# Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process
## 4 Conclusions  
The study was mainly focused on the roles of the aboveground parts of grass strips (green grass, withered grass, and the soil bed surface) in the sediment-trapping process and temporal changes during the sediment-trapping process. The main conclusions are shown below.  
1. The aboveground parts of a grass strip can significantly affect the overland flow regime. The slope of the grass strip used in the experiment was \(5^{\circ}\). At a flow rate of 30 L min\({}^{-1}\) m\({}^{-1}\) and a sediment concentration of 120 g L\({}^{-1}\), a continuous and high instantaneous sediment-trapping efficiency segment occurred in the initial stage of the complete grass strip experiments. The results indicated that overland flow in the three treatments involved transitional flow and turbulent flow but not laminar flow. Overland flow in the CG (complete grass strip) and RG (green grass removed) group experiments was always tranquil, but critical and supercritical flow occurred in the RGW (green and withered grass removed) group experiments.
2. The resistance factor f increased by 0.397 per kilogram of green grass biomass and by 0.418 per kilogram of witnessed grass biomass (1.05 times higher than f for green grass). The green and withered grass biomass generally determined the resistance to overland flow. The resistances of the aboveground parts of the grass strips to overland flow effectively determined the contributions of the parts to sediment trapping.  
Figure 8: Contributions of the structural components of the aboveground parts of the grass strips to the cumulative amount of sediment trapped over different periods.  
3. The soil bed surface played a major role in the initial stage of the sediment-trapping process. As the sediment-trapping process continued, green grass became a more important contributor to sediment trapping. In the initial sediment-trapping stage (the first 5 min), the soil bed surface contributed ~90% of the cumulative amount of sediment trapped \(R(t)\). In the stable stage, the contributions of green grass, witnessed grass, and the soil bed surface to \(R(t)\) were ~50%, ~20%, and ~30%, respectively. Therefore, more attention should be paid to the role of soil bed surface if local rainfall occurs mainly for short periods. The layout of grass strips and modification of the microtopography can be combined to increase the sediment-trapping performance. For areas with long-duration rainfall, attention should be paid to improving and maintaining grass coverage so grass strips can effectively trap sediment. The vegetation module in some water-sediment models should be designed to fully consider the effects of different aboveground part compositions in the vegetation area.  
These results are helpful to improve the ability to accurately assess the effects of vegetation restoration on sediment transport at the hillslope and watershed scales.  
Conceptualization, M.L. and C.P.; methodology, M.L. and C.P.; software, M.L.; validation, M.L., Y.C. and Y.W.; formal analysis, M.L.; investigation, M.L., Y.C. and Y.W.; resources, M.L. and Y.G.; data curation, M.L.; writing--original draft preparation, M.L.; writing--review and editing, M.L. and C.P.; visualization, M.L.; supervision, M.L.; project administration, C.P.; funding acquisition, C.P. All authors have read and agreed to the published version of the manuscript.  
This research was jointly funded by the National Natural Science Foundation of China Project (41771305, 42077059) and the National Key R&D Program of China (2017 YFC0504702).  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors wish to thank all of the technicians involved in field and laboratory work.  
The authors declare no conflict of interest.