QUANTIFYING BEDLOAD TRANSPORT IN EPHEMERAL CHANNELS USING SEISMIC METHODS

The transport of sediment is one of the fundamental geomorphic processes governing the evolution of landscapes. Quantifying the sediment driven by flood events in ephemeral channels is notoriously difficult because of the scarcity, irregular nature, and high intensity of flash floods. Seismic methods are a promising tool with which to characterize such fluvial processes due to their ability to continually and remotely record seismic noise caused by bedload and water turbulence.

We evaluated bedload fluxes using a physics-based model developed by Tsai et al. (2012). The Tsai model relates the power spectral density (PSD) of the Rayleigh waves produced by vertically impulsive impacts from saltating particles to the rate of impacts of fluvial sediment for a given sediment flux. As a test of this model, we collected seismic data during flow events and compared the seismically-estimated bedload flux with high-precision bedload observations. These data derive from a multiyear campaign of monitoring an ephemeral, sand-and-gravel bedded channel in central New Mexico. Based on seismic data analysis, we find bedload transport well explained by signals in the 25-70 Hz frequency range, whereas rainfall generates signals > 100 Hz. Inverting seismic data for bedload flux using the vertical impact model results in underestimates of the observed bedload flux by ~ 2 orders of magnitude. The process of rolling particles, as opposed to saltating particles, and the alluvial characteristics of the riverbed, as contrasted to a rigid bedrock substratum, appear to be the predominant causes for model discrepancy. Due to the inelastic impact of sediment on alluvial sediment, seismic energy is lost and dissipates rapidly. Rolling particles are perhaps a very significant contributor to bedload. Lastly, the bedload impact frequency model may not fully depict the impact of particles onto the riverbed. By thoroughly examining sediment transport mechanisms and considering alternative impulse functions for seismic noise generation, we intend to construct a new physical model, within the framework of the existing models, to quantify bedload transport in the ephemeral environment.