EVENT-SCALE GEOMORPHIC CHANGE EVALUATED ALONGSIDE SEDIMENT TRANSPORT MEASUREMENTS IN A FLOOD-DRIVEN EPHEMERAL CHANNEL

Bedload transport is notoriously difficult to measure consistently. These data are even harder to collect in desert ephemeral channels, where flooding is rare and short-lived. A new sediment monitoring station has been constructed along the Arroyo de los Piños, an ephemeral tributary to the Rio Grande. Here, we measure bedload transport using a variety of techniques, many of them never before used in Southwest US desert channels. Three Reid-type slot samplers serve as the basis against which all other surrogate instruments are evaluated. One of these surrogate instruments is the pipe microphone, which records acoustic response from the impact of bedload on a stainless-steel pipe.

In this presentation, we focus on the geomorphic change observed on the flood event scale. Five flood events of varying discharges were recorded during the first summer of operation, monsoon season 2018. During each event, we observed very early initiation of motion of bedload, at 7 – 10 cm water depth, and the early onset of full mobility at ~ 30 cm depth. The thalweg of the river often migrates across the channel in a matter of hours during these events. Bedload is transported downstream at high rates (up to 16 kg/sm) through this system due to lack of armoring and an effectively unlimited sediment supply.

When taking a broader view, the effect of this ephemeral channel on the mainstem Rio Grande perennial river is obvious. Regular drone flights of the Rio Grande-Piños confluence reveal that the thalweg of the Rio Grande is displaced from bank to bank as large Piños floods deposit gravel at the Arroyo mouth. The Piños sediment monitoring station aims to add much-needed quantitative data on when how often these channels flood and the sediment that is transported during these events.