NSF-BSF: Monitoring bedload transport: Advancing seismic and acoustic surrogate methods in ephemeral channels

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The flow of water in rivers, especially during floods, transports sand, gravel, rocks, and even boulders. This movement of particles (bedload transport) shapes the river channel itself, influences the erosion of bed and banks, and helps give shape to the broader landscape. Measuring bedload transport is difficult, though, because it changes rapidly in both space and time and can be quite dangerous to collect data during the high-energy flood events when most material moves. This project will experiment with new measurement techniques that can be done safely outside of the river channel. The investigators will compare the vibration and sound energy collected by seismic sensors placed near the river bank with direct measurements of bedload transport collected by physical samplers set into the riverbed. Sampling will take place on two ephemeral streams in semi-arid regions of New Mexico, USA and Israel, streams which are usually dry, but which transport a great deal of material when they do flow during flash floods. The data generated by this project will be used to test and expand models of how bedload movement generates seismic energy, which will benefit a wide audience of scientists who want non-invasive methods to monitor river and bedload processes. In addition, because understanding how bedload moves is critical for river management, the results will assist water management agencies such as the US Bureau of Reclamation and the US Army Corp of Engineers. This project also trains diverse US and international students in hydrology, seismology, and geomorphology, with a project team that includes experts from the US, Israel, France, and Germany. Bedload flux is fundamental to river dynamics and landscape evolution, yet the collection of representative measurements of this key parameter is inhibited by its spatial and temporal variability as well as the challenge of sampling in high-energy environments (for example, desert flash floods) without altering transport. Surrogate methods are a promising approach, yet calibration remains a challenge, and the lower grain size limit of detection is uncertain. This project will take advantage of already-built infrastructure to directly measure bedload transport, use these data to calibrate acoustic surrogates, and then use both of these to calibrate and test seismic monitoring of bedload transport. This approach will provide vital field data to enable the advance of fluvial seismology and sediment transport modeling in drylands by testing theoretical models of bedload seismic energy generation. The study sites are ephemeral channels in semi-arid regions of New Mexico, USA, and Israel, at which riverbed slot samplers will monitor bedload flux at multiple locations across the channel. These data will be used to calibrate collocated pipe microphones that collect data throughout the flood. An array of ~ 70-80 short period, 3-component geophone seismometers will collect seismic data near the channel, which will then be compared with the direct and acoustic bedload data. In subsequent years, seismometers will also be deployed along the channel system to study the movement of bedload throughout an entire catchment. The data collection and analysis will refine the overarching theoretical framework of understanding for the transport of sand-rich gravel in semiarid and arid regions of our planet.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.