Estuarine, Coastal and Shelf Science 80 (2008) 31–41

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Estuarine, Coastal and Shelf Science

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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 s1 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 infrequent storm events, roughly 2–3 times each decade. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction order to combine hydrodynamic and coastal geomorphic investigations. 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; Toñisson 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 s1, whereas velocities and decreasing sea ice extent and duration (Alexandersson et al., less than 0.1 cm s1 even in upwelling conditions are common in 2000; Jaagus, 2006; Suursaar and Sooaä¨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; Toñisson 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. Toñisson). velocities’’ measured by the RDCP, and (3) to analyze the relationships

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 negligible 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. Sea level at the So˜rve station is measured 4. Results by tide pole twice daily (at 06 and 12 UTC). 4.1. Meteorological conditions, sea level variations and water 3.2. In situ measurement of waves, currents and sea level variations column properties

A Recording Doppler Current Profiler RDCP-600 from AADI Weather conditions from December 2006 to May 2007 were Aanderaa Instruments applies the Doppler effect to measure rather exceptional (Fig. 2), e.g. 2006 featured the warmest velocity by acting both as source and receiver while bouncing short December (mean air temperature þ6.5 C at Vilsandi versus long- pulses (pings) of acoustic energy off small particles, plankton and term average 0.6 C) since the beginning of instrumental observa- air bubbles. The frequency of the backscattered signals are Doppler tion in the mid-19th century. Air temperature dropped rapidly after shifted proportionally to the average alongbeam relative velocity January 20. The prevailing southwesterly winds were replaced by between the scatterers and the transducers. Four beams are southeasterly and northeasterly winds and the daily mean air simultaneously pinged by transducers with a 25-slant angle in the temperature decreased to 13.6 C on 22 February. By the end of RDCP-600 and three velocity components are calculated from the February, ice covered the entire Vaïnameri Sea and the northern backscattered signals. part of the Gulf of Riga. The RDCP site remained ice-free. The water Divers deployed the self-contained RDCP-600 on the seabed at temperature at the depth of 14 m was below or around zero the location of 58.46N, 21.82E, about 1.5 km offshore (Fig. 1b). The between 16 February and 4 March. A brief advective warm-up upward looking instrument operated from 20 December 2006 to 23 occurred February 20–22, when the water temperature reached May 2007, collecting 3691 h of multi-layer data. The mooring depth 2.5 C and salinity 7.9 (Fig. 2b). was about 14 m, but varied with sea level changes. Excluding a 2-m The air temperature fluctuations were affected greatly by ‘‘blind’’ distance between the instrument and the lowermost prevailing wind directions during the study period. Generally, measurable cell, plus the top 2–3 m in which measurements are warm air advection in winter occurs together with westerly or ‘‘contaminated’’ by wave motions, data from eight depth layers was southwesterly winds, whereas cold air comes from northern obtained. As a 2-m cell size with 50% overlap was used, ‘‘3 m depth’’ and eastern directions (Jaagus, 2006). The study period expe- actually denoted the 2–4 m depth interval, etc. Thus, the instru- rienced fewer northerlies and more southerlies and north- ment is capable to provide only general hydrodynamic information, westerlies compared with the long-term average. The number not the real near-bottom velocities. Each current record contained of storm days, i.e. days when mean wind speed exceeded an average of 300 pings. Each ping had a statistical noise of 15 m s1, was 7 in December and 13 in January. The maximum 9cms1, our set-up yielded estimated standard noise levels as low wind speed of 23 m s1 (gusts up to 33 m s1) was measured on as 0.58 cm s1 for the horizontal currents and 0.29 cm s1 for 15 January (Fig. 2d). Prevalence of southwesterly winds in vertical currents. December and January caused above-average sea levels (Fig. 3a). In addition to current measurements, the RDCP-600 is equipped The maximum at Ristna (þ171 cm) occurred in the evening of with a temperature, turbidity and conductivity (i.e. salinity-) January 14 and the maximum at So˜rve (þ103 cm) occurred in sensor, as well as a quartz-based pressure sensor (model 3187, the morning of January 15. Starting from February, when east- resolution 0.001% of full scale), which enables measurement of the erlies and northerlies prevailed, the sea level dropped below depth of the instrument (or relative sea level variations) and wave average. parameters, such as significant wave height, maximum wave The sea level variations at both So˜rve and Ristna stations and at height, peak period, mean period, wave steepness and wave spec- the RDCP measuring site were generally synchronous (Fig. 3a). trum. Data output also includes time series of signal strength, However, each dataset shows site-specific response to different standard deviations of each beam, and information on instrument wind directions. In addition, both variability and extremes at conditions (e.g. direction, pitch, roll, battery voltage). The coastal stations were greater than on the open sea. The standard measuring interval was set to 1 h. The raw data, pre-processed by deviation of the sea level variation was 35 cm at Ristna, 31 cm at the special software, were stored on a multi-media card. So˜rve, and 23 cm as measured with the RDCP.

