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Ocean & Coastal Management 68 (2012) 18e38

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Ocean & Coastal Management

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Barrier island management: Lessons from the past and directions for the future

A.P. Oost a,*, P. Hoekstra b, A. Wiersma c, B. Flemming d, E.J. Lammerts e, M. Pejrup f, J. Hofstede g,B. van der Valk h, P. Kiden i, J. Bartholdy f, M.W. van der Berg i, P.C. Vos b, S. de Vries b, Z.B. Wang j

aDeltares & Utrecht University, Department of Physical Geography e IMAU, P.O. Box 177, NL 2600 MH Delft, The NetherlandsbUtrecht University, Department of Physical Geography e IMAU, The NetherlandscDeltares, Applied Geology and Geophysics, Utrecht, The Netherlandsd Senckenberg Research Institute, Wilhelmshaven, Germanye Staatsbosbeheer, Groningen, The NetherlandsfDepartment of Geography and Geology, University of Copenhagen, Denmarkg EUCC-The Coastal Union GermanyhDeltares, Morphologic and Sedimentary Systems, Delft, The NetherlandsiGeological Survey of the Netherlands e TNO, Geomodelling Department, Utrecht, The NetherlandsjDeltares & Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands

a r t i c l e i n f o

Article history:Available online 27 July 2012

* Corresponding author. Tel.: þ31 (0)6 22337195; fE-mail address: albertpeter.oost@deltares.nl (A.P. O

0964-5691/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.ocecoaman.2012.07.010

a b s t r a c t

The article focuses on the morphological development of the Wadden Sea barrier island system, withemphasis on West and East Frisian islands on several temporal and spatial scales. In addition, it inte-grates the insights for management purposes. Barrier island management is addressed with respect tomorphology, sediment budgets, safety and natural values. We show that each of these issues is deter-mined to some extent to various spatio-temporal scales and that the management of a barrier island hasto be considered in terms of interactions on various spatial and temporal scales.

Morphology of some of the barrier islands is determined by the pre-existing Pleistocene relief to a fairextent, either directly due to erosion-resistant outcrops on or near the islands, or indirectly by deter-mining the locations where inlet systems or estuaries could develop. Where this is not the case, thelarger part of the sediments are locally reworked Pleistocene or Holocene deposits eroded at the NorthSea coasts of the barrier chain and deposited in the back-barrier area and on the islands as a response tosea-level rise. Hardly any sand is coming in from outside the area. In order to keep up with sea-level risesand has thus to be nourished if coastal retreat is not allowed.

During the long Holocene evolution islands and ebb-tidal deltas have been lined up during theircoastward migration, forming a more or less uninterrupted barrier chain along the Frisian coasts. Thepresent-day approach of mainly focusing on the fixation of the inhabited parts of the chain will mostlikely result in a de-alignment of the various parts of the chain, resulting in increasing erosion of thepromontories.

An inlet system is a sediment-sharing system with a tidal inlet, the ebb-tidal delta, adjacent barrierislands and the tidal basin with channels, shoals, tidal flats and salt marshes. The sand balance ofa barrier island is thus directly linked to tidal inlet system development. A natural change or an inter-vention in the sediment-sharing system by man may thus have repercussions for the island’s develop-ment. Sediment redistribution in the coastal zone may also depend on climate, as is illustrated by therapid growth of the islands after the demise of the Little Ice Age.

On the barrier islands themselves many measures were taken during the past two centuries to ensurecoastal safety. The successful attempts to stabilize the coasts and dunes of the barrier islands resulted ina reduction of sand transport from and along the shoreface to the beach and onto the islands. To someextent this has been restored by applying sand nourishments. However, vertical accretion of the islands isstill largely impossible due to all the older coastal protection measures still present. On the long runsedimentary dynamics are essential if the island is to accrete vertically with sea-level rise, which formsa robust and sustainable strategy to guarantee safety during the next centuries. Massive stabilization alsoreduced the opportunities for pioneer vegetation. Dune belts and tidal marshes have experienced a fastsuccession resulting in a climax vegetation and the loss of the characteristic open landscape.

ax: þ31 (0)88 335 8582.ost).

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A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 19

In order to restore nature sufficient space and time should allow natural processes to develop and tocreate robust ecosystems. Instead of focusing on nature conservation a paradigm shift is needed inbarrier island management towards stimulating the development of natural dynamics. To our opinionthe best solution is to allow the geo-biological processes to take their natural course as much as possible.Examples are given for the various parts of a prototype barrier island. Only where this is not feasibleother management practices should be applied. As a rule of thumb: soft and with respect for morpho-dynamical integrity where possible, hard where really needed. This concept applies both to the variousmorpho-ecological units on the islands and to the morphological developments on larger spatio-temporal scales.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Barrier islands belong to the most dynamic coastal environ-ments where rapid changes may occur in terms of coastal processesand morphological as well as ecological developments (Hayes,1979, 1980; Davis, 1994). Coastal zone management of barrierisland systems is not a trivial issue, but requires a proper under-standing and appreciation of the challenges under discussion.These challenges may vary from changes in the natural system dueto external forcing, to the impact of human measures on thedynamic character and resilience of the coast. Major present-daychallenges are related to: i) coastal safety; ii) natural resources;iii) the tourism industry and iv) the development of ecosystems andassociated ecosystem services. Coastal safety is of paramountimportance for those islands that are vulnerable to flooding andsea-level rise, in particular due to a lack of sediment. Coastal safetyis commonly established by stimulating dune development, bybuilding seawalls, dikes and groynes or by implementing nour-ishments. However, the issue of coastal safety is not limited to thebarrier island “sensu stricto” but also implies the protection ofadjacent wetlands and the mainland coast. In the NW EuropeanWadden region, for example, barrier islands in combination withthe Wadden Sea provide a natural buffer between the highlyenergetic conditions in the North Sea and the mainland coast.Likewise, barriers and barrier island systems in the abandonedparts of the Mississippi delta protect an extensive system ofwetlands and lagoons (Fritz et al., 2007). Once a barrier island iseroded, an accelerated loss in wetlands is observed (Sallenger andWilliams, 1989; Penland et al., 1992).

The use of mineral resources of barrier islands is another activitythat requires careful management. The most important impacts arerelated to sand mining, gas and oil exploitation and the extractionof drinking water. All have in common that they produce a certaindegree of subsidence and result in a greater accommodation spacefor sediment. In addition the non-sustainable extraction of drinkingwater may result in a reduction of the fresh water supplies on theislands and the intrusion of sea water. The increased use of localfresh water supplies is often triggered by a more demandingtourism industry that is important for the economic developmentof the islands. Well-known examples of tourism development arefound along the West and East Coast of Florida (USA), the Algarve(Portugal) and along the West, East and North Frisian Waddencoast. Due to their highly dynamic nature, barrier islands andsurrounding coastal waters also represent valuable assets in theform of ecosystems and ecosystem services. Therefore, ecosystemdynamics and development versus coastal protection and natureconservation should be important considerations in barrier islandmanagement.

Barrier island management thus involves a range of stake-holders with often conflicting interests. On top of that, environ-mental and societal problems are increasingly associated withglobal change issues such as climate change and (accelerated) sea-

level rise. This can be considered as a threat, but can also beinterpreted as an opportunity. Nowadays it is often tried to tacklethe problems by means of an integrated coastal zone management(ICZM) approach. It “involves the comprehensive assessment, settingof objectives, planning and management of coastal systems andresources, taking into account traditional, cultural and historicalperspectives and conflicting interests and uses” (WCC, 1994). Theagents of change are driven by socio-economic demands, naturalprocesses and the impacts of climate change (Misdorp, 2011). It isadvocated that integrated spatial planning should not onlyencompass small-scale adaptations, but should also take a long-term look ahead (Baarse and Misdorp, 2011). In practice,however, the ICZM approach is mostly to react on relatively short-lived developments as far as natural processes are concerned, oftenwithout having a sufficient in-depth understanding of the causesand consequences. As a result, ICZM does seldom take into accountthe various temporal and spatial scales that are relevant tomorphological developments. This is true for most barrier systemsin the world. As an example, problems of coastal erosion on theFrisian barrier islands were tackled in the past by constructinggroins and are nowadays solved by implementing nourishments.Nevertheless, in both cases measures are (were) taken withouthaving a proper understanding of the short- and long-term causesof erosion and accretion (Wang et al., 2012). Furthermore theimpact of human actions on the natural functioning of the coast, inboth time and space, is often poorly understood.

In this paper we will demonstrate that barrier island manage-ment can benefit from a more comprehensive approach that isbased on a careful analysis of the impact of past, present and futureprocesses and developments on barrier island dynamics. Weillustrate this approach for especially the West and East FrisianBarrier Islands and to a lesser extent the North Frisian area. To thisend we address the following three questions:

� What can we learn in terms of ICZM from the natural devel-opment and behavior of theWadden Sea barrier island system?

� What has been the impact of human measures and interven-tions on barrier island development in the last decades andcenturies?

� Is our present-day ICZM strategy a successful, integral andsustainable approach that is able to cope with future aspects ofglobal change or do we have to adopt our coastal policies?

We will argue that the present-day ICZM approach is in favor ofshort-term, local coastal safety and focuses on the maintenance ofbeaches and dunes. However, this approach fails to address someother fundamental issues such as long-term safety, sedimentdynamics and budgets in relation to coastal resilience, ecology andbiodiversity. We advocate that a more balanced approach is desir-able; an approach that better acknowledges the dynamic characterof the islands and involves safety, morphology and ecology. Such anapproach is feasible, but also requires a trade-off between different

A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e3820

stakeholders, evenwithin the community of the environmentalistsif one has to deal with the dilemma of nature conservation orrestoration versus spontaneous nature development due to estab-lishment of the appropriate boundary conditions.

In our analysis the relations between different temporal andspatial scales are crucial. Our central concept is to regardmorphological developments of barrier islands as the product ofdevelopments at a hierarchy of spatial scales. Our concept of scalesassumes a strong coupling between the temporal and spatial scalesinvolved; an increase in spatial scales considered automaticallyimplies that one has to take into account developments at largertime scales (and vice versa). Developments at larger temporal andspatial scales impose boundary conditions on developments ata smaller scale. Throughout this paper, wewill apply this concept ofscale for different objectives.

The first and second question are addressed by analyzing thenatural behavior of the Frisian barrier island system at three relatedscale levels, which partly correspond to three different sources ofinformation: i) the geological record (millennia), ii) the historicalrecord (centuries to the past millennium), and iii) the currentsystem (years and decades to a few centuries). We distinguish thefollowing sequence of scales (see also: Speelman et al., 2009):

� Development of the Wadden Sea barrier island system duringthe Holocene.

At this scale level it will be shown that the present-dayconfiguration of the island chain is mainly the product of Holo-cene (marine) processes that operate(d) in conjunction with thepre-existing Pleistocene topography or relief.

� Interactions of the tidal inletebarrier island system over thepast millennium.

