Seismic Modeling of Bedload Transport in a Gravel‐Bed Alluvial Channel.

Recent theoretical models and field observations suggest that fluvial bedload flux can be estimated from seismic energy measured within appropriate frequency bands. We present an application of the Tsai et al. (2012, https://doi.org/10.1029/2011gl050255) bedload seismic model to an ephemeral channel located in the semi‐arid southwestern US and incorporate modifications to better estimate bedload flux in this environment. To test the model, we collected streambank seismic signals and directly measured bedload flux during four flash‐floods. Bedload predictions calculated by inversion from the Tsai model underestimated bedload flux observations by one‐to‐two orders of magnitude at low stages. However, model predictions were better for moderate flow depths (>50 cm), where saltation is expected to dominate bedload transport. We explored three differences between the model assumptions and our field conditions: (a) rolling and sliding particles have different impact frequencies than saltating particles; (b) the velocity and angle of impact of rolling particles onto the riverbed differ; and (c) the fine‐grained alluvial character of this and similar riverbeds leads to inelastic impacts, as opposed to the originally conceptualized elastic impacts onto rigid bedrock. We modified the original model to assume inelastic bed impacts and to incorporate rolling and sliding by adjusting the statistical distributions of bedload impact frequency, velocity, and angle. Our modified "multiple‐transport‐mode bedload seismic model" decreased error relative to observations to less than one order of magnitude across all measured flow conditions. Further investigations in other environmental settings are required to demonstrate the robustness and general applicability of the model. Plain Language Summary: The conveyance of sediments in rivers, especially of coarse‐grained sediment on the riverbed, is a basic geomorphic mechanism influencing short‐term river ecology, management, and engineering as well as long‐term landscape evolution. To better predict bedload transport rates, researchers have used seismic sensors to record ground vibration due to the impacts of bedload particles on the riverbed. We revised a physics‐based model developed by Tsai et al. (2012, https://doi.org/10.1029/2011gl050255) to estimate bedload sediment transport in an ephemeral channel in the semi‐arid southwestern US. To test the model, we collected nearby seismic signals during flash floods and simultaneously monitored water depth and bedload transport at one‐minute intervals. We modified the original model to better represent alluvial rivers by adding a representation of rolling and sliding particles to the hopping particles already considered in the physics of seismic waves generated by bedload transport. Our modified "multi‐mode bedload seismic model" yields a better result compared to the original Tsai et al. (2012, https://doi.org/10.1029/2011gl050255) model in our test river. The modified model is not only applicable for the ephemeral channels on which this study is focused but also is potentially applicable to a wide range of river conditions in different environmental settings. Key Points: The Tsai et al. (2012, https://doi.org/10.1029/2011gl050255) model underestimates bedload flux at low water depth by 1–2 orders of magnitude in an alluvial channelWe modified the original model to incorporate rolling and sliding by adjusting bedload impact frequency, velocity, and angleThe modified model decreases flux error relative to observations to less than one order of magnitude across all measured water depths [ABSTRACT FROM AUTHOR]