Methodology

Particle size measurements were made using a Par-Tec 200 laser backscatter probe. At each study site, rapid and nondisruptive in situ measurements of the EPSD of suspended sediment were made by immersing the probe at ca. 1 to 2 hourly intervals in the thalweg zone of the channel during flood events. The Par-Tec 200 was typically immersed for ca. 3 min on each periodic deployment, during which time seven replicate measurements of the EPSD of ambient suspended sediment were made. A total of 204 deployments of the Par-Tec equipment were made during 37 storm events at the eight study sites. The operating principles, and full details of the field application of this device are given in Phillips and Walling.12,28 During the in situ measurement process, two representative grab samples of suspended sediment were collected adjacent to the submerged Par-Tec 200 probe. Suspended sediment concentration at the time of in situ measurement of the EPSD was determined by filtration of the contents of one of the samples through a tared 0.45 ¡m filter. Suspended sediment was recovered from the second sample by centrifugation and its APSD measured using the Par-Tec 200 after removal of the organic fraction by treatment with hydrogen peroxide, and chemical dispersion in 0.4% sodium hexametaphosphate solution.

Particle density was investigated using a combination of the physical measure of median effective particle size derived from the Par-Tec 200 laser backscatter probe, and the hydraulically derived estimate of median settling velocity obtained from sedimentation analysis using a Valeport (SK110) Bottom Withdrawal Tube (BWT). The BWT was used to take in situ samples of suspended sediment adjacent to the Par-Tec 200, with sedimentation taking place immediately afterwards by placing the BWT in a vertical stand on the river bank. The median EPD was derived by simple rearrangement of the settling velocity equation of Gibbs et al.31

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