Transport and distribution of bottom sediments at Pirita Beach

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254Fig. 2. Palaeogeographic map of the vicinity of Pirita Beach and the Pirita River mouth. Redrawn based on Tammekann (1940),Raukas et al. (1965), Kessel & Raukas (1967), and an unpublished manuscript Kask (1997). Alvar is a limestone area coveredwith a thin soil layer. In the nomenclature of ancient coastlines, Lim stands for Limnea Sea, L for Litorina Sea, Y for Yoldia Sea,and A for Ancylus Lake. The Roman number indicates the development stage of these water bodies and the Arabic numbers showthe height of the ancient coastline above the contemporary mean water level.temporary beaches similar to Pirita Beach (Orviku 1974;Orviku & Granö 1992).An up to 35 m thick complex of Quaternary sandclaydeposits of different origin, lithological compositionand consistence has accumulated in the ancient PiritaRiver valley during different stages of the Baltic Seaunder the subsequent influence of river erosion, icedynamics, sea level changes, and deposition of sediments.The vertical composition of sediments in theneighbourhood of the former restaurant Rannahoone(see Fig. 3) apparently is representative for a large partof the ancient valley (although several layers are notnecessarily present closer to the borders of the valley).Contemporary dune sand forms the uppermost 2–2.5 mthick layer that overlies older marine sand (thicknessabout 8 m) (Nugis 1971). A thin (0.75–1.60 m) layer ofsilt follows, overlying 4.5–6.5 m thick sandy loam thatapparently formed during the Ancylus Lake Stage. Belowthese deposits lies an about 9.5–10 m thick layer ofloam. The lowermost beds consist of about 2 m thickhighly plastic varved clay, underlain by Cambrian blueclay.The thickness of the Quaternary complex diminishesand its vertical structure undergoes certain changestowards the borders of the ancient valley. At the northernborder (at Merivälja), a layer of till becomes evidenton the surface or below a thin layer of contemporarydeposits. Till is in places also traceable under a thinlayer of younger sediments on the sea bottom at thesouthwestern border in the Maarjamäe area.The rugged landscape features have thus been levelledoff during many millennia. Today the area in questionforms a part of the Pirita(-Mähe) plane (Raukas et al.1965; Kessel & Raukas 1967) that is gently slopingwestwards. The contemporary sandy beach (as well asthe beach that existed south of the Pirita River until the1970s) lies at the western boundary of the plane wherethe seabed is gently sloping until a depth of about7–8 m. The sandy area is limited by the extension ofthe ancient bay, which filled the ancient valley duringdifferent stages of the Baltic Sea and reached far deeperinto the land in the past (Fig. 2), when the relative sealevel was considerably higher.The width of the sandy beach is up to 100 m at thenorthern mole of Pirita Harbour and gradually decreasesnorthwards. Sand is replaced by a till scarp close toMerivälja Jetty at a distance of about 2 km from PiritaHarbour. Noticeable contemporary dunes and valleys236

T. Soomere et al.: Bottom sediments at Pirita Beachonly exist in the immediate vicinity of the sea coast. Majorsea level changes can be recognized within the valleyfrom much higher ancient dunes (Fig. 2), located about1–2 km inland (Künnapuu 1958; Raukas et al. 1965).Contemporary marine sandThe properties of bottom sediments for the entire TallinnBay are described in Lutt & Tammik (1992). Previousstudies of contemporary bottom sediments at Pirita (Lutt1992) are based on 33 sediment samples (Fig. 3) takenwith a vibrocorer (Kudinov 1957). Drill cores extendingdown to 2.1 m into the seafloor (Lutt & Tammik 1992)provide a reliable stratigraphy of the upper layers ofmarine sand at Pirita (Fig. 4).The seabed from the waterline down to depths of2–3 m is covered with fine sand at Pirita. At depths downto 6–8 m relatively coarse silt is found. In deeper areasFig. 4. Geological cross-sections of the beach area. Adaptedfrom Lutt (1992) using the following nomenclature of thegrain size: below 0.01 mm – silt and clay; 0.01–0.05 mm –fine silt; 0.05–0.1 mm – coarse silt; 0.1–0.25 mm – finesand; 0.25–0.5 mm – medium sand; 0.5–1.0 mm – coarsesand. Location of the boreholes is given in Fig. 3.finer silt fractions 1 dominate (Lutt 1992, p. 208). Thelight mineral components (with a specific weight of< 2.89 g/cm 3 ) form more than 95% of the sand mass.Fig. 3. Location of samples used for the construction of crosssectionsA–B, C–D, and E–F (Fig. 4) of the upper sedimentlayers in Lutt (1992), Lutt & Tammik (1992), and for theanalysis of properties of bottom sediments. Filled squaresPR1, PR3, PR7, and PR8 denote the starting points of beachprofiles 1, 3, 7, and 8 in Fig. 7. The box between Rannahooneand Pirita Harbour indicates the area represented in Fig. 8.1 The classification of fractions in Lutt (1992) mostly followsthat of Friedman et al. (1992, pp. 335–340) except thatdifferent fractions of silt (with the grain size < 0.063 mm)are called silty pelite (grain size from 0.01 to 0.063 mm)and pelite (grain size < 0.01 mm). Fractions with the grainsize from 0.063 to 0.2 mm are together referred to as finesand, fractions from 0.2 to 0.63 mm as medium sand,fractions from 0.63 to 2.0 mm as coarse sand, and fractionswith the grain size > 2.0 mm as gravel.237

