Synergistic effects of terraces and check dams on runoff and sediment yields in a slope-gully system in Loess Plateau

A slope-gully system is one of the pivotal units in the primary sediment source area for soil erosion in the watersheds. It is very necessary to restore and rebuild the ecological environment in the Loess Plateau. Among them, both terraces and check dams can be expected as the ecological engineering solutions to effectively control the soil erosion in slope-gully systems. However, only the individual measure has been primarily assessed to reduce the runoff and sediment yields. Consequently, this study aims to quantify the synergistic effects of the integrated configuration of terraces and check dams on the runoff and sediment yield. A series of physical models were established in the slope-gully systems. Three types were divided, including a control group (No measures, CO), a single measure group: terraced measure (T0) and four less length in the slope-gully area that caused by sedimentation from check dam (L1, L2, L3, and L4), as well as a multi-measures group: a combination of terraces and four less length in the slope-gully area that caused by check dam sedimentation (T1, T2, T3, and T4). The simulated rainfall experiments were conducted with a designed rainfall intensity of 90 mm/h and a rainfall duration of 30 min. The runoff and sediment samples were also collected every minute during the soil erosion in the slope-gully system. A systematic analysis was finally made on the variation process of runoff and sediment yields in the simulated rainfall experiments. The results show that the terracing measures decreased the runoff yields by 46.30%-83.59%, and the sediment yields by 25.82%-82.41%, respectively. The increasing siltation depth of check dams was reduced the runoff yields by 7.89% to 33.42%, and the sediment yields by 10.20% to 30.57%, respectively. The combination of terraces and check dams presented a synergistic effect to reduce both runoff and sediment concentration. The magnitude of this synergistic effect increased significantly, as the deposition depths of check dams increased. Specifically, the synergistic effects on the runoff were ranked as follows: 4 m (3.87%)>3 m (3.41%)>2 m (2.63%)>1 m (2.43%). Similarly, the corresponding synergistic effects on the sediment concentration were ranked as follows: 4 m (26.01%)>3 m (24.32%)>2 m (22.54%)>1 m (21.27%). The integrated configuration of terracing and check dams shared a synergistic effect greater than the sum of its individual components. Therefore, it was crucial to follow the principle of integrated treatment for the slopes and gullies, in order to effectively implement the soil and water conservation measures. The synergistic effects were also leveraged to reduce the runoff and sediment yields under the combination of slope and gully measures. There was the decrease in the terraces on slopes and the siltation-induced runoff path length of check dams in gullies. A synergistic effect was also found on the water erosion in a slope-gully system. Anyway, the synergistic effects should be incorporated to develop the soil and water conservation regulations, even to consider in the soil erosion models. This finding can also provide the theoretical reference and scientific support for the scientific allocation of water and soil conservation planning in the slope-gully system on the Loess Plateau.