3.3. Shoreline changes 4.2. Waves

A comparative analysis of coastal changes was based on The RDCP provides both significant (HS) and maximum (Hmax) positional measurements of shorelines, scarp edge contour lines wave heights and several estimates for wave periods. Originally, the and contours of beach ridges, using orthophotos from 2005 and HS was defined as the average height of the 1/3 highest waves and is GPS measurements in 2005, 2006 and 2007. Garmin 12 and roughly equal to visual ‘‘wave height’’. In both RDCP and numerical Garmin 60CS were used in GPS measurements, both of which are wave models H is estimated directly from the wave energy spec- Spffiffiffiffiffiffiffi accurate to within 3 m. Along relatively straight shoreline trum as HS ¼ 4 m0, where m0 is the zero-order moment of the measurements were taken every 10 m with more frequent wave spectrum, which is nevertheless close to the traditional measurements along more indented shoreline. Sea level fluctua- definition (e.g. Broman et al., 2006). tions during measurement were no more than 20 cm above or During the measuring period, the significant wave height below mean sea level, which in Estonian coastal waters is reached 3.16 m on 15 January, was over 2 m five more times in currently near the so-called Kronstadt zero reference (Suursaar January, and 2.24 m on 20 April (Fig. 3d). The significant wave and Kullas, 2006). Due to the steep nearshore topography of the height averaged 0.56 m. The maximum wave heights do not study sites, the influence of sea level variations on coastline provide much additional information, since they represent configuration was insignificant. a computational parameter and behave synchronously with HS. The A topographic survey to assess beach profiles and volume recorded maximum wave height (4.59 m, 15 January) was 1.45 changes was taken at both sites in the summers of 2006 and 2007. times higher than the corresponding HS and the ratio remained Photos were taken at identical locations at the study sites. Shoreline around 1.5 during the rest of the study period. However, the height changes, changes in area covered by beach ridges, and the rates of of the every hundredth wave could be as much as 1.67HS and the erosion and accumulation were calculated using MapInfo software height of single extreme waves can reach as much as 2HS (Soomere, (e.g. Rivis, 2004). 2001). 34 U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41

a Dec Jan Feb March April May 14 C) o

0 2006/07 1966-2005 Air temperature (

-14 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days, from 20.12.06)

b 12 8

C) 9 Salinity 7.6 0 Salinity 6 7.2

3 6.8

0 Temperature 6.4 Temperature ( -3 6 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days, from 20.12.06)

c 360

270

180

90 Wind direction (deg.) 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days, from 20.12.06)

d 24 ) -1 18

6 Wind speed (m s Wind (m speed 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days, from 20.12.06)

Fig. 2. Variations in daily mean air temperature (a; together with corresponding average values for 1966–2005) and wind conditions (c and d; hourly data) at Vilsandi station; variations in water temperature and salinity (b) at the depth of 14 m during the study period (December 2006–May 2007).