Each individual tidal inlet system can be considered to forma sediment-sharing system consisting of a tidal inlet, ebb- and/orflood-tidal delta(s), updrift and downdrift barrier island coastsand the tidal basin, all of which are basically in a hydro-morphodynamic equilibrium with the prevailing conditions.Changes in these equilibria will lead to morphodynamic reactionsin the system until either the original equilibrium is restored ora new equilibrium is established (Dean, 1988; Eysink, 1991;Steijn, 1991; Oost, 1995; Wang et al., 1995; CPSL, 2001, 2005;Kragtwijk et al., 2004; Elias et al., 2005, 2012, acc.; Dastgheibet al., 2008).

� Behavior of an individual barrier island on a time scale ofdecades to a few centuries.

At this scale level we identify different so-called morpho-ecological units, which together comprise the individual island. Inour conceptual approach this island represents a prototype islandsince it illustrates the spatial and temporal coherence anddynamics of the morphological subunits of an island in a schema-tized way. This conceptual framework also appears to be extremelyuseful from an ecological point of view (Löffler et al., 2011).

The paper is organized as follows. In the following section wewill present some main characteristics of the entire Wadden Searegion. We will then continue with discussing the processes anddevelopments at the three different temporal and spatial scalesinvolved and use this information to asses the impact of centuriesof human intervention on barrier island development. Subse-quently the pro- and cons of present-day ICZM strategies areevaluated and an alternative approach is proposed. The paper endswith a discussion and major conclusions.

2. Description of the area

The Frisian barrier island system forms the boundary betweenthe open North Sea and the Wadden Sea and stretches for over450 km along the coasts of the Netherlands, Germany and Denmark(Fig. 1). The system is characterized by relatively short (10e30 kmin length) barrier islands.

The mixed micro- to macrotidal, semidiurnal tidal regime(Fig. 1) is characterized by tidal ranges that initially increase from1.4 m at Den Helder (western Wadden Sea) to 4.4 m at Bremen(central part of the Wadden Sea in the German Bight), beforeprogressively decreasing again to 1.5 m at the Skallingen barrier-spit that marks the northern boundary of the Wadden Sea inDenmark. Mean significant (offshore) wave heights for the calmpart (15 Aprile15 October) were calculated to be around 0.5e1 mand 1e2 m during the storm part (15 Octobere15 April) of theyears 2002 and 2003 (Dobrynin et al., 2010). In the West and EastFrisian area waves and swell are predominantly incident from thewest and southwest and along the North Frisian coast the majorwaves come from the northwest (Bartholdy and Pejrup, 1994; Beelset al., 2007). During storm surges, the maximum recorded set-up inwater level is about 3.5e4 m and the offshore maximum waveheight may be up to 8e11 m (data of several water managementministries).

The islands are separated by 33 adjacent tidal inlets and asso-ciated tidal basins, separated by tidal watersheds (Fig. 1; Dijkema,1980; Hofstede, 2005). Each tidal basin is drained by a channelsystem, which is connected with the North Sea via the tidal inlet(Cleveringa and Oost,1999). The inlets are generally lined by barrierislands on either side, only the first and last inlet being bordered bythe headland of Den Helder in the south and the Skallingenpeninsula in the North.

The modern Wadden Sea can be largely subdivided into twodifferent regions:

1) A chain of barrier islands fronting tidal basins, which extendsfrom the westernmost island of Texel near Den Helder in theNetherlands, up to the area between the Weser and Elbeestuary in Germany, together forming theWest and East FrisianWadden Sea (separated by the Ems estuary).

2) The area between theWeser and Elbe estuary in Germany up tothe Skallingen peninsula in Denmark which forms the NorthFrisian Wadden Sea with a north-south trending series ofislands. In the German Bight true barrier islands are absent dueto the large tidal range (Hayes, 1979, 1980; Jacobsen andMadsen, 1993).

Rivers along the Wadden Sea coast did, and do, not bring insubstantial amounts of sediment. A small amount of sand anda larger amount of fines is imported with the coast-parallelcurrents from the south along the West Frisian coast and fromthe north along the North Frisian coast. The bulk of the sedimentswithin the Wadden Sea, however, consists of locally reworkedPleistocene material selected, transported and imported bymarine processes into the Wadden Sea as demonstrated by themineralogical composition of the sediment (e.g. Winkelmolenand Veenstra, 1974, 1980; Hoselmann and Streif, 2004). Thesediment needed in the back-barrier area, among others tobalance sea-level rise, was and still is largely “taken” from theNorth Sea side of the barrier island chain (Mulder, 2000). Asa result, the barrier islands and tidal basins tend to shift landwardwith rising sea-level rise. The strong coastward retreat of theWest and East Frisian Islands over up to 10 km and the smallerretreat of the North Frisian barrier islands resulted in thereworking of sediments.

Fig. 1. Overview of the Wadden Sea region and overview of tidal range (in m) (based on: Ehlers, 1988; Wiersma et al., 2009).

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3. Barrier islands of the Wadden Sea system: evolution intime and space

3.1. Development of the Wadden Sea barrier island system duringthe Holocene

During the last glacial, most of the southern North Sea was landand the nearest shoreline was off the coast of Brittany (France) inthe south and near the Shetland Islands in the northwest. The riversEms, Weser, Elbe and Eider drained northward via a broad valley(Figge, 1980). After the last Ice Age relative sea-level rise wasinitially rapid (over 1m/century), but decelerated significantly after7500e7000 a BP (Fig. 2; Kiden et al., 2002; Gehrels et al., 2006;Busschers et al., 2007; Vink et al., 2007; Kiden et al., 2008; Pedersenet al., 2009; Baeteman et al., 2011). In close association with therelative sea-level rise, the tidal range increased from initiallymicrotidal conditions everywhere to themore differentiated rangespresently observed along the coast as the water depth in thesouthern North Sea basin increased (Jelgersma, 1979; Franken,1987; Van der Molen and de Swart, 2001).

Around 8000 a BP sea level was still some 20 m below itspresent level (Fig. 2) and the North Sea coastline was much furtheroffshore than its present position, e.g. 10e15 km for the centralWest Frisian Wadden area (Vos and Van Kesteren, 2000). Therising sea level followed (and modified) the pre-existing relief ofthe Pleistocene land surface and thereby constrained the initial

position of the evolving Wadden Sea. Of particular importancewere the following effects:

1) The initial differences in general paleorelief, with the north-south trending North Frisian area being more dominated bya steeper Pleistocene relief and lower availability of loosesediments than the east-west trending East and West Frisianarea. This is, next to lower rates of sea-level rise in the NorthFrisian area, considered to be an important reason for thestronger retreat of the East and West Frisian area, where sea-level rise results in inundation of larger areas in the mainlanddirection.

2) The location and scale of paleo-valleys incised into the Pleis-tocene land surface during the sea-level lowstand. This paleo-channel morphology determined the location, size and inlandpenetration of estuaries, e.g. the Lauwerszee (Oost, 1995), theEms-Dollard estuary, the Jadebusen, and the Weser and Elbeestuaries (Streif, 2004; Wiersma et al., 2009). Moreover: as theestuaries were deeply incised into the mainland, the sea couldeasily invade inland peat areas and convert them into back-barrier basins (e.g. Dollard- and Jade-embayments) therebyenlarging the tidal volume of the estuaries. The increase in tidalprism led to deeper incision of the tidal channels. In particularthe flooding of the paleo-valleys of Ems, Elbe and Weser hada large impact on the coastal development, as the landscapedeveloped into a broad estuarine landscape, which determines

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Fig. 2. Differences in Holocene relative mean sea-level rise (A) for different regionsalong the Belgian-Dutch-German North Sea coast (B), making clear the differences ona regional scale. Sea-level rise in NW Germany has been larger than in the WesternNetherlands and in Belgium. The differences are mainly caused by relative glacio-isostatic subsidence (Mörner, 1979). Figure modified from Kiden et al. (2008), thesea-level curves are from Van de Plassche, 1982 (W Netherlands); Denys andBaeteman, 1995 (Belgium); Kiden, 1995, 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.

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the morphology up to this day. In the West and East FrisianWadden Sea area, the deepest river valleys were floodedaround 8000 a BP (Beets and Van der Spek, 2000). In a fewcases the former paleo-valleys still determine the position ofpresent-day tidal inlets (Wiersma et al., 2009).

3) The occurrence of locally elevated Pleistocene (or older)outcrops and headlands consisting of moraine deposits of theSaale (second-last) glaciation, and sandy meltwater deposits ofthe Weichselian (last) glaciation, especially in the North FrisianWadden area (Bartholdy and Pejrup, 1994; Lindhorst, 2007).Several of the barrier islands evolved around such Pleistoceneor older cores of ice-dislocated deposits, a.o. the islands ofTexel, Amrum, Sylt and Süderoogsand. These headlandscommonly also acted as a source of sediment for the longshore

drift, responsible for a re-alignment and smoothing of the oftenheavily indented coastline. Furthermore there are Pleistoceneheadlands present along the mainland coast, e.g. at Wieringenin the Netherlands and near the coast of Blåvands Huk, north ofSkallingen. Furthermore the presence of sub-marine Pleisto-cene ridges can influence currents and wave impacts, e.g. at thesouthern “spit” of Sylt (Lindhorst, 2007). Smaller barrierislands, sandy shoals or sand spits were likely present in frontof the mainland coast as relicts of Pleistocene headlands(Flemming and Davis, 1994).

4) The low rate of relative sea-level rise in the northern part of theNorth Frisian barrier coast: Pedersen et al. (2009) gave onlyw12 m over the past 8400 years. This led to a much morerestricted erosion of the islands. For instance, the northern spitof Sylt was formed as early as 5000 BP (Lindhorst, 2007),whereas Rømø prograded seaward since 8000 a BP, due toabundant sediment supply (Madsen et al., 2010).

The bulk of theWest and East Frisian barrier island chain formedbetween 6000 and 5000 a BP. Because sedimentation rates wereinsufficient to fill the accommodation space created by the initiallyrapidly rising sea (1 m/century), much of the inundated surfaceevolved into subtidal environments, fringed at the landward side bya narrow zone of intertidal sand andmud flats and salt marshes. Anincrease in tidal ranges resulted in higher tidal volumes, whichmayhave generated an increased net sediment import into the tidalbasins (Vos and Van Kesteren, 2000). However, it was alsoaccompanied by further erosion and deepening of the tidal chan-nels of which the sediments became available to the tidal flats(Hofstede, 2002) and to the ebb-tidal deltas. In this landscape, fensgave way to raised bogs. They started to expand on the mainland ofWest Frisia between 7000 and 6000 a BP (Casparie and Streefkerk,1992; Vos et al., 2011) and on the mainland of East Frisia betweenespecially 7200 and 5600 a BP (Petzelberger et al., 1999; Ecksteinet al., 2010). The onset is attributed to a shift to wetter climateconditions, which were brought about by rising sea level in theearly and mid-Holocene (Eckstein et al., 2010).

