Estuarine, Coastal and Shelf Science 80 (2008) 31–41
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Estuarine, Coastal and Shelf Science 80 (2008) 31–41 Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Field observations on hydrodynamic and coastal geomorphic processes off Harilaid Peninsula (Baltic Sea) in winter and spring 2006–2007 U¨ . Suursaar a,*, J. Jaagus b,A.Kontc, R. Rivis c,H.To˜nisson c a Estonian Marine Institute, University of Tartu, Ma¨ealuse 10a, Tallinn 12618, Estonia b Institute of Geography, University of Tartu, Vanemuise 46, Tartu 51014, Estonia c Institute of Ecology, Tallinn University, Narva 25, Tallinn 10120, Estonia article info abstract Article history: Investigations of multi-layer current regime, variations in sea level and wave parameters using a bottom- Received 30 April 2008 mounted RDCP (Recording Doppler Current Profiler) during 20 December 2006–23 May 2007 were Accepted 5 July 2008 integrated with surveys on changes of shorelines and contours of beach ridges at nearby Harilaid Available online 18 July 2008 Peninsula (Saaremaa Island). A W-storm with a maximum average wind speed of 23 m sÀ1 occurred on 14–15 January with an accompanying sea level rise of at least 100 cm and a significant wave height of Keywords: 3.2 m at the 14 m deep RDCP mooring site. It appeared that in practically tideless Estonian coastal waters, sea level Doppler-based ‘‘vertical velocity’’ measurements reflect mainly site-dependent equilibrium between currents waves resuspension and sedimentation. The mooring site, 1.5 km off the Kelba Spit of Harilaid, was located in vertical fluxes the accumulation zone, where downward fluxes dominated and fine sand settled. As a result of storms in RDCP January and April, the distal part of the accumulative gravel spit advanced by 50 m, whereas a 30–50 m pebble shores retreat of the shoreline in the western and northern parts occurred at Cape Kiipsaare. The location of the Baltic Sea beach ridges shows that the development of the spit occurs through relatively short-period but infre- quent storm events, roughly 2–3 times each decade. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction order to combine hydrodynamic and coastal geomorphic investi- gations. Using the RDCP-600, we have discussed multi-layer The coastal sea near Harilaid Peninsula in NW Saaremaa Island hydrodynamics in previous studies (Suursaar et al., 2005; Suursaar (Fig.1) has the roughest wave regime along Estonian coastal waters, and Aps, 2007). Doppler effect-based instruments, referred by where wave heights may reach 9–10 m (Soomere, 2001; Soomere different manufacturers as DCM (Doppler Current Meter), ADCP, et al., 2008). A quickly developing gravel spit in Cape Kelba and the RDCP, ADV, etc., have proved very useful in studies of frontal famous leaning lighthouse at Cape Kiipsaare are located in the area, structures and vertical fluxes (e.g. Marmorino and Trump, 1992; which lies within Vilsandi National Park. It is a hydrodynamically Geyer, 1993; Yanagi et al., 1995; Wewetzer et al., 1999; Andersen active location, where historical changes in shoreline position and et al., 2007). Nevertheless, technical difficulties and low velocities contour probably reflect changes in wind and wave regime (Orviku have left direct measurement of real vertical velocities et al., 2003; Rivis, 2004; To˜nisson et al., 2008). Anticipated climate controversial. change, manifested in the Baltic Sea region by warmer winters, According to our previous measurements (Suursaar and Aps, higher mean and extreme sea levels, more frequent strong storms 2007), vertical velocities were about 1–2 cm sÀ1, whereas velocities and decreasing sea ice extent and duration (Alexandersson et al., less than 0.1 cm sÀ1 even in upwelling conditions are common in 2000; Jaagus, 2006; Suursaar and Sooa¨a¨r, 2007; BACC, 2008), is the literature (e.g. Lehmann et al., 2002; Kowalewski and Ostrow- likely to intensify shore processes (Kont et al., 2003; To˜nisson et al., ski, 2005). In contrast, both Doppler effect-based velocity 2007). measurements and non-hydrostatic 3D models (e.g. Deleersnijder, A 5-month long record on currents, waves and other hydro- 1989; Kanarska and Maderich, 2003) reveal much higher vertical logical conditions was obtained by the means of a Recording velocities in coastal zones. Doppler Current Profiler (RDCP) placed 1.5 km off the Kelba Spit in The main objectives of the study are (1) to present the results of in situ measurements of hydrophysical variables, currents and waves near the geomorphically active coastal section of the Harilaid Peninsula over a 5-month period (December 2006–May 2007), (2) to * Corresponding author. E-mail addresses: [email protected] (U¨ . Suursaar), [email protected] (J. Jaagus), examine vertical structures of currents and to interpret ‘‘vertical [email protected] (A. Kont), [email