Coastal Protection and Sea Level Rise. Final report. 2001

0Wadden Sea Ecosystem No. 13 - 2001

Coastal Protection andSea Level Rise

Final Report


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Colophon

PublisherCommon Wadden Sea Secretariat (CWSS), Wilhelmshaven, Germany

AuthorsTrilateral Working Group on Coastal Protection and Sea Level Rise (CPSL)

Cover photoJacobus Hofstede

Lay-outCWSS

PrintDruckerei Plakativ, Kirchhatten, +49(0)4482-97440

PaperCyclus 100% Recycling Paper

Number of copies500

Published2001

ISSN 0946-896X

This publication should be cited as:CPSL 2001. Final Report of the Trilateral Working Group on Coastal Protection and Sea Level Rise. Wad-den Sea Ecosystem No. 13. Common Wadden Sea Secretariat, Wilhelmshaven, Germany.


WADDEN SEA ECOSYSTEM No. 13

Coastal Protection andSea Level Rise

Final Report of the Trilateral Working Group onCoastal Protection and Sea Level Rise

2001Common Wadden Sea Secretariat


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Contents

Executive Summary _______________________________________ 7

1. Introduction ___________________________________________ 9

2. National Administrative Structures _______________________ 102.1 Denmark _______________________________________________________________ 10

2.1.1 Coastal defense _____________________________________________________ 102.1.2 Nature protection ____________________________________________________ 102.1.3 Future policy principles _______________________________________________ 11

2.2 Schleswig-Holstein _______________________________________________________ 112.2.1 Coastal defense _____________________________________________________ 112.2.2 Nature conservation __________________________________________________ 112.2.3 Future policy principles _______________________________________________ 12

2.3 Niedersachsen ___________________________________________________________ 122.3.1 Coastal defense _____________________________________________________ 122.3.2 Nature conservation __________________________________________________ 132.3.3 Future policy principles _______________________________________________ 13

2.4 The Netherlands _________________________________________________________ 132.4.1 Coastal defense _____________________________________________________ 132.4.2 Nature protection ____________________________________________________ 142.4.3 Future policy principles _______________________________________________ 14

3. Common Knowledge Basis _______________________________ 153.1 Wadden Sea morphology and sedimentology ___________________________________ 15

3.1.1 Introduction ________________________________________________________ 153.1.2 Morphological elements _______________________________________________ 153.1.3 Sand-sharing system _________________________________________________ 17

3.2 Effects of changes in tidal drainage area ______________________________________ 173.2.1 Dimensions and orientation of the various parts of the system _________________ 173.2.2 Sedimentary characteristics of the system _________________________________ 18

3.3 Effects of fixing of parts of the islands and the mainland coast _____________________ 193.4 The relevance of salt marshes and summer dikes ________________________________ 22

3.4.1 Introduction ________________________________________________________ 223.4.2 Salt marshes ________________________________________________________ 223.4.3 Summer dikes _______________________________________________________ 243.4.4 Conclusions ________________________________________________________ 25

3.5 The relevance of biota for sedimentation- and erosion processes ____________________ 253.5.1 Introduction ________________________________________________________ 253.5.2 Intertidal area ______________________________________________________ 253.5.3 Salt marshes ________________________________________________________ 273.5.4 Dunes _____________________________________________________________ 27

4. Analysis of Changes: Basic Assumptions and Methodology_____ 284.1 Introduction ____________________________________________________________ 284.2 Past ___________________________________________________________________ 284.3 Future _________________________________________________________________ 294.4 Methodology ____________________________________________________________ 30


5. Impact of Changes in Sea Level and Storminess _____________ 315.1 Introduction ____________________________________________________________ 315.2 Physical consequences ____________________________________________________ 31

5.2.1 Flooding time intertidal _______________________________________________ 315.2.2 Surface area intertidal ________________________________________________ 315.2.3 Tidal channel cross-section ____________________________________________ 315.2.4 Salt marsh accretion _________________________________________________ 335.2.5 Salt marsh cliff erosion _______________________________________________ 335.2.6 Barrier retreat_______________________________________________________ 33

5.3 Biological consequences ___________________________________________________ 345.3.1 Benthic biomass _____________________________________________________ 345.3.2 Birds ______________________________________________________________ 345.3.3 Fish_______________________________________________________________ 345.3.4 Seals ______________________________________________________________ 355.3.5 Seagrass ___________________________________________________________ 355.3.6 Salt marsh vegetation ________________________________________________ 35

5.4 Socioeconomic consequences _______________________________________________ 355.4.1 Safety _____________________________________________________________ 355.4.2 Fresh water run-off, harbors/shipping, tourism, agriculture and salinity __________ 37

6. Best Environmental Practice Options ______________________396.1 Introduction ____________________________________________________________ 396.2 Evaluation ______________________________________________________________ 41

6.2.1 Sandy barrier coast ___________________________________________________ 416.2.2 Tidal basins ________________________________________________________ 416.2.3 Salt marshes ________________________________________________________ 426.2.4 Dikes _____________________________________________________________ 436.2.5 Mainland __________________________________________________________ 43

6.3 Conclusions _____________________________________________________________ 456.3.1 Sandy barrier coast ___________________________________________________ 456.3.2 Tidal basins_________________________________________________________ 456.3.3 Salt marshes ________________________________________________________ 456.3.4 Dikes _____________________________________________________________ 456.3.5 Mainland __________________________________________________________ 45

7. Conclusions and Recommendations _______________________477.1 Introduction ____________________________________________________________ 477.2 Conclusions _____________________________________________________________ 47

7.2.1 General conclusions __________________________________________________ 477.2.2 The tidal area _______________________________________________________ 477.2.3 The barrier islands ___________________________________________________ 487.2.4 Socioeconomic impacts _______________________________________________ 487.2.5 Best Environmental Practice ____________________________________________ 48

7.3 Recommendations ________________________________________________________ 49

Annexes________________________________________________ 51Annex 1: CPSL Terms of Reference _______________________________________________ 51Annex 2: CPSL Members ______________________________________________________ 52Annex 3: Technical Terms ______________________________________________________ 53Annex 4: Relevant Running Projects _____________________________________________ 56Annex 5: Literature __________________________________________________________ 59


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Wadden Sea Ecosystem No. 13 - 2001

At the 8th Trilateral Governmental Wadden SeaConference (Stade, Germany), it was decided toinvestigate the possible effects of enhanced sealevel rise and, on the basis of such an investiga-tion, develop proposals for future integrated coast-al defense and nature protection policies. In 1998a trilateral expert group, the Coastal Protectionand Sea Level Rise group (CPSL) was installed withthis remit. The members of the group representcoastal protection and nature protection authori-ties in the three Wadden Sea countries. The re-sults of the work of the CPSL, carried out in theperiod 1 January 1999 till 30 June 2001, are inthis Report.

The remit of the CPSL can be divided into twomain phases. In the first phase a so-called com-mon knowledge basis was elaborated providingthe starting point for the second phase of theproject in which an analysis was carried out ofexpected future developments.

Common agreement about basic facts wasreached regarding Wadden Sea morphology andsedimentology, effects of changes in tidal drain-age area, effects of fixing parts of the islands andmainland coast, the relevance of salt marshes forcoastal and nature protection and the relevanceof biota for sedimentation and erosion processes.

The group acknowledged that the main ele-ments of the Wadden sea system are the barrier

Executive Summary

islands, the tidal inlets, the ebb-tidal deltas, thetidal channels, the tidal flats and the salt marsh-es and that there are strong interactions betweenthese elements. The main driving forces are thetides, the waves and the wind and the main link-ing factor is the sediment transport. All parts ofthis sediment- or sand-sharing system are cou-pled and can be, or strive towards, dynamic equi-librium with the hydrodynamic conditions. Chang-es in any part of the system will cause a sedimenttransport to or from other parts of the system,leading to a new dynamic equilibrium. Therefore,a moderate sea level rise in the Wadden Sea, re-sulting from both natural and man-induced pro-cesses, will be compensated by the import of sed-iment which, in the long term, derives from thetidal channels, shoreface and the beaches anddunes of the barrier islands.

In addition to these hydrodynamical and mor-phological processes the group underlined theimportance of biotic processes for sedimentationand erosion. In this respect the relevance of sea-grass and mussel beds for biodeposition and re-duction of erosion and the role of vegetation inthe formation of dunes were emphasized.

In the second phase of the project the CPSLmade an assessment of the possible impacts ofsea level rise and increase in storminess for threedifferent scenarios, the latter based upon the lat-


Executive Summary

est IPCC model calculations. Scenario 1 assumesa sea level rise of 10 cm in the coming 50 years.Scenario 2, the intermediate and most realisticscenario, takes as a starting point a sea level riseof 25 cm per 50 years. Scenario 3 is the worst-case scenario, under which a sea level rise of 50cm per 50 years is considered. For all three sce-narios the additional impact of increase in storm-iness was evaluated.

For all three scenarios the impact upon selectedphysical, biological, and socioeconomic parame-ters was investigated.

It was concluded that, generally, changescaused by sea level rise will not easily be distin-guishable from changes resulting from the highnatural variability, which is a specific feature ofthe Wadden Sea system. Moreover, there will belarge differences in changes occurring in the dif-ferent tidal basins.

