Towards a Mud Balance for the Trilateral Wadden Sea Area: Mud supply, transport and deposition
Report Towards a Mud Balance for the Trilateral Wadden Sea Area: Mud supply, transport and deposition
Report Colophon
Authors Albert Oost, Ana Colina Alonso, Peter Esselink, Zheng Bing Wang, Thijs van Kessel and Bas van Maren
With contributions from Theo Gerkema, Eelke Former, Julia Vroom, Andreas Wurpts, Yoeri Dijkstra, Matias Duran-Matute, Frank Kösters, Maarten Kleinhans, Bas Borsje, Hans Burchard, Emil Stanev, Peter Milbradt, Piet Hoekstra, Hein Sas and Michiel Firet
Graphic design cover BW H ontwerpers
Photography cover Luchtinspectie Rijkswaterstaat (Ems estuary seen from above the Dollard)
ISBN 978-94-90289-57-7
Report 2021-02
This research has been scientifically supervised by the Wadden Academy and financed by the Programme Towards a Rich Wadden Sea and the Wadden Academy
Published by Wadden Academy © Wadden Academy march 2021
Contact person Klaas Deen Executive Secretary T +31 (0)58 233 90 31 E [email protected] www.waddenacademie.nl
The Wadden Academy is funded by the Wadden Fund. FOREWORD
Muddy sediments are a vital element for the Trilateral Wadden Sea ecosystem and mud, both in the form of suspended particles in the water column as well as substrate on shoals, tidal flats and salt marshes, plays an important role in providing and developing ecosystem-services. For many centuries the import and deposition of mud has significantly contributed to the silting up and evolution of tidal basins in the Wadden Sea in response to tidal processes and long-term sea level rise. Mud accumulates in the high intertidal and supra-tidal zone and contributes to the growth of tidal flats and salt marshes. Likewise, mud dynamics and deposition plays an important role in the development of major estuaries in the Wadden Sea region. Mud has various impacts on the food web of the Wadden Sea. High concentration of fine-grained suspended matter, for example, may become a limiting factor for primary production of phytoplankton (pelagic microalgae) by increasing turbidity levels and reducing the penetration of light in the water column. High densities of microphytobenthos (benthic microalgae) are, however, generally found on the more muddy parts of the tidal flats, amongst others due to higher concentration of nutrients in the pore waters of finer sediments. Fine suspended sediment in the water column may hamper the uptake of food for suspension-feeders such as mussels and cockles, whilst deposit-feeding benthos such as mudsnails prefer muddy substrates.
For already a long time, mud dynamics in the Wadden Sea is heavily affected by human activities and interventions such as land reclamation, channel deepening for navigation, maintenance dredging and more recent attempts to locally extract or demobilize mud in tidal basins and estuaries to reduce turbidity. Surprisingly though a more or less comprehensive overview of mud dynamics in the Trilateral Wadden Sea is lacking. This omission has inspired both the Wadden Academy and “Programma naar een Rijke Wadden Zee” to invite Deltares to launch a study to identify the major sources, sediment pathways and sinks of mud in the Trilateral Wadden Sea. This report provides the first mud balance for the Trilateral Wadden Sea, making use of a range of data sources such as existing literature, bathymetric charts, sediment distribution maps and measurd deposition rates.
We are confident that the report is highly informative and may act as a kind of benchmark study for future work. We wish you pleasant reading!
Prof. dr. Piet Hoekstra Hendrikus Venema, MSc Portfolio Geosciences and Climate Program manager Programma naar Wadden Academy een Rijke Waddenzee (PRW) Towards a Mud Balance for the Trilateral Wadden Sea Area
Summary
The Trilateral Wadden Sea is an extensive barrier-lagoon system composed of three main estuaries and a series of tidal basins, covering Dutch, German, and Danish territory. Although being an important nature reserve, the area also provides important economic services (through fisheries, tourism, and shipping). Because of these economic services, resulting in deepening, land reclamations and flood defences, the area is strongly influenced by human interventions – especially in the three main estuaries (the Elbe, Weser and Ems). The sedimentary processes in tidal basins are dominated by sandy and muddy sediments, with the largest mud content in the shallow tidal flats on the landward side and on the tidal divides. Most of the mud supplied to the Wadden Sea originates from the Straits of Dover, via the so- called North Sea Continental Flow. Additional but smaller contributions are from the IJsselmeer, the Ems, Weser and the Elbe Rivers, local sources and aeolian deposition.
This report provides the first mud balance for the Trilateral Wadden Sea, providing detailed estimates for mud sinks, sources, and transport using a combination of existing literature, bathymetric charts, sediment distribution maps, observed deposition rates, and dredging information. The total mud supply to the Trilateral Wadden Sea is estimated at 12.1 to 16.5 million ton/yr. Mud is mainly deposited on the upper tidal flats connected to the mainland of the Trilateral Wadden Sea, in the tidal marshes, and in the sheltered embayments, providing a sink of sediments. An additional sediment sink is sediment extraction, whereby sediment is dredged and placed on land. The total amount of mud deposition and extraction is estimated at 10.8 to 11.3 million ton/yr. This implies that currently, the mud sources are larger than the mud sinks, but not much larger.
Sand-mud mixtures are either sand-dominated or mud-dominated, resulting in spatial segregation of sand and mud. Especially the inland embayments such as the Dollard, the Leybucht and the Jade Bay are characterised by high mud contents. This segregation is an important characteristic of the system, especially when evaluating the response of the system to sea-level rise and local anthropogenic disturbances. At present, the (mud-dominated) upper tidal flats and salt marshes are accreting at a faster rate than sea-level rise, while some subtidal sections are eroding, especially in the German Wadden Sea. It remains unclear to what extent this redistribution is the result of the observed increase in tidal amplitude or mean sea-level, and therefore related to local human interventions or global sea-level rise.
The most important human interventions influencing mud dynamics are land reclamations and channel deepening. Historic land reclamations resulted in a major loss of sediment sinks where fine sediments naturally accumulate. This loss in sediment accommodation space resulted in more sediment remaining in suspension and therefore higher sediment concentration and more eastward transport of sediment. Both land reclamations and channel deepening (for navigation purposes) lead to amplification and asymmetry of the tides, promoting landward transport of sediments in the estuaries.
iii Towards a Mud Balance for the Trilateral Wadden Sea Area
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table of contents
1 INTRODUCTION 1
1.1 Background 1
1.2 Research questions 1
1.3 Study area 3
1.4 Report outline 4 2 A GENERAL DESCRIPTION OF THE STUDY AREAS 5
2.1 The Dutch Wadden Sea area 5
2.2 The Ems estuary 11
2.3 The Lower Saxony Wadden Sea 13
2.4 The Weser estuary 17
2.5 The Elbe estuary 20
2.6 The Schleswig Holstein Wadden Sea area 22
2.7 The Danish Wadden Sea 30
3 A BRIEF OVERVIEW OF MAIN PROCESSES 35
3.1 Determining factors and timescales 35
3.2 Gross and net fluxes 36
3.3 Residence time of mud 37
4 SUSPENDED SEDIMENT CONCENTRATION 39
4.1 Spatial variations 39
4.2 Temporal variations 43 5 MUD AVAILABILITY IN THE SEDIMENT BED 49
5.1 Dutch Wadden Sea 49
5.2 Ems-Dollard estuary 50
5.3 Lower Saxony Wadden Sea 50
5.4 Weser Estuary 52
5.5 Elbe Estuary 52
5.6 Schleswig Holstein Wadden Sea 53
5.7 Danish Wadden Sea 53
5.8 Sand-mud segregation 54
5.9 Sand and mud in the Wadden Sea: two different worlds? 54 6 TOWARDS A MUD BUDGET 57
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.1 Mud sedimentation in the basins 57
6.2 Mud sedimentation in saltmarshes 65
6.3 Dredging activities 71
6.4 Internal reworking 75
6.5 The North Sea 77
6.6 Synthesis: a mud balance for the Wadden Sea and North Sea 88
6.7 Discussion: uncertainties 90 7 HUMAN INTERVENTIONS 93
7.1 Introduction 93
7.2 Poldering and closure works 93
7.3 Deepening and port construction 94
7.4 Landfills and sediment extraction 98
7.5 Managed realignment and depoldering 101
7.6 Foreland marsh development 103 8 SEA-LEVEL RISE 105
8.1 Past mud sedimentation 105
8.2 Present-day mud sedimentation: effect of tidal amplitude and storms 107
8.3 Future mud sedimentation 109 9 MODELLING PRACTICES: FOCUSING ON MUD 113
9.1 Process-based models 113
9.2 Aggregated models 116
9.3 Idealised models 118 10 DISCUSSION AND CONCLUSIONS 119
10.1 Discussion of the research questions 119
10.2 Testing the hypothesis 125
11 REFERENCES 129
12 WADDEN SEA RESEARCHERS AND PRACTITIONERS 153
1 INTRODUCTION
1.1 BACKGROUND
the basin’s ability to
‘ ’
1.2 RESEARCH QUESTIONS
Towards a Mud Balance for the Trilateral Wadden Sea Area
o i e an o e ie o the b et in the ilate al a en ea base on e istin lite at e an ata o i e a b ie o e ie o the e istin no le e on the yna i s in the ilate al a en ea o i e an o e ie o the a eas in the ilate al a en ea he e the ent on ent ation le els ne ati ely i a t e olo y an e osition ates th eaten sa e na i ability es ltin in hi h aintenan e e in al ate ent an t e ana e ent st ate ies an inte entions to ta le elate oble s s h as in ease t bi ity al es o e osition in ha bo s an na i ation hannels enti y the ain no le e a s
o this en e ha e o late the ollo in esea h estions hat a e at esent the ain so es o the in t o in the ilate al a en ea an ho h is ann ally i o te oss an net an s bse ently e osite hat a e the ain hy o yna i o esses an se i ent t ans o t e hanis s ele ant o the i o t t ans o t an e osition o in the a en ea an o i o tant is the e han e o bet een in i i al ti al basins hat is the nat al an e o on ent ations as a n tion o lo al en i on ental on itions an ho ha e these on ent ations han e in es onse to h an inte entions o ha e these on ent ations been in l en e e to la e s ale inte entions hi h ha e been st t al o e ti e lan e la ation los e o s s bstantial lan ill ith e e ate ials e the e alte nati es to eal ith elate oble s hat is the s atial ist ib tion o in s a e e osits on shoals ti al lats an salt a shes hat in o ana e ent st ate ies inte entions an tools ha e been se so a to eal ith elate oble s in the a en ea o e e thei e e ts eas e in te s o hysi al an e olo i al han es o ost e i ient e e the eas es e the e othe o tions not yet e lo e to eal ith elate oble s an i so hat o l be the e i ien y o these eas es in e in the oble an ith es e t to the osts hat has been the i a t o lon te sea le el ise on the s ly an e osition o an ho ill li ate han e an a ele ate sea le el ise alte these o esses ho is esently o in on hi h elate esea h estion in the t ilate al a en ea an hat is the ty e o o e te o t o e o this esea h e ose e e t iel st ies o el scenario’s)? nalyti al an n e i al o els lay an i o tant ole in n e stan in hin astin an e i tin beha io in the a en ea o eliable an a ate a e these o els hat in o i o e ents a e ne essa y in te s o o ess no le e an i le entation o el alib ation an ali ation
Towards a Mud Balance for the Trilateral Wadden Sea Area
an o e researc es ions a o e a e no e een a resse on e sca e o e a en ea an a ai a e a a an i era re is ins icien o ro i e conc si e ans ers is re or ere ore ri ari ser es as a s ar in oin on ar e sca e na ics an ana e en ra er an ro i in ina ans ers ore s eci ica e ai o es e o o in o eses
i an an e osi s are s a ia se re a e ic in ences no on eir is ri ion a so eir res onse o an in er en ions an sea e e rise ii e con ri ion o o o a se i en a ion ra es in e o e ri a era a en ea is s s an ia iii e so rces an e sin s o are c ose a ance i ea e e rise a ea o a s or a e o n ro o enic se i en e rac ion a ea o a s or a e o
1.3 STUDY AREA e area ic is consi ere is e anis er an an c a en ea area inc in e ree i es aries s eser an e s e a ai a e a a are no er e ai e e eci e o s e e e o en s in e o o in ar er re ions c a en ea area ro e ars ie n e o e a ers e n er c ier onni oo s es ar e c area eas o e a ers e o c ier onni oo e es ar ro er o e eir a er r ) e o ar area an e er an a en area o e a ers e o or so e c in e as ern s c anne c anne ic as eco e ore ica or e o er a onian a en ea since in i in o e connec ion i e es ern s c anne ) e o er a onian a en ea area ro e a ers e o or o eas o e ins eneroo arrier is an an e a e e a en eser e es ar o e eir an e area eas o i o e e e e es ar o e eir an e area nor o i o e a ers e o risc en e a en ea area o c es i o s ein ro e a ers e o risc en o e a o e anis a en ea area ro e a o o e nor ern os ar o e a en area
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 1.1: Satellite image of the Wadden Sea during low tide showing the numbered sub-areas. (albedo39 Satellitenbildwerkstatt e.K., image processing; Brockmann Consult GmbH, scientific consulting; raw data: U.S. Geological Survey).
1.4 REPORT OUTLINE e re or se is as o o s irs e ro i e a enera escri ion o e s areas in ic e isc ss e ain c arac eris ics inc in ain ro na ic ri ers e eo o an e e se i en s a er ) e e isc ss e ain ro na ic rocesses an se i en rans or ec anis s re e an or e i or rans or an e osi ion o in e a en ea a er ) n a er e ro i e an o er ie o e na ra ran e o s s en e concen ra ions a ai a i i in e a er co n) an in a er e e a ora e on e a ai a i i in e se i en e irs ersion o a e or e ri a era a en ea is resen e in a er is e is ase on i era re an a a on e so rces i or o re in ac i i ies an re ainin se i en a ion in e asins se en e e a ora e on e i ac o an in er en ions a er ) an o en ia e ec s o sea e e rise on is e a er ) n a er e e a ora e on n erica ) o e in rac ices oc sin on as a isc ssion on e res s an conc sions are resen e in a er
Towards a Mud Balance for the Trilateral Wadden Sea Area
2 A GENERAL DESCRIPTION OF THE STUDY AREAS
n or er o s e a ance an e nc ionin o in e ri a era a en ea o en ar er an an e e er an s i as eci e o s i e a en area in o se en ar er re ions ein ro es o eas ) e c a en ea e s es ar e o er a on a en ea e eser es ar e e es ar e c es i o s ein a en ea an e anis a en ea e resen a o o ra an re ai in ro na ic con i ions i o a ar e e en e er ine ere is e osi e see c a er ) e eo o ica i an re ) is oric e e o en e er ine a ai a i i in e s s r ace ic is e ec e or an in erna so rce o see c a er ) or is reason o are isc sse re a i e e ensi e ase on a ai a e i era re or eac area
2.1 THE DUTCH WADDEN SEA AREA
2.1.1 Characteristics e es o eas ren in a en area o e e er an s as a o a s r ace area o i in e ir or a i a ra e or irec i es e ac arrier a en ea i se as a o a area o so e an consis s o i a sa ars es o in er i a an s ra i a ) in er an s i a a s an i a c anne s i re )
e area consis s o in e s s e s na e ro es o eas ars ie i er an se a ee a an e ie orn ie in e a o a er aa an i an er a i re ) e era res a er s rea s are en erin e ac arrier e a or ones are e ones ia sse eer an a ers eer an e s e in e s a e is an s a ei er si e an c ear e e o e e i a e as
ser a ions o er e erio s o a increase in e c a en ea is r i in e a ) er eersen e a ) s o e a e ar e in erann a aria i i o ean sea e e s in ers e e ec ion o a resen a acce era ion in sea e e rise a oca sca es eir res s on sea e e rise ra es a ree i re io s in in s i in e a ) er e a an ran a e ) s o e a e in erann a aria i i o ean sea e e o c ) can e ar e e aine e es –eas co onen o e ne in ener ec or is in erann a aria i i is increasin i ear on sea e e recor s re ea in a is increasin consis en as er in in er an s er i e in e s a ion o ar in en risin o i es as er in in er an in s er) – see er e a an ran a e )
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.1 The inlet systems of the Wadden area of the Netherlands (Courtesy Wadden Sea Secretariat).
(Smits et al., 2005), but trends in storms of ≥10 Bft could not be
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.2 Littoral sand drift along the Dutch Wadden coast, computed using wave data from stations IJmuiden (IJM, red arrows), Eierland (EIE, yellow arrows), K13 (magenta arrows), and Schiermonnikoog (SCH, green arrows), in 106 m3/yr (From: Ridderinkhof, 2016).
(until the 80’s)
2.1.2 Geology
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.3 Differences in Holocene relative mean sea-level rise for the Dutch and NW German North Sea coast, making clear the differences on a regional scale. Sea-level rise in NW Germany has been larger than in the Western Netherlands. The differences are mainly caused by relative glacio-isostatic subsidence (Mörner, 1979). Figure modified from Kiden et al. (2008), the sea-level curves are from Van de Plassche, 1982 (W Netherlands); Denys and Baeteman, 1995 (Belgium); Kiden, 1995 and 2006 (SW Netherlands) and Vink et al., 2007 (NW Germany). The sea- level curves depicted here correspond to the mid-lines of the sea-level error bands presented in these studies (From Oost et al., 2012).
e the e s e e ti ll ille ith e t n ul e in ise el ti el e sil n e n e te int ie e ents (e ui e ee n u e s ee) the e enl in the ti l lu e the estu ies ie sins hi h the e e t n the este n en e e the ee est i e lle s e e l e un 8000 ( eets n n e e 000) he esent ti l inlet siti n is in e ses still ete ine the e lle s ( ie s et l 00 ), but most inlets seem to have “drifted” from their i in l l ti ns s e i ll in the este n t the ut h en e ele te leist ene ut s n he l n s nsistin ine e sits the le (se n l st) l i ti n n s n elt te e sits the ei hseli n (l st) l i ti n e e esent he isl n s e el n ie in en e un su h leist ene e sits
he ul the est isi n ie isl n h in e et een 000 000 niti ll se i ent ti n ul n t ill the s e e te the i l isin se ( entu ) n inl su ti l e e in e n ne inte ti l s n n u l ts n s lt shes ne the inl n t the inl n ens e t ise s hi h st te t e n n the inl n est isi et een 000 000 ( s ie n t ee e s et l 0 )
t ut 000 se i ent u ul ti n ul e ee the e ele tin te se le el ise in the est isi n en e e ( i u e ) n inte ti l s n l ts e n e ( n ete en n n e e 00 s et l 0 ) n the ll in illenni tt su si en e in the e ste n t the est isi n en e s el ti el l e n se i ent ti n s insu i ient t ill the sins e e he e es ite e ele tin s lute
8 Towards a Mud Balance for the Trilateral Wadden Sea Area sea level rise os et al , ) n some la es the tidal area as e tendin , e in the oorne, un e and ivel areas os et al , ) n other la es the adden ea lo all silted u and the salt marshes ould advan e sea ard os and an esteren, os et al , )
t about , a , the est risian barrier island hain as still situated several ilometres offshore from its resent osition, meland, m and ers hellin about m further north ard in om arison ith their resent osition ha, a, b os et al , ) rom , a until toda , the hain of ebb tidal deltas and barrier islands has been retreatin land ard at an avera e mi ration rate in the order of m r, and a lar e art of the sediment as de osit ed into the ba barrier areas ost, )
he situation of the est risian adden area at , a is iven in i ure t the mainland, tidal flats mer ed ith tidal marshes and lar e bra ish areas hi h ere flan ed b e tensive bo s linin hi her sand areas sselin , os and nol, ) idall influen ed rivers and streams ere in o en onne tion ith the adden ea
Figure 2.4 Situation around AD 800. Orange = higher sand grounds; brown -= peats; yellow = coastal dunes ;dark green = tidal marshes; light green = tidal flats; blue is subtidal area; contours of present day coasts are given (Oost et al., 2015).
he first lo al di es surroundin smaller areas to safe uard arable fields and a ainst most inter floods, ere resent from around and be ame robabl relativel ide s read in est risia around , a an der e , ost , , ) in e , a di es oriented alon streams ere built in order to hannel the outflo of aters , )
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.5 Sediment characteristics in the Dutch Wadden Sea area. From top to bottom: median and mean grain size in µm, gravel content (relatively high around Texel) and mud content (high north of Ameland and near the mainland. Based on sediment atlas Rijkswaterstaat. Data source: http://opendap.deltares.nl/thredds/catalog/opendatap/rijkswaterstaat/sedimentatlas_waddenzee/Karline Soetaert
a a ontinuous s stem of inter di es had been onstru ted alon the entire est risian adden ea mainland oasts ost, ) eat subsiden e due to a ri ulture led to floodin and before a man lar er ba s rea hed their ma imum area artl oin idin ith the onset of the ittle e e, su essful land re lamation started, from time to time set ba b severe storm sur es ost, ) round a and later, lar e arts of the salt marshes had silted u to su h a level that the ould su essfull be emban ed and e tensive areas of land iddel ee, au ers ee, ivelboe em) ere re laimed van der e , ost, ollmer et al , an eteren and van der e , ) he land re lamations redu ed the surfa e area of the tidal basins, reatin smaller tidal risms hi h
Towards a Mud Balance for the Trilateral Wadden Sea Area in turn resulted in smaller tidal inlet s stems e ause the mainland oast as ro radin sea ard and the barriers ere retreatin land ard, the tidal basins be ame smaller
he lar er art of the sediments of the tidal area onsists of medium si ed uart sands he sand is mainl derived from the orth ea oastal area, hi h, as a result, retreated the mud is mainl riverine or bio eneti of ori in i ure )
2.2 THE EMS ESTUARY
2.2.1 Characteristics he ms estuar is lo ated bet een the etherlands, rovin e of ronin en and erman , o er a on , and bet een the barrier island ottumeroo in the est and or um in the east or the ur ose of this stud e have added the eastern adden ea ith the smaller tidal inlets ee at van de au ers, and hild – see i ure ) eo ra hi all to the ms estuar he ms estuar onsists of an outer estuar and a onne ted tidal river, the lo er ms river he len th of the estuar bet een the barrier islands and the eir at erbrum is m i ure ) he tidal river is a ro imatel m lon , and the outer estuar m he outer estuar onsists of tidal hannels and flats, hi h be ome in reasin l mudd in the land ard dire tion he most sea ard se tion is om osed of dee hannels ith a de th lo all limited b leisto ene substrate ieri et al , ) and sand shoals he outer area has t o main outlets a estern hannel and an eastern hannel t resent, the estern hannel is the main outlet he middle rea h is a t o hannel s stem ith a main hannel the ‘Friesche Gaatje’) and an abandoned tidal channel (the ‘Bocht van Watum’). The outer estuary terminates in a mudd tidal emba ment no n as the ollard
or the lo al meteorolo , the reader is referred to the ha ter on the ut h adden ea
and re lamations arried out in the ast ears have reatl redu ed the intertidal area in e , the si e of the ms stuar the subtidal, intertidal and su ratidal area) u to emshaven de reased b from to m errlin and ieme er, ) he ombined intertidal and su ratidal area de reased b from to m uman interferen es in the estuar have a elerated in the ast ears, ith the onstru tion e tension of three orts emshaven, elf i l and mden), a lar e shi ard a enbur ), and the dee enin of the shi in lanes
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.6 Map of the Ems estuary (including the lower reaches, the middle Reaches, and the Dollard estuary) and lower Ems River (between km 0 and 40). Adapted from De Jonge (2000).
