Grain-size distributions of sandy sediments—sieving versus settling

Abstract

Geografisk Tidsskrift, Danish Journal of Geography 99: 9–17, 1999 Settling analysis are becoming common practice in many sediment laboratories. By analysing sieved sand fractions of varied sample weight in a settling tube, it was found that the optimal sample size for the settling analysis can be calculated as (Sl-units): Sample size = 55.4 dsiv (dtube)2. Grain-size distributions based on settling analysis, differ systematically from those of traditional sieve analysis. The differences between the two methods have been examined in sediments from four different sedimentary environments in the southwest of Denmark. Samples from rivers, the Wadden Sea, the North Sea coast and aeolian dunes gave a mean relationship between sieve diameter and settling diameter: dset, = 0.8169dsiv + 0.3555. This relationship is almost identical to that observed in sediments from the East Frisian Wadden Sea. Thus, the settling diameters are larger than the sieve diameters for sand finer than approximately 2 phi (0.250 mm) and smaller for coarser grain sizes. This trend results from a larger volume diameter in the fine grain-size classes combined with the well known feature that the hydraulic resistance of irregular grains increases as the settling velocity becomes larger. Comparing sieve-based textural parameters with their settling-based counterparts, the following trends were observed: 1) sieve-based grain sizes finer than 2 phi correspond to coarser settling diameters, while coarser sieve-based sand fractions correspond to finer settling diameters; 2) the sorting gets better; 3) generally, fine well-sorted sediments with a negative skewness become more negatively skewed, whereas coarser and less well-sorted sediments become more positively skewed, and 4) the kurtosis of the distributions increases for well sorted sediments. The pattern is very clear for the mean grain size and the sorting, but less obvious, for the skewness and the kurtosis.