Non-Contact Measurements of Rill Geometry and Flow Characteristics Based on Photogrammetry.

Accurate measurements of rill geometry and flow parameters provide theoretical basis for erosion processes on hillslopes. Rill width and depth are basic rill morphological characteristics. Rill flow width and depth are the foundation for calculating hydraulic and hydrodynamic parameters. A soil box (2.0 m long, 0.3 m wide and 0.5 m deep) was subjected to a 140-s movable-bed scouring test under a surface inflow rate of 1 L min^-1with slope gradients of 15° and 20°.

Based on photogrammetry, point cloud and high-precision DEMs of the soil bed at different times were extracted. Measurements of rill depth and width, and rill flow depth and width were made. Variation trends of rill morphology and rill flow characteristics were analyzed and differences between photogrammetry and other measurement methods were discussed. The results showed that rill width and rill flow width could be measured directly from perpendicularly shot photographs after proportional scale calibration. Based on interpolation principal, dynamic variations of rill depth and rill flow depth could be measured. With increasing slope length, rill depth increased while rill flow depth showed no significant trend. Rill width increased and then decreased while rill flow width decreased gradually along the soil bed. Rill widths and depths with slope gradient of 20° were 1.7 and 1.3 times as those with slope gradient of 15°. Rill flow widths in different cross-sections with slope gradient of 15° were 1.7%-13.1% larger than those with slope gradient of 20°, while rill flow depths showed little difference between these two slope gradients. Rill depths and rill flow depths obtained by manual measurements with a steel ruler were 3.3%-5.1% and 91.0%-178.5% higher than those obtained by photogrammetry. Compared with traditional methods and 3D laser scanning technique, photogrammetry has speed, resolution and non-contact advantages and it can also overcome rill wall shield and prevent the occurrence of point cloud “black holes”.

Key words: photogrammetry; soil erosion; rill; movable bed;  scouring test