Large-scale bedforms, sediment distribution and sand mobility in the eastern North Sea off the...

Large-scale bedforms, sediment distribution and sand mobilityin the eastern North Sea o¡ the Danish west coast

Dennis Anthony �;1, J�rgen O. Leth 1

Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark

Received 13 February 2001; accepted 28 August 2001

Abstract

Two side-scan sonar surveys were carried out 9 months apart in a 24-km2 off Thorsminde at the Danish westcoast, were water depth is 12^18 m. Sediment samples were systematically collected in the same area using a 250-msampling grid. The North Sea study area is exposed to strong wind-induced coastal currents and high waves, andthereby characterized by significant sediment transport. The seabed data were used to analyse the large-scaledynamical bedforms found in the area, the mobility of the bedforms, the areal variation of the seabed mean grain-sizeand the grain-size variation across the large bedforms. The compilation of side-scan sonar mosaics revealed very largebedforms, with a mean wavelength of approximately 500 m and heights of 1^3 m. These bedforms indicate a north-going net sediment transport, caused by the Jutland Coastal Current. Large dunes (L= 10^100 m) and smallerbedforms were also recorded, as well as large shoreface-connected ridges with a spacing of 3^4 km. The bedformconfiguration appears to be relatively stable, only small changes having been observed between the two surveys, eventhough a 100-yr cyclone and several storms occurred between the two surveys. The small changes observed, alsoindicate a net northerly sediment transport. The surface sediments reveal a remarkably high positive correlationbetween the mean grain size and the position across the very large flow-transverse bedforms, from coarse sand in thetroughs to fine sand at the crest and lee-side slope of the bedforms. 6 2002 Elsevier Science B.V. All rights reserved.

Keywords: dune; sand wave; North Sea; side scan; grain size; sand ridge

1. Introduction

In the recent years, the Geological Survey ofDenmark and Greenland (GEUS) has carried outseveral investigations along the Danish North Sea

coast. The majority of these investigations werecarried out for the Danish Coastal Authorities(DCA). Apart from a more general marine geo-logical study of the nearshore zone o¡ the DanishNorth Sea west coast (Leth and Anthony, 1999), a24-km2 o¡shore area has been surveyed in greaterdetail with respect to surface sediments and under-lying geology (Anthony and Leth, 2000; Leth andAnthony, 2000). The water depth in the study areavaries between 12 and 18 m. Aspects of the seabedmorphology, sediment distribution and seabedstability are presented in this paper.

0025-3227 / 02 / $ ^ see front matter 6 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 4 5 - 6

1 New address after May 2002: GEUS, Ester Voldgade 10,DK-1350 Copenhagen K, Denmark.

* Corresponding author. Tel. : +45-3814-2000;Fax: +45-3814-2050.

E-mail addresses: [email protected] (D. Anthony), [email protected](J.O. Leth).

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Marine Geology 182 (2002) 247^263

www.elsevier.com/locate/margeo

Areas with sandwaves/subaqueous dunes aswell as shoreface-connected ridges are knownfrom several places in the North Sea (e.g. Ter-windt, 1971; Eisma et al., 1979; Kuijpers et al.,1993; Huntley et al., 1994; Van de Meene andVan Rijn, 2000a; Zeiler et al., 2000). However,it has not been documented before that verylarge-scale dynamic bedforms occur over rela-tively large areas o¡ the central part of the Dan-ish west coast. The objective of this paper, there-fore, is to document the existence and spatialdistribution of these bedforms mapped o¡ theDanish west coast, to describe the hydrodynamicregime responsible for their existence and to dis-cuss the features in a more global context. More-over, on the basis of a side-scan sonar mosaic anda large number of sediment samples, the relation-ship between surface sediment grain size and 2D-bedform position has been analyzed.

2. Physical setting

2.1. Hydrography

The study area is located in the eastern part ofthe North Sea (Fig. 1) which is exposed to rela-tively strong, mainly westerly, winds. A numberof storms normally occur in the winter season.Severe cyclones are more rare, the most recenthaving occurred on December 3, 1999. Accordingto the meteorological records, it was the strongestcyclone to hit Denmark in the last century, withgusts up to 50 m/s at the southern part of theDanish west coast (DMI, 1999). Wind-generatedwaves in the study area, on average up to 5 m inheight (Laustrup et al., 1997), are associated withstrong westerly winds and a large westerly fetch.They are, for the most part, responsible for therelatively large littoral drift.