The peak wave periods reached 11 s (Fig. 3c), which is fairly close 4.3. Wind-driven currents to those of Gudrun in January 2005 (Soomere et al., 2008). The mean wave period reached 7.8 s and it averaged 5.2 s. The latter value is The mean velocity modula of horizontal currents were not large, overestimated, as for the depth of 14 m, the wave periods shorter than ranging between 5.8 and 6.3 cm s1 at different depths (Figs. 3band4). 4 s provided by the RDCP are ignored. The instrument is suitable for The maximum current speeds varied between 30.6 and 34.9 cm s1 wave periods as short as 1 s without aliasing. As the dynamic pressure (Figs. 4band5). This indicates that unlike the Va¨inameri straits, where caused by waves with short wave period is damped rapidly with the current speed can reach 1.5 m s1 (Suursaar et al., 2006)orthe depth, these periods cannot be reproduced in practical deployments. northern coast of Estonia, where upwelling-related coastal jet reached U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41 35

a Dec Jan Feb March April May 15.8 160

RDCP Sea level (cm) Tide gauge 15 80

14.2 0 Instrument depth (m) 13.4 -80 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days from 20.12.06) b 30 ) -1

0 W-E current (cm s -30 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days from 20.12.06) c 12

Peak wave period (s) 3 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days from 20.12.06)

d 3

Signif. wave (m) Signif. height 0 0 102030405060708090100110120130140150 Time (days, from 20.12.06)

Fig. 3. Variations in sea level at Ristna tide gauge and as RDCP ‘‘instrument depth’’ (a), alongshore (W–E) ﬂow component at 10 m depth (b), peak wave period (c), and signiﬁcant wave height (d) for the period 20 December 2006–23 May 2007.

60 cm s1 with merely 5–7 m s1 wind speed (Suursaar and Aps, The water mass was vertically rather homogeneous and the 2007), the Kelba site does not feature strong wind-driven currents. variations in current speed in depth were minor (Fig. 4). The Similar values were reported around the other locations of the Taga- correlation coefficients between the main (u) velocity components mo˜isa Peninsula (Ko˜uts and Laanearu, 2001). of adjacent layers were between 0.95 and 0.98, and 0.8 between the Due to the proximity to the coast, the currents were rather uppermost and the lowermost layers. The data of adjacent v- polarized and modified by the coastline. The clearly prevailing u components had correlation coefficients between 0.73 and 0.90. (W–E) velocity component (Fig. 3b) described about 80% of the total Thus, Figs. 3b and 5 are quite representative for all the depth layers. variability in upper layers and up to 86% in near-bottom layers. The minor features include a slight decrease in flow speed with Although the westward flow was slightly more frequent (51%), the depth and a slight increase in frequency of westward currents with eastward currents were faster, thus yielding resulted eastward depth. In addition, while the mode of the westward current alongshore flow during the study period. direction remained between 275 and 285, the most common 36 U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41

a Vertical velocity (cm s-1) b Horizontal velocity (cm s-1) -1 0 1 02040 2 2

4 4 modulus resultant max 6 st.dev. 6 up down 8 8 Depth (m) Depth (m) 10 10

12 12

c Correlation coefficient d Correlation coefficient 0101 0 0

2 Wu/Cu Wv/Cu 2 Wu/Cv Wv/Cv

4 4

6 6 Depth (m) Depth (m) 8 8

10 10

12 12

Fig. 4. Vertical distributions of mean up- and downward vertical velocity together with standard deviation (a); average modulus and maximum velocity of horizontal flow (b); correlation coefficients (c and d) between velocity components (u and v) of wind (W) and current (C). direction of the eastward current shifted from 105–110 in the downward velocities decreased or even reversed, while during subsurface layer to 90–95 in near-bottom layer. calm periods downward motions were more rapid (Fig. 6). In total, 33% of vertical velocity measurements were >1cms1 Correlation between wind and horizontal flow components was and 3.6% were >2cms1 (Figs. 4a and 6b). As much as 87% of total rather complex (Fig. 4c and d). The v (S–N) component of flow velocity readings were downward. The mean vertical velocity was depended on the corresponding (i.e. S–N) wind component only in about 0.7 cm s1 and decreased slightly with depth (Fig. 6c). During the upper layer (up to 7 m depth). The u (W–E) component of flow stormy periods, and especially in the near-bottom layers, strongly depended on wind over the entire depth range, but the

