Analysis of the Vibration Modes of Impact Geoplates and Implications for Bedload Flux and Grain Size Measurements

Abstract

AbstractSeismic geophones have been frequently used in combination with metal plates to indirectly estimate bedload flux in gravel bed mountain rivers. This indirect method has been calibrated with direct measurements in several conditions and proved effective to provide relevant information on spatial and temporal scales otherwise impossible to explore. Notwithstanding, a detailed description of the vibration modes of the plate is still lacking, limiting the possibility to interpret the signal. Here, we report on two sets of experiments where we explored the geoplate response in terms of frequency and amplitude of different vibrational modes. A first set of dry experiments was designed to investigate the variability of the signal as a function of the impact location on the plate. A second set reproduced standard flow conditions, with single pebbles transported by the flow. Results highlighted the occurrence of two main vibration modes, with a first rapid phase (about 5 ms) characterized by higher frequencies, and a second longer and more stable phase of vibration with a frequency in the range of 300–400 Hz. The signal of single impacts showed a good correlation with pebble mass, with only the number of impacts on the plate depending on the flow conditions. The evaluation of the grain size is a challenge given the variability of the signal generated by similar impacts. We propose a filtering strategy to improve grain size estimation. Our analysis shows that a more detailed understanding of the vibrational modes helps identifying best practices for an improved signal acquisition and elaboration.