# Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process
## 2 Materials and methods  
### Experimental Apparatus and Treatments  
A variable-slope iron flume 10 m long, 1 m wide, and 0.5 m high was used in the experiment, and the flume is artificially planted with grass strips (_Setaria viridis_ (L.) _Beaw._). The flume is shown in Figure 1a. The flume was equipped with water-sediment supply and mixing devices. The experimental sediment was collected from Jixian, Shanxi Province, China. The sediment was typical loess from the Loess Plateau [24], with its physical properties shown in Table 1. The grassland has undergone natural flourishing and widening for five years, and its soil surface is eroded little by the experimental flow rate. The experimental treatments were complete grass strips (CG group), remove green grass and retain withered grass (RG group), and remove all green and withered grass (RGW group). The flume was used at a slope of 5\({}^{\circ}\) with a flow rate of 30 L min\({}^{-1}\) m\({}^{-1}\) and a sediment concentration of 120 g L\({}^{-1}\). Three repeated experiments were conducted for each treatment group. Photographs of the flume prepared for each experiment group are shown in Figure 1b-d, and the experiment group details are shown in Table 2. The three independent repeated experiments for each experimental treatment group were labeled 1, 2, and 3, and they were completely corresponding and fixed in this article. Therefore, the experimental codes were CG1, CG2, CG3, RG1, RG2, RG3, RGW1, RGW2, and RGW3, respectively. Based on the results of our previous experimental study [22], the approximate time required to reach a stable state under the various test conditions was estimated, and the estimated duration was extended appropriately for the formal experiments to ensure that the stable state of sediment trapping by the grass strips could be achieved, and the whole process of sediment trapping can be monitored.  
Figure 1: Photographs of the experimental setup and the three experimental treatments. (**a**) Variable-slope iron flume; (**b**) Completed grass strips; (**c**) Remove green grass; (**d**) Remove green and withered grass.  
The grass strip was sprinkled evenly with water before an experiment was performed until runoff occurred to eliminate differences in soil moisture and infiltration between the different experiments. The flow velocity was measured at five equal intervals and 1 m long sections along the 10 m long grass strip. The flow velocity in each section was measured three times using the dye tracer (KMnO4) method, and the mean flow velocity was calculated. Several groups of the surface velocity of overland flow were measured intermittently during each experiment. The green and witnessed grass in the aboveground parts of the experimental grass strips were collected, respectively, dried at 85\({}^{\circ}\)C to constant weight, and then measured the final weight. The weight result was the total amount of organic matter, which is biomass [25]. The biomass values of green and witnessed grass were 1.170 kg/m\({}^{2}\) and 0.361 kg/m\({}^{2}\), respectively, meaning green and witnessed grass contributed 76.42% and 23.58%, respectively, of the total biomass. Outlet runoff samples were collected at different times during an experiment. Each sample was weighed and then allowed to stand until the supernatant could be removed. The sediment was then dried at 105 \({}^{\circ}\)C to a constant weight [20; 26]. The results were used to calculate the sediment transport rate at the outflow.