protected] (R. Rivis), [email protected] (H. To˜nisson). velocities’’ measured by the RDCP, and (3) to analyze the relationships 0272-7714/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2008.07.007 32 U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41 Fig. 1. Study area (a); location of the RDCP mooring and coastal geomorphic study sites at Harilaid Peninsula (b). between meteorological, hydrodynamic and geomorphic processes observed throughout the 20th century (Orviku et al., 2003; Rivis, of the particular location and to investigate the specific changes in 2004). The earlier accumulative shoreline positions on Cape Kiip- shoreline of the Harilaid Peninsula during the studied period. saare are well-marked by a series of parallel beach ridges that cross the current shoreline at a 30–35 angle. Shore processes during the 2. Study area last century have caused the northwesternmost point of the peninsula to migrate to the northeast and become longer and nar- The study area is located in Saaremaa, the largest island rower. The study site is most influenced by waves from the south- (2671 km2) of the West Estonian Archipelago in the Baltic Sea west, west, northwest and north. The sea bottom is particularly flat (Fig. 1). The main geomorphic features of the coastal zone of the and shallow north and northwest of Cape Kiipsaare: the 5 m isobath island reflect preglacial relief, morphogenesis of the last glaciation is an average of 4 km from the shoreline (Fig. 1b). and postglacial isostatic uplift with a present rate of 2–2.5 mm per year (Vallner et al., 1988). The Baltic Sea near the Estonian coast is 3. Material and methods nearly tideless and the geomorphic role of tidal motions is negli- gible in comparison with waves and currents (e.g. Suursaar and 3.1. Meteorological and sea level observations Kullas, 2006). Harilaid (Fig. 1b) is a trapeze-shaped peninsula with an area of We acquired data on wind speed and direction from the Vilsandi 3.6 km2 and joined to the Tagamo˜isa Peninsula by a narrow meteorological station, operated by the Estonian Meteorological tombolo. The primary landform of the peninsula is a NW–SE and Hydrological Institute (EMHI), which is the closest station to trending glacial ridge. The shape of the submarine portion of the our measurement site, 7 km south of Harilaid Peninsula (Fig. 1). The emerging ridge has been affected by erosion from waves. There are station is located on the western coast of Vilsandi Island (58230N, two study sites located on Harilaid Peninsula. 21490E). The location is the most open among the Estonian The Kelba site – under investigation since 1960s (Orviku, 1974; stations, but low trees and brush to the east and a lighthouse to the Orviku et al., 2003) – is situated in the southern part of the Harilaid west might slightly impede wind. Since September 2003 meteo- Peninsula. This site consists of a series of beach ridges forming an rological stations in Estonia have been equipped with MILOS-520 approximately 1-km long spit. The spit consists mainly of crystal- automatic weather complexes, which provide hourly average wind line pebbles, and the beach ridges within it form distinct incre- speed, gust wind speed and hourly prevailing wind direction. The ments of different age. The beach ridges are typically less than 2 m weather data consisted of hourly wind and air pressure data, as high, only on the root area of the spit the highest beach ridges are well as daily mean air temperature measured from December 2006 3.8 m above sea level. Lagoons and small lakes lie behind the spit. until May 2007. Long-term mean values for the period 1966–2005 The Kelba Spit is exposed to the Baltic Proper between the azimuths were used to describe average temperature and wind regime. of 240 and 330. Hence, large waves can approach the site only from Although the RDCP measures relative sea level variations, sea the southwest, west and northwest. Deeper waters (over 10 m) lie level data from Ristna and So˜rve tide gauge stations (Fig. 1)were west of Kelba. The nearshore bottom is inclined with the 2 m iso- used for background information. The Ristna station is located on bath lying a few meters from the shoreline. the southern coast of Ko˜pu Peninsula (Hiiumaa Island), some 50 km The Cape Kiipsaare study site is located in the northwesternmost distant from Harilaid Peninsula and obviously provides somewhat tip of Harilaid Peninsula (Fig. 1b). The sandy cape is among the most site-dependent values. The tide gauge, which is operated by the rapidly developing coastal areas in Estonia, along which notable EMHI as well, is an old Rohrdanz-type. Hourly data are read changes in shoreline displacement and shore processes have been from paper tapes and later digitalized and transmitted to EMHI U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41 33 headquarters in Tallinn.