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 connection mode between check dam and downstream channel is not considered in the evaluation of sediment interception efficiency of check dam, which may lead to the high calculation result. In this study, in order to evaluate the sediment interception efficiency of check dam more accurately, the control areas of Caoping hydrological station in Wudinghe River Basin 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 combinations of check dam were selected, the check dam types and their connection modes between check dam and downstream channel were investigated and analyzed, and the sediment interception efficiency of check dam was estimated. According to the difference of connectivity between check dams and downstream channels, 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 structure connectivity of sediment between check dam and downstream channel was evaluated. The results show that the structure connectivity of sediment 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. The results show that the sediment interception efficiency would be 9.74–18.78% higher if the connection mode is not considered. And extreme rainfall increased the sediment connectivity between check dams and downstream channels, thus greatly reduced the sediment interception efficiency of check dams.