Assessment of Landslide Risk in Ethiopia: Distributions, Causes, and Impacts

Abstract

The complex geological and geomorphological settings of Ethiopia, consisted of highland plateaus, escarpments, deeply dissected valleys, and flat lowlands, are results of multiple episodes of orogenesis, peneplanation, crustal up-doming, faulting, and emplacement of huge volumes of lava. The broad elevation contrast raging from about −125 m to 4550 m Above Mean Sea Level (AMSL) is an important factor in determining the climate regimes, vegetation types, and even populations’ lifestyles. In Ethiopia landslides, mostly manifested as rockfall, earth slide, debris, and mudflow, are among the major geohazard problems that immensely affects life, infrastructures, and the natural environment. They widely occur in the central, S-SW, and N-NW highland regions. This study discusses the distributions, causes, and impacts of landslides and presents a susceptibility zoning map produced applying the weighted overlay analysis method in the ArcGIS environment. For this purpose, key parameters (lithology, elevation, rainfall, slope angel, land use-land cover, and aspect) were selected and assigned weights by considering their contributions to slope failures. Correlations with inventory data have shown very good matching, where more than 90% of the observed data fall in areas categorized either as moderate, high, or very high susceptible zones, where appropriate risk assessments could be mandatory before approval of major projects.

1. Introduction

Landslide is a phenomenon that represents the downward movements of a wide range of slope-forming materials (soils/rocks) due to gravitational and other driving forces [1, 2]. Considering the characteristics of the sliding materials and mechanisms of movements they can be classified as falls, topples, slides, flows, spreads, or any mixture of these and occur either slowly or suddenly. Situated in the horn of Africa between 33 and 48°E longitude and 3.40 and 14.85°N latitude, Ethiopia is the second African nation with a population of about 115 million (www.worldometers.info) and a surface area of 1.122 million km². The landscape constitutes highlands plateaus, dissected valleys, escarpments, gentle slopes, and flat plains. These land features are results of geodynamic processes associated with the establishment of the East African Rift System (EARS), which is a narrow North-west - South-east (NE-SW) elongated rift with thin continental lithosphere. This rift dissects Ethiopia diagonally into western and eastern plateaus that represent the Nubian and Somalian plates, respectively (Figure 1) [3, 4, 5]. Active rifting processes combined with local and global drivers (like seismicity, hydrometeorological events, and demographic factors) have created a suitable environment for the widespread effects of landslides. It occurs in the mountainous regions of Ethiopia dominantly in the North-Northwest (N-NW), central and South – Southwest (S-SW) highlands, and rift-margins, usually following intensive precipitations and brings variable impacts on life, built infrastructures, and natural environment [6, 7, 8, 9].

Figure 1.

Generalized map of the East African Rift System (the dotted lines show boundaries of the East African Rift System, while the triangles represent volcanic centers (from Riftvolc consortium, 2013).

In this work, the distributions, probable causative factors, and impacts of landslides are described with more emphasis on infrastructures using few selected case studies. Applying different secondary sources, a landslide inventory map is compiled and relationships between the natural attributes (lithology, slope height, slope angle, rainfall, and land use-land cover) and spatial distributions of landslides are assessed. Moreover, a susceptibility zoning map is generated involving the mentioned parameters to which weights were assigned considering their significance to slope failure. Such a map serves as an input to delineate areas according to their importance to various developmental activities and also helps to identify risk potential ones that demand more evaluations and implementation of mitigation measures before major projects are supported.

