LISST Series Instruments for Advanced Measurements of Sediment Concentration, Size Distribution, Settling Velocity and Flocculation

Research or monitoring needs for sediments in water require some combination of these parameters – suspended sediment concentration (SSC), particle size distribution (PSD), the settling velocity distribution of particles, and the state of flocculation. This is known from fundamental principles. The most commonly used sensor has been the turbidity meter. The calibration of a turbidity meter depends on the PSD in water. Since the PSD in water changes in space and time, turbidity-based sensors can lead to serious errors. The relatively new Laser diffraction method employed in the LISST series record SSC and PSD in internally powered autonomous systems.

Modeling transport of sediments requires also the sediment settling velocity spectrum, i.e. the size-dependent settling velocity. These measurements, instead of invoking Stokes’ Law, are required due to the effects of particle shape, and also due to the flocculation. A settling column attached to the original LISST series has been shown to be effective at getting settling velocity spectra. Departure from Stokes’ Law suggests non-homogeneity of particle materials, or the fractal dimension less than 3 (solids). These data can be acquired with a LISST-ST system. Further details of particle shape and flocculation can be studied using a holographic system, LISST-HOLO2. In rivers, where iso-kinetic capture of water samples is established, a velocity sensing LISST-SL2 isokinetically obtains SSC, PSD and velocity in space and time. Such data have confirmed Rouse-like structure of sediments in river profiles, with vertical concentration gradients that depend vary with particle size.

Most recently, an 8MHz point-acoustic-backscatter system has been developed. The LISST-ABS system is an acoustic turbidty-like meter, with superior sensitivity for coarser grains. Finally, a new concept sensor system combines optical and acoustic turbidity data to produce a near-constant sensitivity to broad grain size-range. This development overcomes the key short-coming of turbidity sensors, while retaining a low-cost option for measuring SSC. Furthermore, it eliminates the bothersome need for constant recalibration of turbidity meters and the uncertainly of measured SSC due to changes in underlying sediment PSD.

In this presentation, we shall describe these instruments with example data.