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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
3. (May), Pages 654-665
Sampling Surficial Fluvial Gravels: The Precision of Size Distribution
Percentile Estimates
Stephen Rice (*), Michael Church
ABSTRACT
Grid-by-number ("Wolman") sampling is commonly used to characterize the
texture of surficial fluvial sediments. Assessments of sampling precision
have concentrated on the median grain size, but adequate representation
of other grain size percentiles is paramount in a number of sedimentological
contexts. The field effort required to construct percentile sampling distributions
by replication is prohibitive, and the absence of any general size distribution
for fluvial sediments precludes theoretical calculation of percentile standard
errors. Bootstrapping is used in this paper to determine percentile standard
errors for large samples from two gravel bar surfaces. The improvement
of percentile precision with sample size follows the theoretical expectation,
hence improvement in percentile recision is achieved only at the expense
of much greater sampling effort once sample sizes reach about 400 stones.
However, absolute percentile standard errors differ significantly from
theoretically derived values. Of particular interest is the stability of
coarser percentile estimates in coarse-skewed distributions; D95
estimates may be as precise as D50 estimates for a given
sample size. The sensitivity of the precision of estimated percentiles
to distribution shape is examined using synthetic skewed and bimodal grain
size distributions. The absolute precision of percentile estimates is shown
to vary with the grain size distribution of a particular sediment. Our
field results provide a reference for further work.
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