An extensive offshore sand bar field in the western Baltic Sea

Marine Geology, 18(1975): 197--212 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

A N E X T E N S I V E O F F S H O R E S A N D BAR FIELD IN T H E W E S T E R N BALTIC SEA

N. F. EXON Bureau of Mineral Resources, Canberra City, A.C.T. (Australia) (Received July 2, 1974; accepted October 10, 1974)

ABSTRACT Exon, N. F., 1974. An extensive offshore sand bar field in the western Baltic Sea. Mar. Geol., 18: 197--212. In the breaker zone in southeastern Gelting Bay, east of Flensburg in north Germany, a field of up to twenty parallel relatively small-scale offshore sand bars has developed. The bars, which consist of fine- to medium-grained sand derived from till, are flat and asymmetrical and their steeper sides generally face the shore. Crests are as much as 1000 m long; wave-lengths vary from 7 m inshore to 70 m offshore, and heights correspondingly from 5 to 70 cm. The bars are formed by waves driven by northwesterly winds, and are probably destroyed during severe storms and rebuilt as they wane.

INTRODUCTION A s a n d bar field in Gelting Bay in t h e w e s t e r n Baltic Sea was i n v e s t i g a t e d as p a r t o f a s t u d y ( E x o n , 1 9 7 1 ) at t h e Geologisches-Pal~iontologisches I n s t i t u t of t h e University o f Kiel, u n d e r t h e g u i d a n c e o f P r o f e s s o r E. Seibold. T h e general s e t t i n g o f t h e area was d e s c r i b e d b y E x o n (1972), b u t t h e bar field was n o t . Gelting Bay ( F i g . l ) is a triangular inlet which lies at t h e o u t e r e n d o f F l e n s b u r g fjord, faces n o r t h , a n d has a m a x i m u m w a t e r d e p t h o f 22 m . I t is f l o o r e d with a n d s u r r o u n d e d b y glacial till a n d was s h a p e d b y t h e Baltic Ice S h e e t during t h e last glaciation. T h e c a r b o n a t e - r i c h till in t h e area varies considerably. On l a n d an average till c o n t a i n s 20% w a t e r b y weight a n d t h e solids consist o f 5% gravel, 4 5 % sand, 25% silt a n d 25% clay. T h e sea e n t e r e d the area a b o u t 8 , 0 0 0 y e a r s ago, w h e n t h e w a t e r level was a b o u t 20 m b e l o w t h e p r e s e n t level (see E x o n , 1972). In t h e n e x t 2 , 0 0 0 years t h e sea rose t o w i t h i n 5 m o f its p r e s e n t level, a n d f o r t h e p a s t 2,500 years has b e e n within 1 m o f t h e p r e s e n t level. Wave a c t i o n m o d i f i e d t h e glacial l a n d f o r m s , a n d t h e e r o d e d till was s o r t e d i n t o sand, silt, a n d clay f r a c t i o n s which were d e p o s i t e d as belts in t h e b a y , t h e c o a r s e s t f r a c t i o n s n e a r e s t t h e shore. S o u t h e a s t e r n Gelting Bay is u n i q u e in the w e s t e r n Baltic Sea in t h a t it

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forms a trap for the large waves associated with the frequent northwesterly gales which are most c o m m o n in winter, but is virtually unaffected by waves coming from other directions. Fetch from the sector 270 ° -- 340 ° is 10 -20 km, whereas in other directions it is less than 6 km. These very regular waves ( n o t interfered with by groundswell) break along a line parallel to the gently curving northeasterly-trending coastline on the eastern side of the bay. Essentially this situation has prevailed for the last 6,000 years (Exon, 1972) and a broad wave-cut platform has developed in water shallower than 8 m on the southern and eastern coasts of the bay. There is no efficient outlet by means of longshore currents from the bay, so sand provided by erosion of the till has either accumulated on the platform in water shallower than 2 m, or has been transported into deeper water offshore. More sand has been carried toward the area from t he sandy Bircknack (see Fig.3) by means of longshore currents, but most of this has remained in the sandspits west of Gut Beveroe and northeast o f the bar field. Water less than 2 m deep extends offshore from 250 to 600 m in the east and south, but only from 100 t o 200 m in the west. In this shallow water, waves have f o r me d asymmetrical offshore bars. In the west there are one, two, or three bars on the relatively narrow platform (Exon, 1971), whereas in the east up to t w e n t y shore-parallel sand bars are found.* The sand bar field is 4 km long and up to 500 m wide.