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254Quartz forms about 70% and feldspar about 25% of thematerial.The sand deposit is usually over 2 m thick. In thenorthern part of the beach (profile A–B in Fig. 4) itconsists of coarse or fine silt, or of a mixture of silt andpelite (Lutt 1992, pp. 208–213). A thin-layered structureof the uppermost sediments exists at depths of 2–10 min the central part of the beach (profile C–D in Fig. 4).The layered stratigraphy may reflect the attempts at beachfill (see below); however, such a depositional sequenceof bottom sediments is also a typical feature of sandyareas next to North Estonian river mouths (Lutt 1992,p. 215).Several medium- and coarse-grained sand bodieswere detected in places under a thin sand layer southwardfrom the river mouth (profile E–F in Fig. 4). Finesand forms a thin layer (~ 20 cm) on the sea bottom fromthe waterline down to a depth of about 4 m at about600 m from the coast. At the sampling point, locatedabout 200 m off the coastline, fine sand overlies anotherthin layer (~ 15 cm) of coarse sand and till. Silt forms amore than 2 m thick layer at water depths of 15–20 m(800–900 m off the coast).The transition between fine and coarse sand bodiesis quite sharp at Pirita, whereas the transition betweenfine sand and different silt fractions is generally gradual.Coarser sand bodies are poorly sorted and containappreciable amounts of 0.05–1.0 mm fractions, whereasfiner sand bodies (with the typical grain size of0.05–0.25 mm) usually have a narrow grain size range(Lutt 1992). These properties are common for the NorthEstonian sandy areas (Lutt 1985; Lutt & Tammik 1992;Kask et al. 2003a).Local sediment transportSand transport in the littoral system is usually affectedby a large number of various processes such as waveinducedoscillatory motions, wind-induced transport,coastal currents and wave-induced alongshore flows,variations of water level, sea ice, etc. The effect of ice ismostly indirect and consists in either damage to duneforest or in reducing the wave loads during the iceseason. The tidal range is 1–2 cm in the Baltic Sea andthe tidal currents are hardly distinguishable from theother motions. Water level at Pirita is mainly controlledby hydrometeorological factors. The range of its monthlymean variations is 20–30 cm (Raudsepp et al. 1999), butits short-term deviations from the long-term average arelarger and frequently reach several tens of centimetres.Water levels exceeding the long-term mean more than1 m are rare. The highest measured level at the Port ofTallinn is 152 cm on 09.01.2005 (Suursaar et al. 2006)and the lowest is – 90 cm.Fine sand that predominates at Pirita (see below)can be easily transported by wind when dry; however,winds sufficient for extensive transport are infrequent.The shoreline is parallel to the predominating winds(Soomere & Keevallik 2003). Strong onshore (NW)winds typically occur either during the late stage ofstorms or during the autumn months when sand is wet.Although wind carries a certain amount of sand to thedune forest, the overall intensity of dune building ismodest. The height of the existing dunes is a fewmetres. The dunes are mostly covered with grass andforest. Only sand seawards from the faceted dune faceactively undergoes aeolian transport. The role of aeoliantransport has apparently been larger in the past, assuggested by a photo of a sandstorm at Pirita (Raukas &Teedumäe 1997, p. 291).Coastal currents induced by large-scale circulationpatterns are modest in the whole of the Gulf of Finland(Alenius et al. 1998). Their speed is typically 10–20 cm/sand only in exceptional cases exceeds 30 cm/s in TallinnBay (Orlenko et al. 1984). In the bayheads, such as thenearshore of Pirita Beach, current speeds apparentlyare even smaller (Loopmann & Tuulmets 1980). Localcurrents are at times highly persistent in this area (Ermet al. 2006) and may provide appreciable transport offiner fractions of sand that are suspended in the watercolumn, even though the typical settling time of thesefractions is only a few minutes at the coasts of theViimsi Peninsula (Erm & Soomere 2004, 2006).The magnitude of wave-induced bedload transportgreatly exceeds that of the current-induced transporteven at relatively large depths (8–10 m) in sea areasadjacent to Tallinn Bay (Kask 2003; Kask et al. 2005).Wave action in the surf zone therefore plays a decisiverole in the functioning of Pirita Beach. This is a typicalproperty of beaches located in microtidal seas.Most of the waves affecting Pirita Beach originatefrom the Gulf of Finland. Western winds bring to thePirita area wave energy stemming from the northernsector of the Baltic Proper. The wind regime in theseareas is strongly anisotropic (Mietus 1998; Soomere &Keevallik 2003). Tallinn Bay is well sheltered fromhigh waves coming from a large part of the potentialdirections of strong winds (Soomere 2005a). The localwave climate is relatively mild compared to that in theopen part of the Gulf of Finland or in the adjacent seaareas. The annual mean significant wave height is aslow as 0.29–0.32 m in different sections of Pirita Beach(Soomere et al. 2005).Very high waves, however, occasionally penetrateinto Tallinn Bay and cause intense erosion of its coasts(Lutt & Tammik 1992; Kask et al. 2003b). The significantwave height usually exceeds 2 m each year, but mayreach 4 m in NNW and western storms in the central part238

