FIRST-ORDER ESTIMATES OF HIGH-FLOW STREAM FLOODING DISCHARGE FROM COMPETENCE MEASUREMENTS USING TIME-LAPSE DRONE (SUAS) ORTHOMOSAIC IMAGERY, MESA COUNTY, COLORADO USA

Ephemeral streams dominate the steep topography of the Western Slope of Colorado, but quantifying peak discharge during flooding using traditional stream-monitoring techniques is difficult due to numerous logistical challenges including remote access, lack of funding, numerous canyons, and temporally asymmetric sampling requirements. However, high-volume water discharge and associated sediment flux has implications for flash-flooding and infrastructure damage; thus, any information on peak discharge during flooding events throughout the numerous canyons is useful.

Estimating peak discharge using stream competence is not a new technique, but we leverage previous studies by incorporating drone (sUAS) imagery analyses. Preliminary testing on a midstream gravel bar in the Colorado river over a year-long period with time-lapse imagery demonstrates the viability of the methodology. Specifically, individual cobbles are identified as “existing” or “missing” during time-lapse orthomosaic images before and after peak discharge, and the size distribution of the transported cobbles are used to estimate discharge.

Analyses of a 300-meter section of Ladder Creek, a flash-flood-prone ephemeral-stream system within the 8.5-km-long Ladder Canyon associated with steep topography of the Uncompahgre Uplift in western Colorado USA, was conducted to test the methodology. Sediment-transport behavior was monitored from Spring 2022 through Spring 2024 using time-lapse structure-from-motion (SfM) orthomosaic imagery. For each time step, ca. 1000 images were acquired and subsequently processed with the difference-of-Gaussians SfM method to generate 3-D orthorectified digital models and digital elevation models (DEMs). Individual cobbles and boulders analyzed for movement, along with identification of strand lines associated with maximum stream height, are used to make estimates of maximum stream discharge using standard stream-competency equations. Results of overall sediment transport plus maximum discharge values from 2022-2024 allow for preliminary projections of stream power and discharge within Ladder Creek during flooding events, indicating an potential new additional method for analyzing ephemeral streams.