An important element of the North Sea circu-lation is the Jutland Coastal Current which £owsnorthward along the Danish west coast (Kristen-sen, 1991). In 1997, the Danish Coastal Author-ities deployed an array of current meters acrossthe study area. Current velocity records coveringa 1.5-yr period (1998^1999) are shown in Table 1.Maximum current velocities in the study area are

seen to reach 1.2 m/s in a north-going direction,and about 0.7 m/s in a south-going direction. Thetidal component is in the range of 0.2^0.4 m/s.The peak current velocities in the north-goingdirection are observed during winds from theWSW, and wave-height Hmo = 5.5 m. The theo-retically maximum north-going transport by theJutland Coastal Current would be generated bywinds from the SSW (Kristensen, 1991). A gen-eral North Sea circulation pattern, and, in partic-ular, the initiation of a Jutland Coastal Currentcomparable to the present, are assumed to havebeen established by about 5500 yr BP (Leth,1996).

The spring tidal range in the area is below 1 m,but wind stress can produce a set-up of severalmeters during storms (Nielsen et al., 1996). Theopen North Sea waters are always free of ice, andthe water temperature varies on a seasonal cyclefrom close to freezing point in winter, to about20‡C near the coast in summer. The salinity isnormally in the range of 34^35x. There is nosigni¢cant thermal or haline strati¢cation in thestudy area (Howarth et al., 1994).

2.2. Geology

The main terminal line of the ice sheet duringthe last ice age (Weichselian), was situated imme-diately north of the study area. It is representedby the high coastal cli¡s at Bovbjerg (Petersenet al., 1992) and the Jutland Bank area in theo¡shore (Leth, 1996). Glacial sediments aretherefore widespread onshore. Due to the coastalerosion, since the transgression, the o¡shore gla-cial sediments have been completely reworked lo-cally.

From shallow-seismic surveys and vibrocoringin the study area, it has been shown that thethickness of marine sand is positively correlatedwith the presence of the large bedforms. The max-imum thickness of the marine sand is 5 m (Lethand Anthony, 1999). Below the marine sand, agravel bed is frequently observed. The marine suc-cession overlies Tertiary deposits, primarily com-prising Miocene clays and sands. The Nissum SaltDiapir (Japsen and Langtofte, 1991) is located inthe northern part of the study area. Diapirism has

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D. Anthony, J.O. Leth /Marine Geology 182 (2002) 247^263248

resulted in an uplift of the Tertiary deposits, thatare now outcropping at the seabed. This is alsoevident from the shallow-seismic recordings show-ing a major distinct anticline in the Tertiary de-posits. In the eastern part, £uvioglacial sand andclay are present. Locally, minor channels in¢lledwith marine clay as well as remnants of till arepresent (Leth and Anthony, 1999, 2000).

3. Methods

3.1. Side-scan sonar mosaic

Side-scan sonar surveys were carried out in Au-gust 1999 and May 2000. Survey lines were evenlyspaced at 50 m. Georeferences are in WGS84(UTM coordinates on the maps are in zone 32).

Fig. 1. Location of the study area. The shaded rectangle indicates the position of the side-scan sonar mosaic (track lines notshown). The shown track lines are from the regional shallow seismic survey.

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D. Anthony, J.O. Leth /Marine Geology 182 (2002) 247^263 249

In 1999, 36 lines were run, using a 100-kHzKlein side-scan sonar and Delph recording soft-ware. In 2000, 61 lines were run, using a 100/384-kHz EG p G DF-1000 Side-Scan Sonar, and ISISsoftware. For both surveys, the recording rangewas set to ensure an overlap of the sonar images,and positions were recorded in EUREF89/WGS84 using a DGPS system, with an accuracybetter than 1 m. The side-scan sonar ¢sh wastowed behind the survey vessel, and the algorithmused for calculating the co-ordinates for eachpoint assumed a horizontal seabed, which slightlyreduced the positional accuracy of the mosaics.

