Field and flume measurements with the impact plate: Effect of bedload grain-size distribution on signal response

The Swiss plate geophone is a bedload surrogate monitoring system that has been calibrated in several gravel-bed streams using direct sampling techniques. Past systematic flume experiments have highlighted the dependency of the signal response on the bed roughness and the flow velocity. To further develop the existing calibration approaches and better understand the role of site-specific factors, we conducted calibration measurements of the Swiss plate geophone system in two gravel-bed streams in the Swiss Alps with a crane-mounted net sampler. In parallel, we performed full-scale controlled flume experiments at an outdoor flume facility, where we reproduced the channel characteristics and the transport conditions observed at each field site. We evaluated changes in signal response using relations between the transported bedload mass and the number of detected impulses or packets (representing a single particle impact). Using experiments with single grain sizes, we confirmed that increasing flow velocity reduces the signal response. Additionally, we performed mixed-grain-size experiments and found that the grain-size distribution of the transported bedload can influence the signal response of the Swiss plate geophone system over more than one order of magnitude and lead to a biased estimation of the bedload flux. We show that the effect of the grain-size distribution on the signal response is visible at multiple field sites and discuss possible explanations for this phenomenon.