Testing the use of an image-based technique to measure gully erosion at Sparacia experimental area

The first part of this investigation was aimed at testing the use of a three-dimensional (3D) Digital Terrain Model (DTM) and a quasi-tridimensional (2.5D) Digital Elevation Model (DEM) obtained by a large series of oblique images of eroded channels taken from consumer un-calibrated and non-metric cameras. For two closed earth channels having a different sinuosity the ground measurement of some cross-sections by a profilometer (P) was carried out. The real volume of each channel was also measured by waterproofing it by a plastic film and filling it with a known volume of water. The comparison among the three methods (3D, 2.5D and P) pointed out that a limited underestimation of the total volume always occurs and that the 3D method is characterized by the minimum difference between measured and real volume.This preliminary analysis allowed to conclude that 3D model can be used as benchmark and the 2.5D model is accurate enough to describe the investigated earth channels. In the subsequent part of the investigation the three ground measurement methods were applied for surveying of an ephemeral gully (EG) channel at the Sparacia area. Using the 3D model as benchmark, the morphological and hydraulic variable values of the 24 surveyed cross-sections determined by both 2.5D model and profilometer were compared. This comparison showed that the estimate error is generally less than ±10%. The EG measurements carried out by the three methods agreed with the available literature measurements and supported the applicability both of the empirical relationship between EG length and its eroded volume and the theoretical dimensionless relationship among the morphological variables describing the channelized erosion process. Finally, it was demonstrated that the effect of the distance interval on the EG volume measurement by 3D and 2.5D models is negligible for the investigated EG.