Analysis of shore changes between August 2008 and August 2009 and their relationship to hydrodynamic conditions on Kelba Spit, Saaremaa Island, Estonia

The study is focused on a gravel-pebble spit in Kelba. It is a dynamic region in the magnitude and velocity of hydrodynamic and shore processes, where historical changes in shoreline position reflect the changes in wind and wave climate. The region has probably the roughest wave climate along the Estonian coast. In order to assess the changes in shoreline and configuration of shore formations, the study sites in Saaremaa have been regularly examined since the 1960s. The study of coastal changes was based on orthophotos and GPS measurements. Sonar and leveling surveys were carried out in 2008 and 2009 to assess changes in the volume of deposits. Recording Doppler Current Profiler was deployed near Kelba Spit from December 2006 to May 2007 to measure hydrodynamic variables. In order to assess variations in forcing conditions during the study period (2008-2009) a wave hindcast was performed using a SMB-type wave model for Kelba. Calibrated against the field measurements, the model is forced by wind data from Vilsandi station, and it calculates significant wave parameters for a chosen location. The sea level data from Ristna mareograph was also used. Changes in the area of the study site reached 7800 m² during that period. Erosion made up nearly 2/3 of the changes. Accumulation in volume reached 9200 m³ while loss of the sediments was only 6100 m³. 95% of accumulation took place in the distal part of the spit while 60% of erosion appeared in the proximal part. The year of 2008 started with stormy, ice-free conditions. Although there were no remarkable single storms or storm surges, the wave activity on average was among the strongest over the last two decades. The shore changes were faster than the annual average of the last decade showing that rapid shore changes may happen even without extreme storm events.