Using 3D photo-reconstruction methods to estimate gully headcut erosion

In this paper, for the first time, three-dimensional photo-reconstruction methods (3D-PR) based on Structure from Motion (SfM) and MultiView-Stereo (MVS) techniques are tested for estimating the volume of gully headcut retreat. The study was carried out using 5 small headcuts in SW Spain: two headcuts located along the channel and 3 lateral-bank headcuts. Firstly, the accuracy of the resulting models was tested using as benchmark a 3D model obtained by means of a Terrestrial Laser Scanner (TLS). Results of this analysis showed centimetre-level accuracies with average distances between the two point clouds for the five headcuts ranging from 0.009 m to 0.025 m. Then, using a Digital Elevation Model of Differences approach (DoDs) the volume of soil loss was estimated for every headcut. Total soil loss ranged from − 0.246 m3 (erosion) to 0.114 m3 (deposition) for a wet period (289 mm) of 54 days in 2013. A different dynamic was observed for the main and lateral-bank headcuts, which showed erosion and deposition, respectively. Additionally, the use of historical photographs was explored with the aim of estimating long or medium-term erosion rates in gully heads. Results of this simulation pointed out to a clear decrease in the accuracy of the model when the photos are not acquired sequentially around the headcut. Finally, some methodological advices about the use of this 3D-PR procedure for monitoring small geomorphological features are presented.