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DOI: 10.3990/2.202

 

Jubilee Conference Proceedings, NCK-Days 2012

Quantified and applied sea-bed dynamics of the Netherlands

Continental Shelf and the Wadden Sea

Thaiënne A.G.P. van Dijk

1a,2

, M.H.P. Kleuskens

3

, L.L. Dorst

4

, C. van der Tak

5

, P.J. Doornenbal

1a

,

A.J.F. van der Spek

1a,6

, R.M. Hoogendoorn

1a

, D. Rodriguez Aguilera

1b

, P.J. Menninga

7

and R.P.

Noorlandt

1a,8

1aApplied Geology and Geophysics, Deltares, P.O. Box 85467, 3508 AL, Utrecht, The Netherlands, thaienne.vandijk@deltares.nl 1bSoil and Groundwater Quality, Deltares, P.O. Box 85467, 3508 AL, Utrecht, The Netherlands

2Water Engineering and Management, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands 3Alten PTS, 5651 CD Eindhoven, The Netherlands

4Hydrographic Service, Royal Netherlands Navy, P.O. Box 90701, 2509 LS, The Hague, The Netherlands 5MARIN, P.O. Box 28, 6700 AA Wageningen, The Netherlands

6Netherlands Centre for Coastal Research (NCK), Delft University of Technology, Faculty of Civil Engineering and Geosciences,

P.O. Box 5048, 2600 GA, Delft, The Netherlands

7Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, P.O. Box 80.005, 3508 TA, Utrecht, The

Netherlands

8Department of Geotechnology, Delft University of Technology, P.O. Box 5048, 2600 GA, Delft, The Netherlands

ABSTRACT

Sedimentary coasts and shallow-sea beds may be dynamic. The large-scaled spatial variation in these dynamics and the smaller-scaled behaviour of individual marine bedforms are largely unknown. Sea-bed dynamics are relevant for the safety of shipping, and therefore for monitoring strategies, and for offshore engineering projects and archaeological investigations. To date, sea-bed dynamic studies in the North Sea that are based on high-resolution echo soundings are mostly local. Recently, sufficient time series of modern, digital echo sounder surveys have become available to allow for a Netherlands Continental Shelf-wide quantification ofvertical dynamic trends as well as for the detailed analysis of the morphodynamics of marine bedforms. Results show that (i) tidal channels, estuaries and longshore bars are particularly dynamic, (ii) the shelf offshore is less dynamic in general, and (iii) the most dynamic zones offshore are the zones where marine bedforms occur. The occurrence of superimposed sand banks, long bed waves, sand waves and megaripples is limited to the sandy shelf and sand wave migration rates vary spatially between 0 to 20 m/year. This spatial knowledge of

morphodynamics is used in combination with environmental conditions and sea traffic to validate and to optimise re-survey policies.

INTRODUCTION

Sandy continental shelves and sedimentary coasts may be dynamic. Most sandy sea beds of shallow seas are characterized by marine bedforms of different spatial scales, such as sand banks, long bed waves, sand waves and megaripples. Each of these bedforms has its own dynamic time scale. These dynamics are relevant for navigation safety, especially in seas with critical depths for shipping, such as the southern North Sea. In order to keep nautical maps up-to-date, dynamic sea beds need to be re-surveyed in an appropriate frequency. Guidelines for horizontal and vertical accuracy of the data are provided by the International Hydrographic Organization [IHO, 2008], but no guidelines for the validation and optimization of re-survey policies exist. Other applications are offshore engineering projects, such as wind farms, and archaeological investigations.

Previous empirical studies of seabed morphodynamics focused on the analysis of marine bedforms of small sites with specific local conditions [e.g. Duffy and Hughes-Clarke, 2005; Knaapen, 2005; Van Dijk and Kleinhans, 2005; Winter and Ernstsen, 2007;

Buijsman and Ridderinkhof, 2008; Van Dijk et al., 2008; Dorst et al., 2009; Dorst et al., 2011]. Although recently performed for the

German coastal zone [Winter, 2011], a large-scaled study of the morphodynamics of the Netherlands Continental Shelf (NCS) does not exist. Such a study provides an overview of the spatial variation in seabed dynamics that increases our insight and understanding of the processes of bed evolution.

