Mitigation measures of coastal erosion on the Kotlin Island’s shores in the Gulf of Finland, the Baltic Sea

The intense erosion of shores of the Kotlin Island in the Gulf of Finland during the last 70 years can lead to the complete disappearance of sandy beaches of the western Kotlin Island in the near future. We assess the intensity of the coastal erosion of the western Kotlin Island and propose the method of artificial sand nourishment in order to maintain the sandy beaches. The geological near-shore and onshore surveys revealed the following features of the western Kotlin Island coastal zone: 1) significant sediment deficit; 2) small (about 30 cm) thickness of the active sand layer, 3) predominance of narrow (10—15 m) sand beaches and 4) low and smooth offshore and onshore topography. The parameters of artificial beaches are determined by coupling the models of water circulation, waves, and sediment dynamics. Calculations of currents and waves were performed using a three-dimensional hydrodynamic model of the eastern Gulf of Finland and the SWAN wave model, respectively. The coast deformation due to storms was calculated using the CROSS-P model. Initial data were the existing depth profile, sediment characteristics, wind properties, wave parameters, the height of storm surges, and the duration of a storm. The actual external forcing (atmospheric forcing from the HIRLAM model, open boundary conditions from the HIROMB model, and bathymetry and topography of the beach obtained from geological surveys) is given, the parameters of artificial beach profiles have been calculated to withstand the maximum storm surge, and the annual volume of sand necessary for the conservation of the artificial beaches has been estimated.

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