Wind stress effects on drone-based thermal infrared surface velocimetry
measurements of tidal flow in an estuary
Abstract
We evaluate the effect of surface wind stress on drone-based thermal
surface water velocity measurements of tidal flow in an estuary relative
to in-channel flow velocity measurements. Drones are a useful platform
for capturing imagery of surface flows, with the ability to support many
cameras and sensors. Drone-mounted thermal infrared microbolometer
cameras can retrieve subtle temperature patterns that naturally occur in
many surface flows. These thermal patterns are used as signals for
pattern-tracking algorithms to produce distributed measurements of
velocity across the observed water surface. The effect of wind on remote
surface velocity measurements is relatively unstudied, and herein we
present results demonstrating the impact of wind on surface velocity
measurements. This study demonstrates the feasibility of drone-based
thermal velocimetry in an estuarine channel, while collecting water
velocities with an acoustic current profiler deployed on the channel
bottom within the microbolometer’s field of view. Drone flights were
conducted at Carpinteria Salt Marsh Reserve (California, USA). Wind
speed and direction significantly increased the deviation of drone-based
surface velocimetry measurements relative to in-channel current profiler
measured velocities. Drone-based velocity measurements deviated more
from in-channel near-surface measurements when the parallel wind stress
direction was opposite the tidal flow, while drone-based velocities were
in closer agreement with in-channel velocities when the parallel wind
stress and tidal flow directions were the same. This experiment
demonstrates the feasibility of drone-based thermal surface velocity
measurements in an intertidal setting, while documenting the limitations
of surface-based inferences of at-depth flows due to wind stress.