## 3 Results
### Distributed Hydrological Simulation of the Catchment in the Present Day
Photographs b and c were taken in the locations marked with the red and yellow dots in Figure 1a few meters downstream and upstream of the cross between the Manillas stream and the road, respectively. For instance, extreme erosion or gullies can be located from the simulated map of shear stress (Figure 6c). In the small rectangular area in red of Figure 6c, there is a train of gullies. They are visible in the orthophoto of this region (see the panel at the top of Figure 7a). The five gullies are in-parallel outside the olive tree canopy projection. The simulated shear stress inside the gullies is approximately 5 N-m\({}^{-2}\), as shown in the shear-stress map in the middle of Figure 7a. Thus, the contour level \(|\tau_{b}|=5\) N-m\({}^{-2}\) yields the extension and the path of the gully. The predicted trajectory of the gully is indeed well-correlated with the breaks in the slope of the terrain depicted in the panel at the bottom of Figure 7a. After locating the gullies on the map, we visited the place. We found the deeply incised gullies visible in the in situ photo of Figure 7a, proving the accuracy of the numerical simulation. Another example illustrating the erosional power of the high-speed flow is the rock-bed incision depicted in Figure 7b. The precise location corresponds with the red square in Figure 1a (also, with the left-upper corner of the black rectangle in Figure 6). A few meters downstream of the first cross between the Manillas stream and the road, where the shear stress was as high as 100 N-m\({}^{-2}\) (Figure 6c), the water flow sculpted a single confined channel. The flow eroded a first layer (i.e., 0.7 m) of poorly consolidated sediments (mostly silt and clay) and a second rocky bed (i.e., 1.7 m), leading to the rectangular cross-section of depth \(H=2.4\) m and with \(B=6\) m. The authors of [78] showed that the bankfull dimensions \(B\) and \(H\) of straight rivers adapt to the regime-based bankfull capacity \(Q_{1D}\) and satisfies the classical Manning equation for a uniform flow in a rectangular channel [63]:  
\[Q_{1D}=\frac{B\,H}{n_{1D}}\,R_{h}^{2/3}\sqrt{S_{0}}\quad\text{with}\quad R_{h} =\frac{B\,H}{2\,H+B}\,. \tag{9}\]  
For dimensionally-similar channel planforms, [PERSON] [16] calibrated the one-dimensional roughness parameter \(n_{1D}=0.055\) s\(\cdot\)m\({}^{-1/3}\), which accounts not only for the hydraulic resistance of the walls and the bed but also for the presence of sparse riparian vegetation and secondary head losses. We measured the thalweg elevation using Leica Zeno 20 GPS and obtained the local slope value \(S_{0}=0.0196\). Substituting these values into (9), yields the peak water discharge \(Q_{1D}=44.4\) m\({}^{3}\cdot\)s\({}^{-1}\). Taking into account that the draining area to this cross-section is \(A_{\text{drain}}=4.76\) km\({}^{2}\), the theoretical value of the streamflow given by (5) is \(Q_{\text{gs}}=39.7\) m\({}^{3}\cdot\)s\({}^{-1}\) for the rainfall intensity \(\bar{P}=30\) mm\(\cdot\)h\({}^{-1}\), which differs only by 11%. Upstream of the single channel described above and before the cross between the Manillas stream and the road, see the specific location marked with a yellow dot in Figure 1a, a singular sedimentary process was identified during fieldworks on 25 August 2021. We found ripples over parabolic dunes and ripples above the plane bed, see Figure 7c, provoked by the previous day's flood. The growth of ripples over dunes, see the review on dunes by [PERSON] et al. [79], is usual in deep river flow and reflects a severe inundation in the floodplains of the Manillas stream. Indeed, the high-water marks on the road ditch and the top level of the vegetation debris on the olive-tree trunk yield flow depths of 0.4-0.5 m. Such values are in agreement with the simulated flow depth in Figure 6b, which provides further validation of the model input data and the simulation results. Additionally, the formation of a plane-bed occurs only at high shear stresses [77], which implies a notable sediment transport capacity of overland flows in olive groves.