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 vo lume of suspended sediment in the fluvial environment. In situ measure ment of the effective size distribution is frequently precluded on gro unds of the cost of appropriate equipment or adverse field conditions. An assessment of the potential for using an alternative and simpler s trategy involving collection of bottle samples and subsequent analysis of the samples has been undertaken. A field-portable, laser-backscatt er particle size analyser has been used to assess the representativene ss of measurements made on-site immediately after sample collection, a nd 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 set tled samples together with a concomitant increase in the volume mean p article size upon resuspension. The magnitude of this increase was, in general, positively related to settling time. Following resuspension, flee break-up during continuous stirring caused mean particle size to decline towards the initial on-site value.