2. Geomorphology, climate, and general geology

Ethiopia’s land surface is characterized by wide elevation contrast that varies from about 125 m below sea level to 4550 m above mean sea level which represents the lowest point in the world, Danakil Depression, and Ras-Dashen mountains (Figure 2c). The elevation is the key determinant that defines the climatic conditions of Ethiopia. Accordingly, the country is divided into five climatic zones (Figure 2a) that locally known as Wurch (very cold), Dega (cold), Weyinadega (moderate), Kola (hot), and Breha (very hot temperature zone) [10]. They are distinguished by distinct precipitation and temperature regimes, vegetation and crop types, and even lifestyles of the populations. Wurch, Dega, and Weyinadega climatic zones typically represent the northern, central, and SW highlands as well as rift neighboring plateaus (Figure 2b and c). They are described by medium-high altitudes (mainly above 1500 m amsl), moderate-high precipitation (above 1000 mm/year), and low-moderate average temperatures (below 25°C). Areas like Tarmaber, Meket, Gashena, Semen Mountains, Arsi, and Bale mountains with elevations above 3200 m amsl that intermittently receive snow and hail (personal communications with local people in 2019; World Institute of Conservation & Environment) constitute this category. Meanwhile, Kola and Bereha climatic zones, representing the NE (Afar), western (Humera-Metema), S-SW (Gambela, southern Omo), and eastern Ethiopia, show low altitudes, high-very high temperature (above 30–50°C), and very low precipitation (<500, rarely up to 750 mm/year).

Figure 2.

Climatic zones (a), average annual rainfall distribution (b), and simplified geological (overlain on the topographic) maps of Ethiopia (c). Sources: [10, 11, 12].

The rifting process has defined not only the geomorphology but also the geological settings of Ethiopia, which are discussed in many works [3, 6, 11, 13, 14]. Hence, the formations that underlay the Ethiopian territory differ in composition and age, which ranges from Quaternary to Precambrian (Figure 2c). The oldest Precambrian basement rocks are represented by high-grade ortho- and paragneisses and migmatites as well as low-grade volcano-sedimentary—ultramafic assemblages and granitoids [13]. These Precambrian rocks constitute part of the Pan-African Mozambique belt and are distributed in the northern, western, and southern parts of Ethiopia. These formations have undergone prolonged erosion and denudation during Paleozoic that resulted in undulated terrain over which thick Mesozoic sediments (mainly sandstone and limestone) were deposited. The Jurassic sediments cover wide areas of eastern and some places in central and northern Ethiopia. Uplifting of the Afro-Arabian block during Tertiary has resulted in the eruption of a large volume of lava through fractures and covers a substantial part of the country forming elevated terrains. During this period, sediments deposition took place that cover eastern Ethiopia. Meanwhile, the quaternary period is known for the placement of volcanic lava in areas from Afar depression up to the Lakes Region in the central main Ethiopia rift. Thick Quaternary sediments are distributed in Gambela, Borena, Metema, and few other flat lowland areas (Figure 2c).

From the demographic perspective, areas categorized as Wurch, Dega, and especially Weynadega zones, are the most ideal and preferred for settlement due to the availability of sufficient water, fertile lands, and suitable climate for life. But the spread, frequency, and severity of landslides in these areas are more than in Kola and Breha zones, where the climates are more hostile and flatness of terrain and scarcity of waster do not favor mass movements.

3. Objectives

The basic objective of this study is to examine the distributions, causative factors, and impacts of landslides and acquire a fundamental understanding enabling to develop effective mitigation measures that help to save life and the economy. Accordingly, its specific objectives are: (a) conduct inventory of landslide occurrences across the nation; (b) map links between the spatial distributions and natural attributes that trigger and/or aggravate landslides; (c) assess impacts of landslides on life and infrastructures; d) produce landslide susceptibility zoning map of Ethiopia.

4. Methods and materials

The methodology used in this study comprises—(a) collection and analyses of geological, engineering geological, and geo-hazard data from published and unpublished reports and research publications [11, 15, 16, 17, 18, 19, 20, 21, 22, 23]. All data are compiled in the geographic coordinate system using WGS84 datum; (b) collection of rainfall data—the Chirps gridded data for the year 2015 available online was used after comparing it with the National Meteorological Agency (NMA) data, which was found almost alike; (c) download land use-land cover map from National Aeronautics and Space Administration (NASA) web page; (d) data about past landslides events and their impacts. This includes information about the date and time of occurrences, deaths, injuries, forced resettlements, damages to infrastructure, and possible causes; Government offices, non-governmental organizations (NGOs), private firms, research publications, mass media, and local communities, including elder people with knowledge previous events, have served as sources; (e) 30 m resolution DEM data—important inputs about slope height (elevation), slope gradient, and slope direction (aspect) are extracted. These data are closely linked to rainfall and temperature distributions, soil humidity, soli thickness, vegetation types, and density as well as hydrological features of sloppy areas that determine the scale/rates of mass movements; (f) applying a multi-class scoring system based on assigning of weights to selected parameters contributing to slope failure, produce landslide susceptibility zoning map [24, 25].

