Study On The Soil Erosion Of Towns In Yanting County, Sichuan Province

China is one of the most serious soil erosion countries in the world. There are not only various forms and types of soil erosion in China, also the large area and great intensity. Particularly, the mountainous areas and hilly region at all levels of urban (city) which accounting for more than two thirds of the land area have a comprehensive mountain and urban characteristics of the soil erosion, both natural erosion and manmade erosion, also existing soil erosion of slope lands and development construction. The development construction destroys vegetation, damages soil conservation facilities, causing a large number of exposed earth and loose deposits which becomes the new source of soil erosion and water loss. Reasonable soil and water conservation measures should be targeted to different forms of erosion. This paper has studied the soil erosion of the mountainous towns and the sediment retaining effects of the ditches through field monitoring and simulate rainfall, the main results are as follows:(1) The underlay of the settlements and earth road mainly is impermeable layer, surface runoff and conflux emerge fast, becoming the main source of sediment yield. Through leading surface to field, building water holding constructions and so on measures to control conflux area, speed and flux, and aims to control soil erosion and water loss.(2) The tiptop of soil erosion yield could be 487003.54 t/ km~2·a, under the condition of excavated slope in 60°- 70°.exposed area is more than 80%,and the vegetation along slope surrounding 2-5m has been damaged, the loose soil seriously erosion. The average soil bulk density of project plots and natural plots were 1.664 g/cm~3,1.498 g/cm~3.Because of different slope hydrological characteristics, the slope rock and soil structure, the soil erosion is various.(3) The soil and water conservation effects of fill slopes was: crop cultivation area> weed coverage area> filling the new district> exposed area. The water holding ability of indurated plots is o, without soil erosion. The crop covered slope was the best design to the actual soil and water conservation, also benefit masses.(4) Loose deposits were stacked around sloping ditches, road ditches and river, without any soil and water conservation measures or consolidating, the soil bulk density is 0.82 g/cm~3- 1.25 g/cm~3,slope between 38°-42°,its soil erosion was very serious. The early soil bulk density of dumping deposits(0.82 g/cm~3) is only about 50% contrary to the soil bulk density ,1.70 g/cm~3,before disturbance. The surface soil bulk density of loose deposits is about 10% larger than that of slope. Some native vine roots and herbs are developed, barren resistance, drought tolerance, could grow on suspend rocks, steep slopes or even uncovered rocks. They have a good soil and water conservation effect to loose deposits soil during projects construction. (5) It was the intensive rainfall that caused the main soil erosion and water loss on urban slopelands, surface initial runoff time was speed up by the rainfall intensity, the rainfall intensity was the main factor influence low slope runoff., the relationship between surface accumulation runoff and rainfall intensity and slope gradient is direct ratio. Soil midstream accumulation runoff was less as the slope and rainfall intensity increase. At the same rainfall duration, rainfall was the main factor influencing sediment yield as to slope gradient. At the same rainfall intensity, sediment yield increased with the increasing of slope.(6) In the low rainfall intensity, small flow conditions, with the retaining and adsorption of ditches, TDS decreased 20% to 45%, 25% to 80% of the sediment was storing when in strong rainfall intensity, large flow, the ditches to TDS and sediment was ineffective. TDS content value was relatively high in the runoff 0-20min; the largest sediment content was about runoff after 10 min, and the 30 min sediment content was high after initial runoff. During this time, diffluencing and storing this part of runoff, after other treatment process then let, could reach protecting the water resources purposes.