Using 3D scanner to study gully evolution and its hydrological analysis in the deep weathering of southern China

This study integrated 3D scanner measurement techniques with hydrological analysis to study the evolution of gullies developed in the deep weathering (granite) in the subtropical climate of southern China. Two gullies were repeatedly measured using a 3D laser scanner over a 7.5-year period from 2009 to 2016. The measurements with sub-centimeter accuracy indicated that gully head retreat was the dominant process in the gully development, with a mean rate of 0.46 and 1.10 m yr(-1) for gully A and B respectively. To explore the relationship between gully head retreat (GHR) and hydrologic variables, rainfall with three different thresholds, rainfall erosivity and runoff depth were calculated using different methods. Statistical analysis showed a significant correlation between GHR rates and cumulative erosive rainfall depth with a daily rainfall threshold > 25 mm. Hydrological analysis and field observations suggested that gully head retreats mainly result from mass movement triggered by gravity and soil saturation, although water erosion might also play a role in destabilizing the already unstable gully slopes. This study has implications for restoration of gullies in the area, which requires techniques that go beyond a mere reduction of runoff contribution to gully systems. Further studies are needed to gather more data on the evolution and sediment delivery of gullies, to prove the hypothesis of gravity-driven gully headcut development, and to explore more effective gully restoration techniques.