Mapping and Analysis of Unstable Slopes Using Frequency Ratio and Insar: a Case Study of Deep-Seated Landslides in Debresina, Ethiopia
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Date
2024-05
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Addis Ababa University
Abstract
This research focuses on studying landslides in the Ethiopian highlands, particularly along the edges of the Main Ethiopian Rift. The region's complex topography and tectonic activity have historically made it susceptible to landslides, with significant events recorded in September 2005. Recently, shallow landslides have been observed reactivating along the remnants of older, deep-seated landslides, causing significant damage to agriculture, infrastructure, and residential areas.
The study area's lithological composition includes the Tarmaber formation, making up 37% of the terrain, known for its moderate to extensive fracturing and varying degrees of weathering, with an average uniaxial compressive strength (UCS) value of 243.6 MPa. The Alaje formation, comprising 36%, is characterized by its highly fractured and weathered nature, with weaker UCS values of 79.2 MPa. Additionally, 21% of the area is composed of eluvium-colluvium deposits and 7% of alluvial deposits. These strata overlay an impermeable layer with hydraulic conductivity ranging from 0.0018 to 0.002 cm/sec, consisting of medium to dense sandy silt, which is an altered component of the Alaje formation.
The main focus of this research is to map and analyze instabilities using frequency ratio (FR) and Interferometric Synthetic Aperture Radar (InSAR) methodologies, aiming to create a comprehensive landslide susceptibility map and a displacement map. Six causative factors: aspect, slope, elevation, proximity to streams, and lithology were selected for the input process, tailored to the unique conditions of the site. Results from the FR analysis indicate that lithology, particularly the eluvium-colluvium and alluvial deposits, is the main causative factor for instability.
Furthermore, processing two descending Sentinel-1 Interferometric Wide Single Lock Complex (S1A_IW_SLC) images from 2014 and 2024 via the Sentinel Application Platform (SNAP) facilitated the preparation of the displacement map. InSAR analysis revealed a displacement trend towards the rift margin, ranging from 0.80m to 1.172m over the ten years.
The integration of FR and InSAR analyses demonstrates a strong correlation between displacement and susceptibility, particularly concerning deep-seated landslides, emphasizing the critical nature of these factors in landslide prediction and risk management.
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Keywords
Frequency Ratio, InSAR, Landslide Susceptibility, Displacement, Causative Factor