Rockfall and snow avalanche impact dynamics for road protection design at Svarthola, Senja. A hazard evaluation aided by numerical models and physical formulas

Abstract

Rockfalls and snow avalanches are a major natural hazard and common phenomenon in mountainous areas in Norway, endangering people and infrastructure. Svarthola, on Fv 862, Senja, is a 100 m long unprotected stretch of road that experiences a high frequency of mass movements. The road is situated at a tunnel portal, between a 100 m high cliff and the fjord. As such, it is the only short access route between the northern and western parts of the island, and frequent road closures have created safety and access issues. It is therefore classified by Troms and Finnmark County as one of the most dangerous roads in the region, and the installation of a protection structure is considered necessary. This study focuses on the impact dynamics from rockfalls (kinetic energy, velocity, jump height) and snow avalanches (snow pressure, velocity, flow height) at Svarthola for protection design. An analysis of the hazard at the site was performed through fieldwork and desktop mapping in order to estimate the size of the mass movements relevant for the design. This data were used in numerical modelling to obtain the impact dynamics. The road experiences frequent rock fall inundation, with boulders up to 2 m3 causing damage and road closures. Rockfall analysis shows variable impact dynamics for the different event scenarios, ranging from 1,700 kJ for annual events up to 7,100 kJ for a 100-year event. The analysis showed impact velocities between 22 and 50 m/s. The median jump heights were 0.9-1.2 m, where some rock blocks can jump >20 m if they hit the lower cliff part above the road. The fall height of a block can be up to 100 m. The road experiences a lower frequency of snow avalanches but they are often leading to road closures. Impact dynamics were obtained through numerical modelling and physical formulas. The impact dynamics for a 100-year dry-slab avalanche event with a volume of 16,900 m3 is estimated as follows: snow pressure of 130-270 kPa, impact velocity between 20 and 37 m/s and flow height of 2-3.7 m in the northern part of the road and up to 1.2 m in the southern part. Finally, an analysis of the relationship between the rockfall and avalanche history and climate factors were done with the aim to propose the implications of climate change. Svarthola will most likely experience more rockfall events in the future due to the projected increase in rainfall intensity and frequency. The dry-snow avalanche hazard may disappear completely due to the rarity of snow cover in these low coastal areas, but the likelihood of wet-snow avalanches and slushflows may increase in a wetter climate.