Because the Wadden Sea has a high resilienceto changes it was considered plausible that thesystem will be able to adapt to a sea level rise upto some 25 cm per 50 years (the most realisticscenario), without substantial changes.

Beyond such levels probably a breakpoint willoccur because the capacity of the system to bal-ance the changes will become exhausted. Whensuch a breakpoint, which will differ for differenttidal basins, has been passed, substantial chang-es in morphological and, consequently, biologicalparameters are expected.

One of the major changes will be a reductionof the size of the intertidal area. The group esti-mates that, under the worst case scenario (50 cm/50 years), the size of the tidal flats could decreaseby 15% (720 km2), the tidal basins becoming morethe character of tidal lagoons. An increase instorminess would further enhance this develop-ment. The reduction of tidal flats will have im-portant consequences for biological parameters,most notably bird species depending on the in-tertidal for foraging. A reduction in the popula-tions of such species can be expected, not onlybecause the potential feeding area will be lessthan today but also, and probably more impor-tant, because the feeding time will be less. Forthe worst-case scenario, the CPSL also expectschanges in other morphological and biologicalparameters. It concerns, amongst others, an in-crease of erosion on the barrier islands, a signif-icant erosion of the salt mash cliffs, a decrease inbenthic biomass, a decrease in seagrass and anincrease in typical salt marsh vegetation.

The main socioeconomic consequence envis-aged is an increase in costs for coastal defense.Under the most realistic scenario (25 cm per 50years) an increase of costs for dike maintenanceand strengthening of at least 5 to 15% is expect-ed. Under the worst-case scenario costs to main-tain dike safety may increase by 75% in Germanyand even more in The Netherlands and Denmark.Also the costs for other coastal defense measures,such as sand nourishment and salt marsh workswill increase considerably.

Another important consequence of increasedsea level is that possibilities for discharging freshwater from the mainland into the sea will becomeless and that additional sluicing, pumping and/orfresh water storage capacity is needed.

The CPSL has, furthermore, evaluated a largenumber of coastal defense techniques and strat-egies with the aim of selecting such which may,in the long-term, help maintaining the existingsafety standards and alleviate the expected im-pacts of sea level rise and increase in storminessand which will be beneficial or, at least not nega-tive, for natural assets, such as natural dynamicsand habitat quality.

This evaluation resulted in a list of so-calledBest Environmental Practice measures. These mea-sures should be further investigated for feasibilityin terms of cost-benefit, public perception andlegal aspects.

The CPSL finally formulated several recommen-dations for policy, management and research.

The main message regarding policies is that,as far as such is not already the case, integratedpolicies should be developed for coastal defense,nature protection and economic development inthe coastal zone, anticipating impacts of increasedsea level and storminess. Such policies should alsoaddress strategies for communication with thegeneral public about the expected impacts andthe introduction of additional and new coastaldefense measures and strategies.

With regard to the knowledge basis it is rec-ommended to start a research project in whichdetailed sediment budget studies for the WaddenSea are carried out, encompassing all man-inducedand natural factors, as well as, a research projectin which the interactions between hydrological,geomorphological and biological changes are in-vestigated.

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Climate change and, in particular, its possible ef-fects, have become a central issue in politics andscience in the 1990s. To the layman the notionclimate change has almost become identical withanthropogenically induced increases in the atmo-spheric concentrations of the so-called greenhousegases, most notably carbon dioxide. As a result,increasing temperatures and, consequently, in-creasing water levels are predicted, caused by thethermal expansion of the ocean water and themelting of glaciers and polar ice caps. Also changesin wind climate are expected or have, accordingto some publications, already occurred. Althoughclimate has always changed, the new feature ofthe present situation is the expected speed of thechange. This acceleration may induce significantchanges in the Wadden Sea system.

Questions regarding the consequences of ac-celerated sea level rise and increasing storm lev-els and frequencies for the Wadden Sea ecosys-tem also entered the political agenda of the tri-lateral Wadden Sea cooperation. In 1997, at the8th Trilateral Governmental Wadden Sea Confer-ence (Stade, Germany), it was decided to investi-gate the possible effects of enhanced sea levelrise and, on the basis of such an investigation,develop proposals for future integrated coastaldefense and nature protection policies. In 1998 atrilateral expert group, the Coastal Protection andSea Level Rise group (CPSL) was installed with thisremit. The precise terms of reference of the CPSLare in Annex 1, the members of the group are listedin Annex 2.

In this report the results of the work of theCPSL, carried out in period January 1999 till May2001, are presented. The report consists of twoparts. In Part 1, consisting of Chapters 2 and 3,basic facts are provided. Chapter 2 addresses thenational administrative structures with regard tocoastal protection and nature protection. In Chap-ter 3 relevant facts regarding the main physicaland biological aspects of the Wadden Sea eco-system, about which a common understandingwas achieved within the group, are given. In theintroductory part 3.1 of this chapter the basic geo-morphological principles of the Wadden Sea are

described. Section 3.2 addresses effects of changesin inundation area, for example through outbank-ing of summer polders. In Section 3.3 the impli-cations of fixed coastal defense constructions un-der different sea level rise scenarios are discussedand Section 3.4 addresses the relevance of saltmarshes and summer dikes for coastal protection.In Section 3.5 the relevance of biogenic struc-tures and biostabilisation of sediment are covered.

In Part 2, consisting of the Chapters 4, 5 and 6,an assessment is given of the possible impacts ofsea level rise and an increase in storminess forthree different scenarios. Scenario 1 assumes asea level rise of 10 cm in the coming 50 years.Scenario 2, the intermediate and most realisticscenario, takes as a starting point a sea level riseof 25 cm per 50 years. Scenario 3 is the worst-case scenario, under which a sea level rise of 50cm per 50 years is considered. For all three sce-narios the additional impact of increase in storm-iness is evaluated. In Chapter 4 a description isgiven of changes in water level and storminesswhich have occurred in the past and which mayoccur in the future. In this chapter also the meth-odology applied by the CPSL to evaluate impactsof future changes is explained. In Chapter 5 theexpected impacts of three sea level rise scenarioson physical, biological and socioeconomic param-eters are described, under the assumption thatcurrent coastal defense practices are continued(Business as Usual, BAU). Chapter 6 describes alarge number of coastal defense techniques andanalyses these for the criteria suitability for coastaldefense and compatibility with nature protection.On the basis of the analysis a series of so-calledBest Environmental Practice measures (BEP),which may be used as an alternative or an additionto regular coastal defense practices, is identified.In Chapter 7, the main conclusions and the rec-ommendations of the CPSL are presented. Finally,in chapter 8, a comprehensive summary is given.

The report contains five annexes. In additionto the already mentioned annexes 1 and 2, a com-prehensive glossary of terms is in Annex 3. Annex4 contains a list of relevant running projects. An-nex 5 contains the cited literature.

1. Introduction

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2. National Administrative Structures

very few hard defense constructions and no landreclamation measures are carried out. Most dikesare maintained by local water boards under thesupervision of the DCA and the County of Ribe orthe County of Southern Jutland.

2.1.2 Nature ProtectionThe general laws in Denmark relating to natureconservation incorporate some regulations thatdirectly influence the administration of the coast-al zone. The National Forest and Nature Agency,under the Ministry of Environment and Energy,has overall responsibility for the protection of theInternational Nature Conservation Areas (Ramsar,EU Bird Directive, and EU Habitats). The countiesadminister most of the regulations. They carry outinspections, issue permits and refusals, carry outmaintenance tasks, monitor, plan and dissemi-nate information. Some regulatory measuresworth mentioning include a ban on changes tothe natural conditions in salt and freshwatermarshes, bogs and other areas, a 300 metergeneral protection zone along the coast andconservation regulations for protected dune areas.

Besides the Danish Nature Protection Law, themost significant nature protection regulation inthe Wadden Sea Area is the Executive Order onNature Conservation and a Wildlife Reserve in theWadden Sea. This executive order covers largeparts of the Danish section of the Wadden SeaArea, and is an expression of efforts to establishsustainable development for the region as a na-tional and international nature conservation area,as well as, a way of ensuring that Denmark meetsits obligations for the area including those underthe EU Bird and Habitat Directives. The ExecutiveOrder contains prohibitions that regulate in de-tail aspects such as land and sea traffic, the col-lection of organisms from the sea bed, hunting,civil engineering work including coastal defense,alteration of the terrain, canals, mineral extrac-tion and other technical installations. The Execu-tive Order falls under the jurisdiction of the Min-istry of Environment and Energy and the NationalForest and Nature Agency. The provisions of thisorder make it possible to involve other authori-ties, such as the Ministry of Transport and thecounties.

Finally, the Danish system of planning regula-tions and inter-sector spatial planning is carriedout in practice with regard to the areas that bor-der the Wadden Sea. Such planning results in thedefinition of a framework for future development,which is expressed in guidelines for the adminis-trative procedures of the regional and local au-

2.1 DenmarkThe regulation of the coastal zone in Denmark iscovered by a number of legislative instrumentswhich place the responsibility on several author-ities. For the most part the state authorities areresponsible for the administration of the sea ter-ritory. The ministries with the greatest responsi-bility for coastal defense and nature protectionare the Ministry of Transport and the Ministry ofEnvironment and Energy respectively. The admin-istration of the land territory is mainly carried outby the counties.

