Sediments in River Columns – Particle Size Distribution and Concentrations Measured with a LISST-SL2 Isokinetic Instrument

In this paper, we present measurements made with an iso-kinetic submersible laser diffraction instrument, LISST-SL2 (Sequoia Scientific, Inc., Bellevue, Washington, USA). The instrument measures suspended sediment concentration (SSC), particle size distribution (PSD), velocity, temperature and depth. In contrast to existing samplers, a LISST-SL2 can fully characterize a water column in just a few minutes, or few tens of minutes. All data are processed and displayed in real-time at 1 Hz, so that the user can see in real-time, the vertical profile of SSC, PSD, velocity, and temperature. The LISST-SL2 has been used in several rivers, including in China, US, Europe, and New Zealand. River water columns exhibited a classic Rouse profile: SSC increased with depth, the PSD developed a growing sand mode with depth and concentration gradients in single size particle profiles increased with size. A feature less well known in river sediment transport is the temporal characteristic of sediment transport. Data from the Yangtze river show small fluctuations in concentration of fine particles (clay and silt), and even fine sand, 125 µm. A high variability at long time-scales was seen for sand of 250 µm. This suggests that coarse sand is transported near-bottom in bursts of order 10 seconds long. The implication is that the estimation of flux via the product of mean velocity and mean concentration may underestimate the true transport rate. Furthermore, short-time samples as might be acquired by bottle samplers would not obtain good mean values. These and other findings will be presented.