All data were post-processed with ISIS andDelphMap software, resulting in a mosaic foreach of the surveys with a resolution of 0.2 m.The post-processing of the side-scan sonar dataincluded layback correction, navigational smooth-ing, beam angle compensation and mosaic build-ing. The ¢nal construction of the geocoded acous-tic images of the sea£oor was made using theMapInfo GIS.

Although di¡erent side-scan sonars and soft-ware were used for the two surveys, the two re-sulting mosaics are nevertheless comparable.

The two surveys were primarily carried out inorder to document possible changes in the seabedmorphology of the study area after a stormy win-ter season. The 2000 mosaic was used to producea bedform map and, together with the grain-sizedata, a mean grain-size distribution map.

3.2. Bedform classi¢cation

Large-scale current-induced, £ow-transversebedforms can be classi¢ed according to Ashley(1990), where the wavelength is used as classi¢ca-tion parameter. The large dunes have wavelengthsin the range 10^100 m, whereas the very largedunes have wavelengths s 100 m with no upperlimit speci¢ed. Sandwaves, describing shallow-ma-rine tidal induced bedforms (e.g. Allen, 1984),have been de¢ned by Ashley (1990) as a sub-group of dunes. Since they show characteristicsquite di¡erent from the non-marine dunes, theywill, in this context, be discussed as a group ontheir own. Non-speci¢ed bedforms with wave-lengths in the 100^1000-m interval will be referredto as very large bedforms.

Even larger bedforms with other characteristicswere recorded, which resemble shoreface-con-nected sand ridges (e.g. Swift et al., 1978; Vande Meene and Van Rijn, 2000a). Smaller bed-forms, with an orientation related to wind-gener-ated waves, are classi¢ed as wave ripples, even ifthe bedform wavelength is several meters (Allen,1984).

3.3. Sediment distribution

In October 1998, 348 sediment samples werecollected in the study area. The samples were tak-en with a grab-sampler on a grid with a 250-m net

Table 1Current velocities (depth-averaged and 2 min average) from the study area during a 1.5-yr period in 1998^2000

Distance from shore (km) 4.4 2.8 1.0Mean depth (m) 15 15 8Maximum measured current velocity (m/s) 1.00 1.22 1.62

30.64 30.65 31.03Tidal current velocity (m/s) 0.35 0.41 0.40

30.23 30.23 30.22Example of current velocity with waves from NW and Hmo = 5.15 m (m/s) 0.33 0.33 30.20Example of current velocity with waves from WSW and Hmo = 5.5 m (m/s) 0.95 1.20 1.33

Positive current velocities are northgoing and negative are southgoing. Hmo is mean deep-water waveheight. Data are fromLaustrup et al. (2000).

Fig. 2. (A) Side-scan sonar mosaic from May 2000. (B) Large scale bedforms. The bedform size refers to wavelength. The linesindicating the very large bedforms, show the position and orientation of the lee-slope/bottomset boundary. (C) Mean grain-sizeof surface sediments. The mean grain-size is calculated only for the sand fraction, i.e. the interval 0.064^2.0 mm.

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Fig. 2. (A).

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Fig. 2. (B) (Continued).

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Fig. 2. (C) (Continued).

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spacing. The Sediment Laboratory at the DCAperformed grain-size analysis by wet-sieving forall samples. The grain-size distribution map isbased on the mean grain size of the sand fractionfor each sample. Throughout the contouring pro-cess, the re£ection pattern of the side-scan sonarmosaic was taken into account. Grain-size dataand the side-scan sonar mosaic were linked usinga GIS.

A high positive correlation between the meangrain size and the large-scale bedform morphol-ogy derived from the side-scan sonar mosaic wasfound. For establishing the relationship betweenmean grain-size and cross-sectional position onthe large scale bedforms, only sediment samplesfrom two-dimensional dunes were used in theanalysis. The position of the sediment sampleswas transformed to a normalized distance, calcu-lated from the bottom of the dominant lee-sideand in down-current direction. The position ofthe dominant lee-side bottom was found fromthe side-scan sonar mosaic, de¢ned by a distinctshift in the acoustic backscatter intensity, andveri¢ed by echo-sounder pro¢les. When plottingthe grain-size distributions for the individual sam-ples, the sieve-fractions were proportionally cor-rected because of non-uniform sieving intervals.

