New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande is a crucial part of life in the Southwest U.S.; it and other mainstem perennial rivers allow for development in this semi-arid region. These rivers are often modeled to predict changes and allow for effective management and the largest source of uncertainty in modeling these rivers is associated with the sediment influx from ephemeral tributaries. Studies of ephemeral channels in arid environments are limited; few point measurements, let alone continuous datasets, exist for these flash flood-driven channels.

Evaluating sediment transport by conventional means is not possible in flood-driven channels that are typical of large tracts of land in deserts worldwide. Automatic means are required to monitor these channels. To that end, three Reid-type slot samplers have been deployed at the Arroyo de los Piños sediment monitoring station. These samplers collect bedload in a chamber set below the surface of the channel. Sediment falls through a slot of a specified width and a system of pressure transducers measures the accumulated mass in real time. They represent a 9.5 m wide constricted reach of 1.3% slope draining a 32 km² basin with presumed high yields of sand and gravel. These are transported directly to the Rio Grande in few monsoon season flash floods. The channel bed is unarmored, with coarser pebble-cobble rich bars and finer-grained thalwegs comprised of sand-rich granules. The channel bed contains about equal proportions of sand, granules (2-8 mm) and coarser particles.

While bedload is measured directly using Reid-type slot samplers, a number of novel surrogate methods are deployed to measure bedload indirectly. Here we present our initial analysis of two pipe microphone impact sensors. Pulses are generated from bedload striking the pipe that causes acoustic noise that is recorded by a microphone sealed within the pipe. If the acoustic power exceeds some predetermined threshold, a pulse is counted.

Five flood events, ranging widely in discharge, have been recorded along the Piños station to date. Initial analyses establish that bedload fluxes are very high by global standards (6.5 – 16.5 kg/sm; as expected in unarmored, ephemeral channels). Bedload transport is initiated even by shallow flow events (5 – 10 cm water depth). The pipe microphone timeseries show significant differences both laterally and temporally. At times, the pipe microphone positioned near the left bank received nearly twice as many pulses as the right pipe microphone for a given water depth. These instruments have been successfully used in other channels worldwide; once properly calibrated pipe microphones can be an effective alternative to painstaking manual measurements of bedload transport required by systems like the Reid slot sampler.