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Ocean & 68 (2012) 18e38

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Ocean & Coastal Management

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Barrier 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 a Deltares & University, Department of Physical Geography e IMAU, P.O. Box 177, NL 2600 MH Delft, The b Utrecht University, Department of Physical Geography e IMAU, The Netherlands c Deltares, Applied Geology and Geophysics, Utrecht, The Netherlands d Senckenberg Research Institute, , e Staatsbosbeheer, , The Netherlands f Department of Geography and Geology, University of Copenhagen, g EUCC-The Coastal Union Germany h Deltares, Morphologic and Sedimentary Systems, Delft, The Netherlands i Geological Survey of the Netherlands e TNO, Geomodelling Department, Utrecht, The Netherlands j Deltares & Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands article info abstract

Article history: The article focuses on the morphological development of the system, with Available online 27 July 2012 emphasis on West and East Frisian on several temporal and spatial scales. In addition, it inte- grates the insights for management purposes. Barrier island management is addressed with respect to morphology, 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 has to 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 fair extent, either directly due to -resistant outcrops on or near the islands, or indirectly by deter- mining the locations where systems or could develop. Where this is not the case, the larger part of the are locally reworked Pleistocene or Holocene deposits eroded at the of the barrier chain and deposited in the back-barrier area and on the islands as a response to sea-level rise. Hardly any is coming in from outside the area. In order to keep up with sea-level rise sand 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 their coastward migration, forming a more or less uninterrupted barrier chain along the Frisian coasts. The present-day approach of mainly focusing on the fixation of the inhabited parts of the chain will most likely result in a de-alignment of the various parts of the chain, resulting in increasing erosion of the promontories. An inlet system is a sediment-sharing system with a tidal inlet, the ebb-tidal delta, adjacent barrier islands and the tidal basin with channels, , tidal flats and salt marshes. The sand balance of a 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 the rapid growth of the islands after the demise of the . On the barrier islands themselves many measures were taken during the past two centuries to ensure coastal safety. The successful attempts to stabilize the coasts and of the barrier islands resulted in a reduction of sand transport from and along the shoreface to the and onto the islands. To some extent this has been restored by applying sand nourishments. However, vertical accretion of the islands is still largely impossible due to all the older coastal protection measures still present. On the long run sedimentary dynamics are essential if the island is to accrete vertically with sea-level rise, which forms a robust and sustainable strategy to guarantee safety during the next centuries. Massive stabilization also reduced the opportunities for pioneer vegetation. belts and tidal marshes have experienced a fast succession resulting in a climax vegetation and the loss of the characteristic open landscape.

* Corresponding author. Tel.: þ31 (0)6 22337195; fax: þ31 (0)88 335 8582. E-mail address: [email protected] (A.P. Oost).

0964-5691/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ocecoaman.2012.07.010 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 to create robust ecosystems. Instead of focusing on nature conservation a paradigm shift is needed in barrier island management towards stimulating the development of natural dynamics. To our opinion the best solution is to allow the -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 feasible other 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 various morpho-ecological units on the islands and to the morphological developments on larger spatio- temporal scales. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction level rise. This can be considered as a threat, but can also be interpreted as an opportunity. Nowadays it is often tried to tackle Barrier islands belong to the most dynamic coastal environ- the problems by means of an integrated coastal zone management ments where rapid changes may occur in terms of coastal processes (ICZM) approach. It “involves the comprehensive assessment, setting and morphological as well as ecological developments (Hayes, of objectives, planning and management of coastal systems and 1979, 1980; Davis, 1994). Coastal zone management of barrier resources, taking into account traditional, cultural and historical island systems is not a trivial issue, but requires a proper under- perspectives and conflicting interests and uses” (WCC, 1994). The standing and appreciation of the challenges under discussion. agents of change are driven by socio-economic demands, natural These challenges may vary from changes in the natural system due processes and the impacts of (Misdorp, 2011). It is to external forcing, to the impact of human measures on the advocated that integrated spatial planning should not only dynamic character and resilience of the . Major present-day encompass small-scale adaptations, but should also take a long- challenges are related to: i) coastal safety; ii) natural resources; term look ahead (Baarse and Misdorp, 2011). In practice, iii) the industry and iv) the development of ecosystems and however, the ICZM approach is mostly to react on relatively short- associated ecosystem services. Coastal safety is of paramount lived developments as far as natural processes are concerned, often importance for those islands that are vulnerable to flooding and without having a sufficient in-depth understanding of the causes sea-level rise, in particular due to a lack of sediment. Coastal safety and consequences. As a result, ICZM does seldom take into account is commonly established by stimulating dune development, by the various temporal and spatial scales that are relevant to building seawalls, dikes and or by implementing nour- morphological developments. This is true for most barrier systems ishments. However, the issue of coastal safety is not limited to the in the world. As an example, problems of on the barrier island “sensu stricto” but also implies the protection of Frisian barrier islands were tackled in the past by constructing adjacent and the mainland coast. In the NW European groins and are nowadays solved by implementing nourishments. Wadden region, for example, barrier islands in combination with Nevertheless, in both cases measures are (were) taken without the Wadden Sea provide a natural buffer between the highly having a proper understanding of the short- and long-term causes energetic conditions in the North Sea and the mainland coast. of erosion and accretion (Wang et al., 2012). Furthermore the Likewise, barriers and barrier island systems in the abandoned impact of human actions on the natural functioning of the coast, in parts of the Mississippi delta protect an extensive system of both time and space, is often poorly understood. wetlands and (Fritz et al., 2007). Once a barrier island is In this paper we will demonstrate that barrier island manage- eroded, an accelerated loss in wetlands is observed (Sallenger and ment can benefit from a more comprehensive approach that is Williams, 1989; Penland et al., 1992). based on a careful analysis of the impact of past, present and future The use of mineral resources of barrier islands is another activity processes and developments on barrier island dynamics. We that requires careful management. The most important impacts are illustrate this approach for especially the West and East Frisian related to sand mining, gas and oil exploitation and the extraction Barrier Islands and to a lesser extent the North Frisian area. To this of drinking water. All have in common that they produce a certain end we address the following three questions: degree of subsidence and result in a greater accommodation space for sediment. In addition the non-sustainable extraction of drinking What can we learn in terms of ICZM from the natural devel- water may result in a reduction of the fresh water supplies on the opment and behavior of the Wadden Sea barrier island system? islands and the intrusion of sea water. The increased use of local What has been the impact of human measures and interven- fresh water supplies is often triggered by a more demanding tions on barrier island development in the last decades and tourism industry that is important for the economic development centuries? of the islands. Well-known examples of tourism development are Is our present-day ICZM strategy a successful, integral and found along the West and East Coast of (USA), the Algarve sustainable approach that is able to cope with future aspects of (Portugal) and along the West, East and North Frisian Wadden global change or do we have to adopt our coastal policies? coast. Due to their highly dynamic nature, barrier islands and surrounding coastal waters also represent valuable assets in the We will argue that the present-day ICZM approach is in favor of form of ecosystems and