Mapping river topography in whitewater rapids using bathymetric LiDAR

Abstract

Bathymetric LiDAR captures river topography efficiently for clear and shallow water, but for mountain rivers, whitewater rapids still pose challenges that need to be overcome. This study proposes a novel method to enable the extraction of bottom returns specifically in turbulent whitewater sections. The presented method uses a minimalistic curve fitting approach to subtract the water column of the recorded LiDAR waveform, calculates a residual of reflected energy that is not attributed to the water column, and applies peak detection on the residual to extract previously undetected returns. For the evaluation of the points derived from the additional returns, three different rivers were surveyed with both LiDAR measurements and a total station with a reflector pole. This sets the foundation for an in-depth evaluation and enables the verification of the new underwater terrain points. For the two mountain rivers, the Passer and Fischbach River, the presented method increases the total number of points for the river segments by 2% and 11%. This reflects a median increase of 50 points / m 2 and 48 points / m 2 for the two rivers, respectively. Compared to the reference data, this results in a median reduction in the distance to the reference points from 20 .6 cm to 9 .2 cm and 40 .4 cm to 22 .5 cm . Therefore, we extend full-waveform processing for bathymetric LiDAR applied whitewater rivers, which opens up the field of bathymetric LiDAR for river research in the turbulent environment of mountain rivers.