Performance ed analisi della morfodinamica a seguito di un ripascimento sommerso (feeder berm) e di un reef artificiale (Tecnoreef) realizzati per la mitigazione dei fenomeni erosivi del litorale di Punta Marina (Emilia-Romagna, Italia)

A composite intervention of coastal defense was carried out at Punta Marina (Emilia Romagna) in 2010 to counteract the ongoing erosion. The intervention has been realized between existent hard structures (groins and breakwaters) and it consists of a beach nourishment with slightly finer sand than the native one, a nearshore nourishment (feeder berm) with fine sand (coarser than the native one) and the placement of an artificial reef between the feeder berm and the breakwaters. Several studies of nearshore nourishments have been carried out worldwide but all in quite different environments than the Northern Adriatic littoral, in terms of morphological and sedimentological features, wave climate as well as the nourishment design parameters (sediment volume request, grain size and position on the cross-shore profile). Artificial reefs are used in various countries across the world for coastal management, mainly to enhance the living marine resources, to compensate habitat loss or to promote surf waves but rarely for coastal protection. The artificial reef effects on hydro-sedimentary dynamics and the medium-long term efficacy of the interventions against erosion have not been well studied yet. In order to assess the impacts of this intervention on the coastal processes different field campaigns (topo-bathymetric surveys, multi-beam bathymetric surveys, samples collections, wave data gathering and elaboration) were realized from February 2010 to October 2012. During this period the littoral was exposed to relatively calm wave condition, even if several storms occurred with a main direction from NE-E and two of them classified as “severe” and “extreme” storms. The total sedimentary budget of the beach and nearshore was positive. The results also indicate a progressive and rapid erosion of the feeder berm and an observed gain of sediment shoreward of the nourishment and in part behind the artificial reef. This seabed accretion promoted the wave energy dissipation, contributing to the shoreline stability. At Punta Marina evolutionary trends of the intervention are similar to the North European and American cases, but with different duration of the induced effects: in fact a shorter “wave breaker” effect was observed while a “feeder effect” was still ongoing till the last survey. These differences are probably related to the nourishment characteristics like: smaller volumes, borrow material with a finer grain size than the beach native sediment, different seabed morphological features (absence of sandbars, mild slope), shallower water depth of the dumping area (active also during low-energy storms). During this study small sandbars have been observed; these morphologies were present in the past decades but they disappeared since 90s. For a better understanding of this phenomenon, a study at a regional scale has been performed, investigating the existence and the morphometric characteristics of the sandbars since the 60s to 2000. The results highlighted a strong influence played by hard structures (when built close to the breaker zone) and by the negative sedimentary budget on the sandbar existence, as in the case of Punta Marina. The sandbar reappearance at Punta Marina could be caused by the nourishment or related to the strong storms occurred. It might represent a first sign of a natural littoral dynamic restoration. Otherwise different results have been obtained with the artificial reef placement. As shown in other projects with reefs positioned in shallow water, a substantial settlement due to scour formation has been highlighted, most probably due to wave stress and the generation of a seabed current that disperses the sediment offshore. Moreover, the damage and removal of several reef elements demonstrate their excessive fragility also in mild wave climate like the North Adriatic. Finally, the nearshore nourishment efficacy, even if finer sediment than the beach one were used (not suitable for beach nourishment), could promote virtuous practices for the reuse of harbor dredged sediments like from Porto Corsini (Ravenna harbor) for coastal protection.