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  • Catena 97 (2012) 127–136

    Contents lists available at SciVerse ScienceDirect

    Catena

    j ourna l homepage: www.e lsev ie r .com/ locate /catena

    Spatial patterns, causes and consequences of landslides in the Gilgel Gibe catchment, SW Ethiopia

    N. Broothaerts a,⁎, E. Kissi b, J. Poesen a, A. Van Rompaey a, K. Getahun c, E. Van Ranst d, J. Diels a

    a Department of Earth and Environmental Sciences, K.U. Leuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium b Department of Natural Resources Management, Jimma University, Jimma, Ethiopia c Department of Geography and Environmental Studies, Jimma University, Jimma, Ethiopia d Department of Geology and Soil Science (WE13), Ghent University, Krijgslaan 281/S8, 9000 Gent, Belgium

    ⁎ Corresponding author. Tel.: +32 16 327800; fax: + E-mail address: nils.broothaerts@ees.kuleuven.be (N

    0341-8162/$ – see front matter © 2012 Elsevier B.V. All doi:10.1016/j.catena.2012.05.011

    a b s t r a c t

    a r t i c l e i n f o

    Article history: Received 25 November 2011 Received in revised form 15 May 2012 Accepted 30 May 2012 Available online xxxx

    Keywords: Landslides Landslide inventory Causal factors Soil loss East Africa Ethiopia

    The Gilgel Gibe catchment in SW Ethiopia is one of the areas in East Africa affected by landslides. To better understand the patterns and the causal factors of these landslides, all landslides in a small study area (14 km²) in the hilly parts of the Gilgel Gibe catchment were mapped and analyzed. In total, 60 landslides were mapped. These landslides caused a displacement of 1 million m³ slope material, which corresponds to a mean displaced volume of 50 ton ha−1 y−1 in the last 20 years. Moreover many landslides deliver direct- ly sediment to the rivers and hence increase the sediment load in the rivers. This soil loss to the rivers was estimated at 11 ton ha−1 y−1 during the same period. High annual rainfall (ca. 2000 mm y−1), lithological and pedological properties and to a lesser extent steep (>16°) slopes turn the area into an inherent unstable situation and can be indicated as preconditions for the landslides in the study area. Distance to rivers is significantly the most important precondition, as slopes near rivers are less stable than slopes further away from the rivers. This is mainly caused by river incision and bank erosion which often occur in the area and which can be attributed to increased runoff due to defores- tation over the past 20 years. Therefore recent deforestation caused more shallow landslides but also indi- rectly more deep-seated landslides close to the rivers. Heavy rainfall is indicated as the main triggering factor for almost all landslides.

    © 2012 Elsevier B.V. All rights reserved.

    1. Introduction

    During the second half of the 20th century the number of damag- ing landslides have been substantially increasing worldwide, as well as the number of studies on landslides (e.g. Gokceoglu and Sezer, 2009; Gutiérrez et al., 2010). However, research on landslides in East Africa is still rather restricted, although it is a region where landsliding is a widespread phenomenon. Steep slopes, high annual rainfall, increasing population pressure and deforestation, earth- quakes and extreme rainfall make most areas in East Africa very sen- sitive to landslides (Glade and Crozier, 2005). Moreover, most of the landslides have a significant economic, social and geomorphologic impact (e.g. Ngecu and Mathu, 1999). There are still regions in East Africa where quantitative data of landslides are missing, although these data are necessary to undertake remedial measures and to restrict the negative consequences for the inhabitants.

    In East Africa, landslides have been reported in Uganda (e.g. Kitutu et al., 2009; Knapen, et al., 2006; Ngecu et al., 2004), Kenya (e.g.

    32 16 322980. . Broothaerts).

    rights reserved.

    Davies, 1996; Ngecu and Ichang'i, 1999; Ngecu and Mathu, 1999; Ngecu et al., 2004; Westerberg and Christiansson, 1999), Tanzania (e.g. Lundgren, 1978; Marwa and Kimaro, 2005; Temple and Rapp, 1972; Westerberg and Christiansson, 1999), Rwanda (e.g. Moeyersons, 1989, 2003), D.R.Congo (e.g. Moeyersons et al., 2004, 2010), and Ethiopia (e.g. Abebe et al., 2010; Ayalew, 1999; Ayalew and Yamagishi, 2004; Ayenew and Barbieri, 2005; Coltorti et al., 2009; Fubelli, et al., 2008; Moeyersons, et al., 2008; Nyssen et al., 2002, 2004, 2006; Temesgen, et al., 2001; Van Den Eeckhaut et al., 2009; Vilimek et al., 2010; Woldearegay et al., 2005). All these land- slides are caused by various causal factors, however some factors are common for most of the landslide sensitive areas. These factors can be divided in preconditions, inherent static factors that act as cat- alysts to allow other factors to act more effectively; preparatory fac- tors, which make the slope susceptible to movement without actually initializing it; and triggering factors, which finally initiate the movement (Glade and Crozier, 2005). For East Africa, the most important preconditions for landslides are steep slopes, deep weath- ered soils with high clay content and high annual rainfall. The most important preparatory factors reported for landslides in East Africa are all kinds of human activities such as deforestation and other land use changes but also constructions and excavations. Finally,

