A technique for eliminating water returns from lidar beach elevation surveys

Airborne light detecting and ranging (lidar) systems can survey hundreds of kilometers of shoreline with high spatial resolution (several elevation estimates per square meter). Sequential surveys yield spatial change maps of beach and dune sand levels. However, lidar data include elevations of the exposed, subaerial beach and, seaward of the waterline, the ocean surface. Here, a simple method is developed to find the waterline and eliminate returns from the ocean surface. A vertical elevation cutoff is used, with the waterline elevation (W) above the known tide level because of the superelevation from wave setup and runup. During each lidar pass, the elevation cutoff (W) is assumed proportional (C) to the offshore significant wave height H_s. Comparison of in situ and lidar surveys on a moderately sloped, dissipative California beach yields C ≈ 0.4, which is qualitatively consistent with existing observations of runup and setup. The calibrated method rejects ocean surface data, while retaining subaerial beach points more than 70 m seaward of the mean high waterline, which is often used as a conservative default waterline.