The sea reached the Pleistocene deposits of the southern part ofthe North Frisian area around 6500 years ago. Cliffs were formedand thematerial was transported along the coast. Around 6000 a BPSylt was most likely already an island (Jessel, 2001). Until recently,not much was known about the North FrisianWadden region in theearly Holocene. However, new research (Madsen et al., 2010)suggests that the formation of northern North Frisian barrierislands started some 8000 a BP.

At about 5000 a BP, the decelerating rate of sea-level rise alongthe Dutch west coast and the westernmost part of the West FrisianWadden Sea area (Fig. 2) was exceeded by sediment accumulationrates. Thus, intertidal sand flats grew at the expense of subtidalenvironments (Van Heteren and Van der Spek, 2003; Vos et al.,2011). During the following millennia, sea-level rise deceleratedeven further, and the infilling of the available accommodationspace tipped increasingly in favor of sedimentation exceeding sea-level rise. In the eastern part of the West and in the East FrisianWadden Sea, however, subsidence of the bottom was relativelylarge and sedimentation was insufficient to infill the basinseverywhere (Vos et al., 2011). In some places the tidal area wasextending, e.g. in the Boorne, Hunze and Fivel areas in the WestFrisianWadden Sea (Vos et al., 2011). From time to time, parts of theWadden Sea locally silted up and the salt marshes at the landwardfringe of the tidal basin could advance seaward (see Behre, 2004;Vos and Van Kesteren, 2000; Vos et al., 2011). Raised bogs spreadover a larger area up to 3500 a BP. Some of these peat layers reacheda considerable extent (Behre, 2004). However, most of these freshwater marshes drowned again and clastic tidal sediments were

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deposited on top of them (Vollmer et al., 2001; Behre, 2004; Streif,2004).

At about 5000 a BP, the West and East Frisian barrier islandchain was still situated several kilometers offshore from its presentposition, e.g. in the case of Terschelling about 9.5 km further to thenorth in comparison to its present position (Sha, 1990a,b; Vos et al.,2011). From 5000 a BP until today, the chain of barrier islands hasbeen retreating landward at an average migration rate in the orderof 1e2 m/year. The landward migration of the barrier islands wascaused by erosion on the seaward side due to the rising sea leveland transport into the back-barrier tidal basins of the Wadden Seawhere the sediment deficit created by this sea-level rise more orless was compensated (Oost, 1995; Wang et al., 2012, this volume).

In the southern part of the North Frisian area (a.o. Eiderstedt),the barriers and Pleistocene outcrops in the area created an inlet-segmented coastline around at least 5000 a BP (Vollmer et al.,2001; Meier, 2004; Lindhorst, 2007). On the peninsula of Eider-stedt, the cores of the modern east-west trending sand bars prob-ably consist of eroded Pleistocene material and are a relict of thisbarrier system (Hoffmann, 2004). The sandy barrier system pro-tected the hinterland and around 2500 years ago extensivemarshesformed, consisting of thick sequences of clayey and peaty sedi-ments with small-scale relief due to differential compaction (Meier,2004). Although a large part of these marshes eroded duringseveralmedieval times, its remnants still dominate themorphologyof the area. Seaward of the marshes, tidal flats began to form as, forexample, documented on Hallig Hooge and Pellworm (Vollmeret al., 2001).

3.2. Interactions of tidal inletebarrier island system over the pastmillennium

3.2.1. Coastal evolution between 1200 and 500 a BP (Fig. 3; afterWiersma et al., 2009)

The reconstruction of theWest FrisianWadden area at 1200 a BP(Fig. 3) shows the situation before large-scale human interferencewithin the system developed. The situation in theWest Frisian areais considered to be representative for especially the West and East

Fig. 3. Reconstructed map of the West Frisian Wadden Sea for the situat

Frisian Wadden Sea in this period. At that time the coastal envi-ronments from the mainland up to the Wadden islands were notyet separated from each other by dikes. Towards themainland coastthe tidal flats merged with tidal marshes and extensive brackishareas, which, in turn, were flanked by extensive peat bogs, lininghigher sandy areas (Esselink, 2000). Tidally influenced rivers andstreams debouched freely into the Wadden Sea. Settlements in thecoastal marshes were built on dwelling mounds, which hada negligible influence on the large-scale tidal dynamics. Initially thedwelling mounds made use of the existing landscape and werebuilt on small depositional ridges along marine/estuarine channels.

The first clear ring dikes in West and East Frisia were builtaround 1000e900 a BP and in North Frisia around 900e800 BP(Van der Spek, 1994; Oost, 1995; Ey, 2010), surrounding smallerareas to safeguard arable fields and against most winter floods. Asecond type of dikes was oriented rectangular to themain stream tochannel the outflow of waters and has at least been built since1000e800 a BP (Ey, 2010). Gradually, local ring dikes were con-nected and raised, and by 800e700 a BP a continuous system ofwinter dikes had been constructed along the entire West and EastFrisian Wadden Sea coast and several parts of the North Frisiancoasts (Oost, 1995; Ey, 2010). The diking was partially a response tothe increasing vulnerability to flooding due to peat excavation e

both inside and outside the dikes e and the development ofdrainage systems (e.g. Balgzand, Lauwerszee, Dollard, Jade andNorth Frisian area; Oost and Kleine Punte, 2004; Kühn, 2007;Wiersma et al., 2009). The vulnerability has resulted in some major“invasions” by the sea and the (re-)opening of many embaymentsalong the Wadden Sea (Zuiderzee, Middelzee, part of the Lau-werszee, Dollard, and the Jade Busen; Pons andWiggers, 1960). Thespatial extension of the tidal basins generated a larger tidal prismand larger sediment demand. Before 500 a BP, many larger baysreached their maximum size. Partly coincidingwith the onset of theLittle Ice Age, successful land reclamation started, from time to timeset back by severe storm surges (Oost, 1995). By 500 a BP, largeparts of the salt marshes attached to the mainland which hadbecome high enough due to natural sedimentation had beenembanked and extensive areas of land bordering the inland bays

ion 1200 a BP (based on Vos and Kiden, 2005; Vos and Knol, 2005).

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along theWest (Middelzee, Lauwerszee, Fivelboezem), East (MaadeEinbruch, Schwarzes Brack) and North (Bucht von Sielmönken)Frisian mainland coasts had been reclaimed (Van der Spek, 1994;Oost, 1995; Vollmer et al., 2001; Van Heteren and van der Spek,2003). The land reclamations reduced the surface area of the tidalbasins, creating smaller tidal prisms, which in turn resulted insmaller tidal inlet systems. It was coastal squeeze on a very largescale: while the mainland coast was prograding seaward and thebarriers were retreating landward, the tidal basins became smaller.

Settlements onmost of the islandswere (semi-)permanent fromat least 1000 a BP onwards. It has been documented that several ofthe East Frisian barrier islands were plundered around 1200 a BP(Abel, 1893; Rau,1955). For most of the East Frisian Islands it is clearthat continuous inhabitation was at least present since 800 a BP,based on written historical sources. From radiocarbon dating andpollen analysis, it appears that Ameland and Terschelling have beenused for agriculture in or before 1200 a BP (De Jong, 1984, 1993;Oost, 1995). The names of the villages and excavation on somechurches indicate that some settlements were founded around1100e1000 a BP (van Oosten, 1986; Edes and Glashouwer, 1990).Management and the introduction of animals and plants on theislands initiated morphological and ecological changes. Especiallydune arcs, which provided protection against flooding, wereexploited. Directives protecting the marram grass and dunes datefrom at least as early as 650 a BP (1354 A.D., Rottumeroog; Bloket al., 1899; Oost, 1995).

The Halligen islands in the North Frisian back-barrier area wereat that time still much larger than at present, although sea-levelrise, storm surges and normal tidal and wave action during medi-eval times had already dramatically changed the vulnerable peatylandscape (Hofstede, 1991; Vollmer et al., 2001). From 1200 a BPonwards, dwelling mounds were once more constructed in theNorth Frisian region (Vollmer et al., 2001) as had been done before.Between 1000 and 900 BP, stormeflood layers were deposited ontop of the earliest cultural layers, indicating increased marineinfluence. Flooding is also indicated by the fact that, at the sametime, the peat bogs north of the Garding-Tating beach ridge systemchanged into a tidal marsh (Vollmer et al., 2001). Subsequently, theNorth Frisian marshes were protected by dikes. These requireddrainage, which then resulted in compaction due to dewatering ofthe underlying sediments and peat layers. In addition the peatstarted to oxidize resulting in further lowering of the sedimentsurfaces.

Between 700 and 600 a BP, catastrophic storm surges inundatedextensive areas protected by dikes. In particular, the areas con-sisting of thick clayey and peaty sediments that had experiencedincreased compaction could no longer be protected against the sea.Consequently, parts of former coastal marshes changed into tidalflats. During the middle Ages, people produced salt by burning thepeat in the coastal marshlands under a cover of clay. This peatextraction made the land even more vulnerable to flooding andcoastal erosion (Vollmer et al., 2001). The reconstructions around500 a BP show that small parts of the salt marshes on severalbarrier islands had already been embanked. It is thought to havecommenced as early as 700 BP (Oost, 1995; Schoorl, 2000a).

The high North Frisian barrier islands with a Pleistocene core,such as Amrum and Sylt were inhabited more or less uninterruptedsince pre-historic times (Vollmer et al., 2001).

3.2.2. Changes after 500 a BP (Figs. 4e6; after Wiersma et al., 2009)From 500 a BP onwards, successful land reclamation continued

in the West (Lauwerszee, last remnants of the Middelzee) and East(Harlebucht) Frisian Wadden Sea (Van Veen, 1936; Vollmer et al.,2001). The area of embanked salt marshes gradually increasedand the size of the inland bays consequently decreased.

As early as 500 a BP the first large-scale attempts were made toprotect dune belts on the West and East Frisian barrier islands.However, extensive livestock grazing inhibited the intended stim-ulation of a closed vegetation cover. Serious sand drifts wererecorded on many islands, for instance Texel, Vlieland, Terschelling,Juist and Sylt (Homeier, 1964; Schoorl, 1999a,b, 2000a,b; Jessel,2001; Van Heteren et al., 2006). On most of the islands winderosion must have occurred especially in the inner dunes: marramgrasses benefit from drifting sands along the dune face. In thisperiod, extensive sand drift dikes were built and maintained byplacing brushwood fences, which trapped and stabilized wind-blown sand (Oost et al., 2004). As early as 370 a BP, the islands ofEijerland and Texel were connected by a sand drift dike (Schoorl,1999b). Although such measures suggest a strong influence onthe sedimentary development of the area, the anthropogeniceffects mainly slightly modified the impact of natural processes.Existing technology at that time was largely incapable of stoppinglarge-scale natural developments, such as channel or shoalmigration.

