Size-Dependent Measurement Variability in LISST-200X–Derived Suspended Sediment Particle Size Distributions

Suspended sediment concentration and particle size distribution in rivers are fundamental variables for understanding sediment transport processes, riverbed changes, and hydrological, hydromorphological, and ecological phenomena. The LISST-200X, a field measurement device utilizing laser diffraction principles, offers the advantage of simultaneously observing suspended sediment concentration and particle size distribution at high resolution. However, inherent assumptions in its measurement principle and optical limitations still introduce uncertainty in data interpretation. In particular, the potential for measurement variability to systematically differ across particle size intervals has been identified as a significant issue for the quantitative utilization of LISST data. This study analyzed size-dependent measurement variability in suspended sediment particle size distribution data by simultaneously deploying three LISST-200X units with identical specifications at the same cross-section within a full-scale flume test environment. The experiment was conducted under constant flow conditions. Quartz sand and loess slurries with contrasting particle size characteristics were injected upstream, and temporal changes in suspended sediment concentration and particle size distribution were continuously measured at a downstream measurement section. Particle size distributions were derived using both the spherical model and random shape model provided by the manufacturer to examine the influence of particle shape assumptions on the measurement results. The analysis showed that the three instruments generally exhibited similar temporal variation patterns. However, pronounced variability between instruments persisted in the fine particle size range. Under high-concentration, fine-dominated conditions, optical transmittance decreased substantially, and increased variability was observed not only in the fine size range but also concurrently in some medium size ranges. In addition, significant differences in derived particle size distributions and representative particle sizes were observed depending on the selected inversion model, particularly under fine particle-dominant conditions. These results indicate that LISST-200X–based suspended sediment particle size data can exhibit varying reliability depending on particle size range and measurement conditions, underscoring the need for careful interpretation of laser diffraction–based measurements.

 

This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Research and development on the technology for securing the water resources stability in response to future change Program, funded by Ministry of Climate, Energy, Environment (MCEE) (RS-2024-00397970).