ecosystem services. Therefore, ecosystem short-term, local coastal safety and focuses on the maintenance of dynamics and development versus coastal protection and nature and dunes. However, this approach fails to address some conservation should be important considerations in barrier island other fundamental issues such as long-term safety, sediment management. dynamics and budgets in relation to coastal resilience, ecology and Barrier island management thus involves a range of stake- biodiversity. We advocate that a more balanced approach is desir- holders with often conflicting interests. On top of that, environ- able; an approach that better acknowledges the dynamic character mental and societal problems are increasingly associated with of the islands and involves safety, morphology and ecology. Such an global change issues such as climate change and (accelerated) sea- approach is feasible, but also requires a trade-off between different 20 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 stakeholders, even within the community of the environmentalists 2. Description of the area if one has to deal with the dilemma of nature conservation or restoration versus spontaneous nature development due to estab- The Frisian barrier island system forms the boundary between lishment of the appropriate boundary conditions. the open North Sea and the Wadden Sea and stretches for over In our analysis the relations between different temporal and 450 km along the coasts of the Netherlands, Germany and Denmark spatial scales are crucial. Our central concept is to regard (Fig. 1). The system is characterized by relatively short (10 e30 km morphological developments of barrier islands as the product of in length) barrier islands. developments at a hierarchy of spatial scales. Our concept of scales The mixed micro- to macrotidal, semidiurnal tidal regime assumes a strong coupling between the temporal and spatial scales (Fig. 1) is characterized by tidal ranges that initially increase from involved; an increase in spatial scales considered automatically 1.4 m at (western Wadden Sea) to 4.4 m at implies that one has to take into account developments at larger (central part of the Wadden Sea in the German ), before time scales (and vice versa). Developments at larger temporal and progressively decreasing again to 1.5 m at the Skallingen barrier- spatial scales impose boundary conditions on developments at that marks the northern boundary of the Wadden Sea in a smaller scale. Throughout this paper, we will apply this concept of Denmark. Mean significant (offshore) wave heights for the calm scale for different objectives. part (15 Aprile15 October) were calculated to be around 0.5e1m The first and second question are addressed by analyzing the and 1e2 m during the storm part (15 Octobere15 April) of the natural behavior of the Frisian barrier island system at three related years 2002 and 2003 (Dobrynin et al., 2010). In the West and East scale levels, which partly correspond to three different sources of Frisian area waves and swell are predominantly incident from the information: i) the geological record (millennia), ii) the historical west and southwest and along the North Frisian coast the major record (centuries to the past millennium), and iii) the waves come from the northwest (Bartholdy and Pejrup, 1994; Beels system (years and decades to a few centuries). We distinguish the et al., 2007). During storm surges, the maximum recorded set-up in following sequence of scales (see also: Speelman et al., 2009): water level is about 3.5e4 m and the offshore maximum wave height may be up to 8e11 m (data of several water management Development of the Wadden Sea barrier island system during ministries). the Holocene. The islands are separated by 33 adjacent tidal and asso- ciated tidal basins, separated by tidal watersheds (Fig. 1; Dijkema, At this scale level it will be shown that the present-day 1980; Hofstede, 2005). Each tidal basin is drained by a configuration of the island chain is mainly the product of Holo- system, which is connected with the North Sea via the tidal inlet cene (marine) processes that operate(d) in conjunction with the (Cleveringa and Oost, 1999). The inlets are generally lined by barrier pre-existing Pleistocene topography or relief. islands on either side, only the first and last inlet being bordered by the of Den Helder in the south and the Skallingen Interactions of the tidal inletebarrier island system over the peninsula in the North. past millennium. The modern Wadden Sea can be largely subdivided into two different regions: Each individual tidal inlet system can be considered to form a sediment-sharing system consisting of a tidal inlet, ebb- and/or 1) A