Abstract
AbstractMeasuring two‐dimensional (2‐D) patterns of flow in rivers at high resolution over large areas is challenging using traditional velocity‐measurement methods, which provide data at specific locations or cross sections. Large‐scale particle image velocimetry (LSPIV) based on imagery obtained from fixed camera platforms can measure flow velocity on the surface of rivers and is generally accurate compared to near‐surface velocity measurements obtained by traditional methods. The proliferation of inexpensive small unmanned aerial systems (sUAS) equipped with high‐resolution cameras and onboard GPS has the potential to facilitate measurements of flow patterns in rivers using LSPIV, but few studies have assessed the accuracy of sUAS‐derived LSPIV compared to fixed‐platform LSPIV and in‐stream velocity measurements. This study assesses the accuracy of sUAS‐based LSPIV for measuring 2‐D mean surface velocities as well as quasi‐instantaneous 2‐D velocities obtained from successive image frames. For persistent 2‐D flow, mean velocities derived from sUAS‐based LSPIV match those obtained by stationary camera platforms, and velocities measured by both LSPIV methods agree with near‐surface velocities measured by an acoustic Doppler velocimeter. Quasi‐instantaneous velocities are degraded by camera movement and low pixel resolution, but capturing the evolution of 2‐D flow structures is possible in certain circumstances. The results confirm that sUAS‐derived LSPIV provides accurate, high‐resolution measurements of mean surface velocities over large spatial areas of persistent 2‐D flow and can characterize evolving 2‐D flow structures under favorable conditions. sUAS‐based LSPIV is a valuable new method for mapping of 2‐D patterns of surface flow in rivers—an issue explored in a companion paper.
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