Temporal-spatial analyses of collapsed gully erosion based on three-dimensional laser scanning

Abstract: Three-dimensional laser scanner (Leica ScanStation 2) was used to measure Liantanggang collapsing hill and erosional gully in Wuhua County of Guangdong Province for 6 times during 2011-2013. About half a year as an interval, and through ArcGIS, high-solution analyses of measurements was conducted to study the spatial and temporal variations of the erosional processes and sediment yield in the watershed of Liantanggang. The average erosional volume was 499 m3 in rainy season, and was 291 m3 in dry season. During the 3 years measurement, the average annual erosional amount was 833 m3 and the erosional modulus was 222 408 t/(km2·a). The heavy rainstorm more than 100 mm within 24h greatly influenced the collapses. Rainfall of rainstorm during each measuring period was positively correlated with the erosional amount. The intense erosional zones located on the colluvial cones at the elevations between 111-131 m, accounting for 55.6% of the total erosional amount, which indicates that the erosional rate in the middle-lower parts of the watershed is the greatest. The slopes of 40°-50° was the largest erosional amount, then the slopes of 50°-60°, and both them accounting for 49% of the total erosional amount of 1 019 m3. The sediment yield per square meter increased with the gradient, which means the erosional processes are more intense in steeper slopes. The average maximum erosional amount per square meter was located on the slopes of 70°-80°. The average erosional depth for each measuring period was 0.105 m. During the period of all the measurements, the severe erosion was found in the main and branch channels including their sides, and at the gully heads. The accumulative erosional depth was greater than 1 m, and the maximum depth was more than 2.5 m. Within the period of five measurements, the erosional modes showed as in the first 3 measuring periods, rapid down-cutting, lateral and retreated erosions were the main modes. In the last 2 measuring periods, there were dominantly gravitational erosions with small-scale landslips. The total erosional amount firstly declined and then increased. The watershed now is in its maturity stage. Morphometric terrain may be the major factor affecting its current evolution. Topographic condition may lead to the transformation between gravitational erosion and hydraulic erosion. The combination of gravitational and hydraulic erosions and their alternations may make the erosional volumes to be fluctuating changes.