The Inventory data in the study area was collected using a detailed field survey and Google Earth image interpretation. As noticed in Google Earth image interpretation, landslide incidence in the study area is a common problem since 2006 and has impressed 13 Villages such as Ineget weyin weha, Gete Semane, Angot, Abare weha Tayita, Yewaye, Wendeye, Mahile Goncha, Inegode, Sekela Genbore, Debre yakob, Debre Birhan, Adise, and Fela Yegarida.
4.1 Landslide in Gete Semane Village
According to local people, the first small landslide with crack development was occurred in 2014, which forced to relocate one family from their village (Fig. 4a). According to local people, a small landslide in this village continuously occurred in the rainy season, and curved cracks developed from year to year. This village has been affected again by a reactivated landslide in 2018 (Fig. 4b) and caused damages in a house and 48 hectares of cultivated land that has been covered by various crops like maize, teff and bean and different fruits like coffee, almond, lemon, buckthorn, orange, and papaya. This slide has occurred due to stream erosion, gully erosion, man-made activities, and due to the presence of spring, heavy rainfall, and unconsolidated deposit. The type of slope failure in this village is soil slide with a width of 10–800 m, depth of 1–15 m, depletion zone of 1– 25 m, runout distance of 18–950 m, and accumulation zone of 0–800 m. Landslide in this village will pose risks in the future because various tension cracks have been developed continuously from the toe to the top parts of the slope with an average length of 1–241 m and depth of 0.1–0.3 m. These cracks caused a change in the streamflow directly into the cracks. The flow of water into the crack has caused the slope material to saturate by raising the groundwater level. The saturation of soil material can weaken the strength of the soil and result in slope failure. Various technical remedial and preventive measures like plantation of the eucalyptus tree, fruits, rock riprap, and terracing were applied but it is not possible to stop land sliding because of imbalance in the driving force with the constructed support and the plantation was above the failure zone. This further adds weight to unstable slopes.
4.2 Landslide in Yewaye (Yekimcha) Village
According to the local people, this area was severely affected by a debris flow in 1961 because of intensive and prolonged rainfall and damaged in 15-hectare farmland with crops. According to the local people, various measures like a plantation, and terracing were applied. It was effective for many years but it has been reactivated in 2018 because of overgrazing of the steep slope area, weathering, surface erosion, river undercutting, intensive agricultural activity, irrigation, and heavy rainfall. Landslide in this area includes transitional soil slide, debris flow/fall, earthfall, and rockfall. Landslide in this area has a depth of 0.1–12 m, scarp height of 0.1–75 m, an accumulation zone of 0–75 m, a width of 10–200 m, and a runout distance of 20–370 m. These landslides caused damages in six-hectare farmland that has covered by various crops and permanent fruits. According to the local people, a landslide in this area occurred after 57 years ago. This area is under the risk of land sliding in the future because many cracks with an average length of 10–20 m and depth of 0–0.25 m are developed parallel to the scarp.
4.3 Landslide in Sekela Genbore
Landslide in this village has occurred due to stream initiated gully erosion, removal of slope toe by the river since 6/8/2006 (Fig. 6B) as observed in Google Earth image interpretation. Starting from this year, the study area, stream formation, and gully erosion increased from year to year as can be observed from Google Earth image interpretation (Fig. 6B). A few years ago, a permanent and large gully has been formed. The stream has been removing the slope toe support during the rainy season. This caused curved crack development, and then the slope falls as a curved surface and slides down towards the stream. The moving material moved away by the flowing stream. In 2018, a large soil slide is occurred because of springs, gully and stream erosion, improper ditch design, overgrazing, intensive agricultural activities, weathering, and heavy rainfall. Landslide in this area includes transitional, rotational, weathered rockslide and ground subsidence related landslide with average landslide depth of 0.5–6 m, the width of 15–300 m, depletion zone of 0.1–15 m, an accumulation zone of 0–360 m, and runout distance of 0–370 m. These caused damages in seven-hectare farmland with crops and five houses are under risk. Various measures like plantation of the eucalyptus tree, terracing, rock riprap, and gabion wall were applied, but it is not effective because the measures are not applied in their-appropriated location. Landslide in this area has been occurred since 2006 and active until 2018 due to the removal of the slope toe by the river undercutting during the rainy season.
