Measuring the grain‐size distributions of mass movement deposits

Abstract

Mass movement deposit grain-size distributions (GSDs) record initiation, transport and deposition mechanisms, and contribute to the rate at which sediment is exported from hillslopes to channels. Defining the GSD of a mass movement deposit is a significant challenge because they are often difficult to access, are heterogeneous in planform and with depth, contain grain sizes from clay (<63 μm) to boulders (>1 m), and require considerable time to calculate accurately. There are numerous methods used to measure mass movement GSDs, but no single method alone can measure the entire range of grain sizes. This paper compares five common methods for determining mass movement deposit GSDs to assess how their accuracy may affect their applicability to different research areas. We applied an automated wavelet analysis (pyDGS), Wolman pebble counts, survey tape counts, manual photo counts and sieving to three different mass movement deposits (two debris flows, one rockslide) in Tredegar, Wales and the Longmen Shan, China. We found that pyDGS and survey tape counts produced comparable GSDs to sieving over a single order of magnitude.PyDGS required calibration to achieve accurate results, limiting its use for many applications. In Tredegar, Wolman pebble counts over-estimated grain sizes in the lower 80% of the distribution relative to the other four methods used. We demonstrate that method choice can introduce significant uncertainties, particularly at the edges of the distributions, such that D 16 values differ by up to a factor of five. These methodological uncertainties limit GSD comparisons across studies, particularly where these are used to infer processes within deposits. To minimize these challenges, the methods chosen should be both carefully reported and consistent with the research question.

Comparison of three grain size measuring methods applied to coarse-grained gravel deposits

“…2a) we excavated between 32 and 61 kg of bulk-sediment (Table 1) and yielded corresponding values between approximately 0.4-5.5 %. Although these (e.g., site A; Table 1) are in cases slightly larger than suggested for an ideal survey (Church et al, 1987), they can be regarded as acceptable (Guerit et al, 2018;Watkins et al, 2020;Harvey et al, 2022). Note that grains <2 mm were then removed from the datasets for further analyses to ensure a consistent comparison between the different measuring methods, because grains <2 mm cannot be measured by hand with callipers and are barely detectable on photos taken with the setup (camera, distance) used in this study.…”

A Novel Efficient Method of Estimating Suspended‐To‐Total Sediment Load Fraction in Natural Rivers

“…ADCPs can be utilized for the simultaneous measurement of flow and suspended sediment (Noh et al., 2022, 2023; Son et al., 2021). For bed grain‐size estimation, one method is to use image‐processing software packages, such as pyDGS (Buscombe, 2013) and Basegrain (Detert & Weitbrecht, 2012); however, sieving is the only reliable method that can be used for sand or finer grains (Harvey et al., 2022). If sieving is the only option, it is advantageous to create a dictionary of the median size of bed material on the probable areas before applying the above methods.…”

Introduction to the Women in Geomorphology special issue

“…Field (26) Atkins et al, 2023;Bernatek-Jakiel & Nadal-Romero, 2023;Biausque et al, 2022;Brooks et al, 2022;Bywater-Reyes et al, 2022;Chin, 2022;Gervasi et al, 2021;Glover et al, 2022; Goodman et al, in press;Guiney & Lininger, 2022; Gurnell et al, 2022;Harvey et al, 2022;Hinshaw et al, 2022;Hooke, 2022;Konlechner & Hilton, 2022;Kooijman et al, 2022;Lane et al, 2022;Lefebvre et al, 2022;Marchetti et al, 2022;McKeon et al, 2022;Mol & Grenfell, 2022;Mossa & Chen, 2022;Opalka et al, 2022;Patton et al, 2022;Quock et al, 2022;Regalla et al, 2022;Reitman et al, 2022;Sampietro-Vattuone et al, 2021;Schoch-Baumann et al, 2022;Souza Fonseca et al, 2022;Spiske et al, 2022;Thompson et al, 2022;Uphues et al, 2022;Valentine et al, 2022;Van Soest et al, 2022;Wohl, 2022;Zhang et al, 2022 (Continues)…”

“…Papers addressing process and form in coastal environments include examinations of dune morphodynamics and evolution (Hovenga et al, 2021; Konlechner & Hilton, 2022; Kooijman et al, 2022); the influence of sorting and bedforms on the magnitude of coastal eolian transport (Uphues et al, 2022); coastal changes in response to an extreme storm (Spiske et al, 2022); estuarine mangrove dynamics (Glover et al, 2022) and the morphology of estuarine bedforms (Lefebvre et al, 2022); geotechnical properties of salt marsh soils (Brooks et al, 2022) and tidal wetland development (McKeon et al, 2022); seasonal morphodynamics of intertidal bars (Biausque et al, 2022); and wave impacts and cliff erosion (Thompson et al, 2022). Papers addressing hillslopes and mass movements include interactions between raindrop impacts and particle settling velocities in sheet flow (Bako et al, 2022); a review and synthesis of soil piping (Bernatek‐Jakiel & Nadal‐Romero, 2023); a review of erosion plot studies in the Brazilian Cerrado (Souza Fonseca et al, 2022); seasonal variations in gully erosion (Agostini et al, 2022); influences on Mediterranean badlands formation (Nadal‐Romero et al, 2022); numerical models of landslide effects on the cosmogenic isotope signal of fluvial sediment (Schide et al, 2022); surface slip on strike‐slip faults (Reitman et al, 2022); post‐fire patterns of rock weathering (Mol & Grenfell, 2022); precursory indicators of alpine rockfall (Hendrickx et al, 2022); grain‐size distributions of mass movement deposits (Harvey et al, 2022); influences on landslide initiation based on field observations and numerical modelling (Patton et al, 2022); physical experiments on debris‐flow erosion (Roelofs et al, 2022); and comparison of digital elevation models built from different data sources (Atwood & West, 2022). Glacial geomorphic studies are represented by papers examining topographic controls on ice flow and glacier front recession (Davies et al, 2022); moraine dynamics in the context of climate change (Rowan et al, 2022); and streamlined sub‐glacial bedforms (McKenzie et al, 2022).…”