EP007-05
Modeling how rivers digest boulders: long residence times and coarse riverbed surfaces lead to preferential wear of boulders through in-place abrasion
Monday, 7 December 2020: 10:42
Virtual
Jeff Prancevic, University of California Berkeley, Earth and Planetary Science, Berkeley, United States, Leonard S Sklar, San Francisco State University, Department of Geosciences, San Francisco, CA, United States and William E Dietrich, University of California Berkeley, Earth and Planetary Science, Berkeley, CA, United States
Abstract:
Landslides and rockfall produce boulders that are relatively immobile in many mountain streams. The scarcity of such boulders at the outlets of many rapidly eroding mountain ranges suggests that boulders are often ground into much smaller particles before being exported. This observation is inconsistent with laboratory measurements of particle wear rates in tumbling mills. For example, assuming that particles lose 10% of their mass per km transported (a high rate compared with most lab measurements), then a 1 m boulder would still be a 0.7 m boulder after moving 10 km. However, in tumbling experiments, all particles are transported at a similar rate. In rivers, boulders move much more slowly than the gravel that’s moving over them, allowing them to abrade in-place by more mobile sediment. Here, we use numerical simulations to show that this in-place abrasion of slow-moving boulders can increase their effective abrasion rate by up to two orders of magnitude.
Using an input size distribution based on observed bedrock landslides in the Southern Alps of New Zealand, we track the size evolution of landslide-derived sediment as it’s transported 10 km in a steady-state long-profile model with lateral inputs. The model uses existing theory for size-selective transport and bed-armor development to separately consider breakdown of particles in transport and breakdown of particles on the bed surface. With moderate catchment-averaged erosion rates (0.1 mm/yr), our model predicts that meter-scale boulders can take roughly one thousand years to be transported just a few hundred meters. Most of that transport is predicted to occur only after boulders have worn down to smaller sizes. Indeed, due to their slow transport rate, in-place abrasion is expected to wear large boulders down to mobile gravel over those few hundred meters. These model predictions help us understand how rivers deal with large landslide-derived boulders and provide a mechanistic explanation for rapid fining downstream of landslide deposits.