2.1.1 Coastal DefenseThe national Coast Protection Act of 1988 is

the main legislation regulating all coastal defensemeasures. This law is mainly a procedural code,stating the procedure that the relevant authori-ties are obliged to follow when an application orpublic initiative for building or altering coastaldefense constructions comes up. The DanishCoastal Authority (DCA), an organization underthe Ministry of Transport, and the county author-ities, are responsible for legislation.

The overall principle is that the responsibilityfor establishing and maintaining protection mea-sures lies with the persons who profit. On the otherhand, landowners do not have an immediate rightto protect property. Each new measure has to beconsidered appropriate by several authorities. Themain considerations within the framework of theCoast Protection Act are whether a constructionis necessary, can fulfil its purpose, will cause un-desirable side effects or conflict with nature pro-tection rules. The county considers project draftsand decides whether the project in question issuitable to be passed on for further considerationor can be rejected immediately. The local munic-ipality and other relevant authorities are alwaysasked to comment on projects considered suit-able, and approval for all such projects has to beobtained from the DCA within the Ministry ofTransport. In general there is no public obligationto undertake coastal defense. In particularly ex-treme conditions the political bodies have con-sidered it a public duty to enforce or erect dikesat the Wadden Sea coast financed by public fundsin part or in full through the issue of special con-struction laws.

The most significant element of the coastaldefenses along the Danish Wadden Sea coast con-sists of the existing sea dikes made of sand andclay. These stretch along approximately 115 kilo-meters of coastline, protecting an area of approx-imately 600 square kilometers (Fig. 3.3). There are

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marshes from erosion and, eventually, disintegra-tion.

Coastal defense (coastal protection and coast-al flood defense) is regulated in the Schleswig-Holstein State Water Act and the Master PlanCoastal Defense (the technical and financial con-cept). In principle coastal defense devolves on thepersons who profit. However, flood defense thatis in the interest of the public is a public obliga-tion. Depending on the measures, the responsibil-ity lies with state or municipal administration orwith local water boards. Coastal protection thatis in the interest of the public (i.e. protection ofsettlements against land loss) devolves on the stateadministration.

The Schleswig-Holstein State Ministry for theRural Areas, Agriculture, Food and Tourism is re-sponsible for legislation, general planning and fi-nancing. Two regional offices for the rural areasimplement the state coastal defense programs.Public participation in state coastal defense is reg-ulated in the Schleswig-Holstein State decree onthe implementation of public plans. Since 1998the integration of different uses and interests hasbeen realized with the establishment of the In-tegrated Coastal Defense Board. This advisoryboard is chaired by the Secretary of State for theRural Areas and consists of representatives fromall relevant public and private parties involved inthe different aspects of coastal defense manage-ment.

The organization and administration of (pub-lic) coastal defense in Germany is in the responsi-bility of the respective states. However, as coastaldefense has national consequences, capital mea-sures are co-financed by the federal governmentwith 70% of total eligible costs (the other 30%are matched by the states). The maintenance ofexisting state coastal defense structures, on theother hand, is financed 100% by the state. Mu-nicipalities and/or local water boards that are re-sponsible for coastal defense measures in theirarea normally have to contribute between 5 and20% to the costs. The rest is financed by state(and federal) government. Finally, a small but in-creasing financial contribution to coastal defensecomes from the European Union.

In all, in Schleswig-Holstein (Wadden Sea andBaltic Sea coast) about 40 to 45 million EURO isspent on coastal defense each year.

2.2.2 Nature conservationNature conservation is regulated in the Schles-

wig-Holstein State Nature Conservation Act andthe State Act for the National Park Wadden Sea of

thorities. The general public is always involved inthe planning procedure. In certain situations theplanning act also dictates that larger construc-tion projects should be subject to an EIA (envi-ronmental impact assessment).

2.1.3 Future policy principlesThe agreements contained in the Wadden Sea Planare currently being incorporated into the nation-al and regional plans, regulations and adminis-trative practices.

The two Wadden Sea Counties and the DCAare conducting a technical reassessment of thesafety of the dikes in the Wadden Sea Area in thelight of changes in sea level. This will form thebasis for the various water boards to make a de-cision as to whether to upgrade the safety levelof those dikes for which they are responsible. Inpractice this is mainly expected to take placethrough reinforcement of existing dikes.

In general there seems to be a trend towardsmore emphasis on the regulation of use and pro-tection of the Danish coasts. For instance a spe-cial 3 km broad planning zone has been imposedto the land territory. A greater part of applica-tions for defense measures to protect uninhabit-ed areas is now rejected because it is consideredimportant to protect the natural coastal process-es. At the same time it is considered very impor-tant to maintain and if necessary improve the pro-tection of the inhabitants in areas threatened byflooding.

The protection methods in the Danish WaddenSea continue the tradition of using green dikes.The use of other hard constructions along the westcoast is getting rare in favor of sand nourishment.

2.2 Schleswig-Holstein2.2.1 Coastal defense

The Wadden Sea coastline of Schleswig-Holsteinmeasures about 553 km, 297 km of which aremainland, the rest island coasts (Fig.3.3). About425 km of the coastline are protected by State(355 km) and other dikes (70 km). In all, these seawalls protect an area of 3,472 km2 against flood-ing during severe storm surges. In these coastallowlands 253,000 people live, and economic val-ues of about 31 billion EURO are concentrated.Further, on the islands Sylt and Fhr, sand nourish-ment is conducted to prevent coastal retreat. Thethird major coastal defense activity (apart from seawalls and nourishment) in the Wadden Sea ofSchleswig-Holstein are salt marsh works, nowa-days carried out mainly to protect existing salt


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Schleswig-Holstein. Responsible for environmen-tal legislation, financing and general planning inSchleswig-Holstein is the State Ministry for theEnvironment, Nature and Forests. Two state envi-ronmental offices, the Schleswig-Holstein StateOffice for Nature and the Environment and theSchleswig-Holstein State Office for the NationalPark Wadden Sea of Schleswig-Holstein (NPA) areresponsible for research, preparation of plans, and(partly) permits. Finally, several regional environ-mental offices, as well as, the counties are re-sponsible for monitoring and permits.

The Wadden Sea of Schleswig-Holstein (exclu-sive of the islands) is a national park, adminis-tered by the NPA. In 1999, the national park wasextended with a whale protection area in the NorthSea west of the island Sylt. The NPA is, amongstothers, responsible for research and permits in thenational park. On the Wadden Sea islands severalnature reserves exist that are mainly managed byenvironmental NGOs.

2.2.3 Future policy principlesThe old philosophy of executing coastal defense

(building sea walls) in order to reclaim fertile landalready ceased in the early fifties. The last sea wallaiming at this purpose was constructed inSchleswig-Holstein in 1954 (Friedrich-Wilhelm-Lbke-Koog). Afterwards, the policy for coastaldefense turned into achieving the same level ofsecurity for all state dikes (i.e. each sea wall hasthe same probability of breaching). The sixties,seventies and early eighties were characterized bya strong belief in engineering (hard) solutions forcoastal defense. However, this attitude changedinto trying to use more natural techniques andmaterial, e.g. sand nourishment, to combat coastalretreat. In 1995 a common salt marsh manage-

ment plan was established by coastal defense andenvironmental authorities that aims at an eco-logically sound protection and management of saltmarshes (see also 3.4.2), salt marshes being bothan important (natural) coastal defense structureand an ecologically sensitive and valuable habi-tat.

In future, the coastal defense policy will prob-ably increasingly include risk analyses for singleflood units (risk being defined here as the productof the probability of dike breaching and the dam-age potential in the flood unit). Further, more at-tention will be paid to public participation andthe integration of other interests in coastal de-fense policy (integrated coastal defense manage-ment).

2.3 Niedersachsen2.3.1 Coastal defense

The legal basis for coastal defense in Niedersach-sen is the Niedersachsen Dike Act. It contains bothregulations for design, maintenance, supervisionand usage of dikes, forelands, dunes and othercoastal defense structures and responsibilities ofthe authorities and the water boards. Main ob-jective of the Dike Act is protection of man, set-tlements, public, industrial and infrastructure fa-cilities, as well as agricultural areas, against flood-ing. The overall principle is that all persons whoprofit from protection are in charge of maintain-ing the dikes. They are organized in water boardswhich have to do the maintenance and construc-tion works on the mainland dikes, except for somewhich are under State responsibility. The State,moreover, is responsible for all coastal protectionstructures on the islands and the storm surge bar-riers.


Salt marsh works atNordstrand (FRG).

(Photo: J. Hofstede)

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The Ministry of the Environment is in chargeof the general guidelines and principle issues forcoastal defense, as well as, the master planning.It also supervises the regional authorities, whohave to fix the dimensions of the dikes in a legalact and grant permissions for extensive coastalprotection measures. The local authorities workon the rural district level and are supervised bythe regional authorities. They are responsible forall other supervision tasks including the waterboards and permissions according to the Dike Act.

The technical planning of coastal protectionmeasures and the maintenance of the state coastaldefense structures is done by the NiedersachsenAgency for Water Management and Coastal De-fense (NLWK), which is supervised by the minis-try. Special aspects of planning and applied sci-ence in coastal engineering are carried out by thecoastal research station of the NiedersachsenState Board of Ecology subordinated to theministry.