5. Inventory, distribution, and impacts of landslide

This landslide inventory has identified more than 600 locations across the nation, where landslides occurrences are clearly observed, very few of them are even known with a history of repeated events. Moreover, it reflects localities, where potential landslide risks are imminent [7, 8, 9, 15, 16, 17, 18, 19, 20, 21, 22, 23, 26, 27, 28, 29]. The distribution of inventory data well correlates with lithology, elevation, structural, rainfall, and seismicity maps. Only considering the patterns, landslides occurrences are tentatively classified into four blocks, Block A–D (Figure 3). Block A represents the N-NE parts of the country, including the eastern part of the western plateau, western rift escarpments, and some places on the rift floor. It stretches from north of Mekele through Michew, Woldiya, Dese, Kombolcha, Kemisse, Shewarobit and continues to the south of Debrebirhan. Major parts of this block are underlain by Tertiary and Quaternary volcanic, whereas Mesozoic sediments are distributed at the NE part of the block covering limited areas of SE Tigray.

Figure 3.

Landslide inventory map (left) and landscape of NE part of Ethiopia (right).

Dese and its surrounding are the most well-known areas, where recurrent landslides cause impacts on settlements, roads, and other properties (Figure 4a and b). At many places, emerging springs from near surfaces are observed which indicate shallow groundwater. So, steep terrain, undercutting of stream banks, slope erosion, and shallow groundwater are key factors that trigger/aggravate displacement of slope materials. Meanwhile, huge volcanic blocks that are almost detached from the parent rocks are observed at the southern end of the block, in Mushmado village, Say-Debir district, about 8 km from Lemi town (Figure 4c). The probability that these blocks would crumble into the valley side is very high if triggered by extreme hydrometeorological, seismic, or other events and will put life, infrastructures, and farmlands in the valley under very high rockfall risk.

Figure 4.

Panoramic views of landslides: (a) partial settlement of house foundation, in Dese town; (b) debris slide threatening the Addis Ababa-Dese main road, Kewet district, Debresina town; (c) rockfall risk in Mushmado village, Saya-Debir district, North Shewa zone.

Block B encompasses areas between 8 and 13°N latitude and 36.5 and 39°E longitude. Many zones in East and West Gojam (Gozamin, Gonch-Siso Ense, Hulet-Ej-Ense, Shebel-Berenta, Awabel, Aneded, Machakil, Dejen, Adet, Sekela), East Wollega (Ambo, Gedo, Weliso), and Gonder (Lai-Armachoho, Ebinat, Guangua, Quarit) are found here. Moreover, such rivers like Abay, Tekeze, Beshilo, and their main tributaries that formed deep valleys are also among the risk vulnerable areas. The dominant landslide types are rockfall and rock/soil slides, to some extent mudflows. Their impacts on infrastructures and farmlands are quite significant.

The landslides in the Abay gorge, between Dejen and Gohatsion main road, have long and repeated histories, and this economically vital route passes through the 40 km wide Abay (Nile) valley (Figure 5). Subsurface investigations carried out within this valley revealed the depths to the slip planes mainly vary are the range of 14–25 m [22]. Even though deaths are not reported, unofficial sources disclosed that the cost of monitoring and road maintenance exceeds 1.5 million USD/year.

Figure 5.

View of landslide occurred in Kurar village, Dejen side (a), the same route, but on the Gohatsion side (b–d): road under maintenance in June 2010 (b), rockfall and debris slide damaged it in August 2010 (c), the site was visited in September 2019 (d).

