Characterizing geologic and climatic controls on rockfall hazards using an inventory and integrated kinematic 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 accomplish across extensive areas. Here, we combine these methods with a rockfall inventory spanning 2005 to 2022 to assess geologic and climate controls on rockfall activity in the steep and forested postglacial valleys proximal to Skagway, Alaska, where recent rockfall activity has imperilled public safety, infrastructure, and tourism. The inventory reveals rockfall activity throughout the year with peak activity in early spring that coincides with a rapid rise in minimum daily temperatures. 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 405 field-derived joint orientations to inform a kinematic toppling failure model that identifies source zones upslope of abundant talus slopes that we mapped from field observations and lidar analyses. We coupled the predicted source zones with RAMMS:Rockfall to simulate 197 800 rockfall runout events for scenarios with varying block size and ground cover. The runout predictions highlight zones of low and high rockfall propagation susceptibility that are negatively correlated with hillslope roughness which results from the combined influence of joint orientations that generate bedrock benches and the spatial pattern of glacial erosion. High-hazard segments of the ridgeline exhibit distinct bedrock cliffs and slope-spanning talus slopes that result from the accumulation of rockfall activity over millennia. Taken together, our findings illustrate past controls on rockfall location and timing to inform mitigative measures.