Potential of mobile application based on structure from motion (SfM) photogrammetry to monitor slope fast erosion by runoff water

Abstract

Runoff water may cause soil erosion, being this a concern affecting multiple areas. Monitoring tools can help design efficient solutions to prevent erosion. This is usually done using reliable tools such as terrestrial laser scanners (TLS). However, for very fast erosive processes, the rate at which changes occur is greater than the scanning time and this is a challenge. Structure from Motion (SfM) photogrammetry may solve this problem since it is possible to obtain 3D models from a few photographs, reducing the acquisition time to seconds. SfM technique was used in this paper to monitor ravine formation along a test carried out in the laboratory on a small slope (870 cm2) made of dry sand and subjected to a controlled water jet for 12 min. A mobile phone was used to take pictures at 2, 5, and 10 min of the test. All photos were taken at a distance of 80 cm from the slope. Photos were also taken before and after the test. In total, 5 sets of 9 photographs were taken. Photogrammetry-based Eyescloud3D application was used to obtain point clouds from the images taken with the mobile phone. These were later processed with CloudCompare software to analyse the geometry evolution of the erosion channel created by the runoff water. Because of the rapidly evolving system, it was not possible to control the lighting conditions when taking photographs. As a consequence, shadows prevent a good reconstruction where the erosion channel is deeper and narrower (less than 1 cm). Finally, a TLS was used to scan the final state of the slope and compare the result with that obtained using the SfM-based application. Similar results were obtained for 80% of the length of the erosion channel. However, deficiencies in the SfM reconstruction were found in the narrow areas mentioned above.