The Halligen area in the North Frisian Wadden Sea, which wasespecially vulnerable due to peat extraction for the production ofsalt, was struck by several catastrophic storm surges (e.g.1634,1717,1825 A.D.) and large areas of land were lost, largely shaping thepresent-day Halligen landscape (Figs. 4e6; Vollmer et al., 2001).Only a small percentage of the inundated and eroded salt marshbetween Rømø and Eiderstedt could subsequently be reclaimed(Vollmer et al., 2001).

The tidal waters flowing through the inlets in combination withwave- and wind-driven currents continuously exchange enormousamounts of mainly sandy sediments between the North Sea, theislands, the ebb-tidal delta, the inlet and the back-barrier basin(Oost, 1995). Net sedimentation occurs in the back-barrier basins atthe expense of the open North Sea coast, which is on averageretreating in a landward direction (e.g. Juist, Texel). A closer look,however, reveals large differences from island to island, indicatingthat on top of the long-term regional trend in coastal behaviorother processes are locally important. One of the clear examples isthe western side of Vlieland during the 16th to 18th century. Due tostrong sediment transport into the back-barrier area the ebb-tidaldelta retreated followed by the west coast of the island. Retreat ofthe island was as much as 1.5 km in 150 years (Abogado Rios, 2009,unpubl. res.). Another clear example can be found on the adjacentbarrier islands of Ameland and Schiermonnikoog in the WestFrisian Wadden Sea. From reconstructions (Oost, 1995) it appearsthat after 150 a BPAmeland retreated landward. In the same periodthe barrier island of Schiermonnikoog, by contrast, shifted seaward,especially after a reduction of the tidal volume of SchiermonnikoogInlet with a third in 1969 (Biegel and Hoekstra, 1995; Oost, 1995).Also, the eastern side showed a growth of more than 6 km, whereasAmeland only grew by about 1 km towards the E (Oost, 1995).

Both developments result from the fact that the tidal inletsystem is a sediment-sharing system. In the back-barrier area,roughly speaking, vertical accretion of the shoals keeps pace withsea-level rise and hence, if tidal amplitude remains constant, thetidal volume will mainly depend on the area of the back-barriertidal basin. The volume of the back-barrier tidal channelsdepends directly on the tidal volume that has to be imported andexported through these channels. If this tidal volume decreases, thetidal current velocities will decrease as well and net sedimentationof sand and clays will occur in the channel until the tidal flow willestablish a new equilibrium. In this morphodynamic equilibriumsedimentation in the channel is balanced by erosion and e apartfrom migration e the channel will maintain stable dimensions(Biegel and Hoekstra, 1995). Something similar applies to the ebb-tidal deltas, the sediment volumes of these are also related to the

wadden 1500Dunes and higher shoals

Intertidal area and salt marsh

Embanked area and high marsh

Peat bogs

Pleistocene and older

Water

Fig. 4. Reconstructed map of the Frisian Wadden Sea for the situation 500 a BP (1500 A.D.; German and Danish peats not reconstructed; after Wiersma et al., 2009; based on:Aagaard et al., 2007; Behre, 1999; Behre and Schmidt, 1998; Geognostische Karte, 1847; Homeier, 1962, 1963, 1969a,b,c,d,e, 1972, 1974, 1979a,b, 1982a,b,c, 1983; Homeier and Luck, G.,1969; Lang, 1958; Meier, 2006; Meyer, 1975; Oost, 1995; Schoorl, 1999a,b, 2000a,b; Vollmer et al., 2001; Vos and Kiden, 2005; Vos and Knol, 2005; WWF, 1987).

A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 25

tidal volume in the basin: the sand volume decreases withdecreasing tidal volume. If the tidal volume decreases, part of thesand eroded from the ebb-tidal delta is transported into thechannels and into the back-barrier area. The relationships betweensand volume of the ebb-tidal delta and volume of the tidal channelsthough are not linearly correlated with tidal volume. This mayresult in a mismatch between the amount of sediment thatbecomes available due a reduction in size of the ebb-tidal delta inresponse to a reduction in tidal volume (or tidal prism) and theamount of sediment required for the morphological adaptation ofthe channels within the tidal basin, depending on the configurationof the basin and the infill of the back-barrier channels. If this resultsin a surplus of sediment, this sand will be transported along thedowndrift barrier island coast, most frequently in the form ofshoals. After longshore and cross-shore dispersion, this surplus ofsediment will contribute to the seaward progradation or stabili-zation of the shoreline. Schiermonnikoog Inlet has a long history ofshrinking tidal basins and decreasing tidal volumes since about 600a BP (Oost, 1995) due to land reclamation projects in the tidal basin(mainly Lauwerszee embayment) resulting in net progradation ofthe barrier island in contrast to Ameland, where hardly any recla-mation occurred in the back-barrier basin of Ameland Inlet.

In the past 150 years, a series of notable morphological changesoccurred (Ehlers, 1988). These changes primarily concern island

growth and dune development reflecting sand supply from the ebbdeltas and shoreface. The islands of Wangerooge, Spiekeroog, Bal-trum, Norderney, Juist, Schiermonnikoog and Ameland started togrow rapidly eastward and sometimes were eroded at their west-ward side. Dunes became larger on many places on the islandsalong the entire Frisian barrier chain. In the North Frisian Waddenarea, Amrum grew and the Havsand and Juvre Sand in the inletbetween Sylt and Rømø merged with Rømø. Fanø extendednorthwards with the formation of new dune ridges, because ebb-tidal deltas were turned southward by the littoral drift andsubsequently moved landward by the waves. The Skallingenbarrier-spit eroded and retreated after the 1960s (Ehlers, 1988).

3.3. Behavior of an individual barrier island on a time scale ofdecades to a few centuries

In recent morpho-ecological studies of theWest and East Frisianbarrier islands, the concept of the so-called prototype island isdeveloped and applied (Fig. 7, Löffler et al., 2011). The prototypeisland illustrates and integrates most of the common morpholog-ical aspects of the natural islands of the West and East Frisian partof the system. It demonstrates both the temporal and spatialcoherence between the large-scale morphological units, whichform the island under natural conditions. The large-scale

wadden 1850Dunes and higher shoals

Intertidal area and salt marsh

Embanked area and high marsh

Peat bogs

Pleistocene and older

Water

Fig. 5. Reconstructed map of the Frisian Wadden Sea for the situation 150 a BP (1850 AD; German and Danish peats not reconstructed; after Wiersma et al., 2009; based on: Aagaardet al., 2007; Behre, 1999; Behre and Schmidt, 1998; Geognostische Karte, 1847; Homeier, 1962, 1963, 1969a,b,c,d,e, 1972, 1974, 1979a,b, 1982a,b,c, 1983; Homeier, and Luck, G., 1969;Lang, 1958; Meier, 2006; Meyer, 1975; Oost, 1995; Schoorl, 1999a,b, 2000a,b; Vollmer et al., 2001; Vos and Kiden, 2005; Vos and Knol, 2005; WWF, 1987).

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morphological units are characterized by developments on a timescale of decades to centuries, and a spatial scale of tens of squarekilometers. For that reason, this morphological concept alsoprovides a robust conceptual framework for the ecology of theislands, because each large-scale morphological unit is character-ized by a range of abiotic conditions relevant to both flora andfauna. Such abiotic conditions are expressed in terms of e forexamplee the exposure towaves and currents, wind and salt spray,the average bed level, the amount of relief, the role of surface waterand groundwater, the availability of nutrients, and the compositionof the sediment or substrate. The prototype island (Fig. 7) is char-acterized by five large-scale morphological units, which mutuallyoverlap and influence each other (Hoekstra et al., 2009; Löffleret al., 2011).

1) Island heads

These encompass the updrift coast of the island adjacent to thetidal inlet. This is the area where the sandy shoals of the ebb-tidaldelta merge with the island and eolian sands are mobilized on thebeach plains from where they are blown into the dune belts (see:FitzGerald et al., 1984, 1994; Cheung et al., 2007). In the long-term,processes of accretion and erosion alternate with each other(Oertel, 1977). Depending on the long-term sand surpluses ordeficits in the adjacent ebb-tidal delta the downdrift coast of the

island will demonstrate accretion (e.g. Schiermonnikoog) orerosion (e.g. Norderney), respectively. Within this long-termdevelopment there commonly is a shorter cycle of sedimentationdue to the merger of shoals (Joustra, 1971; Oost, 1995; Israel andDunsbergen, 1999) and (channel-driven) erosion associated withthe autonomous dynamics of the ebb-tidal delta. With increasingsize of the tidal volume and hence the size of the ebb-tidal delta,the duration of the cycle will increase, from decennia for tidalvolumes of a few 100 $ 106 m3 to more than a century for tidalvolumes of the order of several 1000 $ 106 m3 (Oost, 1995; Schoorl,1999a,b; Oost et al., 2004).

2) Dune arcs

This unit comprises large dune complexes that form an arcwith curved and partly parallel dune ridges, encircling formerbeach plains with tidal marshes along the central island coast.The exact nature of the rows of dunes is still under debate, butdune belts at the accretion side of the island consist of primarydunes (Van Heteren et al., 2006), whereas those at the erosivesides of the islands consist of parabolic dunes, formed by thereworking of primary dunes. The large dune complexes havebeen stabilized by natural vegetation development. The dunearcs provide coastal protection to major human settlements onmany of the islands.

wadden 2000Dunes and higher shoals

Intertidal area and salt marsh

Embanked area and high marsh

Peat bogs

Pleistocene and older

Water

Fig. 6. Map of the Frisian Wadden Sea for the situation 0 BP (2000 A.D.; after Wiersma et al., 2009).

A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 27

3) Washover complexes

A (former) washover complex is normally found at the end ofa dune arc with an alternation of foredunes, remnants of dunes, andlocal depressions. Shallow creeks form a connection between theNorth Sea and theWadden Sea during storm surges. The associatedsupratidal plains form low depressions, separated from the beachby a beach ridge or high berm and accommodate a pioneer vege-tation, tidal salt marshes and low dunes (Hoekstra et al., 2009).

Fig. 7. The prototype West and East Frisian barrier island is characterized by thepresence of five large-scale morpho-ecological units: (1) island head; (2) dune arc; (3)washover complex; (4) island tail; (5) Beach and shoreface (after Löffler et al., 2011).

However, almost all of these wide channels/openings at the end ofthe dune arcs have disappeared in past centuries due to thedevelopment of sand drift dikes.

4) Island tails

Downdrift tails of the islands are limited in width up to a fewkilometers. Dunes alternating with (former) washover fans andchannels, fringed by tidal marshes at the back-barrier side(Dijkema, 1989), form the other end of the island (Leatherman,1976; Hoekstra et al., 2009). In general, these areas are dynamic(beach) plains where periods of accretion and erosion alternate. Inaddition, a “wagging’ of the island tail is also possible due to the(cyclic) behavior of the tidal channels in the downdrift located inlet(Cheung et al., 2007). Here, low dunes and e in their shelter e saltmarshes may also occur. If dynamics are somewhat reduced due todune development the marshes can develop strongly and quickly(e.g. island tail of Terschelling).

