## 3 Results
### Benefits of NbS to Mitigate Soil Erosion Risk
The NbS that can reproduce the desired model parameters can only be selected and sized after the highly resolved CFD simulation of the flow depth \(h\) (1) and the velocity vector \(\mathbf{u}\) (2). The simulated hydrograph in Figure 12a (blue line) highlights the dynamic and transient nature of the surface waters across the draining basin. Its shape changes substantially regarding the current situation (black line). Figure 11: Optimal targeted placements of specific NbS to provide the most benefits and maximise their effectiveness for flood management and soil conservation, as described in Section 3.4. The photos illustrate the actions at each location, as indicated by the frame colour. Overall, we observe the mixed benefits of increasing the in-channel roughness in the drainage network and the installation of a flood barrier to create the artificial wetland. They protect against flooding by slowing down the flow and storing surface runoff. Hence, the rising limb exhibits two stages separated by a steady plateau associated with the wetland filling. After one hour of rainfall, the peak discharge in the urban area is only \(Q=4.2\) m\({}^{3}\).s\({}^{-1}\), much lower than for the actual scenario CN89-A, which is \(Q=48.5\) m\({}^{3}\).s\({}^{-1}\). The map of water depth at \(t=1\) h (Figure 12b) shows that the Manillas stream did not inundate the town. The early infilling of the wetland and the absence of flow downstream can also be appreciated in such a map. Forty minutes later, the water discharge reaches a steady state with \(Q=17.6\) m\({}^{3}\).s\({}^{-1}\) until \(t\approx 2.5\) h. For this low value of the water discharge, the town remains again protected against flooding; see the map of flow depth in Figure 12c. At \(t\approx 2.5\) h, the wetland achieves its maximum capacity, and the water starts to flow downstream of the wetland barrier. Then, the second stage of the rising limb occurs up to reaching the peak water discharge \(Q_{\rm max}=39.2\) m\({}^{3}\).s\({}^{-1}\) at the end of the rainfall (i.e., \(t=3\) h). The map of flow depth at that time is shown in Figure 12d. Later, the water  
Figure 12: (**a**) Simulated hydrographs in the Manillas stream, near the town, for the current state (black) and the possible future (blue) after implementing the combined NbS B and D. (**b–e**) Snapshots of the simulated flow depth (in meters) with NbS B and D at \(t=60\), 100, 180, and 200 min corresponding, respectively, with the streamflows \(Q=4.2\), 17.6, 39.2, and 39.1 m\({}^{3}\).s\({}^{-1}\) (squares in panel a). (**f**) Maximum infiltration rate (in mm-h\({}^{-1}\)) achieved at the end of the rain (i.e., \(t=180\) min) for the future scenario with NbS B and D.  
discharge remains constant near the urban area for about 30 min (i.e., up to \(t=200\) min). However, at that time, the rain had already ceased, runoff vanished in the watershed (see the flow depth map in Figure 12e), and the discharge monotonously decreased, vanishing approximately at \(t=6\) h.  
The peak discharge for the combined NbSs B and D, \(Q_{\text{max}}=39.2\) m\({}^{3}\)-s\({}^{-1}\), decreased with respect to the present value of 51.6 m\({}^{3}\)-s\({}^{-1}\) as much as using uniquely NbS B, which yields 41.7 m\({}^{3}\)-s\({}^{-1}\) (CN64-B in Table 1). The infiltration was responsible for the attenuation of the peak streamflow. The infiltration rate was at a maximum at that time, reaching the values depicted in the map of Figure 12f. In the agricultural area, the highest infiltration rates were around 7 mm-h\({}^{-1}\) and occurred in the northern part. In the headwaters, the infiltration decreased to 2-5 mm-h\({}^{-1}\) because of the effects of the slope in the curve number (recall Figure 5c). Though such values are low regarding the larger precipitation rate of 30 mm-h\({}^{-1}\), the cumulative volume of water infiltrated during the whole hydrological event of 6 h is relevant.