199

In Gelting Bay the m a x i m u m tidal range is about 30 cm and tidal currents are unimportant. During westerly storms the water level can fall a metre in the western Baltic whereas during normal easterly storms the water level can rise a metre. Wave action during easterly storms erodes the offshore bars along the west coast of Gelting Bay and the cliffs behind them; at such times the till can be laid bare in wide areas (Kannenberg, 1951). Waves driven by northwesterly storms similarly attack the east coast, but much of their energy is dissipated on the broad sand bar field. FIELD WORK AND LABORATORY METHODS

To document changes in the bar field during the last 30 years sets of aerial photographs were examined. A 1969 photograph (original scale 1:3000) of a typical area shows the general form and size of the bars {Fig.2). Maps drawn from aerial photographs flown in 1964 and 1969 (Fig.3) show little change in the general distribution of sand bars, although most individual bars could not be identified in both sets of photographs. There has been little change in the distribution of the sand bars since 1936, when the first aerial photographs were taken. Glacial erratic boulders project through the sand bars in places, and most of them had not moved between 1964 and 1969. In 1970 three profiles normal to the coast and across the sand bars were measured with the aid of a diver. It was possible to walk out from the beach or use a rubber boat and, using steel-tipped rods with flags and a long rope marked in metres, to measure the water depth at intervals of 2 or 2.5 m. Water level changes over the few hours needed to measure a normal 200- to 300-m profile are unimportant in periods of calm weather, and no corrections were applied. The profiles were continued out from the beach to beyond the sand bars, where the underlying till was revealed (generally in 1.5 to 2 m of water). Surface samples or push-box cores were taken at regular intervals on selected bars. In two profiles the sand thickness was measured at regular intervals, either by digging to the till surface or by use of a water-jet lance. Water was forced through the lance by a pump m o u n t e d on a rubber boat, and rapidly penetrated the loose sand of the sand bars. The more compact till slowed the lance, which allowed the sand thickness to be measured either directly on the lance or by lowering a scale into the hole. Polymer relief casts of sand bar cores were made to determine the internal structure (see Exon, 1972, for technique). Grain-size analyses of surface samples, and from top and b o t t o m of cores, were made using a sediment balance.

* Offshore bars have been widely described and discussed and Reineck (1970) provided a s u m m a r y and literature review. Wave-formed bars in the practically tideless Baltic Sea are fairly well k n o w n (e.g.,Werner, 1963; Brand, 1955), but normally not more than three bars are found. T o distinguish the unique occurrence at Gelting Bay the terms "sandwave" and "sandwave field" were used by Exon (1971, 1972), but the distinction is rather artificialand the terms "sand bar" and "sand bar field" will n o w be used.

200

A c o m p u t e r p r o g r a m d e v e l o p e d by Dr. E. Walger c o n v e r t e d the data into s e d i m e n t grain-size parameters. GENERAL FEATURES OF THE SAND BARS AND CONTROLS ON THEIR FORMATION As the general distribution of the bar field has varied little in the past t h i r t y years, the forces creating the sand bars have evidently n o t changed, alt h o u g h the bars themselves have m o v e d . T h e bars are very sensitive to disturbance b y inflow o f fresh w a t e r f r o m small streams, or waves r e f r a c t e d f r o m m a n - m a d e structures, as is s h o w n by the gaps in, or drastic r e d u c t i o n of, the field s o u t h o f the old d y k e wall o f f Gelting N o o r (inlet}, o f f the groynes

Fig.2. Aerial photograph of part of the sand bar field.

