Hazard assessment in rockfall-prone Himalayan slopes along National Highway-58, India: rating and simulation

A massive disaster occurred in June 2013 in Kedarnath, India, due to cloudburst and extremely heavy rain along the Chorabari glacier. The resulting flash floods further aggravated the instability of natural and hill cut slopes at different places on the downstream side. The village Rambara that existed in close proximity of Kedarnath was swept away under flow of debris and water. The immediate surrounding area, which housed over a hundred and fifty shops and hotels, was completely washed away leaving no trace of civilization. This calamity in Uttarakhand is considered as India’s worst natural disasters after the tsunami in December 2004. On the downstream of the affected areas lie other pilgrim destinations that witness innumerable footfalls every year. Investigation of the health of the slopes on the routes to these destinations is therefore very important to ensure minimal damage to humans and machinery. The Himalayan terrain is a tectonically active mountain belt, having a large number of unstable natural and road cut slopes. Such slopes with rugged topography lie in the high seismic vulnerability zone. Further, the instability is aggravated by natural and anthropogenic activities increasing at a rapid and uncontrollable rate. In the light of the Kedarnath tragedy, more advanced research is being conducted along the National Highways to monitor and prevent slope/structure failures. This study was conducted to evaluate the hazard potential along National Highway-58, near Saknidhar village of Devprayag district by analysing rockfall using hazard rating systems and numerical simulation. Rockfall hazard rating systems were applied to evaluate the conditions of the slopes and to identify the associated risks. Based on the field and laboratory analyses, the parameters required for numerical models were determined. The bounce height, roll-out distance, kinetic energy and speed of the detached blocks were determined by using a competent rockfall simulator. The results obtained were used to identify rockfall risk in the region. Optimization strategies were applied during investigation by modifying the slope angle, ditch width and ditch angle to assess the possibility of a hazard to occur in different scenarios. The simulation studies revealed that an increasing slope angle could significantly increase the kinetic energy of the rock blocks. However, an increase in the ditch angle and the ditch width reduces the energy of moving blocks. The maximum bounce height above the slope varied from 0.003 m to 0.8 m for 10-kg blocks, whereas the maximum velocity and the maximum kinetic energy under such circumstances were 7.882 m/s and 379.89 J, respectively. The barrier capacity was found to be 233.18 J for 10-kg falling blocks at a height of 10.02 m. From the optimization studies, it was found that the risk can be reduced by up to 13 % if the slope of 70° has a ditch angle of 15° while on a flat ditch, the maximum risk will be at an angle of 65°. If the ditch angle is increased, the vertical component of the falling blocks is more effective than that in case of a flat ditch. These optimization studies lay foundation for advanced research for mitigation of rockfall hazards in similar potential areas.