The study of the factors controlling rainfall-induced landslides at a failure-prone catchment area in Enugu, Southeastern Nigeria using remote sensing data

Abstract

Using remote sensing data and geographic information system (GIS) analytical techniques, we investigated instability at Iva Valley where there is an anomalous concentration of landslide scars. The area has a fairly rugged topography caused to a large extent by the headward erosion by the east-flowing Ekulu River. Mass wasting is more pronounced on the poorly consolidated sandstone (Ajali) formation which (as a major aquifer) serves as the source of the Ekulu River and its tributaries. At the lower portion of the formation and the uppermost part of shale (Mamu) formation (fractured in some places), inter-bedded thin mudstone layers provide the sandstones additional resistance to erosion, which however leads to steeper slopes and very deep and narrow stream channels. Digital elevation models show that the landforms are dominantly lowlands and lowhills with landslides concentrated on river tributaries where failures on the incised slopes create further instability. The overlying sandstone formation seems inherently unstable because of its semi-consolidated and friable nature. The fractures (orientated NE-SW) in the underlying shale formation and the occasional removal of lateral support by mining or road construction probably aggravate the instability. Additional stress on the unstable sandy mass resulting from increase in self-weight and pore pressure during rainfall readily induces failure, which then steepens the slopes and renders them vulnerable to more collapse behaviors that create clusters of scars. The results show good agreement with field observations and literature data indicating the importance of remote sensing analysis, ASTER, and SRTM imagery in geomorphologic interpretations.

Stability Analysis of Partially Submerged Landslide with the Consideration of the Relationship between Porewater Pressure and Seepage Force

“…According to (8) and 9, the resultant of the porewater pressures acting on four sides of the underwater slice equals the buoyancy. Based on (16) and (17), the resultant of the porewater pressures acting on three sides acting on the partially submerged slice is equivalent to the vector sum of the buoyancy and the seepage force.…”

“…One approach, considering the effect of water on the underwater slices of partially submerged landslides in the limit equilibrium analysis, is based on porewater pressures. The resultant of the porewater pressures is based on (8) and (9). All forces are resolved into both the parallel and perpendicular directions to the slice base ( Figure 5(a)), resulting in the following equations:…”

“…where is the cohesion of the slip surface, is the length of the slip surface, is the friction angle of the slip surface, and Fs is the safety factor. In fact, substituting (8) into (18), (20) can be obtained, and substituting (9) into (19), (21) can be obtained. Therefore, there are two equivalent approaches for considering the effect of water on the underwater zone of the partially submerged landslide in limit equilibrium analysis.…”

“…The combined seepage-slope stability analyses in Mountain Toc of Italy by Paronuzzi et al [7] demonstrated that the decreases in safety factors caused by filling and drawdown of the Vajont reservoir and heavy rainfall were about 12% and 3%. The significant examples related to the water level fluctuation and rainfall have been recorded and discussed by researchers, such as rainfall-induced landslides of the Iva Valley in Southeastern Nigeria [8], landslides triggered by the July 2011 intense rainstorm in Korea, the June 2013 extreme rainfall in India and the October 2013 heavy rainfall associated with the typhoon in Japan [9][10][11], and landslide events associated with the water level fluctuation and rainfall [12][13][14][15][16][17][18][19].…”

Prediction of Spatial Likelihood of Shallow Landslide Using GIS-Based Machine Learning in Awgu, Southeast/Nigeria

“…Landslides are among the critical natural hazards that have seriously impacted the landscape and that increasingly threaten the lives and resources of society in Enugu State, Nigeria [1]. Furthermore, landslides cause a loss of fertile soil and land degradation [2].…”