Block C represents south-southwestern Ethiopia and the landslide occurrences are identified within 36–39°E longitude and 5–8°N latitude. It includes Ziway, Shashemene, Hawasa, Hosaina, Adami-Tulu, Jima, Dila, Sodo, Agremariam, Koso Jinka, Sawla, Arbaminch Zuria, Chincha, Gofa, Gidole, Konso, Bako-Gazer, Basketo, and many others places. Along the rift margins, where slope gradients are relatively high, the landslides are manifested by rockfall, debris, and mudflows. A massive landslide that occurred in Gidole, about 55 km SE of Arbaminch town, is a good example that demonstrates how severe is the economic, social, and environmental impacts of landslides in the region (Figure 6).

Figure 6.

Panoramic view of a landslide body in Welaite village, 2 km NE of Gidole town observed in March 2011 (a), and the same body observed in March 2016 (b). Note that in 2011 its width was about 40 m whereas in 2016 it expanded to about 200 m.

This recent occurrence within the deeply excavated zone (up to 25 m) started in 2009 following intensive rainfalls that saturate the subsurface. The road construction intended to connect Gidole with the Arbaminch-Konso main road has affected the toe parts of the old landslide zone and resulted in the release of shallow groundwater that triggered that landslide. To prevent mass movement slope regarding, about 250 m long retaining walls and drainage ditches were constructed. But due to the large extent of the sliding zone these measures did not change the situation, rather doubled the project cost. So, construction across the failed was abandoned in 2013.

The landslide observed in Alem village, Dodota district, in September 2019 has severely damaged a section on the Dera-Asela main road (Figure 7a). The mudflow occurred on May 28, 2018 (Figure 7a and b) following heavy rainfalls has triggered the sudden movement of a huge volume of earth mass from the head of the landslide and buried houses with 22 people in Western Arsi Zone, Tulu-Gola village, of which 14 were from the same family (May 30, 2018, the Ethiopian reporter).

Figure 7.

Road collapse at Alem village, Dodota district, along with the Dera-Assela road (a) and mudslide that killed 22 people and domestic animals in Tulu-Gola village, Western Arsi zone (b and c).

Block D mainly constitutes the eastern part of the Main Ethiopian Rift, such as different districts of East Shewa, Arsi, Harage, Diredawa, and Jigjiga zones. Accordingly, Adama, Chole, Cheleleka, Merti, Fentale, Golelcha, Mechara, Lome, Asebe-Teferi, Bedeno, Kersa, Deder, Chiro, Haromay, Melka-Jilo, Fedis, Gursum, and the areas with landslide records. Rockfall, rock slide, and debris flows are the widely observed landslide phenomena. At many places, the landslides are associated with highly weathered and fractured volcanic (ignimbrites and basalts) with steep slope gradients (up to 75°).

In general, this inventory survey has provided tangible information about the spatial distribution, main causative factors, and impacts of landslides. Meanwhile, lack of well-organized records about the types and extents of damages, at this stage it is impossible to give any credible estimations of the economic and environmental losses caused by landslides. Abay A. [30] estimated the losses from 1998 to 2003 to be 135 death, 3500 displaced households, and 1.5 million USD worth of property damages. B. Abebe, et al. [8] stated that landslides that occurred between 1993 and 1998 have claimed hundreds of human lives, damaged over a hundred kilometers of asphalt roads, destroyed many houses, farmlands, and natural vegetations. Similarly, a compilation of data from mass media, newspapers, different reports, and affected communities, (including Fana Broadcasting Corporation; Ethiopian Broadcast Corporation (EBC); Walta Information Center; GSE unpublished technical reports published in 2003–2019) revealed that only between 2016 and 2020 more than 302 people and 1500 domestic animals were killed (Table 1).

