Weights of evidence modeling for landslide susceptibility mapping of Kabi-Gebro locality, Gundomeskel area, Central Ethiopia

Kabi-Gebro locality of Gundomeskel area is located within the Abay Basin at Dera District of North Shewa Zone in the Central highland of Ethiopia and it is about 320Km from Addis Ababa. This is characterized by undulating topography, intense rainfall, active erosion and highly cultivated area. Geologically, it comprises weathered sedimentary and volcanic rocks. Active landslides damaged the gravel road, houses and agricultural land. The main objective of this research is to prepare the landslide susceptibility map using GIS-based Weights of Evidence model. Based on detailed field assessment and Google Earth image interpretation, 514 landslides were identified and classified randomly into training landslides (80%) and validation landslides (20%). The most common types of landslides in the study area include earth slide (rotational and translational slide), debris slide, debris flow, rock fall, topple, rock slide, creep and complex. Nine landslide causative factors such as lithology, slope, aspect, curvature, land use/land cover, distance to stream, distance to lineament, distance to spring and rainfall were used to prepare a landslide susceptibility map of the study area by adding the weights of contrast values of these causative factors using a rater calculator of the spatial analyst tool in ArcGIS. The final landslide susceptibility map was reclassified as very low, low, moderate, high and very high susceptibility classes. This susceptibility map was validated using landslide density index and area under the curve (AUC). The result from this model validation showed a success rate and a validation rate accuracy of 82.4% and 83.4% respectively. Finally, implementing afforestation strategies on bare land, constructing surface drainage channels & ditches, providing engineering reinforcements such as gabion walls, retaining walls, anchors and bolts whenever necessary and prohibiting hazardous zones can be recommended in order to lessen the impact of landslides in this area.

Landslide Hazard Zonation Along Milot-Kukës Motorway, Albania

In this paper it is briefly described the landslide hazard zonation before and after the construction of the Milot-Kukës motorway, Albania. The studied area is a mountains zone with extremely complicated morphological and geological framework, characterized by steep slopes, intensively fractured and highly to completely weathered rocks. Due to intensive excavations done during its construction, many of the slopes are destabilized and nowadays, have become unstable. During and after the rainfalls, on both sides of the motorway, several landslides such as earth slide debris flow, rock falls and rocks slides have occurred and have increased the risk due to natural hazards. For this reason, the motorway’s area is analyzed in terms of landslide hazard zonation by using the high-resolution satellite imagery and factors data in a GIS environment. During 2015-2017, a 1:10000 scale engineering geological map is compiled and was used to identify the landslides location, lithological characteristic, slope features, geotechnical conditions and land use situations. As a result, the studied area was divided into five categories from very low to very high-risk zones. Moreover, based on the analysis results of the landslide hazard zonation before the motorway’s construction, it was concluded that the excavation works had a considerable influence in increase of hazard level, particularly on the instability of the slopes.

Landslide Interpretation of the Northeast Flank of Kohala Volcano, Hawaii

The Hawaiian Island volcanic edifices have shed at least 15 giant submarine landslides, each classified as either a slump or debris avalanche. Controversy exists regarding the number, size, and type of landslides on the northeast flank of Kohala Volcano. This study provides a new interpretation for the Kohala flank based on contour and balanced cross-section analysis. Specifically, contours indicate that there is a landslide extending from the summit to the coast between Pololu and Waipio Valleys. The contour evidence also shows that the slide plane is planar and dips less steeply than the topographic slope. Balanced cross sections show the slide plane to be approximately 950 m deep immediately downhill from the zone of depletion, and the slide plane presumably reaches the surface at the base of the coastal cliffs on the northeast coast of Kohala mountain. The lower part of the landslide once extended from the coast to approximately 10 km offshore, but this portion now has been completely removed, apparently as a debris avalanche. Removal of this distal landslide mass created a 200 to 450 m headwall that is now topographically represented by sea cliffs. This newly identified slide/debris avalanche is informally named the “Kohala landslide.” Based on cross-cutting relations of landslide faults with Hawi series lava flows, the upper slide part of the landslide moved sometime between 270 and 60 ka. The age of the lower, debris avalanche part is even less certain and depends on whether canyons cut in the seafloor after the avalanche movement were eroded in the subaerial or submarine environment.

Failure of the Alexander Dam Embankment and Reconstruction Using Drainage Mitigation On Kauai, Hawaii, 1930–1932

Alexander Dam is a hydraulic fill earth dam and the second-highest embankment dam in Hawaii, having been built in 1929–1932 on the south side of the Hawaiian island of Kauai to provide irrigation for McBryde Sugar Company Ltd. It was constructed across Wahiawa Stream mauka (Hawaiian for “stream that comes from the mountains,” literally “toward the mountains”), upstream of Kalaheo, to store 800 million gallons (5 million m3) of water to irrigate sugarcane fields. The embankment dam was intended to have a maximum height of 125 ft (38 m), a crest length of 620 ft (189 m), and a maximum base thickness of 640 ft (195 m). The total design volume was 580,000 yd3 (443,120 m3) and consisted of hydraulic fill sluiced to the dam site and supporting shell material. On March 23, 1930, a 60-ft- (18.3-m) wide section of the core pool suddenly dropped ∼30 ft (9.1 m) and moved downstream, rapidly draining the core pool and enlarging the mass. The embankment was at a height of 95 ft (29 m) and 78 percent complete when the failure occurred. The failure occurred so quickly that it killed six workers and injured two others on the downstream face. The volume of slide debris was ∼275,000 yd3 (210,100 m3). Thirty feet (9.1 m) of the embankment's core stood near vertical after the failure, leading engineers to believe that the materials making up the downstream shell had consolidated sufficiently to inhibit internal drainage. The embankment was rebuilt by emplacing a 40-ft- (12.2-m) high rock buttress across the downstream toe, widening the downstream shell, and installing tile drains to facilitate internal drainage. The retrofitted structure was completed in December 1932 and remains in service some 85 years later.

Observations from the large, rapid Yigong rock slide – debris avalanche, southeast Tibet

The Yigong rock slide – debris avalanche (YRA), which occurred on 9 April 2000, received worldwide attention as one of the largest nonseismic landslides in recent years, with a volume of 0.3 × 109 m3. Sixty-two days after this landslide event, a catastrophic flood happened because of landslide dam failure. One of the special features of this debris avalanche is liquefaction, which plays an important role in the entrainment and long run-out distance and high-speed movement of the debris avalanche. Numerous sand boils were found in the deposition zone, providing strong evidence for liquefaction. The YRA provides the first actual evidence for a theoretical model where the mechanisms of excess pore pressure and liquefaction induced by undrained loading, and entrainment and dissipation control the run out and deposition of the debris avalanche. The damage mode to trees and the presence of debris cones or molards with a rounded top is proven to be the result of strong air waves and eddies. These features all imply that the YRA is a solid–liquid–air mixed-debris avalanche.