The Dike Law defines the main dikes, dunes,storm surge and all constructions that enhancetheir stability (e.g. groynes, revetments) as ele-ments of coastal protection. These elements haveto be maintained and, if necessary, reinforced. Theforeland (salt marsh in front of a dike) has to bepreserved in a defined width and maintained, in-cluding all technical constructions, as a protec-tion element for the main dike.

2.3.2 Nature conservationThe legal basis for nature conservation are the

Federal and the Niedersachsen Nature Conserva-tion Act, the State Act for the Wadden Sea Na-tional Park of Niedersachsen and the legal act forthe Nature conservation area Dollart. Except forthe estuaries of Ems, Jade, and Weser, the wholeNiedersachsen part of the Wadden Sea has beendesignated as a national park in which the unin-habited parts of the East Frisian islands are in-cluded.

The general objective for the protection of thetidal flats and the adjacent sublittoral, the dunes,and the salt marshes as valuable habitats, is topreserve the species composition and natural pro-cesses, including the natural morphology and dy-namics of this area.

Under the responsibility of the Ministry of theEnvironment, the National Park Administration, asa subdivision of the Weser-Ems district authority,develops general guidelines and executes the reg-ulations of the National Park Act. In addition, thelocal authorities at the rural district level executethe regulations of nature conservation in areasabove MHTL except those areas with highest pro-

tection. The other legal regulations remain in ex-istence. Special aspects of nature conservation areassessed by the Niedersachsen State Board of Ecol-ogy.

2.3.3 Future policy principlesThe future coastal defense policy will be fo-

cussed on the development of probabilistic de-sign codes instead of the present deterministicones. The evaluation of the feasibility of risk anal-yses for coastal protection structures will also bea matter of high importance.

On the islands a long term investigation pro-gram is carried out for the evaluation of sustain-able future protection concepts in areas wherestructural erosion occurs. The future managementof the foreland areas will be further harmonizedby creating regional management plans that in-tegrate demands of coastal defense and natureconservation.

2.4 The Netherlands2.4.1. Coastal defense

The complete mainland coast of the Dutch Wad-den Sea, about 200 km, is defended by dikes. Alsothe polders at the Wadden Sea site of the islandsare protected by dikes (Fig. 3.3). The dike lengthon the different islands varies from a few to 30kilometers. Dunes form the coastal protection ofthe 155 km of North Sea coasts of the inhabitedislands. Since 1990 sand losses from the NorthSea coast have been compensated for with sandnourishments and this will also be the case in thefuture. On Texel and Vlieland, the sandy coast re-ceives additional protection by breakwaters madeof basalt and concrete. On the tips of parts of theinhabited islands natural coastal development isallowed within certain limits. No or very limitedmaintenance is carried out on the uninhabitedeastern part of Schiermonnikoog and the unin-habited islands Rottumerplaat en Rottumeroog.

On the national level the Ministry of Transport,Public Works and Water Management and theMinistry of Agriculture, Nature Management andFisheries are responsible for coastal defense re-spectively nature protection of the Wadden area.The Ministry of Public Housing, Spatial Planningand the Environment presents the guidelines forlocal and regional plans in a national directive,the PKB-Waddenzee (Planological Core DecisionWadden Sea).

The protection of the North Sea coast of theislands is executed by the Directorate-General forPublic Works and Water Management of the Min-istry of Transport, Public Works and Water Man-


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agement. The Provinces along the coast each havea Provincial Consultative Body for the Coast (POK).In this body national, provincial and municipalauthorities and regional water boards discuss allissues concerning coastal defense and give rec-ommendations to the Minister of Transport andPublic works. The province Groningen has no POK.Here coastal affairs are discussed in a provincialcommittee on water management. Only the un-inhabited barrier islands Rottumerplaat and Rot-tumeroog belong to this province.

The responsibilities of the different parties aredivided as follows:

the central government safeguards the posi-tion of the coastline and combats structuralerosion;

the water boards are responsible for the de-sign and maintenance of the sea defenses, ex-cept on most of the Wadden islands, wherethe central government is responsible both forcoastline preservation and for the sea defens-es;

the provincial authorities are responsible forthe overall coordination and for the integra-tion with other areas of policy, such as physi-cal planning. The provincial authority alsochairs the POK.

Matters which may arise in the POKs include

the position of the coastline, to be maintainedby sand nourishments;

the annual program of nourishment works;

plans for alternative methods of coastal pro-tection;

plans for developments within the coastalzone, e.g. nature development projects.

POKs increasingly pay attention to the links be-tween coastal protection, nature development,recreation and physical planning. Therefore, in thePOK for the Wadden Sea area, the central gov-ernment is not only represented by the Ministry

of Transport, Public Works and Water Manage-ment but also by the Ministry of Agriculture,Nature Management and Fisheries.

2.4.2 Nature protectionAlmost the entire Dutch Wadden Sea is a natureconservation area by law. Exceptions are stripsalong the inner parts of the islands and the mainshipping channels. The entire area, including themain parts of the islands and the coastal zone ofthe North Sea, is part of the core and/or naturedevelopment area in the so-called ecological mainstructure. Most of the area has been designatedas EU Habitats and Bird Directive area. The entireisland of Schiermonnikoog has the status of Na-tional Park.

From the central government the Ministry ofAgriculture, Nature Management and Fisheries isresponsible for nature conservation. Provinces andmunicipalities deal with nature protection in theirregional and local plans. The guidelines for thoseplans are given in a national directive, the PKB-Waddenzee (Planological Core Decision WaddenSea) of the Ministry of Public Housing, SpatialPlanning and the Environment. Specific areas areowned and managed by nature conservation or-ganizations.

2.4.3 Future policy principlesCoastal management is gradually changing frombuilding sea defenses and damming off coastalinlets to dynamic preservation. Coastal manage-ment is more and more directed at working withthe natural dynamics of the coast instead of try-ing to control it. In this way, not only safety butalso ecology and human use are taken into ac-count. Sand nourishments are considered a goodmethod for dynamic preservation of the coast. Inthe back barrier area the strict divisions betweendry and wet, high and low, fresh and salt are sub-ject of discussions and projects have started aim-ing at restoring gradients.

For the management of the Wadden Sea itselfnature conservation laws and EU-directives be-come more and more important leading to con-flicts between nature conservation and human use.


15


3. Common Knowledge Basis

3.1 Wadden Seamorphology and

sedimentology3.1.1 Introduction

The objective of this chapter is to give an over-view of the morphology of the Wadden Sea barri-er coast. The Wadden Sea, with a tidal range from1.4 to 3.5 m, fringes the Dutch, German and Dan-ish coasts over a distance of nearly 500 km with amaximum width of approximately 35 km. The tidalwave in the North Sea moves from the S(W) tothe N(E), that is from Den Helder in The Nether-lands to Esbjerg in Denmark. Towards the NorthSea the Wadden Sea is bordered by some 20 largeand many small barrier islands, peninsulas andsandy shoals. Behind these islands lies the largesttidal flat area in Europe. The mainland coast con-sists of dikes, some salt marshes and, especiallytowards the northern Wadden Sea, a few Pleis-tocene cliffs. Especially north of the Elbe river rem-nants of older mainland deposits form small is-lands in the tidal basins, these are the so-calledHalligen.

3.1.2 Morphological elementsThe Wadden Sea is characterized by 33 adjacenttidal inlet systems, each consisting of the follow-ing morphological elements: (1) barrier island, (2)tidal inlets, (3) ebb-tidal delta and inlet, (4) tidalchannels, (5) tidal flats and (6) salt marshes (Fig.3.1).

Barrier islandsBarrier islands lie on the seaward side of the Wad-den Sea. They are formed and sustained by thecombined action of wind, waves and tides andrepresent a sediment sink. Normally a barrier is-

land consists of a shoreface, beach, dunes andoverwash areas. However, some barriers, e.g. Jap-sand, Norderoog- and Sderoogsand in Schleswig-Holstein lack dunes. Today, on most barriers alsoreclaimed salt marsh areas (polders) exist.

Tidal inletsThe barrier islands are separated from each otherby tidal inlets. Tidal inlets represent the transportroutes through which the tidal waters (loaded withsediment) enter and leave the tidal basins with

Tidal flats

Tidal channels

Tidal inlet

Ebb-

tida

l del

taBa

rrie

r is

land

Figure 3.1:Morphological elements

of the Wadden Seasystem.

16


each tidal cycle. A dynamic equilibrium exists be-tween the tidal currents and the cross-sectionalarea of the inlet channel (mainly controlled bythe scouring potential of the currents).

Ebb-tidal deltasThe sediment that is transported by ebb-tidal cur-rents is deposited at the seaward outlet, causedby decreasing current velocities. In result, an ebb-tidal delta develops. However, the erosive forcesof deep water waves coming in from the NorthSea, limit the sediment volume of the deltas. Adynamic equilibrium exists between these erosiveforces and the tidal accumulation (Oost, 1995a;Hofstede, 1999). Because the tidal channels of theinlet and the delta are strongly interrelated, theyare normally treated as one element.