The resent day a ro imate maintenance de ths o the a roach channels to the orts are m ( emshaven) m ( el ijl) and m ( mden) re uirin re ular maintenance dred in . The lo er ms iver as dee ened rom a ater de th o m belo W to m belo W bet een the s and (van aren et al. b). This dee enin has led to stron tidal am li ication ossibly am li ied by the resence o the eir at erbrum constructed in ( chuttelaars et al. ). The tidal ran e at a enbur ( m ) has increased rom . m in to . m in ith a major lo erin o the tidal lo ater level ( rebs and Weilbeer ). ntil the tidal ran e ea ed at mden ( m do nstream o the eir at erbrum) because the tide as dam ened urther in the u stream direction. ince then ho ever the o er ms has been dee ened urther and the tide am li ied u streams o mden and no adays the tidal ran e at a enbur e ceeds the tidal ran e at mden by .
2.2.2 Geology For the eneral local eolo y the reader is re erred to the cha ter on the utch Wadden ea.
The aleo valley incised durin the leistocene sea level lo stand determined the location dimensions and inland enetration o estuaries such as the ms ollard (Wiersma et al. ). t as illed early durin the rise o the sea. ri inally lar e eatlands surrounded this dro ned river valley. ome land areas have o ened to the sea in an early sta e. They ere looded illed u ith ine rained sediment and became land a ain. ther areas such as the ollard and the eybucht ere breached durin medieval times. eat as artially eroded. nother art as com ressed under the ei ht o ne sedimentary de osits hich radually illed u the lar e art o the areas. urin the ormation o the ollard the estuary idened considerably in the direction o mden.
Towards a Mud Balance for the Trilateral Wadden Sea Area
2.3 THE LOWER SAXONY WADDEN SEA
2.3.1 Characteristics The est to east trendin Wadden area o o er a ony has a total sur ace area o . m ithin the ational ar ieders chsisches Wattenmeer. The bac barrier Wadden ea consists o tidal marshes su ra inter and subtidal lats and tidal channels (Fi ure . ).
The described area consists o (Fi ure . ) . nlet systems in the mesotidal area rom est to east Westerems sterems orderneyer ee at Wichter e ccumer e t umer Balje arle and Blaue Balje . more eastern coast ith e islands in the su ratidal area rom est to east ade Weser obinbalje and Westertill ordertill.
n eneral this area resembles stron ly the utch Wadden area both or its a earance and the mor ho enesis. everal resh ater streams are enterin the bac barrier the major one is the river ms hereas the more eastern hal is also stron ly in luenced by the Weser and lbe estuaries. ost o the inlets have islands at either side and clearly develo ed ebb tidal deltas but east o inseneroo the increasin tidal ran e results in small su ratidal shoals instead o islands. bservations over the ast century sho that increase in the o er a ony Wadden ea is around . mm yr ( tations Bor um lte Weser Wan eroo e West and orderney see also Wahl lbrecht et al. ). Tidal am litude increases rom est to east rom . m at station Bor um dstrand u to m in station Wan eroo ost (B ). lso over time tidal am litude has increased ith some . . mm yr ( tations Bor um lte Weser Wan eroo e West and orderney). The mean si ni icant ave hei ht decreases rom . m in the Westerems to . m in the Blaue Balje ( oast at data). lon the o er a ony orth ea coast the littoral dri t o sandy sediments is directed to the east and calculated to be . . m yr (Fi ure . idderin ho ). For indiness storminess ave conditions and related storm sur e conditions alon the Wadden ea the reader is re erred to the utch Wadden ea ara ra h . . Wave conditions and related ocean res onses in the area o ast Frisian Wadden ea and outhern orth ea are addressed by Grashorn et al ( ) and chloen et al. ( ) res ectively.
everal lar e en ineerin interventions have been carried out in the area. The most im ortant are the reclaimin o lar e arts o the sterems area and the arle embayment and the dee enin o the Westerems the Weser and the lbe hich led to increases in tidal am litude in a lar e art o each estuary. nother im ortant eature has been the rotection or s on most o the barrier islands. The estern ti s o Bor um orderney Baltrum ie eroo and Wan eroo e are rotected by roynes and stone or ( W Thoren ). The mainland is di ed and in ront o it marsh develo ment or s have been constructed on many laces resultin in oreland salt marshes. n the barrier islands only the inhabited areas are surrounded by a closed chain o sand dri t di es and bac barrier di es.
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.7 The inlet systems of the Wadden area of Lower Saxony (Courtesy, Wadden Sea Secretariat).
Figure 2.8 Littoral drift along the North Sea coast of Lower Saxony, computed using wave data from stations Schiermonnikoog (SCH, green arrows), Elbe (ELB, yellow arrows) and Fino1 (FN1, magenta arrows), in 106 m3/yr (From: Ridderinkhof, 2016).
Towards a Mud Balance for the Trilateral Wadden Sea Area
2.3.2 Geology ter the last ce e relative sea level rise as initially ra id but decreased over time (see Fi ure . ). The risin sea level ollo ed and modi ied the mostly entle leistocene relie and determined the initial osition o the Wadden ea. The aleo valleys incised durin the leistocene sea level lo stand determined the location dimensions and inland enetration o estuaries such as the ms ollard and Weser estuaries and the adebusen ( trei Wiersma et al. ). ome o the areas ere illed ith eat and could be incised relatively easily and be converted into bac barrier embayments (e. . arle and ade embayments) thereby enlar in the tidal volume o the estuaries or bac barrier basins o hich they ere art. n the ast Frisian Wadden ea area the dee est river valleys ere looded around a B (Beets and an der e ). The resent day tidal inlet osition is in a e cases still determined by the ormer valleys (Wiersma et al. ). ocally elevated leistocene outcro s and headlands consistin o moraine de osits o the aale (second last) laciation and sandy de osits o the Weichselian (last) laciation ere resent ormin the resent day Geest runden o o er a ony. maller barrier islands sandy shoals or sand s its may have been resent in ront o the mainland coast as relicts o leistocene headlands (Flemmin and avis ).
The bul o the ast Frisian barrier island chain ormed bet een a B . nitially sedimentation could not ill the s ace created by the ra idly risin sea ( m century) and a mainly subtidal area ormed rin ed by a narro one o intertidal sand and mud lats and salt marshes near the mainland. t the mainland ens ave ay to raised bo s hich started to e and on the mainland o east Frisia bet een es ecially a B ( et elber er et al. c stein et al. ).
n the ollo in millennia a ter B subsidence o the bottom in the ast Frisian Wadden ea as relatively lar e and des ite a deceleratin absolute sea level rise sedimentation as insu icient to ill the basins every here ( os et al. ). n some laces the tidal area as e tendin ( os et al. ). n other laces the Wadden ea silted u locally and salt marshes could advance sea ard (see Behre os et al. ).
t about a B the ast Frisian barrier island chain as still situated several ilometres o shore rom its resent osition ( os et al. ). From a B until today the chain o ebb tidal deltas and barrier islands has been retreatin land ard and a lar e art o the sediment as de osited into the bac barrier areas.
The situation o the ast Frisian Wadden area at a B is iven in Fi ure . . t is the eriod be ore man started to se arate the various coastal environments rom each other by di es. t the mainland tidal lats mer ed into tidal marshes and lar e brac ish areas hich ere lan ed by e tensive bo s linin hi her sandy areas ( sselin ). Tidally in luenced rivers and streams ere in o en connection ith the Wadden ea.
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.9 Situation around 800 AD. Orange = higher sand grounds; brown -= peats; yellow = coastal dunes ;dark green = tidal marshes; light green = tidal flats; blue is subtidal area; contours of present-day coasts are given (Oost et al., 2017).
The irst local di es surroundin small areas to sa e uard arable ields a ainst ty ical inter loods ere resent rom around B and became robably relatively ide s read in Frisia around a B ( an der e ost y ). ince a B di es alon streams ere built in order to channel the out lo o aters ( y ). By a B a continuous system o inter di es had been constructed alon the entire ast Frisian Wadden ea mainland coasts ( y ). ue to eat subsidence as a result o a riculture areas ere looded and be ore a B many lar er bays reached their ma imum si e. artly coincidin ith the onset o the ittle ce e success ul land reclamation started rom time to time set bac by severe storm sur es ( ost ). round a B and later lar e arts o the salt marshes silted u so hi h that they could be emban ed and e tensive areas o land ( eybucht aade inbruch ch ar es Brac arlebucht) has been reclaimed ( ollmer et al. an eteren and van der e ). and reclamations and the retreat o the barrier islands reduced the sur ace area o the tidal basins creatin smaller tidal risms hich in turn resulted in smaller tidal inlet systems. s technolo y u to the end o the th century as lar ely inca able o sto in lar e scale natural develo ments many stron east ard shi ts o inlets and ebb tidal deltas occurred ( iemeyer ).
Towards a Mud Balance for the Trilateral Wadden Sea Area
Grain si es are mainly ine sandy on the tidal lats hereas medium to coarse rained sediments are resent in most o the inlets and around the islands (Fi ure . ). lon the mainland coasts and in the estuaries (es ecially more u stream) and in the ade Busen ine rained sediments are dominant resultin in lar e mud rich areas.
Figure 2.10 Median grain size distribution in m-classes of the Lower Saxonian area as given in the functional bottom model (Milbradt et al. 2015).
2.4 THE WESER ESTUARY
2.4.1 Characteristics The Weser is the second lar est river in Germany dischar in into the orth ea. The len th o the estuary rom the tidal eir is a ro imately m (Fi ure . ). The Weser estuary consists o the o er Weser bet een the eir and the city o Bremerhaven and the uter Weser sea ard o Bremerhaven. The tide in luenced tributaries are those o the chtum esum unte and Geeste. The uter Weser has a unnel sha e oriented to the north est.
The tidal ran e has increased in Bremen since the irst dee enin rom a ro . . m to a current level o . m due to the dee enin o the o er and uter Weser. o adays mean tidal ran e rises rom . m at the li hthouse lte Weser (a ro . m ) to . m at Bremerhaven (a ro . m ) and to . m at Bremen slebshausen (a ro . m ). Bet een and the o er Weser as dee ened to m chart datum. n the eriod the uter Weser as dee ened to m chart datum.
Bottom sediments o the o er and uter Weser consist mainly o medium and ine sands. n the o er Weser lar e ronounced bed orms (dunes su erim osed by ri les) are resent due to currents. They have avera e crest hei hts o m at mean len ths o m hile ma imum hei hts o . m can be ound. ea ards o ordenham these bed orms disa ear. There the so called “Nordenham mud section” (km 55– ) is resent. This is the turbidity ma imum one here muddy sediments dominate containin u to silt and clay and or anic matter (Grabemann and rause ller ). ear bed concentrations e ceed . l (Fan er et al. iethm ller et al. Grabemann and rause ). The turbidity ma imum one is ormed due to the combined e ects o tidal asymmetry (Grabemann et al. an et al. ) and non tidal estuarine ravitational circulation ( iethm ller et al. ).
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.11 An overview of the Weser estuary (BIOCONSULT and NLWKN, 2012)
he unnel sha ed uter eser has t o main channels se eral secondar channels tidal ullies and e tensi e tidal lats ( an e et al n this sand area the to o ra h chan es continuousl due to the stron h drod namics e ore the main na i ation channel in the inner estuar as esta lished and ro nes and trainin alls ere i atin the main channels (earl th centur c clic ariations led to an alternatin re erence o the estern ( edder arder rm or eastern ( urster rm channel ( an e et al ediments o the channels o the uter eser are mainl ine and medium sands n the ottom o the
Towards a Mud Balance for the Trilateral Wadden Sea Area na i ation channel e tensi e lat sections alternate ith scoured areas or stretches ith sand a es ha in crest hei hts u to 5 m and len ths o u to m e endin on the location tidal lats and ullies consist o sand silt and all other sediment mi tures
ikes ere irst uilt on the lo er reaches o the eser i er a out ar e areas o the lood lain ere thus se arated rom the estuar he eser estuar ot its resent sha e in the iddle es herea ter orti ied em ankments and dikes ere uilt ( ra emann n the eriod 5 the irst re ulation o the ri er course as undertaken in order to uarantee assa e or sea oin essels ith a dra t o less than 5 m s a result the tidal a e could enetrate almost unim eded till remen n the th centur additional chan es and im ro ements ere made to ada t to increasin shi si es and to counter the reaction o the estuarine s stem ( a le rainin orks and alls etties and ro nes ere uilt to sta ili e the course and rotect the em ankments and shorelines ( o ers ue to the enetration o the tide le els ere allin resultin in lo er round ater ta les u stream o remen n the eriod a eir as uilt in emelin en to sto this de elo ment n the s the storm sur e arriers ere uilt at the mouth o the tri utaries unte esum und chtum
or meteorolo ical conditions the reader is re erred to the section in the descri tion o o er a on
Table 2.1 Overview of river deepening and correction measures in the Lower and Outer Weser (SKN = nautical chart datum; from Lange et al., 2008)
2.4.2 Geology or the eneral o er ie o the de elo ment o the eser area the reader is re erred to the cha ter on the eolo o the o er a on area adiocar on measurements and ollen anal sis indicates that the North ea reached the eser area around
Towards a Mud Balance for the Trilateral Wadden Sea Area
2.5 THE ELBE ESTUARY
2.5.1 Characteristics he l e is the lar est ri er in erman dischar in into the North ea rom the eir at eesthacht until it lo s into the North ea the km lon estuar is dominated tides rom the eesthacht eir the l e is tide in luenced and kno n as the inner l e ( nnenel e u to u ha en rom thereon it is kno n as the uter l e ( ussenel e he inner l e consists o a o l e ( nterel e startin at the eir at eesthacht and the o er l e (Niederel e endin at u ha en ithin am ur the nterel e has a num er o ranches such as o e l e ose l e hl rand Northern l e (Norderel e eiherstie outhern l e ( derel e some o hich ha e een disconnected dikes
he l e s t o main ranches Northern l e and the hl rand reunite south o ltona ltstadt in am ur cit t kilometer (see ottom o i ure the Northern and the outhern l e used to reunite hich is h the a there is seen as the startin oint o the o er l e (Niederel e t u ha en the o er l e lo s into the ussenel e hich connects to the North ea he National ark am ur isches attenmeer e ists since and has a sur ace area o ca ha t is art o the l e estuar and includes the islands Neu erk charhoern and Ni ehoern
Figure 2.12 Overview of the Elbe estuary. TIDE factsheet
n the earl th centur the estuar as still relati el natural ( i ure and i ure t that time the l e estuar near am ur as still relati el shallo ith a ater de th o a out m lot o small islands ormed a delta close to am ur ince then the l e has een dee ened times and is no assa le u to am ur or shi s that need a de th o 5 m
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.13 Development of high water and low water levels at St. Pauli station with running mean and the various management measures in the Elbe estuary (Meine, 2011).
Figure 2.14 High water and low water levels along the estuary in 1900 (blue) and 2010 (red) (after Meine, 2011).
n the tidal am litude as still some m at am ur ( i ure t the mouth o the l e estuar tidal ran es are circa m ( u ha en rom there it increases u to am ur aint auli to m a ter hich it decreases to some 5 m in the direction o the eir at eesthacht he lood eriod is shorter than the e eriod a rocess associated ith a lood dominated as mmetr in tidal currents hich causes tidal um in and u stream sediment trans ort
or meteorolo ical conditions the reader is re erred to the cha ter on chles i olstein
Towards a Mud Balance for the Trilateral Wadden Sea Area
2.5.2 Geology or the eneral o er ie o the eolo the reader is re erred to the cha ter on chles i olstein
urin the lster ce e ( ears a o melt ater eroded channels u to m dee hich ere later illed ith sands urin the aale ce e ( – ears a o land ice reached the area ands e les and oulder cla ere de osited at oth sides o the l e alle and ormed a hei ht et een mainl to m a o e resent da sea le el n the lo er areas north o the l e u to m thick eatlands ere ormed (a o iether oor immelmoor olmmoor hmoor lasmoor ittmoor and endor er oor
urin the eichsel ce e ( 5 – ears a o laciers ended Northeast o the l e n these da s the ri er lo as di erted to the North est and the l e acted as a mar inal lacial melt ater s stem lo in arallel to the eichselian ice ront ee alle s ere ormed such as those o the lster ille andse and innau o ether ith the aleo alle o the l e the determine the resent da landsca e o the am ur re ion ter the eichsel ce e sea le el rose once more and tidal ater could enter the l e alle and turn the alle into a road estuar his led to the eneration o ri er shi ts and island ormation he mud lats near the l e estuar are elie ed to ha e een a o e sea le el durin oman histor and to ha e een inundated hen the shoreline sank durin the th centur
2.6 THE SCHLESWIG HOLSTEIN WADDEN SEA AREA
2.6.1 Characteristics he south north oriented orld erita e adden area o chles i olstein has a total sur ace area o km and ran es rom the mouth o the ri er l e u to the anish order the island lt he adden ea o chles i olstein itsel has a total area o some 5 km and consists o small islands hich ma or ma not e inha ited the so called alli en ( o the area or urther e lanation see ha ter tidal marshes ( tidal lats (5 and tidal channels ( i ure 5
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.15 The inlet systems of the Wadden area of Schleswig Holstein: Norderhever-Heverstrom to Hörnum Tief (Courtesy, Wadden Sea Secretariat).
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.16 Detailed overview of the inlet systems of Schleswig Holstein.
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.17 Littoral drift along the Schleswig Holstein coastline, computed using wave data from stations Helgoland (HLG, magenta arrows) and Sylt (SLT, green arrows), in 106 m3/yr (From: Ridderinkhof, 2016).
’ 1990’s
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.18 Development of the MHW, mid-tide and MLW over the period 1940-2007 as a mean of the stations: List, Hörnum, Wittdünn, Dagebüll, Husum, Büsum and Helgoland (MELUR, 2013).
Figure 2.19 Observations of the annual highest storm surges at Husum gauge (MELUR, 2013)
s s s s s s s s s s s 19 000 s s s s s s s ss s s s s 00 s s s s s s s s s s s
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.20 Annual hours of storm surges higher than +2.0 and 2.4 m NN at tidal gauge station List on Sylt over the period 1900-2004 (MELUR data, courtesy of Hofstede). NN is the reference level in the German Ordnance system, and is identical to Dutch NAP (both approximately corresponding to MSL).