4. Results

4.1. Bedforms

Fig. 2A shows the side-scan sonar mosaic of thesurvey area as recorded in May 2000. The mosaicis composed of 61 lines, each with a length ofapproximately 8 km. The total area amounts to24.2 km2 The E^W-oriented large-scale features inthe central part of the mosaic are very large bed-forms with a characteristic wavelength of approx-imately 500 m. Their height is in the range 1^3 m.The dark areas on the mosaic are relativelycoarse-grained troughs, and the gentle increase

in intensity (¢ner grain size) in a northward direc-tion is interpreted to be the bedforms stoss-sides.The abrupt change from high to dark intensityindicates the lee-side/bottom-set intersection ofthe bedforms. The bedforms can thus be inferredto migrate in a northerly direction which is con-¢rmed by information from repeated high-resolu-tion bathymetric surveys made by the DCA. Theinsert in Fig. 2B shows a cross-section of twobedforms for the line marked A^B. The gradientsof the very large bedforms are very gentle, and aregenerally not more than approximately 2^4‡, evenon the lee-side, although locally some steeperslopes may occur. These features will, in the fol-lowing, be referred to as the Thorsminde Bed-forms. In both the northern and southern partof the survey area other very large-scale bedformsare found, but direction and wavelength are am-biguous. The nature of the Thorsminde Bedformswill be dealt with in Section 5 Discussion.

The areal distribution of the bedforms in thesurvey area is shown on Fig. 2B. Besides theThorsminde Bedforms, areas with large dunes(L= 10^100 m) and wave ripples (L6 2 m) areshown. The large dunes, mostly found in clusters,are most widespread in the western part of thearea, where depths are up to approximately18 m. The strike direction is generally E^W,which is the same as for the Thorsminde Bed-forms. In general, bedforms with wavelengths of2^10 m (Wsmall to medium dunes) have not beenobserved. The wave-ripples are, with few excep-tions, located on the lee-side troughs of the largebedforms. The strike direction of the wave-ripplesis consistently NNE^SSW, re£ecting incomingwaves from WNW.

Seabed features recorded in the troughs be-tween the Thorsminde Bedforms are illustratedin Fig. 3. The sonographs reveal outcroppingeroded clay, glacial lag deposits with bouldersand wave ripples. Fig. 3A shows an example ofseabed erosion with outcropping layered (Mio-cene) clay. The seismic records from this area in-

Fig. 3. Side-scan sonar close-up views (May 2000) of the troughs of the very large dunes. The highlighted boxes in the key mapshow the precise position of the SSS-mosaic examples labeled A^E. The grid size of the map is 100U100 m, and the scalebar onthe sonographs is 10 m. See text for explanation.

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Fig. 3.

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dicate that the clay layers are dipping eastward,which corresponds well with the N^S strike direc-tion of the outcropping claybeds and the west-ward facing erosional scarps. Some of the erosion-al scarps are not visible on the mosaic ^ the scarpsnot facing the side-scan sonar produce low back-scattering or acoustic shadow in the side-scan im-agery. Fig. 3B shows large-scale wave ripples(LW2 m) and coarse-grained lag deposits. Themaximum clast size of the lag deposits on theleft side are approximately 0.5^1 m, but in thecentral part, a 3-m boulder is seen (the whitearea at the right side of the boulder is an acousticshadow). Fig. 3C shows erosion in clay ^ prob-ably Miocene or glacial clay. Fig. 3D and E showwave-ripples with varying wavelength (LW1.5 mand LW0.8 m). The locally exposed residual sedi-ments and glacial lag deposits underlying theThorsminde Bedforms indicate that the amountof sand available for bedform generation is lim-ited, and that the bedforms become locallystarved.