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254section of the river mouth during a two-year period(Loopmann & Tuulmets 1980). Since the river flowonly transports about 400 m 3 of sediments annually(Lutt & Kask 1992), a large amount of marine sandapparently was carried and accumulated to the rivermouth (Loopmann & Tuulmets 1980).Substantial development works were performed atPirita, related to the construction of the Olympic sailingharbour in the mid-1970s. The basin of the harbour wasexpanded considerably. The combined marina and a riverharbour were designed for 750 vessels. The northerngroin was lengthened and completely reconstructed.Another breakwater was erected to protect the southernside of the harbour.Both marine hydrodynamic features and sand transportproperties of the river flow were notably changed,although the seaward extension of the harbour is closeto that of the earlier groins (cf. Figs 1, 5, 6). The waterdepth at the harbour entrance is about 5 m (EVA 1998).The coastal processes at different sides of the harbourare therefore practically disconnected. The river flowslows down considerably in the harbour basin, whichacts as a sediment trap, and a much smaller amount ofvery fine sediments reaches the sea. The material stillsupplied to the sea is deposited into areas deeper than4–5 m, far offshore from the equilibrium beach profile.The fluvial material thus has almost no chance to enterthe active sand body of the beach.The sea coast south of the Pirita River mouth wasreclaimed using dredged sand to provide land for theOlympic village. The sandy beach was largely modifiedby the seawall of the new road to Pirita. There is aminimum amount of sand in this area now and we ignoreit for estimates of the sediment budget.To summarize, the extensive coastal engineeringactivities have largely blocked the littoral drift andsupplies of sand to Pirita Beach. The remaining sourcesof coarser material are the till cliff at the northern end ofthe beach and the dunes. The intensity of their erosiondepends mostly on the joint occurrence of high waterlevel and intense waves. This misbalance of the supplyof different fractions is expected to lead to a decrease inthe dominant grain size and an overall worsening of thesand quality from the recreational viewpoint. On theother hand, the Olympic Harbour also prevents the lateralsand loss from Pirita Beach towards the bayhead andthus stabilizes the beach to some extent.Changes in the beachThe coastline and the whole beach apparently werestable for at least a century until the 1970s. There wereno identifiable changes to the coastline between therestaurant and Merivälja Jetty in 1940–1976 (Loopmann& Tuulmets 1980). No records of substantial changesalso seem to exist from the last 200–300 years duringwhich the largest events in this area have generally beendocumented.Noticeable changes only occurred in the vicinity ofthe Pirita River mouth owing to the joint influence ofthe groins and dumping of sediments from the basin ofthe Olympic Harbour. The width of the beach in itsmiddle section in the 1960s (60–70 m, Soomere et al.2005) was larger than nowadays (cf. also Raukas &Teedumäe 1997, p. 291). There are no reliable data tojudge if this feature was natural or was induced by thegroins. The width of the dry beach was relatively largein the southern part of the beach already by the 1950s(Fig. 5) and increased up to 200 m along the northernmole by the 1970s (Loopmann & Tuulmets 1980). Alimited shift of the coastline seawards occurred at thesouthern mole and in a short section at Maarjamäe.The combination of the engineering structures formsa probable background of bathymetric changes near PiritaHarbour, which were first identified in the mid-1970s.The isobaths at 2 m and larger depths were shifted by40–50 m shorewards between 1958 and 1976 in a largearea north of the harbour (Paap 1976). This processreflects a loss of a large volume of sand from areasadjacent to the new harbour at certain depths.To mitigate the consequences of coastal engineeringstructures, an estimated amount of about 65 000 m 3 of thesand dredged from the basins of Olympic Harbour wasdumped in the central and northern parts of Pirita Beachand into shallow water near Merivälja Jetty in the 1970s.It was expected that waves would transport the sandsouthwards to Pirita Beach. As sand in over 1 m deepwater is not necessarily transported alongshore at Pirita(Soomere et al. 2005), it is unclear how much sandactually reached the beach. As the sand dredged fromthe harbour was quite fine (Orviku & Veisson 1979),a large part of it apparently was transported seawardsand the width of the beachface decreased already by the1980s (Kask 1997). Also, a certain amount of quarrysand was placed near Rannahoone about 500 m north ofthe Pirita River mouth. This considerably increased thesand volume and apparently prevented the central andsouthern parts of Pirita Beach from erosion.The images in Figs 5 and 6 reflect the presence of asubstantial amount of sand in underwater bars in 1951.Comparison with later images (Photo 4) suggests thatthe amount of active sand in bars has been reduced sincethen. Because the width of the contemporary beachis considerably smaller, a large amount of sedimentsapparently has been lost seawards. The nature of changesis different in different parts of the beach.242

