# Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process
## 3 Results and Discussions
### Resistances to Overland Flow Provided by the Grass Strips  
The mean velocity was calculated from the measured overland flow surface velocity using Equation (5), and the Darcy-Weisbach friction factor \(f\) and Manning roughness \(n\) were calculated using Equations (9) and (10), respectively. Box plots of the \(f\) and \(n\) distributions for the different experiments are shown in Figure 6a,b, respectively. There  
Figure 5: Box plots of (**a**) the mean flow velocity \(V\), (**b**) Reynolds number Re, and (**c**) Froude number Fr data for the different experiments. The red line in the middle of each box is the median. The blue lines below and above the red line indicate the first quartile and third quartile, respectively. The black lines above and below each box are the maximum and minimum values (excluding outliers), respectively. The red plus signs beyond the black horizontal lines are outliers. The vertical length of a blue box is the interquartile range. were marked differences between the \(f\) and \(n\) values for the different experiments. The more the material blocked overland flow on the hillslope, the greater the resistance to overload flow was. The mean values and error ranges for \(f\) and \(n\) were 7.26 \(\pm\) 0.71 and 0.1344 \(\pm\) 0.0092, respectively, for the CG group; 2.61 \(\pm\) 0.17 and 0.0773 \(\pm\) 0.0024, respectively, for the RG group; and 1.10 \(\pm\) 0.12 and 0.0486 \(\pm\) 0.0029, respectively, for the RG group. Removing the green grass decreased the mean \(f\) from 7.26 to 2.61, i.e., by 64.05%, and decreased the mean \(n\) by 42.49%. Removing the witnessed grass decreased \(f\) by a further 20.80%, to 1.10, and decreased \(n\) by a further 21.35%, from 0.0773 to 0.0486. Removing the green grass decreased the total amount of sediment trapped \(R_{m}\) by 50.49%, and removing the witnessed grass decreased the \(R_{m}\) further by 19.25%. It can be seen that the contributions of the aboveground parts to the total amount of sediment trapped was mainly determined by the resistance of the aboveground parts of the grass strips to overland flow. Figure 6: Box plots of the Darcy–Weisbach resistance factor \(f\) and Manning’s roughness \(n\) for the grass strip in the different treatment experiments. Note: Some outliers far from the boxes are not shown to improve clarity. The outliers may have been caused by large measurement errors. In plot (**a**), outliers with \(f>\) 30 (eight points) are not shown. These were all found in the complete grass strip experiments. Two of the points were for test CG1, four for testCG2, and two for test CG3. These outlying values were caused by overland flow being much more disturbed in the complete grass strips than in the other strips, making the measurements more unstable for the complete grass strips than the other strips. Outliers with \(n>\) 0.3 in plot (**b**) are not shown and corresponded to the eight outliers for plot (**a**). Green grass and withtered grass increased \(f\) by 4.65 and 1.51, respectively, and the ratio between these values was 3.08:1. The green to witnessed grass biomass ratio was 3.24:1. These two ratios were very similar. It can be seen that the resistance to overland flow provided by green and withered grass was essentially determined by the biomass. Theoretically, the moisture content is markedly higher for green grass than withtered grass. If green and withered grass had the same shape, resistance to overland flow would be higher for green grass than withtered grass. In fact, f was increased by green and withered grass by 0.397 and 0.418 per kilogram of biomass, respectively, meaning f was increased \(\sim\)1.05 times more per unit of biomass by withered grass than by green grass. This would mainly have been because the green grass was mostly upright, but the withered grass was mostly lodged or semi-lodged. Resistance to overland flow was therefore increased more by the same biomass of withered grass than of green grass.