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

Present bulk-sampling recommendations for the characterisation of fluvial s ediments based upon ellipsoids of revolution do not take into account varia tions in particle shape and sorting. Disc-shaped particles with equivalent b-axes to spheres have lower relative masses and require more relaxed sampl ing criteria compared to existing recommendations, whereas heavier cube- an d rod-shaped particles require more stringent criteria. Empirical data from the River Reds, Northumberland, UK, an upland gravel-bed channel, indicate that samples are dominated by discs. However, particle shape was not const ant in every grain-size fraction; blocky material (cubes and rods) was more frequent in the >63 mm fraction in comparison to the other finer fractions . Data based on the weight of the D-99.9 particle indicate that a larger sa mple weight is required when the coarsest size category of that sample is d ominated by blocky material. Sorting also affects the size of the sample re quired to characterise a deposit; the more poorly sorted the deposit, the l arger the sample has to be or the finer should be the truncation,sr ain siz e. This is particularly evident in samples with a high matrix concentration . Two sample reference Lines, developed from these data using the 0.1% by m ass criteria, recommend larger minimum sample sizes (or finer truncation) i n comparison to other published standards. Data from a second site support these results, indicating that they may be applicable to other poorly sorte d upland gravel-bed streams. Analysis of the size category variance demonst rates that the new guidelines returned the lowest variance levels, suggesti ng that they should be used where particularly accurate sampling procedures are required. (C) 1999 Elsevier Science B.V. All rights reserved.