2.6.2 Geology s s s s s s s s s s s s s s s s s s s ss s s s s 00 s s s s s s s 110 s s s s s s s s s s s s s s ss s s s s s
s s s s s 00 000 00 s 00 ss s 00 00 00 s 009 011 s ss s s s s s s s s s s s s
Towards a Mud Balance for the Trilateral Wadden Sea Area
s s s s s s s 1 s s s s s s eest Islands and extensive “Außensänden” in Nordfriesland, and the s 199
s s s s s s s s s s s s s s s 199 s 00 s s s s s s s s s s s s s s s s s s s s s s s s s s s s s s s ss s s s s s s s s 19
1 000 s s s s s s s s s s s s s s s s s s 1991 001 00 00 00 s s s s s s 00 s s s s 001 s 1 000 1 100 s s s s s s s s s s s s s 001 s s s s s s s s s s s s s s s s s s s s s 199 199 1 s s s s s s s s s “Außensänden” remained ap s
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.21 Geological map of the Schleswig Holstein Wadden Sea area (Falk, 2001).
he lar er part of the sediments of the tidal area onsists of medium si ed uart sands ut it is learl visi le on i ure that the hannels of ever, Aue, and espe iall rnum ief and ister ief are hara teri ed oarse rained sediments indi atin erosion into the leisto ene moraine deposits or transport of oarser materials in melt ater of the ontinental i e sheets , the sturd oulder la s his mi ht e an explanation to the eneral la of meanderin in the deeper inlet hannels he sand is mainl ori inatin from the North ea oastal area, hi h, as a result, retreated the mud is mainl riverine or io eneti of ori in i ure Ahrendt, ,
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 2.22 Median grain size distribution in µm-classes of the Schleswig Holstein Wadden Sea area as given in the functional bottom model (Milbradt et al. 2015).
2.7 THE DANISH WADDEN SEA
2.7.1 Characteristics he south north oriented anish adden ea area has a total area of some m i ure aursen and ri e, It onsists of four inlet s stems, from south to north ister ister ief , uvre , nude and r d , situated et een lt, m , and , an and the allin en peninsula i ure everal small ompared to erman and ut h adden area fresh ater rivers are enterin the a arrier he ma or one is the
Towards a Mud Balance for the Trilateral Wadden Sea Area
arde enterin the r d via an open estuar In r d and ister , the rate of sedimentation ex eeds relative sea level rise In vardsen, a,
Figure 2.23 The inlet systems of the Wadden area of Denmark: Lister Dyb to Grådyb (Courtesy, Wadden Sea Secretariat).
idal amplitude is m stdire toratet, aursen and ri e, he mean si nifi ant ave hei ht de reases from ister a m to r d a m oast at data Alon the est oast of utland littoral drift is dire ted to the south, amountin to m r i ure idderin hof, revailin esterl inds ex eed m s for of the time and m s for of the time he stron littoral drift for es the main inlet hannels south ards I and I, until the e urrent ill ut throu h the e tidal
Towards a Mud Balance for the Trilateral Wadden Sea Area delta and forms a more dire t route In the pro ess an elevated e tidal delta sand od is left south of the ne e hannel and transported to ards the oast aves arthold and e rup,
Figure 2.24 Littoral drift along the Danish coastline, computed using wave data from stations Fanø (FNO, red arrows) and Sylt (SLT, green arrows), in 106 m3/yr (From: Ridderinkhof, 2016).
servations indi ate that in the anish adden ea is mm r in the period omparison to the avera e rate of sea level rise of mm r in the th entur at s er , su ests a eleration in re ent ears la en er et al , o ever, are should e ta en, as shorter time series ma i nore lon term flu tuations and thus lead to fault on lusions illin h et al , Indeed, the a eleration is not o served in North risian part of hles i olstein see there ,
ive out of six I stations in utland indi ate that ind has een in reasin over the period to ith mean ind in reasin from m s to m s in the period la en er et al , revailin hi h ind ener from the is oin ident ith the hi h ave ener periods la en er et al , everal lar e en ineerin interventions have een arried out in the area he most important are the dams to lt and m and the tidal road to and Alon the mainland oastal prote tion or s and salt marsh or s are present Alon the r d mainland oast the har our to n of s er has een developed he ater a to the har our of s er has een deepened and is maintained dred in
Towards a Mud Balance for the Trilateral Wadden Sea Area
2.7.2 Geology urin the se ond last I e A e aalian the area as la iated, and moraines ere deposited hi h e ame later elevated urin the last I e A e ei hselian the terminal line of the i e sheet as a m east of the present da adden ea elt aters drained to the est into the North ea asin sea level a out m lo er than at present As a result, out ash plains formed et een the older moraines i ure a o sen, arthold and e rup,
Figure 2.25 Geological map of the Danish Wadden Sea (Jacobsen, 1993)
Towards a Mud Balance for the Trilateral Wadden Sea Area
he east and north of enmar experien ed a post la ial isostati ad ustment to the removed i e sheet In om ination ith the post la ial eustati sea level rise, this resulted in re ression trans ression alternations arthold and e rup, It is enerall elieved that the adden area has een isostati all sta le durin the olo ene, at least durin the present Initiall the post la ial sea level rise velo it as hi h, ut raduall slo ed edersen et al ave onl m over the past ears his led to a mu h more restri ted erosion of the oast or instan e, the northern spit of lt as formed as earl as the sea ard part has een eroded indhorst, , hereas m pro raded sea ard sin e , a , due to a undant sediment suppl adsen et al , from the updrift e tidal delta et een lt and m et een r sea level rise as some m entur ith li el several hi hstands After r sea level rise as slo and a ain li el pun tuated several hi hstands
As a result of the floodin the out ash deposits have een re or ed into arrier islands n the mainland the out ash deposits and the moraines ere eroded and formed liffs o all tidal marsh deposits formed hi h have een partiall re laimed
It is not ompletel lear hen the arrier islands ere formed It is thou ht that an li, the eastern part of an , and , and ordsand formed a s stem of old arrier islands arthold and e rup, A aard et al , he re ent arrier islands formed in the period he development of oresand is onsidered to fill the ap in the ro of the ne , more esterl lo ated arrier islands, hi h onsist of allin en the most re ent arrier island , an , oresand and m stin spe toratet, lt is an erodin leisto lio ene ore otes liff and surroundin s hi h has an even more estern position than the anish islands more to the north
he lar er part of the anish adden ea onsists of fine to oarse sand he area of mudflats, mixed flats and tidal marshes ith a hi h mud ontent is rather small
Towards a Mud Balance for the Trilateral Wadden Sea Area
3 A BRIEF OVERVIEW OF MAIN PROCESSES
In this hapter e dis uss the main h drod nami pro esses and sediment transport me hanisms relevant for the import, transport and deposition of mud in the adden ea esides, e dis uss the residen e time of mud inside the asins, and the ex han e of mud et een individual tidal asins and et een the su parts of the rilateral adden ea s stem
3.1 DETERMINING FACTORS AND TIMESCALES erman et al re o ni ed three important fa tors and three important time s ales here pro esses operate that determine oth the suspended parti ulate matter in the ater olumn and the mud ontent of sediments in the adden ea see i ure
Figure 3.1 Schematic diagram of the most important processes affecting mud dynamics in the Wadden Sea. After: Herman et al. (2018). The light blue arrows indicate the mud sources, the dark blue arrows show mud extraction, by salt marshes and landfills and the white arrows show mud transport between the basins and the North Sea and between the individual basins.
he fa tors are • he quantity of mud, some of hi h is in suspension ut most of hi h is stored in the sea ed, in an either eas or diffi ult to erode state • he properties of mud hi h determine oth hori ontal and verti al transport i e sedimentation, erosion and transport , mainl hara teri ed settlin velo it , riti al shear stress for erosion and the erosion rate • he hydrodynamic forces a tin on mud, steered the tide, ind, aves and fresh ater dis har e see also i ure
he time s ales are hort tidal to sprin neap tidal da s , intermediate seasonal to multi ear, and lon ears, i e morpholo i al time s ale
Towards a Mud Balance for the Trilateral Wadden Sea Area
At the short time s ale, most pro esses related to deposition, resuspension and transport ex han e et een the adden ea and North ea of suspended material ta e pla e At the ver lon times ale de adal e o serve intera tions et een ma or h drolo i al han es e han es in tidal volume e ause of land re lamations, losures of uider ee and au ers ee, mi rations of tidal divides and mud ontent of the sediments o ever, the o servations also point to a third time s ale, hi h ran es from seasonal to multi ear, here the surprisingly long ‘memory’ of the system resides. There are year to ear variations oth in mud ontent of the sediment and on entrations, hi h have auto orrelations up to more than one ear, and here, e ause of this, avera e flu tuations ma diver e from the lon time avera e for a num er of su essive ears his time s ale is often ne le ted in on eptuali ations of mud d nami s, ut it appears to pla an important role in the time d nami s of the o servation series
Note that this does not ex lude other fa tors that an e important from a mana ement point of vie , su h as influen e of iota, fisheries and dred in erman et al explain that these influen es operate mainl throu h han es in the main fa tors mentioned a ove see also i ure
Figure 3.2 Schematic and generic diagram of the most important processes affecting SPM levels. By: Herman et al. (2018).
3.2 GROSS AND NET FLUXES In multiple inlet s stems li e the adden ea, lar e amounts of sediment are ex han ed ith the ad a ent sea durin e and flood or an inlet, the amount enterin ith flood is enerall une ual to the amount leavin ith e , ivin a residual flux of suspended sediment over a tidal period he residual flux is ver small ompared to the ross flux and is
Towards a Mud Balance for the Trilateral Wadden Sea Area highly aria le in spa e and in time er ema et al. assi et al. . hange pro esses et een the adden ea and surrounding systems in lude the orth ea oastal one and fresh ater dis harge. ross e hange rates et een the orth ea and ea h of the tidal asins are in the order of ton yr and net import rates are roughly to of the gross e hange rate erman et al. .
The residual flu of ater and sediment in the adden ea is in the east ard dire tion generating a residual transport o er the tidal di ides. The arsdiep inlet is the first inlet en ountered y the north ard plume along the ut h oast and is hen e a fa oura le lo ation for import au et al. er ema et al. . ost of this sediment ill not e e ported again in the do ndrift tidal inlet ut rather e transported east ard a ross the atershed uran atute et al. . The large ontri ution of ind to residual transport o er tidal di ides as first esta lished using numeri al models tane et al. uran atute et al. assi et al. ur hard et al. errling and inter . ts importan e as orro orated through o ser ational data y an eerden urg in the estern adden ea. The ontri ution of the ind to residual flo and transport o er intertidal areas of the estern adden ea as analysed in detail y olosimo et al. ho sho ed that a single and e en moderately strong ind e ent may transport the same amount of sediment transported during ee s of tide dominated onditions. s a result of the dependen e of residual transport of sediment on storms sediment is mostly deposited in the east ard erman ight during periods ith high ind elo ities sters et al. .
epresentati e estimates of o erall transports are still la ing e ause of the high short and long term aria ility indu ed y the ind and the logisti diffi ulties in measuring ontinues flu es o er a tidal di ide. esidual flu es therefore need to e omputed using numeri al models. o e er numeri al models themsel es introdu e un ertainties espe ially in residual uantities as net transport is often a small differen e et een t o mu h larger gross sediment flu es assi et al. mits et al. . s ill e ela orated in more detail in hapter important sour es of un ertainties in net sediment transport arise from the meteorologi al onditions definition of ind limate aria ility s hemati ation of a e dri en transport resuspension geometry of the asins in luding model resolution and sediment parameter settings. etermining a erage residual flu es therefore re uires running a large amount of meteorologi al years and model parameter settings hi h is hallenging from a omputational point of ie . n order to impro e our no ledge on the net e hange et een asins more long term measurements in om ination ith dedi ated modelling is needed.
3.3 RESIDENCE TIME OF MUD hile ad e ti e e hange of ater et een the adden ea and the orth ea has a time s ale of ee s only idderin hof ad e ti e e hange of mud an ha e mu h longer time s ales e ause large uantities of mud are stored ithin the ed and an e resuspended in the future erman et al. . ased on tra er studies it is estimated that the residen e time of mud in the ut h oastal one in the ater olumn and the sediment ed om ined is multiple years an essel et al. .
Towards a Mud Balance for the Trilateral Wadden Sea Area
The residen e time of mud has not een uantified in a similar ay for the adden ea. o e er as large parts of the system are depositional the residen e time of mud in the ed must e mu h longer in the adden ea than in the fairly energeti orth ea. ud has een depositing here for millennia forming su stantial mud uffers ithin the adden ea area. large part of these deposits is no losed off from the sea through onstru tion of di es. e ertheless ithin the adden ea area su stantial olumes of mud are still present of hi h a part an e eroded during energeti onditions e.g. hea y storms .
isma et al. estimated the time s ales of transport and deposition of mud in the adden ea using an unsta le isotope of lead measured in suspended and ed sediments. The transfer time the a erage time needed for dissol ed to e ome adsor ed onto parti les is . days the remo al time the a erage time needed to remo e parti ulate from the ater olumn is . days and the residen e time the a erage time during hi h remains in the adden sea is . days. This is relati ely short refle ting the rapid e hange of ater and suspended matter et een the analysed flats in the adden ea and the oastal orth ea and the fre uent settling and resuspension of suspended matter.
mits et al. e plain that the residen e time of mud ithin the adden ea determines at hi h times ale the dynami amount of mud ill su stantially ary. They estimated the residen e time ased on model simulations y omparing mud transport flu es net and gross and mud a aila ility in the ed. They sho ed that the residen e time largely depends on the fall elo ity of the mud parti les espe ially related to the density and si e of flo s eing signifi antly larger for parti les ith a large settling elo ity on a erage days ompared to parti les ith a small settling elo ity on a erage days sin e the fast settling parti les intera t more ith the sediment ed. ased on their results they estimated an a erage lo er limit of the residen e time of days and an upper limit of years. The lo er limit in the order of half a year ill pro a ly e a etter indi ation for relati ely dynami areas lose to the inlets hile the upper limit in the order of a de ade represents the residen e time in lo dynami areas su h as the salt marshes along the mainland oast.
There is also an assumed transport of from est to the east in the adden ea aane et al. . sing a physi al mi ing model of the ut h adden ea erhagen al ulated the adaptation time of the sediment uality in arious parts of the adden ea. y using the inter annual trend of lead on entration in the surfa e sediments he ould uantify the time an parti le needs to tra el from the estern to the eastern parts of the ut h adden ea. ording to the results it ta es a out years for and asso iated ontaminants to tra el through the ut h adden ea from the est to the east.
Towards a Mud Balance for the Trilateral Wadden Sea Area
4 SUSPENDED SEDIMENT CONCENTRATION
uspended mud on entrations sho a ery ide natural range oth spatially and temporally. id term ariations an ha e natural and anthropogeni auses. amples of natural auses are the a e limate meteorologi al ariation ind air pressure effe ts y mi rophyto enthos ater salinity and temperature ariations erti al mi ing and hanging properties of fine sediments y for instan e flo ulation. n the other hand human inter entions ha e an impa t y arrying out dredging a ti ities and regulating fresh ater output into the asins. uman a ti ities su h as land re lamations and e tra tion of mud an also influen e the long term ariation of suspended mud on entrations. n this hapter e riefly dis uss o ser ed spatial and temporal ariations of suspended mud on entrations rought a out y natural fa tors. n hapter the human inter entions are dis ussed.
4.1 SPATIAL VARIATIONS
4.1.1 Introduction The de elopment of suspended mud on entrations is ompli ated e ause settling elo ity and riti al shear stress for erosion are not dire tly lin ed to parti le si e. lo ulation pro esses influen e settling elo ity hereas onsolidation and diatom o ers of ed sediment influen e erosion properties.
4.1.2 The larger picture nalysis of data erman et al. olle ted as part of the ut h national ater monitoring programme T re ealed that ithin the orth ea transe ts off eeland off oord i and off Ters helling is primarily determined y the ater depth the time a eraged on entration depends linearly on the logarithm of depth le eld et al. . o e er most adden ea stations ha e a mean on entration ell a o e that of the orth ea le els and a dire t orrelation ith the depth does not e ist. The in e.g. a hannel station is also influen ed y pro esses in the intertidal tane et al. there y o s uring any depth dependen y in the o ser ations. multi year trend analysis of did re eal the e isten e of multi year aria ility in i.e. alues may e lo er or higher for a period of multiple years o er larger spatial domains su h as the adden ea orth ea orthern part orth ea outhern part et . . o e er this multi annual aria ility may differ per spatial domain i.e. a period of se eral years ith ele ated on entrations in one area does not orrespond ith multi year trends in another area ithout any auto orrelation et een areas.
4.1.3 Basin variations rain si e fra tions in tidal asins are sorted a ording to an energy gradient see e.g. yand i lemming and iegler . The general trend sho s a de rease ith oth tidal and a e energy and hen e in grain si e from the inlet to the land ard side of the asin. ud d . mm here log d is thus dominant in the ater olumn in the lo est energy parts of the tidal asin.
Towards a Mud Balance for the Trilateral Wadden Sea Area
rain si es a. . mm diameter are sometimes alled the suspension population as the differen es et een urrent elo ities needed for entrainment of a grain or to suspend them in the ater are ery lo an i n . Thus a distin tion an e made et een finer sediments which are almost totally transported in suspension (the ‘suspension population’) and a so called ‘bed load population’ of coarser grains. The latter group is partly transported o er the ottom and ill only e suspended at fairly high urrent elo ities . m s . nder normal onditions su h urrent elo ities an o ur for longer periods during the tidal y le in the deeper hannels ut only o ur for a short period in the shallo er hannels gullies shoals marshes ree s et . ear the inlet and in some parts of the main hannel the urrent elo ities are suffi iently high that e en oarse sand ill e pi ed up.
o ing from the inlet to ards the end of the hannel systems under normal onditions the grain si e of parti les suspended in the ater olumn de reases ostma . The t o main sediment populations suspension population and ed load population are learly separated here. The oarser sands of the ed load population are mainly to e found near the inlet and sho a ui de rease to ards the a arrier. rom hannel lags the shells and stones hi h are left ehind after the ashing out of sands do n to sand of a. . mm the distri ution is learly determined y the urrent elo ities in the hannel. The suspension population in the hannel in reases in on entration ith in reasing distan e from the inlet highest on entrations are found loser to the end of the hannels igure . .
ithin this suspension population there is also a ery lear de rease in the median grain si e to ards the end of the hannels. The same is also true for the median grain si e and the lay ontent at the sediment surfa e of the hannels ysin . The o ser ations strongly suggest that the suspension transport in hannels fun tions to sort the arious grain si e fra tions. lthough tidal urrents dominate the igger hannels a e and ind generated tur ulen e does influen e the smaller hannels ostma . t is important to reali e that also the shoals e ome muddier in the dire tion of the mainland y settling lag and s our lag effe ts. part of this material ill e rought a from the shoals to ards the hannels. This implies that the on entrations of suspended matter do not ha e to orrelate ompletely ith the lo al urrent elo ities in the hannels ysin .
n this report e argue that the sorting of mud is determined y the urrents and a es oth in the smaller hannels and on the shoals e ause . ele tion to grain si e is learly isi le in the ater olumn of the hannels . n the hannels parallel to the mainland in igure . stations and the grain si e learly de reases to ards the end. t the same time the intertidal to supratidal oast is ery ri h in mud. This an only e e plained y deposition of the oarser suspended material of the mud ri h ater and transporting the finer material to the end of the hannel . odel studies sho that in su tidal areas the on entrations of mud are lo near the inlet due to tidally dri en ed shear stress. lso mud on entrations are high to ards the end of the hannels due to a om ination of tidal and a e dri en shear stress. This is in good a ordan e ith field data. n general it is o ser ed ieu enhuis and predi ted an rooi en and ang that mud ill e en ountered some m
Towards a Mud Balance for the Trilateral Wadden Sea Area
distance from the inlet in the case of the meland inlet ( igure . ieuwenhuis an rooi en and ang ).
Figure 4.1 Suspended sediment concentrations of various grain sizes in the Borndiep channels as measured in September 1958 (Postma, 1961).
Figure 4.2 Mud content of the sediment versus distance from the gorge (Nieuwenhuis, 2001).