Only recently, the coverage of multiple datasets (time series) of digital bathymetric data, which are required for the study of sea-bed morphodynamics, has become sufficient for the NCS to perform this study. In addition, the horizontal precision of these modern data is adequate for the detailed and quantitative analyses of bed changes and bedform mobility.

The aim of this paper is to present the vertical sea-bed dynamics of the Netherlands Continental Shelf and the Wadden Sea, based on a quantitative analysis on a 25 x 25 m resolution. Detailed analyses of selected locations serve to describe the local morphology and dynamics of individual bedforms.

DATA AND METHODS

Bathymetric data

All data that are contained in the digital Bathymetric Archive System (BAS) of the Hydrographic Service of the Royal

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224 Net wer Rijk com bea the Inte T ext thre than not han seri ech dat and use D bath Inv m. Fig grid only a ti 4 therlands Navy re used in this kswaterstaat fo mprise both sin am echo sound

Order 1 st ernational Hydr The overlap of ents and differ ee datasets; at n 5 datasets ex t exist, historica nd-written fair ies. The data d ho sounding be asets used in th d June 2010. Th ed in this study Digital Elevatio hymetric data b verse Distance W This 25 m reso gure 1. Number d node (UTM3 ly one dataset w ime series of 2 d y below the 10 study. This da r the shallower ngle-beam ech dings (MBES), tandards for rographic Organ f surveys creat rent periods. M few locations ist (Figure 1). I al echo soundin sheets in ord density and prec

ams) differ per his study were he fair sheets da is at most 1 obs on Models (D by interpolating Weighting algo olution was cho

r of digital data 31 WGS84 co-o was available, fa datasets. m isobath [Ri atabase include r coastal zone. D ho soundings (

that were coll hydrographic nization [IHO, es time series Most time serie

on the NCS, ti In locations wh ngs were digitiz er to create a cision (horizon r method and in acquired betw ate from before servation per 3 DEMs) were c g to a 25 x 25 m orithm with a se osen to still rep

asets included in ordinates). In fair sheets were

igholt et al., 20 s data acquired Digital data in B SBES) and mu ected according surveys of 2008]. of various spa s comprise two ime series of m here time series zed from plotte bathymetric t tal positioning n time. The dig ween the late 19 e that. Data den

x 5 m. reated from th m grid, using the

earch radius of present sand wa n a time series most areas wh digitized to cre 010] d by BAS ulti-g to the atial o or more s did ed or time and gital 980s nsity hese e 100 aves (hundre introdu soundin this res the firs

Verti

Beca differen insuffic meters (m/yr) regress (Figure in time covered per sur and ne Becaus differen vertica subtrac stacked node (F

Geom

For individ is sepa truncat details, per here eate Figu serie and b and o time green Jubilee eds of meters uction of inte ngs in track lin solution, but ar st place.

cal nodal dy

ause we deal w nt numbers o cient to use a . For the quan for each grid n sion of all bed e e 2). Based on v e, linear regress

d by the time se rvey, because th ever were a pro se the correctio nces between l dynamics, cting the aver d combination o Figure 3).

metry and m

the detailed dual sand waves arated into bed ting a Fourier , see Van Dijk

re 2. Illustratio es, displaying tw

blue) distribute one set of mult series for nod n, blue).

Conference Pr s in length) rpolation artef nes. Megaripple re often not cap

ynamic anal

with numerous of surveys an verage values ntitative analysi node, we devel elevations in the visual inspectio sion is justified eries. We used he periods of a oblem for the on for tides an surveys that a we corrected aged vertical of surveys from

mobility of in

morphological s and long bed dform types o approximation et al., 2008]. on of different b wo sets of sing ed in track line tibeam echo so de (0,0) compr roceedings, NC well and to facts of singl les cannot be c ptured by singl

lysis

overlaps of v nd different of bed elevat is of vertical d loped a fully au e stacked time on of the nodal d within in the p an averaged da acquisition are precision of th nd ship movem are larger than d for this di dynamics for m the vertical dy

ndividual be

l and dynami waves, the bath of different spa n at certain fr

This way, the

bathymetric dat gle-bean echo s es that do not e oundings (green rises three dat

K-Days 2012 minimize the le beam echo characterized at e beam data in various extents, periods, it is tion change in dynamic trends utomated linear series per node bed elevations periods that are ate of collection relatively short he calculations. ments provided n the (natural) iscrepancy by each specific ynamics at each

dforms

c analyses of hymetric signal atial scales by requencies [for more dynamic tasets in a time soundings (red exactly overlap n). The stacked ta points (red, e o t n , s n s r e s e n t . d ) y c h f l y r c e d d