Region	Landslide affected district (woredas)	Death
Tigray	Hintalo-Wajirat, Hawzen, Atsbi-Wenbera, Degua-Temnbie, Enderta, and Samri-Shart	NR
Amhara	Harbu, Ambassel, Guba-Lafto, Kalu, Dawint, Delanta, Werebabu, Bati, Bugna, Kutaber, Dese-Zuria, Artuma-Farsina, Jille, Efratana-Gidim, Debresina, Kewet, Wagide, Mafud, Mezezo, Chefie-Golana, Dawe-Rahmedo, Gozamin, Gonch-Siso Ense, Hulet-Ej-Ense, Shebel-Berenta, Adet, Sekela, Awabel, Machakil, Dejen, Lai-Armachoho, Ebinat, Guangua, Quarit	11 deaths
Oromiya	Wolmera, Ambo, Guder, Were-Jarso, Kuyu, Jeldu, Tikur, Golelcha, Dodotanasire, Merti, Boset, Aseko, Sude, Dugda-Bora, Wenchi, Welesona Gora, Chela, Chole, Guba-Korcha, Chiro, Dendi, Deder, Kombolcha, Babile, Tullo, Jeju, Daro-Lebbu, Dobba, Seke-Chekorsa, Dedo, Omo-Nada, Goma, Limu-Kosa, Tiro-Afeta, Haromaya, Girawa, Gursum, Chelenko, Bedno, Horo-Guduru	73 deaths and 20 injuries
Southern Nations and Nationalities People (SNNP)	Aleta-Wondo, Kokir Gedebano, Ameya, Gorro, Gumer, Enemorna-ener, Soddo, Meskanena-mareko, Silti, Esara-Tocha, Ela, Marekagena, Decha, Gimbo, Aroresa, Bensa, Dale, Yiga Dera, Shebedino, Yirgachefe, Derashe, Arbaminchzuria, Amaro, Gofazuria, Basketo, Bako-Gazer, Gidole, Konso	102 deaths in one incident
Others	Addis Ababa	116 deaths

Table 1.

Summary of landslide inventory showing affected districts and death and injury reported from 2016 to 2020.

The landslide in different parts of the country is associated related with three distinct geological setups—(a) landslides developed within the Territory volcanic environment where saturated pyroclastic materials and clay are present as intercalations within the volcanic flows that cover a wide area of the Ethiopian highlands; (b) landslides formed within the sedimentary terrain and the presence of siltstone, shale, and marl as intercalations within the limestone sequence. These are common in the Abay (Nile) valley, in areas south of Mekele (Northern Ethiopia); (c) presence of unstable colluvial materials (silt and clay with gravel and boulder matrix) in areas of relatively gentle terrain covering different formations. Overall, the intercalation within the volcanic and sediments acts as rupture surfaces that aggravate easily displacement of landmasses whenever absorb more fluid in the rainy season.

6. Landslide causative factors

The root causes that initiated or accelerated landslide observed at various locations could be associated with the following factors—(a) presence of physically incompetent (soft) earth materials that make up slope surfaces or elevated terrains and also effects of structural discontinuities in areas; (b) intensity and duration of rainfall and effects flooding, erosion a well as groundwater level fluctuations; (c) slope heights and (elevation) and slope angles, which favor mass movements; (d) poor earthwork practices during infrastructure developments (constructions of roads, bridges, dams/reservoirs), and quarrying for mine exploitations. These works involve the removal of earth masses from one place and dumping it into another place which causes either mass deficiency or excess load or both; the effects destabilize slop balances; (e) demographic factor expressed by fast population growth that accompanied by a continuous struggle for resource share. Such struggles put too much pressure on the natural environment and aggravate slope movements; (f) passiveness to enforce code of land-use practices and make accountable those who violate norms; (g) lack of awareness (illiteracy) among rural communities about the influence of landslides in their livelihoods; (h) absence of alternative means of subsistence for rural youth community who have little access to land ownership. So, they rely on over-using of the natural environment that leads to intensive land degradation. Except the natural factors, the human-related ones seem to be fully manageable if better awareness is created, job opportunities are improved and extreme poverty is reduced, land use and land administration codes and practices are enforced, and traditional community practices on land and forest preservations are fully respected. These measures play their role to improve communities’ resilience to cope up with the impacts of landslides. The spatial associations between landslide and seismicity are explained in different works [4, 31, 32, 33]. In the Ethiopian context, the occurrences of landslides and earthquake epicenters that are practically concentrated within the rift system and surrounding plateaus are found to have very close correlations. But no instrumental records are available that justify the contribution of ground vibrations to triggering landslides.