Towards a Mud Balance for the Trilateral Wadden Sea Area
n the tidal flats current elocities are generally low. s soon as the tidal flat is flooded current elocities drop sharply. The coarser grains are deposited near the channel edge forming the channel le ees. uring the ebb phase the highest current elocities occur when the flats are (almost) dry. onse uently suspension transport dominates on the intertidal shoals with the e ception of the le ees. ysin and iegel ( ) agreed with ostma ( ) that wa e action on the tidal flats dominated and that currents were not important. owe er obser ations show that currents also play an important role
The grain si e decreases in general going from the islands towards the mainland ( ostma amps lemming and yandwi lemming and iegler sediment atlas ). This suggests that also currents must be of influence otherwise the sheltered areas near the islands would probably be muddier. The decrease in grain si e towards the mainland (see lemming and yandwi ) can partially be e plained from the settling lag and scour lag effects ( ostma an traaten and uenen ) and partially by the sorting in the tidal channels as discussed before. n comparable sub en ironments the median grain si e decreases from west to east ( ysin ). ince the tidal flats east of chiermonni oog are far less sheltered than the ones south of meland or chiermonni oog this cannot be e plained by wa e effects alone (only wa es would lead to highest mud content directly south of the islands. This suggests that currents li ely in combination with storm induced setup setdown lead to eastward transport of fines (such as the pre alent eastward wind dri en transport coinciding with storm setup during winds from the to . amps ( ) found that during storm surges the tide and wind dri en currents determine the direction of sediment transport on the shoals. odel studies indicate that areas where mud deposition occurs are mainly determined by the tide. en though in shallow water depths wa es determine how much and how fast mud can be suspended the tidal currents determine the actual transport ( ieuwenhuis ).
ll in all it can be concluded that grain si e fractions in tidal basins are deposited according to an energy gradient ( lemming and iegler yandwi ).
4.1.4 Influence of hydrodynamic conditions in the wider area uspended sediment concentrations are not simply a factor of entrainment and deposition i.e. the water bed e change (as discussed abo e). lso processes in the water column (settling ersus ertical mi ing) play an important role. This is uite e ident in estuaries where estuarine turbidity ma ima are determined by many hydrodynamic and sediment dynamic processes ( urchard et al. tane et al. ).
ori ontal salinity gradients generate hori ontal gradients in the near bed hydrostatic pressure (dri ing a near bed flow directed to areas with low salinity) and a sloping free water surface (dri ing a seaward directed current near the surface). n most estuaries this gi es rise to a ertical gra itational circulation with landward directed currents near the bed and seaward directed currents near the water surface. ith sediment concentrations typically
Towards a Mud Balance for the Trilateral Wadden Sea Area higher close to the bed this gra itational circulation leads to net landward transport of sediment.
This salinity dri en circulation increases non linearly with the water depth. n water depths of se eral meters (pristine conditions in many natural estuaries) such a salinity dri en circulation is not important. t water depths e ceeding m a pronounced salinity dri en circulation often de elops in the transition one of salt and fresh water. t water depths e ceeding m it often becomes a dominant mechanism for sediment transport.
n summary the relati e importance of these processes for trapping is determined by topography flu ial and tidal forcing — because the interaction determines the degree of mi ing and stratification — and composition. ud transport is steered by hydrodynamic conditions in the wider area and with important memory effects. Typically mud transport is dri en by supply (sin s and sources) rather than by transport capacity as is the case with sand.
4.1.5 Fluid mud layers uring the past decades the accumulation of flocculated cohesi e sediments and the formation of wea ly consolidated mud deposits including fluid mud gained attention. luid mud is deposited in depressions in troughs of suba ueous dunes in the form of mud drapes during slac water or as large scale bed co ers in areas with abundant mud supply. ntrainment is controlled by local production of turbulence which is in turn influenced by local morphology.
luid mud may de elop in areas where the deposition rate is ery high as mud consolidation times scale uadratically with the thic ness of the deposit. This occurs for instance in coastal systems where mud transport con erges in energetic areas (resulting in regular resuspension of the mud). ud particles in the water column are ept in suspension by turbulent mi ing and therefore the strong turbulence damping at the interface between the fluid mud and the water column promotes settling of mud into the fluid mud. This leads to positi e feedbac mechanisms in which muddy fluid mud dominated areas become progressi ely muddier.
n the adden ea fluid mud is primarily confined to the estuaries (most prominent in the lower ms i er) and arious harbours. owe er fluid mud may also de elop in na igation routes in the tidal basins. egular infilling of the access channel to the ferry platform at olwerd has led to a rapid increase in dredging uantities in the last years.
4.2 TEMPORAL VARIATIONS
4.2.1 Tidal variations uring slac tide in subtidal areas mud deposits on the bed as a thin (se eral mm thic ) layer resulting in low concentrations around the turn of the tide. uring the flood and ebb phase resuspension occurs and mud concentrations in the water increase ( igure . and igure . ). ue to the asymmetry of the tides current elocities in a channel may be either characteri ed by a shorter duration of the flood or ebb period which results in higher current elocities and a higher chance of erosion ( isser e oer and ost ). n addition
Towards a Mud Balance for the Trilateral Wadden Sea Area
–
Figure 4.3 Sediment concentrations during the tide on the locations 9 (large channel Borndiep) and 6 (small channel eastward of Veerdam, for the different fractions (from: Postma, 1961). The numbers at the lines correspond to the fractions: 1: total suspended matter; 4: fraction < 63 µm; 5: Fraction < 32 µm; 6: Fraction < 16 µm; 7: Fraction < 8 µm.
Towards a Mud Balance for the Trilateral Wadden Sea Area
4.2.2 Seasonal variations
Figure 4.4 Monthly relative deviation from the long-time average concentration of SPM in the different systems of the Netherlands continental shelf. From: Herman, et al. (2018).
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 4.5 Annual variation of the percentage of sediment <16 micron in the top 0.5 cm of the intertidal flats near the mainland (average of 15 separate observations). The smooth line is a third order polynomial fit (Oost, 1995; data from Kamps, 1956).
4.2.3 Annual variations likely. There are no clear signs of ‘clouds of elevated SPM’ drifting into the Wadden Sea
4.2.4 Long-term variations developments
Towards a Mud Balance for the Trilateral Wadden Sea Area to su tidal area hich restricts ud de osition. or the s estuary it has een sho n that such decrease in sedi entation s ace for fines results over the course of several decades the SS ay increase considera ly van Maren et al. . iven the historic develo ent over the ast illennia here any e ay ents and tidal flats silted u and ere diked it can e e ected that tur idity ay have increased considera ly ithin the re aining Wadden Sea and its estuaries. o ever direct roof is not availa le.
Figure 4.6 Light attenuation coefficient k in the Western Wadden Sea, using data of Giesen et al. (1990), suggesting a decreasing visibility from 1920 to 1990. The R2 correlation coefficient is only 0.18 because of the large scatter in the k-values (van Maren et al., 2016).
Towards a Mud Balance for the Trilateral Wadden Sea Area
5 MUD AVAILABILITY IN THE SEDIMENT BED
n this cha ter e descri e the s atial distri ution of ud in surface de osits on shoals tidal flats and salt arshes in tidal asins and estuaries of the Wadden Sea.
5.1 DUTCH WADDEN SEA i ks aterstaat collected an e tensive set of sea ed sa les fro the Wadden Sea including tidal asins inlets and arrier island coasts i ks aterstaat . Mud is al ost co letely a sent in sa les fro the orth Sea each the e tidal delta and the ain tidal channels. The ud content increases in sa les fro the lee side of the arrier islands and in the direction of the ainland.
The overall sedi ent distri ution in the Western utch Wadden Sea is characteri ed y a strong sand ud segregation hich has also een o served and descri ed in various earlier studies e.g. Post a van Straaten and uenen de lo er van edden arts . We o serve distinct differences in the ud de osits et een the Western art corres onding to the Marsdie i erlandse at and lie inlets and the astern art u to the s ollard estuary . n the Western art shoals are ainly su tidal and consist of fine sand ith a of a out μ . The central arts of the asins ainly consist of sandy channels ith lo ud content. Surface ud de osits can e found along the ainland coastline here surface sedi ents have a ty ical ud content of u to . n addition a andoned channels ra idly filled in ith fine sedi ents after the closure of the uider ee in and these ud de osits are still resent in the u er ed olina lonso et al. .
n the astern utch asins ore ud rich intertidal and su tidal shoals are resent. ear the coastlines and the atersheds the ud content suddenly increases to high values over a li ited distance. or the ack arrier area S of eland it as o served that ud content is higher in the eastern art of the eland tidal asin than in the estern art ieu enhuis . This ight e related to the distance fro the inlet van Prooi en and Wang and to a i u ed shear stress during the tidal cycle hich is higher in the estern ack arrier area ieu enhuis . urther ore here filterfeeders are resent local atches ith high ud content are o served in areas ith other ise relatively lo ud content ost . y closing off the tidal asin au ers ee in an i ortant ud sink as re oved. s a conse uence the atershed et een Schier onnikoog and the ainland eca e uddier in the years olina lonso . ol er et al. sho ed that the intertidal flats in the utch Wadden Sea have eco e slightly sandier in the eriod the edian grain si e of the sedi ent has increased ith an average yearly increase in ean grain si e of . μ and the ud fraction has decreased ith an average reduction of . er year .
Towards a Mud Balance for the Trilateral Wadden Sea Area
5.2 EMS-DOLLARD ESTUARY n the s ollard the ud content is relatively high in . The tidal flats of the ollard ty ical ud content is . The ain channel of the ollard the roote at . This difference is ost ronounced hen co aring the ud content in the roote at ith the erings laat located south est of the roote at . The lo er s iver and the den navigation channel roo et al. .
roo et al. sho ed that oth finer and coarser sedi ents are resent a PS analysis sho ed t o eaks around μ and μ . n the Middle eaches ig. . the edian grain si e is larger in the dee er areas in oth the ollard and the lo er s iver this difference is less ronounced.
Figure 5.1: Mud content in the upper bed. Based on Sediment Atlas data (1989-1996, all measurements were carried out in summer).
ualitative co arison of the ud content of ith the ud content of revealed the follo ing • The ud content on the tidal flats of the ollard ay see s to have decreased. • The ed of the ain tidal channel in the ollard the roote at a ears to have eco e uddier. • Most of the lo er s iver sho s an increase in ud content.
ven though the and sa les ere collected in a si ilar eriod Se t – ec and ug – ct in and so e of the o served changes et een and ay have to e attri uted to seasonal fluctuations roo et al. .
5.3 LOWER SAXONY WADDEN SEA s e lained in Section . . the tidal flats in the o er Sa ony Wadden area ainly consist of fine sand ith ediu lo ercentages of ud . o ever along the ainland coasts and in the ey ucht and ade ay ud content in the u er ed ranges u to and fine grained sedi ents are do inant see also igure . .
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 5.2 Mud content in the bed sediments of the German Bight. Raw data retrieved from: www.geoseaportal.de/mapapps/resources/yrpps/projekt_aufmod/
Figure 5.3
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 5.3 Spatial pattern of the mean sediment grain-size in the back-barrier area of Spiekeroog Island. Note the landward-fining trend. Sediment grain sizes are indicated in phi classes. The phi unit (ø) is a logarithmic transformation of millimetres into dimensionless numbers, according to the formula: ø =2log-d, where d = grain diameter in millimeters. The grain size decreases with increasing phi number. (From Flemming and Ziegler, 1995).
5.4 WESER ESTUARY
5.5 ELBE ESTUARY
Towards a Mud Balance for the Trilateral Wadden Sea Area
5.6 SCHLESWIG HOLSTEIN WADDEN SEA
5.7 DANISH WADDEN SEA
Figure 5.4 Map of the Danish Wadden Sea (at low water) according to a classification by means of remote sensing: 1. Land and ocean. 2. Coastline. 3. Mudflat. 4. Muddy or mixed flat. 5. Wet or moist sand flat. 6. Dry sand flat. 7. High sand. Retrieved from: Bartholdy and Folving (1986).
Towards a Mud Balance for the Trilateral Wadden Sea Area
5.8 SAND-MUD SEGREGATION
Figure 5.5 Statistical distribution of the observed values of sediment mud content in the SIBES data base (left) and of the station means over the period 2008-2013 (right). Values (%) were natural log-transformed. Very low observations close to detection limit (< 0.1%) were discarded. From: Herman, et al. (2018).
5.9 SAND AND MUD IN THE WADDEN SEA: TWO DIFFERENT WORLDS?
Towards a Mud Balance for the Trilateral Wadden Sea Area
the sediment. This stimulates the development of benthic diatoms which may “glue” the
separate “worlds”: one of
Towards a Mud Balance for the Trilateral Wadden Sea Area
6 TOWARDS A MUD BUDGET
n this chapter we wor towards a first indication of a mud budget for the Trilateral adden ea. This sediment budget is based on observed mud sedimentation volumes in the tidal basins section . and salt marshes section . dredging volumes and anthropogenic sediment e traction section . and rewor ing of older deposits section . . ombined with a balance for sediment flu es in the orth ea section . we derive an integrated mud budget for the adden ea and ad acent orth ea section . .
6.1 MUD SEDIMENTATION IN THE BASINS
6.1.1 Dutch Wadden Sea basins large scale sand and mud budget of the utch adden ea has been presented by olina lonso et al. in which an estimate was given for the volume changes by both mud and sand fractions for the past years. The sediment budgets were calculated by multiplying the bed level changes with the measured volumetric mud content in the bed . formula by van i n was used to estimate the bul density of the sand mud mi ture which depends on the local sediment composition. The findings were validated against sediment cores.
igure . and Table . show the calculated volume budgets and the spatial sedimentation erosion patterns. n the estern utch adden ea a more or less linear infilling of mud has been observed since the closure of the uider ee. arge mud sin s are abandoned channels as a result of the closure and tidal flats near the mainland coast. n the astern utch adden ea a sudden increase of import of mud was observed after following the closure of the auwers ee in . olume changes in roninger ad have been much smaller compared to the other basins igure . Table . .
ud sedimentation seems to be largely constant in space and in time: we observe a spatial sedimentation erosion pattern that is uite persistent in time. onse uently it is li ely that compaction has played an important role. Therefore we assume a mean mud dry density of g m representing slightly consolidated mud typical for surface deposits . ith this the utch adden ea basins currently act as a mud sin of . ton yr e cluding salt marshes .
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.1 Volume budget of the Western Dutch Wadden Sea: a) bathymetry changes based on Vaklodingen data, b) calculated volume changes in the basins, c) estimated sand contribution to the bathymetry changes d) estimated mud contribution to the bathymetry changes. From: Colina Alonso et al. (2021).
Table 6.1 Calculated volume changes in the Dutch Wadden Sea basins (Colina Alonso, 2020)
Total volume changes [×106 Sand volume changes [×106 Mud volume changes [×106 m3] m3] m3] Western Dutch Wadden Sea . . 94.9 (1932-2015) Inlets of Ameland and . . 23.5 Schiermonninkoog (1971-2011) Groninger Wad (1985-2011) . . 0.52
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.2 Volume budget of the Eastern Dutch Wadden Sea: a) bathymetry changes based on Vaklodingen data, b) calculated volume changes in the basins, c) estimated sand contribution to the bathymetry changes d) estimated mud contribution to the bathymetry changes. From: Colina Alonso (2020).
6.1.2 Ems Estuary Bed level changes The net volume changes in the ms estuary also contribute to the net sediment budget and have been established by lias and van aren . The largest gross volumetric changes in the ms estuary occur in the tidal channels connecting the ms estuary with the orth ea. The two outlet channels in the west of the estuary are laterally migrating resulting in large volume changes but small net volume changes. lso one of these two outlet channels estereems is deepening while the other channel uibertgat is degenerating. dditionally the flood tidal delta is accreting with . m yr with primarily sandy sediment.
The two largest areas with net bed level changes within the estuary are a degenerating tidal channel the ocht van atum and the emshoornplaat. This latter is now an e tensive shallow area but used to be a channel connecting the astern part of the estuary with the western part. n the past years m was deposited here corresponding to m yr. The part of the emshoornplaat with largest net bed level changes is primarily muddy and it is estimated lias and van aren that with mud the annual mud deposition here is . . m . ith a mud dry density of g m which we assume as an average value representative for the estuaries this sin constitutes . – . ton of mud. The sand volume with a density of g m is e uivalent to . – . ton of sand. The annual sedimentation in the ocht van atum is . m of almost completely muddy sediment therefore corresponding to . ton yr. The ma or sediment
Towards a Mud Balance for the Trilateral Wadden Sea Area
– –
The sediment budget
–
Table 6.2 Sediment sinks in the Ems estuary [106 ton/yr], with a sink defined as net accretion, extraction during maintenance dredging, and sand mining, over the approximate period 1960-1990 and 1990 - 2011
1960-1990 1990-2011 Total Ems(mud) >1.8 (extraction) 1.9-2.4 – Total Ems (sand + mud) >6.1 4.8-6.7
6.1.3 Lower Saxony
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.3 Calculated mud contribution to the bed level changes in the Wadden Sea basins of the German Bight, for the period 2000-2010.
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table 6.3 Calculated mud sedimentation in Lower Saxony, Jade Bay and the Weser estuary (excl. extraction)
Total mass changes Sand mass changes Mud mass changes (2000- (2000-2010) [ 106 (2000-2010) [ 106 2010) [ 106 ton/yr] ton/yr] ton/yr] Lower Saxony excl. Jade Bay 12.3 11.7 0.6 Jade Bay 0.9 Outer Weser 22.1 20.8 * 1.3
6.1.4 Weser Estuary
6.1.5 Elbe Estuary
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table 6.4 Overview of the average import from the Elbe river and the North Sea of mud, the removal by dredging and the net result in ×106 ton/yr
Source Weights Ton/yr Net sedimentation of fluvial mud in the Elbe estuary +0.165
Net sedimentation of marine mud in the Elbe estuary +0.161
Net total sedimentation of mud in the Elbe estuary +0.326
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.1.6 The Wadden Sea area of Schleswig Holstein
Table 6.5 Calculated mud sedimentation in the Meldorf Bight
Mud mass changes (2000- Sand mass changes (2000-2010) 2010) 106 ton/yr] [ 106 ton/yr] Meldorf Bight area 0.2 * North Frisian German Wadden Sea 1.4 22.2 Total Wadden Sea area of Schleswig Holstein 1.6 *
6.1.7 Danish Wadden Sea basins
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.2 MUD SEDIMENTATION IN SALTMARSHES
6.2.1 Introduction
6.2.2 Methods
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.4 Overview of the subdivision of the Wadden Sea into seven sections in which the annual mud deposition is estimated.
Mud BDD = Total Sediment BDD(1 – SandMASS FRACTION)
Mud BDD = Total Sediment BDD (Clay + Silt)VOLUME FRACTION× specific weight (Clay + Silt)
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.2.3 Results
Total area
Table 6.6 Overview of tidal marsh types in the Wadden Sea in ha (Esselink et al. 2017).
Landform Netherlands Lower Hamburg Schleswig- Denmark Total Saxony Holstein Year of survey 2008/2012 2004 2009/2014 2011/2012 2010/2012 Island salt marsh Barrier connected (foreland incl.) 4,640 3,670 310 1,700 3,280 13,600
De-embanked (summer) polder 90 150 40 280 Mainland salt marsh
Barrier connected 780 1,340 2,120
Foreland type 4,000 5,460 8,580 2,520 20,560
De-embanked summer polder 360 380 740 Hallig salt marsh
50 2,180 2,230 Total 9,140 9,660 350 13,240 7,140 39,530
Surface-elevation change (SEC)
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.5 Comparison of SEC and vertical accretion rates in Wadden Sea salt marshes and summer polders. The graph is a compilation of studies based on both SEC from levelling and accretion data from studies which used SEB and marker-horizon techniques including clay-layer thickness measurements (Nolte et al. 2013a). SECs are based on overlapping and varying periods in time, mostly starting around 1990 or a more recent date. The graph presents a boxplot with median and 25 and 75 percentiles (grey). The vertical lines give the minimum and maximum values or 1.5 times the interquartile range of the data. Black dots show outliers. The horizontal blue bar gives the range of the long-term increase of MHT level since about 1900 (figure from Esselink et al. 2017).
Grain-size distribution and bulk dry densities
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table 6.7 Overview of SEC and vertical accretion rates in Wadden Sea salt marshes. The table presents data that were selected for the boxplot in Figure 6.5.
