Landslide is a geological phenomenon which gets triggered by factors such as rainfall, earthquake and human activities such as deforestation, excavation, blasting etc. India is a country which has witnessed the deleterious effect of many disastrous landslides. North eastern parts (Himalayan Region) and Western Ghats of India are more prone to landslides every year mainly due to heavy rainfall and thus resulting in large casualties and huge economic loss. Therefore there is a need to precisely assess and manage this landslide hazards. In the present study, a methodology has been implemented to analyze rain induced slope failures by taking a landslide prone area in Western Ghats, Kerala as a case study. Many previous landslides are reported in this region due to heavy rainfall during monsoon and steep slopes of the area. A combined hydrological geotechnical model has been set up to evaluate rain induced slope failures in a deterministic framework using Microsoft Excel and it is used to back evaluate a slope failure which occurred in the study area during 2007 monsoon. Rainfall data, hydrologic properties and geotechnical properties of the study area is collected. Rainfall data and the infiltration capacity at the site is an important input to the model.

Initial moisture content distribution, saturated moisture content, residual moisture content and saturated conductivity of the soil deposits are used to obtain soil water characteristic curves (SWCC). Pore water pressure and hydraulic conductivity of soil is estimated from SWCC for the initial state of stability. An implicit finite difference backward solution is used to solve partial differential equation for variably saturated flow and change in pore water pressure and moisture content as a result of a rainfall event is obtained. Top boundary condition is either flux controlled or head controlled. At each time step, top compartment of soil is checked for its saturation. If it is saturated, suction or pressure head is taken as
zero and it is head controlled. If it is not saturated then potential infiltration rate is compared with infiltration capacity of soil at each time step and minimum of these two values is taken as actual flux and top boundary is flux controlled. Bottom boundary is controlled by water table. If water table is of shallow depth, model depth is taken as depth up to water table and suction is zero at this bottom boundary. If water table is deep, suction is assumed based on capillary forces and type of soil. As a result of rainfall, suction in soil is either reduced or eliminated. A reduction in suction can also initiate landslide. Thu change in pore water pressure (negative and positive) with respect to time and depth is obtained and this result from hydrological model is given to geotechnical (stability model) in the same spreadsheet.