5) Beaches and shorefaces

Beaches, upper and lower shoreface along the open North Seacoast of the islands form the natural link with other morpho-ecological units (Guillèn and Hoekstra, 1996; Hoekstra et al.,1999; Houwman, 2000). Often these beaches are characterized bya considerable width and a rather low gradient (1:100) (compare

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050Year

Number

Fig. 8. Population development of the barrier island Norderney (data: historical &internet sources). The boom of the population occurred after 1797 when the islandbecame a bathing resort.

A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e3828

Wijnberg, 1995; Ruessink, 1998, 2010; Ruessink et al., 1998, 2011, inpress).

3.4. Major human measures and coastal interventions in pastcenturies

3.4.1. Influence of man in the past 150 years (Figs. 5 and 6)After 150 a BP, wide-spread land reclamation occurred. Over

the period 500 a BP to the present the tidal flat area decreased byup to 58% of which a large part occurred in the past century(Delafontaine et al., 2000). Parts of the West and East FrisianWadden Sea were also used for harbor development (e.g. Ems,Weser and Elbe estuary and Jade Bay). Next to that, dredging,canalization and deepening of the larger estuaries (Ems, Weser,Elbe) significantly influenced the hydrography, hydrodynamicprocesses and morphological development of these waters (see forinstance: Winterwerp, 2009).

Another important intervention in theWest FrisianWadden Seawas the closure of the Zuiderzee in 1932 A.D. It led to a sedimentdemand of 500e1000 $ 106 m3, which is stilled by graduallyimporting sediment from the North Sea side of the barriersystem, especially from the ebb-tidal deltas (Sha, 1990a; Elias,2006; Elias et al., 2012, acc.). Since that time the westernmostWadden Sea in front of the Zuiderzee has been adapting to the newhydrodynamic conditions, and over the period 1927e1997 nearly400 $ 106 m3 of sediment have accumulated in the back-barrier,especially in the abandoned tidal channels (Berger et al., 1987). Alarge part of it was derived from the North Sea coastal zone, causingstrong retreat of the ebb-tidal delta of Texel Inlet. Together withthat, retreat occurred of the adjacent coasts of the barrier island ofsouth-Texel (800 m in 50 years) and Northern Holland, wherea large ebb-delta channel formed near the coast (Oost et al., 2004;Elias, 2006). From the developments, it can be judged that theeffects of the Afsluitdijk will continue to influence the area forcenturies (Elias, 2006; Elias et al., 2012, acc.). The process is ex-pected to lead to serious ebb-delta erosion (Elias et al., 2012, acc.).Retreat of the ebb-tidal deltas may on the long run also exertinfluence on the barrier coasts since shelter decreases and sandmay become less easily available (Hofstede, 1999a,b).

Smaller enclosures such as part of the embayment of Schier-monnikoog Inlet in 1969 A.D. have resulted in an infill of the mainback-barrier channel connected to it and erosion of the ebb-tidaldelta, all tuned to the decrease of tidal prism from 300 to200 $ 106 m3. A substantial part of the sediment became availableto the downdrift island of Schiermonnikoog. The adjustmentprocesses took at least about 4 decades (Biegel and Hoekstra,1995). Also in the North Frisian Wadden Sea hydrodynamicaland morphological effects were produced by the construction ofthe dams connecting Rømø (1949 A.D.) and Sylt (1927 A.D.) to themainland, and the construction of the Eider storm surge barrier in1973 (Reise et al., 1996). In the case of Rømø the location of thedam did not fully coincide with the position of the tidal watershed(or drainage divide). Subsequently, the tidal prism of the areasNorth and South of the dam was redistributed; a process whichwas followed by an adjustment in local channel morphology.Furthermore, subsidence of some tidal inlet systems and a subse-quent increase of sediment demand resulted from sand miningwithin the tidal systems (above �13 m MSL) and due to theexploitation of gas and oil. Volumes of subsidence are of the orderof 0.1e10 $ 106 m3.

On the barrier islands, the increase in tourism during thisperiod resulted in the development of seaside resorts along theWadden Sea coasts and a growth of the island population (Fig. 8).Permanent structures were built in highly dynamic locations andwere soon threatened by coastal retreat (Ehlers, 1988). In

response, new coastal defense structures were developed, whichwith the exception of nourishments, reduced the mobility ofsediment and the morphological development of the islands, suchas (Table 1):

� Dikes (at least since 650 a BP on Texel; Oost et al., 2004, butperhaps even 1000 a BP on some of the islands (Oost, 1995);

� Protection (since at least 650 a BP on Rottumeroog; Oost, 1995)and planting of marram grasses (since at least 350 a BP onNorderney);

� Sand drift fences and formation of sand drift dikes (since atleast 370 a BP on Texel);

� (Sub)littoral stone defense works (since at least 300 a BP in theMarsdiep Inlet);

� Groynes (since about 200 a BP);� Revetments and seawalls (since 150 a BP on Norderney;Thorenz, 2011);

� Harbor moles (since at least 150 a BP) and massive dischargesluices;

� Sand nourishments (since 60 a BP Norderney island in 1950/51;Kunz, 1990).

Substantial were the hard protection measures taken at thenorthwestern and western heads of the islands of Ameland, Bor-kum, Nordeney, Baltrum, Spiekeroog and Wangerooge (Oost, 1995;Thorenz, 2011). The constructions have halted the process of tidalinlet migration (e.g. Norderney, Nummedal and Penland, 1981). Insome cases, for instance Ameland and Texel inlets, this causedextreme deepening (Oost, 1995; Schoorl, 1999b). In addition togroynes and revetments, seawalls were erected to protect thebarrier coast in places vulnerable to erosion (cf. Elko and Davis,2006). The earliest were built of basalt; later, asphalt mountedwith stones was used in order to increase the roughness of thesurface and decelerate the waves. All these measures also influ-enced sediment transport along and perpendicular to the islandcoasts. In the same period some barrier islands have been given upfor maintenance by man. For instance, in 1991 it was decided bythe Dutch Ministry of Transport, Public Works and WaterManagement that the uninhabited barrier island Rottumeroogwould no longer be managed and to let nature take its course. Dueto considerable protest of several stakeholders it lasted until 2001before the advice was really implemented and it was decided tostop all management of the dunes and let natural forces of windand water take over.

Table 1Some data on revetments, poles placed in rows perpendicular to the coast line and groynes at the North Sea coast of the West and East Frisian barrier islands (mainly afterThorenz, 2011).

Begin of building Revetments (km) Poles (km) Begin of building Number of groynes

Island Year 1900 1997 1900 1997 Year 1900 1997

Texel 1952e1957 e 0.6 24Vlieland e e 1854 13 18Ameland 1960e1969 e 0.6Borkum 1874 2.8 6.5 e e 1869 21 35Juist 1913 e 1.4 e e 1914 e 7Norderney 1857 2.0 4.7 0.5 e 1846a 12 32Baltrum 1873 e 1.7 1.8 0.2 1873 14 14Spiekeroog 1874 1.6 1.9 0.8 e 1873 12 12Wangerooge 1874 3.1 5.7 e e 1818b 14 23Total 9.5 21.9 3.1 0.2 73 123

a Two woodwork groynes were build in that year, but destroyed in 1848, stone groynes were build since 1860.b Ten woodwork groynes were build between 1818 and 1834, but given up in 1850, stone groynes were build since 1874.

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Also, dune areas developed quite differently due to humaninterventions (Nordstrom et al., 2007). Whereas natural dunes arelow and irregular in form (e.g. Spiekeroog) the artificial dunes orsand drift dikes are high and often strongly interconnected (e.g.Schiermonnikoog). Partially they were formed to protect theinhabited areas and partially to close off the open parts of the coast.Once formed these were often repeatedly re-enforced and grew to5 m or more above mean sea level, especially when the first dunerow could be kept, which became quite normal once nourishmentsbecame important to maintain the coastline. The dunes were alsoused as a reservoir for drinking water. In the Netherlands, theproduction has been restricted, but on the East Frisian Islands, theproduction is often more than the annual precipitation leading toa gradual lowering of ground water tables. Furthermore, dunes areused on many places as recreational areas. Infrastructure, holidayhouses, hotels and campings have been constructed, often near theshore outside of safe zones, leading to extra demands for safetyfrom flooding.

In recent decades, there has been a growing tendency to usesand nourishment rather than hard structures to actively protectthe coast. A good example is Sylt where at Hornum, concretetetrapod barriers were placed in 1960e1967 in cross- (jetty) andlongshore direction (dune foot revetment; Sistermans andNieuwenhuis, 2002). Also, seawalls were built in Westerland. Themeasures proved to be counterproductive in the long run since theycontributed to disrupt longshore sediment transport, thus gener-ating lee-erosion south of the jetty. The seawall at Westerlandsuffered from severe damage during storm surges as a result offoreshore lowering in front of the structure. Thereupon authoritiesstarted in the early 1970s with beach nourishments and flexiblesolutions such as geotextile revetments. These measures did notdispel the need for hard protection but improved their efficiencyand life expectancy (Sistermans and Nieuwenhuis, 2002; http://www.ubcwheel.eu/index.php/gpdb/article/1733). An advantage ofnourishments is that the beaches can maintain their dynamicstates, keeping natural processes of erosion and sedimentationmore intact. For many of the North Frisian barrier islands theavailability of nourishment sand is limited.

Effects of nourishments are (Holzhauer, 2011; Oost, 2012) are:

1) The influence of dredging of sand

Depending on the depth on which sand for nourishment isdredged this may influence the development of the coastal zone. Inthe West Frisian area sand is dredged on water depths of 20 m andmore, thus trying to avoid effects on the coastal development. Thisis different per country. This is illustrated by the erosion of the

southern spit of Skallingen and the ebb-tidal delta of the SkallingenInlet (Grådyb). The processes can partly be attributed to thedredging of the navigation channel and the associated dumping ofthe sand at the southern side of the inlet, which is thereby lost forthe sediment circulation around the southern part of Skallingenand on the ebb-tidal delta.

2) The effects of nourishments on the morphology of the dunes atthe beachIt has been shown that on the West Frisian Barrier islands an

amount of sediment is blown into the dunes from the beach,which is in volume 44% of the volume of the nourishments.Depending on the management, several reactions are possible(Arens, 1999; Arens et al., 2007, 2008, 2010):A) no dynamics in the dune face area, only minor migration of

the dune foot. No sediment is transported into the dunesbehind it and no primary dunes do develop.

B) Dynamics are mainly restricted to the zone in front of thedune face and the top of the dune face. Embryonic dunesdevelop, which may develop to new dunes. The dunesbehind the dune face do not receive sands and are hencenot rejuvenated.

C) Dynamics are slight to strong, and sand transported by thewind can reach the area behind the dune face. Due to thedynamics embryonic dunes can form which may developinto new dunes and older dunes are rejuvenated by theimport of calcareous sands.

