.E. Ephraim

    Granitoids of the Older Granite suites in southeastern Nigeria

    Adv. Appl. Res.

    (2012)
  • R. Fell et al.

    Guidelines for landslide susceptibility, hazard and risk zoning for land-use planning

    Eng. Geol.

    (2008)
  • F. Guzzetti et al.

    The rainfall intensity–duration control of shallow landslides and debris flows: an update

    Landslides

    (2008)
  • O. Hungr et al.

    Review of the classification of landslides of the flow type

    Environ. Eng. Geosci.

    (2001)
  • O. Igwe et al.

    The effect of water-saturation on the stability of problematic slopes at the Iva Valley area, Southeast Nigeria

    Arab. J. Geosci.

    (2014)
  • O. Igwe et al.

    The effect of relative density and confining stress on shear properties of sands with varying grading

    Geotech. Geol. Eng.

    (2012)
  • Cited by (17)

    View all citing articles on Scopus
    View full text