Comment on esurf-2021-19

Mikaël Attal

doi:10.5194/esurf-2021-19-rc1

Abstract

Abstract. The size of grains delivered to rivers by hillslope processes is thought to be a key factor controlling sediment transport, long-term erosion and the information recorded in sedimentary archives. Recently, models have been developed to estimate the grain size distribution produced in soil, but these models may not apply to active orogens where high erosion rates on hillslopes are driven by landsliding. To date, relatively few studies have focused on landslide grain size distributions. Here, we present grain size distributionsÂ (GSDs) obtained by grid-by-number sampling on 17 recent landslide deposits in Taiwan, and we compare these GSDs to the geometrical and physical properties of the landslides, such as their width, area, rock type, drop height and estimated scar depth. All slides occurred in slightly metamorphosed sedimentary units, except two, which occurred in younger unmetamorphosed shales, with a rock strength that is expected to be 3â10Â times weaker than their metamorphosed counterparts. For 11Â landslides, we did not observe substantial spatial variations in the GSD over the deposit. However, four landslides displayed a strong grain size segregation on their deposit, with the overall GSD of the downslope toe sectors being 3â10 times coarser than apex sectors. In three cases, we could also measure the GSD inside incised sectors of the landslides deposits, which presented percentiles that were 3â10 times finer than the surface of the deposit. Both observations could be due to either kinetic sieving or deposit reworking after the landslide failure, but we cannot explain why only some deposits had strong segregation. Averaging this spatial variability, we found the median grain size of the deposits to be strongly negatively correlated with drop height, scar width and depth. However, previous work suggests that regolith particles and bedrock blocks should coarsen with increasing depth, which is the inverse of our observations. Accounting for a model of regolith coarsening with depth, we found that the ratio of the estimated original bedrock block size to the deposit median grain size (D50) of the deposit was proportional to the potential energy of the landslide normalized to its bedrock strength. Thus, the studied landslides agree well with a published, simple fragmentation model, even if that model was calibrated on rock avalanches with larger volume and stronger bedrock than those featured in our dataset. Therefore, this scaling may serve for future modeling of grain size transfer from hillslopes to rivers, with the aim to better understanding landslide sediment evacuation and coupling to river erosional dynamics.

Highlights

Grain size is an essential parameter for understanding sediment transport and associated processes in river evolution or in hazards related to sediment pulses
We formulate two hypotheses: first, we suggest that Eq (1) could be generalized to landslides of intermediate size and depth and, that the landslide deposit D50 should increase with rock strength, σc, and the source material’s median size, Di, but decrease with drop height, H ; second, we hypothesize that materials mobilized by shallow landslides coarsen with the landslide scar thickness, T (i.e., Di increases with T ), due to a reduction with depth of the fracture density of the bedrock (Clarke and Burbank, 2011) and/or of the degree of physical and chemical weathering experienced by particles (Cohen et al, 2010; Anderson et al, 2013; Sklar et al, 2017)
We propose that the variability in landslide D50 can be reconciled with the fragmentation scaling of Locat et al (2006) (i.e., D50 decreases with the ratio of drop height to bedrock strength, as in Eq 1), when accounting for regolith coarsening with depth (e.g., Cohen et al, 2010)

Summary

Introduction

Grain size is an essential parameter for understanding sediment transport and associated processes in river evolution or in hazards related to sediment pulses. There are many processes that control the grain size distribution (GSD) delivered to rivers, and they are poorly understood (Allen et al, 2015). Models have been proposed that describe how weathering in the critical zone reduces the original size distribution of bedrock before the grains reach the surface In active orogens with high erosion rates (> 0.5 mm yr−1 ), landslides are likely the main providers of sediments to rivers (Hovius et al, 1997; Struck et al, 2015; Marc et al, 2019), and a large fraction of sediment may reach the river only partially weathered.

Results

Discussion

Conclusion