A mixed solution for a highly protected coast (Punta Marina, Northern Adriatic Sea, Italy)

Spatial data collected over three years are presented to assess the impacts of a mixed intervention carried out in a low sandy microtidal highly protected beach (Punta Marina, Italy). Coastal erosion started in the 1980s, principally due to reduction of fluvial sediment discharges, wave attack, and both anthropic and natural subsidence. The intervention consisted in a small beach nourishment, shallow feeder berm construction, and the placement of an artificial reef to retain the sand fill. Topo-bathymetrical surveys with Real-Time Kinematic (RTK) GPS, multi-beam surveys, sediment samplings, wave data records, dive inspections and numerical simulations were undertaken over a 3-year monitoring period to analyse the process-response mechanism of this intervention, to assess the main factors affecting the performance of the mixed intervention and the possibility of using fine sediments for nourishment practices. The results reveal a generally positive impact characterised by short-term beach stability that was mostly due to the feeder berm effect. The beach nourishment had a short but direct effect on the beach width, while the feeder berm dissipated wave energy and successively fed the nearshore with eroded sediment. On the other hand, the artificial reef did not retain the sand, but rather induced rip currents that increased eddy circulation and, therefore, promoted the loss of offshore sediment. Settlement and scouring problems were also observed and the structure's failure induced safety problems. The results confirm that a feeder berm operation carried out with fine sediment represents a good solution, although particular attention must be paid to the relationship between sediment grain sizes, volume, and location of the nourishment, as they are the key factors for good performance. Finally, the cross-shore sediment transport represents a critical factor that controls the coastal evolution of beaches bounded by hard structures and assimilated to the littoral cell. Consequently, it should be better integrated within the nourishment design and model setting.