Bathymetric LiDAR captures river topography efficiently for clear and shallow water, but for mountain rivers, whitewater rapids still pose challenges.

This study proposes a novel method to enable the extraction of bottom returns specifically in turbulent whitewater sections. The method outlined uses a curve fitting approach to subtract the water column from 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 (Pielach River in Lower Austria, Passer River in South Tyrol and Fischbach River in Tyrol) were surveyed with both LiDAR measurements and a total station with a reflector pole. This provides the foundation for assessing the accuracy of the new underwater terrain points. 

Each site was surveyed using a bathymetric laser scanner mounted on a UAV platform. In all cases, a RIEGL VQ-840-GL topo-bathymetric laser scanner providing a wavelength of 532 nm and full-waveform recording was used.

Compared to the reference data, this results in a median reduction in the distance to the reference points from 20.6 to 9.2 cm and 40.4 to 22.5 cm. Because it closes significant gaps in the data where no bottom points were detected.

In conclusion, this study extends the application of full-waveform processing for bathymetric LiDAR to whitewater rivers, which opens up the field of bathymetric LiDAR for river research in the turbulent environment of mountain rivers.

This research article is published in the WILEY Online Library and can be found here.