Inferring rockfall frequency-magnitude relationships and talus accumulation times from lichenometric study of talus deposits, Glenwood Canyon, CO, USA

Rockfall is a frequent hazard in mountainous areas of the world. Rockfall hazard assessment and mitigation design typically require design volumes of blocks and design frequencies of events. However, constraining the frequency of rockfall is challenging due to limited data, limited timespans of data collection, and difficulty of estimating dates for past rockfalls. This paper presents a study of rockfall frequency from natural rock slopes in Glenwood Canyon, Colorado using lichenometry measurements of Lecanora novomexicana and Lecidea atrobrunnea lichens from 1070 boulders from six talus deposits. The lichen ages are used to develop frequency-magnitude relationships for rockfall, to calculate recurrence intervals for selected volumes, and to estimate the time required to develop the observed talus deposits. Power laws fit to frequency-magnitude distributions of lichen ages yield b parameters ranging from −0.61 to −1.04. Based on these distributions, recurrence intervals for selected rockfall magnitudes include: sub-annual to annual for 0.1 m3, a few years to decades for 1 m3, and several decades for 10 m3 rockfalls. These estimated recurrence intervals appear to agree with observed signs of rockfall activity from the studied slopes and with rockfall data from the limited historical record for Glenwood Canyon. Mapped lichen ages indicate no preferential spatial distribution of rockfalls, implying the majority of rockfall events are small relative to the scale of the deposit. A comparison of the estimated volumetric rockfall accumulation rates with estimated total talus deposit volumes suggests that each of the studied deposits has a depositional history extending back longer than the beginning of the Holocene.