a b 360 24 360 36 Current speed (cm s Wind speed (m s

270 18 270 27 direction speed 180 12 180 18

90 direction 6 -1 90 9 ) speed -1 ) Wind direction (deg.) Current direction (deg.) 0 0 0 0 14.01 15.01 14.01 15.01 Date Date

c 15.2 170 d 5 16 Peak wave period (s) RDCP depth Height Ristna Sea level (cm) 4 Period sea level 14.8 130 12 3

2 14.4 90 8 1 Instrument depth (m) Max wave height (m) 14 50 0 4 14.01 15.01 14.01 15.01 Date Date

Fig. 5. Variation is wind speed and direction at Vilsandi station (a), current speed and direction at 4 m (b), sea level together with air pressure-corrected instrument depth (c), and maximum wave height and wave period (d) during the storm event on 14–15 January 2007. Time expressed as UTC. U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41 37

a Dec Jan Feb March April May 5 15

4 10 Turbidity (NTU)

3 5

2 0 Wave height (m) 1 -5

0 -10 0 102030405060708090100110120130140150 Time (days, from 20.12.06)

b 3 0 ) Signal strength (dB)

-1 2 -5

1 -10

0 -15

-1 -20 Vert. Velocity (cm s

-2 -25 0 102030405060708090100110120130140150 Time (days, from 20.12.06)

c 0 .. 11 m 10 m 6 m 4 m stormy period

calm periods stormy period Cumul. vert. vel. (km) -110 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (days, from 20.12.06)

Fig. 6. Variations in turbidity and maximum wave height (upper and lower graphs in a), backscattered signal strength and vertical velocity (upper and lower graphs in b); original data together with smoothed using 2-day moving average at 11 m depth; cumulative vertical velocities for selected depth layers (c).

responsible wind component was v (Fig. 4c). Surprisingly, there was beside the spit. In the proximal part of the spit, the shoreline has no direct association between currents and u (W–E) wind compo- receded by up to 15 m along a 400 m stretch accounting for nent. These features can be attributed to bending of the Kelba approximately 3500 m2 of land loss. The eroded volume in the coastline from meridional direction to parallel. Secondly, we must proximal part of the Kelba Spit is relatively small, as the nearshore consider general wind-driven ﬂow patterns, which are visible in sea is only 2 m deep and beach face is covered by cobble and small oval-shaped lakes and bays. Depending on wind direction, boulder sized sediments. Such large sized sediments act as wave pair of gyres can develop within the basin and compensatory ﬂow breakers and most of the energy generated by the swash does not can be against the wind in certain locations (e.g. Suursaar and reach the higher elevations of the spit. We estimate that the erosion Kullas, 2006). in the proximal part has been approximately 4000–5000 m3, whereas accumulation in the distal part has been nearly 13 000 m3. 4.4. Changes in coastline between summers 2005 and 2007 Recent measurements in Kiipsaare study site show retreat of the shoreline and sandy scarp in the western part of Cape Kiipsaare The shoreline measurements taken in Kelba in summers 2005, (Fig. 7b), while the shoreline in the northeastern part of Cape 2006 and 2007 show an advance of the spit as a result of accu- Kiipsaare has been quite stable. The GPS measurements taken mulation (Fig. 7a). The distal part of the spit has shifted from north between summer 2005 and summer 2007 show a clear retreat of to northeast. The distal top of the spit has advanced up to 50–60 m, the shoreline in the western (up to 15 m), northwestern (30–50 m), and its area has increased by about 4500 m2. The end of the spit northern (up to 130 m) and northeastern (10–20 m) parts of Cape consists mainly of gravel eroded from both the proximal part of the Kiipsaare. The land area has decreased by approximately 17 500 m2. spit itself as well as from less than 2 m deep nearshore sea bottom The sandy scarp on the northern and northwestern shore has 38 U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41