chain of barrier islands fronting tidal basins, which extends flood-tidal delta(s), updrift and downdrift barrier island coasts from the westernmost island of near Den Helder in the and the tidal basin, all of which are basically in a hydro- Netherlands, up to the area between the and morphodynamic equilibrium with the prevailing conditions. in Germany, together forming the West and East Frisian Changes in these equilibria will lead to morphodynamic reactions Wadden Sea (separated by the estuary). in the system until either the original equilibrium is restored or 2) The area between the Weser and Elbe estuary in Germany up to a new equilibrium is established (Dean, 1988; Eysink, 1991; the Skallingen peninsula in Denmark which forms the North Steijn, 1991; Oost, 1995; Wang et al., 1995; CPSL, 2001, 2005; Frisian Wadden Sea with a north-south trending series of Kragtwijk et al., 2004; Elias et al., 2005, 2012,acc.;Dastgheib islands. In the true barrier islands are absent due et al., 2008). to the large (Hayes, 1979, 1980; Jacobsen and Madsen, 1993). Behavior of an individual barrier island on a time scale of decades to a few centuries. along the Wadden Sea coast did, and do, not bring in substantial amounts of sediment. A small amount of sand and At this scale level we identify different so-called morpho- a larger amount of fines is imported with the coast-parallel ecological units, which together comprise the individual island. In currents from the south along the West Frisian coast and from our conceptual approach this island represents a prototype island the north along the North Frisian coast. The bulk of the sediments since it illustrates the spatial and temporal coherence and within the Wadden Sea, however, consists of locally reworked dynamics of the morphological subunits of an island in a schema- Pleistocene material selected, transported and imported by tized way. This conceptual framework also appears to be extremely marine processes into the Wadden Sea as demonstrated by the useful from an ecological point of view (Löffler et al., 2011). mineralogical composition of the sediment (e.g. Winkelmolen The paper is organized as follows. In the following section we and Veenstra, 1974, 1980; Hoselmann and Streif, 2004). The will present some main characteristics of the entire Wadden Sea sediment needed in the back-barrier area, among others to region. We will then continue with discussing the processes and balance sea-level rise, was and still is largely “taken” from the developments at the three different temporal and spatial scales North Sea side of the barrier island chain (Mulder, 2000). As involved and use this information to asses the impact of centuries a result, the barrier islands and tidal basins tend to shift landward of human intervention on barrier island development. Subse- with rising sea-level rise. The strong coastward retreat of the quently the pro- and cons of present-day ICZM strategies are West and East over up to 10 km and the smaller evaluated and an alternative approach is proposed. The paper ends retreat of the North Frisian barrier islands resulted in the with a discussion and major conclusions. reworking of sediments. A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 21

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

3. Barrier islands of the Wadden Sea system: evolution in position of the evolving Wadden Sea. Of particular importance time and space were the following effects:

3.1. Development of the Wadden Sea barrier island system during 1) The initial differences in general paleorelief, with the north- the Holocene south trending North Frisian area being more dominated by a steeper Pleistocene relief and lower availability of loose During the last glacial, most of the southern North Sea was land sediments than the east-west trending East and West Frisian and the nearest shoreline was off the coast of Brittany (France) in area. This is, next to lower rates of sea-level rise in the North the south and near the Shetland Islands in the northwest. The rivers Frisian area, considered to be an important reason for the Ems, Weser, Elbe and Eider drained northward via a broad valley stronger retreat of the East and West Frisian area, where sea- (Figge, 1980). After the last Ice Age relative sea-level rise was level rise results in inundation of larger areas in the mainland initially rapid (over 1 m/century), but decelerated significantly after direction. 