Environmental Effect Of Warp Land Dam Project In Loess Plateau

Warp land dam is an effective water conservancy project to control soil erosion in hilly and gully region of Loess Plateau。By the end of 2015,more than 110,000 warp land dams of various types had been built in the Loess Plateau,and the measured sediment transport capacity of Tongguan Hydrologic Station in the Yellow River decreased from 1.592 billion tons per year from 1919-1959 to 55 million tons in 2015.The sediment reduction of warp land dams accounted for about 20%of the total sediment reduction of water and soil conservation measures.At the same time,the artificial plain formed by the siltation dam was moist and fertile,strong in drought resistance and high in yield,with an increase of 3～10 times over the slope However,warp land dams have changed the natural topography and waterflow conditions,and the effects of unsaturated loess engineering properties,water circulation and salt accumulation,dam collapse and slope instability caused of warp land dams have caused great losses to people’s lives and property.The main findings are as follows1.The unsaturated characteristic effect of loess caused by warp land dam project is remarkable.To be compared to undisturbed soil,backfill soil’s construction has been reconstituted.Unsaturated Characteristics’changes of Loess manifested in the following areas Under the Drying condition,suction stress of undisturbed soil reach 2114.471 kPa when moisture content is 12%,but when the moisture content of backfill soil is 12%,the suction stress is only 2.52 kPa.During Drying process,the largest suction stress of backfill soil is1958.463kPa(Volume moisture θ=1.8%);Under the Wetting condition,suction stress of undisturbed soil is 586.94 kPa when moisture content is 12%,but when the moisture content of backfill soil is 12%,the suction stress is only 2.52 kPa.During Wetting process,the largest suction stress of backfill soil is 492.75kPa(Volume moisture θ=1.8%).When the Volume moisture content is similar,in drying conditions,saturation permeability coefficient of backfill soil is3.1e-4 cm/s,and 1.72e-7 cm/s of undisturbed soil,and in Wetting conditions,saturation permeability coefficient of backfill soil is2.13e-4 cm/s,and 5.14e-8 cm/s of undisturbed soil During Drying process,the air-entry pressure of undisturbed soil is 90.91kPa,and the backfill soil’s air-entry pressure is 5.46kPa;During Wetting process,the air-entry pressure of undisturbed soil is19.23kPa,and the backfill soil’s air-entry pressure is only 2.45kPa.Hysteresis of undisturbed soil is 71.68kPa,and backfill soil’s hysteresis is only 3.01kPa2.The effect of soil salinization is obvious due to the change of water circulation in local area of warp land dam system.The warp land dam project of Gu Tun basin has changed the topography,geomorphology and hydrogeological structure of the river channel,and reconstructed the local water circulation system of surface water and groundwater.Under natural conditions,the buried depth of groundwater level is large,and the phenomenon of soil salinization is difficult to occur.After the construction of warp land dam upstream perennial water storage or formation of reservoirs,groundwater’s level became higher and higher,and backfill soil capillary rise height of 1.346 m,resulting in soil salinization effect3.There is a high risk of collapse in silt dam.Through field investigation,ground geophysical exploration and numerical simulation,it is considered that the leakage problem at the junction of the east side of the dam and the trafficway is more serious,and there is a high risk of collapse in the warp land dam,and the causes of the risk of collapse are the poor drainage in the dam and the high groundwater level4.Anti-seepage reinforcement of dam and Strengthening drainage are effective ways to prevent and control the risk of dam collapse and soil salinization.Firstly,the original drainage corridor and drainage ditch on the west side of the dam body are desilted to ensure the smooth drainage;secondly,in view of the present situation of no drainage measures on the east side of the dam body,in addition to adding corresponding drainage corridor and drainage ditch in the lower part of the east side of the dam body,the culvert should be arranged horizontally to connect with the drainage ditch,so that the water in the backfill soil layer can be discharged smoothly,the groundwater level of the dam ground is reduced,the soil salinization is controlled and the safety of the dam body is improved;thirdly,use comprehensive measures to ameliorate and prevent soil salinization.For example,to turn over tillage before the rainy season,reduce the rising height of water salt,cover straw to restrain water evaporation,and improve the quality ratio of soil organic matter to improve plant growth and so on.