differed remarkably from that in So˜rve (88 cm over the average) or of the RDCP (78 cm over the average), and should be considered non-representative. For the ﬁrst time, the same effect during the extreme storm in January 2005 was reported by Suursaar et al. (2006). The sea level on the Kelba coast probably reached 100–120 cm above average, which is nevertheless quite a remark- able event. In addition, strong wave action should be considered. Wind waves in shallow water are limited by wind speed, time and fetch, but also by depth. The water depth at the RDCP location (corrected with the Vilsandi air pressure data) varied between 13.5 and 14.7 m (Fig. 3), to which 0.3 is added to account for the sensor height over the seabed. About 1 km west of the instrument, the depth drops to 25 m and after 7 km it reaches 50 m. Access of high waves is permitted only from directions between 250 and 300 with corresponding fetches of 150–250 km. The fetch is 5–20 km to the SE, S and SW, and just 2–5 km to the N, NE and E. Nevertheless, the directions of strongest possible winds (Soomere, 2003) coincide nearly with the directions of the longest fetches. According to the model simulations, the signiﬁcant wave height can reach as much as 9–10 m at Harilaiu Bank (Soomere, 2001), which is 10–15 km from our measuring site, where the depth drops sharply from 20 m to about 70 m. During Gudrun in January 2005, the roughest wave conditions in the Baltic Sea were estimated to have been off the coast of NW Saaremaa, where HS reached 9.5 m and the peak wave period exceeded 12 s according to assessments made by different operational wave model simulations, as well as on measurements in the northern part of the Baltic Proper (Broman et al., 2006; Soomere et al., 2008).

5.2. Interpretation of ‘‘vertical velocities’’

The curves in Fig. 6 reveal that the RDCP do not deliver the real vertical velocities of water. Our previous measurements at different locations in the Estonian coastal seas (Suursaar et al., 2005; Suur- saar and Aps, 2007) were not long enough for explaining their origin. Although the nearshore measuring sites located above gentle bottom slopes, where up- and downwelling or upsloping could occur, it was not possible to explain the surprisingly large 1 Fig. 7. Changes in shoreline and contours of old beach ridges at Cape Kelba (a), (order of 1–2 cm s ) values by true hydrodynamic processes. changes in shorelines and scarp contours at Cape Kiipsaare (b) between summers Unusually high vertical velocities compared to those of ‘‘ordinary’’ 2005, 2006 and 2007. models are also suggested by non-hydrostatic 3D hydrodynamic models (Deleersnijder, 1989; Hervouet, 2003; Zalesny et al., 2004). Nonetheless we assume that in addition to the ‘‘real’’ vertical receded by 35–50 m (Fig. 7b). As the mean height of the scarp is movements of water (which are very small), the data largely a little bit over 1 m, the approximate amount of eroded sediment is include the influence of particle movement that are not neutrally about 3000–4000 m3. At the same time, the shore has advanced up buoyant: air bubbles, suspended matter, detritus, marine organ- to 25 m (about 10 000 m2) in the southwestern part of the study isms, the influence of orbital motions of waves, etc. (Ott, 2005; site, about 100 m south of the Kiipsaare lighthouse. Suursaar and Aps, 2007). The Doppler-based technology presumes that scattered parti- 5. Discussion cles flow at the same speed as water currents, but this is valid only for horizontal motions. The relatively low vertical velocity values 5.1. On the wind waves and their influence on sea level included some ‘‘useful’’ signal, which rose above the white noise measurements at Ristna spectrum in the low frequency spectral band (Suursaar et al., 2005). The source of that signal can vary at different locations, but in the Ristna tide gauge locates in the middle of the beach in a small nearshore of the Harilaid Peninsula, the ‘‘vertical velocity’’ seems to embayment. It seems, that wave run-up over the beach during reflect primarily the equilibrium between resuspension and extreme SW storms may bias sea level recording for up to 30–50 cm settling of suspended mineral matter (Figs. 6 and 8). (Figs. 3a and 5c), which may render this tide gauge unsuitable. The Correlation analysis shows that the vertical velocity is primarily effect of super-elevation due to set-up and swash is a well-known associated with turbidity through the side-parameter called signal process (e.g. Longuet-Higgins and Stewart, 1964; Stockdon et al., strength (Figs. 8 and 9). The intensity of the backscattering signal 2006). During the January 2007 storm, the highest sea level at depends on properties of the water, as well as on particles within. Ristna (171 cm) was comparable to the maximum at Pa¨rnu During storm events, downward ‘‘vertical velocity’’ decreases with (176 cm). Normally, Pa¨rnu Bay has the greatest sea level variability depth and changes even to upward flux in near-bottom layer, while in the Estonian coastal sea (Suursaar et al., 2006; Suursaar and during calm periods downward motions prevail over the whole Sooaä¨r, 2007) with a record surge of 275 cm. Such a high value at depth range (Fig. 6c). Although the resulted vertical (i.e. downward) Ristna, which was 160 cm over the 5-month periods average, velocities are faster during calm periods, both signal strength and U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41 39

