*3.2.2 Land use and land cover*

Landslides are caused by a variety of factors, including land use and land cover [25]. Thereby, vegetation covers play an important role in slope stabilization through a different mechanism. The land use land cover of the study area (**Figure 6b**) is characterized by agricultural land, bare land, sparsely vegetated land, and moderately vegetated land categories.

### *3.2.3 Slope morphometry*

Slope morphometry is the steepness of the slope [4]. Slope morphology brings to bear major controls on landslide types and the severity of associated damages to life and property. It is classified into five classes (**Figure 6c**) such as escarpment/cliff (>45°), steep slope (36–45°), moderately steep slope (26–35°), gentle slope (16–25°), and very gentle slope (<15° [11]).

### *3.2.4 Relative relief*

The difference between a facet's maximum and minimum elevation is known as relative relief. Inside the apiece facet, the relative relief map depicts the local relief of maximum height between the ridge top and valley floor (**Figure 6d**). This relative relief lies on very high, high, medium, and moderate classes in the area based on the slope geometry classification system, and very high-class value range from elevation greater than 300 m, high 201–300 m, medium 101–200 m, and moderate has an elevation of 51–100 m. Therefore, an area with high relative relief is more susceptible to slope failures compared with low relative relief [15, 34, 35].

#### *3.2.5 Lithology*

Types of lithologies are the main influencing factors that contributed to the occurrences of landslides in the study area. The dominant lithological units are highly weathered basalts, ignimbrite, tuff, colluvium, alluvial soil deposits, and residual soils overlying the bedrocks (**Figure 5b**). The main criterion in the allocation of the rating for lithological subclasses is the response of the rocks to the weathering processes and erosion. The degree of weathering may vary based on the rock types

### *Landslide Assessment and Hazard Zonation in the Birbir Mariam District, Gamo Highlands… DOI: http://dx.doi.org/10.5772/intechopen.108122*

and mineralogical compositions. Basalt and ignimbrites form steep slopes because they are hard, massive, and resistant to erosion. Soft rocks, on the other hand, such as tuff, are more susceptible to weathering and erosion, as well as slope instability (**Figure 5b**).

#### *3.2.6 Structural discontinuities*

The association between structural discontinuity and rock slopes is a critical factor. The orientation of structural discontinuities was determined by collecting data facet-by-facet from the exposed rock mass and determining its affiliation to slope proclivities. Based on field observations and preliminary analysis joints and faults were considered the main geologic structures for further hazard/susceptibility/evaluations. Facet wise the structural data were collected and ratings were assigned based on the proposed slope susceptibility evaluation factors.

#### *3.2.7 Rainfall manifestation*

The average annual rainfall in the study area is 1372.8 mm. The amount of rain that falls has a significant impact on the slope's stability [25, 29, 43]. The instability of slopes increased as the amount and intensity of rainfall increased, which is obvious and reasonable. Due to the nature of the materials exposed in the slopes and the slopes' drainage characteristics, this is not always the case. As a result, rainfallinduced features are useful indicators for assessing the impact of rainfall on slope instabilities. Rainfall-induced slope manifestations (such as gully formation, toe erosion, and stream bank erosion) were taken into account when determining the rainfall rating (**Figure 7a**).

#### *3.2.8 Human activity*

Man-made activities, in addition to natural triggering parameters, increase the slopes' potential for instability [44]. Developmental activities such as road construction and cultivation activities are examples of man-made activities that affect slope stability conditions. Such anthropogenic activities increased the moisture content of soil or rock masses, as well as a reduction in slope stability (**Figure 7b**).