Sector Site Period SEC / accretion Method References (mm/yr) Foreland NL Friesland_West (B’ pollen) 1992-2014 21.3 Levelling van Duin et al. 2016 NL Friesland_mid 1992-2014 15.3 Levelling van Duin et al. 2016 NL Friesland_East 1992-2014 15.2 Levelling van Duin et al. 2016 NL Peazemerlannen 2007-2015 10.5 SEB van Duin and Sonneveld 2016 NL Groningen_West 1992-2014 8.5 Levelling van Duin et al. 2016 NL Groningen_Mid 1992-2014 9.8 Levelling van Duin et al. 2016 NL Groningen_East 1992-2014 10.2 Levelling van Duin et al. 2016 NL Dollard 1984-2012 6.5 Levelling Esselink et al. 1998, 2013 LS Leybucht 1989-1994 18.2 Sedimentation plates Erchinger et al. 1994, 1996 LS Wûrster Kûste 1960-1997 16.0 Levelling Michaelis 2008 SH Reg_Marien 1988/90-2009 7.0 Levelling Suchrow et al. 2012 SH Reg_NissHH 1988/90-2009 7.0 Levelling Suchrow et al. 2012 SH Reg_NordBu 1988/90-2009 9.1 Levelling Suchrow et al. 2012 SH Reg_EiderN 1988/90-2009 9.7 Levelling Suchrow et al. 2012 SH Reg_Tümlau 1988/90-2009 0.0 Levelling Suchrow et al. 2012 SH Reg_Ehsten 1988/90-2009 2.5 Levelling Suchrow et al. 2012 SH Reg_Hering 1988/90-2009 4.1 Levelling Suchrow et al. 2012 SH Reg_DiekKo 1988/90 -2009 2.6 Levelling Suchrow et al. 2012 Barrier-connected NL Vlieland 2002-2008 7.2 SEB Esselink unpubl. NL Terschelling 1992-20xx 1.1 SEB de Groot, pers.comm. NL Schiermonnikoog 1994-20xx 3.4 SEB de Groot, pers.comm. HH Scharhörn/Nigehörn 2011-2015 3.5 SEB Hellwig, pers. comm. SH Trischen 2004-2013 3.7 SEB Stock et al. 2014 DK Langli 2001-2009 0.4 SEB Kuijper and Bakker 2012 DK Skallingen 1992-1997 2.8 clay thickness, SEB Van Wijnen and Bakker 2001 Hallig SH Hamburger Hallig 1995-2010 0.9 SET/SEB Stock 2011 SH (Nordmarsh-)Langeneß 1986-2011 1.6 Cs-137 and Pb-210 Schindler 2014 SH Süderoog 1963-2007 3.9 sediment traps, Cs-137, Deicke et al 2009 SEB SH Nordstrandischmoor 1986-2011 3.2 Cs-137 and Pb-210 Schindler 2014 SH Hooge 1986-2011 1.4 Cs-137/Pb-210 Schindler 2014
6.2.4 Mud sedimentation
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table 6.8 Grain-size distributions and bulk dry densities in Wadden Sea salt marshes. Grain-size distribution in italics and shaded cells refer to volumetric distributions; other values are mass percentages (100×g/g). Bulk dry densities either refer to the same depth as the grain-size distribution, or to the compacted salt marsh sediment at greater depth.
e tor ite ept ineral ra tion ( ) B l dr densit o o o r e ( ) ( ) ores and ilt la pper deeper la er la er
Foreland oord Friesland olte ollard Wiertse a and artners e t r in er et al e t r in er et al B s er eller e er eller r in er et al ie sander oo eller et al Wester e er eller et al a r er alli olte an e issen oo eller et al re pper l n a ederson and Bart old n ded pper l n a ederson and Bart old r d pper l n a ederson and Bart old Barrier onne ted eland ls ot pers o ier onni oo n a ls ot et al and pers o F r er et al Fano pper l ederson and Bart old allin en Bart old et al an li pper l ederson and Bart old
Towards a Mud Balance for the Trilateral Wadden Sea Area
Table 6.9 Mud sedimentation in the Wadden Sea salt marshes, specified for seven sections (Figure 6.4), and the three main salt-marsh types. QSRarea refers to the subdivision of the Wadden Sea into 23 tidal areas in Fig. 5 of Esselink et al. (2017). For further explanation, see text.
area e tion ars rea edi ent B and ra tion d d otal d t pe in rease ( ( a) ( r) ( r) ( ) ( ol ) ( ) (ton r) (ton r) ) arrier oreland otal 306211 s arrier oreland oreland alli otal s 151382 a arrier oreland otal a 384903 ade oreland otal ade 229320 arrier Weser oreland otal 179778 Weser l e arrier oreland otal l e 107801 arrier oreland alli otal 3051618 arrier oreland otal 199710
rand total 1864263
6.3 DREDGING ACTIVITIES
6.3.1 Dutch Wadden Sea
6.3.2 Ems Estuary
Towards a Mud Balance for the Trilateral Wadden Sea Area
Figure 6.6 Dredging volumes for the Ems estuary since 1925: sediment extraction (mainly mud) in the Ems estuary and the lower Ems River, sediment dispersal, and sand mining (from van Maren et al. (2016). Total dredging volumes before 1960 are from de Jonge (1983); dredging volumes after 1960 are from Mulder (2013) for the Ems estuary and from Krebs (2006) in the lower Ems River (until 2006; after 2006 a constant value of 1.5×106 m3 is assumed).
Towards a Mud Balance for the Trilateral Wadden Sea Area
approach channel) increased up to the mid 1970’s, but became close to 0 in the early 1990’s
6.3.3 Weser Estuary Dredging
Figure 6.7 Overview of data on dredging. Data 1998-2010 based on BIOCONSULT and NLWKN (2012); data 2007- 2018 WSV data für dritten. Differences in overlap most likely due to exclusion of sand mining in latter series.
Sand mining
Permanent storage of mud outside the estuary
Towards a Mud Balance for the Trilateral Wadden Sea Area
he muddy sediments dred ed rom the harbours are brou ht on land because o contamination ith harm ul substances e ore 199 contaminated ine rained material rom the harbours as deposited on disposal sites on land and on a placement site in the urster rm uter eser) ince 199 remen stored dred ed material on land at the inte rated dred ed material disposal site in remen eehausen ince 001 also remerha en used this site ependin on the source, stora e i ures di er some hat rom 0 17 to 0 00 10 m yr and , 01 , 01 ) ore as remo ed in the period 00 – 00 and 011 hen sediments ere transported to the o er hine urthermore, since 011 more material can be remo ed because it can be stored in the lu ter deposal site at otterdam n 01 the total need or permanent deposition has been estimated to be some 0 1 10 m yr due to the enlar ement o the aiserschleuse and the more intense use o the ischereiha enschleuse , 01 ) his i ure as used or permanent stora e on land includin otterdam deposition) since 01
6.3.4 Elbe Estuary he tidal lbe has ma or players in ol ed in dred in o the area the ater and hippin uthorities o runsb ttel, u ha en and o ambur and the ambur ort uthority n total dred in olumes ha e been ro in up to more than 1 10 m since the deepenin o 1999 ntil 1999, be ore the last lbe shippin lane depth increase, some 10 m sediments ere dred ed annually in the lbe and the ambur harbour i en a speci ic density o 0 m enit er and an der tar, 01 ) this amounts to an annual dred in o 1 1 1 7 10 ton yr he amount o dred in material produced in the lbe and the ort o ambur has increased rom million m in 000, up to 10 m by 00 i , 00 ) ince the be innin o the years 000 on a era e some 9 10 m is dred ed ambur ort uthority, 017)
n combination ith all other dred in and minin some 1 10 m is annually dred ed in the entire lbe stuary in the period 000 01 n a era e some 11 o the dred ed sediment is remo ed rom the system emo al o the mud is as ollo s
1) Mud deposited on land 1 10 m or 0 10 ton yr is treated on land et band et al , 00 ) raditionally, the dred in material rom the ort o ambur is stored on land, to hei hten areas t the end o the 1970s, the en ironmental impacts o this practice became clear o reduce these e ects a dred in material treatment as de eloped and implemented in the 19 0s, the so called nla e rbeits emeinscha t r die einhaltun der lbe, 199 ) he nla e treats some 1 10 m per year o the most contaminated dred in slud e t is separated in sand, ine sand and a silt clay raction he contaminated ines are deposited in special depots on land he uncontaminated ine raction is used as a clay layer on top o di es otal permanent deposition is 1 10 m i , 00 ) his is mainly mud
2) Mud deposited at Buoy E3 t as decided to place part o the li htly to uncontaminated sediments e en more sea ard he dumpin location is at 0 m depth o charh rn and o el oland near uoy
7 Towards a Mud Balance for the Trilateral Wadden Sea Area
Dispersal of remaining dredged sediment: deposition in the seaward part of the estuary
Figure 6.8 Dredging volumes for the Elbe Estuary since 1998: (mainly) sand extraction in the Estuary, sand extraction from the harbours; mud extraction from the Estuary plus harbours and sediment dispersal by various means in the Estuary and the harbours. Data WSA and Hamburg Port Authority
6.4 INTERNAL REWORKING
Outwash of fines during reworking of sand
Towards a Mud Balance for the Trilateral Wadden Sea Area
Pleistocene deposits
Holocene clay-rich deposits
Towards a Mud Balance for the Trilateral Wadden Sea Area
the various areas won’t add up exactly. How large this discrepancy is, is
Figure 6.9 Cross-section running from the North Sea coast of Ameland, through the Wadden Sea onto the mainland of Friesland, showing the distribution of fine-grained deposits. Note the decrease in basin length of the original basin versus the present-day basin bounded by dikes (Van der Spek, 1994).
6.5 THE NORTH SEA
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.5.1 Major sediment sources ost sedi ent enters the orth ea ro the orth tlantic etween cotland and orway characteristic settling velocity s and through the trait o over characteristic settling velocity . to several s is a and al , uls et al., . edi ent co position suggests that which is transported along the elgian and utch coasts is largely supplied y sedi ent eroded ro the rench and ritish cli coasts rion and ll er, ettweis and van den ynde, . he annual input shows large interannual varia ility which pro a ly re lect di erences in re uency and duration o stor s van lphen, .
nother i portant source are the resh water inputs, such as l e, eser, s, hine, ha es, elland, Hu er, ees, yne, orth, ssel and the oord ee anaal Huthnance et al., . iver input o shows a strong interannual varia ility due to the interannual changes in discharge, in relation to cli ate varia ility. lso, strong variations occur over the year, with axi a coinciding with pea discharges. urher ore, suspended atter is entering the orth ea ro the althic ea, erosion o the otto and coasts, at ospheric deposition and, ainly in the su er hal year iologic pri ary production appen erg and anger, a le . .
Table 6.10 Overview of the estimates of the various sources of SPM input in the North Sea basin. Values vary due to differences in measurement techniques and natural fluctuations. Total input in the tidally influenced realm is given irrespective of deposition or dredging; data for landfill are added as negatives. It should be noted that Eisma and Irion (1988) made calculations for <63 mu, whereas Puls (1997) and Schubert (2006) looked at <20 mu which constitutes some 85% of all fines. The estimates of Puls (1997) being in general higher than those of Eisma and Irion (1988).
Source of SPM Input (106 ton/yr) Source orth tlantic uls is a and rion over trait . c anus and randle elegra is et . . . al uls a ite et al. an lphen is a and rion althic . is a and rion rosion o sea ed . . an lphen is a and rion nglish li erosion uls is a and rion ll rivers . . . is a and rion c ave t ospheric deposition . is a and rion ri ary production is a and rion xtraction l e , xtraction s and storage ud otterda . an lphen har our Total input SPM matter in the 44.8 – 95.3 North Sea Basin
t is assu ed that all studies addressed the suspended ineral atter.
Towards a Mud Balance for the Trilateral Wadden Sea Area
6.5.2 Residual transport patterns he ain ud transport towards the adden ea area ollows the long year residual current patterns in the orth ea is a, is a and rion, leveld et al., igure . and igure . . he concentration in the open orth ea is ainly deter ined y the distance to the coast, water depth, sedi ent co position o the sea ed, eteorology and the a ount o plan ton. he varia ility is up to two orders o agnitude appen erg and anger, . sing re ote sensing i ages ietr a et al. characteri ed the water types, relating the suspended atter to strati ication in the southern orth ea area igure . . hey showed strong seasonal and neap spring varia ility and highlighted the i portant role o tides and winds in controlling strati ication and distri ution.
Figure 6.10 Fronts and residual currents in the North Sea during a) summer and b) winter (ICONA, 1992).
wo i portant paths, relevant to the adden ea suspended atter i port, are recogni ed one which we will re er to as the orth ea ontinental low and the other eing the ast nglia plu e. oth transport plu es are separated y clearer waters originating ro the traits o over. e will uanti y the transport rates in oth these plu es in ore detail in the ollowing sections ro their source to their ter inus.
6.5.3 The North Sea Continental Flow he is the residual current ro the over trait along the rench, elgian and Holland coast which reaches exel nlet and su se uently oves along the adden ea coast a el . igure . . long the Holland coast its northward otion is strengthened y the hine
Towards a Mud Balance for the Trilateral Wadden Sea Area
region o resh water in luence through de lection o resh water asses y the oriolis orce. he density di erences generate a vertical circulation current with landward directed otto currents and seaward directed sur ace currents. s sedi ent particles settle, they are transported landward y these otto currents, e ectively eeping the tur id waters close to the coast.
Figure 6.11 Monthly average surface SPM based on SeaWiF’s data 1998. Insets show histograms of the pixel values, with x-axis giving the SPM pixel value and y-axis relative frequency distribution (Pietrzak et al., 2011).
Dover Straits he largest contri ution o ud originates ro the over traits which i port so e . ton yr, hal o which lows along the coast and hal into the . with a range o . . ton yr see c anus and randle, . ettweis et al. esti ated the lux ro the over trait using a co ination o satellite i ages and a nu erical odel at illion ton yr o which . – . illion ton enters the see also a le . .
Belgium High concentrations are easured along the elgian coast during al ost all onths o the year, ut especially in epte er to arch average onthly concentrations ay e as high as g l aaphorst et al., . t or s a tur idity axi u in ront o ee rugge. he high tur idity near ee rugge has een explained with a co ination o salinity and
owards a Mud alance for the rilateral Wadden Sea rea
able .11 verview of the estimates for the SPM flux in the S F from France to the Marsdiep Inlet. Sediment sources are positive, sediment sin s landfill or net deposition are negative. nderlined rows give local transport in transects perpendicular to the coast eastward positive .
able .1 Sediment flux in the S F orth of otterdam
owards a Mud alance for the rilateral Wadden Sea rea
etherlands, olland coast
utch Wadden Sea
– between Nauw’s and Sassi’s estimates therefore seems most and Nauw et al. (2014)’s estimate for the Marsdiep flux
owards a Mud alance for the rilateral Wadden Sea rea
he NS be omes narrow (some 10 m but with ariations o er the seasons) in the North Sea borderin the adden Sea. er hi h on entrations ha e been obser ed in the upper 10 m of the water olumn in the North Sea near the risian nlet where the adden Sea area is relati el narrow ( a. 10 m) and sediments are mudd ( aaphorst et al. 1 ). he transport of parti ulate matter from the North Sea into the adden Sea is assumed to o ur lar el in bottom urrents whi h ha e a landward omponent ( ostma 1 4 isser 1 ). he adden Sea is fillin in with fine sediments resultin from (1) a net residual landward but annuall ar in residual flow dri en b the dominantl westward dire ted winds ( ui sman and idderin hof 200 ) and (2) the low ener onditions in the adden Sea ompared to the North Sea result in hi her SS durin the flood than durin ebb eneratin residual transport from the North Sea to the adden Sea (Nauw et al. 2014). urin more ener eti storm onditions wa e dri en sediment remobilisation ma lead to export of sediment. he ex han e between the North Sea and adden Sea an be simplified as follows
1. he adden Sea is an area of net lon term S M sedimentation (see espe iall se tion .1) trappin part of the mud pro ided b the North Sea. 2. Sediment in the adden Sea is resuspended durin storm onditions. ependin onto the storm hara teristi s and the tide this resuspended mud an be deposited on the tidal marshes ( amps 1 2) be transported to the North Sea or remain in the adden Sea inter and subtidal area. urin storm periods the adden Sea ma therefore fun tion as a temporal sour e of mud for the North Sea.
t is diffi ult to uantif the eastward han e of the sediment flux in the NS based on data i en onstraints in spatial o era e duration and fre uen of obser ations. s an alternati e we start with our best estimate for sediment transport alon the olland oast (10.0 14. million m r – see able .11) and orre t this for sin s and sour es in the eastward dire tion resultin sediment fluxes are erified with existin numeri al model results.
Subtra tin the estimated sedimentation in the ut h adden Sea ( 1. 10 ton r – see se tion .1) and addin mud deli eran e throu h the fsluitdi (0.4 10 ton r) this results in an eastward transport of . 1 .1 10 ton r at the transe t o er S hiermonni oo . ne of the few omprehensi e model studies shows that o er a 210 m lon S N profile at the east side of the barrier island S hiermonni oo the eastward transport of suspended sediment (in both the NS and the ast n lia lume ombined) is in the order of 11. 10 ton r ( ut ar and Mal here 201 ) whi h is omparable in ma nitude to the al ulated transports at Marsdiep.
Wadden Sea of ower Saxony i en the losses due to retrie al and sedimentation in the ms (1 10 ton r) and in the adden Sea area of ower Saxon (1. 10 ton r) the sediment transport towards the eser area an be estimated to be some . 11. 10 ton r of mud ( able .1 ). Near to the ast risian barrier islands sediment transport follows the oast and pro eeds all the wa up to the ade ( i ure .12). More seaward west of el oland the paths han e towards a S N tra e tor ( ut ar and Mal here 201 ).
owards a Mud alance for the rilateral Wadden Sea rea
able .1 verview of the estimates for the SPM flux in the S F from Marsdiep Inlet to enmar . Sediment sources are positive, sediment sin s landfill or net deposition are negative. nderlined rows give local transport in transects perpendicular to the coast eastward positive .
he lbe Weser elgoland triangle
Wadden Sea of Schleswig olstein
owards a Mud alance for the rilateral Wadden Sea rea
Wadden Sea of enmar
Figure .1 esidual suspension transport paths for the scenario Put ar and Malchere , 1 .
otal gross availability of mud
owards a Mud alance for the rilateral Wadden Sea rea
Figure .1 Sea bed upper cm fine sediment micron . athymetry given in blac lines depth in m. istribution determined by combination of grain si e analysis, satellite data during storms and numerical model results obyrnin et al., 1 .
6.5.4 East Anglia Plume
owards a Mud alance for the rilateral Wadden Sea rea
able .1 verview of the estimates of the SPM availability of the ast nglia Plume. otal input in the tidally influenced realm is given irrespective of deposition or dredging data for permanent deposition are added as negatives. nderlined rows give local transport in transects perpendicular to the coast.
owards a Mud alance for the rilateral Wadden Sea rea
6.6 SYNTHESIS: A MUD BALANCE FOR THE WADDEN SEA AND NORTH SEA
able .1 verview of net mud and sand sedimentation yr in terms of million ton yr and million m yr in the three ma or estuaries. Metric tons were recalculated into m and vice versa by using a sediment density of 1 g m for sand and g m for mud, based on the average experiences within the estuaries.
≤0.03 ≥1.9 ≤0.5
owards a Mud alance for the rilateral Wadden Sea rea
Figure .1 isual presentation of the estimated mud budget for the trilateral Wadden Sea. ellow numbers indicate annual mud transport fluxes, white numbers annual deposited mass of mud, and blue number anthropogenic extraction rates all in million ton yr . ocal storage during uiet periods in the near coastal part of the orth Sea could not be uantified.
• • • •
owards a Mud alance for the rilateral Wadden Sea rea
• • • •
able .1 verview of sediment sin s net mud sedimentation per year in the rilateral Wadden Sea.
