# Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process
## 3 Results and Discussions
### Overland Flow Regime  
Equations (5)-(7) were used to calculate the mean velocity \(V\), Re, and Fr, respectively, for overland flow in the CG, RG, and RGW group experiments, and the ranges of values found are shown in Table 3. Re was \(>\)500 for all of the experiments, meaning that overland flow under the experimental conditions we used involved transitional and/or turbulent flow but no laminar flow. The Fr values for most of the experiments were \(<\)1, meaning that overland flow was tranquil flow. This may have been because the supercritical flow was hardly to occur at the gentle slope of 5\({}^{\circ}\) and the flow rate of 30 L min\({}^{-1}\) m\({}^{-1}\). [PERSON] et al. performed simulated rainfall tests using rainfall intensities between 60 and 181 mm h\({}^{-1}\) and slopes of between 3.6% and 39.6% using plots with four different types of vegetation  
Figure 4: Cumulative amount of sediment trapped plotted against time for the different grass strip treatment experiments. and found Fr values of 0.02-0.47 [32]. This indicated that overland flow was tranquil, similar to the conclusions drawn from the results of our study. The range of hydraulic parameters cannot indicate the specific differences of the flow regimes in each experimental group. The distributions of \(V\), Re, and Fr in each group were shown in Figure 5. The median \(\pm\) error overland flow velocity \(V\) (m s\({}^{-1}\)) values for the CG, RG, and RGW group experiments were 0.0951 \(\pm\) 0.0027, 0.1516 \(\pm\) 0.0054, and 0.1954 \(\pm\) 0.0069, respectively. The median lines and box ranges in Figure 5a indicate that the overland flow velocity was higher for the RGW group than the CG group. Resistance caused by witnessed grass decreased the flow velocity by \(\sim\)22%, and resistance caused by both green and witnessed grass decreased the flow velocity by \(\sim\)51%. The median Re values were slightly lower for the CG group than the RG and RGW groups and were similar for the RG and RGW groups (Figure 5b). Under the influence of deposited sediment, the overland flow regime was constantly changing during the sediment-trapping process. The Fr distributions indicated that most measurement results of the overland flow in each group experiment were tranquil flow. A more complete aboveground part structure caused the flow velocity to be lower and Fr to be lower (Figure 5c). There are some outliers in Figure 5. The outliers of the box plots in this study were identified via the following process. The lower and upper quartiles (D\({}_{1}\) and D\({}_{3}\)) are called the 25% and 75% positions of the ascending sequence, respectively. Interquartile range (IQR), D\({}_{3}\) - D\({}_{1}\), is called as the interquartile range in statistics. In a box plot, the data of >D\({}_{3}\) + 3/2 \(\times\) IQR or <D\({}_{1}\) - 3/2 \(\times\) IQR are identified as outliers. Outliers are therefore related to the IQR range of the sample sequence, that is, determined by the concentration degree of sample points. The smaller the IQR range is, the more sample points may be identified as outliers. Removing the green and withered grass caused the Fr for overland flow to occasionally be >1, indicating that the overland flow changed from continuous tranquil flow in the RG and RGW groups to critical and supercritical flow occurring in the CG group. Water was applied evenly to the grassland plots before each experiment until runoff occurred to eliminate differences between the soil moisture contents in the different experiments. No infiltration occurred during the experiments. In one experiment, changes in the overland flow velocity and regime were mainly caused by the sediment deposited during the experiment, which was explained in some previous publications [12; 23]. The differences between the overland flow regimes in the different experiments were mainly caused by resistances provided by green grass, withered grass, and the soil surface, which changed the overland flow velocity and regime and finally caused deposition of sediment.