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A Sampling Frame for Improving Pebble
Count Accuracy in Coarse Gravel-Bed Streams
Improved
sampling techniques are needed to increase the accuracy of pebble
count particle-size distribution measurements in gravel-bed streams.
Researchers Kristin Bunte and Steven Abt of the Engineering Research Center at Colorado State University
have developed a 60 by 60 cm sampling frame (Figure 1) to standardize
the sampling process of selecting particles from sand to cobbles
in gravel-bed rivers.
Figure
1. Eight-piece 60 by 60 cm collapsible sampling frame constructed
from aluminum bars. An adjustable grid of thin white elastic bands are stretched
horizontally and vertically across the frame to define the exact
particles to sample. When the frame is placed onto the stream bed,
the elastic bands stretch across the cobbles that protrude above
the bed.
Pebble
counts are prone to operator errors introduced through subjective
particle selection, serial correlation, and inaccurate particle-size
measurement. Errors in particle-size measurement can be minimized
by using a gravel template. Operator influence on particle selection
can be minimized by using a sampling frame 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 them equal to the dominant large particle
size (≈D95).
The
traditional way of particle selection in a pebble count by a blind
touch at the tip of the boot allows an operator to have a large
influence, voluntarily or involuntarily, on particle selection.
Selecting a particle where the operator places his/her feet is not
an objective means of particle selection, because when wading in
coarse gravel and cobble-bed streams, an operator may be reluctant
to step on top of a slippery cobble or boulder for risk of insecure
footing. Thus, cobbles and boulders are less likely to be selected.
Whereas
foot placement tends to bias against cobbles, the blind touch aspect
using the tip of the finger tends to bias against small particles
surrounded by large particles. This happens because as the finger
reaches down, the sides of large neighboring particles are more
likely to be touched before the finger touches smaller particles.
Using the grid, fine particles can be precisely identified visually
and the operator can then concentrate on retrieving just that particle.
Reducing
operator bias leads to reduced variability between operators. Samples
from two operators that varied substantially in heel-to-toe walks
were nearly identical when using the sampling frame.
Complete details can
be found in: Bunte, K., and S.R. Abt, 2001. Sampling frame for
improving pebble count accuracy in coarse gravel-bed streams. Journal
of the American Water Resources Association, 37(4): 1001-1013.
Kristin
Bunte, Fluvial Geomorphologist,
Colorado State University, Fort Collins, CO; (970) 491-3980; kbunte@engr.colostate.edu.
Steven
R. Abt, Professor, Civil
Engineering, Colorado State University, Fort Collins, CO; (970)
491-8203; abt@lance.colostate.edu.
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