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. Reliable sediment flux forecasts are necessary for a variety of applications such as sedimentation engineering, river restoration, and flood risk mitigation. Quantifying bedload driven by flood events in ephemeral channels is notoriously difficult because of the scarcity, irregular nature, and high intensity of flash floods. Seismic methods appear to be a promising tool to characterize such fluvial processes, as they continually and remotely record the noise caused by bedload and water movements.

We evaluated the performance of the Tsai et al. (2012) physics-based model bedload fluxes by comparison to continuous monitoring of bedload flux. 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 bedload 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 multi-year campaign of monitoring an ephemeral, sand-and-gravel bedded channel in the arroyo de Los Pinos, 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. We explore three hypotheses to explain this mismatch. First, the process of rolling and/or sliding particles, as opposed to saltating particles, may be the predominant cause of the model discrepancy. Rolling particles are perhaps a very significant contributor to bedload. Second, the alluvial characteristics of the riverbed, as contrasted to a rigid bedrock substratum, produce seismic waves in a different manner. Seismic energy is lost and quickly dissipated as a result of the inelastic impact of bedload particles, primarily so on a finer-grained substratum. Also, the bedload impact frequency model may not fully depict the impact of particles onto the riverbed. By thoroughly examining bedload transport mechanisms and considering alternative impulse functions for seismic noise generation, we intend to construct a new physics-based model, within the framework of the existing models, to quantify bedload transport in the ephemeral environment.