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202 southwest of Quisnis peninsula, off the Wackerballig jetty, and off the little stream in the far south (Fig.3). The field is limited to the north by the Gut Beveroe sandspits, and disappears on the sheltered southerly coast because little sand has accumulated there. Wavelengths increase from the shore outwards, and tend to be longer in the more exposed northerly areas than in the southwest (15 -- 50 m in the northeast, and 10 -- 30 m southwest of Wackerballig jetty}. Furthermore the axial length of individual sand bars is as much as 1,000 m in the northeast, but seldom exceeds 200 m in the southwest. In the more exposed areas of the western Baltic Sea, the frequent rework° ing of the sand bars by sizeable waves means that biotic cover cannot establish itself. Under these conditions the shape of individual bars is very regular (e.g., Brand, 1955). However, on the eastern shore of Gelting Bay wave action is quite limited during the warmer months, a period when growth of sea grass and seaweed is at a maximum. Sea grass and Mytilus communities have established themselves on the seaward flanks of many of the sand bars, and have tended to stabilize them. During winter storms shoreward sand movement on the less densely covered parts of the bars occurs, and this has destroyed the natural regular shape of many of the waves (see Fig.4}. Field observations show that sand is c o m m o n l y scoured away above and below Mytilus banks. The sand bars are eroded by longshore currents caused by wind drag or the escape of piled-up water from the bay. This effect is greatest inshore, and there the bars are very low, or glacial till may even be exposed. Small scour channels perpendicular to the shore are cut through many bar crests south of Gelting Noor. These channels {generally < 5 m wide and 10 -- 20 cm deep) are presumably cut by outflowing bottom currents when wind drives the surface water directly onshore. They are readily recognizable on aerial photographs (e.g., Fig.2) where they have eroded dark areas of sea grass and Mytilus and exposed the light sand beneath. In November 1969 channel cross-sections, although normal to the shore, tended to be strongly asymmetrical, with erosion concentrated on the northeast side where Mytilus banks were being undercut. Presumably the outflowing undercurrents had first cut the channels normal to the shore, but later outflow had a more northerly direction, causing lateral cutting in the existing channels. Profiles B and C (Fig.4) show that in general the shape of the underlying till surface and that of the sand bars are independent. Where the bars are best developed the sand body averages 60 cm in thickness. From the three sections and other data Exon (1972) estimated that about 500,000 m 3 of sand lie on the southeasterly platform. The shapes of the sand bars in the profiles were measured, and the parameters wavelength, wave height, ripple index (length/height), and s y m m e t r y (horizontal distance from crest to shoreward trough/wave length) were plotted against the distance of the bar crest from the shore. Wavelength was also plotted against bar height. All these graphs are shown in Fig.5. Overall, wavelengths vary from 7 to 70 m, heights from 4 to 55 cm, ripple indexes from

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205 80 to 370, and s y m m e t r y from 0.3 to 0.63 (symmetry values < 0 . 5 indicate asymmetry with steep side shoreward; values > 0.5 steep side seaward). In detail the various graphs with these sand bar shape data show: (a) Wavelength and bar height both increase with distance from shore, but height increases more rapidly than length inshore, and length more rapidly offshore. (b) The ripple index is greatest (i.e., the bars are flattest) inshore, reaches a minimum in the middle distances, and increases somewhat offshore. (c) The bars of profile C (Fig.4) tend to be asymmetrical in the seaward direction (steep side seaward) near shore, and strongly asymmetrical in the shoreward direction further offshore. The bars of profile A are very irregular, but generally also show decreasing s y m m e t r y values {change from steep side seaward to steep side shoreward) with distance from shore. When frequency of occurrence is plotted against wavelength and against bar height, there is a marked difference in the patterns. The wavelengths give a unimodal curve, with three-quarters of lengths lying between 12 and 33 m and one third between 15 and 20 m. Bar heights show no clear maximum but frequency decreases steadily with height. Thus 15% of the bars are less than 5 cm high and 10% between 25 and 30 cm. Grain-size m o m e n t data obtained from surface samples and pushbox cores on five sand bars have been plotted against bar shape in Fig.6, and show: (a) The sand is well sorted. (b) Although grain-size is related to bar shape for most sand bars, this relationship varies from bar to bar. However, in most cases the coarsest sand is in the troughs. (c) Curves for subsurface samples at Wackerballig in summer (bar 10515) and a u t u m n {bar 10423) appear to be out of phase with the surface curves, which suggests slow migration of the sand bars with time. Plots of skewness against mean grain-size (Fig.7) show that data points for each bar sampled lie in separate fields. Plots of skewness against standard deviation (Fig.7) show a greater spread of points and the various bars are n o t as distinct. Plots of standard deviation against grain-size (Fig.7) also allow the various bars to be distinguished. The Wackerballig offshore samples (10514), with coarse grains and appropriately high standard deviation, are just as distinctive as in the skewness plots. Relief casts were made from pushbox cores 15 to 30 cm long taken across sand bars 10423 and 10515 (Wackerballig onshore: the same bar at different times of the year) and are illustrated in Fig.8. These show that day-to-day migration of the bars takes place by means of small-scale ripple migration in units mostly less than 5 cm thick. The structure is complex, cross-bedding units dip both onshore and offshore, and some laminae are parallel to the bar surface. In this particular area wave action was not hampered by Mytilus or sea grass cover. In cores from sand bar 10423 taken in a u t u m n 1969, fresh structure was visible to depths of 5 -- 10 cm on the seaward side, but only to 1 or 2 cm on the leeward side, showing that reworking was strongest on the seaward slope.

