Waves, currents and their relationships with coastal processes near Kelba Spit (Saaremaa Island) on the basis of RDCP measurements

Abstract

A 5-month long record on currents, waves and other hydrological variables was obtained by means of Recording Doppler Current Profilers (RDCP) 1.5 km off the Kelba spit of Saaremaa Island over a period from December 2006 to May 2007. A parallel study of coastal changes was based on orthophotos and the measurements of beach ridges, as well as on the GPS measurements made in 2005, 2006 and 2007 to identify changes in the shoreline and scarp contours and positions. The results suggest that the major changes in coastline at Harilaid Peninsula occur mainly during infrequent but strong storms, e.g. such as the January storm (Gudrun) in 2005. During the measurement period, a strong storm event occurred, too. At Vilsandi station, the maximum wind speed reached 23 m/s (gusts up to 33 m/s) on 15 January 2007 and the maximum sea level at Ristna tide gauge reached 171 cm. The RDCP mooring site fell into accumulation zone, where downward fluxes dominated and fine sand settled. The currents at the RDCP location were not large, but the significant wave height reached 3.2 m in the 14 m deep mooring site and the maximum wave height was 4.6 m. It appeared that in Estonian coastal waters, the Doppler-based vertical velocity measurements reflect the site-dependent equilibrium between resuspension and sedimentation. The GPS measurements taken between summers 2005, 2006 and 2007 show that the tip of the Kelba spit has shifted from north back to north-east. As a result of the single storm, the distal part of the spit has advanced by 50-60 m and its area has increased by 4500 m2. At Cape Kiipsaare, as it also has happened during previous large storms, a clear retreat of the shoreline in the western, north-western (30-50 meters), and north-eastern parts has occurred.