Remote Sensing of Salt Marsh Tidal Velocity with UAS-based Infrared Thermal Imaging

Remote sensing of water velocity and channel discharge in tidally-forced ecosystems has the potential to improve upon the safety and accuracy of conventional stage-based and acoustic velocimetry methods, especially under extreme conditions. Infrared Quantitative Image Velocimetry (IR-QIV) delivers accurate velocity and discharge measurements by using a georeferenced thermal infrared camera to track temperature gradients that naturally appear in flowing water (Schweitzer & Cowen, 2021). This project expands on past work that used a more sensitive cooled infrared camera mounted in a fixed, oblique field of view. Uncooled microbolometer thermal infrared cameras are light and efficient enough to be deployed on a UAS, providing an orthogonal viewing angle of the water surface that significantly simplifies georectification and velocity conversion. The tidal interface of a coastal salt marsh presents an ideal setting to test IR-QIV from a UAS due to dramatic thermal gradients between marsh and ocean water, which are more easily visible to the less-sensitive uncooled microbolometer. Experiments consisting of ten-minute flights at various points in the tidal cycle were conducted at the University of California's Carpinteria Salt Marsh Reserve (CSMR) in November 2021 and May 2022 during periods of minimal riparian inflow to the marsh, and supplemental acoustic velocity profile and in-situ temperature measurements were collected for statistical comparison with UAS imagery results. CSMR was heavily impacted by sediment deposition from 2018 debris flows following the Thomas Fire, which severely impacted this ecosystem and altered its hydrology. This experimental setting also provides continuous meteorological and water level data immediately adjacent to the experiment location, enabling time-synchronized corrections for wind speed and free surface elevation. A time series of in-situ temperature variance in this tidally-dominated salt marsh is also presented to explore what points in the tidal cycle exhibit the highest thermal gradients and thus the best imaging conditions. This project presents one of the first comparisons of accuracy between UAS-based infrared imagery velocity measurements and in-channel acoustic instruments within the frame of view.