7500e7000 a BP (Fig. 2; Kiden et al., 2002; Gehrels et al., 2006; 2) The location and scale of paleo-valleys incised into the Pleis- Busschers et al., 2007; Vink et al., 2007; Kiden et al., 2008; Pedersen tocene land surface during the sea-level lowstand. This paleo- et al., 2009; Baeteman et al., 2011). In close association with the channel morphology determined the location, size and inland relative sea-level rise, the tidal range increased from initially penetration of estuaries, e.g. the Lauwerszee (Oost, 1995), the microtidal conditions everywhere to the more differentiated ranges Ems-Dollard estuary, the Jadebusen, and the Weser and Elbe presently observed along the coast as the water depth in the estuaries (Streif, 2004; Wiersma et al., 2009). Moreover: as the southern North Sea basin increased (Jelgersma, 1979; Franken, estuaries were deeply incised into the mainland, the sea could 1987; Van der Molen and de Swart, 2001). easily invade inland peat areas and convert them into back- Around 8000 a BP was still some 20 m below its barrier basins (e.g. Dollard- and Jade-embayments) thereby present level (Fig. 2) and the North Sea coastline was much further enlarging the tidal volume of the estuaries. The increase in tidal offshore than its present position, e.g. 10e15 km for the central prism led to deeper incision of the tidal channels. In particular West Frisian Wadden area (Vos and Van Kesteren, 2000). The the flooding of the paleo-valleys of Ems, Elbe and Weser had rising sea level followed (and modified) the pre-existing relief of a large impact on the coastal development, as the landscape the Pleistocene land surface and thereby constrained the initial developed into a broad estuarine landscape, which determines 22 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38

Cal. yrs BP drift, responsible for a re-alignment and smoothing of the often 10000 8000 6000 4000 2000 0 heavily indented coastline. Furthermore there are Pleistocene present along the mainland coast, e.g. at m in the Netherlands and near the coast of Blåvands Huk, north of NAP Skallingen. Furthermore the presence of sub-marine Pleisto- cene ridges can influence currents and wave impacts, e.g. at the -5 southern “spit” of (Lindhorst, 2007). Smaller barrier islands, sandy shoals or sand spits were likely present in front -10 of the mainland coast as relicts of Pleistocene headlands 1 (Flemming and Davis, 1994). 4) The low rate of relative sea-level rise in the northern part of the -15 3 North Frisian barrier coast: Pedersen et al. (2009) gave only 2 w12 m over the past 8400 years. This led to a much more -20 1: Belgium restricted erosion of the islands. For instance, the northern spit 2: SW-Netherlands of Sylt was formed as early as 5000 BP (Lindhorst, 2007), -25 4 3: W-Netherlands whereas Rømø prograded seaward since 8000 a BP, due to A 4: NW-Germany abundant sediment supply (Madsen et al., 2010). -30 The bulk of the West and East Frisian barrier island chain formed between 6000 and 5000 a BP. Because sedimentation rates were N insufficient to fill the accommodation space created by the initially rapidly rising sea (1 m/century), much of the inundated surface evolved into subtidal environments, fringed at the landward side by a narrow zone of intertidal sand and mud flats and salt marshes. An Elbe increase in tidal ranges resulted in higher tidal volumes, which may North Sea have generated an increased net sediment import into the tidal 4 basins (Vos and Van Kesteren, 2000). However, it was also Weser accompanied by further erosion and deepening of the tidal chan- 3 Netherlands nels of which the sediments became available to the tidal flats (Hofstede, 2002) and to the ebb-tidal deltas. In this landscape, fens gave way to raised bogs. They started to expand on the mainland of West between 7000 and 6000 a BP (Casparie and Streefkerk, 1992; Vos et al., 2011) and on the mainland of between Germany especially 7200 and 5600 a BP (Petzelberger et al., 1999; Eckstein et al., 2010). The onset is attributed to a shift to wetter climate conditions, which were brought about by rising sea level in the 2 early and mid-Holocene (Eckstein et al., 2010). 1 The sea reached the Pleistocene deposits of the southern part of Belgium the North Frisian area around 6500 years ago. Cliffs were formed Schelde and the material was transported along the coast. Around 6000 a BP B Sylt was most likely already an island (Jessel, 2001). Until recently, Meuse 0 100 km not much was known about the North Frisian Wadden region in the Fig. 2. Differences in Holocene relative mean sea-level rise (A) for different regions early Holocene. However, new research (Madsen et al., 2010) along the Belgian-Dutch-German North Sea coast (B), making clear the differences on suggests that the formation of northern North Frisian barrier a regional scale. Sea-level rise in NW Germany has been larger than in the Western islands started some 8000 a BP. Netherlands and in Belgium. The differences are mainly caused by