D) Dynamics are very strong. Depending on the beachdynamics embryonic dunes may form or not. Due to (arti-ficial) openings in the dunes sand can reach the olderdunes, which are thus developing in a natural way.

3) The effects of strange sand (Stuyfzand et al., 2010)

The sand has to be dredged somewhere else, consequentlyintroducing sand that in terms of hydraulic or geochemical behaviormay be different from the local sands, thereby affecting the habitats.Studies revealed that indeed the sand differs in grainsize andgeochemical characteristics and thus may be of influence.

4) Other effects (Holzhauer, 2011)

In recent studies it could not be demonstrated that nourish-ments on the foreshore did influence significantly the currentvelocities, bottom roughness, sediment characteristics (also on thebeach), infauna (also on the beach) and epifauna. Sediment nour-ishments did also not influence the sanderling, which is dependingon the infauna of the beach.

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3.4.2. The impact on morpho-ecological units of an individualisland

Human interventions in the last two centuries have seriouslyaffected themorphologyand ecologyof the islands.Major effects are:i) the large-scale erosion, movement and deposition of sedimentaround islands in relation to morphological adjustment due toclosure works and large constructions; ii) the lack of mobility ofsediment being fixed by constructions or “trapped” in, for example,sanddrift dikes and iii) themore recent artificial supply of sand in theframework of nourishment projects to protect the coast. Allmeasureshave in common that they are related to “sediment management”within the system and that they have significantly altered the coastaldynamics of, in particular, the West Frisian barrier islands. We willnow discuss the way in which these developments are visible in thelandscape of the morpho-ecological units of an island.

1) Island heads

Depending on the sand surpluses (e.g. Schiermonnikoog) ordeficits (e.g. Norderney) in the ebb-tidal delta the downdrift coastof the island adjacent to the tidal inlet may receive sediment orerode. As erosion is being hindered by stonework and nourish-ments to protect the nearby inhabited dune arcs the net effect isoften an (updrift) elongation of the islands. Also, the island headsbecome either bulwarks or at least semi-stable areas where duneerosion is largely prevented. One has to keep in mind that duneerosion is often a source of sediment for new dune buildingprocesses. So by interrupting the natural cycle of erosion anddeposition the impact of eolian processes is reduced.

2) Dune arcs

The large dune complexes that form an arc have been stabilizedby coastal protection works, planting of marram grasses andforestation. It resulted in a reduction of eolian transport in thedunes. The formation of high sand drift dikes at the dune face onmany of the Frisian barrier islands resulted in further reduction ofeolian transport into the so-called grey dunes behind them. Thecharacteristic vegetation of the grey dunes of grasses and lichensdepends on poor nutrient conditions and a slight sand import. Thereduced dynamics led to the formation of shrubbery. This is furtherenhanced by the increase of atmospheric nitrogen deposition andprecipitation and the decrease of grazing (Van Dobben and Slim,2011, acc.). On many parts the characteristic open landscape ofthe island is being replaced by a climax vegetation and a process ofrejuvenation is lacking (Löffler et al., 2011). Present natureconservation strategies, such as the removal of top soils andshrubberies do not address the causes of the problem e the lack ofdynamic processes e but simply are dealing with the symptomsand, consequently will not be sustainable. On the long run it will beimpossible to provide the envisaged nature restoration, since themeasures taken are short-lived, often have a very local characterand also depend on seed banks that are becoming poorer in species(Löffler et al., 2011).

3) Washover complexes

The wide openings at the end of the dune arcs have all beenclosed off by sand drift dikes at the North Sea side and mostly bydikes at the Wadden side, except for the one on Spiekeroog. On theWest Frisian Islands the storm surge disaster of 1953 had asa consequence that many sand drift dikes were built over the fulllength of the islands to link the individual dune systems and toprotect the coast. This strategy was part of a major master plan toclose off the entire West Frisian Wadden Sea; a plan that was later

on abandoned again. Due to this, large areas of beach plain and tidalmarshes have changed into polders.

4) The island tails

On the West Frisian Islands the presence of the sand drift dikesprevented the transport of water, sediment and nutrients to theinterior or even Wadden coast of the islands. The sand drift dikeswere also intensively maintained by planting marram grass andplacing sand fences further limiting the eolian transport across theisland. This lack of natural dynamics led to fast development andsuccession of the vegetation. After it was decided that the sand driftdikes on the uninhabited island tails would no longer be main-tained new openings formed in the sand drift dikes on somelocations but this has not seriously improved the situation. Thecontrast to the East Frisian barrier islands is huge now. Large sanddrift dikes are absent there and we see a much more naturalenvironment with highly dynamic ecosystems.

5) Beaches and shorefaces

Beaches and upper and lower shorefaces are often nourished orkept with stoneworks. In this way the natural retreat of the barrierislands thereby slowly cannibalizing themselves over the pastthousands of years has come to a halt. The erosive coasts havebecome accretional static coasts. This implies less reworking ofsediment from the dune face: sand, which is eroded from the duneface or the beach, is replenished with sand dredged from deeperwaters. This sand has commonly not the same sedimentologicalproperties as the original beach sands (compare Guillèn andHoekstra, 1996), often contains a lot of shell fragments and, asa result, is often less suitable for eolian transport.

4. Barrier island management: discussion

4.1. Introduction

In this discussion we will answer the three major questions thatwe addressed in the introduction of this paper.

4.2. What can we learn in terms of ICZM from the naturaldevelopment and behavior of the Wadden Sea barrier islandsystem?

An overarching theme in barrier island management is the roleof coastal (morpho)dynamics in determining safety (or the oppo-site: creating risks), in providing the boundary conditions for thedynamics of ecosystems and in delivering ecosystem services, suchas for the environment and the tourism industry. Coastal zonemanagers of the Wadden Sea barrier island system have to beaware of the following morphological conditions and sedimentbudget considerations:

1) Presence of outcrops and inherited Pleistocene topography andpaleomorphology

Up to this day the Pleistocene initial topography is a dominantfactor in the development of the north-south trending North FrisianWadden area, which is characterized by a steeper Pleistocene reliefand lower availability of loose sediments. On a smaller scale deeplyincised river valleys, which formed during the Pleistocene low-stand, determine to a large extent the present-day positions ofseveral major estuaries such as Elbe, Weser/Jade and Ems (Streif,2004). Furthermore Pleistocene outcrops determine locally theformation of the coastal zone such as the southern spit of Sylt, and

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near Skallingen (Blåvands Huk). The rigid clays deposited or over-ridden by ice during the Ice Ages such as boulder clays are known tobe resistant to erosion. It has been argued that the boulder clayoutcrops on and in front of the southwest part of Texel Islandactually form the reason why Texel forms the knick point betweenthe N-S trending Holland coast and theW-E trending Frisian barriercoast (Schoorl, 1999a). The resistance against erosion also explainsthe seaward position the barrier islands of Amrum and Sylt (Fig. 1).The influence of the rigid clays on channels, which are incised intothe Pleistocene sediments, and their possible influence on islanddynamics may also be of importance. An example is a pipelinewhich was entrenched through a Pleistocene clay ridge in theLangeoog Inlet in the East Frisian Wadden Sea and which wassubsequently covered with sand (Grann, 1997). Much to thesurprise of the engineers the sand was quickly eroded, leavinga hanging pipeline and leading to a reorientation of the mainchannel via the pipeline trench. The boulder clay which had orig-inally been present was more resistant to erosion than the under-lying sand and had hindered the reorientation of the inlet channel.Currently the dredging of a deeper channel thalweg is consideredthrough the boulder clay W of Borkum, which may have its effectson the barrier island: special attention is given to it by the stateauthorities.

However, the larger part of the islands coastal morphology is theresult of repeated reworking of sediments and thus totally gov-erned by the present-day natural forces and interactions betweenthe various morphological units. In such areas morphodynamicsare determined by Holocene sands and clays and not so much thePleistocene relief. In that case the following points are ofimportance:

2) Holocene line-up of barrier islands and ebb-tidal deltas

During its long Holocene evolution, since 5000e6000 yrs BP theislands and ebb-tidal deltas along the North Sea side have beenlined up during their Holocene coastward migration, forminga more or less uninterrupted barrier chain along the Frisian coasts.Momentarily there are management plans to push the coast locallyseaward, or to keep the coast in its present position. Other parts ofislands or entire islands or ebb-tidal deltas are allowed to continuetomigrate towards themainland as they have done during the largepart of the Holocene. As yet, it is not clear what would be the effectsof allowing such disturbances in a chain of lined-out barrier islands.If retreat of parts of the chain becomes strong it will most likelyresult in increasing erosion of the promontories and disturbancesin the coast-parallel sand transports.

3) Balance between sea-level rise and sedimentation

The observation that around 5000 a BP sediment accumulationrates locally exceeded the rate of sea-level rise, whereas in otherplaces areas drowned, indicates that the balance between sea-levelrise and sedimentation is rather subtle (Flemming, 2002). It impliesthat the availability of sediment from the North Sea side and thepossibility to settle in the back-barrier area both play an importantrole. The availability is partly determined by the variation in tidalvolume and wave climate (Van Goor et al., 2003; Grunnet et al.,2005), and for the other part by changes in the North Sea facingcross-shore coastal profile. The potential for sediment to settledepends on the hydrodynamic conditions in the back-barrier basin.During the sedimentary infill and shrinking of the back-barrierbasin towards the present day, the energy gradient between themainland coast and the inlet became steeper, thus making it moredifficult for especially finer-grained sediments to deposit andaccumulate (Flemming and Nyandwi, 1994). If the idea is correct, it

may have consequences for the demand of sediment when sea-level rise accelerates. As long as sedimentation can keep up withsea-level rise within the back-barrier basin, the sediment demandfrom the North Sea coast (and thus erosion) will increase linearlywith the rate of sea-level rise. However, if sedimentation cannotkeep up with sea-level rise, which is expected to occur at lowerrates of sea-level rise for larger basins than for smaller (Oost et al.,1998), wave and tidal energy within the basin increase leading toenlargement of the tidal channels and even more difficult condi-tions for sedimentation on the shoals (Flemming and Barholomä,1997; Van Ledden, 2003; CPSL, 2010). A tidal basin might thenstart to function as a source of sediment instead of a sink.Depending on the original dimensions of the tidal system thechanging demand for sediment in the back-barrier area may thushave different implications for the management of the North Seacoasts of the barrier islands.

