Drivers and features of rapid suspended sediment composition changes in the small urban River Setun

This article discusses the results of field experiments in the lower reaches of the Setun River, the largest tributary of the Moscow River within the city of Moscow, based on the LISST-200X diffractometer, which measures suspended sediment concentration (SSC) and particle size through laser diffraction. The research was conducted in early 2024, and involved high-frequency (10-second interval) long-term recordings (49 h in total) of sediment transport characteristics. The combination of these measurements with sampling for optical and gravimetric turbidity allowed for the identification of limitations in the use of such measurement tools. The reproducibility of the granulometric composition of suspended sediments based on high-frequency monitoring was found to be worse than that of their concentrations. The LISST-200X data, on average, overestimated the particle size by nearly two times compared to laboratory measurements, which can be partially explained by the inclusion of larger particles (over 500 µm) in the measured range; however, it consistently reproduced relative changes in granulometric composition. During the experiments on the Setun River, short-term (up to 95 min) increases in turbidity and particle size (plumes) were identified, likely of anthropogenic origin, characterized by hysteresis relationships between SSC and sediment composition. In all cases, during the rise in SSC, the size of suspended sediments was lower than during its decline. This result highlights a previously unexplored phenomenon of sediment transport downstream from point sources into channel flows, that shows hydraulic sorting along the river length, where lighter particles move faster than larger and heavier particles (including organic ones). The obtained estimates are significant for both monitoring anthropogenic impacts and advancing the theory of river sediments.