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209 The underlying 20 cm o f sediment was bioturbated, showing t hat the bar had been d o r m a n t earlier. Structure is c o m m o n l y preserved at the b o t t o m of the cores, suggesting that bioturbation does n o t reach m ore than 20 to 25 cm below the surface. The abundance of sea grass fragments in t he troughs shows that b r o k en o f f stems settle there preferentially. The cores from sand bar 10515, taken in summer 1970 on the same bar in much the same position, contain very little fresh structure. The bar had been essentially inactive for some time, and burrowing crustaceans and worms had t h oroughl y mixed the sediment.

Wave action and its effects Walden (1960) p l o t t e d wave data for the lightship " F l e n s b u r g " ("F.S. Flensburg" in Fig.l) which lay several kilometres nort h of Gelting Bay, for the six years 1949--1954, and the data gathered over this relatively long period can be regarded as fairly representative. The exposure of the lightship to waves from the sector 2700--340 ° is much the same as that of southeast Gelting Bay, and wave heights and periods in both areas must be similar at the same time. The wind and waves came from this quadrant on 31% of days. The distribution of readings for various sized waves interpreted for southeast Gelting Bay, i.e., excluding waves which could n o t reach this side of the bay, is tabulated in Table I. The most c o m m o n appreciable waves are around 0.5 m high, but waves to 1 m high can be expected several times a year, and still larger waves must be r e c k o n e d with. F r o m Walden's (1960) wave-height and period data it is possible to calculate th e theoretical m a x i m u m current velocities developed in various water depths for various wave sizes (Table II). These values cannot be regarded as accurate, but do give an indication of velocity relationships. Although on dry land many of the glacial tills are firm and resistant to erosion, in the sea the upper few decimetres of till absorb a great deal of water and become soft and readily erodable. The well-known curves of Hjulstrbm (1935) show that velocities of 15 cm/sec can erode fine and medium sand, and velocities of 30 cm/sec fine gravel. Thus the larger waves are capable o f eroding glacial till down to water depths of 10 or more metres. TABLE I Waves in Gelting Bay

Wave height (m) % of readings

From all directions

Only produced by northwest winds

< 0.25

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1 (0.75--1.25)

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210 TABLE II Effect of waves in outer Flensburg Fjord (1) Wave height H (m)

(2) Period T (sec)

(3) Wave velocity v (m/sec)

(4) Wave length ~ (m)

(5) Base of erosion ~/2 (m)

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Feels bottom at k/2 where orbital velocity is 4% that at surface (Bascom, 1959). Water depth assumed to calculate (7).

(7) Maximum horizontal bottom velocity = 0.4V ( h - - 0"062)0"72 (Johnsen, 1961).

