Sediment-reducing benefits by runoff regulation under engineering measures in steep slope of abandoned soil deposits in Chinese loessial region

It is of significant sense to quantitatively distinguish the different role of decreasing runoff and changing flow-sediment relationship in sediment control, which will be helpful to profoundly understand the soil and water conservation benefits of runoff regulation measures. In order to explore the water-sediment effects and associated control efficiency of different engineering measures and their combinations in regulating runoff yield and sediment export from the deposit slope, a series of simulated runoff scouring experiments under the control of different engineering measures were conducted on a steep deposit slope (36°) along the Shenmu-Fugu Expressway in the loessial region of China. Several types of traditional engineering measures were designed in the study, among which individual measures included level terraces, level trenches, and fish-scale pits. In order to optimize the layouts of engineering measures on the slope, combined measures of level terraces + level trenches, level trenches + fish-scale pits, and level trenches+ fish-scale pits were also designed. Closed runoff plots with a size of 2.5×12 m2 were established on the deposit slope, clean water was added at the top of the plot with an inflow rate of 25 L/min to simulate surface runoff processes. Runoff and sediment was funneled through a PVC tube down to a bucket with a radius of 0.49 m and a height of 1 m. Runoff and sediment samples from the plots were collected by were taken using1000 mL plastic bottles every 1 min in initial 3 min of each run and then every 2 min for the rest of the duration, for determining sediment concentration. Therefore, the runoff and sediment process was calculated under different scenarios. The results showed that slope runoff and erosion were well controlled by engineering measures, and the control ratio of runoff commencement time, runoff, sediment export, and mean sediment concentration varied from 2 to 20, 0.45 to 0.78, 0.20 to 0.59, and 0.38 to 0.79 under different situations, respectively. Overall, sediment reduction induced by decreasing runoff was the main cause for sediment reduction benefits by engineering measures. However, sediment reduction induced by flow-sediment relationship changes was highly dependent on the behavior of decreasing runoff in reducing sediment output. For level terrace-based measures, sediment reduction induced by flow-sediment relationship changes was positively and linearly correlated to sediment reduction induced by decreasing runoff. The sediment regulation effects of altering flow-sediment relationship initiated, when sediment reduction induced by decreasing runoff exceeded a certain threshold. For level trench-based measures, there was a quadratic function relationship between sediment reduction induced by flow-sediment relationship changes and sediment reduction induced by decreasing runoff. Furthermore, a climax existed for sediment reduction induced by flow-sediment changes. In terms of sediment control, the level trench-based measures were more efficient than level terrace-based measures. In the case of the combination of level trench and fish-scale pit, the potential of decreasing runoff and altering flow-sediment relationship in reducing sediment output was well exerted with the efficiency coefficient being 55%. It indicated that relative equilibrium between the two causes for sediment reduction was kept at a high level under this combination. Therefore, the application of different engineering measures and their combinations should be considered based on specific soil and water conservation benefits and erosion control goals. The study provides theoretical references for the optimization of engineering measure in soil and water conservation management of steep deposit slope.