Influence of particle shape and sorting upon sample size estimates\ud for a coarse-grained upland stream

摘要

Present bulk-sampling recommendations for the characterisation of fluvial sediments based upon ellipsoids of revolution\uddo not take into account variations in particle shape and sorting. Disc-shaped particles with equivalent\udb\ud-axes to\udspheres have lower relative masses and require more relaxed sampling criteria compared to existing recommendations,\udwhereas heavier cube- and rod-shaped particles require more stringent criteria. Empirical data from the River Rede,\udNorthumberland, UK, an upland gravel-bed channel, indicate that samples are dominated by discs. However, particle shape\udwas not constant in every grain-size fraction; blocky material (cubes and rods) was more frequent in the\ud>\ud63 mm fraction\udin comparison to the other finer fractions. Data based on the weight of the\udD\ud99\ud:\ud9\udparticle indicate that a larger sample\udweight is required when the coarsest size category of that sample is dominated by blocky material. Sorting also affects the\udsize of the sample required to characterise a deposit; the more poorly sorted the deposit, the larger the sample has to be\udor the finer should be the truncation grain size. This is particularly evident in samples with a high matrix concentration.\udTwo sample reference lines, developed from these data using the 0.1% by mass criteria, recommend larger minimum\udsample sizes (or finer truncation) in comparison to other published standards. Data from a second site support these results,\udindicating that they may be applicable to other poorly sorted upland gravel-bed streams. Analysis of the size category\udvariance demonstrates that the new guidelines returned the lowest variance levels, suggesting that they should be used\udwhere particularly accurate sampling procedures are required.