In situ measurements of settling velocity and particle size distribution with the LISST-ST

In situ techniques are essential for a proper characterisation of suspended sediment. The Laser In Situ Scattering and Transmissiometry-Settling Tube (LISST-ST) uses laser diffraction to measure both the in-situ size and settling velocity distribution of suspended sediment. The instrument is equipped with a settling tube in which a water sample is captured. By preprogrammed scanning of the sample the LISST-ST keeps up a settling record of the water sample for either 12 or 24 hour. The instruments' facility to measure stand-alone offers a relatively easy way to collect a substantial data set. Size distributions and volume concentrations are calculated from the laser diffraction patterns using an algorithm based on Mic scattering theory. From the time series of the volume concentrations, settling velocities are estimated using a model based on Stokes' law. The instrument was deployed in the Hollandsch Diep and Haringvliet, two fresh water basins in The Netherlands. Measured settling velocities are in the order of 0.005 mm/s for the finest fractions (<7 mum) and 0.3 mm/s for the larger particles (>96 mum and <186 mum); corresponding effective densities are in the order of 600 to 30 kg/m(3). Settling velocities in the upstream Hollandsch Diep are higher than those in the downstream Haringvliet; the data indicate that due to flocculation within the system large flocs of a relatively low density are formed in the downstream area. In general small particles have a relatively small volume concentration, but because of their high density, they dominate the mass concentration. The mass flux is dominated by the larger particles, because despite their low density these particles have a relatively high settling velocity.