7. Landslide risk susceptibility zoning

Landslide susceptibility zoning maps are useful tools to differentiate areas that are suitable for agriculture, infrastructure development, national parks, or other purposes as well as delineate risk-prone areas that should be either protected or rehabilitated before approval of any developmental projects [24, 34, 35]. In Ethiopian landslide, mapping and risk zonation were carried out in specific hazard affected areas, mostly in the highlands and rift regions, using ground survey and remote sensing data [8, 22, 27, 28, 30, 36, 37, 38, 39, 40]. However, in this work attempt is made to produce a landslide susceptibility zoning map of the country and correlated with the inventory data acquired through extensive fieldworks mainly by the Geological Survey of Ethiopia, where the lead author has been working for a long time. The field observation data was also used for validation purposes. Thus, the parameters for analyses were selected based on the expert’s decision to which weighted values were assigned according to their contributions or influence to slope instabilities [24, 25]. The weights given to involved parameters are as follows: For lithology, elevation, and rainfall—20% each, for slope angle and land use-land cover—15% each, and for aspect—10%. Initially, each of these parameters was sub-divided into five categories, which represent the very low, low, moderate, high, and very high landslide susceptibility zones.

Then using the weighted overlay method in the ArcGIS environment, the map displayed in Figure 8 is generated. The spatial coverage of each class was calculated by multiplying the corresponding raster counts by the grid pixel sizes and dividing a single class value by the total areal coverage and then multiplying by 100%. Accordingly, about 49.1% of Ethiopia’s land surface is susceptible to landslides, of which 39% moderate, 10% high, and 0.1% very high-risk zones. Similarly, 50.9% of the territory is categorized either as very low (5.9%) or low (45%) susceptible zones (Table 2).

Figure 8.

Landslide susceptibility zoning map of Ethiopia and known landslide occurrences.

No	Susceptible zone	Areal coverage (sq. km)	Country coverage (%)
1	Very low	66,287	5.9
2	Low	504,791	45.0
3	Moderate	437,421	39.0
4	High	112,152	10.0
5	Very high	1448	0.1
	Total coverage	1,122,104	100

Table 2.

Landslide susceptibility zoning.

8. Conclusions

This assessment clearly indicated that landslides are major threats to life, infrastructures, and the natural environment. Natural and human-induced factors (existences of poorly consolidated, easily erodible, saturated and soft earth materials, high slope gradients, intensive or continuous precipitations with subsequent flooding and erosion, scarcity or absence of vegetation cover in sloppy terrains, ground vibrations or seismicity, and continuous growth of population with poor land-use practices) are among the key causes that exposed about 49% of the country to landslide risks. Unfortunately, until the road sector sensed the real challenges posed by a landslide and the ever-increasing rates of fatalities and environmental losses became evident, the issue has never been taken seriously. Hence, it is quite important to proceed with landslide risk assessments to identify and prioritize areas based on their extents, frequency of occurrences, the severity of consequences, as well as nature of different elements exposed to risk. This could be possible through careful considerations of updated landslide inventory data/maps and introducing varieties of risk susceptibility models based on integrated analyses of high-resolution remote sensing and ground observation data, which represent distributions of natural and human-related factors. Ultimately, such comprehensive assessments will play a positive role to ease consequences on life, infrastructures, and the natural environment. It is important to underline that the existing trends of land-use practices are completely inadequate to manage impacts of human-induced landslides that occur very widely. Therefore, implementing zero tolerance for improper land uses through stringent monitoring and enforcement of relevant policies, guidelines, directives, and respecting important social norms must be taken as fundamental tasks of all concerned bodies.

Acknowledgments

We are very grateful to geoscientists of the Geological Survey of Ethiopia (Leta Alemayehu, Habtamu Eshetu, Yewunesh Bekele, Biruk Abel, Abaynesh Mitiku, Tekaligene Tesfaye, Yekoye Bizuye, Debebe Nida, and many others), Addis Ababa University, Ethiopian Roads Authority, National Disaster Risk Management Commission (NDRMC), and other who put tremendous efforts to travel to various parts areas of the country and collect invaluable data used in this assessment. We also extend our sincere appreciation to those who put direct or indirect contributions to this piece of work.