In the context of the OMES project we applied the particle settling velocity equation for spheres from Gibbs et al. (1971), which incorporates the Stoke's law, to determine a model of sedimentation of SPM from the Scheldt estuary. The classical method of sedimentation column was applied in combination with a microscopical image analysis technique using natural water samples from different locations. Important discrepancies between experimental and theoretical results were obtained by applying the Stoke's law and Gibbs' equation relating settling velocity and diameter. This was not surprising since the complexity of estuarine SPM, changes in floc densities and shape factor influence the SPM behaviour. In addition, to measure particle volumes by image analysis we used the new method introduced by Billiones et al.(1999a) based on some approximations such as mean d and the relationship between h and d (h = 34*d), which could be potential sources of errors. To verify which fractions of particles generated the observed irregularity in obtained size distribution ( compared to the theoretically expected), we made balance calculations, both in volume and particles number, on the amounts of the different size classes of particles present in each experiment and observed at different sedimentation times. The balance was calculated by subtracting the total amount of the different size fractions in the sampled and evacuated volume at each sampling time, from the total amount of this fraction at t0. Generally, the total balance tended to the equilibrium when we removed the bigger fractions, and the balance for each fraction was generally negative for the small fractions (4, 8 and 11 µm) and positive for the bigger ones. High frequency of occurrence was observed after the expected sedimentation time for the bigger fractions and before the expected time for the smaller ones. These facts pointed to an important role of the big fractions in the observed discrepancy. To verify if the microscopical image analysis used did not entail any systematic over- or underestimation of particles volume, total deviation in the balance for each size fraction (expressed as percentage of that fraction at t0) was plotted as a function of SPM concentration in the stations. No increase in deviation was seen with increasing SPM concentration indicating that image analysis did not produce any systematic error. The chemical analysis of the fractions obtained generally showed an homogeneous sedimentation of SPM in autumn and early spring contrary to the slower sedimentation of phytoplanktonic organic matter than the bulk organic matter, found in summer by Hannouti (unpublished data). We concluded that under slack conditions, SPM will sediment differently than predicted by Gibb's equation. The small particles will show to some degree a good agreement with the precited equation.
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