SAMPLING FRAME FOR IMPROVING PEBBLE COUNT ACCURACY IN COARSE GRAVEL‐BED STREAMS¹

Abstract

Improved sampling techniques are needed to increase the accuracy of pebble‐count particle‐size distributions used for stream studies in gravel‐bed streams. However, pebble counts are prone to operator errors introduced through subjective particle selection, serial correlation, and inaccurate particle‐size measurements. Errors in particle‐size measurements can be minimized by using a gravel template. Operator influence on particle selection can be minimized by using a sampling frame, 60 by 60 cm, in which sampling points are identified by the cross points of thin elastic bands. Serial correlation can be minimized by adjusting the spacing between the cross points and setting it equal to the dominant large particle size (=D95). In a field test in a cobble‐bed stream, the sampling frame developed in this study produced slightly coarser size distributions, particularly in the cobble range, than the traditional heel‐to‐toe walk that selects particles with a blind touch at the tip of the boot. The sampling frame produced more similar sampling results between two operators than heel‐to‐toe walks. The difference between the two sampling methods is attributed to an unbiased selection of fine and coarse particles when using the sampling frame.

Sampling surface and subsurface particle-size distributions in wadable gravel-and cobble-bed streams for analyses in sediment transport, hydraulics, and streambed monitoring

“…However, when data were plotted, the ratio of the same percentile particle-sizes between heel-to-toe samples and sampling along a tape showed a systematic decrease with bedmaterial particle size (Fig. 4.5) (Bunte and Abt 2001). On fine gravel beds, heel-to-toe samples had coarser D 16 , D 50 and D 84 particle sizes than sampling along a tape.…”

“…Physical shape of the operator can also play a role in the variability of sampling results between operators in heel-to-toe pebble counts. Bunte and Abt (2001) compared sampling results obtained from heel-to-toe walks in a cobble-bed stream (D 50 = 69 mm, D max > 720 mm, sorting coefficient s I = 1.7 φ) between two operators of different size. The operator with a small boot size (Operator B) was more prone to avoiding cobbles and boulders, and produced particle-size distributions with fewer coarse particles than the operator with a large boot size (Fig.…”

“…Operators performing pebble counts in mountain gravel-bed streams are often faced with submerged beds and swift flow, however, and need a device that overcomes the shortcomings of a measuring tape in underwater pebble counts and that mitigates the typical sampling errors associated with heel-to-toe walks. For this reason, Bunte and Abt (2001) developed a sampling frame, following a suggestion made earlier by Marcus et al (1995).…”

Possible threshold controls on sediment grain properties of Peruvian coastal river basins

“…It has been shown that using a standard frame with fixed dimensions to assist gravel sampling reduces user-biased selections of gravels (Marcus et al, 1995;Bunte and Abt, 2001a). In order to reduce this bias, we substituted the frame by shooting an equal number of photos at a fixed distance (ca.…”

A Bayesian analysis of physical habitat changes at tributary confluences in cobble‐bed upland streams of the Acheron River basin, Australia

“…Therefore it is important to reduce bias by using a sampling grid [Bunte and Abt, 2001a;Marcus et al, 1995]. A grid was placed on the stream bed along transects, and diameters of particles beneath the grid intersections were measured with a template in half y units (y = log 2 (D pass )).…”