Results
Sediment Erosion and Deposition: The measurements of bed depth made along the axis of the flume confirmed that erosion occurred predominantly, throughout the sampling period, at about 4.8 m, adjacent to the plug. The rate of erosion decreased with time and a total decrease of about 5 cm in bed height was observed after 8 h (Fig. 2).
Figure 2: Percentages of tracer Rh (filled symbols) and Pt (open symbols) in sediment cores at various distances along the central axis of the flume after an 8 hour experimental run; LL=core taken off the central axis; the red arrow indicates the location of the plug. The dashed line indicates the axial depth profile with erosion at x=4.8 m and deposition at x »6.5 m.
Transport in Suspension: Fig. 3 a-c show results for 0.5 h in the Mersey 3 experiment and they illustrate the viability of the sampling and particle tracking technique. For the total SPM, the axial concentrations near-bed were generally greater, with a maximum of 550 mg L-1 (Fig. 3a), than the concentrations at 0.4H. The ICP-MS analyses were used to calculate the percentage of tagged plug sediment in the SPM (Fig. 3b) which, in turn, allowed estimation of the concentration of plug SPM (Fig. 3c). The highest near bed concentrations of tagged SPM were in the vicinity of the plug, where erosion occurred and they declined exponentially downstream. The variability in Rh concentrations is due to dilution of the plug material by the “background” untagged Mersey sediment. The estimation of the relative proportion of Rh in SPM samples allowed the development of a particle mixing model based on the proportional mixing of the two particle types, i.e. Mersey fine sand and the sediment spiked with Rh (and Pt). The particle mixing model arising from the chemical determinations will be an important concept for integration into the hydrodynamic measurements and predictive modelling.
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Figure 3: (a)-(c) results from Mersey 3 experiment after 0.5 hour of flume run; (a) axial concentrations of suspended particulate matter (blue=near bed; red=0.4H); (b) percentage of plug particles in the SPM at the two depths; (c) concentration of plug particles in the SPM; (d) results of partitioning experiments for Ni, showing the percentage of particulate Ni versus SPM concentration, with the full lines indicating the values of the partition coefficients, Kd = [Ni]P/[Ni]S (L kg-1), where [Ni]P and [Ni]S are the concentrations of particulate and dissolved Ni, respectively.
Publications
Couceiro, F., Turner, A. and Millward, G.E. (2007). Adsorption and desorption kinetics of rhodium (III) and platinum (IV) in turbid suspensions: Potential tracers for sediment transport in estuarine flumes. Marine Chemistry 107, 308-318.