    http://dx.doi.org/10.1016/j.catena.2012.05.011 mailto:nils.broothaerts@ees.kuleuven.be Unlabelled image http://dx.doi.org/10.1016/j.catena.2012.05.011 Unlabelled image http://www.sciencedirect.com/science/journal/03418162

  • Fig. 2. Example of a recent, shallow landslide in the Gilgel Gibe catchment. Landslide area is ca. 700 m², landslide volume is ca. 1400 m³. (August 24, 2009).

    128 N. Broothaerts et al. / Catena 97 (2012) 127–136

    most important triggering factors are earthquakes and extreme rainfall.

    The Gigel Gibe catchment (Fig. 1), located in the South-West of Ethiopia, is one of the regions in Ethiopia that are heavily affected by landslides (Fig. 2). Landslides in this region have been reported by Ayalew (1999) and Abebe et al. (2010). These landslides cause many social, economic and geomorphologic problems. One of the problems is the filling up with sediments of the Gilgel Gibe reservoir, con- structed for the Gilgel Gibe I hydro-electric power plant (Devi et al., 2008), for which the landslides may be an important sediment source. However, quantitative data on the landslides in the Gilgel Gibe catch- ment are absent as well as clear insight into the local causes for the landslides. Hence, more research about these landslides is needed to undertake specific measures against the landslides and to restrict their consequences for the Gilgel Gibe reservoir and on the socio- economic level in general.

    Therefore this study investigate the spatial and temporal patterns, causes and consequences of landslides in the Gilgel Gibe catchment, and assesses the contribution of the landslides to the high sediment load in the rivers of the Gilgel Gibe catchment.

    2. Study area

    Landslides are reported in all hilly areas of the Gilgel Gibe catch- ment (Abebe et al., 2010; Ayalew, 1999). This research concentrates on a study area of 14 km², located in the upper parts of the Gilgel Gibe catchment (Fig. 1). The study area has altitudes ranging between 2000 and 2800 m.a.s.l., and is bounded by latitude 7°29′30′′–7°26′00′′ N and longitude 36°51′30′′–36°53′00′′ E. According to the geological map of Ethiopia (Tadesse et al., 2003), the study area consists of volca- nic rocks of the Eocene and Paleocene, rhyolites, trachytes, rhyolitic and trachytic tuffs, ignimbrites agglomerates and basalts. However, the spatial occurrence of the different geological materials is very complex and heterogeneous and not known in detail. The major soil types in the study area are Nitisols, Acrisols and Vertisols

    Fig. 1. Location of the study area in Ethio

    (FAO-Unesco, 1974). Mean annual rainfall in the study area is ca. 2000 mm (National Meteorological Agency of Ethiopia, 2009). Around 60% of the rainfall occurs in the rainy season, lasting from May until September. Mean annual air temperature is ca. 19 °C (National Meteorological Agency of Ethiopia, 2009).

    In the study area, almost all the land has been taken into cultiva- tion. The main land use type in the study area is agricultural cropping, mainly wheat, teff, barley, faba bean, sorghum and maize. Next to these cropping activities, the farmers keep certain plots as grazing land for their livestock. The plots are mostly small and enclosed by hedges or tree rows. Total forest cover in the study area is ca. 11%, however this is mainly bush vegetation which is extensively used

    pia and in the Gilgel Gibe catchment.

    image of Fig.�1 image of Fig.�2

  • 129N. Broothaerts et al. / Catena 97 (2012) 127–136

    by the farmers. Only the highest parts of the study area are still covered by forest.

    3. Materials and methods

    3.1. Data collection

    In order to get more insight in to the spatial pattern of the land- slides in the study area, an intensive field survey was made during August and September 2009, at the end of the rainy season. The focus was on the recent landslides. They included active landslides, which are currently moving (which include first-time movements and reactivations), and landslides which were not moving at the mo- ment of investigating (inactive landslides) but showed clear signs of movements within the last 20 years. In addition to these recent land- slides, also ancient landslides could be observed in the field. They include all landslides that have not been active for a very long time (more than 20 years

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