Besides the Thorsminde Bedforms, displayingwavelengths in the order of 500 m, even largerfeatures are found in the study area. These areoutlined in Fig. 4. The height of these featuresreaches 4^5 m, their spacings being 3^4 km.They are oriented oblique to the Thorsminde Bed-forms. Echo-soundings and seismic records fromelsewhere along the Danish west coast (Leth andAnthony, 1999; see part of survey lines in Fig. 1)indicate that these features with spacings of morethan 1 km extend to approximately 10 km to thesouth, 5^15 km to the north and at least 5 km tothe west of the study area. They may be prefera-bly de¢ned as shoreface-connected ridges, al-though the origin of these feature is not fullyunderstood. The general distribution of theseand other large-scale bedforms in other parts ofthe Danish sector of the North Sea will be pre-sented in a later paper.

4.2. Sediment distribution

4.2.1. Areal variation of mean grain sizeFig. 2C shows the grain-size variation of the

surface sediments in the study area. The bounda-ries between the di¡erent mean grain size group-

Fig. 4. Con¢guration of the shoreface-connected ridges, strik-ing oblique to the very large bedforms.

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ings were slightly adjusted on the basis of theside-scan sonar information. The overall sono-graphic re£ection patterns con¢rm the sedimentvariability indicated by the grain-size distribution,especially because seabed gradients in the studyarea are relatively low.

The general picture is that the crest region ofthe Thorsminde Bedforms consists of ¢ne sand. Itis £anked by a wide belt of medium sand on thestoss-side and a narrower belt of medium sand atthe lee side. The troughs consist of coarse (andvery coarse) sand, as well as gravel. Residual sedi-ments (glacial lag deposits in the form of bouldersand clay beds) are also locally present in thetroughs, where they are partly covered by sand.Similarly, as in the case of the very large bed-forms, the shoreface-connected ridges shown inFig. 4, are in their central part almost entirelycomposed of ¢ne sand.

4.2.2. Grain-size variation across the ThorsmindeBedforms

The systematic variation in mean grain size (inthe sand fraction) across the Thorsminde Bed-forms is shown in Fig. 5. A best ¢t polynomialregression line and a schematic cross-section ofthe bedforms have been added.

In the trough, adjacent to the steeper slope, thecoarsest sediment is found; mean grain sizes hereare 0.6^0.7 mm. Some gravel was commonly alsopresent (not shown). Moving across the bedformin the down-current direction, the mean sedimentgrain size on the stoss-side decreases rapidly to-wards the top, and more moderately around thecrest and uppermost part of the lee-side £ank.The ¢nest sediments are found on the lee-slope,where the mean grain size is 0.1^0.2 mm. To high-light this trend, only sediment samples from well-de¢ned very large 2D bedforms were used

Fig. 5. Mean grain-size variation across the very large bedforms.

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(marked as triangles in Fig. 2C). The remainingscatter is probably due to the fact that the data iscomposite, originating from di¡erent very largebedforms, and because at the sampling positions,the bedforms were not strictly two-dimensional inall cases.

The grain-size distributions used for the con-struction of Fig. 5, are shown in Fig. 6. The in-dividual samples are placed on a vertical scalecorresponding to the horizontal scale of Fig. 5.Beside sample Nos. 3 and 7, the individual sam-ples do not show clear evidence of a bimodal sedi-ment distribution, though some of the samplesdisplay skewness. However, the fact that the sam-ples were sieved with a relative coarse sieving in-terval will normally reduce the possibility of ¢nd-ing individual sub-populations.

Still, the general impression of the grain-sizedistributions as a whole, points to the presenceof sediment sub-populations. These are indicatedon Fig. 6, with following sieve fraction means:(A) 0.59 mm, (B) 0.39 mm, (C) 0.27 mm and(D) 0.21 mm. The extension of these sedimentsub-populations across the very large bedformsoverlaps, suggesting that gradual mixing of thesubpopulations causes the shift in mean grainsize. To con¢rm that we actually are dealingwith a mixing model (cf. Flemming, 1988a),more detailed grain-size analysis would, however,be required.

4.3. Sand mobility

The survey area of May 2000 had previouslybeen partly surveyed in August 1999. The 1999survey covered the central and eastern sector ofthe 2000 survey area. The general picture is thatthe con¢guration and position of the ThorsmindeBedforms remained almost unchanged, eventhough a stormy winter including an exceptionally

Fig. 6. Grain-size distributions of individual samples acrossthe bedforms with 350^700 m spacings. The left side scale issimilar to the normalized distance of Fig. 5. The numbers onthe right side are the sample numbers. The assumed subpo-pulations are indicated by the letters A, B, C and D, whereasAB, BC and CD are mixtures.