T. Soomere et al.: Bottom sediments at Pirita BeachIn the southern part, between the restaurant andPirita Harbour, the width of the beach and the total sandvolume have increased. This has resulted from beachrenourishment and the influence of Pirita Harbour, whichalmost completely blocks the southward littoral drift.This apparently is a long-term trend, since even the mostviolent storms in 2001 and 2005 have not affected thebeach width. The total width of the dry beach is up to100 m. In elevation the beach extends up to 2 m abovethe mean water level. Waves did not reach the dune toeeven during extreme water levels. This sector of beachtherefore seems relatively stable and is characterized bygradual sand accumulation.The central part of the beach (adjacent to therestaurant and extending to about 1 km from the PiritaRiver mouth) is in a near-equilibrium state. The bluff atthe back of the beach is at times eroded to some extentin strong storms. Relatively intense aeolian sand transportinto the dune forest occurs in this sector (Kask & Kask2002). Although this may have an adverse effect on thedune forest, it can be interpreted as an expression of anexcess of sand and a generally healthy state of the beach.The northern part of the sandy beach, an about 1 kmlong sector, is changing actively. Strong western stormsand high storm surges (Photo 2) caused extensiveregression of the bluff in 1999–2005. The changes aremarked to the north of the mouth of a small stream(that discharges to the sea about 900 m north of PiritaHarbour; see its location in Fig. 3) and are substantialin the northernmost part of the sandy beach. Therecession of the bluff was on average 1–2 m (at a fewsections even 3–5 m) in 1999–2001. The most intenseerosion occurred at the interface between the sandyand till coasts. A large amount of sand was carriedinto the dune forest. Several dozens of pine-trees weredestroyed and this additionally weakens the stability ofthe dunes.The section of the coast between the northern end ofthe sandy beach and Merivälja Jetty embraces a till scarpabout 400 m south of the jetty, which is intensivelyeroded during high water level conditions. Seawardsfrom the scarp there is a gently sloping sea floor, witha width of the dry area up to 100 m in low waterconditions, mostly covered by pebbles, cobbles, andboulders. This section illustrates intense sediment transitin high water level conditions.Approximately 50% of Pirita Beach therefore suffersfrom substantial damage at times. One reason behindan apparent intensification of beach processes is thehigh frequency of storms experienced since the 1960s(Alexandersson et al. 1998). Still, a major threat to thebeach is human activity that has considerably weakenedthe sand supply to the beach. Another potential threat isthe influence of long waves from fast ferries on theseaward end of the equilibrium beach profile (Soomere2005b). Given the circumstances, one cannot expect thatPirita Beach will further develop in a stable manner,although the natural variability of storms may providerelatively long periods during which the evolution isfairly slow.Quantification of changesThe changes in the beach that have occurred since themid-1970s can be roughly quantified by the comparisonof maps stemming from different decades or the resultsof topographical surveys.If the sand volume were constant at Pirita, thecoastline would shift seawards and a gain of dry landwould occur owing to the postglacial rebound. A roughestimate based on the comparison of the location of thewaterline on a topographic map (1 : 25 000) from 1979and the digital Estonian Base Map (1 : 50 000) from theend of the 1990s combined with an estimate of theeffect of land uplift, suggests that the net sand loss atPirita is about 1000–1500 m 3 annually (Soomere et al.2005). The accuracy of the underlying information, however,is equivocal and this estimate should be consideredas indicative only. A more detailed analysis of historicaldata is in progress and will be published elsewhere.The above estimate only accounts for the changes inthe sand within the equilibrium profile and in the tillcliff, and ignores the potential changes in the amount ofsand in berms and foredunes. The spatial patterns of thesechanges, inferred from beach profiles measured regularlywithin the framework of national coastal monitoring, areconsistent with the qualitative analysis above (Suurojaet al. 2004). The erosion of the dry beach and thescarp is most evident in the northernmost section. Forprofile 1 (Fig. 7; see location of the profile in Fig. 3) thewhole coastal profile is shifted by 10–20 m landwards.The central part of the beach is close to an equilibriumstate (cf. profile 3 in Fig. 7). There is some sandaccumulation in the southernmost beach sector asindicated by profiles 7 and 8.A complementary estimate of the loss of dry sandcan be derived based upon the comparison of isolinesobtained from two subsequent topographic surveys ofthe beach. A comparison of the entire topography wouldbe more exact, but the relevant data are not available.The results of two high-resolution (1 : 500) topographicalmappings from 1997 and 2006 suggest that nonet changes in the dry land area occurred during thisperiod, but systematic landward shift of the isolines of0.5, 1.0, and 1.5 m took place (Fig. 8, Table 2). Suchshifts suggest that the sandy beach has lost a certain243