–
6.7 DISCUSSION: UNCERTAINTIES
owards a Mud alance for the rilateral Wadden Sea rea
• • • • • • •
owards a Mud alance for the rilateral Wadden Sea rea
7 HUMAN INTERVENTIONS
7.1 INTRODUCTION
7.2 POLDERING AND CLOSURE WORKS were reclaimed (‘poldered’)
owards a Mud alance for the rilateral Wadden Sea rea
e a wer ee a d e rad al di i o e idal mar e i ma e e pec ed a le edime a io pace wa a aila le ( ee al o a der pe a d a der al lemmi a d a dwi a d ai a d ar olom ) a d co ce ra io i e adde ea will a e i crea ed ( a are e al )
7.3 DEEPENING AND PORT CONSTRUCTION
7.3.1 Historic developments e m e ar pro ide acce o ree por ( em a e el i l a d mde ) a d a lar e ip ard ( ape r ) e pre e da appro ima e mai e a ce dep (all dep elow local ord a ce le el i c por a d ormal ll i erma ) o e approac c a el o e por are m ( em a e ) m ( el i l) a d m ( mde ) re iri re lar mai e a ce dred i e lower m i er wa deepe ed rom a wa er dep o m below HW to ~8 m below HW between the 1930’s and 1994 (van Maren et al )
e ir re la io o e e er ri er co r e occ rred i e period i order o reac a mi im m dep or ea oi e el wi a dra o le a m a re l e idal wa e co ld pe e ra e almo impeded ill reme e ce r addi io al c a e a d impro eme were made o adap o i crea i ip i e a d o co er e re po e o e e ari e em rai i wor a d wall e ie a d ro e were il o a ili e e co r e a d pro ec e em a me a d oreli e ( o er ) e o e pe e ra io o e ide le el were alli re l i i lower ro d wa er a le p ream o reme e period a weir wa il i emeli e o op i de elopme e e orm r e arrier were il a e mo o e ri arie e e m d c m e wee a d e ower e er wa deepe ed o m c ar da m e period e er e er wa deepe ed o m c ar da m
e earl ce r e l e e ar wa ill rela i e a ral a ime e l e e ar ear am r wa rela i el allow wi a wa er dep o a o m lo o mall i la d ormed a del a clo e o am r i ce e e l e a ee deepe ed ime a d i ow pa a le p o am r or ip wi a dep o m ra ormi e e ar rom a allow m l i c a el em o a i le deep c a el em a or deepe i pro ec were e ec ed i ( rom o m) a d ( m)
7.3.2 Impact on tidal propagation eepe i a d lo o i er idal area (la d reclama io ) i l e ce e idal propa a io ll ial e arie i e ili ri m are el aped eir i e decrea e e po e iall i e la dward direc io i el ape lead o a i crea e i e idal ra e d e o co er e ce o idal e er – ere ore ma e arie are c arac eri ed a i i iall i crea i idal ra e i la dward direc io e idal ra e decrea e ric io rom e ea ed a d wa er ora e i i er idal area ric io o decrea e wi i crea i wa er dep a d ere ore c a el deepe i lead o a i crea e i e idal ra e a d a i crea e i propa a io eloci o e idal wa e o o i er idal area red ce wa er ora e a d ere ore la d reclama io ma lead o idal ampli ica io mo e arie c a el are deepe ed a d i er idal area reclaimed re l i i a i crea e o e idal ra e i ma e arie i e pa o ear e c a e i idal d amic i o e well doc me ed
owards a Mud alance for the rilateral Wadden Sea rea
n heav l n l en ed est ar es n l d n the lbe ms and Weser dal han es n these s stems w ll be s mmar ed n the ollow n se t ons
nt l 19 0 the t dal ran e n the ms est ar and ms r ver n reased rom m at the mo th to 9 m at mden a ter wh h the t dal ran e de reased n the landward d re t on he t dal ran e n the o ter ms est ar ( to mden has not han ed m h s n e then w th the lar est han e be n the n rease rom m to 3 m at el l n the er od 19 0 1980 ee en n o the lower ms ver however has led to stron t dal am l at on oss bl stren thened b the resen e o the we r at Herbr m onstr ted n 1899 ( h ttelaars et al 013 and a os t ve eedba me han sm asso ated w th the m ort o ne ra ned sed ment (W nterwer et al 013 he t dal ran e at a enb r ( m 0 has n reased rom 1 m n 19 0 to 3 m n 010 w th a ma or lower n o the t dal low water level ( rebs and We lbeer 008 nt l 1990 the t dal ran e ea ed at mden (4 m seaward o a enb r be a se the t de was dam ed rther n the est ar d re t on n e then the t de s am l ed stream o mden and w th n 0 ears the t dal ran e at a enb r has be ome 0 lar er than the t dal ran e at mden
e ore dee en n the t dal ran e o the Weser est ar de reased n the landward d re t on rom abo t 3 m on the seaward entran e w th a t dal l m t near remen he t dal ran e n remen n reased rom a ro 0 m s n e the rst dee en n to a resent level o 4 1 m d e to the dee en n o the ower and ter Weser ( h hard and holle 019 owada s the mean t dal ran e n reases rom 9 m at the l htho se lte Weser (a ro m 11 to 3 8 m at remerhaven (a ro m and to 4 1 m at remen slebsha sen (a ro m 8
lso the t dal ran e n the lbe est ar sed to de rease n the landward d re t on rom nearl 3 m at the mo th o the lbe est ar ( haven to 1 m ( ollens e er stream o the ort o Hamb r n 1911 he t dal ran e at the mo th rema ned a rl onstant n t me n reas n a ro matel 30 m between 1911 and 00 (W nterwer et al 013 n between the mo th and the ort o Hamb r (stat ons ollmar and h la the t dal ran e n reased w th 30 and 0 m (res e t vel between 19 and 19 8 ollow n the dee en n s o 19 and 19 he t dal ran e at Hamb r n reased w th 1 m and s n e 19 1 the h hest t dal ran e s observed at the ort o Hamb r (nowada s 3 m he t dal ran e o the nner lbe ( ollens e er has been stead l n reas n e eed n m n 199 ( eme er 1998 he n reas n ran e n the nner lbe does not d s la a lear orrelat on n t me w th h man ntervent ons ( n ontrast to ntervent ons n the o ter est ar where dee en n d re tl res lted n an n rease n t dal ran e
What these three s stems have n ommon s that the r t dal ran e de reased n the landward d re t on be ore dee en n d e to r t on and ener loss o the t dal wave b t sw t hed to a landward n rease as a res lt o hannel dee en n t s also noteworth that han es n the t dal ran e o the o ter est ar show abr t han es related to dee en n whereas the t des n the nner est ar es res ond m h more rad all
9 owards a Mud alance for the rilateral Wadden Sea rea
7.3.3 Impact on estuarine circulation se ond mod at on o the h drod nam s s related to a sal n t dr ven r lat on n e t on 4 1 4 we e la ned that sal n t dr ven r lat on (or rav tat onal r lat on n reases non l nearl w th the water de th n water de ths o several meters ( r st ne ond t ons n man nat ral est ar es s h a sal n t dr ven r lat on s not m ortant t water de ths e eed n 10 m a rono n ed sal n t dr ven r lat on o ten develo s n the trans t on one o salt and resh water t water de ths e eed n 0 m t o ten be omes a dom nant me han sm or sed ment trans ort
he han e n sal n t dr ven r lat on res lt n rom a mod ed water de th an onl be establ shed thro h model based h nd asts nl or the ms est ar s h a h nd ast has been done (van Maren et al 01 a and onl or a a rl small han e n water de th n both the lbe ms and Weser sal n t dr ven lows are rrentl m ortant or sed ment trans ort (see also ne t se t on t an there ore be ass med that dee en n res lted n (enhan ed sal n t dr ven r lat ons that be ame m ortant d e to hannel dee en n
7.3.4 Impact on sediment dynamics The sediment concentration increases from several 10’s of mg/l in the outer estuary to 10’s o l n the lower ms ver he o ter ms est ar has be ome more t rb d n the ast de ades (de on e et al 014 st mates or th s n rease are t all to a a tor h her (at least n the ollard van Maren et al 01 a b t oss bl also n w der se t ons o the o ter est ar (de on e et al 014 h s n rease was robabl the res lt o a loss o sed ment s n s (van Maren et al 01 a 01 and ntera t on w th the lower ms ver (de on e et al 013 ee en n o the a rwa s and ma ntenan e dred n robabl ontr b ted less to the n rease n t rb d t he h h on entrat on n the o ter est ar are the res lt o t dal m n b t also o sal n t dr ven rav tat onal r lat on (van Maren et al 01 a he sed ment on entrat on n the lower ms ver has n reased m h more resentl be n several orders of magnitude larger than in the 1950’s (de Jonge et al., 2014). h s n rease most l el res lts rom dee en n o the r ver hannel res lt n n a eedba me han sm w th dee en n lead n to de ormat on o the t des n t rn lead n to more sed ment m ort wh h rther de orms t des and there ore stren thens sed ment m ort (W nterwer et al 013 – see also re 1 e ormat on o the t des s n the orm o am l at on and han e n as mmetr (towards a more lood dom nant t de w th van Maren et al ( 01 b stress n the m ortan e o the n rease n am l at on and stra ( 019 the role o han n t dal as mmetr
The Elbe estuary has two ETM’s (estuarine turbidity maxima): one at the tip of the salt wedge ( 0 m land nward and one rther landward n the t dal reshwater one o the lbe ( a enber et al 199 r hard et al 004 tanev et al 019 he o ter M s the res lt o sal n t nd ed res d al lows the nner M rom t dal m n ver nterann al t mes ales the os t on o the M n the lbe est ar de ends on the resh water d s har e ( a enber and rabemann 001 he o ter est ar o the lbe s dom nantl de os t onal wh le the nner est ar s eros onal res lt n rom t dal m n ( et al 014 e os t on r mar l ta es la e on the t dal lats whereas eros on dom nates n t dal hannels he sed ment on entrat on n the lbe has n reased n the er od 1999 00 relat ve to 199
9 owards a Mud alance for the rilateral Wadden Sea rea
199 around the port of amburg (20 ) but even more in the freshwater part landward of the port (120 ) ( erner, 200 ). There are no specific studies relating the change in sediment concentration to detailed interventions as established for the Ems estuary. interwerp et al. (201 ) argue that changes in around the port of amburg are uite limited because of the dredging strategies around the port of amburg (a large amount of sediment is dredged and disposed on land or at sea).
Figure .1 raphical presentation of the conceptual model of Winterwerp et al. 1 relating tidal deformation to channel deepening. he initial increase in tidal range as a result of deepening leads to more sediment import, hence higher sediment concentrations and a reduction in the apparent hydraulic drag, which in turn further strengthens tidal deformation.
The formation of the ETM in the eser estuary is also the combined result of estuarine circulation and asymmetries in mixing. Mainly fine sediments deposit in the main channel, often as a fluid mud, at a location which varies seasonally ( esse et al., 2019). There is no information on historic changes in the sediment concentration in the eser estuary, or on the impact of human interventions on . owever, as for the Elbe estuary, the location of the ETM varies over years resulting from variability in fresh water discharge ( appenberg and rabemann, 2001).
or all three estuaries, the location with high turbidity (the ETM) is formed by landward transport by tidal pumping and by salinity driven circulation, balanced by seaward transport governed by freshwater flow. Especially the ETM of the Elbe and eser is susceptible to the river discharge. In the Elbe two ETM’s exist, one formed by salinity driven flows and one by tidal pumping. or all three estuaries the sediment concentrations have increased during a period in which intertidal areas were reclaimed and channels were deepened. The increase may have been partly mitigated by sediment extraction as a maintenance dredging strategy. owever, only for the Ems estuary historic hindcasts have been carried out revealing how human interventions lead to higher suspended sediment concentrations. More research is needed to relate mechanisms responsible for the increase in sediment concentrations in the Elbe and eser to the various human interventions.
9 owards a Mud alance for the rilateral Wadden Sea rea
7.3.5 Maintenance dredging is osal of sediments rimarily leads to a redistrib tion of sediment, rather than an increase in s s ended sediment concentrations van Maren et al., a , beca se the est arine sediment concentrations effectively decrease when sediment settles in the orts or fairways. bse ent dis osal restores the sediment concentration to a level com arable with a sit ation witho t settling in orts and fairways, at least on large time and s atial scales. lose to dis osal sites, the sediment concentrations increase. n to of this as a secondary effect maintenance dredging can be considered as an e isodic stirring mechanism, leading to tem orally higher s s ended sediment concentrations com ared to a sit ation witho t dredging i.e. com arable to a storm event . veraged over time, this effect constit tes an increase in the s s ended sediment concentration.
igh sediment concentrations negatively im act biota thro gh loss of visibility es ecially elagic rimary rod ction or b rial smothering es ecially im acting benthic organisms b t also vegetation . redging therefore negatively im acts ecology close to dis osal sites, where a m d layer is de osited on the bed and the t rbidity tem orally increased. Its im act over larger s atial scales remains n nown. There are indications, for instance, that the sediment concentrations aro nd the ort of amb rg Elbe are relatively low beca se of the dredging wor s carried o t by the ort of amb rg interwer et al., . or the Ems est ary the large scale effect of dredging and dis osal on t rbidity are robably limited van Maren et al., a – the observed grad al increase in t rbidity negatively im acting rimary rod ction is robably not directly related to other factors.
7.4 LANDFILLS AND SEDIMENT EXTRACTION andfills im ly a retrieval of sediment from the est ary and sho ld be considered an artificial sin in which sediment is ermanently extracted from the adden ea system. If vol mes which are th s stored are significant, the infl ence on the s s ended m d concentrations may be noticeable.
tarting in the late th cent ry landfills with m d have ta en a flight in the th cent ry as harbo rs were extended and fairways were dee ened. irstly, sediment had to be dredged to dee en the areas. econdly, maintenance dredging was needed to ee the de ths. This is es ecially tr e for the three big est aries of the Ems, eser and Elbe, each showing their own develo ments. n estimated m of m d has been stored on land along the Ems in the ast years million m yr between and and since and . million m yr between and see van Maren et al., and references therein . This is com arable in magnit de to the total net sedimentation of sand and m d in the western art of the tch adden ea area in the same eriod. or the eser and Elbe these fig res are not well nown.
owards a Mud alance for the rilateral Wadden Sea rea
Figure . he iederung den iepe is being filled via a 1 m long pipe transporting the dredging sludge over the ms ade channel period 19 1989 . Source reative ommons ttribution . International icense Photo 1 9 . pg
century. Already in the 1930’s substantial amounts of mud were stored in landfills along
Figure . omparison of volumes of retrieved mud as part of total sedimentation left and total maximum influx of suspended matter right excluding supply by the ast nglia Plume, since this is most li ely largely insignificant
owards a Mud alance for the rilateral Wadden Sea rea
igure .3 shows the current significance of the retrie al of mud from the system between 11 and 1 of the total de ending on the definition . tracting mud lowers the sus ended sediment concentration of the ad acent estuary igure . – this was numerically in estigated for the ms estuary by an aren et al. 01 . traction may also ha e a downdrift im act de ending on the scale of e traction otentially influencing a larger art of the adden ea ast of the e traction as less mud is a ailable for natural sedimentation in the downdrift area of the sediment e traction. Any im act of large scale e traction may remain unnoticed for a rolonged eriod because the sediment stoc within the adden ea is large. At the moment it is unclear whether such e traction influences the sedimentary and ecological de elo ment of the adden ea area more to the east of the estuary and what associated res onse timescales are.
Figure . ffect of sediment extraction from all ports and fairways on SS in the ms stuary. From van Maren et al. 1 .
he results of this research indicate that the annual su ly of mud which does not all settle in the adden ea is only 1.1 to 1. times larger than the annual de osition including e traction. his would im ly that with an increase in e traction or mud de osition rates mud a ailability may become limiting on the scale of the trilateral adden ea in the longer run. er a shorter eriod the influence would robably be limited because the gross influ is at least 1 orders of magnitude larger than the net annual flu es. n nown is howe er what the cumulati e effects of multi le e traction sites will be es ecially in combination with the antici ated increase in mud de osition rates resulting from . or instance it might be ossible that the e tent of the mud areas would gradually diminish as the mud is trans orted to those laces where it can still be de osited. otential shortages in mud would become manifest in the more north eastern arts of the trilateral adden ea not the western adden ea. ud may therefore become a scarce commodity which should become an integral art of sustainable long term sediment management. easures aiming at using e tracting mud should account for its im act on the large scale sediment budget on the le el of the trilateral adden ea. ontinuous long term obser ations of sus ended sediments and
100 owards a Mud alance for the rilateral Wadden Sea rea
7.5 MANAGED REALIGNMENT AND DEPOLDERING
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Figure . ealignment pro ects and cumulative area in the estuaries ms Weser lbe and the Wadden Sea outside the estuaries data mainly sselin et al., 1 , P , 1 and several other sources .
• • •
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• • •
Figure . rtist impression of alternative ways to heighten the low lying land of ast roningen by se uential flooding of large areas. “Rijzend Land” of Bureau Lamaland, see http www.lamaland.eu pro ects ri end land
7.6 FORELAND MARSH DEVELOPMENT
o ard a ud Balan e for e rila eral adden ea rea
called “farmer’s method”). Since the 20 called “Schleswig method”) to establish foreland salt m
i ure ema i o er ie of an e in e ar ifi ial draina e em af er e radi ional main enan e a di on inued e am le from i e in e e erland and erman ar in i ua ion B en ral ar of e di e or field drain a e il ed u ream draina e i lo ed do n ream drain dee en and de rea e in id e re ul in oil a erlo in au e lo al re la emen of e e a ion e ondar ioneer e e a ion and lo mar e e a ion elin e al modified from an ee en ee e al
o ard a ud Balan e for e rila eral adden ea rea
8 SEA-LEVEL RISE ne of the research estions was “ a a een e im a of lon erm ea le el ri e on e u l and de o i ion of mud and o ill lima e an e and a elera ed ea le el ri e al er e e ro e e o our o inion ” ere we shortl disc ss this estion b com aring the ast resent and f t re sit ations.
8.1 PAST MUD SEDIMENTATION large art of the res onse to long term sea le el rise in the olocene has alread been disc ssed in ha ter 2. ere we will foc s on the changes in sedimentation rates thro gh time in the adden Sea. efore h mans started to infl ence the de elo ment of the mainland coastal one a large art of the adden Sea area was a egetated tidal landsca e intermittentl flooded d ring high tides or storms. idal marshes and tidal m dflats were to se eral tens of ilometres wide ig re . ). ntil di es were constr cted on a large scale d ring the iddle ges these tidal marshes formed abo t the half of the adden Sea eise 200 ).
d bro ght in d ring high water conditions robabl largel de osited on the e tensi e tidal areas ro iding an e tensi e sin for fine sediments. lemming and andwi ) arg e that the original nat ral m d gradient stretched o er a larger distance than at resent ig re .2) res lting in an o erall coarsening of the bed of the adden Sea.
i ure Re on ru ion of e a and e ri ian adden ea land a e around ran e i er lei o ene round ro n ea dar reen al mar li reen in er idal and and mudfla lue reen i al mar le ee and rid e ello dune and ea rid e o and nol
0
hen alle s or eat areas were flooded the eat was eroded or com acted and strong sedimentation of m d occ rred ig re . ). fter the initial inc rsion of the sea the area silted and became land once again. he ra id infill of emba ments s ch as the ollard the iddel ee and the arleba oint to a er large a ailabilit of m d. or the olocene de osits in the tch adden Sea it has been calc lated that in total some 0 m of m d was de osited an der S e and eets ). f it is ass med that the ma or art has been de osited d ring the ast 000 ears corres onding to an a erage de osition ol me of some 2. 0 m r for the tch adden Sea area to roningen. his is abo t twice as high as resent da al es. he e lanation is most li el two fold
1) he adden Sea area was m ch larger with incised alle s and emba ments which ro ided large areas with er iet conditions for m ds to be de osited. 2) he s l of m d might ha e been higher. orests were cleared b h mans leading to stronger erosion and the sea bottom was still reacting on the ra id flooding after the last ce ge locall eroding the bottom. lso coastal retreat has occ rred o er the ast 000 ears in the est and ast risian adden area and o er the ast 000 ears along the olland coast and arts of the orth risian adden area.
0
8.2 PRESENT-DAY MUD SEDIMENTATION: EFFECT OF TIDAL AMPLITUDE AND STORMS or resent da conditions the nat ral landward net sediment fl allows for the tidal flats to ee ace with sea le el rise. en more enninghoff and inter 20 ) concl ded that most of the erman adden Sea basins were accreting o er the eriod – 20 with rates higher than the obser ed mean sea le el rise in the erman ight while sim ltaneo sl the s btidal mean de th increases. or the eriod of in estigation this adden Sea stee ening is antified to a eraged al es of . mm r for the tidal flats and −20.0 mm/yr for the channels.
Sedimentation rates increase with increasing tidal am lit de ig re . ). or the mesotidal flat areas behind barriers the linear fit of the ositi e correlation between the mean tidal range and sedimentation rates has an 2 of 0. ig re . ). his s ggests that tidal am lit de ma infl ence the rate of sedimentation on a tidal flat as also s ggested b enninghoff and inter 20 ). owe er for the est aries and ade a obser ations are less clear. ith increasing tidal am lit de the elocit of the water increases and brings larger amo nts of sediments in s s ension which are trans orted from the channels towards the shoals. bo e the shoals the mean settling distance with res ect to is rather constant ofstede 20 ) res lting in faster sedimentation rates with increasing tidal range.