The clay is s w e p t a w a y in suspension and the sand is t r a n s p o r t e d along t h e b o t t o m by various currents, and in the shallower w a t e r is s w e p t inshore b y breaking waves. Sand a c c u m u l a t e s in w a t e r shallower t h a n 2 m, where wave energies are lower. Western Baltic Sea sand bars are n o r m a l l y (e.g., near Heiligenhafen -- Brand, 1955) strongly a s y m m e t r i c a l , with steeper lee slopes. The strong r e w o r k i n g caused by t u r b u l e n c e at the f o o t o f the lee slope means that o n l y coarser material can r e m a i n there, a n d finer material a c c u m u l a t e s higher on the bars. The bars have a wavelength o f 50 - - 75 m a n d a h e i g h t of 1 - - 1.5 m. The crest o f the o u t e r m o s t bar is as m u c h as 2 m b e l o w sea level. The d e e p e r internal s t r u c t u r e o f t h e bars, which are generally a m e t r e or m o r e thick, is unk n o w n , b u t p u s h b o x cores (Werner, 1 9 6 3 ; N e w t o n , 1 9 6 9 ) have s h o w n t h a t the u p p e r m o s t 30 c m is c o m m o n l y c h a r a c t e r i z e d by small-scale ripple lamin a t i o n a n d b i o t u r b a t i o n . These features develop during periods o f relative calm. In t h e less e x p o s e d Gelting Bay the o u t e r m o s t sand bars m a y be as long (70 m) as the bars in the o p e n sea, b u t t h e y are usually flatter ( 6 0 - - 7 0 cm) as a result of longer periods o f relative calm during w h i c h m o d i f i c a t i o n b y smallscale ripple migration is possible. The nearer the shore the s h o r t e r and lower

211 are the bars. The original wavelengths have been preserved, but the heights of the bars have been extensively modified by small-scale ripple migration of sand from the crests to the trough, and vary much less than wavelength does with distance from shore. Fig.5 shows that the sand bars were steepest and most asymmetrical in the middle distance at the time they were measured. This suggests that rebuilding of the bars had been most active in these areas, and thus that most waves had 'been breaking there. Water depths over the crests were between 60 and 80 cm, suggesting t h a t waves a little over 0.5 m high were involved (waves break in water where the depth is 1.3 times their height -- Bascom, 1959). These were probably the very common 0.5 m high open sea waves (Table I) somewhat shortened and heightened by b o t t o m drag. DISCUSSION The formation of such an extensive field of highly regular wave-formed sand bars as that in Gelting Bay (similarly extensive fields have not, to my knowledge, been previously described) seems to depend on two things. Firstly, the Baltic Sea is virtually tideless, and thus no matter what the tide, waves of similar size and orientation must attack much the same part of the sea bed. Secondly, effective waves come from virtually one direction only, the northwest, which also happens to be perpendicular to the shore line, so that little refraction occurs and wave energy is evenly distributed along the shore. Thus exceedingly regular bar fields could develop which, once they had formed, were not attacked by waves coming from different directions. The outstanding problem with these sand bars remains their genesis -either by gradual growth by small-scale ripple migration, or suddenly during major storms. The author favours erosion during storms and rebuilding as they wane, and envisages their development as follows. The post-glacial sea-level rise slowed several thousand years ago, and thereafter wave action cut a broad platform on which sand, derived from the till which was being eroded, accumulated. Once enough sand had accumulated a bar formed. The breaking of waves on the bar caused scouring on the lee side, but as the waves reformed they deposited their sand load, forming a second bar shoreward of the first. Gradual erosion of the till surface behind the bars, and massive erosion caused by particularly wild storms when the bars were destroyed and the whole platform attacked, caused the platform to widen. As it widened more and more bars could be built up. Because much energy was expended on the outer bars less wave energy was available to form the inner bars and hence the size of the sand bars diminishes inshore. It is suggested that even now during major storms the entire sand mass is reworked, and only as the storms wane, and wave sizes diminish, do the sand bars again form. Such storms may occur only once or twice a century. Vicrocorer investigations could show if the lower parts of the bars consist of large scale cross-beds, which would indicate that the bars had, in fact, formed at the conclusion of a storm, afterwards being modified by ripple migration.

