LSPIV Measurements of Two-Dimensional Flow Structure in Streams Using Small Unmanned Aerial Systems: 1. Accuracy Assessment Based on Comparison With Stationary Camera Platforms and In-Stream Velocity Measurements

Measuring 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.