# 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
This was finally reflected in the effect of the green and withered grass on the total amount of sediment trapped by the grass strips. To date, it is still unclear whether a unified \(f\)-\(S\) relation exists for granular surfaces or vegetation-covered slopes, or what causes the different \(f\)-\(S\) relation under shallow overland flow conditions. Inspired by channel or pipe hydraulics, resistance f to overland flow is frequently expressed by the Re as follows: \(f=K\times\text{Re}-b\), where \(K\) and \(b\) are regressed parameters. \(K\) equals 24 for a smooth surface under a laminar flow regime [33]. It means that slope steepness would have no relation with \(f\) because Re is a product of unit flow rate and kinematical viscosity and has nothing with slope [34]. The steeper slopes evolve to rougher surfaces, compared with shallower slopes, and this increase of roughness with slope balanced the increase of flow velocity with slope, resulting in a slope-independent condition [35]. The hypothesis of slope-velocity equilibrium implies that the use of hydraulic equations, such as Manning and Darcy-Weisbach, cannot easily be applied in hillslope scale runoff models [32; 36]. Manning and Darcy-Weisbach equations were often used in overland flow studies. In order to compare with the results of previous similar studies, the traditional Manning and Darcy-Weisbach equations were used in this study. However, the use of Manning and Darcy-Weisbach equations for the calculation of the friction factor is limited. The resistance calculation formula of steep hillslope needs to be further improved. Temporal Variations in the Contributions of the Aboveground Parts of the Grass Strips to Sediment Trapping  
As mentioned above, the ratio of the contributions of green grass, withered grass, and the soil bed surface to the total amount of sediment trapped \(R_{m}\) when the grass strips each reached a stable state were 5:2:3. This ratio was for when the sediment-trapping process reached a stable state so does not reflect the contributions of the aboveground components to sediment trapping in other stages. In reality, rain sometimes falls for a short time, and water and sediment transport over a hillslope will finish before sediment trapping reaches a stable state. It is therefore important to investigate temporal variations in sediment trapping by aboveground grass strip components. Temporal variations in the cumulative amount of sediment trapped \(R(t)\) in the CG, RG, and RGW group experiments are shown in Figure 7. The \(R(t)\) curves for the three groups of experiments in the first 10 min were very similar, indicating that the soil bed surface played an important role in the early stage of sediment trapping. After some sediment had been deposited, the \(R(t)\) curves for the different treatments continued to increase, and differences in the effects of green grass, withered grass, and the soil bed surface gradually became apparent. As the stable state of the sediment-trapping process for each experimental group gradually was reached, differences between the cumulative amounts of sediment trapped in the different experimental groups became relatively stable after 50 min. The cumulative amounts of sediment trapped at different stages of the different experiments are shown in Table 3.  
The vertical bars in, from top to bottom, green, red, and blue, in Figure 8 show the proportions of the cumulative amount of sediment trapped \(R(t)\) caused by green grass, withered grass, and the soil bed surface. For example, the fourth bar from the left in Figure 8 indicates the contributions (as percentages) of each aboveground part to the cumulative amount of sediment trapped \(R(t)\) in the previous 30 min (i.e., \(t=30\) min). The lengths of the different colored parts of the bars were different at different times, indicating that the contributions of the different aboveground parts of the grass strips to \(R(t)\) changed as the duration of the sediment-trapping process increased.