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254Fig. 7. Beach profiles 1 (a), 3 (b), 7 (c), and 8 (d) at Pirita on9 Sept. 2003 (solid lines) and in 1994 (dashed lines) based ondata from Suuroja et al. (2004). The profiles begin from thecoastal scarp marked by a circle; refer to Fig. 3 for theirlocation. The horizontal dashed line shows the mean waterlevel.amount of sand. As the landscape of the beach mayeasily undergo considerable changes in the immediatevicinity of the waterline, changes below the 0.5 melevation may be temporary and are not accounted for.Sand loss may only occur from between the waterlineand the dune scarp, the latter being represented by the1.5 m isoline at Pirita.The overall state of the beach, however, is not good:the isoline shifts indicate the loss of sand from largesections of the beach. The change in the areas of therelevant elevation is between 2000 and 5000 m 2 . Thelargest changes have occurred at the elevations of1–1.5 m. The relevant isolines are shifted 2–3 m landwardson average. As the typical width of the stripof dry sand is about 50 m, it means that, on average,at least 2 m 3 (about 5% of dry sand volume) of sandhas been lost from each metre of the sandy area. It isFig. 8. Changes in the location of isolines of surface elevationin the southern part of Pirita Beach. Solid and dashed linesshow the 0, 0.5, 1, and 1.5-m isolines according to surveysof 1997 by REIB Llc (Map 1 : 500 with technological networks.Harju County, Tallinn, Pirita District, Pirita Beach.Contract No. 06-612) and of 2006 by Hectare Llc (Mapwith technological networks. Survey of trasses of PiritaBeach. Contract No. TT-0249), respectively. The shadedarea represents changes in the positions of the isolinesbetween 1997 and 2006. The seaward shift of the 0.5–1.5-misolines reflects sand accumulation in this area. The representedarea is denoted by a box between Rannahoone and PiritaHarbour in Fig. 3.interesting to notice that this loss is roughly equal to theestimated loss of finer sediments owing to wakes fromfast ferries (Erm & Soomere 2004, 2006). Given thelength of the surveyed section is 1800 m, the net loss ofsand is about 3000–4000 m 3 and corresponds to theannual loss of sand volume of about 400 m 3 .Table 2. Changes in the dry land area and in the area of sectionsbordered by the isolines of 0.5, 1.0, and 1.5 m along a 1800 mlong beach section at Pirita during 1997–2006Isolineelevation, mLoss, m 2 Gain, m 2 Balance, m 20.0 1860 1860 00.5 2600 360 – 22401.0 4970 230 – 47401.5 5400 1550 – 3950244

T. Soomere et al.: Bottom sediments at Pirita BeachCONTEMPORARY BATHYMETRY ANDBOTTOM SEDIMENTSMapping of sediment texture and bathymetryThe detailed sand budget of a beach is usually establishedbased on long-term surveys. The situation is simpler forbayhead beaches such as Pirita Beach that (i) mainlyserve as end points of littoral transport for which thelateral losses owing to alongshore transport are negligible,and (ii) which are subject to small seasonal changes andoverall moderate wave activity. Such beaches are usuallyclose to an equilibrium state. Many of their propertiescan be described with the use of a small number ofparameters (e.g. Dean & Dalrymple 2002). One of thebasic quantities is the typical grain size.During the field campaign of 2005–2007, bathymetryand sediment texture were mapped in the nearshorebetween the waterline and the 11 m depth contour fromthe northern mole of Pirita Harbour to Merivälja Jetty.The total length of the study area (Fig. 9) along the waterlineis about 2.5 km.Granulometric (textural) analysis of Pirita Beach wasperformed on 51 samples from the upper 30 cm layer ofbottom sediments taken during the 2005 campaign overthe entire study area, and on 18 samples from the nearshoretaken in 2007 (Fig. 9). The majority of the samplingpoints in 2005 are located along five profiles transversalto the coastline. Each profile contains about 10 samplesfrom the waterline to the 10 m isobath. A selection fromthe standard set of sieves with the grid size of 2.0, 1.0,0.63, 0.5, 0.25, 0.2, 0.125, and 0.063 mm (DIN 4022)was used in the analysis performed in the laboratory ofthe Estonian Geological Survey and in the EstonianEnvironmental Research Centre. The correspondingφ -values are – 1.0, 0, 2 3 , 1.0, 2.0, 12 , 3.0, and 4.0.3The mean grain size M φ, its value in physical unitsd , and the standard deviation 50σφof the grain size arefound, as recommended in Dean & Dalrymple (2002,Ch. 2), from the values of φ84and φ : 16φ84+ φ16φ84−φ16M φ= , σ φ= ,2 2assuming the log-normal distribution of the grain size.The values of φ 84and φ 16represent the grain sizes,from which 84% or 16% of the entire sediment mass hasa larger grain size. Their approximate values (Table 3)are found from the cumulative distributions of the grainsize (Fig. 10).Bathymetric mapping was conducted by Gotta Ltdwith the echo sounder Bathy 500 MF and the DGPSsystem Trimble DSM 132 in April 2006. The measurementswere performed along 20 transects perpendicularto the coast and separated by about 100 m, and 3transects along the coastline. The spatial resolution ofsampling along transects was about 1.5 m.BathymetryThe available beach profiles (Fig. 7) confirm that thenearshore of Pirita Beach is generally very gently sloping(Suuroja et al. 2004). The slope of the profiles in theshallow water (depth < 1 m) of the northern part of thebeach is about 1 : 100 and well matches the estimates ofthe properties of the equilibrium beach profile (Soomereet al. 2005). The same average slope holds for profile 3Table 3. Parameters of sand at Pirita Beach based on surveysof 2005 and 2007 (Soomere et al. 2007; Kask & Kask 2007).See text for explanation of symbolsFig. 9. Sampling points of bottom sediments at Pirita in 2005(profiles 1–6) and 2007 (points in the nearshore, marked byempty circles). Isobaths of – 2, – 5, – 10, and – 20 m are shownbased on the digital 1 : 50 000 Estonian Base Map from theEstonian Land Board (www.maaamet.ee).AreaPropertyφ16φ84 M φ d 50, mm σ φVicinity of the 1.1φ 2.53φ 1.815φ 0.284 0.715waterlineEntire beach 2.16φ 3.84φ 3.0φ 0.125 0.84Nearshore (depth< 1 m, 2007)2.5φ 3.68φ 3.1φ 0.117 0.59245