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Ju me the smo poi the

Ve

T Net reg dyn ord disp extr or a I con may inc in Dyn abs S san Her extr app ind mig rela lon san lon san Fig dyn line com mo nat ubilee Conferen garipples (cont sand wave or l oothed bedform nts are then de geometry and d

ertical dynam

The quantified therlands Conti gions of contras namic trends ar der to amplify t play the full remes may rang aggradation. In general, th ntinental shelf. y be distingui ludes estuaries, the Wadden s namic trends i solute values of Second, areas o ndy parts of th

re, values typ remes of an ab pearance of be dicates that the m

gration and/or atively high dyn ng bed wave fie nd wave field in ng bed waves. nd waves seem gure 3. Applied namics of the N e) are corrected mbination of su rphodynamics tural seabed dyn

ce Proceedings tained in multi long bed wave m signal, locatio etermined in a s dynamics of ind

RESU

mics trends

d vertical no inental Shelf (F sting dynamics re simplified in the dynamic co range of dyn ge between few he coastal zon Three zones o ished. First, th , tidal inlets wi sea and near-sh

in these enviro f 0.1 and 0.35 m of moderate dy he NCS, where pically range b bsolute 0.3 m/ edform patterns measure of vert growth of indi namics are the eld north of Te n the Southern

The tidal ridge m to be abse d correction in NCS. (a) The c d by subtractin urvey overlap in which the namics are reve

s, NCK-Days 20 ibeam data), w dynamics, are r ons of crest, tro semi-automated dividual bedfor

ULTS

odal dynamic Figure 4) provi on the NCS. In nto classes of ontrasts. Thus, namic trends, w w decimeters pe ne is more d of contrasting ( he highly dyna ith ebb-tidal de hore Zeeland, onments typica m/yr with extrem ynamics occur o e rhythmic bed

between -0.1 /yr in the sand s in the vertic tical dynamics i ividual bedform

sand wave fiel exel and Vliela n Bight includin es 75 km offsh ent, also disp

n calculating t calculated dyna ng the average (red dashed li patchwork is ealed. 012

which may obsc removed. From ough and inflec d way, from wh rms is calculate

trends for ide an overview n this map, vert

absolute value this map does which apart f er year degrada

dynamic than (natural) dynam amic coastal z eltas, tidal chan and breaker b ally range betw mes up to 1.5 m offshore on mo forms are pres to 0.1 m/yr w d wave fields. cal dynamics m is controlled by ms. Clear field ld west of Texe and, and the en ng tidal ridges hore Texel, wh lay large vert

the vertical no amics (black so dynamics for e nes) (b) Resul removed and cure m the ction hich ed. the w of tical es in not from ation the mics zone nnels bars. ween m/yr. ostly sent. with The map y the ds of el, a ntire and here tical dynam resolut Thir m/yr; l bedform in thes deeper north o that th been su of low field w patches the Vla Othe anthrop artefac promin Water Wadde the con

Bedfo

Figure Contin signifi fields classe betwe odal olid each lting the mics, although th tion data of digi

d, offshore are light blue) occu ms are absent. A se zones, the l parts of the N of de Wadden he shore-face co usceptible to m seabed dynam west of Texel s along the coa akte van de Raa er small parts pogenic areas,

ts caused by th nent in the ship

Route East) an en islands. The ntrasts of dynam

orm size and

e 4. Vertical nental Shelf (ab ficantly more d

offshore are th es is to amplify en -0.35 and 0.

hese results ma itized fair sheet

as of very low ur mostly in part Although the dy argest areas of NCS, farther off islands is unex onnected ridge migration or gro mics appear in a

and the offsho ast offshore De an (ebb-tidal del

of high vertica such as sand he data in the tim

pping lane in t nd in the zones effect of data p mics for the sep

d mobility

nodal dynamic bsolute values) dynamic than t e most dynamic the morphodyn 35 m/yr. V ay in part be d ts. w sea-bed dynam rts of the NCS w ynamics map is f low dynamic fshore. The low

xpectedly stab es that occur he

owth/decay. In a zone between ore sand banks en Haag, offsho elta of the Weste

al dynamics in extraction site ime series. The the centre of t

of low dynami precision is also parate survey ov

c trends of th show that the the shelf, and c zones on the s namic contrasts

Van Dijk et al.

