Rockfall Hazard Assessment Using RAMMS for the SE Facing Escarpment of Manikaran, Himachal Pradesh, India

Abstract

Several rockfall events occurred in the Manikaran town and surrounding area of Kullu district, Himachal Pradesh, India, due to heavy monsoon rains and snow melting on the mountain peaks. One major rockfall event occurred near the historical Gurudwara and Shiv Temple located in Manikaran town on August 2015, killing nearly more than 10 people and injuring 15 people sleeping in the Sarai of Gurudwara. Every year during monsoon season numerous rockfall events happen in Gargi Village which is situated uphill approximately 1.5 km from the base of Parvati River at Manikaran and many a times rock blocks topple up to Manikaran. The SE facing escarpment of Manikaran town is prone to landslide activity which needs to be monitored in order to prevent loss of life. In this study, we present geological investigation and geomorphological zonation mapping of the rockfall site which was done depicting forest cover, terrain material, identification of the detachment rock mass and certain possible invasion areas of future landslide blocks. Remote sensing and GIS platforms were used to map older scarp retreat in the main rockfall body of Manikaran Landslide for the past 40 years. We used RAMMS (Rapid mass movements: Rockfall Module) for rockfall trajectory simulations of rockfall event of August 2015 in Manikaran town. Based on field investigation of boulders (of rockfall) lying on the uphill slope, two boulder sizes are selected for simulation one for small volume of rocks (6.4 m3) and second for large volume of rocks (32 m3). We found that once rockfall starts it take ~8.2 s for 32 m3 rocks to reach the base at Manikaran town. The jump height of rock blocks during rockfall is very high >40 m which cannot be stopped by building any engineering design wall. The maximum total kinetic energy of >6000 kJ for 6.4 m3 and >200,000 kJ for 32 m3 has been observed in the numerical analysis with the maximum runout distance up to 1200 m. Due to pilgrim rush to Manikaran Gurudwara and Shiv Temple, the periodic rockfall events during monsoon season poses a grave danger to the life of people and infrastructure of Manikaran town. We prepared a rockfall hazard map of Manikaran town based on the results of RAAMS which demarcate unsafe, moderately safe and safe zones in terms of rockfall events from the SE facing escarpment. We propose an early warning landslide system to be installed in the Manikaran town for real-time monitoring and prediction of rockfall events.