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254Fig. 10. (a) The content and(b) the cumulative distribution ofdifferent grain size fractions atPirita based on samples collectedin 2005 and 2007. Refer to Fig. 9for sample locations. The averagedproperties are shown for sandalong the waterline (grey bars,squares), in the area with adepth of ~ 1 m (nearshore in 2007,white bars, triangles), and for theentire beach (filled bars, circles).The horizontal dashed lines onpanel (b) represent 16% and 84%of the sand mass.in the central part of the beach. The mean slope is somewhatlarger, about 1 : 150 in the vicinity of the restaurant(profile 7). The gentlest slope (about 1 : 200) is foundin the southern part of the beach along profile 8. Theseslopes not necessarily reflect the mean slope of theequilibrium beach profile that extends out to about 2.5 mdeep water (Soomere et al. 2005).The water depth increases monotonically along theprofiles in the southern and northern sections (Fig. 7).Shallow-water sand bars are apparently more frequentin the central part of the bay, however, the situationmay be different for different seasons and depths. Therelative height of the bars on profiles 3 and 8 is up to0.5 m.The bathymetry survey reveals that a gently slopingnearshore strip stretches between the waterline and the2-m isobath (Fig. 11). Its alongshore profile is nearlyhomogeneous in the northern half of the beach wherethe distance of the 2-m isobath from the coastline is220–230 m. The strip widens to about 300 m in thevicinity of Pirita Harbour. Its mean slope is from about1 : 100 in the northern part of the beach up to 1 : 150 inthe southern part.The water depth increases relatively fast between theseaward border of the above strip and the 4-m isobath(Fig. 11). This increase is also homogeneous along thecoast: the distance of the 4-m isobath from the coastlinevaries insignificantly, between 400 and 450 m (Fig. 12).Located at depths of 4–7 m is a relatively gently slopingsubmarine terrace, some 500–800 m wide. The slopeof this terrace is 1 : 200 in the central part of the beach.The water depth increases relatively fast from 7 to overFig. 11. Bathymetry of the nearshore of Pirita Beachaccording to the water depth survey in April 2006. The waterdepth along profiles A–E is given in Fig. 12. The coastline isshown based on the digital 1 : 50 000 Estonian Base Map fromthe Estonian Land Board (www.maaamet.ee).246

T. Soomere et al.: Bottom sediments at Pirita BeachFig. 12. Beach profiles (A–E) from the middle of the equilibriumprofile to the 10 m isobath in April 2006. Refer to Fig. 11 forthe location of profiles.10 m at a distance of about 700 m in the southern part ofthe beach. The width of the area with a depth of < 7 mextends to 1200 m in the central part of the beach.A prominent feature of the submarine slope is anelongated elevation of moderate height (about 30 cm)extending through a large part of the deeper (> 4 m)section of the study area. This feature, probably a largesand bar, is stretched obliquely with respect to the depthcontours (Figs 11, 12) and to the till formations (Fig. 2).As it can be identified also on the older maps from the1980s, it probably is a long-term feature of the beach.Distribution of bottom sedimentsThe sediments on the nearshore submarine slope consistmostly of fine sand in accordance with the data in Lutt(1992) and Lutt & Tammik (1992). The predominatinggrain size constituents are fine (84%) and medium sand(13%). The proportion of silt is about 5.5%. Its contentis largest (46%) in quite a deep area (at the seaward endof profile 4 in Fig. 9 at a depth of about 12 m) whereusually fine sediment accumulates. Coarse sand alsoforms a small part of the sediments (3.9% on average).There is very little gravel in the study area (0.8% onaverage, maximally 11% in the immediate vicinity ofthe till scarp). The sand is better sorted in the shallownearshore where σφ≈ 0.6, but poorly sorted in the restof the beach.The average grain size in the nearshore of Pirita Beachis close to 0.12 mm, in accordance with domination offine sand in most of the beach. An approximate value ofthe scale factor of the relevant equilibrium beach profileis thus A = 0.07 (Table 7.2 of Dean & Dalrymple 2002).The closure depth ranges from 2.4 to 2.6 m for differentsections of the beach (Soomere et al. 2005). Thereforethe width W of the equilibrium profile is expected to beabout 250 m and its mean slope 1 : 100 at Pirita. Thisestimate well matches the results of the bathymetricsurvey. Notice that profiles in Figs 11, 12 cover only theseaward part of the equilibrium profile that ends at adistance of about 200 m from the coast and does notextend down to the underwater terrace located at depthsof 4–7 m.Several variations of the mean grain size and thecontent of the fractions should occur naturally withinthe study area. As waves sort the sediments and thefiner fractions are gradually transported offshore, deeperareas usually host the finest sediments and the coarsermaterial is concentrated in the vicinity of the breakerline and at the waterline (Dean & Dalrymple 2002).Coarser sand is found along the waterline (where themaximum content of medium sand is up to 84%) andfiner components in deeper areas indeed (Fig. 13). Themean grain size along the waterline is much larger thanin the rest of the study area (Fig. 10, Table 3).The proportion of coarser sand generally decreasesoffshore (Figs 13, 14). The explanation is evidently relatedto the above-discussed highly intermittent nature ofwave activity at Pirita (where mean wave conditions arevery mild but severe wave storms may occur). Thebreaker line is poorly defined there and the relevantband of relatively coarse sand is not always apparent.This feature is not entirely typical but also not verysurprising (Dean & Dalrymple 2002, Ch. 2.3.2). It mayplay an important role in the planning of beach nourishmentactivities because material with the grain size muchsmaller than the one at the waterline may be lost relativelyfast. Moreover, relatively coarse and well sorted sand isFig. 