Sediment Connection Modes Of Check Dams And Its Effects For Sediment Interception Efficiency In The Loess Hill And Gully Region

The Loess Plateau is the area with the most serious soil erosion in china,and its control of soil erosion has always been a focus of attention and research.As an important engineering measure of soil and water conservation,the check dam plays an irreplaceable role,however,the effect of the connection mode between check dam and downstream channel is not considered in the evaluation sediment interception efficiency of check dam,which leads to the high calculation result.In this study,in order to evaluate and analyze the sediment interception efficiency of check dam in the more refined way,the control areas of Caoping hydrological station in Chabagou watershed and Ganguyi hydrological station in Yanhe River Basin were taken as study areas,and 24 typical dam-controlled watersheds with different land use types,4 typical small watersheds with different degrees of check dams'management were selected,the check dams types,connection modes between check dam and downstream channel were investigated and analyzed,and the sediment interception efficiency of check dam was estimated based on the connection degree of sediment.The main conclusions are as follows:1)There are 367 and 2258 check dams in Chabagou watershed study area and Yanhe River basin,respectively,and the number of check dams per unit area is 1.97/km~2 and 0.38/km~2,respectively.According to the difference of connectivity between check dams and downstream channels,check dams can be divided into 11 types:check dam with 100%sediment interception efficiency,check dam with water,check dam with spillway,check dam with shaft or horizontal pipe,check dam with dam body damaged,check dam with discharge,check dam with discharge and shafts,check dam with discharge and spillway,check dam with discharge,shaft and spillways,check dam with discharge and dam body damaged,Gully land consolidation project.2)There are 11 types of connection modes between check dam and downstream channel:disconnected,connected with spillway,connected with shaft or horizontal pipe,connected with spillway and shaft or spillway and horizontal pipe,connected with shaft,horizontal pipe and spillway,connected with dam body damaged gap,connected with discharge canal,connected with discharge canal and shaft,connected with discharge canal–spillway,connected with discharge canal–shaft and spillway,connected with discharge canal–dam body damaged gap.Based on the actual control area percentage of dam land,the connectivity of sediment structure between check dam and downstream channel was evaluated.The results show that the connectivity of sediment structure is mainly affected by the discharge canal and its length.The sediment interception efficiency of dam system is greater than the sum of single dams.If there is no drainage channel at the bottom of the key check dam,the sediment interception efficiency of the whole dam system will be greatly increased.3)From 2017 to 2020,the erodibility rainfall events of Chagou watershed study area ranged from 9 to 19 times,the average sediment blocking modulus ranged from 1586.27 t/km~2to 4617.10 t/km~2;From 2010-2020,the erodibility rainfall events of Yanhe River basin study area ranged from 19 to 84 times,the average sediment blocking modulus ranged from 178.08t/km~2 to 877.18 t/km~2.The ecological restoration in Yanhe River basin study area is better than that in Chabagou watershed study area,but due to the disturbance of human activities such as production roads and oil pipelines,some of the minimum sediment yield rainfall is smaller than the latter.4)According to the variation of sediment deposition thickness in the dam land,the spatial distribution types of sediment depositon in dam land can be divided into 5 categories:upstream<downstream,upstream>downstream,upstream only,downstream only and irregular.With the advancement of watershed ecological restoration,the type of spatial distribution of sediment deposition changes from upstream<downstrea?upstream>downstream?irregular?upstream only/downstream only?no sediment deposition,which is also a process of gradual decrease of sediment connectivity.Influenced by natural factors such as rainfall,topography,collapse and landslide,and human factors such as land use types,farming methods and engineering measures in the dam controlled watershed,the spatial distribution characteristics and transport distance of sediment deposition in the dam land are obviously different under different rainfall conditions.5)In the Chabagou watershed study area,the sediment interception efficiency of check dam is between 81.69–83.61%without considering the influence of connection mode,while is between 61.97–65.46%under considering the influence of connection mode.In the Yanhe River basin study area,the sediment interception efficiency of check dam is between 48.05–55.51%without considering the influence of connection mode and between 38.57–45.32%under considering the influence of connection mode.The result show that the sediment interception efficiency will be 9.74–18.78%higher if the connection mode was not donsidered.Under extreme rainstorm,the sediment interception efficiency of check dam in the Chabagou sutdy area without considering the influence of connection mode is 81.85%,while that considering the influence of connection mode is 61.80%,and the actual sediment interception efficiency of check dam under the extreme rainstorm condition is 26.23%.The sediment interception efficiency with extreme rainfall events has decreased by 35.44%compared with that with only considering the connection mode.Indicating that extreme rainfall increases the sediment connectivity between check dams and downstream channels,thus greatly reducing the sediment interception efficiency of check dams.