Debris flow in the Nenmara, Kerala, India: insights from geophysical testing, subsurface investigation and numerical analysis

Debris flows pose significant threats due to their high velocity and fluid-like consistency. This research evaluates the intricate failure mechanisms of the rainfall-induced debris-flow event in Nenmara, Palakkad district, Kerala, India, on August 16, 2018, through detailed investigations. A geophysical (Multi-channel Analysis of Surface Waves (MASW)) test was carried out to obtain the shear wave velocity (V_s) of substrata. The dewpoint potentiometer and ring shear test were used to assess unsaturated soil strength and residual shear parameters to analyse the progressive failure mechanism of the landslide using the numerical model LS-RAPID. The mineralogical studies in the Nenmara region reveal that the soil originated from charnockite rocks containing quartz and clay minerals. The low V_s of 197 m/s at 2 m depth indicates the loose and unconsolidated soil layer at the site. The debris flow initiates when the pore water pressure ratio (r_u) rises to 0.40 with a peak velocity of 11.9 m/s and 13.9 m/s in the X and Y directions, which led to the demolition of 3 buildings and the loss of 8 lives. The deterministic analysis reveals that r_u above 0.30 can trigger a landslide near the Nenmara location. The rainfall threshold analysis suggests that 148 mm of daily or 210 mm of continuous rainfall over five days can trigger landslides around the Nenmara region. This research combines geophysical, geotechnical, and numerical simulations to make a substantial contribution to disaster management in comprehending the mechanism of debris flow by identifying triggering factors, and it will help to find the appropriate mitigation measures for future hill area development.