# Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process
## 3 Results and Discussions  
### Sediment-Trapping Process  
It can be observed from Figure 2 that the sediment transport rate at the outflow first increased and then became stable but that the rate increased markedly differently in the different experiments. The more complete the aboveground grass strip structure was, the longer the system took to reach a stable state (55 min for the CG group, 40 min for the RG group, and 30 min for the RGW group). The sediment transport rate at the outflow for the CG group is shown in Figure 2. It shows that the sediment-trapping efficiency was high for the first \(\sim\)30 min of the CG group experiments. No clear gentle increase stage occurred at the beginning of the RG group experiments or RGW group experiments (Figure 2). This indicated that only a little amount of sediment was deposited in the early stages of the experiments, the CG group strips had high levels of roughness, the resistance to overland flow was high, and the flow velocity was low. ISTEs are plotted against time in Figure 3. ISTEs were high in the initial stages of the experiments. For example, ISTEs for the first sampling points in the CG, RG, and RGW group experiments were 0.9950 \(\pm\) 0.0027, 0.8957 \(\pm\) 0.0077, and 0.7984 \(\pm\) 0.0111, respectively. The results indicated that ISTEs were usually higher in the initial stage, and the high ISTEs usually lasted longer for the grass strips with more complete aboveground structures than for the grass strips with less complete aboveground structures. The initial ISTE for the grass strips reached 0.7984 even with the green and withered grass removed. This was still a high ISTE, indicating that soil bed resistance played a major role in the grass strips trapping sediment in the initial stage. The cumulative amount of sediment trapped \(R(t)\) (kg) is plotted against time in Figure 4. \(R(t)\) increased rapidly in the early stages of the experiments but increased little in the later stages. The more complete the grass strips were, the faster the rate of \(R(t)\) increased in the early stages, and the higher the \(R(t)\) was in the stable stages of the experiments. [PERSON] et al. proposed a formula to calculate the sediment transport capacity of overland flow using slope, flow rate, and median particle size by flat plexiglass bed surface tests [31]. Under our experimental conditions (flow rate 30 L min\({}^{-1}\) m\({}^{-1}\), median particle size 39.9 \(\upmu\)m,  
Figure 3: Instantaneous sediment-trapping efficiency plotted against time for the different grass strip treatment experiments. Figure 2: Sediment delivery rate at the outlet (\(Sd_{out}\)) for the different grass strip treatment experiments. slope 5\({}^{\circ}\)), the sediment transport capacity of overland flow was \(\sim\)5620 g min\({}^{-1}\), which is greater than the sediment inflow rate in our study (3600 g min\({}^{-1}\)). Therefore, under the silt-laden inflow conditions (3600 g min\({}^{-1}\)), the sediment deposition would not occur on a smooth and flat 5\({}^{\circ}\) slope bed surface but would occur if the effect of microtopography, gravel particles, and vegetations exist. The bed surface has been covered by some deposited sediment at the stable-state stage in experiments. Although it was not as smooth and flat as the plexiglass bed surface, ISTE fluctuated around 0 (Figure 4), indicating that almost no sediment was deposited in the grass strips during the stable state stage, which also mirrored the findings of [PERSON] et al. (2018). Sediment deposition in the early stage of the RGW group experiments would therefore have been caused by the microtopography resistances and gravel resistances of the soil bed surface. ISTE also decreased when the soil bed surface was gradually covered by deposited sediment. \(R_{m}\) (kg) was the total amount of deposited sediment when the stable state stage of sediment trapping was reached.