#### *3.2.9 Landslide hazard zonation*

For landslide hazard zonation, the slopes in the study area were divided into individual land facets. For this purpose, topographical map on 1:50,000 scales was utilized to delineate the land facets. A total of 106 slope land facets were delineated (**Figure 4**). Slope geometry includes relative relief and slope morphometry. From a total 110 km2 , 60.5 km2 (55%) fall in to very high relative relief (>300 m), 24.2 km2 (22%) fall in to high relative relief (201–300 m), 14.3 km2 (13%) fall in to medium relative relief (101–200 m), and 11 km2 (10%) fall in to moderate relative relief (51–100 m) (**Figure 6d**). Slope morphometry defines the steepness of the slopes. In the study area, about 18.87% of slopes fall under the category of escarpment/cliff (>45°) and 29.25% of slopes are a step (36–45°). The remaining slope falls under moderately steep slope (26–35°), gentle slope (16–25°), and very gentle slope (<15°) which account for 30.19, 15.09, and 6.60%, respectively (**Figure 6c**).

**Figure 7.**

*a. Rainfall-induced surface manifestation map and b. anthropogenic developmental activities affecting slope stability.*

The most dominant lithological units of the study area are highly disintegrated basalt. Spheroidal weathering was dominantly observed in the northwest and central parts of the study area (**Figure 2**). Faults, joints, and fractures are the dominant geological structures affecting the slope material. Residual soils and colluvial deposits are the most common slope materials (**Figure 5b**). Based on the map 58.11% of the total area is covered by soil mass, 28.3% is covered by disintegrated rock mass, and 18.87% is covered by medium-strong rock mass (**Figure 5b**).

Based on the land use-land cover, the major portion of the slopes is covered by agricultural land (43.4%). Moreover, areas covered by moderately vegetated, sparsely vegetated, and barren land account for 27.34, 18.87, and 10.39%, respectively. Surface indications such as damp, wet, dripping, and flowing water were considered for each facet to assess and evaluate the groundwater conditions. Watermarks, algal growth, and other features were also noted. As a result, each land facet was given a score (**Figure 6a**). A record of the rainfall in the study area shows that the months from April to June and from July to October received more rain. Rain-induced slope manifestations such as gully erosion, toe erosion, and stream bank erosion were also taken into account. Slope toe erosion, stream bank erosion, and gully erosion all accounted for 24.53, 20.75, and 54.72% of the total erosion (**Figure 7a**).

Manmade activities which affect the slope stability in the study area include cultivation activity, road construction, and unsafe dumping of materials. Based on field data-intensive cultivation activity covered about 43.4%, steep rock-cut for road construction constitute 26.4%, and unsafe dumped materials covered about 22.6%. About 7.6% of the total area falls under no human activities (**Figure 7b**). The study

*Landslide Assessment and Hazard Zonation in the Birbir Mariam District, Gamo Highlands… DOI: http://dx.doi.org/10.5772/intechopen.108122*

**Figure 8.**

*a. Landslide hazard zonation of Birbir Mariam and b. validation of present landslide hazard zonation map.*

area's landslide hazard was determined facet-by-facet using an evaluated landslide hazard, which indicates the cumulative likelihood of instability. The area was divided into three zones based on the values of the assessed landslide hazard: high, moderate, and low hazard zones (**Figure 8a**).

### **4. Discussion**

The selection and validity of the conditioning factors are of major importance for the correct result of the slope susceptibility evaluation parameter. Intrinsic and external causative parameter rating schemes were therefore considered to be responsible for slope instability. The high-hazard zones are mainly located in the central part and north-eastern side of the study area (**Figure 8a**). These localities are dominantly agricultural lands subjected to many anthropogenic activities. Most of the main road that crosses the area falls within the high-hazard zone areas. Along the main road, it is common to observe slope failures also in the form of earth slide rock falls (**Figure 8a**). Such failures mostly occurred following heavy rains. It was also reported that the main road experienced frequent failures and maintenance in the past. The problem still exists in the areas decease a lack of proper understanding of the causes and failure mechanisms of failures in the area. The southern and northeastern parts of the area fall under the moderate hazard zone, while the West and North West areas fall under the low hazard zone (**Figure 8a**). On the landslide hazard map of the study area, 18.87% (20.76 km<sup>2</sup> ) is classified as high hazard, 54.72% (60.19 km<sup>2</sup> ) is categorized as a moderate hazard, and 26.41% (29.05 km<sup>2</sup> ) is divided into a low hazard (**Figure 8a**).
