Variability of Wolman Pebble Samples in Gravel/Cobble Bed Streams

Abstract

Wolman [1954] pebble sampling is the most commonly used method to estimate surface bed grain sizes in gravel-bed streams. A few studies documented different results between individual operators or repeated measurements within the same channel-reach obtained by this method. We tested potential differences in pebble sample distributions and related grain-size percentiles (D 10 , D 50 and D 90) between two fluvial geomorphologists and two almost inexperienced students in three channel-reaches and one gravel bar. None of sampled locations provided statistically consistent particle-size distributions and related percentiles when comparing measurements of all operators. The samples of experienced fluvial geomorphologists were most consistent for the channel-reaches with assumed widest range of particle sizes; a post-hoc test documented significant differences for the gravel bar and the lower plane bed reach. Medians of particle-size distributions for the gravel bar were equal for three of four operators; the fourth operator probably included also coarser particle population between the channel bed and bar. It implies that 100 sampled particles are most likely sufficient only for D 50 estimations and homogenous sediment populations (i.e. well-sorted gravel bars). In any other case, much larger number of particles should be sampled in gravel/cobble bed streams to obtain narrower confidence limits of related grainsize percentiles.

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

“…We then measured 200 grains per sample site upon collecting the data by hand and calliper and on photos. This number is sufficient to calculate accurate percentile values for moderately-to well-sorted material (Daniels and McCusker, 2010;Galia et al, 2017;Eaton et al, 2019), as is the case in the Finsterhennen gravel pit (Pfander et al, 2022). Upon sieving, the minimum representative weight of the sample to be collected was estimated based on the length of the largest b-axis measured with a calliper.…”

“…Measuring by hand and calliper involves the risk of a selective bias and under-sampling (e.g., Fripp and Diplas, 1993;Marcus et al, 1995;Wohl et al, 1996;Galia et al, 2017), and it may yield less precise results for large, small, or irregular-shaped grains (Fripp and Diplas, 1993;Marcus et al, 1995). Additionally, the shape of grains can lead to a misidentification of a specific axis, e.g., for rounded or spherical grains that might have similar long axes (Yuzyk and Winkler, 1991).…”

Large-Scale Hydromorphological Characteristics Of The Proglacial River Katun (Ob Headwaters)

“…Additionally, a hydromorphological survey based on the River Habitat Survey (Environment Agency 2003) adapted to the circumstances was conducted by the author along the Katun River during an excursion in September of 2019 (Schletterer et al 2021b). Surveys were carried out in the middle and lower reaches, including bedload characterisation with Wolman-Counts (Wolman 1954;Nikora et al 1998;Galia et al 2017) along the banks (Fig. 4).…”