Seismic monitoring of bedload in monsoon floods in gravel bed arroyos in central New Mexico

The mid-Rio Grande is situated in a semi-arid region with sediment supplied largely by short-lived, powerful floods in ephemeral rivers. Due to the very large amounts of bedload influx relative to perennial rivers in other regions, it is challenging to acquire accurate estimates of bedload flux in these environments. In recent years, seismic analyses have emerged as a promising method for studying fluvial processes with the intention of isolating signals from bedload transport. To explore these methods in ephemeral rivers, 5 broadband seismometers and 66 nodes were deployed alongside a well-instrumented channel in central New Mexico and its tributaries. Alongside the seismic equipment, the channel is instrumented with bedload samplers and surrogates near the outlet and pressure transducers in upstream tributaries. Data from these hydrologic instruments combined with seismic power measurements suggest that seismometers may be an effective tool for providing accurate estimates of bedload transport at a safe distance from powerful flows. Data from dozens of seismic nodes during two floods in July 2020 show that seismic power calculated on one minute intervals at frequencies between 25 and 70 Hz have a strong correlation with bed shear (R2 between 0.75 and 0.8), particularly on the falling limb of floods. Additionally, data collected on a single seismometer from 7 floods between 2018 and 2020 show a consistent correlation between seismic power at these frequencies and bedload flux measured by bedload samplers (R2 of 0.73). These correlations remain strong even at very shallow flow depths during which bedload was moving. Through the application of these seismometers, characteristics of monsoon floods can be measured with little risk of injury to researchers and without potential for lost and damaged equipment.