Testing Green DRR Solutions by Dune Reconstruction at a Rapidly Eroding Coast in the Adriatic Sea

In the current scenario of climate change and increased sea-level the occurrence of high water level storms threatens coastal landscapes that lye at a low elevation. Naturally coastal dunes provide the so-called first line of defence. At eroding coastlines their reconstruction offers the opportunity to build dynamic coastal defences, in opposition to more static approaches like sea-walls. In order to properly design a revegetated dune, modeling must be undertaken to identify the optimal dune height and width able to withstand the effect of an exceptional storm. The primary role of the plants is to modify the velocity profile above them: this implies to introduce in the model credible values of the drag coefficient that also reflect the spatial distribution of the different plant species and the density of stems per unit area. Numerical tests of the effectiveness of dune reconstruction and revegetation as a Disaster Risk Reduction (DRR) measure were undertaken on the Bellocchiocoastline, south of Porto Garibaldi, in the northern Adriatic sea coast of Emilia-Romagna. The work was developed as a part of the EU FP7-RISC-KIT project, which elected the site as the pilot case study after selection of a number of highly exposed hotspots (Armaroli and Duo, 2017). A detailed digital elevation model was producedmerginga topographic Lidarfor the emerged area, a nearshore bathymetric Lidar up to -4.5 m water depth and a multibeam bathymetric survey for the offshore up to – 10 m. The storm selected to test the efficiency of the natured-based DRR occurred on 5-6 of February 2015, with a maximum offshore significant wave height of 4.66 m and a peak of 1.2 m of water level above MSL. These conditions approximately correspond to a return period of 50 years. The two-dimensional domain extends 3000 m alongshore and 3600 m cross-shore. The resolution of the ranges from 18m alongshore and 21m cross-shore at the offshore open boundary at (10 depth) to 1m cross-shore and 8m alongshore in the area where the dune is located. The reconstructed dune was built mimicking the current dune but enlarging it and increasing its crest, according to literature knowledge of comparable dune systems in the region. The vegetation was introduced according to spatial distribution of local species assuming either the current stem density or doubling it. Two indicators were chosen to assess performance, respectively the Maximum Water Volume (MWV) and the sediment volume variation (SVV). The results outline a reduction of inundation with the reconstructed dune (Fig. 1a), which is still breached and overtopped at some points. If the vegetation is reconstructed on the dune there is considerable decrease of inundation (Fig. 1b). Minimum changes are observed between normal and high density of vegetation for the reduction of MWV (Fig. 1c). The high-density vegetation set-up provides instead the most efficient solution for SVV reduction. Further tests will be undertaken at a number of European sites in the context of the EU H2020-ANYWHERE Project, encompassing also different vegetation taxonomy and distribution.