| Transforming slope into terrace is one of the most important soil erosion measures in the loess plateau region.It is also an inevitable choice to guarantee agricultural production and food security.However,due to the frequent occurrence of extreme rainfall,especially in July 2013,there was a a thousand year frequency severe rainstorm in Yan watershed,resulting in a large scale collapse of the terrace.Aiming at the phenomenon,the terraces in the loess area are studied,combined with the remote sensing image data,five hydrological stations in Yan watershed,25 rainfall stations and field soil samplesas in this paper.Using the Kring method to analyze the rainfall characteristics of Yan watershed in July 2013.The results of the study can help to investigate the effects of rainfall on collapse and provide scientific theoretical basis for the protection of terraced fields in the loess area.The main conclusions are as follows:(1)The damage rate of Yangou watershed is 69.87% and the terraced fields collapsed seriously.Ridge,different surface width and different land utilization have different ability against storm.The field ridge with a height greater than 10 cm is stronger than that in the field with less than 10 cm.Terraced fields with a width of more than 5m are more powerful than narrow terraces with a width of less than 5m.The order of different land utilization to withstand severe storm ability is,maize >miscellaneous cereals> fruit trees.(2)The laboratory test showed that under the condition of the rainfall intensity of 150mm/h and the rainfall of 750 mm,the collapse type of the terraced slope was shallow.There are five stages in the course of the rainfall along the terrace slopes.In turn,the five stages are respectively surface erosion,water of field surface,floodwater flows over fields and forms drop sill,penetration of drop sill to form erosion gully and overall collapse.As the rainfall continued,the infiltration rate of terraced slopes decreased from 78.9% to 25%,moisture content increased from 7.86% to 36.3%,pore water pressure increased from 0 Kpa to 3.6Kpa,soil suction force decreased from 14.6Kpa to 7.4Kpa.At the end of the rainfall,the displacement to the medium of slope,the oe of slope and the top of slope decreased to 15.5cm,6cm and 3.2cm respectively.(3)The numerical simulation results show that the pore water pressure,effective stress and displacement of the interior soil in the terraced field are positively correlated with rainfall intensity and rainfall.Compared with the three rainfall intensity(30mm/d,90mm/d and 150mm/d),it is known that the greater the rainfall intensity,the faster the soil pore water pressure,the effective stress and the displacement increase in the terraced slope when the rain is at the same time.The influence depth is 0~3m,and only when the rainfall duration is longer,the change is more obvious.(4)The minimum safety factor of slope and terraced field has the same tendency with the rainfall.Both decrease with the increasing of rainfall and the slope shows better sTableility.Under the conditions of 30mm/d,90mm/d and 150mm/d,the slope protection against rainfall is 20 d,11d and 9d respectively,and the terrace to resist rainfall time is 17 d,9 d and 5 d respectively.The reason is that the terraced field increases the infiltration area and single-stage terrace slope... |
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