Comparison of Measured Bedload with Predictions from Transport Equations in an Unarmored Ephemeral Channel

Abstract

The Arroyo de los Pinos is a tributary of the Rio Grande that transports relatively coarse sediment into the river annually through flash flood events. This coarse-grained sediment can lead to problems for downstream infrastructure, such as sedimentation in reservoirs and increased channel maintenance requirements for flow conveyance. Over the past five years, a comprehensive database of bedload, suspended sediment, and meteorological-hydrologic measurements have been developed at the confluence of the channel to the Rio Grande. Bedload flux is monitored by three Reid-type slot samplers at 1-minute resolution, flow stage is continuously monitored with pressure transducers, and surface flow velocity is measured periodically using large scale particle imagery velocimetry (LSPIV) to produce a stage-discharge rating curve. Bed material samples have been collected and sieved, and channel geometry has been mapped in detail using drone imagery and structure from motion (SfM) photogrammetry. This dataset enables assessment of predicted bedload using a wide range of well-established equations including Meyer-Peter and Müller, Wilcock and Crowe, Einstein, Parker, Ackers-White, and Engelund-Hansen which are calculated and compared in HEC-RAS and BedloadWeb. Crucially, we can compare the quality of prediction from these methods against the observed bedload transport at a range of flow depths between 5 – 50 cm (discharge at 0.25 – 10 m3/s). The Pinos dataset provides an excellent opportunity to compare a range of transport equations and consider their relative performance in ephemeral, semi-arid, flash flood driven fluvial systems. Successful equation selection will enable the extension of our temporally-limited direct bedload measurements to approximate annual bedload yields from the Arroyos de los Pinos, as well as from other similar ephemeral tributaries to the Rio Grande and elsewhere. The best fitting bedload transport equations for the Arroyo de los Pinos are the Meyer-Peter and Müller and the Wilcock and Crowe equations.