New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Obtaining measurements of water velocity during flash flooding events is both difficult and dangerous. Safety is of the utmost importance when conducting research in the field, particularly in remote areas with limited vehicle and support access. Traditional methods of measuring water velocity are insufficient, especially when considering the unpredictability of shot-lived ephemeral flash floods. New, automated methods should be evaluated for their accuracy.

We evaluated two methods to measure surface water velocity: Doppler velocity radar and Large-Scale Particle Image Velocimetry (LSPIV). The Doppler radar records an average of the surface velocity by directing a beam of radio wave energy at an approaching target. The frequency shift of the reflected energy is proportional to the radial velocity of the target object relative to the velocimeter. LSPIV analysis consists of recording a video of a flood and analyzing each frame for changes in the water surface. Individual particle tracking produces an array of surface velocity vectors. Using cross section and reference target surveys, continuously monitored stage data, and estimates of the ratio of depth-averaged velocity to surface water velocity for a given relative roughness, continuous cross-section-average velocity can be estimated for the flood event, and from this the entire discharge hydrograph.

The Arroyo de los Pinos is an ephemeral tributary to the Rio Grande. In the 2021 flood season we have collected velocity measurements, both video and radar, across a large range (32- 125 cm) of water depths. To test the accuracy of our two remote methods, we have compared between them and to measurements made in the channel using an electromagnetic velocity meter. The LSPIV was able to calculate velocities that are within the accepted values in hydrometry. A drawback to using LSPIV occurs with lighting. We recorded one flood this season at nighttime with illumination from vehicle headlights, which does not initially appear sufficient for particle tracking with the LSPIV software. However, we hope to address this issue with stronger standard and infrared lighting in hopes that this program will allow for a safer collection of previously unavailable velocity data for ephemeral rivers worldwide.