Toward a general calibration of the Swiss plate geophone system for fractional bedload transport

Abstract. Substantial uncertainties in bedload transport predictions in steep streams have triggered intensive efforts to develop surrogate monitoring technologies. One such system, the Swiss plate geophone (SPG), has been deployed and calibrated in numerous steep water courses, mainly in the Alps. Calibration relationships linking the signal recorded by the SPG system to the transported bedload can vary substantially between different monitoring stations, likely due to site-specific factors such as the flow velocity and the bed roughness. Furthermore, recent controlled experiments have shown that site-specific calibration relationships can be biased by elastic waves resulting from impacts occurring outside the plate boundaries. Motivated by these findings, here we present a hybrid calibration procedure derived from flume experiments and an extensive dataset of 308 calibration measurements from four different field monitoring stations. Our main goal is to investigate the feasibility of a general, site-independent calibration procedure for inferring fractional bedload transport from the SPG signal. First, we use flume experiments to show that sediment size classes can be distinguished more accurately using a combination of vibrational frequency and amplitude information than by using amplitude information alone. Second, we apply this amplitude-frequency method to field measurements to derive general calibration coefficients for ten different grain-size fractions. The amplitude-frequency method results in more homogeneous signal responses across all sites and significantly improves the accuracy of fractional sediment flux and grain-size estimates. We attribute the remaining site-to-site discrepancies to large differences in flow velocity, and discuss further factors that may influence the accuracy of these bedload estimates.