An assessment of the effects of sample collection, storage and resuspension on the representativeness of measurements of the effective particle size distribution of fluvial suspended sediment

Flocs/aggregates can comprise a significant proportion of the total volume of suspended sediment in the fluvial environment. In situ measurement of the effective size distribution is frequently precluded on grounds of the cost of appropriate equipment or adverse field conditions. An assessment of the potential for using an alternative and simpler strategy involving collection of bottle samples and subsequent analysis of the samples has been undertaken. A field-portable, laser-backscatter particle size analyser has been used to assess the representativeness of measurements made on-site immediately after sample collection, and subsequently in the laboratory after settling and resuspension. On-site measurements of bottle samples were broadly representative of the in situ effective size distribution. Flocculation occurred within settled samples together with a concomitant increase in the volume mean particle size upon resuspension. The magnitude of this increase was, in general, positively related to settling time. Following resuspension, floc break-up during continuous stirring caused mean particle size to decline towards the initial on-site value.