0
However, it’s most likely that waves have only played a minor
8.3 FUTURE MUD SEDIMENTATION limate han e may ompli ate thin s rther he potential rowth o tidal lats with sea level rise depends on the availa ility o sediments as ildin material n order to assess the t re development o the adden ea it is espe ially important to o tain realisti estimates or sediment availa ility and d ets wo e e ts are o prime importan e, as predi tions or wave and storm limate s est only sli ht han es with lar e n ertainties
lready in the past ent ry pre ipitation and evaporation patterns have een han in verall, r n o o rivers in reased However, d rin the s mmer period reshwater r n o is red ed stron ly d e to the in reasin re en y o e tremely hot and dry periods y ontrast, d rin a t mn winter pre ipitation in reased leadin to stron r n o into the adden ea system t remains nknown whether this leads to an in rease in s spended matter in l he in reased o t low o resh water will enhan e the est arine ir lation in the est aries and alon the orth ea oast leadin to a reater e i ien y to trap m d his mi ht in rease the on entrations o m d in these waters and the availa ility o m d or the system
lo al sea levels have een risin with some mm yr d rin the irst hal o the past ent ry in e lo al sea level rise is a eleratin resently, it is some mm yr i re an endor et al , n the adden ea sea level rise is slower, most likely d e to re ional eolo i al, la io isostati and meteorolo i al e e ts t is e pe ted that lo al sea level rise will ontin e or ent ries to ome and it is oreseea le that also in the adden ea, in the near t re sea level rise may pro eed at a i ker pa e
he present day knowled e o the morpholo i al response o tidal asins to sea level rise is ins i ient to on idently estimate the role o the sea level rise on the m d d et n m er o s enarios may e orm lated related to the response o sand and m d deposition rates nder a elerated
he present day adden ea asins keep pa e with sea level rise, in l din the hori ontal distri tion o sand and m d he re ired ann al m d mass is then the prod t o the areal e tent o the adden ea, the present day m d ontent, and the rate o his is raphi ally displayed or the t h and erman adden ea in i re , allowin assessment at whi h rate o m d availa ility e omes limitin he rrent m d s pply is – million ton yr, o whi h million ton is ann ally e tra ted he remainin – million ton ompletely ontri te to verti al a retion to ompensate sea level rise or rates etween and mm yr his is in etween pro e tions o mm yr and mm yr or the adden ea ermeersen et al , he s enario ass mes that H emissions de line a ter whereas the s enario represents a ontin ed rise o H emissions thro ho t the st ent ry n reality, a sealevel rise in etween s enario and is most likely his then implies that the availa ility o m d is ins i ient to keep pa e with , ass min that the the spatial m d distri tion remains as it is today or , already million ton yr o m d deposits is re ired in the t h adden ea in order to keep pa e with ea level ise y that time e tra tion rates remain the same, only – million ton o sediment remains availa le to ann ally deposit with demandin million ton to keep pa e with nder these onditions, m d will e ome a limitin a tor, espe ially in the eastern adden ea he system will either drown or e ome more sandy dependin on the availa ility o sand, whi h is not ela orated on here
ote that in the lon term, the s pply o m d o ld possi ly han e, e e a se o sea level rise, han es in wind or wave limate, han es in reshwater dis har e, h man interventions s h as oastline han es and sand minin , temperat re in rease res ltin in di erent m d omposition and settlin velo ity et till it remains nknown how these pro esses and onditions wo ld han e m d s pply his is the more important, sin e we see that s pply mi ht e ome limitin in the t re
–
9 MODELLING PRACTICES: FOCUSING ON MUD
9.1 PROCESS-BASED MODELS
9.1.1 Morphological models also indicated as “complex” and “quasi–realistic” in the litera
underl in ph sical processes and mechanisms or e the o ser ed morpholo ical and sedimentolo ical de elopments t is important to reali e that e en process ased models are not realit ut aim to represent realit as closel as possi le dependin on the model purpose his representation o realit is as ood as the underl in equations and model deplo ment ome o these equations are ell studied and ell no n e h drod namics some contain a considera le mar in o error and un no ns e sediment transport
e part o the sediment transport model ormulation is the ed oundar condition or the ad ection di usion equation or sediment concentration his part o the model ormulation is also essentiall di erent or mud than or sand or sand the equili rium concentration or equili rium concentration radient in the ertical direction calculated rom a sediment transport ormula is prescri ed or mud the erosion and deposition luxes are calculated usin the rone artheniades t pe ormulation n the classical a o usin this ormulation i ith critical ed shear stress or deposition smaller than that or erosion there is an essential di erence ith the ormulation or sand or sand an equili rium concentration can e calculated as a continuous unction o the lo condition characteri ed e the ed shear stress and or mud it is not the case o e er inter erp and an esteren recommend settin the critical ed shear stress or deposition in initel lar e i e let deposition al a s ta e place remo in this di erence et een the ormulations or sand and or mud
urthermore mud transport is more complicated than sand transport as sediment parameters such as settlin elocit and critical shear stress or erosion are not directl lin ed to particle si e ut ma chan e in time due to processes o locculation o suspended sediment in luencin settlin elocit and consolidation o ed sediment in luencin erosion properties lso sand transport is steered mainl actual and local h drod namic conditions determinin transport capacit and radients herein hereas mud transport is more steered h drod namic conditions in a much ider area and ith important memor e ects picall mud transport is dri en suppl sin s and sources rather than transport capacit as or sand and mud interaction ma es the sediment transport modellin e en more di icult in areas li e the adden ea
orpholo ical models such as el t ha e reached a sta e that the can e used to in esti ate h dro and morphod namics and reatl impro e our undamental understandin o the processes dri in sediment transport see lias and an der pe esser an der e en lias and ansen an der e en illustrated that lon term centuries morphod namic simulations are capa le o reproducin concepts and equili rium relationships ased on measurements and la orator experiments similar indin s ere presented in the studies o i ma et al a arciano et al ast hei et al issana a e et al a esser demonstrated throu h a reement et een modelled and measured morphod namic eha iour o illapa a that a process ased numerical model could reproduce the most important ph sical processes in the coastal one o er medium term ear timescales lso recent modellin o morpholo ical chan es o the andmotor me a nourishment alon the olland coast illustrates that the el t model can capture morphod namic de elopments on a
decadal scale ui endi et al uisman et al tudies that aimed to predict the morphod namics o the adden ea inlets e meland nlet ere less success ul e oc ert es e lias et al an et al ne o the important lessons learned rom the a o e models is that these are er ell capa le to predict the morphod namic e olution a ter lar e scale distortion o the s stem e construction o the andmotor ut cannot easil predict the smaller scale natural e olution o inlets unless a undant ield data and model de elopment are a aila le esser pplications o process ased models or simulatin impact o to the adden ea tane et al issana a e et al o stede et al echerer et al ha e onl demonstrated limited success
9.1.2 Turbidity models ur idit models ha e other requirements or h drod namic model cali ration than morpholo ical models ext to the proper reproduction o pea s in lood and e elocities and their as mmetr and pea s in current and a e induced ed shear stress also residual currents and the residence time o ater masses are important hese are in luenced tidal as mmetr ut also ind speed and direction and densit radients induced salinit temperature or the sediment concentration itsel i su icientl hi h o reproduce densit radients a approach is adopted de ault or suspended sediment modellin hereas or morpholo ical models o ten still a approach is applica le rom h drod namic models usin the tur ulence model also the ertical mixin intensit is no n rom hich in com ination ith the particle settlin elocit the ertical suspended sediment concentration pro iles can e computed and hence near ed and near sur ace residual uspended articulate atter transport
idderin ho as one o the irst to stud residual currents in the estern adden ea usin a numerical model er since models ha e een de eloped ith increasin resolution and complexit in orcin and processes hereas ocus as initiall most on tidal transport more and more also the e ect o densit radients on transport and mixin as studied lso the anal sis extended rom a era e orcin conditions to aria le conditions ith re ard to ater le el set up and set do n ind speed and direction a e hei ht resh ater dischar e includin rain and e aporation and heat alance ota l in shallo areas such as tidal di ides residual transport is steered more ind and a e induced transport than tidal transport
he pro ect pro ides an excellent tri lateral o er ie o recent ad ances in h drod namic and suspended sediment modellin in the adden ea urchard et al or the estern adden ea uran atute et al discuss the residual circulation and resh ater transport ased on the numerical model stud hi hli htin the importance o ind dri en circulation his model is a aila le oth in and el t ased on this h drod namic model a suspended sediment transport model as de eloped as reported assi et al he e aluate the sensiti it o luxes on tidal and meteorolo ical orcin n important outcome o their or is that the residual transport is not onl sensiti e to meteorolo and eometr ut also to su tle ariations in sediment parameters used as model input settlin elocit erodi ilit his dependence on o ten limitedl no n input
parameters introduces an uncertaint associated ith the predicta ilit o net luxes ater this model as extended in el t ith a second ed la er o consolidated mud his model introduces more realistic time la s and dela ed response o the s stem an essel and an uren et al ut does not reduces the dependenc o residual luxes on model input parameters ecentl this model as con erted to and its resolution as urther increased room et al or the ms ollard area a model is a aila le ased on the same philosoph an aren et al a and n unstructured suspended sediment transport model or the erman i ht ith hi h resolution in the three main estuaries as recentl de eloped tane et al usin the model
he adden ea ide models are a le to reproduce the main eatures o o ser ed suspended sediment d namics such as a era e concentration le els spatial radients tidal and seasonal ariations and storm pea s o e er the oodness o it or a direct comparison ith lon timeseries o hi h requenc o ser ations e ith is o ten open or urther impro ement e an aren et al his indicates that some processes are still lac in or not ell descri ed in these models ut also o ser ations on suspended sediment concentrations ha e inherent uncertainties ett eis et al part rom the three adden ea ide models de eloped in el t and a lar e num er o small scale models ha e een de eloped in the past ears or a speci ic part o the adden ea hich o ten attain a etter data model comparison than the lar e scale models hese models are o ten pro ect and question speci ic and the are onl operational or a limited amount o time
ot ithstandin their limitations these models ha e the i ad anta e o er ield o ser ations that it is eas to deri e a mud alance rom them as the are s noptic lso it is eas to s itch some processes on or o and in esti ate the sensiti it to ariations in h dro meteo orcin i e to identi the main dri ers or mud import and export he results assi et al an essel and room et al su est that the interannual aria ilit in mud d namics and transport in the utch adden ea is lar e here ore a era in o er a multi ear period is required to o tain a mean mud alance ma or shortcomin o complex numerical models is that net sediment transport in s stems ith small residual sediment luxes relati e to the ross tidal sediment luxes is sensiti e to model input meteorolo eometr and sediment settin s his limits the predicta ilit o the residual transport requirin cali ration a ainst a re ated parameters such as net transport uch o ser ations are ho e er rarel a aila le and onl limitedl accurate
9.2 AGGREGATED MODELS re ated models or semi empirical models are also no n as eha iour oriented models hese models ma e explicit use o empirical relationships to de ine a morpholo ical equili rium n important assumption is that the morpholo ical s stem a ter a distur ance throu h natural e olution or human inter erence al a s tends to de elop to a state satis in the empirical equili rium relationships he model ti e et al ti e and an o nend et al a hich is used as an important tool or determinin the e ects o inter entions in the adden ea is a t pical example o this t pe
o model he model uses a schematisation in hich a tidal inlet s stem is di ided into the main morpholo ical elements e tidal delta channels in the asin and intertidal shoals and lats hese elements exchan e sediment ith each other and ith the ad acent coast to de elop a morpholo ical equili rium as de ined the empirical relationships he model is eas to handle and simulates lon term de elopments his ma es it suita le to stud the e ects o an oor et al and lar e scale human inter entions ra t i et al on the morpholo o the adden ea ince these models are dependin on the asic assumption concernin morpholo ical equili rium su icient historical data or the cali ration and alidation is essential
ecentl o nend et al a demonstrated that the di erence et een and a process ased model such as el t is in the le el o spatial and temporal a re ation rather than in the extent o empiricism he model is ased on the same principles as a process ased model in the case that the suspended sediment transport is dominant oth models tr to represent the same ph sical processes ut at di erent le els o a re ation he di erence et een the t o models is in the ormulations or the exchan e et een the ottom sediment and the ater column n el t this is arran ed ia the ottom oundar condition in mode or a ormulation deri ed rom an as mptotic solution o the ad ection di usion equation alappatti and reu denhil an in mode n oth cases the local equili rium concentration or sediment needs to e calculated his requires a sediment transport ormula that relates the transport capacit to the stren th o the lo and the properties o sediment sediment transport ormula is o ten deri ed considerin the rele ant ph sical processes ut it al a s contains parameter s to e cali rated ith o ser ations rom the ield and or la orator n that sense a sediment transport ormula is thus empirical ne must also consider the uncertainties associated ith the application o such a ormula his is sho n the act that there are not one ut man sediment transport ormulas a aila le n indication o the uncertaint is i en an i n a ho su ested that sediment transport estimates and measurements can di er easil a actor o n a sin le ormulation is used or the exchan e et een the ottom and the ater column or each lar e morpholo ical element e the hole e tidal delta his is ased on morpholo ical equili rium relationships o the elements and a re ated h drod namic parameters re lectin the a re ation in time and space hese relationships are li e the sediment transport ormula ased on ph sical considerations to determine rele ant h drod namic parameters and morpholo ical relationships and o ser ations rom the ield to cali rate the parameters in the relationship he local equili rium concentration is related to the ratio et een the equili rium olume and the actual olume he empirical relationships or the morpholo ical equili rium also contain uncertainties ust li e the empirical aspects o a sediment transport ormula in el t
he existin models onl consider a sin le raction sediment o e er recent research an and an der pe an et al made clear that sand as ell as mud are important or the adden ea in its response to sea le el rise and that the models produce ood results onl ecause the parameter settin s in the models mimic sand mud transport ith a sin le raction ormulation xtension o the models ith a multi raction sand mud transport module is needed in order to ha e the parameters in the model clearl
related ith the parameters in the process ased models complicatin actor is that or sand transport ma e assumed to e close to equili rium i e the transport capacit o the lo ut or mud transport is t picall limited suppl o a ed module eepin trac o the ed composition and hence the a aila ilit o mud is essential or mud models hereas or sin le raction sand models a transport ormula to compute the equili rium concentration su ices
9.3 IDEALISED MODELS n idealised model is a process ased model that ma es use o simpli ied ph sical and mathematical descriptions to anal se the eha iour o a morphod namic s stem he di erence ith the complex models is that the do not pursue ull description o eometr orcin and ph sical processes ut tr to reduce these to essential principles dealised models themsel es ha e reat aria ilit in complexit as ell see also the re ie in e art and immerman n example o a simple idealised model is the conceptual model o ostma on land ard sediment transport in the adden ea ore recent and more complex idealised models de eloped or the adden ea include the schematised model o an rooi en and an and those de eloped oos et al and ee et al
dealised models can e used to stud speci ic phenomena such as re ime shi t in estuaries recent example o an idealised model is lo de eloped i stra uildin on the models de eloped hernets et al and chuttelaars et al uch ideali ed models ha e een used to understand the transition to hi h tur idit in the ms stuar inter erp and an inter erp et al i stra model de eloped or in esti atin the response o mud lats to seale el rise is ud lats lmilad et al
10 DISCUSSION AND CONCLUSIONS
10.1 DISCUSSION OF THE RESEARCH QUESTIONS n this section the research questions ormulated in hapter ill e addressed ased on the results ormulated in hapters to
he main source or inor anic mud in lux in the trilateral adden ea is sediment rom the o er traits hich is transported alon the rench el ian and olland coasts the orth ea ontinental lo he estimates sho that ia the some ton r o mud is transported to ards and alon the adden ea area up to the eser l e el oland trian le ther sources are local ri ers hich suppl an additional ton r o mud to the adden ea area and its estuaries rimar production and eolian dust deli er an additional ton r he resultin total mud suppl is estimated at to ton r he a era e decadal deposition and extraction o mud i e excludin sand ran es et een and ton r hich means that presentl the suppl is times lar er than demand extraction and deposition ote that ross import rates are orders o ma nitude lar er than the net import i e the demand ut it is the net import that contri utes to the mud ud et it is assumed that suspended sediment transport ia the stops in the eser l e el oland trian le perhaps leadin to the ormation o the el oland mud ield then net deposition is ton r the amount o mud settlin in the orth risian adden ea impl in suppl is times lar er than demand
n case o a temporal mud shorta e local mud stoc s in the orth ea and in the adden ea could ma e up the di erence et een the suppl and the demand temporaril closin the im alance o e er this ould not hold or the lon term alance since these mud stoc s are limited
ore suspended matter mi ht e deli ered ia the ast n lia lume ut the ate o sediment rom this sediment source is poorl no n here are indications that at least some ton r o the ma reach the especiall in the orth risian area e ic et al he is thou ht to e responsi le or stron sedimentation in deeper ater leadin to mudd o shore areas art o the mud deposited o shore ma e intermittentl released durin storms and transported to the re ion
t is important to note that this is the irst time that the mud alance has een estimated or the rilateral adden ea and that the results should there ore e interpreted as such
oth in the adden ea and alon the orth ea coast the ariations o er a tidal c cle a neap sprin c cle the annual ind in luence and interannual ariations in storminess all lead to mar ed chan es in the a aila ilit o suspended matter n eneral sediment is transported rom the orth ea into the adden ea idal currents transport mud to the intertidal areas dri en hori ontal and temporal tidal as mmetries and la e ects ome o this material ma e exported durin storm e ents edistri ution ithin the adden ea and transport et een tidal asins is stron l o erned ind dri en lo s durin storm conditions oth a es and ind dri en currents remo ili e ine sediments i ure deposited durin lo ener conditions and transport sediment in the east ard direction n addition densit radients and ertical mixin can stron l in luence mud transport hich is especiall the case in the estuaries
n eneral the natural ran e o suspended mud concentration ma ar due to the a o e descri ed ariations o er to orders o ma nitude n a era e there is a narro one o hi her mud concentrations alon the coasts rom olland up to ins eroo e the easternmost arrier island o the adden ea o o er axon rom the eser up to lt this transport pattern is not present rom lt on ards it is restored e ertheless on a era e concentrations are er lo o the order o summer period m l inter period nto the adden ea area the concentrations o suspended matter increase in the channels ith increasin distance rom the inlet ostma an rooi en and an o e the tidal lats e see a similar pattern
ithin the eser and l e estuaries there is clearl de ined tur idit maximum one present here a com ination o estuarine circulation and tidal pumpin enerates hi h tur idit up to se eral m l Concentrations may be as high as 10’s of g/l in the hyperturbid lower Ems i er
or the second part o the question e re er to uestion
he t o inter entions that ha e most stron l impacted the sediment concentration are land reclamation and channel deepenin
eclamation of naturally accreting land the e tensi e pristine intertidal and supratidal areas south of the adden ea introduced a ma or loss in sediment sin s ediment transported from west to east is no longer able to settle resulting in an increase in the suspended sediment concentrations an aren et al 01 hus with the closure of large basins such as the uider ee and the auwers ee and the gradual di ing of the tidal marshes it may be e pected that less sedimentation space was a ailable see also an der pe and an der al 1 lemming and yandwi 1 and ai and artholom 000 his would result in a higher concentration of fines in the water column eclamations also influence the tidal dynamics especially in the estuaries arrowing of the estuaries increases the amplification and flood dominance of the tides contributing to landward transport of sand and mud
eepening of tidal channels for na igation purposes also increases the range and flood dominance of the tides contributing to landward sediment transport n an area with hori ontal salinity gradients primarily the estuaries deepening will also strengthen a salinity dri en residual circulation cell promoting near bed landward sediment transport or all three ma or estuaries in the trilateral adden ea the combined effect of deepening has led to a substantial increase in suspended sediment concentrations with the Ems ri er as the most e treme e ample resulting in a 100 fold increase in sediment concentrations
arge scale managed realignment may reduce the mud related problems in the adden ea ee also the answer to uestion
