The southwest United States’ climate is prone to monsoon flooding. Arid air from the west draws in moisture from the south, leading to thunderstorms, flash flooding, and debris flow risk. According to NOAA, Arizona and New Mexico receive more than 50% of their annual rainfall during the July–September monsoon season. Measuring the discharge can be dangerous due to the volume and velocity of the water, as well as suspended sediment. The use of small unoccupied aircraft systems, or drones, has grown as a reliable, safe, and affordable measurement tool. Operated by trained staff, the sUAS can capture high-resolution video footage above the water’s surface. Using large-scale particle image velocimetry, researchers can derive turbulence characteristics to better estimate water level, velocity, and flow discharge.
In “Discharge Estimation Using Video Recordings from Small Unoccupied Aircraft Systems” for the Journal of Hydraulic Engineering, researchers Jennifer G. Duan, Frank L. Engel, and Ammon Cadogan evaluate the accuracy of discharge estimation methods using turbulence dissipation rate derived from imagery taken from sUAS in the field. The team identified eight field sites and used four methods of computing average velocities from TDR. Learn more about how this study and how it can help accurately calculate seeding and flow conditions using video images at https://doi.org/10.1061/JHEND8.HYENG-13591. The abstract is below.
Abstract
Measurement of river discharge during flooding events has especially been a challenging and dangerous task in the southwestern US, where flows can be flashy, laden with sediment, and at high velocity. Small unoccupied aircraft systems (sUAS) can be deployed to access unsafe field sites and capture imagery for measuring surface flow velocity and discharge. This paper compares flow discharge estimation at eight field sites — at or near USGS gauging stations — using time-averaged surface velocities and the turbulence dissipation rate (TDR) derived from large-scale particle image velocimetry (LSPIV) analysis of sUAS videos with conventional measurement techniques conducted by professional USGS hydrographers. Sites characteristics include both natural and engineered channels. The conventional measured discharges were treated as the reference discharges for evaluating the accuracy of the LSPIV discharge estimates. This study evaluated four approaches to estimate the depth-averaged or cross-sectional averaged velocity: constant-velocity index, logarithmic law, power-law, and the entropy method. Results showed the discharges can be accurately calculated by using any of these methods, and that choice of method depended on width to depth ratios.
Practical Applications
Accurate measurement of water quantity is of vital importance to water resource managers, forecasters, and the public. Often, such as during floods, conditions at the river can be very dangerous to the crews responsible for such measurements. Small unoccupied aircraft systems (or drones) are proving to be an excellent tool for quantifying river flows using methods that do not involve directly entering flooding rivers. By using video collected from drones, we show that it is possible for practitioners to accurately measure flow discharge during in rivers and canals. We evaluate four methods for completing the task, and offer suggestions based on our findings. Although more research is needed to perfect the methods, we find that it is possible to accurately measure river flows using video from sUAS, and thus potentially improve safety for those put in harm’s way.
Get the details on using drones to analyze flooding in the ASCE Library: https://doi.org/10.1061/JHEND8.HYENG-13591.
