Suitability Analysis On Spatial Differentiation Of Gully Controlling Engineering In Typical Regions Of Loess Plateau

The serious loss of soil and water in the Loess Plateau has not only caused the reduction of land nutrition and cultivated land resources,giving rise to the decrease of grain output,but also affects the efficient use of water and soil resource.The deterioration of the ecological environment is therefore exacerbated.The channel part is the both the main source and main outlet of soil erosion and sediment.Meanwhile,due to the advantages of convenient transportation and abundant water resources,this part is more suitable for the agricultural cultivation.In order to effectively control soil erosion to increase the available cultivated land area,since the 1950 s,the Loess Plateau has carried out large-scale channel treatment projects,of which the silt dam project has achieved significant benefits.In this project,a silt dam at a suitable location in the channel is constructed so that the long-term sediment in the upstream of the dam can form available arable land under natural condition.However,the sedimentation period is pretty long.Since 2009,in the areas around Yan’an City,Shaanxi Province,a new pilot project for channel management has been carried out to improve the canals and ditches by manual excavation so as to rapidly increase the area of cultivated land and grain output.The land reclamation project comprises dam land construction,dilapidated dam reparation,and deserted gully land remediation,in which the dam land construction is similar to the traditional silt dam construction.Both the design of the channel system and the construction of the silt dam are fairly large-scale engineering projects.So,how to choose the proper site before building a dam? And how to assess the suitability of dam construction for terrain conditions in a particular area? Besides,how to evaluate the effect of natural siltation and the benefits of dam construction projects in different landform types in the Loess Hilly Region? Further research is needed for all the problems above.In accordance with the problems,this paper starts from the perspective of terrain analysis.On the basis of relevant methods for digital terrain analysis and the construction idea of silt dam,the topographic data of small watersheds in different landform types can be obtained to establish a digital channel model.And typical channel shape indicators can be extracted,including slope,catchment area,channel ratio drop,area-elevation integral value,etc.Further analysis is conducted to reveal the relationship between the indicators and small dam construction.In this article,the GIS secondary development technology has been used to simulate the trench treatment process of the terrain,in which the silt/land area and storage capacity under different dam height conditions are calculated.Thereby,the dam height-area-storage relationship curve is established and used to analyze the spatial differentiation characteristics in different geomorphological types and the suitability of trench treatment projects in different geomorphological types.The main results are shown below:(1)The method in this research is based on the secondary development technology of GIS.The channel governance terrain simulation system developed by this method can be used for quick simulation of the channel treatment engineering process and acquisition of watershed dam height-area-reservoir relationship curve.Compared with traditional methods such as field survey or estimation,computer simulation has the advantages of fast calculation speed and high precision of results.(2)There are obvious spatial differences in topographic indicators related to trench management projects in different geomorphological types.The average and distribution of slope of Hill Area Ⅰ and Hill Area Ⅱ are similar.The geomorphological development is in mutual stage and therefore the channel ratio is small.The average slope of the Hill Area Ⅲ is slightly smaller than that of the Hill Area Ⅰ and II,but larger than Hill Area Ⅳ and V.The slopes distribution mainly lies in 15~25°.The development stage of the hill is mutual but closer to infancy.The gradient of the channel is greater than 10%.There are similarities in the slope distribution of Hill Area Ⅳ and Hill Area Ⅴ.The slope of the valley is small,while the valley is wide,which means the geomorphology is well developed and transit from maturity to the old age.Even some small watersheds have already been in the old age,where the channel ratio is low.These topographical features will affect the suitability and engineering design of trench treatment engineering.(3)The dam height-area-tolerance relationship curves of small watersheds with different geomorphological types are significantly different.Under the same dam height conditions,the area and storage capacity of each plot reveals the relation: Hill Area Ⅴ > Hill Area Ⅳ > Hill Area Ⅰ > Hill Area Ⅱ > Hill Area Ⅲ.The terrain conditions in Hill Area Ⅰ and Hill Area Ⅱ meet the requirements of dam construction.And due to the small amount of dam construction,it can reach a certain scale in silt/land construction and suitable for trench engineering treatment.The topographic conditions of Hill Area Ⅳ and V are basically in line with the dam-building conditions.The channel topography is gentle,but the water outlet of the basin is wide,so the amount of dam body engineering work is too much,which could be solved by considering trim of the trench.The channel ratio of the Hill Area Ⅲ is more than 10%.According to the higharea-storage relationship curve of the dam foundation,the silt/land area in such case is the smallest.Therefore,the construction of the silt dam should be cautious.