4.4 Landslide in Ineget Kebele/Village
According to local people, the earth flow type of landslide occurred in 2018 in this village, which caused damages to two houses, the death of eight people, and destroyed the 2.5-hectare crop and permanent fruit covered farmland. In this area, slope failure has occurred at the soil-rock interface when the slope soils are fully saturated due to heavy and prolonged rainfall. This slope failure has initiated by agricultural activities, steep slope gradient, shallow colluvial soil deposit overlaid on fractured, and highly weathered basalt unit and triggered by heavy and prolonged rainfall. It has a runout distance of 900–910 m with an average width of 15–20 m.
4.5 Landslide in Tora Meda Village
In this village, soil slide is predominantly found and caused the destruction of farmland that has covered by various crop types, eucalyptus trees, and fruit. In addition to this, more than five houses are at risk. The rotational slide occurred on thick unconsolidated soil because of the removal of slope toe by stream erosion and gully erosion. In Tora Meda area, landslide incidence is a common problem in Gofer and Yigagira area and has landslide dimensions with depletion zone of 1.5–12 m, accumulation zone of 0–20 m, runout a distance of 11–300 m, the width of 7–60 m and a depth of 1–12 m.
4.6 Landslide in Debre Yakob Village
In this village, the landslide has occurred as soil slip with a curved failure surface because of stream erosion of the slope toe and spring emanation at the soil-rock interface. According to local people, a landslide in this village has been occurred since 2012 and is reactivated in 2018 that caused damages on two hectares of cultivated land, which was covered by “teff”, wheat, chickpea, and eucalyptus tree. Since the soil is unconsolidated, less permeable, and has high water absorption, it has failed when the soil is fully saturated. Earth fall has occurred at the interface of soil and rock units. Hence, the soil at the top of the slope is unconsolidated and the interface rock is highly weathered, the slope materials have failed at any season as earth fall.
4.7 Landslide in Inegode Village
Inegode is one of the areas that has been severely affected by landslide incidence, especially along the Tigidar river catchment. Landslide in this village has occurred since 2006 and active still now. The rotational, transitional, rock toppling, rockfall, and talus fall are the common landslide in the area. It resulted in damages to farmland, crops, and eucalyptus trees. A landslide in this area has a depletion zone of 0.5–30 m, an accumulation zone of 0–200 m, runout distance of 8-203m, a width of 6–289 m, and a depth of 0.75–6 m. Hence, the shape of the slope is concave upward, which can impound water during the rainy season and cause the groundwater level raised. Thus, various springs have been exposed at the various point of the slope toe and flow permanently. This causes a decrease in the shear strength of slope material and initiates large-scale landslide occurrence in the area. Most of the landslides in this area have occurred on thick unconsolidated colluvial deposits, which is overlaid on highly weathered basalt and unconsolidated tuff rock unit.
4.8 Landslide in Debre Birhan Village
Debre Birhan is one of the areas that have been severely affected by landslide incidence before 2006 (Fig. 10) and active still now as observed in Google Earth image interpretation. Landslide in this area is damaged three-hectare farmland with crops because of Bina river erosion of the thick unconsolidated soil deposit, agricultural activities, and the presence of erodible soil, springs, heavy and prolonged rainfall.
Landslide in this village has dominantly occurred as rotational soil slide in four sites of Bina river catchment. Different sizes of landslides have occurred along the river when the river channel is changed. The river channel is changed when the landslide blocked the old river channels (Fig. 10). The landslide in this site has a width of 20–120 m, a run-out distance of 1,990 m, a depth of 2–12 m, an accumulation zone of 4-10m, and a depletion zone of 1.5–6 m. Although they tried to provide various treatments like a plantation, gabion walls, and rock riprap, it is not well effective. Because the preventive methods have not been provided based on the causative factors condition.
4.9 Landslide in Angot Village
Angot is one of the areas that have been severely affected by rockfall, debris, and earthflow types of landslide incidence. This slide is caused by heavy rainfall, spring, stream overflow, agricultural activities, river cut, slope shape, and slope escarpment. Since the area is characterized by steep escarpment and highly concave upwards, rainwater can be impounded and infiltered into the sloping ground, which is then exposed at the slope toe.
The flowing of shallow groundwater inside the slope causes slope material saturation, piping, and pore water pressure development. These all caused soil strength reduction and a slope material movement. Earthflow and debris flow are the common landslide incidence in the area but later the whole slope materials are developed tensional cracks, curved cracks, and uneven surface. The spring that was blocked by a landslide has been exposed at the slope toe and flows towards the river. This will also cause piping and slope toe erosion. The combination of all landslide caused damages in 120-hectare farmland that has covered by various crops and permanent fruits. The slowly moving ground was blocked the spring water and has been interrupted the irrigation activities.