## 3 Results
### Benefits of NbS to Mitigate Soil Erosion Risk
The most probable values of the shear stress developed in the agricultural area of the watershed. This fact highlights the soil-loss mitigating effect of NbS B and NbS B C because the major modes are lower than the threshold for erosion. The minor mode for NbS B, about \([0.27,0.44]\) N\(\cdot\)m\({}^{-2}\), is also one order of magnitude lower than presently. Consequently, the relative area where the dangerous threshold for erosion \(\tau_{bs}>\tau_{c}\) occurred decreased from about 86% to 25% in NbS B. In the case of NbS C, the potential area for erosion also falls to 39%. The minor peak arising in the probability of the shear stress at \(\tau_{bs}\sim 15\) N\(\cdot\)m\({}^{-2}\) for NbS C corresponds to the high velocities, of the order of 1 m\(\cdot\)s\({}^{-1}\) (Figure 10b), in the unaltered channels of the drainage network (recall the velocity map in Figure 6b for the present day). Note the positive aspects of increasing the resistance to flow in the channels (NbS B), which reduces the minor mode of the velocity to \([0.16,0.24]\) m\(\cdot\)s\({}^{-1}\) (Figure 10b) and could favour sediment trap processes to restore deep incised channels. Synergetic Benefits of the Wetland, In-Channel Macro-Roughness and Cultivation Systems in Olive Orchards  
Considering the independent benefits of each nature-based solution, we combine three to maximise their effectiveness in terms of flood mitigation and soil conservation. Furthermore, we choose the optimal targeted placement of NbS because funding is always limited. We seek to delay the flood, attenuate the peak discharge in the urban area and favour infiltration processes across the watershed. At the same time, we want to prevent soil erosion in olive orchards, promote mechanisms for gully aggradation rather than gully erosion and reduce the catchment sediment budget. Figure 11 shows a map of the optimal location of an NbS to provide the most benefits. The results of the distributed hydrological simulations described in the previous sections helped site and classify the actions (which were unknown a priori) as follows:  
* First, the black rectangle in Figure 11 depicts the installation of a flood barrier with the crest of the earthen embankment at 406 m, the maximum height of 6 m and the dam crest length of 245 m. It should create an artificial wetland (purple area in Figure 11) as in previous simulations of type D.
* Second, from simulation type B, we delimited the extension of the main channel and the inundated floodplains along the Manillas stream (yellow area). Further, the tributary streams in green correspond to simulated flow depths larger than 0.5 m \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **A** & **B** & **C** & **D** & **E** \\ \hline
**CN100** & 87.8 & 25.4 & 39.9 & 86.9 & 86.7 \\
**CN89** & 87.2 & 25.1 & 40.1 & 86.9 & 87.3 \\
**CN64** & 86.4 & 25.3 & 39.4 & 91.7 & 93.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relative area (%) where the dangerous threshold for erosion \(\tau_{bs}>\tau_{c}\) holds. outside of the Manillas stream. Then, ephemeral gullies in red were located using criterion \(0.03<h<0.5\) m from Soil Science Society of America [51]. * Third, the river floodplains in blue, located in the headwater and near the town, are an extinct, ephemeral wetland and a stream confluence requiring restoration. * Fourth, in the olive groves, we propose different cultivation systems for bed slopes lower (brown area) and larger (greenish blue area) than 10 degrees. The following step in AFC [19] is choosing the model physical parameters at the targeted placements to achieve the optimal values of the objective functions. According to the minimum values of the peak water discharge and the erosion-prone area given, respectively, in Tables 1 and 2, we set the Manning roughness parameter \(n=0.2\) s\(\cdot\)m\({}^{-1/3}\) in the green, yellow, and red areas, and \(n=0.025\) s\(\cdot\)m\({}^{-1/3}\) in the olive groves for the CN64 setup (recall Section 2.2.4).