14. Properties of different grain size fractions along beachprofiles 1–6 in Fig. 9: (a) silt, (b) fine and medium sand.247

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254perceived to be of the largest recreational value. In otherwords, beach fill with fine sand would lead to a decreasein the beach quality.The potential sources of coarser material are locatednorth of the beach whereas fine sediments are suppliedby the Pirita River mostly to its southern part. Thisfeature was apparently enhanced by the pumping ofsediments dredged from the river mouth starting fromthe 1960s. Also, fine sand is more easily transportedsouthwards by littoral drift. Thus it is natural that morefine sand is found in the southern sections of the beach(except at the waterline) and that the share of coarsersediments is larger in the northern part of the beachwhere unsorted material is regularly abraded from thetill cliff and where gravel and coarse sand are mostlyfound. Also, the portion of medium sand is larger in thenorthern sections of the beach (Fig. 13), although a localmaximum of its proportion is in the central part of thestudy area.The alongshore variation of the size fraction propertiesalong isobaths (Fig. 15) suggests that the entirebeach is not in perfect equilibrium. Sediments are moreheterogeneous in the northern sections of the beach(Figs 13, 14). This can be explained by the combinationof supply of unsorted sediments from the till scarp andthe overall limited amount of sand in this area. Theheterogeneity may partly be induced by the sand barstretching over the depths of 4–8 m. The dumping of thematerial dredged from the river mouth and harbour basinsin the 1970s may also have contributed to the formationof local variations of the grain size and the content ofdifferent fractions.An interesting feature is the difference of spatialdistribution of sand fractions in relatively shallow(down to depths of 5–6 m) and deeper parts of the studyarea. The proportion of coarser sand decreases fromnorth to south in the shallower part of the coastal slope(Figs 13, 15). On the other hand, the proportion of siltincreases northwards at depths greater than 6–7 m(Fig. 15). This peculiarity may reflect the above-describedselective blocking of the natural sand supply or dumpingof fine sediments dredged from the Pirita River mouthto the vicinity of Merivälja Jetty.On the basis of the maps of the distributions of meangrain size, the beach can be divided into four areas:! The northernmost part has a relatively large proportionof inhomogeneously distributed coarser sediments.! Relatively coarse sand is found along the waterline.! Most of the study area at depths from about 1 m to6–8 m and extending to about 1800 m from PiritaHarbour has a more or less homogeneous distributionof sand fractions with a clear domination of fine sand.! The deepest part of the nearshore (depths > 6–8 m)mostly hosts even finer sand.Fig. 15. Properties of different grain size fractions interpolatedfrom sediment samples: (a) along the coastline, (b) along the2-m isobath, (c) along the 5-m isobath, and (d) along the 10-misobath. The letter C stands for coarse sand, M for mediumsand, F for fine sand, and S for the fraction with the grain size< 0.063 mm. Refer to Fig. 9 for sample locations.CONCLUSIONS AND DISCUSSIONHistorical as well as recently collected data wereanalysed to specify the previous and the current state otPirita Beach and to estimate the sand budget there. PiritaBeach has apparently been stabilized by the postglacialuplift and natural sediment supplies until the middle ofthe 20th century. The littoral transport of material erodedfrom the coast of the Viimsi Peninsula and originatingfrom the Pirita River were the major sand sources.Aeolian sand loss is minor in this area.The results of bathymetric and textural studies forma basis for the quantification of the sediment transportprocesses. The overall changes of the predominating grain248

T. Soomere et al.: Bottom sediments at Pirita BeachFig. 13. Distribution of the dominant fractions in the study area: (a) fine sand (0.0630.125 mm), (b) medium sand (0.1250.2 mm). The background bathymetry is from thesurvey of April 2006 (Fig. 11). The coastline is shown based on the digital 1 : 50 000 Estonian Base Map from the Estonian Land Board (www.maaamet.ee).249

Estonian Journal of Earth Sciences, 2007, 56, 4, 233–254Photo 1. View of southern Pirita Beach and Olympic Harbour into which thePirita River discharges. Photo by A. Kask.Photo 3. Remnants of the revetment originally constructed at the dune toe at thenorth end of Pirita Beach at the beginning of the 1980s. Photo by A. Kask.Photo 2. Erosion and loss of pine forest on the low dune during a strong storm inJanuary 2005. Photo by I. Kask.Photo 4. Aerial photo of Pirita Beach in 2000. Courtesy of the Estonian LandBoard.250