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severe cyclone had passed between the two sur-veys.

The physiography of selected areas of the tran-sitional zones between the mobile sandy seabedand the lag deposits/troughs are shown in the so-nographs of Fig. 7 for both the 1999 and the 2000surveys. They reveal the di¡erences between thetwo surveys. In Fig. 7A, an eroded seabed of lay-ered Miocene clay, partly covered by sand, isseen. Between the 1999 survey (Fig. 7A1) andthe 2000 survey (Fig. 7A2), the sand cover hasincreased, and the transition between the sand

and the clay has become more di¡use. Fig. 7Bshows glacial deposits with lag sediments seen inthe upper part of the sonograph. In the lower partof the sonograph, these deposits can be observedto be overlain by sand. The boundary of the sand/lag transition recorded in the 1999 survey is stillrecognizable in 2000. It has shifted its positionslightly northward and has become more di¡use.Fig. 7C and D show close-ups of the lower lee-side and adjacent trough of two bedforms. Again,apart from a slightly northward shift, the boun-daries of the transitional zones have remained al-

Fig. 7. Side-scan sonar mosaics for August 1999 (index 1) and May 2000 (index 2), showing di¡erences in sand distribution. Thehighlighted boxes in the key map show the precise position of the various examples labeled A^E shown left. The grid size is100U100 m. See text for explanation.

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most unchanged in both the 1999 and the 2000mosaics. A band of ¢ner sand is seen at the lee-side base of both bedforms on the 2000 mosaic. InFig. 7E, a part of an area of large 3D-dunes and aWNW^ESE trough of a Thorsminde Bedformsare seen. The large 3D-dunes exist on both mo-saics, although they are a little di¡use and patchyon the 2000 mosaic. On the 1999 mosaic, the sedi-ment transport direction for the large dunes seemsto be southerly.

5. Discussion

The side-scan sonar mosaics from the area o¡Thorsminde illustrate the presence of very largebedforms (referred to here as Thorsminde Bed-forms) indicative of net northerly sediment trans-port throughout the study area. Asymmetries ofsome of the large dunes as well as relative highvelocities measured for both northerly and south-erly directions of the Jutland Coastal Current(JCC) indicate, however, that sediment transporttowards the south may also occur. The largedunes and Thorsminde Bedforms are assumed tohave been generated and maintained by the tideand the wind-induced JCC, especially during epi-sodes when the north-going current is strong. Ac-cording to Miller et al. (1977), the threshold forsediment motion under unidirectional currents, isin the range 0.67^0.86 m/s for the ¢ne-sand frac-tion (velocity 1 m above seabed), which applies tothe surface sediment on most of the study area.This critical threshold will, however, be lowerwhen waves are present (Freds�e and Deigaard,1992), depending on the wave characteristics.

The nature of the Thorsminde Bedforms isambiguous. They are £ow-transverse bedforms,slightly asymmetric in shape, and they migrateslowly. This would normally classify them asvery large dunes. As the wavelength (L) of dunesnormally is said to scale with £ow depth (h), givenby the relation L= 6h (Yalin, 1964), the Thors-minde Bedforms are much larger than dunes nor-mally seen at waterdepths of 12^18 m. The largescatter in global ¢eld data, especially for marineor marine in£uenced dunes (sandwaves), (e.g. Al-len, 1984, ¢gs. 8.18 and 11.25) may leave, how-

ever, space for other relationships as well. Theheights of the bedforms (1^3 m) compared totheir wavelength (350^700 m) are also less thanone would normally ¢nd for dunes (e.g. Flem-ming, 1988b). In addition, the lee-side slope ofthe bedforms is very gentle, but sediment starva-tion as well as depth-limited £ow can reduce bed-form height (Flemming, 2000). The grain-size ofdunes is also supposed to play a major role incontrolling the size of dunes. According to Flem-ming (2000), who has suggested a grain-size/wave-length relation found in a deep water dune ¢eldstudy, very large dunes would hardly be devel-oped in ¢ne sand. If the Thorsminde Bedformsin fact are dunes, this relation might not be validin a depth limited £ow, as the Thorsminde Bed-forms consists of ¢ne sand in their central part.

