Enhanced Surf Zone and Wave Runup Observations with Hovering Drone-Mounted Lidar

We demonstrate that a hovering, drone-mounted laser scanner (lidar) paired with a survey-grade satellite and inertial positioning system measures the wave transformation across the surf zone and the resulting runup with accuracy almost equal to a stationary truck-mounted terrestrial lidar. The drone, a multirotor small uncrewed aircraft system (sUAS), provides unobstructed measurements by hovering above the surf zone at 20-m elevation while scanning surfaces along a 150-m-wide cross-shore transect. The drone enables rapid data collection in remote locations where terrestrial scanning may not be possible. Allowing for battery changes, about 17 min of scanning data can be acquired every 25 min for several hours. Observations were collected with a wide (H-s = 2.2 m) and narrow (H-s = 0.8 m) surf zone, and are validated with traditional land-based survey techniques and an array of buried pressure sensors. Thorough postprocessing yields a stable (sigma over bar =1.7 cm) back beach topography estimate comparable to the terrestrial lidar (sigma over bar =0.8 cm). Statistical wave properties and runup values are calculated, as well as bathymetry inversions using a relatively simple nonlinear correction to wave crest phase speed in the surf zone, illustrating the utility of drone-based lidar observations for nearshore processes.