关键词: 斜坡单元, 划分方法, 平均曲率, 流域分割, ArcGIS

Abstract:

In recent years, slope units have been widely utilized in landslide susceptibility mapping and geological hazard assessment. Slope units automatically derived from high-quality DTMs, partition the territory into hydrological regions between drainage and divide lines. The division method of slope units conducts subwatershed segmentation on elevation and the reversal to extract ridge lines and valley lines, then overlays these terrain feature lines. However, these resultant units are unable to match with geomorphology background where intermountain basins or large open valleys exist. In this article, according to the basic morphometic system of elevation and the derived variables, slope is derivative of elevation while curvature is that of slope, thus the break of slope should be indicated by curvature instead of elevation. The disability of indicating variation of slope is regarded as the main reason for the limitation in the conventional method. According to the theory of watershed segmentation of mean curvature, the division method of slope units is improved by overlaying watershed boundaries on curvature and the reversal in ArcGIS. Firstly, the DEM should be smoothed twice with a 5×5 average filter in order to reduce the effect of noise and small scale variation in the DEM. The curvature is then calculated from the filtered DEM using the Arctool named Curvature. After that, watershed boundaries are generated mainly through computing flow direction, scouting sink and dividing watershed in the curvature image, and this image should be reversed for watershed boundaries on the reversal curvature. Finally, slope units can be separated from each other by these two kinds of boundaries which are further combined into the boundaries of slope units. Giving an example from Huachi County of Gansu Province, elevation layer in the resolution of 20 m was used as input data, and both two methods were then applied for slope unit division. Compared with the conventional method, the revised approach not only uses ridge lines and valley lines to segment slope units, but also utilizes tableland boundaries and open valley boundaries to separate horizontal surface from the inclined. The revised approach may give a new definition that slope units should be divided by ridge lines, valley lines, tableland boundaries and open valley boundaries. Also the revised method utilizes watershed segmentation on curvature instead of subwatershed division on DEM where filling sinks perhaps causes the serious change of reversal elevation. Furthermore, the resultant units have relatively uniform size and shape, more than 80% of them with the area from 0.1×10⁴ to 6×10⁴m² and about 60% between circle and triangle shape, which are more suitable for landslide hazard evaluation. It is worth mentioning that the revised method would be more competitive than the original in some regions with large area of horizontal surface, just like Loess Tableland and reservoir surface.

Key words: slope units, division method, mean curvature, watershed segmentation, ArcGIS