Comparison between bedload flux from inverse modelling of seismic ground motion data and direct monitoring by Reid bedload samplers

Quantification of bedload flux along with its boundary conditions is essential to advance our understanding of rivers and to reduce human and economic threats. However, gaining continuous high resolution empirical data is challenging. Seismic sensors can provide time-resolved quantitative bedload flux data, given adequate data processing. The mechanistic model by Tsai et al. (2012) predicts the power spectral density (PSD) of Rayleigh waves caused by impacts of saltating particles on the river bed, allowing to invert seismic signals to obtain bedload flux. Here we test the robustness of bedload flux inversions of seismic observations against in-situ continuous monitoring of bedload flux and select bedload grain sizes made in the Nahal Eshtemoa, Israel. Proper testing is further ensured by wave propagation (the Green’s function) being fully constrained from active seismic survey experiments. We find that there is a discrepancy of approximately one order of magnitude between the measured and reconstructed bedload flux. We support that this discrepancy can be due to the largest grains, though constituting an infinitely small fraction, generating considerable seismic signals but not being caught by the bedload samplers. It is also possible that this discrepancy is due to model simplification regarding grains in motion. Based on these findings we support that seismic observations may be complementary rather than redundant to in-situ measured bedload flux, because they may give constraints on the fraction of large grains, which is challenging to monitor otherwise.