225 due to the

low-mics (around 0 where rhythmic s scarcely filled cs occur in the w-dynamic area le in the sense ere could have addition, areas the sand wave s, and in small ore Voorne and ern Scheldt). n Figure 4 are es, and due to latter are most the map (Deep ics north of the o recognized in verlaps.

he Netherlands coastal zone is that bed form shelf. Choice of s; values range -0 c d e a e e s e l d e o t p e n s s m f e

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226 D info bed typ F wit of m/y to o mo Van S Tex iden nev ave m. 200 12. T isla (Fig the rate on the A coa per num estu offs (dif nod dyn goo des Wa con Wa Fig and 6 Detailed analy ormation on th dforms. Three es of bedforms Firstly, the sand th an average w 1.4 m. With an yr to the northe other sand wav stly less than 5

n Dijk et al., 20

Secondly, five xel and Vlielan

ntified by Kna ver reported. T erage waveleng Net migration 09 (Figure 5), r 4 m/yr to the no Thirdly, the s ands were ana gure 6). The av average heigh es and vertical the NCS is lim average migrat A large-scaled astal zone of rformed by Win mber of annua uaries) and a s fshore. Although fference betwee de (in meters) namic trends (i od agreement scribes highly d adden islands nnects very we adden presented gure 5. Profiles d Vlieland from yses of indiv he geometry, m

areas that are are here descri d wave field we wavelength of 3 n average sand east, this area is ve fields on the 5 m/yr [Van Dij 011].

long bed wave nd were analy

aapen et al. [2

The long bed gth of 1125 m a n rates, based o range from 10.5 ortheast. shoreface-conne lyzed based o verage wavelen ht is 4.3 m. Pres morphodynam mited. Changes tion rate is 1.0 m

DISCU

study of vertic the German nter [2011]. His al datasets nea small number h his results are en the maximu and are thus in m/yr), the o with our find dynamic estuar and areas of ell to the verti d in this paper. s (SE to NW) o m the 1990, 2003 vidual bedform mobility and m characterized ibed.

est of Texel com 45 m and an av wave migratio exceptionally d e NCS, where m jk et al., 2008; es in the dyna zed. Long bed 2001], but mig waves north and an average on 3 surveys b 5 to 18.4 m/yr, ected ridges n on profiles of ngth of the ridg sent-day knowl ics of shorefac in dimensions m/yr to the sou

USSION

cal bed level c Continental Sh s calculations ar ar the coast (u of datasets (le e presented as b um and minimu not directly c overall pattern dings of the N

ries and tidal low dynamic cal dynamics m of long bed wa 3 and 2009 data ms provide m morphodynamics by three diffe mprises sand wa verage wave he on rate of 16 to dynamic compa migration rates Dorst et al., 20

amic field north d waves were gration rates w of Texel have wave height of between 1990 with an averag north of Wad two datasets o ges is 4614 m ledge on migra e-connected rid are negligible thwest. changes of a w helf was rece re based on a la up to 30 for ess than 5) far bed elevation ra um bed levels) comparable to of dynamics i NCS. Winter inlets between s offshore, wh map of the Du aves north of T asets. more s of erent aves eight o 19 ared s are 011; h of first were e an f 3.4 and ge of dden only and ation dges and wide ently arge two rther ange per our is in also the hich utch A re time se [2010] (cm), t levels directly highly-corrob For connec Germa more d 100-20 find, a North-1996, banks ridges shorter As th dynam - grain prox over corre - curre sand and rate mod Mole [199 The such as the old also af data re when c Texel Fig ridg bedf Jubilee egional bed dy eries from 1926 . They express thus the differe

in the time s y comparable t -dynamic tidal orates our findi the shoreface-c cted ridges at an Wadden isla dynamic [Antia 00 m/yr is a fac and 4 to 40 tim America, whic and references at Spiekeroog at Ameland an r and lower. he controlling p ics, we hypothe n size affects th xy for spatial va rlay of median g espond very we ent velocity se d waves [for wa sediment transp of sand wave deled sediment en and De Swa 99] to our sand w vertical dynam s data density a der datasets, m ffect the locatio solution of one comparing to an

