Hydraulic modeling of Natural Water Retention Measures for flood risk mitigation in Sicily

Flood events show a worrying growth trend, in severity and frequency, compared to the past, in particular in the Mediterranean countries. In accordance with the recent Natural Restoration Law (2024), Natural Water Retention Measures (NWRMs) are configured as tools capable of integrating engineering interventions with nature-based approaches to mitigate flood risks. To the authors’ knowledge, there is a lack of studies on the quantitative assessment of the effectiveness of NWRMs. In this context, the present study proposes a methodology based on hydraulic modeling for the evaluation of the effects of some NWRM at catchment area scale for flood risk mitigation, considering two case studies in Sicily, i.e. the catchment areas of the Eleuterio and Belice rivers. Starting from the study of the land use and the lithology of the considered Sicilian catchments, the most appropriate NWRM proposals are defined, which are reforestation in uncultivated land and low- and no-till practices in agricultural areas. Multiple intervention scenarios are proposed to identify the most effective measures for the case studies: i) reforestation, assumed on increasing percentages of the areas allocated to such intervention, i.e. 25%, 55%, 85% and 100%, considering both the initial and final state of growth of planted tree species; ii) conservation agriculture techniques (low-till and no-till practices) in fields intended for arable and similar crops or/and in orchards, vineyards and olive groves; iii) the combination of the above mentioned interventions. By using a rainfall-runoff model based on the Curve Number method, four hydrographs at the river mouth are obtained for each scenario and for the no intervention case, corresponding to the return periods of 5, 50,100 and 300 years. The implementation of the considered NWRMs produces the reduction of the peak flow rates with respect to the no intervention case. The obtained hydrographs are then used as inputs for the 2D hydraulic model developed in HEC-RAS,  and flood maps are obtained. As expected, for both case studies, the best performances are obtained with the combination of reforestation and conservation agriculture techniques, with reduction in the water depth of flooded areas up to 34.52% for a return period of 5 years and 16.12% for a return period of 300 years, and in the extension of flooded areas up to 52.74% for a return period of 5 years and 8.35% for a return period of 300 years. Moreover, reforestation appears to provide the larger contribution to flood risk reduction.