Regarding the gradients of the ThorsmindeBedforms, they have a resemblance to the tidal-induced, low-gradient dunes formerly classi¢ed asClass V^VI sandwaves by Allen (1980). Thesehave lee-side slopes in the range of 3^4‡, andare primarily in£uenced by bi-directional currentswell above the threshold for sediment transport.Since the Class V and VI sandwaves have a com-plex small-scale internal structure, this could ex-plain the absence of internal structures in the seis-mic recordings of the Thorsminde Bedforms. Thelower boundary of the bedforms, however, is wellde¢ned on the seismic records (Leth and An-thony, 1999).

The grain-size variation across the ThorsmindeBedforms yields evidence for a characteristic pat-tern of mean grain-size variability in the sandfraction. In the troughs, the mean grain size isup to 0.6^0.7 mm, and the mean grain size grad-ually ¢nes up the stoss-side of the bedforms. The¢nest sediments, having mean grain sizes of 0.15^0.2 mm, are found at the crest and on the lee-slope. A similar type of grain-size/morphology re-lationship has been found on shoreface-connectedridges (e.g. Swift et al., 1978) and on linear tidalsand ridges southwest of Florida (Davis et al.,1993). Whereas the former have been associatedwith combined tide and storm e¡ects, the latterare predominantly controlled by strong tidal cur-rents. Stolk (2000) reports a relationship betweenmorphology and grain size of dunes in the Flem-

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ish Bank area of the southern North Sea, but herethe relationship was found to be more or less theopposite. The surface sediment distribution in thisarea, however, was found to vary considerablyaccording to the in£uence of waves and verystrong tides (Houthuys et al., 1994). Terwindt(1971), who also studied large dunes (sandwaves)in the southern North Sea, found signi¢cant var-iations in the grain-size distributions in di¡erentdune areas showing contrasting relationships. Thereason for this may be the di¡erence in hydrody-namic regime, rather than di¡erences in sedimentsource and supply, as the general mean grain-sizespectra are similar. The Thorsminde Bedforms aresituated in a micro-tidal setting exposed to veryhigh wave energy generated by the prevailingwesterly storms which also control the wind-sen-sitive Jutland Coastal Current. The Flemish Bankarea is, on the other hand, exposed to a macro-tidal regime (tidal currents up to 1 m/s), with alimited westerly wind fetch. The well-de¢ned rela-tionship between grain-size distribution and bed-form morphology found for the Thorsmindedunes is therefore suggested to be governed byepisodic strong currents, and a relatively high-en-ergy wave climate.

Further north, i.e. o¡ Hanstholm and Hirt-shals, Kuijpers et al. (1993) also found sandwaves,providing evidence for northeast-going currentswith velocities in the order of 0.7^1.0 m/s. Thepeak current measurements by DCA have veri¢edthat these high current velocities also occur in ourstudy area (Laustrup et al., 2000). According tohydrodynamic modelling, the highest transport bythe JCC is reached during winds from SSW (Kris-tensen, 1991). Coincidence of maximum tidal cur-rent velocity and peak wind stress are assumed tobe important in order to achieve such high, coast-parallel current velocities. The large dunes wouldthen re£ect the last extreme dynamic event withsigni¢cant sediment transport, and the Thors-minde Bedforms would re£ect the long-term netsediment transport.

The formation mechanism of the shoreface-con-nected ridges in the study area is poorly under-stood. The ridges, characterized by a spacing of3^4 km and an oblique angle to the coastline(southward-opening angles of about 60‡), are ob-

viously not directly related to the JCC like the£ow-transverse bedforms. Along the Dutch coast,shore-face connected ridges have also been ob-served and some of these are interpreted to havebeen formed, or at least maintained, by thepresent hydrodynamic regime (Van de Meeneand Van Rijn, 2000b). Vibrocores from the cen-tral part of the ridges in our study area (Leth andAnthony, 2000), however, indicate a shift in thedepositional environment during the developmentof the ridges. The main part of the sand bodyconsists of sand with thin mud drapes and someorganic content (dated 1^3 kyr BP from C-14 dat-ings), which is overlain by a relatively thin (6 1m) sand cover rich in shells. This would suggestthat the basal shoreface-connected form of theridges is sub-recent. Due to the relatively highsediment transport in the study area, the shore-face-connected ridges are presumably not justrelics, but may be maintained by the present hy-drodynamic regime. Therefore, they are presum-ably relatively long-lived features that also mayplay a role in littoral processes and coastal devel-opment.

