We currently lack quantitative field metrics to decipher how often boulders move by fluvial or debris flow processes. This project aims to develop and run numerical experiments to test whether the frequency of boulder movement can be deciphered from cosmogenic exposure dating. Preliminary modeling has shown that, in some cases, the number of boulder movement events can be determined from the pattern of exposure around the circumference of a boulder. The proposed work would include systematic investigation of likely controlling variables (e.g., boulder size, erosion rate, and inheritance) on the mean and spatial variance in nuclide concentration around and within a mobile, eroding boulder. Results from this effort would form the backbone for future field sampling campaigns aimed at measuring boulder movement, potentially applicable to a wide range of environments (e.g., river channels, debris flow fans, moraines).
Boulders line the beds of many rivers and are an important control on rates of landscape evolution, yet their long-term behavior is not well understood. The recurrence interval of large, boulder-mobilizing events is often beyond the historical record, leaving the probability of these events poorly constrained. The project aims to improve our ability to predict the frequency of boulder moving events for hazard mitigation and habitat restoration. It could potentially lead to the ability to quantify infrequent, large magnitude flood and debris flow events with recurrence intervals greater than ~ 1000 yrs, which can cause significant loss of life and property.
Large, relatively stable boulders line bottoms of many river channels and the boundaries of glaciers. Although boulder movement is often rare, determining the frequency of movement events is important for diverse applications such as mitigating flood and debris-flow hazards in mountain terrain, restoring rivers for aquatic habitat, and understanding the extent of glaciers in the past. In this study we explored the use of cosmogenic exposure age dating to determine the age, erosion rate and mobilization frequency of boulders. Through a series of numerical experiments we modeled the evolution of the mean and standard deviation of surface nuclide concentrations around a boulder surface. Stable boulders have distinctive radial distributions of surface concentration in comparison to those that are periodically mobile, and this can be used to establish boulder stability or mobility. Mean nuclide accumulation rates around the surface of a boulder increase as the radius passes below approximately 1.5 e-folding lengths (~1.2 m) of neutron flux intensity, whereupon nuclide accumulation on the underside of the boulder becomes non-negligible (~10%). The normalized standard deviation of surface concentration systematically decreases with increasing boulder mobilization events, and can be used to constrain the total number times a boulder has moved. Using non-dimensional scaling relations between surface concentration statistics, boulder size, and time, we developed a method to estimate the minimum age and erosion rate of a mobile boulder. This approach has the potential to quantify boulder residence times in river systems, boulder erosion rates therein, to predict the recurrence times of rare floods and debris flows, and to identify stable boulders on glacial moraines. Mackey, B.H. and Lamb, M.P., 2013, Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating, Journal of Geophysical Research - Earth Surface, in press.
