Long term and short term coastal line changes of the Eastern Gulf of Finland. Problems of coastal erosion

摘要

Traditionally the coastal zone of the easternmost (Russian) part of the Gulf of Finland has not been considered as an area of active litho- and morphodynamics, but a recent study has shown that the easternmost part of the coastal zone suffers from erosion. Within some coastal segments the shoreline recession rate reaches 2 — 2.5 m/ year. As well as determining the hydrodynamic reasons for recent erosion acceleration, important geological and geomorphic features of coastal zone which influenced the lithodynamics were established. The Kurortny District of St.Petersburg is located along the northern coast of the Gulf of Finland to the west of the St. Petersburg Flood Protection Facility. It has special importance as a unique recreation zone of the North-West of Russia. Coastal erosion is one of the most serious problems of the area. The analysis of historical materials, archive aerial photographs and modern high-resolution satellite images have shown that advancing parts of coast are almost non-existant with most sections of the coast being eroded and further retreating. Field monitoring between 2004 and 2007 showed intense damage to sandy beaches during autumn and winter storms and progressive erosion of the dunes system. Among the most important natural reasons for the erosion processes are that the coastline is open to storm waves induced by westerly and south-westerly winds, the geological structure of coastal area (easily eroded Quaternary deposits) and a sediment deficit. In some areas sediment loss was the result of the submarine coastal slope morphology (a steep slope of a narrow submarine terrace within the area of sediment drift discharge), with erosion of an alongshore submarine sandy terrace and erosion runnels at the depth 8-12 m. The situation becomes worse due to anthropogenic impact. The southern coastal zone dynamics are also very active. According to an aerial and satellite photos analysis from 1975-1976 to 1989-1990, sandy beaches to the west of Lebyazhye village were eroded up to 30 m, and near Bolshaya Izora village up to 70 m. The comparison of coastine GPSsurvey with old nautical and topographic charts published in the 1980s shows the considerable change.