Characterizing rockfall hazard with an integrated kinematic analysis and runout model: Skagway, Alaska, USA

Abstract. Rockfall is common in steep terrain and poses a hazard to nearby communities. While rockfall triggering mechanisms are highly variable and difficult to quantify, the susceptibility of rock slopes to planar, wedge, or toppling failure can be readily assessed using kinematic analysis. As such, valley slopes with favourable joint orientations exhibit high rockfall susceptibility although the potential for rockfall runout to impact infrastructure and public safety depends on the morphology of downslope terrain. Integrating rockfall susceptibility and runout models with maps of talus deposits accumulated from past rockfall events is an effective combination of tools to inform mitigation but can be difficult to realize across extensive areas. Here, we combine these methods with a historic rockfall inventory to assess rockfall hazard in the steep and forested postglacial valleys proximal to Skagway, AK, where recent rockfall activity has imperilled public safety, infrastructure, and tourism. Our field investigations identified two steeply dipping orthogonal joint sets that favour toppling failure along NW-facing hillslopes in the lower Skagway River valley as well as the NW-facing valleys that bound nearby Dyea Bay and Nahku Bay. We used new and existing lidar data and >300 field-derived joint orientations to inform a kinematic toppling failure model that identifies likely zones of rock toppling. The predicted source zones are positioned upslope of abundant talus slopes that we mapped from field observations and lidar analyses. Along the prominent ridgeline on the eastern margin of Skagway, we used RAMMS:Rockfall to model nearly 200,000 rockfall runout events for four scenarios that account for variations in clast size and ground cover. The runout predictions highlight distinct zones of low and high rockfall hazard along the ridgeline that result from changes in hillslope morphology set by the combined influence of joint orientations and the pattern of glacial erosion. High-hazard segments of the ridgeline exhibit distinct bedrock escarpments and slope-spanning talus slopes that result from the accumulation of rockfall activity over millennia. Our findings reveal controls on past and future rockfall activity and can be used to inform mitigative measures.