Tidal channelsThe same accounts for the tidal channels and thesub- and intertidal flats which constitute a tidalbasin. Sub- and intertidal channels function astransport routes for the tidal water masses. Hence,the same dynamic equilibrium between currentsand cross-section exists as for the tidal inlet chan-nels. Through the channels sediment is transport-ed towards the tidal flats with the flood currents.

Tidal flatsAt the tidal flats this material may become set-tled as a result of decreasing current velocities,i.e. the tidal flats act as a tidal sediment sink. Be-cause the (energy-rich) waves from the North Seaare almost completely dissipated at the shorefaceand ebb-tidal deltas (Niemeyer, 1986), only local(storm) waves limit the tidal accumulation on thetidal flats. Similar to the ebb-tidal delta, a dynamic

equilibrium seems to exist on tidal flats betweenthe erosive forces of storm waves and tidal accu-mulation (mainly controlled by the time of tidalinundation, Ch. 3.1.3).

Salt marshesIf sedimentation on the tidal flats exceeds ero-sion (i.e. no dynamic equilibrium), eventually asupratidal salt marsh may form. Salt marshes areinter- and supratidal areas of fine sediments sta-bilized by a halophytic vegetation cover. Bound-ary conditions for establishing and sustaining saltmarshes are an adequate supply of fine sediments,a low energy environment which allows for sedi-mentation (see above), regular saltwater inunda-tion and, finally, a moderate sea level rise to bal-ance accumulation and prohibit vegetation suc-cession. Normally (mainly controlled by the timeof tidal inundation) tidal accumulation exceedssea level rise and succession occurs. Nowadays,most of the mainland salt marshes in the WaddenSea are artificial, i.e. developed by salt marsh ac-cretion enhancement techniques (drained brush-wood groyne fields, Hofstede, 1996).

In conclusion, the elements of the tidal sys-tems that constitute the Wadden Sea show strongmutual interactions. All elements influence thelocal tidal currents and wave regime and thus thelocal sediment redistribution patterns. Empiricalrelationships document that the morphologicalstructure of all elements strongly depends on theprevailing hydrodynamic (tidal and wave) condi-tions. In general, the tidal currents seem to con-stitute a positive (accumulative) force in most el-ements, whereas waves are a negative (erosive)factor. Only at the barriers waves may act as a


Figure 3.2:Sediment transport

between Wadden Sea andNorth Sea and along the

islands. From: Louters andGerritsen, 1994.

Outer Delta

Tidal Basin

Watershed

17


positive force during fair weather conditions. Themorphology and morphodynamics of the WaddenSea are described in detail by, amongst others,Ehlers (1988) and Oost (1995a).

3.1.3 Sand-sharing systemEach tidal system can be considered to form amore or less separate (closed) sand-sharing system(Dean, 1988). However, it should be taken intoconsideration that adjacent tidal basins may in-fluence each other across the tidal watersheds(Fig.3.2) (Oost & Dijkema, 1993). All parts of asand-sharing system are coupled and can be in,or strive towards, a dynamic equilibrium with thehydrodynamic conditions. Changes in any part ofthe system will primarily be compensated by sed-iment transport to or from the other parts of thesame system. When changes are temporary andlimited, the old dynamic equilibrium will eventu-ally be restored. For example, a moderate increasein sea level rise induces a stronger accumulationon tidal flats and salt marshes as a result of long-er tidal inundation (i.e. the sediment having moretime to settle). As a result, the elevation of theflats and salt marshes increases and the time oftidal inundation decreases again until the old dy-namic equilibrium is restored. If changes are morepermanent or intense, a new equilibrium will beestablished (e.g. reduction in the cross section oftidal channels caused by the permanent reduc-tion in tidal prism due to land reclamation). Espe-cially in the last situation sediment may be im-ported from or exported to areas outside the sand-sharing system.

To compensate for the observed secular sea lev-el rise (Chapter 4), each year several million m3 ofmaterial are deposited in the Wadden Sea. In thisway the same altitude with regard to mean sealevel is maintained or, in other words, the dynam-ic equilibrium between hydrography and morphol-ogy is maintained. In the long term most of thissediment is derived from the shoreface, beachesand dunes of the barriers islands, western Jutlandand northern Holland. As a consequence, gener-ally, barrier islands tend to retreat in response tosea level rise (Bruun, 1962). However, on the Wad-den Sea barrier islands a number of factors (ex-cessive sediment supply by the littoral sand trans-port, Pleistocene subsurface, coastal protection)may counteract this morphological response. Forexample, the western coasts of Fan and Amrumseem to be rather stable, while Rm is even ex-panding seaward as a result of a strong littoralsediment supply. Sylt is maintained in its positionthrough regular sand nourishments and the west-ern part of Norderney is kept in place by massive

coastal protection measures. A further morpho-logical result of the wave-driven littoral sandtransport is the long-term eastward movementof most of the West- and East-Frisian Islands.

3.2 Effects of changes intidal drainage area

Sea level rise, as well as, human interventions maylead to a change in tidal drainage area. From hu-man interventions it is known that changes indrainage area of a tidal inlet system may affectdimensions and orientation of the various partsof the system, as well as, sedimentary character-istics.

3.2.1 Dimensions and orientation ofthe various parts of the system

If changes in tidal drainage area result in changesin tidal volume, the dimensions and orientationof various parts of the system may also be influ-enced. A decrease in size of a tidal drainage areawill generally result in a decrease in tidal volume.This decrease in tidal volume will result in a de-crease of the channel dimensions and the ebb-tidal delta sand volume. An enlargement of a tid-al drainage area will lead to a larger tidal volumeand consequently bigger channel dimensions andebb-tidal delta sand volume.


An Example of a Change inTidal Volume

In 1969 the Lauwerszee, an embayment of theWadden Sea, was diked and the tidal prism ofthe tidal inlet Zoutkamperlaag decreased from305 million m3 to 200 million m3. Before theclosure of the Lauwerszee, a dynamical equi-librium was maintained in the ebb-tidal deltaand the backbarrier area of the Zoutkamper-laag Inlet. The reduction of the tidal prism ledto the development towards new equilibriumdimensions. In the period 1970-1987 26 mil-lion m3 of sand was transferred from the ebb-tidal delta. Also sedimentation in the inlet anderosion of the ebb-tidal delta occurred as a re-sult of reduced tidal currents. In the backbarri-er area the main backbarrier channel was par-tially filled and the eastern watershed shiftedas the channel east of it filled up. The changesin the Zoutkamperlaag inlet system thus alsoinfluence the adjacent, downdrift inlet system.In the period 1987-1993 the inlet itself becamereoriented into a more downdrift orientation,as might be expected for a decrease in tidalprism.

18


Under equilibrium conditions, the net amountsof sediment which are trapped or eroded are nor-mally small. Gradual changes in, for instance, sealevel rise or bottom subsidence, cause a slight dis-traction of the dynamic equilibrium between hy-draulics and morphology. As a reaction more sed-iment will be trapped or eroded in order to re-store the dynamic equilibrium situation.

After sudden changes in the size of tidal ba-sins, for instance by embankments, sediment def-icits or surpluses may become so large that it takesdecades to reach new hydraulic and morphologicequilibria. Those bigger changes in one basin mayalso affect neighboring basins. Examples of suchsudden changes are the closures of Zuiderzee enLauwerszee which took respectively 60 and 25years erosion and sedimentation before more orless new dynamic equilibrium states were reached(see Box).

3.2.2 Sedimentary characteristics ofthe system

As a result of the sorting processes, caused bydecreasing dynamics with increasing distancefrom the inlet, the grain size of the sediments de-creases in this direction. The finer sediments will


also settle in the more sheltered embayments. Theembankment of mainland areas in the past andresulting decrease of tidal basin dimensions hasled to a decrease of the surface of those muddierenvironments. As a result, muddy environmentsare rare nowadays. Flemming and others showedfor the Spiekeroog backbarrier area that, due tothe embankments, the hydrodynamic energy in theremaining area increased, resulting in a decreaseof the mud content in front of the dike throughtime.

In the Netherlands dikes around some summerpolders have been opened to establish new tidalmarshes. Because those polders are situated justabove MHW-level, sedimentation of mud duringstorm set-up is expected. Due to their relativelysmall sizes and height above MHW level, the open-ing of the summer polders has no effect on tidalvolume, height of storm-surge levels or sedimentbudget.

Changes in grain size may have a profound in-fluence on the species composition of the sedi-ment (compare also 3.5) and, consequently, for-aging animals.

Beach wall in Westerland,Sylt (FRG).


19


3.3 Effects of fixing ofparts of the islands and the

mainland coastAround 1000 AD the inhabitants of the higher dryareas in the coastal zone started to colonize thelower lying peatlands. To keep the storm tides outthey started to surround part of the lands by dikes.From the 13th century on, polders were created.Windmills made it possible to change inland lakesinto polders. In the last centuries also measureswere taken to stabilize the sandy coast itself. Inorder to prevent erosion, groynes perpendicularto the coast were built. At other places the coastwas embedded in stones. More recently sand nour-ishment was introduced as a means of combatingerosion.

In The Netherlands coastal defense measureswere also undertaken at the uninhabited parts ofthe islands. Starting in the 14th century sand duneswere stabilized by planting marram grass and in-dividual dunes were connected to each other inorder to get an elongated uninterrupted dune ridgeall along each island. In Germany and Denmarkcoastal protection is mostly restricted to safe-guarding the inhabited parts and infrastructureof the islands. On other parts of the islands al-most no protective measures were taken. One ex-ception is the island of Sylt where the entire 38km long beach has been nourished at least oncesince 1984.