gure 6. Profiles ges north of dforms have nei

Conference Pr namics study f 6 to 2006, was c sed dynamics in ence between t series. Althoug o our dynamic channels corr ings. connected ridge the north coas ands, also migr

a, 1996]. The

ctor 100 – 200 mes higher than h range from n therein]. Alth seems similar nd Schiermonn parameters of th esise that: he presence of s ariation in bedf grain sizes and ell;

ems the domin ave length, see port potential s s and long be transport rates

rt, 2001] and a

wave migration

mic trends are se nd precision. L may underestim on of crests and dataset results nother dataset. s (SSW to NNE the Wadden ther grown nor

roceedings, NC for the Wadde carried out by V n terms of the the latest and gh these value trends in m/yr responds well es, in comparis st of Spiekeroo rate landwards, maximum mig higher than th rates reported nearly stable to hough the envir

r to the shore nikoog, the for

he spatial varia

sand waves and form migration d bed morpholog

nant factor for also Van Sante seems related to ed waves, whe s of the North an overview stu n rates. ensitive to data Low-resolution d mate the bedfor

d troughs. In g s in a higher ver E) of the shoref islands indica r migrated in al K-Days 2012 en Sea, using a Vonhögen et al. net deposition the lowest bed s are also not r, the pattern of and therewith

son, shoreface-og, one of the , but are much gration rate of e migration we from ridges in 6 m/yr [Antia, ronment of the eface-connected rmer banks are

ation in sea-bed d also may be a rates, since an gy & dynamics dimensions of en et al., 2011] o the migration n we compare Sea [Van der udy of Bearman a quality issues, data, especially rm heights and eneral, a lower rtical dynamics eface-connected ate that these lmost a decade. a . n d t f h -e h f e n , e d e d a n s f ] n e r n , y d r s d e

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Van Dijk et al.

Jubilee Conference Proceedings, NCK-Days 2012 227

APPLICATIONS

Without knowledge of the sea-bed dynamics, re-survey policies of continental shelves and rivers cannot be validated. The above analysis was used to validate and to optimize the survey-policy of the Netherlands Hydrographic Office. Hereto, we combined the morphodynamics and the predicted grounding dangers for shipping, based on shipping intensity, predicted water depth and the probability of unknown objects at the bed. An ongoing study on the river- and sea-bed dynamics for Rijkswaterstaat includes the investigation of cause and effects, including the interaction between dredging activities and sand waves.

Other applications are advice on sea-bed stability for regional planning and offshore engineering projects, such as the allocation, construction and maintenance of offshore wind farms. Bedform mobility may affect the depth of wind piles into the sea bed by maximally the bedform height, which may be up to several meters. Finally, knowledge on the morphodynamics of tidal channels and sand banks is crucial in refining the 3D geological model of the subsurface, the evolution of the marine palaeolandscape and the prediction of archaeological remnants, such as shipwrecks and subrecent archaeology (e.g. WOII-remnants). Sea-bed dynamics also play a role in the burial and exposure of shipwrecks, so establishing the affected depths and time-scales of these dynamics provide information on the preservation potential of these archaeological items.

CONCLUSIONS

Vertical nodal dynamic trends of the Netherlands Continental Shelf typically vary between -0.35 and 0.35 m/yr. The coastal zone is significantly more dynamic than the offshore zone. Especially estuaries, tidal inlets, tidal channels and breaker bars are dynamic (abs. values of 0.1 – 0.35 m/yr, with extremes up to 1.5 m/yr). Offshore the most dynamic parts are the fields of mobile marine bedforms, such as sand waves and long bed waves, where vertical bed levels are caused by bedform migration and/or growth (levels between -0.1 and 0.1 m/s with extremes up to an abs. 0.35 m/yr). Deeper parts and north of the Wadden islands, vertical dynamics are very low (~0 m/yr).

Vertical trends in sea-bed dynamics are crucial in the re-survey policies of hydrographic surveyors. By combining the (predicted) sea-bed dynamics with predicted grounding dangers for sea traffic, monitoring policies can be optimized so that measuring efforts are low while safety is still the highest. To offshore engineers, managers and marine archaeologists, morphodynamics support the decision-making on regional planning, marine protected areas and archaeological values and the potential risks to offshore structures and maintenance.