4) Climate change

As discussed, the striking coincidence of so many positiveshoreline changes after 150 BP makes it very likely that somecommon processes were responsible. Since the period after 150a BP marks the end of the Little Ice Age (A.D. w1450e1850),a change in circulation following this period has been suggestedas a driving mechanism for the observed changes (Ehlers, 1988).Partially it will, of course, also be the resultant of human inducedpoldering of mainland areas resulting in a reduction of tidal volumein inlets and surpluses of sand, which became available to theislands. Furthermore, the decrease of dynamics on inhabitedislands and the furthering of the development of vegetation willhave enhanced local dune development, especially since AD 1960.However, given the fact that also uninhabited islands showa different development from the period before 150 BP, thehypothesis of the Little Ice Age aftermath might well be correct. Inthat case the adaptationwill not continue indefinitely: wide-spreadcoastal aggradation might give way to wide-spread erosion onceagain just as in the past (e.g. Lindhorst, 2007). Coastal managersthen will be facing larger net erosion in the future. In this light itmight be wise to store as much sand as possible on the islands byallowing natural processes.

5) Reactions of the sediment-sharing system

An inlet system is, as discussed in chapter 3, primarily a water-and sediment-sharing systemwith a tidal inlet, the ebb-tidal delta,adjacent barrier islands and the tidal basin with channels, shoals,tidal flats and salt marshes. Barrier island development is thusdirectly linked to tidal inlet system development. A natural changeor an intervention by man may have repercussions for other partsof the system. The initial increase and subsequent decrease of thetidal volumes of inlet systems due to sedimentation and dikingmust have influenced the development of the North Sea coasts ofthe barrier islands strongly, by initiating coastal erosion whensediment was needed and sedimentation when surplus sedimentsbecame available. This is illustrated by the differences in coastaldevelopment of the barrier islands of Ameland and Schiermonni-koog. It should be realized that thus caused accretionary orerosional tendencies on the islands will eventually come to an end,when the sediment-sharing system is moving towards equilibriumin the long run (decades to centuries).

Sometimes, however, a closure triggers a strong increase insediment demand in the back-barrier basins and thus results incoastal erosion ofe in the first onsete the ebb-tidal deltas, as is thecase with Texel inlet. The effects are often ignored, as managementis often focused on the immobilization of the island and the

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protection of the inhabitants, infrastructure, harbors, polders, andpreservation of the dunes and tidal marshes (i.e. everything aboveMSL). Nourishing of sand or building groynes and revetments maycounteract erosion of the island caused by the sediment demand inthe back-barrier area, but may not prevent the erosion and retreatof the ebb-tidal delta (Elias et al., 2012, acc.). This, in turn, may leadto less shelter and sediment supply to the islands and hence togreater difficulties in maintaining them (FitzGerald et al., 1994;Abogado Rios, 2009). Examples are the Texel and Skallingen inlets.

6) Dynamics on the barrier island between the larger morpho-ecological units

The larger morpho-ecological units on a barrier island are inclose interaction with each other. In this the beach and foreshoreplay an important role as conveyor belt of sediment between theother units in a coast-parallel direction. Next to that naturaldynamics will allow for coast-perpendicular wind and waterflowdriven transports of sediments, thus allowing the island to move ina back-barrier direction or to accrete vertically.

4.3. What has been the impact of human measures andinterventions on barrier island development in the last decades andcenturies?

Human measures and interventions have strongly influencedthe development of the barrier islands, in terms of sedimentdeliverance to the island (see above), safety from flooding andnatural development of the island itself. Originally safety was foundon natural relief of the dunes and dwelling hills. After that closedduneedike rings were maintained, of which the dune vegetationwas somewhat protected. Gaps between the dune arcs were leftopen allowing eolian transport and inundation transport so thatparts of the island could accrete in a natural way.

From the 17th century onwards sand drift dikes were formed atthe dune face on the West Frisian islands and parts of the East andNorth Frisian islands. Also, marram grasses and trees shrubsstabilized the dunes. Before that vegetation density was mostlylower and there was ample possibility for sediment transport bywind or water on many locations, resulting in a more dynamiclandscape. Due to plantation the vegetation trapped the trans-ported sand more effectively and erosion was avoided better.Where needed the coast was protected by stoneworks, forminga bulwark at the western side of many islands. The fixation of inletsand sandy coasts limits the possibility of inlet migration andlongshore barrier island dynamics (compare Elko and Davis, 2006).Due to all the anthropogenic attempts to stabilize the islands, alsosand transport from the shoreface to the beach and onto the islandshas been reduced.

The later beach and shoreface nourishments allowed for sandtransport along and perpendicular to the island coasts and into theback-barrier and to the ebb-tidal deltas. Thus the integrity of thesediment-sharing system was to some extent restored. However,the approach was mainly derived from the rather reactive coastalprotection policies aiming at restoration of erosion andmaintainingthe sand drift dikes. Only locally eolian (Terschelling beach pole10e15) or inundation transport (Spiekeroog Leegde) of sedimentsonto the islands is allowed. Sufficient sediment import is, however,on the long run essential everywhere outside of the dike-dune ringif the island is to accrete vertically and to keep up with rising sealevels. As such allowing sediment import onto the island has to bepart of a robust and sustainable strategy to guarantee safety in thecenturies to come.

The massive stabilization also reduced the opportunities forpioneer vegetation. Dune belts and tidal marshes have experienced

a fast succession (Löffler et al., 2011) resulting in a climax vegeta-tion and the loss of the characteristic open landscape. It was rec-ommended in earlier studies (Williams et al., 2009; Feagin et al.,2010), that a more sustainable strategy of barrier island manage-ment and nature development should be based on a natural degreeof substrate mobility. To that end the focus for managing barrierislands should be on maintaining the integrity of ecosystemservices, their functioning and natural process development. In2005 the United Nations Millenium Ecosystem Assessment (MEA)divided the ecosystem services in four categories: production, suchas the production of food and water; regulating, such as the controlof climate and disease; cultural, such as spiritual and recreationalbenefits and supporting, such as nutrient cycles and crop pollination(DeFries et al., 2005). Traditionally, coastal management mainlyfocused on strengthening especially one or more of the production-, regulating- or cultural services -, paying little attention to thesupporting services (e.g. Morton, 2008). As a result, many examplesof traditional coastal management projects show a negative impacton supporting services, undermining sustainability of the system.This is also the case on most of the Wadden barrier islands.

4.4. Is our present-day ICZM strategy a successful, integral andsustainable approach that is able to cope with future aspects ofglobal change or do we have to adopt our coastal policies?

There is a clear connection between the various spatial scalesconsidered which are strongly coupled to the temporal scales overwhich the characteristic developments take place. This applies aswell to the regional development during the Holocene of the barrierisland chain, as to the interconnectedness of the barrier island withthe adjacent tidal inlet systems, as to the local large-scalemorphological units, which together compose the barrier island.But it also applies to relations between the various spatial scales:larger scales to a large extent set the stage where smaller scaledevelopments can take place. This interconnectedness implies thatthe management of a barrier island cannot be seen in isolation buthas to be considered in terms of interactions on various spatial andtemporal scales. Until now it is not taken strongly into considerationin the ICZM approach that morphodynamics of a barrier island isalso connected to the developments which take place on largerspatio-temporal scales and the relative strong influence by man onthese in the past two centuries. This may lead to surprises like forinstance accelerated coastal erosion due to changes in the netsediment supply to the coast, or fast retreat of an ebb-tidal delta,leading to changes inwave attack on the barrier island.We advocateto take into account the bigger picture of relevant developments onlarger spatio-temporal scales and to make this a consciouslyconsidered part of barrier island management. The list given in theabove discussion (4.2) indicates that exact outcomes of suchdevelopments may differ from island to island. In general, a hierar-chical order of influences should be considered (Fig. 9):

1) Is the Pleistocene paleomorphology important to the island?

If the answer is yes it should be taken into account in terms ofless free morphological development than in the situation of onlyHolocene sediments (predominantly sand and some gravel, shellsand clay) influencing the development. In the latter case theintegrity of the barrier-ebb-delta chain should be considered.Although the effects of forming seaward bulwarks while at thesame time allowing other parts of the chain to retreat coastward areyet not fully understood it is clear from experience that this maywell lead to unwanted accelerations in local erosion. It thus maybecome more cost-efficient (also in terms of ecosystem services) tonourish those other retreating parts of the barrier chain, for

Yes

Yes

No

Restrictions development on smaller scalese.g. Sylt, Amrum & Texel; Elbe, Weser & Ems Estuary

Hardly restrictions to development on smaller scales

Yes

NoPeriodic sand supply to barrier island

Barrier island has sand surpluses or deficits for decades tocenturies, e.g Texel

NoNo

Reduced natural dynamics and decay of natural values, e.g. Borkum, Ameland

Natural dynamics prevail, resilient

Pleistocene

paleomorphology

important?

Inlet system not

in dynamic

equilibrium?

Human influence

on morpho-

ecological units

strong?

Soft where possible, hard

where really needed

Fig. 9. Scheme to evaluate ICZM-policies for the West and East Frisian Barrier Islands.

A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 33

instance in the form of ebb-delta nourishments in channels whichare threatening to erode a barrier island.

2) Is the inlet system not in dynamic equilibrium?

If the answer is yes the net sand deliverance to an island mightbe either showing a long-term (decennia to centuries) surplus ordeficit, which is temporary. Such insights may influence the coastalnourishment and development schemes of barrier islands. Nour-ishments will be necessary even when the inlet is in equilibrium ifthe coast is to be maintained on its position.

3) Is human influence on morpho-ecological units strong?

Often the answer will be yes as the supporting ecosystemservices of many units are restricted due to all coastal protectionmeasures and adverse effects of nourishments, especially withrespect to dune development. Moreover, the increasing tourism onthe islands is based on use of production, regulating and culturalecosystem services, taking a gradual increase of demands forgranted. This situation is not sustainable on the long run. We willexpand on this below.

In the past decade the United States National Park Service (NPS)called for natural processes to be maintained and for human-altered systems to be restored on barrier islands to allow naturalprocesses to function and natural landforms to develop and evolve.For Fire Island it is stated that: “Storms and associated processes suchas waves, tides, currents and relative sea-level change are criticalelements for the formation and evolution of [ ] barrier islands, sanddunes, back-barrier sand flats and lagoons and vegetated wetlands.Processes such as wave run-up, overwash and barrier breaching,which occur during elevated storm surges, are all necessary processesin enabling the efficient transfer of sediments, nutrients and marinewater from the Atlantic Ocean across the barriers.” (Williams andFoley, 2007). In order to restore nature sufficient space should be

available to allow natural processes to develop and to create robustecosystems. The same applies to the factor time: the system willneed time to tune and adjust to changing external conditions (inthe order of several decades up to centuries). Furthermore, itshould be realized that developments may not always occur on theexact same place as original (before human intervention). There-fore, instead of focusing on nature conservation a paradigm shift isneeded in barrier island management towards stimulating thedevelopment of nature dynamics. Stopping management of Rot-tummeroog, allowing natural forces to take over, leading to a strongmorpho-ecological development of the area is an example of this‘working with nature’. To our opinion the best solution is to allowthe geo-biological processes to take their natural course as much aspossible. Only where this is not feasible other management prac-tices should be applied. As a rule: soft and with respect for mor-phodynamical integrity where possible, hard where really needed. It isour expectation that this e to a certain extent e will also regulatethe booming use by tourism of the barrier islands. Examples are thedevelopment of the National Park Schiermonnikoog and the barrierisland of Spiekeroog, where large parts of the islands are naturallydeveloping. But even there morphodynamics is often unnecessarilyreduced due to earlier restrictive measures.

