Measuring rill erosion using structure from motion: A plot experiment

In this paper the results of a plot experiment on rill erosion are reported. The rill network, manually incised on the soil and further shaped by a clear inflow discharge, was surveyed using the three-dimensional photo reconstruction (3D-PR) technique which allows to obtain a digital terrain model (DTM) by a large series of oblique images of the channel from consumer un-calibrated and non-metric cameras. The three-dimensional (3D) DTM and the quasi-tridimensional (2.5D) model were generated by Agisoft Photoscan software. For a single rill channel, the reliability of the 3D image-based ground measurements of morphological and hydraulic variables was positively tested by the corresponding measurements carried out on the rill gypsum cast. Moreover, the morphological and hydraulic variables were measured at 11 transects of the plot by a profilometer P, the 2.5D and 3D models. Using the 3D model as reference, the analysis showed that the reliability of the measurements by P and 2.5D methods was comparable and the error was generally lower than 15%. The rill measurements carried out by the three methods agreed with the available literature measurements and supported the applicability of both the empirical relationship between rill length and its eroded volume and the theoretical dimensionless relationship among the morphological variables describing the channelized erosion process. The analysis also showed that the effect of the distance between two consecutive cross sections on the rill volume measurement by 3D model is negligible. Finally, the shaping effect of different flow discharges on three straight rills incised into the plot was tested. In particular, three increasing flow discharges (0.15, 0.35 and 0.5 1 s(-1)) were used sequentially in two rills while the maximum discharge of the sequence (0.51 s(-1)) was used into the third rill. Both the grain-size distribution of the eroded sediment and the total amount of soil loss were affected by the discharge sequence.