T. Soomere et al.: Bottom sediments at Pirita Beachsize are fairly minor at Pirita. This feature allows usinga fixed value of the shape parameter of the equilibriumbeach profile for this beach.Major changes in the sand budget occurred when(i) Miiduranna Port and its fairway largely blocked thelittoral transport, and (ii) when Pirita Olympic Harbourdrastically decreased the river-induced sand transport tothe littoral system.The deficit of relatively fine river-transported material(about 400 m 3 annually) has to some extent beencompensated by the pumping of the material dredgedfrom the river mouth to the beach.The blocking of littoral drift of coarser sediments byMiiduranna Port and Merivälja Jetty probably is themain reason for the sediment deficit in this area. Thedecrease in the littoral transport also led to an overallsediment deficit southwards from Merivälja Jetty,and evidently to more intense erosion of the scarp inthe northernmost part of Pirita Beach. This change,apparently, is not compensated by the total blocking ofthe lateral sand transport by Pirita Harbour. It can bespeculated that the specific deficit of coarser materialleads to undesirable gradual decrease in the grain size inthe southern part of the beach.Since 1997, apart from the above sediment deficit,another 400 m 3 has been eroded annually from the drybeach. The erosion process of the beachface, dunes, andscarp in the northernmost part of the beach may havebeen less intense in the past, because the littoral transportpreviously carried more sand to this area.In general, sand supply from the north, sand dredgedfrom the Pirita River mouth and the basin of the OlympicHarbour, beach fill activities in the 1970s, and the relativeland uplift have kept Pirita Beach more or less stableduring the recent decades. Other beach protectionmeasures such as construction of a revetment in thenorthern part of the beach have been ineffective.Extensive damage by storms in November 2001 andJanuary 2005 to the beach scarp and to the duneforest behind it, accompanied by seaward sand loss (cf.Orviku et al. 2003), suggest that the natural recoveringmechanisms of Pirita Beach are no longer sustainableand that the beach has become very vulnerable. Thepositive influence of the beach fill from the past isprobably over, and an increase in the net sand loss fromboth the dry beach and the nearshore is likely in thefuture. Therefore, construction activities in the vicinity ofthe beach or its sand supply channels, even if designed asbeach protection measures, may lead to adverse effectsand to an increase in the net sand loss. For example, ifthe till cliff north of the beach will be further stabilizedby construction of a seawall, less material will be suppliedto the beach and the net sand loss will likely increase. Itis also likely that a rigid wall will enhance the sedimentdeficit in the northernmost section of the sandy beach.A feasible way of restoring the sand balance at Piritaand to make the beach stable consists in increasing thesand volume at the beach. The fastest result would be byutilizing classical beach renourishment methods. Thiswould involve placing high-quality, relatively coarsesand (extracted, e.g., from Naissaar Harbour, Kask &Kask 2007) either on the dry beach area or in theimmediate vicinity of the shoreline. An artificial dune ofmoderate height (about 1.5 m) along the existing coastlinewould effectively protect the dune forest and notdistort the sea view (Soomere et al. 2006). The dumpingof sand to the nearshore is ineffective owing to therelatively small closure depth. Filling the beach withsand from the Pirita River mouth, from the OlympicHarbour basin or from the coastal slope of Pirita Beachshould be undertaken with great care (Soomere 2007),because the sand there is relatively fine. Another feasibleway, which would bring to good results, consists ofsand bypassing to the southern side of Miiduranna Port.Doing so would eventually compensate the sedimentdeficit along the coast at Merivälja, reduce the coastalerosion in this area, and deliver medium and coarse sandto Pirita Beach.ACKNOWLEDGEMENTSThis study was supported by the Estonian ScienceFoundation (grant No. 5762) and EU-supported SAINNOVE contract No. 1.0101-0208, and was partiallyperformed within the Marie Curie Research and Trainingnetwork SEAMOCS (MRTN-CT-2005-019374) and theEco-Net network “Wave-current interactions in coastalenvironment”. A large part of the work was donewhen one of the authors (TS) was visiting the Centreof Mathematics for Applications, University of Oslo,in the framework of the Marie Curie Transfer ofKnowledge project CENS-CMA (MC-TK-013909).Field data gathered in 2005–2007 within the contracts“Environmental studies of the coastal zone of PiritaBeach and formulation of technical conditions of beachprotection” (City of Tallinn) and “Study of bottomsediments in the harbour of the Island of Naissaar, in thestrait between the islands of Aegna and Kräsuli, and atthe nearshore of Pirita” (R-Project Ltd) have been usedin the analysis. AK thanks Hectare Llc for permissionto use their survey data for Fig. 8 and Sten Suurojafor providing historical data about beach profiles forFig. 7. Useful comments, corrections, and suggestionsof the reviewers T. Healy and J. Vassiljev are gratefullyacknowledged.251

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