In situ particle size spectra and density of particle aggregates in a dredging plume

An in situ laser particle sizer, the LISST-100, was used to describe the sp atial variation of beam attenuation coefficient, in situ particle size spec tra and aggregate densities in a dredging plume in the sound Oresund betwee n Denmark and Sweden. The results proved that the above mentioned parameter s varied significantly within the investigated length of the plume (approxi mately 2 km). It is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution, and at the same time how the mean density of particles/aggregates decreases significantly. This shift in the state in w hich the particles exist in the water effectively changes the optical respo nse of the mass of particles suspended in the water. It is shown that adequ ate correlations between mass concentrations and beam attenuation coefficie nts can only be obtained if parameters describing the in situ quality and s tate of the mass of particles, e.g. standard deviation of the size spectra and volume concentration, is included in the regression. From the spatial v ariation in mean density and in situ particle size, it was possible to calc ulate the spatial difference in settling velocity. It was found that the di fference in settling velocity was only about a factor of 1.7, because incre asing in situ particle size was counter-balanced by decreasing mean density . Furthermore, the time-scale of flocculation within the plume was found to be in the order of 50 min. (C) 2000 Elsevier Science B.V. All rights reser ved.