a Correlation coefficientb Correlation coefficient 0 0.8 0 0.8 0 0 vert.vel/turbid. 2d smooth vert.vel/signal s. 2d smooth 2 2

4 4

6 6 Depth (m) 8 Depth (m) 8

10 10

12 12

Fig. 8. Vertical distribution of correlation coefﬁcients between vertical velocity and turbidity (a), and vertical velocity and signal strength (b), calculated between both the original data and smoothed with 2-day moving averages.

turbidity are considerably lower. It shows that upward flux from 5.3. Role of storm events in shore changes bottom is weak due to absence of resuspension, while some, though gradually lesser previously resuspended or laterally trans- According to the geomorphic observations in Harilaid Peninsula ported matter is still present in water column and is settling over the last decades (Orviku et al., 2003; Rivis, 2004), the Kelba undisturbedly. The correlations between both signal strength and Spit has developed by gradual accumulation of new beach ridges vertical velocity, as well as between turbidity and vertical velocity, that reshape and elongate the spit in its distal direction (Fig. 8a). In were stronger in near-bottom layers (Fig. 9). Smoothing of the recent years, a clear retreat of the shoreline in the proximal part of series yields stronger correlation (Figs. 6 and 8), but r ¼ 0.44 for the the Kelba Spit has occurred as a result of strong erosion of the older 3691 pairs of original near-bottom data is highly significant as well. beach ridges and lack of sediment on submarine shoals. During Signal strength results equally by both turbidity and wave action some of the strongest storms, waves have rolled over the spit. (Fig. 9), and the corresponding correlation coefficients were The coastline in summers 2005 and 2006 were nearly identical between 0.5 and 0.7 over the entire depth range. Influence of (Fig. 7a). As there were no other noteworthy storms other than current speed on turbidity is considerably lower than in waves those in winter 2006–2007, the coastline changes measured (Figs. 6 and 9). between the GPS surveys in summers 2005, 2006, and 2007 reflect Storm events see an initial increase in both wave height and that their impact and the influence of calm summer conditions was signal strength followed shortly thereafter by an increase in negligible. There were distinctive storm episodes on 11–15 and 26– turbidity and vertical velocity (Fig. 6). Signal strength probably 27 January, 19 March and 20–21 April (Figs. 2d and 3). Strictly includes also effects due to the fraction (e.g. small air bubbles), speaking, as we have no land or satellite data between these which is not registered by the turbidity sensor. Wave action storms, it is not possible to attribute the above described coastal during the first autumn storm caused relatively slow increase changes to any of these separately. However, considering that the in turbidity, as the bottom sediments were somewhat consol- wind-driven current cannot be a notable forcing for reforming the idated. After the strong storm in January, turbidity remained at spit of pebble and gobble, the major contribution must appear as a high level for a long period. Each new wave event caused the combined effect of both waves and high sea level. Taking into a new rise in turbidity, as sufficient mobile sediment was account that wave energy is proportional to the wave height available. (Fig. 3d) squared, the waves on 14–15 January were roughly twice Upon retrieving the instrument in May 2007, the mooring frame as powerful as the ones during the other storm episodes. Also, the was covered by 20–30 cm of fine sand. The existence of the nearby sea level was about 1 m higher then (Fig. 3a). Thus, we can assume Kelba Spit also confirms the mooring site as being in the accumu- that although each of the storm events had its contribution, the lation zone. However, the settling conditions depend greatly on main changes occurred on 14–15 January (Fig. 5). depth. While accumulation of gravel and pebble occurs in the The earlier notable systems of beach ridges in Kelba also highly dynamic surf zone of the nearby spit, finer fractions may developed as a result of powerful storms too – in January 2005 deposit off the coast (Amos et al., 1997). Obviously, the settling is not constant in time (e.g. Heiskanen and Tallberg, 1999). The settled matter originated both from the resuspended particles at the site, and to a greater extent, from sediment laterally transported by currents from the nearby locations, where abrasion and erosion dominate. Such areas where the coastline gradually shifts eastward under strong wave action (Orviku et al., 2003) exist in the northern part of the Harilaid Peninsula. We assume that northwards, where erosion exceeds accumulation, the ‘‘vertical velocity’’ might be upward. Hence, the vertical velocity feature of the RDCP is not suitable for measuring real water motion, but may function as a kind of virtual sediment trap (Fig. 6c), nevertheless providing valuable information of geomorphic nature. The RDCP may offer some other applications in less turbulent seas. For example, Zedel et al. (2003) have interpreted the ADCP data Fig. 9. Correlation coefficients between modula of wind (A), water flow (F), wave measured in the Norwegian Sea to detect both horizontal and height (W), near-bottom turbidity (T), signal strength (S) and vertical velocity (Z). F, S vertical movements of herring schools. and Z 3 m from the bottom. 40 U¨. Suursaar et al. / Estuarine, Coastal and Shelf Science 80 (2008) 31–41

