Rainfall-Induced Landslides: Case Study of the Marappalam Landslide, Nilgiris District, Tamil Nadu, India

Rainfall-induced landslides are common natural hazards in tropical regions. The present study investigates a rainfall-induced landslide that occurred in 2009 in the Marappalam area of Nilgiris district, Tamil Nadu, India. It was triggered by a high-intensity short period of rainfall that occurred after a prolonged low-intensity rainfall at Marappalam. The effect of rainfall infiltration on the unsaturated residual soil slope was investigated by transient seepage analysis using the finite difference method. The fluid–mechanical interaction of unsaturated soil has been obtained using coupled fluid flow analysis. The analysis was carried out in two stages. The first stage saw prolonged low-intensity rainfall of 710 mm over 8 months. The second stage showed a short-duration, high-intensity rainfall of 719 mm in just 10 days, which caused a landslide in Marappalam. It was observed that the first rainfall event saturated the Marappalam slope to a degree of saturation of 0.6. The saturation level increased during the second rainfall, which advanced the wetting front to the lower layer of the Marappalam slope and saturated the slope to the degree of saturation of 0.8. The wetting front reached the soil–rock interface at the end of the second rainfall event, increasing soil saturation and seepage force, accompanied by reduction in matric suction and the subsequent development of positive pore water pressure. The development of positive pore water pressure decreased the effective stress, thus reducing the shear strength of the soil along the soil–rock interface, causing landslide in Marappalam. The study also described the relationship between rainfall and landslide occurrence based on daily and antecedent rainfall in conjunction with past landslide events occurring in the Marappalam region. The rainfall-threshold relationship provided the optimum level of daily and 5-day antecedent rainfall required to trigger landslides in the Marappalam region, which can be used for early warnings at the regional level.