As stated above, many barrier islands originally consisted of fivemajor morpho-ecological units, which mutually overlap andinfluence each other. Each of these units is characterized by its owndynamics, morphology and ecology. The interconnectedness andthe morphological development of the various units are to a largeextent governed by the (possibility of) transport of fresh and saltwater, wind, sediments, nutrients and salts. All of these factors orprocesses are vital to nature development. Many barrier islands, orparts of them, have changed and are used so strongly by man thatrestoration of natural conditions is near to impossible. Neverthe-less, in many places there are alternatives for the present-daymanagement, which envisage a more integrated approach (Löffleret al., 2011). Restoring interconnection between the various units

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and allowing natural developments to once more take their coursewill be beneficial for barrier island safety, maintenance and naturedevelopment. All will benefit by the resulting increase of resilience,which is reached by increasing the ability of (part of) the islands toexchange nutrients, waters and sediments by natural forces. Thedevelopment of such new approaches is nowadays without risk tosafety, since today it is technically feasible to take rapid, correctivemeasures such as sand nourishments, stopping of eolian processes,closing openings etc.

The following alternative barrier island management optionsare available (see also Löffler et al., 2011):

1) Island heads

At many island heads where sandy ebb-delta shoals merge withthe island, erosion and sedimentation alternate over many decadesoften with an overarching longer term deficit or surplus of sedi-ments and sometimes the possibility of Pleistocene paleoreliefinfluences (see above discussion). Taking this into account, reac-tivation of dunes, perhaps in combination with removing the topsoils of the valleys, is thought to have the following effects: i)rejuvenation of lime-rich soil; ii) increase in the dynamics of thearea; iii) increase in the ability for the more inland area to build upvertically with sea-level rise (increasing resilience); and iv)improving the conditions for the establishment of dry pioneer plantspecies of the xeroseries and for the wetter hygroseries. Where neterosion dominates, larger sand nourishments and new duneformation may provide the desired level of safety (see Arens andMulder, 2008).

2) Dune arcs

The dune arcs offer safety toman (Roelvink et al., 2009, in press).If there are many rows of dunes between the beach and theinhabited area, this offers locally possibilities for eolian sandtransport. The transport can easily be stopped at a sufficientdistance from the inhabited areas. In this way: i) sedimentation inthe (inland) dunes is stimulated again, allowing vertical aggrada-tion; ii) pioneer species have more possibilities to survive; and iii)larger dune massifs may come into existence, providing larger freshwater buffers and supplying the dune valleys with groundwaterrich in dissolved carbonates. Any sediment losses thus caused onthe beach have to be compensated by nourishments if the beachposition is to be maintained. As illustrated by Landry (2007) andCostanza and Farley (2007), it is possible to optimize nourishmentschemes by taking the value of houses, recreation and nature intoaccount. In this way a coupling between dune arcs and beacheswould be restored. Also, dune arcs may be allowed to form againwhere they were once present, either by actively assisting in thefirst steps of the process (e.g.West Vlieland) or by bringing back thewashover complexes and removing sand drift dikes (see below).New dune arcs such as the one currently forming in the eastern partof Spiekeroog which are not enclosed by dikes on the Wadden Seaside also provide ample conditions for tidal marsh developmentwith gradients from high to low, salt to fresh, sandy to clayey.

3) Washover complexes

Many of the islands once had one or more washover complexes,which were mostly closed by sand drift dikes. A good survivingexample is the so-called Spiekeroog Leegde. Restoration of suchwashover complexes is only possible if it does not form a threat tohuman safety. Restoration will often consist of the removal ofa sufficient length of sand drift dike and removing part of the topsoils, preferably in places where the washovers were originally

present. If washover complexes are restored then: i) pioneerspecies will re-occur; and ii) additional sand is deposited on thebarrier islands from the North Sea side, resulting in a higher resil-ience of these natural plains with respect to sea-level rise (VanDongeren and Van Ormondt, 2007) and iii) the washovercomplexes will act as a gateway for eolian transport across theisland, further stimulating aggradation.

4) Island tails

Often the island tails are among the more natural parts of theWest and East Frisian barrier islands and need little maintenanceor restoration. Locally on the West Frisian Islands sand drift dikesare present which are relatively easy to reactivate, for instance byopening up these dunes, so that: i) small flooding openings form(Ten Haaf and Buijs, 2008); ii) eolian sand drift may increase thetopographical level in the area; and iii) tidal marshes may form.Also, a more natural development of the island tail leads to morenatural and ecologically diverse dune system (De Jong et al.,2011).

That such an approach may also form a partial answer toaccelerated relative sea-level rise is illustrated by the developmentson Skallingen in the North Frisian Wadden Sea. At the Skallingenpeninsula coastal protection was abandoned some 20 years ago.This has allowed the dune ridges to be breached during severestorm surges leading to the deposition of largewashover fans. It hasbeen shown that this process results in local gain of sediment andwashover fans, which subsequently built up to new dune ridgesperpendicular to the coast (Christiansen et al., 2004; Nielsen andNielsen, 2006).

5) Beach and shoreface

One of the characteristics of a natural barrier island is theseasonal and decadal alternation of sedimentation and erosionphases of the beach. The present-day state of technology is suchthat the beach’s position can be safeguarded. This can be done bymeans of hard coastal engineering works, which obstruct orprevent natural beach morphodynamics and sediment dynamics ingeneral. However, given the strong interconnectedness of theislands with the back-barrier and ebb-tidal deltas, and the strongconnection between the beach and the rest of the island, the bettersolution seems to be nourishments (Hoekstra et al., 1994, 1999; seealso Williams and Foley, 2007). Thus far nourishments have beencarried out to primarily maintain the coastline. However, thepresent-day state of technology make it possible to steer nourish-ments in terms of location, frequency, volumes and form, so thatnot only safety is maintained but also nature is furthered andpartially restored (see also Van der Wal, 1999). Large volumes ofbeach or ebb-delta nourishments may, for instance, lead to the(temporal) formation of new dunes and habitats, but hinder thedevelopment of older more landinward dunes (Arens, pers. com.).Such approaches are especially interesting on the NW parts of theisland coasts where through natural developments strong sedi-mentation and erosion patterns occur on a decadal scale. It shouldbe realized however, that under natural conditions sand supply atmany other coastal locations is periodic but very limited. Nour-ishing large amounts of sandmay severely disturb themore naturalprocesses of sand transport and coastal development in suchplaces.

For the West and East Frisian islands a simple scheme can bededuced from the various scales when addressing coastal zonemanagement (Fig. 9). If questions are answered with a “yes” thanthe influence should be considered in barrier island management.The outcome however is always similar: on the long run barrier

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islands and their nature may become more resilient when naturalprocesses are furthered as much as possible.

5. Conclusions

The morphological development of the West and East Frisianbarrier islands is the outcome of a hierarchy of morphologicaldevelopments on several spatio-temporal scales. In this the largerscale settings to a large extent determine which smaller scaledevelopments can take place. The natural development andbehavior of the Wadden Sea barrier island system depends on:

� The presence of outcrops and inherited Pleistocene topographyand paleomorphology, which may dictate to some extent thedevelopment of the islands, with promontories of rigid Pleis-tocene deposits such as on Sylt and Texel as the most strikingexamples.

� The Holocene alignment of the chain of barrier islands and ebb-tidal deltas, which is currently under pressure as man tries tokeep some (important parts) of the islands in position whereasother parts are allowed to erode.

� The balance between sea-level rise and sedimentation, whichmay become disturbed in the future when sea-level rise is toofast for the back-barrier basin to keep up with.

� Other adaptations to climate change such as changes in winddirections, which may have led to sediment surpluses for theislands during the past 150 years.

� Reactions of the sediment-sharing inlet system on theincreases and decreases in tidal volume due to changes in theback-barrier areal extent leading to surpluses or deficits ofsediments on the barrier islands which form part of the sedi-ment-sharing system.

� Dynamics on the barrier island between the larger morpho-ecological units, which in their natural form do allowfor both coast-parallel and coast-perpendicular sedimenttransports.

The impact of human measures and interventions on barrierisland development in the last decades to centuries has led todisturbances in the sediment-sharing inlet systems and hence tosurpluses or deficits of sediment becoming available to many of thebarrier islands. Furthermore some parts of the barrier chain havebeen allowed to retreat landward whereas other parts are kept intheir position, which may lead to local increases in erosion. On thebarrier islands itself human measures have led to an increase insafety from flooding and at the same time a decrease in naturaldynamics and hence supporting ecosystem services. Natural valuesare decreasing on many locations on the Frisian barrier islands;especially pioneer species rich ecosystems, which are stronglydepending on natural dynamics. All in all there has been a lot offocus on strengthening the production-, regulating- or culturalecosystem services, but little attention was paid to supportingservices.

Sand nourishments to some extent add to the natural devel-opment of the islands in terms of sand becoming available to otherparts of the islands. However as they are primarily aimed atmaintaining safety it can be concluded that the present-day ICZMstrategy is not a successful, integral and sustainable approach thatis able to cope with future aspects of global change. Adaptation ofthe coastal policies is needed in the direction of restoring naturaldynamics where possible. Taking into account the influences ofdevelopments on larger spatio-temporal scales the aim should beto restore the functioning of the larger eco-morphological unitswherever possible and allow sedimentary dynamics. As a rule: softand with respect for morphodynamical integrity where possible, hard

where really needed. In this way large parts the islands can accreteto cope with sea-level rise, safety will be maximized and pioneerconditions will not disappear on the islands.

Ethical statement

The work described in or manuscript has not been publishedpreviously. It is not under consideration for publication elsewhere.Its publication is approved by all authors and tacitly or explicitly bythe responsible authorities where the work was carried out, andthat, if accepted, it will not be published elsewhere includingelectronically in the same form, in English or in any other language,without the written consent of the copyright-holder.

Acknowledgements

This paper is partly based on a previous document or positionpaper (in Dutch) initiated by the Wadden Academy (part of theRoyal Dutch Academy of Arts and Sciences) to discuss relevantgeoscientific issues of the Wadden Sea (see Speelman et al., 2009).The position paper addresses three major subjects: the geologicaldevelopment and structure of the region (including the presenceand development of mineral/natural resources), the Holocenecoastal evolution and “present-day” coastal morphodynamics. Theposition paper was almost entirely based on data from the Dutchpart of the Wadden system. Our present paper focusing on barrierislands is based on an extended and strongly edited version of theposition paper on the Holocene evolution updated with our studyon the Holocene evolution of the trilateral Wadden Sea on requestof the CommonWadden Sea Secretariat (CWSS; see Wiersma et al.,2009) and our insights on barrier island management (see Löflleret al., 2011).

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