Often the introduction of hard constructionsresults in an accelerated erosion at the edges ofthe defended parts. Consequently, the defendedparts need to be extended with more construc-tions. An example of this is given by the sea de-fense along the island Vlieland in the western partof the Wadden Sea (Fig. 3.3). The first groyneswere built in the 19th century on the western partof this island. In the course of time next to exist-ing groynes new groynes were built in easterlydirection. Nowadays the complete North Sea coastof this island is embedded in groynes. Another ex-ample is the holiday resort Westerland at the NorthSea coast of the island Sylt. Here, in the year 1907,a 70 m long wall was constructed at the upperbeach to protect a hotel. As a result of intenseerosion at its ends, this wall had to be lengthenedto about 850 m until 1954. Strong erosion due towave reflection at the foot of the wall was com-bated by revetments and, since 1960, by large tet-rapods. However, the beach in front of the wall

still suffers from erosion and, since 1972, regularsand nourishments are carried out to compen-sate for this loss. Also the island of Norderneyhas a long history of subsequent enforcements.These were carried out mainly on the western part.

Physically, the Wadden system forms an en-semble of islands, inlets, outer deltas and a seriesof adjacent tidal basins with channels, flats andsalt marshes. They are interacting through thelongshore transport of sediment. The Wadden sys-tem can be said to have a closed sand economy(compare 3.1.3). If part of a tidal basin in such asystem becomes deeper, for instance by relativesea level rise, the system re-establishes equilibri-um by importing sediment and by internal sedi-ment redistribution (from the channels to theflats). As the ebb tidal delta is in equilibrium withthe tidal volume of the tidal basin, the ebb tidaldelta cannot be a net sediment source. Conse-quently the sediment import of the Wadden Seawill eventually lead to a net loss of sediment fromthe North Sea coast. In case of an acceleratedsea level rise more sand will be transported fromthe North Sea, mainly the coastal zone, to theWadden Sea. If the islands are fixed by hard con-structions the question arises at what costs thoseconstructions must be safeguarded.

Another, more recent, method of fixing thecoast are sand nourishments. With this methodnatural processes along sandy coasts are takeninto account and characteristic aspects of thecoast are safeguarded. Compared to hard con-structions it is a more flexible method because itcan easily be replaced by other defense measures.In many locations sand nourishment offers acheap and sustainable method for coastal defense.Elsewhere, for instance at places with deep tidalchannels, sand is rapidly carried away. In suchsituations hard defense elements, possiblycombined with sand nourishments, may be moresuitable.

In fact, the artificial sand nourishments com-pensate the loss of sand from the North Sea coastto the Wadden Sea. The question is whether inthe case of an accelerated rise in the sea level inthe future there will be enough sand for the coastto maintain this method of coastal defense. Atsome places already a gradual steepening of theunderwater shore can be observed. In the longrun, a reduction in the amount of sand availablein the underwater landward shore zone might leadto accelerated regression of the coastline.


20


Texel

Noorderhaaks

Vlieland

Den Helder

Emden


21


Figure 3.3:Coastal defense: Main

dikes, other hardconstructions and sand

nourishments.


Emden

Heide

22


3.4 The relevance of saltmarshes and summer dikes

3.4.1 IntroductionSalt marshes are tidal areas of fine sediments sta-bilized by a halophytic vegetation cover (see also3.1.2 and 3.5.3). They are favored in sheltered (lowenergy) tidal environments with an adequate sed-iment supply and a moderate sea level rise. Forcoastal defense purposes salt marshes may be de-fined as the area between the dike-foot and theMHW-level. Summer polders are former saltmarshes protected from inundation by lower stormsurges through lower sea walls, the summer dikes.

Most of the salt marshes along the mainlandcoastline of the Wadden Sea are artificial, i.e. havebeen developed through management techniques.Until the mid of the 20th century the main pur-pose of salt marsh works was to reclaim new fer-tile agricultural land. Later, in the 1960s and 70s,the main argument became coastal defense (Hof-stede & Schirmacher, 1996). In the mid of the1970s nature conservation in the Wadden Seabecame more important and, consequently, thenature function of salt marshes.

In this section the relevance of the salt marsh-es and summer dikes for coastal protection andnature conservation will be discussed on the ba-sis of an integrated analysis of costs, benefits, riskof flooding, cultural-historic and natural valuesand public perception .

3.4.2 Salt marshes

Coastal protection functionsA salt marsh in front of a sea wall (also calledforeland) reduces the wave energy and transfersthe energy-impact of storm waves from the dikestowards the edge of the salt marshes. After dikebreaching, a salt marsh prevents the establish-ment of a scour hole within the breach andprevents water to flow through the breach duringsucceeding tides. Further, the salt marsh providesmaterial (clay and salt marsh sods) for dikereparation and maintenance. Finally, salt marshesreduce the energy input and prevent damage atthe outer dike foot. Therefore, in Niedersachsen,Schleswig Holstein and Denmark higher saltmarshes render the building of revetmentssuperfluous. These arguments underline theimportance of salt marshes for coastal defense.For example, in the Niedersachsen Dike Act (2.3.1)and the Schleswig-Holstein State Water Act (2.2.1)it is written that salt marsh managementtechniques for coastal defense purposes are apublic affair.

GrazingIn Germany, grazing of salt marshes has long beenregarded necessary for enhancing salt marsh sta-bility and reducing the amounts of flotsam. Thishas created a controversy with nature protectionaims, which were directed at achieving a naturalvegetation by abandoning grazing. Several recentinvestigations have made clear that, even with-out grazing, the shear strength of the salt marsh-es suffices to prevent erosion of the surface (Erch-inger et al., 1994). The same authors concludedthat very intensive grazing may even reduce sheerstrength.

With regard to reducing the amount of flot-sam there is still controversy. Most investigationscould not demonstrate a relationship betweengrazing and amounts of flotsam washed ashore(Gerlach, 1999). According to an evaluation inNiedersachsen intensive grazing might significant-ly reduce the amounts. Investigations in the Ley-bucht have shown that the amount of flotsamincreased due to a reduction of grazing (Erchingeret al., 1996). It was concluded that, in general,grazing might influence the flotsam potential andthat further investigations have to be carried out.

ErosionA more pertinent threat to salt marshes is clifferosion. Most salt marshes are nowadays situatedin an exposed high-energy position as a result ofthe former practice of artificial salt marsh cre-ation and subsequent reclamation. Without pro-tective measures a large part of these salt marsh-es would probably erode and in the end disinte-grate. This problem will intensify if the input ofenergy by waves and tides into the Wadden Seaincreases. If the height of the tidal flats does notincrease enough to balance the expected sea lev-el rise, water depths in front of the salt marsheswill increase. This, in combination with an increas-ing storminess, might enable higher waves toreach the salt marshes and induce cliff erosion.

Intensive investigations into the stability ofartificially created salt marshes under an increas-ing sea level rise have been carried out in the Neth-erlands (Dijkema et al., 1990; Dijkema, 1992). Theresults indicate that vertical accretion at the lowermainland salt marshes is high enough to com-pensate for a sea level rise of about 1 to 2 cm peryear. Although at this moment the higher main-land salt marshes could not balance such a sealevel rise, it is expected that the higher floodingfrequency will induce a stronger accumulation (see3.1.3). The critical zone will be the higher mudflats in front of the salt marshes. Here, no signif-icant accumulation could be observed during the


23


last decade, nor can a significant increase in sed-imentation be expected as a result of higher wa-ter levels. As a consequence, the gradient betweenthe salt marshes and the mud flats might increase.This, in combination with an increasing wave at-tack, could lead to cliff formation and a horizon-tal erosion of existing salt marshes. Dijkema et al.(1990) concluded that future management tech-niques to stabilize existing salt marshes shouldpay most attention to the fronting mud flats.

Wave climateResults of model investigations have shown thatthe effect of salt marshes on wave damping mainlydepends on the water depth and the wave char-acteristics. The waves are breaking the first timeon the salt marsh and the wave energy is reducedbefore the waves reach the dike. Generally, thiseffect is getting smaller with increasing water level(Niemeyer & Kaiser, 1999; Zimmermann et al.,1999). If local water depths exceed 2.5 m, resultsof model investigations indicate no significanteffect of salt marshes on waves and wave run-upat the outer dike slopes (Zimmermann et al., 1999).Hence, in the case of a design storm surge, nodirect positive effects of salt marshes on the re-duction of wave impact at the outer dike slopemay be expected. In the Danish part of the Wad-den Sea there are dikes with security levels of 30-50 years. Secondly, the tidal range is smaller thanin Germany and The Netherlands. This means thatthe design water level for these dikes is lower thanin Germany and The Netherlands. As a conse-quence, the water depth under design storm in

Denmark is lower, so the salt marsh has an influ-ence on the proceeding design waves.

In Germany also the perception that local in-habitants have of salt marshes as an essential fac-tor in flood protection must be taken into account.For them, any reduction in salt marsh maintenanceis a defeat in the continuous battle against thesea and, consequently, a reduction in safety.

