Active Gully Morphological Characteristics And Transformation Models In The Loessial Hilly-gully Region

Research on active gullies morphological characteristics have important significance to reveal erosion processes and provide theoretical basis for erosion prediction model and erosion prevention. Current high-resolution DEM can be used to accurately extract gully morphological characteristic indicators, but it is difficult to obtain, while DEMs easy to obtain cannot meet the requirement to extract gully morphological characteristics accurately. This study randomly selected 31 typical gullies at different watershed locations and gully orders in Chabagou watershed. The extraction method of gully morphological characteristic indicators was established based on 0.1 m high resolution DEMs extracted from 3-D laser scanning technique and 5 m low resolution DEMs; gully spatial distribution was analyzed; gully volume estimation models were established; transform models of gully morphological characteristic indicators between high and low resolution DEMs were established and the universality of the transformation models was validated. The main results were as follows:(1) The accuracy of the technology was discussed compared with manual measurement method. The results indicated that gully morphological characteristic indicators extracted from 0.1 m resolution DEM and manual measurement had no significant difference, indicating 0.1 m resolution DEM could be used to extract gully morphological characteristic indicators accurately. In addition, 3-D laser scanning technology make up for the deficiency of manual measurement method, which was dangerous and difficult to measure in the field.(2) The extraction method of gully morphological characteristic indicators based on stream network extraction theory in the ArcGIS was proposed. And the differences between 0.1 m high resolution DEMs extracted from 3-D laser scanning technique and 5 m low resolution DEMs were analyzed. The results showed that gully length extracted from 0.1 m resolution DEM had no significant difference with that extracted from 5 m resolution DEM, while gully width, depth, surface area and volume extracted from 0.1 m resolution DEM had significant difference with those extracted from 5 m resolution DEM. Thus, it is necessary to establish transformation model of gully morphology characteristics extracted from 5 m resolution DEM based on 0.1 m resolution DEM.(3) The gully spatial distribution in the study area was analyzed and gully volume estimation models were established. Gully fundamental morphological characteristics covering gully length, gully width and gully depth and five derivative morphological indicators including gully head curvature, gully width depth ratio, gully bottom to top width ratio, gully surface area and gully volume were extracted to analyze the spatial distribution of gully morphological characteristics. The results showed that active gully length were between 50~70 m, gully width were between 10~20 m, and gully depth were between 5~10 m. Gully length decreased gradually from upstream to downstream in the study watershed and gully depth was greater in the downstream of watershed. Gully bottom to top width ratio and gully width depth ratio decreased gradually from upstream to downstream, indicating that U-shaped cross-sections were widely distributed in the upstream of watershed and upper position of a gully; while V-shaped cross-sections were widely distributed in the downstream of watershed and lower position of a gully. Gully cross-section shapes at gully scale and watershed scale were similar, namely gully cross-sections had self-similarity. The Pearson correlation analysis among gully morphological characteristic indicators showed gully length, width, depth and gully surface area had a very significant correlation with gully volume. Thus, gully volume estimation models based on gully length and surface area were established which can be relatively simple to estimate gully erosion amount.(4) The transform models of gully morphological characteristic indicators between 0.1 m resolution and 5 m resolution DEMs were established. The model validation results showed that the correlation coefficient of gully width, surface area and volume transform model were all greater than 0.6 and effectiveness coefficient were all greater than 0.5, indicating that the accuracy of three transform models were all acceptable. To test the universality of the model, three models were validated gullies in the Zhifanggou watershed, and results showed that the accuracies of gully width and surface area transform models were all acceptable.