Marcinkowski, T., Szmytkiewicz, M, 2013. Performance of submerged breakwaters as improvement of beach fill effectiveness in Gdynia, Poland

The study is focused on improvement of beach fill effectiveness due to construction of submerged breakwaters at an extremely sediment-hungry coastal segment, situated in the Gulf of Gdansk in Gdynia municipality. The studied coastal segment stretches over 500 metres between a cliff headland belonging to a local nature reserve and a walkway pier. The segment has been heavily eroded, resulting in disturbances to the operation of the wharf, as well as the local urban infrastructure. Geophysical records show that the sediment layer in the coastal zone hardly exists. To reduce the vulnerability of this coastal section a 3-segment submerged breakwater was constructed in 2006. It was situated 150 m offshore and the crest was positioned 0.5 m below MWL. The modeling was aimed at precise mapping of structures' effect and was carried out using MIKE 21/3. Bathymetric maps from 2005 – 2010 were used to verify the calculated sea bottom changes. The comparison of wave heights and water circulations, calculated both with and without the presence of the structures, showed that during storms the wave energy reaching the shore is reduced by 45% for the mean water level when the structures are present. For storm surges of 1.1 m the effectiveness of the structures is reduced to some 20%. For the mean water level the reduction of water circulation velocity is 20 – 25%. The presence of submerged breakwaters slow down the migration of sediment; the increase of residence time of beach fill was estimated between 15 and 20%.