(Suursaar et al., 2006; Toñisson et al., 2008), in 2001, and 1999 (e.g. between the GPS surveys in summers 2005, 2006 and 2007 Rivis, 2004). During the more powerful January 2005 storm, in resulted from the stormy period in winter 2006–2007, and addition to strong accumulation in the distal part of the spit, the sea mainly from the storm event on 14–15 January 2007. The level was extremely high and waves rolled over the spit, levelling location of the beach ridges shows that the development of the the crests of older beach ridges and carrying material to the lagoons spit occurs through relatively short and infrequent storm (Toñisson et al., 2008). In 2007, the sea level and waves were not as events, roughly 2–3 times each decade. high and fresh beach ridges formed mainly at the coastline. The storm began in the morning of 14 January with moderate winds (10–17 m s1), which turned by evening from S to SW, and later to Acknowledgements NW (Fig. 7a). The greatest changes at Kelba occurred from the evening of the 14th January until the morning of the 15th (Fig. 5), This work was supported by Estonian Science Foundation when the wave action was strongest (4–4.5 m) and both the wave- Grants Nos. 7609 and 7564. The authors express their gratitude to generated and wind-driven nearshore currents (up to 35 cm s1 at Dr. Georg Martin, Kaire Kaljurand and Arno Po˜llumaë, who assisted the RDCP site and probably more near the coast) were directed as divers in the RDCP mooring, and to Dr. Robert Szava-Kovats for eastward, leading to elongation of the tip of the spit (Fig. 7a). some valuable suggestions in preparing the manuscript. Obviously, wave action is the main hydrodynamic agent near Harilaid Peninsula, while wind-driven currents only provide References a background drift effect for material suspended by waves (Fig. 5). Compared to wave action, longshore sediment transport by Alexandersson, H., Tomenvirta, H., Schmith, T., Iden, K., 2000. 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