## 1 Introduction
The developed methodology has two stages: (i) first, the diagnosis of the causes leading to inundation and soil erosion using the highly resolved Computational Fluid Dynamics (CFD) simulation of the current situation; (ii) second, the quantification of the hydraulic effects of the possible measures for natural flood management (NFM) and soil loss mitigation by simulating parameter variation in future scenarios. The output products identify the optimal model parameters across the catchment (i.e., controlled variables) that minimise the objective functions, namely: the peak water discharge at the basin outlet and the shear stresses responsible for soil erosion. The approach is standard in Active Flow-Control (AFC), a \"multidisciplinary science and technology thrust aimed at altering a natural flow state into a more desired state\" [19]. The last step is the selection of a realistic NbS that produces the optimal values of the controlled physical variables at the targeted placement. We consider NbSs compatible with sustainable production techniques in olive growing [20] and complementary innovations developed worldwide, as described below. Isolated NbSs cannot fix all the problems alone, and the ideal future scenario should exploit several actions' synergetic benefits. To this end, we apply standard techniques in AFC [19] and CFD [21], which consist of parameter variation and geometric modelling with sub-metric spatial resolution, uncommon yet in evaluating the effectiveness of an NbS [10]. There is presently no standard approach to determining how many NbSs to use and where to locate them in NFM [22]. A limited number of studies applied the typical engineering protocol with spatially-resolved, physically-based 2D-SVE simulations [23]. A frequent limitation of the existing design routines is the coarse resolution of the computational grid because of two reasons: first, the coarse mesh impedes the detailed geometrical modelling and hydraulic simulation of an individual NbS at a given location; second, the grid size affects the simulation results of rainfall/runoff events in a magnitude as large as physical factors [24]. We overcome such drawbacks using the latest advances in distributed hydrological modelling that allow simulating catchment-scale problems at extremely high spatial resolution in the Graphics Processing Unit (GPU) [25]. We adopt the software IBER+ [26], among alternatives as SERGHEI-SWE [27], TRITON [28], and LISFLOOD-FP [29]. Here we show that IBER+ allows the development of a whole system model incorporating the agricultural areas, the drainage network, urban areas, and civil engineering infrastructures by a multiple-scale grid resolution and proper mapping of physical parameters. Hence, the following issues are addressed at once: simulating rainfall-runoff processes [30]; urban flood mapping [31; 32]; soil-erosion risk [14]; and the effectiveness of nature-based mitigation measures [33; 34; 35]. However, the current limitation of single GPU computations in IBER+ restricts the catchment area to 20 km\({}^{2}\) for extremely high spatial resolutions of 1 m\({}^{2}\). Scaling up the analysis to larger draining basins (<1000 km\({}^{2}\)) is now possible with SERGHEI-SWE through exascale High-Performance Computing (HPC) technology and multi-GPU [27]. The proposed approach permits fully exploiting the available Digital Elevation Model (DEM), resolving many microtopographic features controlling surface flow, soil erosion and sediment transport connectivity. To the best of the authors' knowledge, the design of nature-based interventions and the quantification of their hydraulic benefits in reducing flood and soil loss hazards in catchments with soil uses dominated by olive growing have not been undertaken beforehand. Such objectives lie in the scope of NbSs, which are \"actions to protect, sustainably manage, and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits\" [36].