Sediment pulse propagation and identification using a low-cost sensor network: a hydropeaking study on the Spöl river, Switzerland

Hydropeaking in rivers changes the flow regime, increases river clogging, mobilizes fine sediment, and causes major stress to fish, macroinvertebrates, and aquatic plants that suffer from the rapid water level fluctuations. One in four medium- to large-sized rivers in Switzerland is affected by hydropeaking. In this study, we investigated the effect of hydropeaking on fine sediment transport during an experimental flood on the Spöl river, a tributary of the Inn river, in the canton of Graubünden, Switzerland. The study was a proof-of-concept for new smart turbidity sensors, which were developed in our laboratory, calibrated, and tested in mixing tank experiments in 2021 and again in 2022 with a range of different sediment types. These sensors were deployed at two locations on the Spöl during an experimental flood release by the upstream Ova Spinne hydropower dam. The collected data reveal sudden sediment concentration increases and decreases (pulsing) as the discharge increases steadily throughout the day. The highest concentration of sediment is much larger (4-5 g/L) than would be expected and appeared with the onset of the flood and again with the peak discharge. Our findings also reveal clockwise and counter-clockwise hysteresis loops in the stage-concentration relation, which point to a switch in the sediment supply between supply limited and unlimited conditions during the experimental flood. This study shows that high spatial- and temporal-resolution monitoring of suspended sediment is possible with a low-cost sensor network. The applications of such a network are plentiful: from identifying sediment source activation and transport in small streams, glacier networks and deltas, to environmental monitoring of maximum sediment concentration levels for the survival of fry fish, for prevention of river bed clogging, and for pollutant monitoring (binding to sediments).