Sampling Procedures for Coarse Fluvial Sediments

Procedures for sampling the bed material of gravel-bed rivers consist of five independent steps: selection of site and time, collection of a sample, determination of grain size, computation of size distribution, and presentation of results. Because several choices of methods are available at each step, much confusion exists in the literature. Simple geometric arguments, illustrated with a model based on densely packed cubes, are used to develop weighting factors to convert from one sampling procedure to another. Comparison with several sets of field data shows that the conversion method is applicable to gravel deposits. Differences remaining after conversion are attributable to random differences between samples, or to population differences. The grid-by-number procedure is the only surface-oriented procedure directly comparable (equivalent) to customary bulk sieve analysis. It is, therefore, the only correct sampling procedure for the study of surface-dependent phenomena. Additional field data are presented to compare three equivalent procedures.