Engineering geological investigation and runout modelling of the disastrous Taliye landslide, Maharashtra, India of 22 July 2021

The Taliye landslide, a hill slope debris flow, is one of the recent large-scale landslides in India in terms of mortality and socio-economic predicaments. In general, the Taliye area exhibits a moderately dissected plateau relief, and the landslide was initiated from a north-westerly slope of a west trending major ridge and flowed through a paddy field to bury houses located just below it. The landslide had a total runout of 563 m and maximum width of 230 m at the toe. The heavy rainfall and presence of a narrow stream may be the prime triggering factors, but the affected area had also been exposed to human interference for farming as well as for habitation. Given its socio-economic relevance and landslide proneness of this area, a post-event engineering geological investigation and numerical modelling of this landslide have been carried out to have a closer look at its type, causes of failure, back calculation of run-out characteristics, as well as to calibrate frictional parameters for this area. The numerical modelling package, rapid mass movements (RAMMS) was used to undertake back analysis of this debris flow by referring the original flow shape. For the precise preference of Voellmy frictional parameters, dry-Coulomb friction (µ) and viscous-turbulent friction (ξ) coefficients, this study applies the popular receiver operative characteristics (ROC) technique as the model validation tool. The landslide may have initiated as a talus or translational failure of shallow soil on the hillock, but later might have transformed into a debris flow by entraining materials from the paddy field. Amongst the different combinations of frictional parameters, the model with µ of 0.06 m/s2 and ξ of 1450 m/s2 has emerged as the best with an area under the curve (AUC) value of 0.883 for the ROC assessment. With these calibrated frictional parameters, the maximum flow velocity of this debris flow was simulated to be in the order of 5.26 m/s taking place at the middle reaches, and the maximum flow velocity and pressure were derived as 21 m/s and 0.92 kPa, respectively, concentrated in the hillock zone. Although it was a rain-induced failure, the investigation suggests that improper slope management practices might have amplified its magnitude and got manifested as devastating landslide. Therefore, human activities in these hilly regions shall be supported with systematic landslide hazard evaluation, and here in this case, the calibrated frictional parameters may be useful for debris flow modelling and landslide risk reduction in the area.