Measuring rill erosion by laser scanning

• A Terrestrial Laser Scanner (TLS) was used for a field investigation of the rill.
• Three methods for recognizing the channel network from DEM were tested.
• Capability of TLS to detect and to map the rill formation at plot scale was assessed.
• Geometric characteristics of rill detected by the TLS and by manual were compared.
• Total eroded volume estimated by two methods (manual and TLS) showed a good agreement.

This paper reports the results of a field investigation on the rill morphology and the corresponding soil loss, using a Terrestrial Laser Scanner (TLS), at the Masse experimental station (Umbria, Central Italy). Laser scanning is a survey method that allows determining the three-dimensional position of a large number of points (point cloud) through the measurement of angles (azimuth and zenith) and distances. For this paper five scans were made of some rills, which formed during particularly erosive rain in the plots of the Masse station. The point cloud was first interpolated by the natural neighbor method to create a discrete 0.02 m × 0.02 m square cell grid Digital Elevation Model (DEM) used for mapping the flow network. The same cloud was also used to generate a continuous modeling of the surface (by the Triangulated Irregular Network model, TIN) to quantify the total eroded volume and the morphological characteristics of the rill formations. Three methods were applied to the DEM for detecting the channel network: the slope method (Horn, 1981; Wood, 1996), the constant drop method, described by Broscoe (1959) and the method of landform curvature (Tarolli et al., 2012). The comparison has shown a good agreement between the three methods, but Broscoe's method seems to be more accurate for rill recognition. The morphological characteristics of the rill formations derived by the TIN model (i.e. length, width, depth and volume) were compared with the corresponding characteristics obtained manually using a profilometer. The analysis showed a good agreement between the width at the top measured by the two methods, a general overestimation of the maximum depth and of the cross section areas and an underestimation of the rill length when the manual method is used. Lastly, the comparison between the volumes obtained by the TLS survey and by the manual method showed that the total volume calculated by the manual measurements overestimates that evaluated by the TLS, with a good agreement between these variables.