212 It is unlikely t h a t such features w o u l d be preserved in the geologic record. This w o u l d require s u d d e n p r o t e c t i o n f r o m wave a c t i o n ( t e c t o n i c m o v e m e n t s , d e v e l o p m e n t of a p r o t e c t i n g sandspit), and d e p o s i t i o n of silt or m u d on the bars. If t h e y were f o u n d in the r e c o r d t h e y w o u l d be difficult to distinguish f r o m n o r m a l large-scale c u r r e n t ripples. With their ripple i n d e x o f a r o u n d 100, t h e y are f l a t t e r t h a n m o s t such ripples, b u t still fall within t h e possible field (see fig.4.10 in Allen, 1968). I f o u t c r o p were very good, and t h e p o s i t i o n of the p a l a e o - c o a s t l i n e w e r e k n o w n , the f a c t t h a t t h e large-scale ripples w o u l d parallel the shore and diminish in size s h o r e w a r d , and t h a t t h e y w o u l d show c r o s s - l a m i n a t i o n o r i e n t e d b o t h s h o r e w a r d a n d seaward, w o u l d be diagnostic of a s a n d bar field. ACKNOWLEDGEMENTS This w o r k was carried o u t at the suggestion o f P r o f e s s o r E. Seibold, during a t w o y e a r s t a y at the University o f Kiel. Drs. R. N e w t o n , F. Werner, and M. S a r n t h e i n gave advice a n d practical assistance in the field a n d the l a b o r a t o r y . S t u d e n t divers B. F l e m m i n g a n d R. Fritsche h e l p e d with t h e field w o r k , a n d Frl. W. R e h d e r with t h e l a b o r a t o r y w o r k . Dr. E. Walger advised on t h e grainsize w o r k a n d p r o v i d e d a suitable c o m p u t e r p r o g r a m . I wish to t h a n k all these colleagues for t h e i r u n s t i n t e d h e l p and e n c o u r a g e m e n t . This article is p u b l i s h e d with the p e r m i s s i o n o f the D i r e c t o r , Bureau o f Mineral R e s o u r c e s , Canberra, A.C.T. {Australia). REFERENCES Allen, J. R. L., 1968. Current Ripples. North-Holland, Amsterdam, 433 pp. Bascom, W., 1959. Ocean waves. Sci. Am., August, 1959. Brand, G., 1955. Sedimentpetrographische Untersuchungen zum Erkennen der Sandwanderungsvorg~nge am Strand, im Flachwasser und dem daran anschliessenden Seegebiet. Meyniana, 4: 84--111. Exon, N. F., 1971. Holocene Sedimentation in and near the Outer Flensburg Fjord (westernmost Baltic Sea). Thesis, Geologisches Institut der Universit~t Kiel, Kiel (unpublished). Exon, N. F., 1972. Sedimentation in the outer Flensburg Fjord area (Baltic Sea) since the last glaciation. Meyniana, 22: 5--62. HjulstrSm, F., 1935. Studies of the morphological activity of rivers as illustrated by the River Fyris. Bull. Geol. Inst. Uppsala, 25: 221--527. Johnsen, R., 1961. Wechselbeziehungen zwischen der Welle und dem strandnahen Unterwasserhang. VerSffentl. Forschungsanst. Schiffart, Wasser-Grundbau, 9: 1--102. Kannenberg, E. G., 1951. Die Steilufer der schleswig-holsteinischer Ostseekiiste. Schr. Geogr. Inst. Kiel, 14(1): 101 pp. Newton, R. S., 1969. Internal structure of wave-formed ripple marks in the nearshore zone. Sedimentology, 11 : 275--292. Reineck, H. E., 1970. Marine SandkSrper, rezent und fossil. Geol. Rundsch., 60(1): 302--320. Seibold, E., Exon, N., Hartmann, M., KSgler, F., Krumm, H., Lutze, G. F., Newton, R. S. and Werner, F., 1971. Marine geology of Kiel Bay. VII Int. Sediment. Congr., 1971: Guidebook to Sedimentology of Parts of Central Europe, pp.209--235. Walden, D. H., 1960. Der Seegang bei den Feuerschiffen "Flensburg", "Kiel", und "Fehmarnbelt". Dtsch. Wetterdienst, Seewetteramt EinzelverSffentl., 26:50 pp. Werner, F., 1963. Uber den inneren Aufbau yon Strandwallen an einem Kustenabschnitt der EckernfSrder Bucht. Meyniana, 13: 108--121.