Nature protectionDuring the last decades a growing environmentalconcern has led to a new appreciation of the Wad-den Sea salt marshes as areas of very high eco-logical value.

The main aim of the German National Parks isthat salt marsh flora and fauna be governed bythe geomorphological structure of the habitat andthat natural processes can take place (Stock, 1997;Bunje, 1997). For coastal defense this would im-ply the abandonment of (most) management tech-niques. For example, for reasons of nature pro-tection, generally, clay may only be taken fromsalt marshes in case of emergency. In Schleswig-Holstein the following compromise between thecoastal defense and environmental demands wasrealized (Hofstede & Schirmacher, 1996): The com-mon goal of both coastal defense and environ-mental authorities is to preserve existing saltmarshes. Where no salt marshes exist in front ofsea walls, they should be created. The techniquesused to reach this goal depend upon local circum-stances and must be carried out as ecologicallysound as possible. If local circumstances allowsuch, technical measures are abandoned.


Salt marsh cliff, Nord-deich (FRG).


24


In Schleswig-Holstein the area with intensivegrazing of sheep has decreased from 95% in 1989to 45 % in 1995. (Stock et al., 1996). Also artifi-cial drainage has decreased. In accordance withthe Schleswig-Holstein salt marsh managementplan (Hofstede & Schirmacher, 1996) artificialdrainage in ungrazed salt marshes within theSchleswig-Holstein National Park has beenstopped.

Also in the Niedersachsen National Park therehas been a progressive reduction of grazing. Pres-ently 60% of the salt marshes are unused, 24%are extensively used and 16% are heavily grazed.Also artificial drainage has been reduced consid-erably. With the aim of integrating coastal pro-tection and nature protection interest an ad-hocProject Group was established in 1997. With re-gard to the management of the foreland thisgroup, which has in the meantime finalized itswork, recommended that only in specific casesgrazing and mowing would be possible with theaim of reducing flotsam (Striegnitz, 1999).

Most salt marshes in The Netherlands have anature protection function. On the minority(mainly privately owned) salt marsh areas agri-culture is the main use. The policy for the saltmarshes is to establish a differentiated pattern ofgrazing (no grazing to intermediate grazing pres-sure) and to stop heavy grazing with the aim ofenhancing the diversity of flora and fauna. To thisend also drainage has been reduced considerably:The amount of clay from digging activities in theDutch mainland salt marshes has decreased from500,000 m3 to 150,000 m3 in the past ten years.This reduction is mainly due to the reduction ofthe size of ditches and of the maintenance fre-quency. The reduction of artificial drainage willbe further continued, both by the cessation ofdrainage and the introduction of better tech-niques, such as the reduction of the number ofditches. Reduction of drainage could lead to lesssedimentation. Therefore, artificial drainage is re-duced to such an extent that no unacceptable ero-sion occurs.

In the Danish Wadden Sea there is, so far, noexplicit nature protection policy for the saltmarches which has been implemented in a man-agement plan.

The policies laid down in the Stade Declarationare considered as management guidelines. TheWadden Sea Nature Conservation and WildlifeReserve Executive Order aims to promote sustain-able management, as far as the natural dynamicsin the evolution of the landscape is not influencedunnecessarily. This can be regarded as an imple-

mentation of the Stade Declaration.Furthermore, the Danish Nature Protection Act

prohibits changes in the state of salt marsh areas.This prohibition includes changes in present use(amongst which agricultural use), which leads toany change in the condition/state of the area. Theregulation preserves the present situation with adifferentiated pattern of extensive and ungrazedareas. Only few areas are grazed intensively. Thelegislation and the limited actual demands keepartificial drainage and maintenance activities topractically zero. In general the morphological sit-uation in the zone between existing foreland andmudflat is in equilibrium and, consequently, coast-al protection activities are presently at a low lev-el. It is practice (as a public task carried out bythe Ministry of Food, Agriculture and Fishery) tomaintain existing salt marshes in front of dikes asa couple of hundred meters broad foreland. Thisis done mainly with brushwood groynes.

3.4.3 Summer Dikes

Coastal protectionSummer dikes only exist in the Dutch and Nieder-sachsen parts of the Wadden Sea. In The Nether-lands they have no coastal protection function.

The present summer dikes in Niedersachsenhave a function in the collection of flotsam. Theyalso avoid penetration of water in the dike footup to medium storm surges and decrease the waveenergy input on the main dike. Up to mediumstorm surges flotsam accumulates at the summerdike and is collected here. Should summer dikesbe removed, a berm and a road would have to beconstructed at the foot of the main dike for thesame purpose. Moreover, the construction of theouter slope of the main dike in Niedersachsen dif-fers from the situation in The Netherlands. In TheNetherlands the outer slope and the foot of themain dike are well protected with a hard con-struction whereas in Niedersachsen there is onlya clay layer. Also here adaptations would benecessary in case summer dikes are removed.

In Denmark there is no hard construction onthe outer slope or the foot of the main dike andthere are no summer dikes. Instead, the salt marsh/foreland is maintained in areas where erosion isobserved.

Wave climateIn The Netherlands summer dikes are not regard-ed relevant for the design of the main dike. Sum-mer dikes in Niedersachsen are part of the fore-land which is mentioned in the dike law. There is,however, discussion about the possible safetyfunction of summer dikes and its relevance for


25


the design of the main dike. This discussion is rel-evant for the option of opening or removing sum-mer dikes for nature conservation purposes (seefurther below). On the basis of model calculationsand hydraulic model tests Niemeyer and Kaiser(1999) concluded that, at design water level, sum-mer dikes have only a small wave damping effect.Mai et al. (1998) found in physical and numericalmodel tests a reduction in wave of more than 20%in case the water depth over the crest of the sum-mer dike was lower than the relative wave height.These authors concluded that, due to the reduc-tion of wave height and, to some extent on thewave period, the wave load on the main dike issignificantly reduced. For very high water levels(water level above crest more than 2.4 times waveheight) the influence of summer dikes is negligi-ble.

Nature protectionIt is the Dutch policy to outbank summer poldersby opening summer dikes so as to increase thetotal salt mars area. The background is that in thepast large areas of salt marsh have been embankedfor agricultural purposes. It is not intended to re-move the whole summer dike. Only several open-ings are made.

In Niedersachsen the option of outbankingsummer polders was discussed in the frameworkof compensatory measures for the constructionof the Europipe. The discussion mainly focussedon the role of summer dikes for coastal and flooddefense (see above) and has, as yet, not been fi-nalized.

3.4.4 ConclusionsFrom the above it is concluded that existing saltmarshes have important functions in coastal pro-tection and that maintaining existing salt marsh-es has several advantages compared to dikes with-out salt marshes. Salt marshes have a significanteffect on wave damping up to medium storm surgelevels but their effect at high storm surges is lim-ited. In the public opinion, however, salt marshesare still considered an important safety element.

Also from the nature protection point of viewmaintaining salt marshes is the preferred option.There have, however, been differing positions asregards salt marsh maintenance, i.e. the intensityof grazing and drainage. With regard to grazing,there is broad consensus about the fact that thisis not necessary for the stability of salt marshes.

There is still discussion about the relevance ofgrazing for reducing the amounts of flotsam. In-vestigations have shown that either there are nocausal links between grazing and amounts of flot-

sam or that only intensive grazing would have asignificant effect on flotsam amounts.

Summer dikes, which only exist in Niedersach-sen and The Netherlands, have a coastal protec-tion function in Niedersachsen. Their removalwould imply adaptations to the foot of the maindike, i.e. the construction of a berm and a roadfor removal of flotsam and strengthening of theouter slope.

Summer dikes have only a limited function inflood defense, although there is still controversyabout the question to what extent summer dikesare relevant for the design of the main dike. Inthis discussion also the perception of the localinhabitants plays an important role.

3.5 The relevance of biotafor sedimentation- and

erosion processes3.5.1 Introduction

The influence of biota on marine sedimentationand erosion processes is usually ignored. This iscertainly not justified for the Wadden Sea whereinnumerable individuals of plants and animals in-fluence the muddy and sandy intertidal flats. De-pending on abundance and species compositionthis biota influences transport, sedimentation anderosion of the sediments. Moreover, the vegeta-tion in salt marshes and dunes is the most impor-tant factor for the retention of mud respectivelythe formation of dunes and their protectionagainst wind erosion.

3.5.2 Intertidal area

Biogenic sedimentationChanges in the sedimentation and erosion pro-cesses and in the sediment composition causedeither by biological activities or biogenic struc-tures are defined as biogenic sedimentation. De-pending on the local conditions biogenic sedimen-tation can prevail physical sedimentation as re-ported from the tidal basin of List (Sylt) (Bayerl etal., 1998) or the Meldorfer Bucht (Gast et al.,1984)and can contribute to the raising of tidal flats(Thiel et al.,1984).

Wadden Sea organisms living on or in the sed-iment actively contribute to sedimentation bydeposition (biodeposition) or by stabilization (bio-stabilisation) of the sediment. More passiveepibenthic biogenic structures, such as musselbeds, affect the local hydrodynamic conditions,enhance sedimentation or prevent erosion. On theother hand biota is also able to increase the erod-


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ibility of the sediment or to destruct the surfaceby bioturbation, burrowing activities and resus-pension of particles.