The strong current velocities of the JCC willnormally be associated with wind-generatedwaves. The waves in the area can be high (s 5m) due to a long fetch associated with strongwesterly winds. Wave action produces ripples,seen in the lee-side troughs of the ThorsmindeBedforms. The orientation of these wave ripplesis the same all over the study area, even thoughthe orientation of the Thorsminde Bedformschanges. Wave-ripples are generally not observedon top of the dunes, presumably because of the¢ne grain-size, and hence smaller spacings whichare below the acoustic resolution of the side-scansonar. The stronger currents on top of the verylarge bedforms could also have a negative e¡ecton the development of the ripples. From otherstudies (Allen, 1984), it is known that wave-ripplelength increases at coarser grain sizes.

The two side-scan sonar mosaics, with a stormywinter and a severe cyclone in between, indicate arelatively low net north-going sediment transport.The individual Thorsminde Bedforms were, with-out problems, recognized on both mosaics, and itwould probably be easy to recognize and follow

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them on side-scan images for several years. How-ever, it should be noted that the littoral drift inthe surf zone is south-going ^ as visualized in theaccumulation patterns around coastal structures.The Thorsminde Bedforms as well as the shore-face-connected ridges are likely to interact withsediment processes in the surf zone and probablyalso the shoreline position, although this aspecthas not been investigated yet.

6. Conclusions

Several large-scale bedform types were found inthe North Sea o¡ the Danish west coast nearThorsminde. The largest bedforms were shore-face-connected ridges with 3^4-km spacings and3^4 m of height. They have an angle oblique tothe coast and continue far outside the study area.These features are presumably stable on 100^1000-yr scale.

Very large bedforms (called the ThorsmindeBedforms) with an orientation transverse to theJutland Coastal Current, are widespread in thestudy area. These bedforms have spacings in theorder of 350^700 m and heights mainly between1 and 3 m. These bedforms have many similaritieswith dunes, especially the marine sandwave type.Therefore, we suggest to classify them as largesandwaves ^ a sub-group of very large marinedunes, formed under the in£uence of tidal (andwind induced) currents and waves.

Clusters of large and medium dunes were alsofound, displaying an orientation generally trans-verse to the Jutland Coastal Current. Whereas thelarge sandwaves migrates northward, re£ectingthe net direction of sediment transport, some ofthe smaller dunes are indicative of periodicalsouthgoing sediment transport.

The mean grain-size variation across the sand-waves shows a clear gradual shift from coarsesand in the troughs to ¢ne sand at the crest andlee-side slope. The mechanism involved is prob-ably a progressively mixing of individual sedimentpopulations across the sandwaves, but this has tobe veri¢ed by more detailed grain-size analyses.

The stability of the seabed is relatively high onan annual time scale. High-resolution side-scan

sonar mosaic images from August 1999 andMay 2000 shows only minor changes in the bed-form con¢guration. A minor northward shift ofthe lee-slope/bottomset boundary of the sand-waves did occur, but the distribution and orienta-tion of the bedforms was unchanged, even thougha stormy winter and an exceptionally severe cy-clone passed between the two surveys.

Acknowledgements

Field work and dataprocessing of this study wascarried out as a part of a geological mappingprogram covering most of the Danish North Seaoffshore area. Our work was initiated and partlyfinanced by the Danish Coastal Authorities. Wethank them for their collaboration. We are grate-ful to Antoon Kuijpers and Niels Vinther foruseful suggestions on the manuscript. We alsoappreciate discussions with Christian Christiansenon the grain-size variation. Burg W. Flemmingand an anonymous reviewer are acknowledged fortheir careful reviewing and valuable comments.

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