he a ailable E measurements as well as the obser ations in other parts of the world ha e shown that there is a strong bi modal distribution of the mud content of bed sediments erman et al 01 Colina lonso et al in prep with a mode at a low percentage of mud and a mode at a relati ely high percentage his suggests that sand rich and mud rich sediment mi tures beha e ery differently and therefore also respond differently to e ternal changes e g human inter entions sea le el rise he locations where mud settles are characterised by low energy hydrodynamic conditions and are in general persistent dditionally the presence of mud itself also promotes mud sedimentation Colina lonso et al in prep for different reasons hese reasons may be physical related to bed roughness and erodibility of the mi tures and/or biological stic iness of benthic diatoms growing o er mud beds ud sedimentation is important in abandoned channels at mudflats on distance of 10 m from the inlets in tidal marshes and in uiet embayments and in the estuaries around the turbidity ma imum and in fairways and harbours with o erdepth
1 1
he most widely applied sediment management techni ue is dredging redged sediment is disposed in within the system disposed offshore or brought on land ffshore disposal or bringing sediment on land can be considered e traction of suspended sediments
isposal of sediments primarily leads to a redistribution of sediment rather than an increase in suspended sediment concentrations an aren et al 01 a because the estuarine sediment concentrations effecti ely decrease when sediment settles in the ports or fairways ubse uent disposal restores the sediment concentration to a le el comparable with a situation without settling in ports and fairways at least on large time and spatial scales Close to disposal sites the sediment concentrations increase n top of this dredging can be considered a permanent stirring mechanism leading to higher suspended sediment concentrations compared to a situation without dredging i e comparable to an estuary with or without permanent wa e induced resuspension f dredged material is released close to the dredging location important return flows may occur enhancing local sediment concentrations substantially
andfill or deposition far outside the adden area of dredged sediments will result in an artificial e traction of sediment from the estuaries and a decrease of the concentrations in the estuaries f the dredged sediments are hea ily contaminated as is the case near amburg Elbe and in the eser then there is no other option than to store such sediments on land as they are detrimental to the ecology of the adden area owe er if clean mud is deposited on land to heighten the land or fill former sand pits this should be regarded as a loss to the system he uestion arises if the benefits lower concentration in the estuaries outdo the effects less mud a ailable for ecosystem ser ices in downdrift basins t the same time a reduction in mud a ailability may also be considered positi e as it may lead to a reduction in maintenance dredging and higher primary production rates t the present rates of the reduction in mud a ailability may not lead to a substantial negati e ecological impact owe er the large scale and long term impact of sediment management strategies needs to be accounted for when designing such strategies
andfills are beneficiary for the estuary but also for conditions on land Estuarine mud is composed of nutrients and historical landfills especially in the areas around Emden and the lower Ems i er show that the uality of the agricultural land significantly impro es n the near future the etherlands will also e periment with landfills or semi natural flooding to impro e the soil uality of agricultural lands second positi e effect of landfills / artificial flooding is that the land grows ertically reducing flood ris s less disrupting method to enrich agricultural lands is through mechanic spreading of marine mud as an alternati e for manure he subsoil of the low lying coastal areas in the north east of the etherlands consist of compacted peatland which is poor in nutrients E periments with mechanic spreading of limited olumes of mud sufficiently low to allow agricultural functions to be continued lead re eal that the soil uality impro es owe er the olume of e tracted mud is too low to significantly influence the mud balance
n alternati e to storage on land is managed realignment n the past realignments were often accidental when di es were breached and had to be gi en up e g aesummer lannen
1
owadays with the increased uality of the di es realignment is a choice anaged realignment or de emban ment can be done ia the reintroduction of tidal storm surge influence to polders by breaching or remo ing di es n total more than 0 realignment pro ects ha e been reali ed in the trilateral adden ea and some 1 00 ha has been con erted into adden area Esselin et al 01 edimentation rates in these areas are slightly higher or comparable to a erages found in the near tidal marshes padenl nder pit e mm/yr olwerder summer polder 10 mm/yr 01 owe er sometimes sedimentation rates are ery high especially in estuaries n e ample is the leinsieler late in the eser with a sedimentation rate of 00 mm/yr 01 ccasionally sedimentation rates may be ery low e g ildtpollen 1 mm/yr a er et al 01 resulting in a lag relati e to mean high water le el rise but that is rare he reasons for differences in sedimentation rates are the hydrodynamic conditions full tide or dampening concentrations wa e effects ele ation and inundation of the site the slope of the terrain and the number of breaches
p to present most of the realignment schemes ha e been rather small in areal e tent E en at sedimentation rates of 10 mm/yr this does not allow for large scale permanent storage of mud o that end larger scale realignment pro ects are needed he silting up of a basin of 10 m with 1 cm/yr would re uire 0 1 10 m /yr of mainly mud resently se eral possibilities are being considered from a totally open landscape without di es to strongly regulated tidal basins
The most comprehensive and futuristic plan is “Rijzend Land” of Bureau amaland igure t en isages to open polders surrounded by inland di es which function as sedimentation basins hen the area has become high enough it can be reclaimed or if sea le el rise is too fast left to nature t the same time other realignment pro ects are still silting up ariants of the plan ha e been brought forward by the bureau and other sta eholders
o store considerable amounts of dredged mud and thus reduce the concentrations se eral of these large scale realignment initiati es would be needed along the estuaries uch managed realignment will mainly be implemented on agricultural land which will turn into nature reser es ut depending on financial compensation farmers are li ely willing to sell their land lternati ely the land may be e ploited economically while being regularly flooded his includes recreation nature reser es as abo e shell fish farming or salty agriculture ecently oggema 0 0 showed that higher alue future crops ta e up much less land and deli er the same amount of income ut e en these lands retain some economic alue additional in estments will be needed
1
s ar ued in hapter this is difficult to predict t is anticipated that as lon as sand flats can eep up ith LR the ill provide shelter for the more inland mudflats o ever ith increasin rates of LR the sand flats increasin l la ehind and ecome some hat deeper ith reference to L and ave action and tides ill ecome stron er t is not no n hat ill happen ith mud sedimentation second point is that more mud ill e needed to eep up ith accelerated LR s alread at present mud suppl is limited it ma e e pected that for the more east ard asins this ma ecome pro lematic in the lon er run t is not no n to hat e tent sea level rise ill enhance coastal erosion at over trait and thus mud influ tron er fluctuations in run off due to chan in precipitation patterns ma lead to increased mud suppl via the rivers o ever this ma e hindered if river ater levels increase due to sea level rise hich mi ht lead to mud entrapment in the river eds
ove a certain critical limit sand tidal flats are e pected to e no lon er a le to eep up ith LR and the ill start to dro n Then ave ener on the mudflats ill increase This ma lead to erosion or reduced vertical accretion
ee hapter
lar e num er of models developed for different purposes and different spatial and temporal scales summarized in hapter
10.2 TESTING THE HYPOTHESIS
Hypothesis I: Sand and mud deposits are spatially segregated, which influences not only their distribution but also their response to human interventions and sea-level rise
rom hapter and it appears that overall stron sand mud se re ation in the sediment ed of the utch adden ea has een o served man researchers ostma van traaten and uenen de lopper van Ledden arts etailed data indicate that the se re ation is not onl e plained the ath metr ith sand channels and mudd shoals ut is also o served on intertidal flats here sediments tend to e either mud rich avera e ca mud or mud poor avera e ca erman et al This imodal distri ution of the mud content as sho n to e relativel sta le over decennia olina lonso The pro a ilit of findin ed sediments in et een these t o modes is much lo er The intermediate samples sho lar e fluctuations in rain size distri utions and ma e the result of samplin mi ed eds of flaser and linzen structures cla rich and cla poor la ers of sediment hich alternate Thus as a first appro imation one mi ht thin of the ac arrier area as consisting of two separate “worlds”: one of mud rich sediments that are enerall cohesive and one of sand rich sediments that are non cohesive f course the idea of t o separate orlds is an over simplification and it should e noted that in oth orlds cohesive and non cohesive the ed sediments e ist of oth sand and mud oreover these t o orlds interact in their morphod namic development These interactions ta e place at various scales t small rain rain scales sand and mud particles mutuall influence their erosion rates ith mud particles reducin sand erosion rates hile an addition of sand stren thens a pure mud ed – see inter erp and van esteren t lar er scales sand shoals shelter fine rained tidal flats from ave induced erosion hereas simultaneousl deposition of mud on intertidal areas reduce the tidal prism and therefore flo velocities in the dominantl sand tidal channels
The e istence of imodalit is pro a l rou ht a out oth h drod namic conditions and the stron l differin sedimentar ehaviour of sand and mud in terms of entrainment transport and settlin ostma an rooijen and an urthermore it appears that mud settles preferentiall in mud fields over periods of a centur or more olina lonso et al in prep possi le e planation is the or anic matter on mud hich stimulates enthic diatom development and hence sediment indin thus increasin entrainment velocit
The e istence of imodalit in the erman adden ea could not et e studied in this project ecause the availa le data consists of interpolated data and the ori inal datasets ere not availa le o ever since the imodalit is also o served on other areas ith sand and mudflats around the orld it is assumed that it is also true for the erman and anish adden ea
e implications of t is o ser ation are far reac ing pon accelerated sea le el rise it does not suffice to onl consider t e a aila ilit and transport of sand or more landward areas t e a aila ilit of mud s ould also e considered see furt er sea le el rise
Hypothesis II: The contribution of mud to total sedimentation rates in the whole trilateral Wadden Sea is substantial
e ed sediment in t e adden ea consists of appro imatel mud mass see e g ost et al owe er large inter entions ma lead to a larger contri ution t an t is of mud to total deposition is was s own for t e western part of t e utc adden ea after closure of t e uider ee w ere a out olume of t e deposited sediment consisted of mud see igure and a le e contri ution of mud to deposition in t e astern part of t e utc adden ea was largest after closure of t e auwers ee olume since igure and a le n realit mud contri utes e en more to t e net deposition rates ecause t e and do not account for salt mars sedimentation rate w ere mud constitutes of t e deposit n t e ms stuar mud contri utes mass to infilling since see a le n t e sandier parts of t e ower a on coast and t e ort risian erman adden ea mud contri utes muc less mass see a le and a le uc more mud deposits in some of t e s eltered em a ments e uc t ade a elt of ig t and t e salt mars es e estuaries of t e l e and eser are primaril mudd ut uantitati e alues for t e mud contri ution to total sedimentation are not a aila le
erall we conclude t at mud su stantiall influences morp od namic e a iour and t e sediment udgets in t e adden ea
Hypothesis III: The sources and the sinks of mud are closely balanced.
is pot esis is confirmed for t e current sedimentation rates e potential influ of mud originating from t e aforementioned sources is estimated at ton r of mud ased on a literature re iew and an additional data anal sis we a e s own t at recent mud deposition consisting of net sedimentation and e traction uman is estimated at ton r t is s ows t at at t e moment t e sources are onl appro imatel times t e net total sedimentation owe er two t ings s ould e noted: ud sedimentation in t e erman ig t as een calculated ased on data of t e period onl w ic means t at we cannot conclude w at t e long term mud udget of t e adden ea loo s li e erefore we recommend e tending t is anal sis to a longer period of time e mud sources are still difficult to uantif w ic creates uncertaint in t e calculations erefore we strongl recommend furt er researc on t eir magnitude and pat wa s owe er it is also clear t at alt oug suppl ma e ig l aria le it
will not e an order of magnitude larger t an current estimates t implies t at in general suppl and deposition of mud are uite compara le in si e
Hypothesis IV: Sea-level rise may lead to a shortage of mud
e deposition rate of mud is e pected to increase wit sea le el rise it t e present da rate of or a slig t increase and current mud e traction rates t ere is no s ortage of mud owe er t e mud alance is nearl closed see pot esis w ic implies t at a critical rate of e ist w ere mud deposition rates are so large t at a s ortage in mud ma de elop – especiall in t e east of t e adden ea is critical rate of is now estimated at mm r ut t is num er needs to e uantified in more detail wit additional studies n t e s ort term t is response is furt er complicated t e sediment uffers in t e adden ea e stoc of mud in and around t e adden ea is so large t at sedimentation ma continue for a limited period of time at rates of e ceeding mm r owe er as t e a aila le mud stoc s will ecome depleted in time t is uffer capacit onl pro ides a dela ed response to
ma or uncertaint is to w at e tent drod namic conditions c ange for instance due to t e drowning of more seaward sand s oals or parts of arriers uc a c ange will ecome li el for more e treme rates of and lead to a reduction in mud deposition rates
Hypothesis V: Anthropogenic sediment extraction may lead to a shortage of mud.
uman mud retrie al is estimated at ton r w ic is a out of t e net mud suppl t e ort ea ontinental flow t e ri ers primar production and aeolian dust com ined is million ton ear can e considered large compared to t e ton r of mud w ic naturall deposits in t e adden ea asins and t e saltmars es nder current conditions t e amount of mud a aila le in t e entire trilateral adden ea is sufficient owe er on a large scale and o er a longer period of time t ere is a limit on t e amount of mud t at can e e tracted especiall considering t e e pected acceleration of see pot esis on t e scale of t e entire adden ea ocall enoug sediment ma e a aila le especiall in t e estern adden ea ut in time sediment s ortages ma de elop in t e eastern adden ea ustaina le sediment management strategies including sediment e traction sc emes s ould account not onl for t e local impact ut also t e large scale long term implications
11 REFERENCES –
– –
–
level change. A methodological review with particular reference to ‘A new Holocene sea level curve for the southern North Sea’ presented by K.
–
a er . i er . and sselin . . onitoring effecten van ver weldering in de ildtpollen – . indrapport. Aand rapport A rapport Altenburg and ymenga ecologisch onder oe eanw lden A cologisch onder oe and Advies ries in opdracht rys e ea.
antelmann A . ie andschaftsentwic lung an der schleswig holsteinischen est uste dargestellt am eispiel Nordfriesland. ine un tionschroni durch f nf ahrtausende. ie K ste .
artholdy . . hristiansen and H. Kun endorf. . ong term variations in bac barrier salt marsh deposition on the S allingen peninsula – the anish adden Sea. arine eology – .
artholdy . and e rup . . Holocene evolution of the anish adden Sea Sen enbergiana aritima .
echerer . l ser . mlauf . and urchard H. . stuarine circulation versus tidal pumping sediment transport in a well mi ed tidal inlet. ournal of eophysical esearch ceans – .
echerer . Hofstede . r we . ur iani K. Schul . and urchard H. . he adden Sea in transition – conse uences of sea level rise. cean ynamics . .
eets . and an der Spe A. . he Holocene evolution of the barrier and the bac barrier basins of elgium and the Netherlands as a function of late eichselian morphology relative sea level rise and sediment supply. Netherlands ournal of eosciences .
ehre K. . . ie ver nderungen der Nieders chsischen K stenlinie in den let ten ahren und ihre rsachen. robleme der K stenforschung im S dlichen Nordseegebiet .
ehre K . . oastal development sea level change and settlement history during the later Holocene in the lay istrict of ower Sa ony Niedersachsen Northern ermany. uaternary nter national .
enninghoff . and inter . . ecadal evolution of tidal flats and channels in the uter eser estuary ermany. cean yn. – .
enninghoff . and inter . . ecent morphologic evolution of the erman adden sea. Scientific eports https doi.org . s .
NS Schuchardt and Scholle b . mwelt und naturschut fachliche Studie um Sedimentmanagement on ept f r die bremischen H fen pp.
NS and N KN . Sediment anagement Strategies in the eser stuary – Study in the framewor of the nterreg pro ect . pages. remen ldenburg.
SH . e eiten alender . Nr . S N .
ui sman . . idderin hof H. . ater transport at subtidal fre uencies in the arsdiep inlet. . Sea es. doi . .seares. . . .
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
– –
–
–
–
–
–
–
–
–
haben “Erosionsfestigkeit von Hellern”. Verbundbericht,
deichvorlandes in Forschungsvorhaben ‘Erosionsfestigkeit von Hellern’. –
Esselink, ., . . i ke a, . eents and . Hage an. . Vertical accretion and rofile changes in abandoned an ade tidal arshes in the ollard estuar , the etherlands. ournal of oastal esearch – .
Esselink, ., . aniels and . eerstand. . e hoogte van de ollardk elders in . eschri ving van de uitgangssituatie in het kader van het elderherstel rogra a roningen. atara ort. ra ort . Ecologisch nder oek and dvies, Vries. .
Esselink, ., . os, . aniels, .E. van uin and . . Veeneklaas. . Van older naar k elder tien aar k elderherstel oarderleech. ra ort , Ecologisch nder oek and dvies, Vries. .
Esselink ., van uin .E., un e ., re er ., Fol er E. ., Frikke ., lahn ., de root .V., Hecker ., Hell ig ., ensen ., rber ., etersen . and tock ., . alt arshes. n adden ea ualit tatus e ort Eds. loe er . et al., o on adden ea ecretariat, ilhel shaven, er an . ast u dated . . . htt s sr. addensea orldheritage.org re orts salt arshes.
E , ., . nitiation of dike construction in the er an cla district. adden ea Ecos ste , , .
Falk, . ., . ie al ogeo or hologie ausge hlter tandorte der schles ig holsteinischen ordseek ste i Fr h und ittelholo n hesis Fachbereich – elt und esellschaft der ech nischen niversit t erlin ur Erlangung des akade ischen rades oktor der atur issenschaften, h hesis, .
Fanger H. ., eu ann ., h . and ieth ller ., . E , eine ntersuchung ber die r bungs one der nter eser, ie ste, , .
Fett eis, ., and . van den E nde, . he ud de osits and the high turbidit in the elgian utch coastal one, southern bight of the orth ea. ont. helf es. – .
Fett eis, ., echad, ., van den E nde, ., . n esti ate of the sus ended articulate atter trans ort in the southern orth ea using ea iF i ages, in situ easure ents and nu erical odel results. ont. helf es. , – . htt d .doi.org . .csr. . . .
Fett eis, . . ieth ller, . Verne , . ecker, . ackers, . ae e, . ha alain, . lae s, . laus, . o , . eloffre, . e reiter, F. ruine, . Fl ser, . r nler, F. ourdin, . afite, . au , . echad, . ttgers, . ottolichio, . van Engeland, . Vanhaverbeke, H. Vereecken, . ncertainties associated ith in situ high fre uenc long ter observations of sus ended articulate atter concentration using o tical and acoustic sensors. rogress in ceanogra h . htt s doi.org . . ocean. .
Figge, ., . arte der edi entverteilung in der eutschen ucht, , eutsches H d rogra hisches nstitut Ed. , Ha burg.
–
–
Gehrels, W.R., Marshall, W.A., Gehrels, M.J., Larsen, G., Kirby, J.R., Eirıksson, J., Heinemeier, –
–
–
–
– –
–
–
–
–
–
–
–
–
–
KNMI. 2014. KNMI’14. Climate Change scenarios for the 21st century –
–
Kragt i N. . itman . . ti e M. . . an ang . . 2004. Mor hological res onse of ti al asins to human inter entions. Coastal ngineering 1 20 –221.
Kre s M. 200 . ater uality as ects for o timisation of maintenance re ging in the ms stuary. resentation 200 at m en in erman.
Kre s M. eil eer . 200 . he ms ollart stuary. ie K ste 4 2 2 2 2.
Kui er . . . an . . a er. 2012. elo an a o e groun erte rate her i ory an a iotic factors alternate in sha ing salt marsh lant communities. ournal of erimental Marine iology an cology 4 2–4 1 –2 .
K hn . 200 . as att im Nor erhe er ereich als untergegangene Kulturlan schaft. In. isher . 200 Kulturlan schaft Nor seemarschen eite. .
Kystenins e toratet 1 . Morfologis u i ling i a eha et ister y s ti e an somr e og a esha fronten .
afite .. him ell . .. Nash. . .. u ont. . .. uault M. .. rocho s i. N. . .. am oy. .M.. Collins. M. .. 1 . u as I us en e material flu es through the trait of o er. y ro ynamics an iogeochemical lu es in the astern Channel lu es into the North ea. M NC I econ nnual e ort. M 00 C . . 1 10 .
amalan 201 . li het gri e gou Integraal er ennen ont er en on er oe o hogen lan ou gron sli als motor oor regionale lan scha ont i eling. or rogramma Naar een ri e a en ee.
amalan N . i en an htt .lamalan .eu ro ects ri en lan
ang . chu ert . an Mar ofs y M. 1 . ata Inter retation an Numerical Mo eling of the Mu an us en e e iment eriment 1 ournal of eo hysical esearch 4 C10 14 1 14 .
ange . M ller . an iechotta . 200 . he eser stuary. ie K ste 4 ICC 200 2 2
angen erg . . fi enmay . on torch an . n ermann. 1 . torm relate sea le el ariations along the North ea coast. Natural aria ility an anthro ogenic change. Continental helf es. 1 . 21 42.
aursen K. an ri e . 201 . he a en ea enmar . he etlan oo .1 14 ringer erlag. I 10.100 4 00 1 1 1
eggett . . Coo er N. an ar ey . 2004. Coastal an stuarine Manage ealignment – esign Issues. on on CI I .
esser . . oel in . . an Kester . . .M. an telling . . 2004. e elo ment an ali ation of a three imensional mo el. Coastal ngineering 1 – 1
esser . . 200 . n a roach to me ium term coastal mor hological mo elling. h hesis. N C I elft ..
140
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–
–
–
–
–
–
— –
–
–
–
–
‐
–
–
–
–
–
tidal Delta of „Het Friesche Zeegat‟,
–
–
–
–
–
–
–
–
–
–
–
–
– –
–
–
–
–
–
– –
–
–
–
–
–
Ausschließlichen Wirtschaftszone „AWZ“”.BAW
–
–
Wiertse a an artners. . n erzoe naar e sa enstellin an lei te ieu e tatenzi l en e o eli he en oor e toe assin in een i . ra ort. aa e en n enieurs ureau Wiertse a an artners B. . ol ert. .
Winter er . . an an esteren W. . . . ntro uction to the h sics of cohesi e se i ent in the arine en iron ent. A ster a lse ier.
Winter er . . an Wan Z. B. . an in uce re i e shifts in s all estuaries theor . cean na ics – .
Winter er . . Wan Z. B. an Brac el A. an ollan . an sters . . an in uce re i e shifts in s all estuaries a co arison of ri ers. cean na ics – .
Wolters . ar utt A. an Ba er . . . alt arsh restoration e aluatin the success of e e an ents in north est uro e. Biolo ical onser ation – .
Wur ts . an orn . . ears of e erience ith flui u efinition of the nautical otto ith rheolo ical ara eters. erra et A ua . .
Zeiler . il ra t . l ß A. an . alerius . o ellin ar e cale e i ent rans ort in the er an Bi ht orth ea . ie ste .
Z arts . . Bo e estel hei en echanische o el isseri in e Wa enzee. a ort ZA . B
12 WADDEN SEA RESEARCHERS AND PRACTITIONERS
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