The anatomy of effective discharge: The dynamics of coarse sediment transport revealed using continuous bedload monitoring in a gravel-bed river during a very wet year

Indirect, passive approaches for monitoring coarse bedload transport could allow cheaper, safer, higher-resolution, longer-term data that revolutionises bedload understanding and informs river management. Here, insights provided by seismic impact plates in a downstream reach of a flashy gravel-bed river (River Avon, Devon, UK) are explored in the context of plate performance. Monitoring of a centrally-situated plate (IP1) during an extremely wet 12-month period demonstrated that impacts were related to discharge as a measure of transport potential (R²=0.38) but that factors other than transport limitations are important. Analysis of discrete flow events revealed consistent rising-limb and falling-limb impact spikes biased toward the latter for larger events. Such patterns may result from disruption of the upstream armour layer (rising limb) and supply enhancements related to both upstream mass bank failures and/or flood routing of non-local sediment sources (falling limb). Installation of additional impact plates indicated that plate IP1 was indeed dominantly related to instantaneous discharge, that a three-plate lateral array somewhat better explained impact variability (R²=0.49), and that the bedload track shifts laterally with discharge. Aggregating event-total IP1 impacts against volumetric discharge further increases explanation as intra-event and stochastic bedload factors are subsumed but left 26% unexplained variance related to the unsampled bedload mass, inter-event supply differences, and attributes of plate performance. Annualising the data created an impact-based 'effective discharge' for this extremely wet year that was closer to morphological bar-full in magnitude than bankfull, but the preceding results imply this outcome is related as much to supply limitations as transport limitations. Overall, passive approaches offer a liberating prospect for bedload monitoring, capable of producing insights only achievable through high resolution, extended time periods. Such results could potentially inform threshold conditions and geomorphological effectiveness of flows for future river management strategies.