ACKNOWLEDGEMENT

All data were provided by the Hydrographic Office, Royal Netherlands Navy, and Rijkswaterstaat, Ministry of Infra Structure and the Environment. Most of this work was carried out within a research project financed by the Ministry of Defense. Within this project, the Maritime Research Institute of the Netherlands (MARIN) quantified the grounding dangers for shipping.

REFERENCES

Antia, E.E. (1996), Rates and patterns of migration of shoreface-connected ridges along the sourthern North Sea coast, Journal of Coastal Research, 12, 38-46.

Bearman, G. (Ed.) (1999), Waves, tides and shallow-water processes, second ed., 227 pp., Butterworth-Heinemann, Oxford.

Buijsman, M.C. and Ridderinkhof, H. (2008), Long-term evolution of sand waves in the Marsdiep inlet, II: relation to hydrodynamics, Continental Shelf Research, 28, 1202-1215.

Dorst, L.L., Roos, P.C. and Hulscher, S.J.M.H. (2011), Spatial differences in sand wave dynamics between the Amsterdam and the Rotterdam region in the Southern North Sea, Continental Shelf Research, 31, 1096-1105.

Dorst, L.L., Roos, P.C., Hulscher, S.J.M.H. and Lindenbergh, R.C. (2009), The estimation of sea floor dynamics from bathymetric surveys of a sand wave area, Journal of Applied Geodesy, 3, 97-120.

Duffy, G.P. and Hughes-Clarke, J.E. (2005), Application of spatial cross correlation to detection of migration of submarine sand dunes, Journal of Geophysical Research, 110.

IHO (2008), IHO Standards for Hydrographic Surveys, edited, International Hydrographic Bureau, Monaco.

Knaapen, M.A.F. (2005), Sandwave migration predictor based on shape information Journal of Geophysical Research, 110, 9. Knaapen, M.A.F., Hulscher, S.J.M.H., de Vriend, H.J. and Stolk, A.

(2001), A new type of sea bed waves, Geophysical Research Letters, 28, 1323-1326.

Righolt, R.H., Schaap, J., Dorst, L.L. and Vos, E.M. (2010), Needle in a haystack, in Management of massive point cloud data: wet and dry, edited by P.J.M. Van Oosterom, M.G. Vosselman, T.A.G.P. Van Dijk and M. Uitentuis, p. 104, Nederlandse Commissie voor Geodesie, Delft.

Van der Molen, J. and De Swart, H.E. (2001), Holocene wave conditions and wave-induced sand transport in the southern North Sea, Continental Shelf Research, 21, 1723-1749.

Van Dijk, T.A.G.P. and Kleinhans, M.G. (2005), Processes controlling the dynamics of compound sand waves in the North Sea, Netherlands, Journal of Geophysical Research, 110.

Van Dijk, T.A.G.P., Lindenbergh, R.C. and Egberts, P.J.P. (2008), Separating bathymetric data representing multi-scale rhythmic bedforms: a geostatistical and spectral method compared, Journal of Geophysical Research, 113.

Van Dijk, T.A.G.P., Van der Tak, C., De Boer, W.P., Kleuskens, M.H.P., Doornenbal, P.J., Noorlandt, R.P. and Marges, V.C. (2011), The scientific validation of the hydrographic survey policy of the Netherlands Hydrographic Office, Royal Netherlands Navy, 165 pp, Deltares.

Van Santen, R.B., De Swart, H.E. and Van Dijk, T.A.G.P. (2011), Sensitivity of tidal sand wavelength to environmental parameters: A combined data analysis and modelling approach, Continental Shelf Research, 31, 966-978.

Vonhögen, L.M., Van Heteren, S., Marges, V.C., Wiersma, A.P. and De Kleine, M.P.E. (2010), Comparison between the internal dynamics of the Dutch Wadden Sea and the volumes involved by subsidence areas, paper presented at NCK days 2010, West-Kapelle, Netherlands.

Winter, C. (2011), Macro scale morphodynamics of the German North Sea coast, Journal of Coastal Research, SI 64, 706 - 710.

Winter, C. and Ernstsen, V.B. (2007), Spectral analysis of compound dunes, paper presented at 5th IAHR Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2007), Taylor & Francis, University of Twente, Enschede, The Netherlands

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