The role of sand in an impact-based bedload monitoring system

Monitoring bedload transport is essential for understanding the morphology of rivers, yet traditional direct measurement techniques offer limited spatial and temporal resolution. Impact plates are a valuable addition for monitoring the transport of gravel-sized particles, although they require extensive field calibration. Recent research has therefore focused on developing globally applicable calibration approaches derived from controlled laboratory experiments. These efforts mainly aim to reproduce field-like flow velocities and roughness conditions. One aspect that remains largely unexamined is the influence of coincident sand transport. To address this gap, we conducted laboratory flume experiments that included additional sand transport, thereby replicating natural conditions during flood events more closely. Three sand feeding rates and seven gravel-sized diameter classes were investigated under high flow velocities, as they occur in mountainous rivers and creeks. The results were then compared with clear-water experiments. Signal and video analyses revealed that frequency-based signal properties, particularly centroid frequency, are strongly affected by the transported sand, whereas amplitude-based properties are more robust. Moreover, sand introduces biases in transport rate predictions derived from impulse counts. This study highlights the need to account for sand transport when calibrating impact-based monitoring systems, ultimately supporting more effective riverine sediment management and future revitalization measures.