Airborne LiDAR Measurements and Model Simulations of Tides, Waves, and Surface Slope at the Mouth of the Columbia River

Abstract

Airborne light detection and ranging (LiDAR) measurements and model simulations are used to investigate the temporal and spatial variability of the water surface elevation at the mouth of the Columbia River. A series of 15-km transects repeated in 2-3-h flights over a two week period resolve processes at a range of scales that are important to the dynamics of the river mouth region, including tides, surface slope, surface gravity waves, and wave setup. Water surface elevations agree well with a nearby tide gauge with an average difference of 0.01 m and an root mean square error of 0.39 m. Significant wave heights derived from the LiDAR measurements agree well with in situ wave drifter measurements and are observed to react to both bathymetry and spatial/temporal variations in currents, increasing up to 228 % on large ebb tides. Surface slopes varied from 2.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to -2.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> over the course of a typical tide to as large as 6.3 × 10-5 to -4.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> during a larger tide. The magnitude of the setup and set down due to wave height amplification during ebb tide was estimated to be 4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> . These observations demonstrate that many of the key dynamical variables at river mouths can be determined from airborne remote sensing measurements of water surface elevation and suggest that forthcoming altimetry products, such as those from the Surface Water Ocean Topography altimeter, may be able to provide new insight on monitoring in these complex regions.