Statistical Approach to Bed-Material Surface Sampling

Abstract

Surface bed-material sampling by grid methods produces particle frequency distributions that are not directly applicable to sediment-transport models. Appropriate conversion models are required to turn these distributions into volumetric frequency distributions. A conversion model has been widely applied in the past, in which grid-by-number particle distributions coincide with volume-by-weight distributions. This model was derived from oversimplified assumptions on particle shapes and sizes. In this paper the validity of that model is considered. A conversion model based on the statistical properties of homogeneous mixtures is presented, in which limitation is not introduced on the number of size fractions and on particle dimensions. The derivation of the new model allows for the application of the old one only in those circumstances, where an operational bias in sample collection is necessarily introduced. Results from the model proposed here are in good agreement with experimental results from other authors and with an extensive set of new experimental results obtained from the photographic analysis of bimodal and trimodal mixtures.