BiodepositionBiodeposites are fecal pellets (feces and pseud-ofeces) being deposited at the sea floor (Haven &Morales-Alamo, 1972). Biodeposition influencesthe sediment composition and, subsequently, thehabitat and community structure of the intertidalecosystem. Austen (1997) found that up to 80%of the sediment volume of mudflats of the Knigs-hafen (Sylt) was formed by fecal pellet.; On flatswith mixed sediment the rate was 1 50% andon sand flats 1 13%. Biodeposites change thesediment composition by agglomerating fine par-ticles (e.g. clay, silt, organic debris) to pellets withthe size of sand grains. Because these pellets showthe same sedimentation behavior as sand, finematerial is deposited at locations where, undernormal physical conditions, only sand may be ex-pected. The fine material can be worked into thesediment by bioturbation. Biodeposites, moreover,do not take part in the current induced transportprocesses of their original particle size (Thiel etal., 1984). Pellets can mainly be found in the sed-iment layers near the surface, whereas in deeperlayers they are normally converted into homoge-neous mud (Bayerl et al., 1998; Austen, 1997).Maximum deposit rates of 10.5 mm/month werecalculated for cockles, 0.1 mm/month for the Bal-tic tellin and 0.04 mm/month for the soft clam(Thiel et al., 1984).

Very high biodeposition rates are common inmussel beds. For the period 1975 to 1978, whenmussel beds covered about 41.5 km2 of the DutchWadden Sea, Oost (1995b) calculated an amountof 7.7 million tons of sediment bound in the eu-littoral mussel beds and 9.8 million tons in sublit-toral beds. To a certain degree biodeposites willbe exported from mussel beds so that mixed sed-iment can be found in the vicinity (Oost, 1995b).The substantial loss of mussel beds in the Dutchand Lower Saxonian Wadden Sea during the 1980sand 1990s caused a decrease of areas with mixedsediment (Obert, 1995; Oost, 1995b). It is con-cluded that mussel beds have an important influ-ence on the amount of fine material and the sed-iment balance of the Wadden Sea.

BiostabilisationBiostabilisation is defined as the stabilization ofsediment by organisms (Thiel et al., 1984) e.g. themucous coating of upper sediment layers bybenthic diatoms, filamentous networks of blue-green algae or microbial mats. The mucous excre-


tions of these organisms have a cohesive effecton the sediment particles and decease the rough-ness of the sediment surface (Asmus et al., 1994;Paterson et al., 1994) which results in an increasedcritical velocity for erosion processes. Field experi-ments in an intertidal area of the Oosterschelde(SW Netherlands) have shown that in higher en-ergy parts of the intertidal flats strong erosion mayoccur in places where the algae cover is chemicallydestroyed (De Boer, 1981). Laboratory experimentshave revealed that a diatom population that wasallowed to re-establish itself for 24 hours afterstirring, increased the critical velocity by severaltens of percents, as compared to freshly stirredand redeposited sand. Stabilization by diatoms cantherefore be an important factor in the stabilityof sediment in intertidal and shallow subtidal ar-eas. In addition the cellular network of filamen-tous blue-green algae causes a mechanical con-solidation of the sediment (Stal, 1994). Anoxicsediment surfaces like the black spots in the Nied-ersachsen Wadden Sea, which have very low abun-dance of microphytobenthos, show a decrease ofthe critical threshold velocity and can easier beeroded (Austen & Witte, 1997).

Biogenic structuresBiogenic structures also have effects on the sed-imentation processes. By decreasing current ve-locities or water turbulence these structures actas sediment traps for fine grained material. Mus-sel colonies form a semi-rigid framework whichtends to bind the sediment, thereby protecting itfrom erosion. In seagrass meadows the propor-tion of fine particles is higher than in the sur-rounding flats and the meadows are slightly ele-vated (Asmus & Asmus, 1998). The loss of sublit-toral seagrass in the 1930s increased the erosionin the areas where the losses occurred (Reise,1998). A high abundance of tube building poly-chaete worms can change the small scale topog-raphy of the flats by forming a layer of mud onthe former sandy sediment surface or by develop-ing a distinct structure of consolidated embossedpatterns and erosive troughs (Heuers et al., 1998).

BioturbationOne effect of bioturbation is the transport of fine-grained material, e.g. biodeposits, from the sur-face into deeper, less erodable sediment layers.But bioturbation can also enhance the erodibility:(1) by increasing the bottom roughness; (2) by ac-tively bringing grains in suspension; (3) by sortingof sediment. The increase in erodibility enhancesthe sediment transport. Some forms of bioturba-tion tend to destroy small-scale structures on the

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surface. Intensive bioturbation can obliterate rip-ples formed during one tide.

On the higher parts of the tidal flats organismshave sufficient time to completely modify the orig-inal depositional structures. One of the most im-portant bioturbators in the Wadden Sea is the lug-worm. The animal lives in a U-shaped burrow atdepths of 15 to 20 cm, almost continuously in-gesting sand. From time to time the animal ex-cretes the ingested sand at the surface. It has beencalculated that the top 30 cm of the tidal flatsediments can be reworked completely by lugwormseach year (Cade, 1976). The grains that are toolarge to be swallowed are concentrated at thedeepest point of the U-burrow. In this way exten-sive layers of shells, especially of mud snails, areformed.

3.5.3 Salt marshesIn the pioneer zone (the transition between tidalflats and salt marshes) and in salt marshes, plantsadapted to the extreme environmental conditionsare the principal biotic component influencingsedimentation. Where a tidal flat is elevated tosome decimeters below the mean high water lev-el, the pioneer plants glasswort and common cord-grass settle, followed with increasing elevation bythe sea aster and the annual sea-blite (Dijkemaet al., 1990). Glasswort does not significantly in-fluence sedimentation, but enables other plantsto settle, while the common cord-grass is knownto enhance sedimentation. When the elevation ofthe pioneer zone increases, the vegetation coverof pioneers becomes denser and other plants startto settle.

The border between the pioneer zone and thelower marsh, i.e. the area around or above themean high-water level is characterized by the ap-pearance of marsh grasses. By decelerating theflow of the current during flooding, the vegeta-tion of halotolerant plants enhances sedimenta-tion rates to maximum values (Dijkema et al.,1990). In this way large amounts of sediment areretained and stabilized by the root systems of themarsh plants; the marsh accretes vertically. In ad-dition, erosion is strongly reduced. When the marshbecomes even higher, sedimentation rates de-crease due to the decreasing number of floodingsand also due to the lower sediment supply of in-dividual floodings. As a result of sedimentationthe lower marsh zone evolves into a middle marshzone with a characteristic plant community. Above

this zone the upper marsh commences with nor-mal grassland plants.

Besides the deposition of fine silt and clay inthe middle and higher marshes during (very) highwater, beds of sand and shells may be depositedduring storms over kilometer wide areas. Moresubstantial shell hash deposits, situated furtherinland, are the result of activities of birds such aseiderducks, oystercatchers, gulls and crows.

3.5.4 DunesAlong the Wadden Sea coast dune formation, sta-bilization and protection against erosion, is closelyconnected to plants which are able to settle insuch a barren sandy environment. Dunes withoutvegetation are unstable, for example smallbarkhans (Sicheldnen) on the beaches or verylarge, moving transversal ridges (Wanderdnen)at the inner borders of the dune areas on someislands, which occur when the vegetation cannotcope with the moving sand or after destruction offixed dune systems (Doing, 1983; Ellenberg, 1982).The establishment of more permanent dunes startswhen, under favorable conditions, e.g. sufficientrainfall and presence of some organic matter, pi-oneer plants settle on sand which is piled up bythe wind in the lee of shells, plants or flotsam.Sand couch grass and lime grass are the halotol-erant pioneers which are able to start dune suc-cession on the beaches, accumulating sand to pri-mary dunes with maximum heights of about 2 m.The second step of dune development, the whitedunes, which can be piled up to ridges of up to 10m, is a result of the growth of sea marram whichis the most effective dune forming plant. Theplants are able to fix the sand with their strongvertical straws and long horizontal root systemsand are able to grow through the sand when theyare covered during storms. Layer by layer theyclimb up with the sand which is deposited due tothe drop of the wind speed in their presence (El-lenberg, 1982). More landward, where the mov-ing sand calms down and the accumulation ofhumus, decalcification, leaching of nutrients andacidification starts soil development, gray andbrown dunes, densely covered with characteristicplant societies, form the next stages of dune suc-cession (Neuhaus & Petersen, 1999). Here, vege-tation mainly prevents wind erosion. In all suc-cessional stages and in all dune areas where wind,waves, rainwater, animals or man damage the veg-etation, sand drifts may occur.

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4.1 IntroductionIn the foregoing chapters an overview has beenpresented of current Wadden Sea nature protec-tion and coastal defense polices and the commonknowledge basis regarding geomorphology andrelated biological processes and coastal defensetechniques in the Wadden Sea. In this chapter anoverview will be given of changes in water levelsand storminess which have occurred in the past(section 4.2) and which are expected to occur inthe future (section 4.3). For the latter the scenar-ios as elaborated by the Intergovernmental Panelon Climate Change (IPCC) will be used. In the fi-nal section 4.4 a description is given of the meth-odology applied by the CPSL for the

Documents

Coastal Protection and Sea Level Rise. Final report. 2001