DEM Based Investigation Of Loess Shoulder Line

China loess plateau is world-wide famous for its peculiar and unique landscape, in which the loess shoulder lines zigzag on the surface making a distinct separation between loess positive terrain with the negative one. More evidences from both field investigation and quantificational analysis show that the shape and spatial distribution of loess shoulder line could be regarded as an effective clue to make a deep study of loess landform, especially the spatial distribution pattern of loess terrain and soil erosion. However, there are very few works was done on the classification, derivation, spatial distribution as well as geomorphological significance of loess shoulder line.A systematic and comprehensive classification to loess shoulder line is proposed for the first time based on different methods in this paper. Those classification principles include: cause of formation of shoulder line, spatial distribution pattern, configuration, amount, notability, evolution speed and the mode of expand. For example, abiding the spatial distribution pattern principle, the shoulder line could be divided into four basic types: parallel pattern, branch pattern, skew pattern, separate pattern. The classification is the cognition of loess shoulder line from different view point, and is the basis for constructing an overall concept of loess shoulder line.A DEM based shoulder line extraction method is also developed in this paper. This algorithm is mainly based on the slope configuration and filter calculation: By judging the local relief relationship cell by cell on a certain profile, with two separate adjustable thresholds, a series of shoulder line candidate points are gathered. Those separate points could be connected based on the principle of mathematical morphologic method. The result shows, based on 5 meter resolution DEM, most of the distinct shoulder line on Loess Yuan, Loess Liang, and Loess Liang-Mao landform could be well derived. Comparing with the result from 25 resolution DEM, the extracted shoulder line is more accurate.In order to describe the shoulder line quantitatively, a shoulder line shape representation index system is set up at three scale levels. They are drainage scale level, the channel scale level and the slope scale level. The drainage scale indexes describe the shape of the shoulder line on whole drainage system. The channel scale indexes are used when the information of one or more shoulder lines is demanded. The information includes not only the configuration information but also some statistical information that closely related to the local relief, like shoulder line density and elevation range. The slope scale indexes depict the shoulder line from their concrete spatial modality, i.e. amount, curvature and average elevation. Every index on each scale is calculated, and then, a comparison between different relief regions is made in this study. The results show that the calculated index varies from one study area to another and close related to the landform itself. Those indexes can depict the character of shoulder line effectively. Three most significant channel scale indexes are selected to study their spatial-and-temporal variation. The indexes, including mean slope on shoulder line, fractal dimension value and mean gradient, are calculated in 48 different study areas in northern Shannxi respectively. Those 48 study areas are located in different landform regions, i.e. Loess Liang, Loess Mao, gully area, etc. And then, a distinct spatial distribution pattern is achieved by means of a spatial interpolation of the index. Should-line's index is closely associated with loess landforms. The index of fractal dimension value representing the complexity and self-similarity of shoulder line is closely correlated to the channel network density. The mean slope on shoulder line is bigger in Loess Yuan region than in Loess Liang and gully area. However, with in the gulley area, the mean surface slope become bigger and bigger along with the erosion process, and the biggest value is achieved in Suide area, in which have the highest channel network density and the most serious erosion problem. The mean shoulder line slope can be regarded as an index of shoulder line evolution potential. Those big value areas are in Loess Liang and Loess Mao-Liang area. The result is accordant to the relief developing law.A physical experiment is made to simulation the development of loess shoulder line temporally. A 35m×26m loess watershed is build in experiment hall. After nine times carefully designed manmade rainfall, the development of landform and shoulder line of the watershed is revealed. The whole process is recorded by high-precision DEMs of 10 millimeter resolution by indoor photogrammetry method. This provides the possibility to study the shoulder line variation in erosion process. The average slope gradient becomes steeper along with the erosion and the channel development in every DEM. The average elevation on the shoulder position is bigger after each rainfall process. The study of the process gives an even deeper and comprehensive understanding to the development of loess shoulder line. Furthermore a relationship between shoulder line index and soil erosion is constructed, which is of great significance in both theoretical study and practical application.