relative glacio- At about 5000 a BP, the decelerating rate of sea-level rise along isostatic subsidence (Mörner, 1979). Figure modified from Kiden et al. (2008), the sea-level curves are from Van de Plassche, 1982 (W Netherlands); Denys and the Dutch west coast and the westernmost part of the West Frisian Baeteman, 1995 (Belgium); Kiden, 1995, 2006 (SW Netherlands) and Vink et al., Wadden Sea area (Fig. 2) was exceeded by sediment accumulation 2007 (NW Germany). The sea-level curves depicted here correspond to the mid- rates. Thus, intertidal sand flats grew at the expense of subtidal lines of the sea-level error bands presented in these studies. environments (Van Heteren and Van der Spek, 2003; Vos et al., 2011). During the following millennia, sea-level rise decelerated the morphology up to this day. In the West and East Frisian even further, and the infilling of the available accommodation Wadden Sea area, the deepest valleys were flooded space tipped increasingly in favor of sedimentation exceeding sea- around 8000 a BP (Beets and Van der Spek, 2000). In a few level rise. In the eastern part of the West and in the East Frisian cases the former paleo-valleys still determine the position of Wadden Sea, however, subsidence of the bottom was relatively present-day tidal inlets (Wiersma et al., 2009). large and sedimentation was insufficient to infill the basins 3) The occurrence of locally elevated Pleistocene (or older) everywhere (Vos et al., 2011). In some places the tidal area was outcrops and headlands consisting of moraine deposits of the extending, e.g. in the Boorne, Hunze and Fivel areas in the West Saale (second-last) glaciation, and sandy meltwater deposits of Frisian Wadden Sea (Vos et al., 2011). From time to time, parts of the the Weichselian (last) glaciation, especially in the North Frisian Wadden Sea locally silted up and the salt marshes at the landward Wadden area (Bartholdy and Pejrup, 1994; Lindhorst, 2007). fringe of the tidal basin could advance seaward (see Behre, 2004; Several of the barrier islands evolved around such Pleistocene Vos and Van Kesteren, 2000; Vos et al., 2011). Raised bogs spread or older cores of ice-dislocated deposits, a.o. the islands of over a larger area up to 3500 a BP. Some of these peat layers reached Texel, , Sylt and Süderoogsand. These headlands a considerable extent (Behre, 2004). However, most of these fresh commonly also acted as a source of sediment for the longshore water marshes drowned again and clastic tidal sediments were A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 23 deposited on top of them (Vollmer et al., 2001; Behre, 2004; Streif, Frisian Wadden Sea in this period. At that time the coastal envi- 2004). ronments from the mainland up to the Wadden islands were not At about 5000 a BP, the West and East Frisian barrier island yet separated from each other by dikes. Towards the mainland coast chain was still situated several kilometers offshore from its present the tidal flats merged with tidal marshes and extensive brackish position, e.g. in the case of about 9.5 km further to the areas, which, in turn, were flanked by extensive peat bogs, lining north in comparison to its present position (Sha, 1990a,b; Vos et al., higher sandy areas (Esselink, 2000). Tidally influenced rivers and 2011). From 5000 a BP until today, the chain of barrier islands has streams debouched freely into the Wadden Sea. Settlements in the been retreating landward at an average migration rate in the order coastal marshes were built on dwelling mounds, which had of 1e2 m/year. The landward migration of the barrier islands was a negligible influence on the large-scale tidal dynamics. Initially the caused by erosion on the seaward side due to the rising sea level dwelling mounds made use of the existing landscape and were and transport into the back-barrier tidal basins of the Wadden Sea built on small depositional ridges along marine/estuarine channels. where the sediment deficit created by this sea-level rise more or The first clear ring dikes in West and East Frisia were built less was compensated (Oost, 1995; Wang et al., 2012, this volume). around 1000 e900 a BP and in around 900e800 BP In the southern part of the North Frisian area (a.o. ), (Van der Spek, 1994; Oost, 1995; Ey, 2010), surrounding smaller the barriers and Pleistocene outcrops in the area created an inlet- areas to safeguard arable fields and against most winter floods. A segmented coastline around at least 5000 a BP (Vollmer et al., second type of dikes was oriented rectangular to the main stream to 2001; Meier, 2004; Lindhorst, 2007). On the peninsula of Eider- channel the outflow of waters and has at least been built since stedt, the cores of the modern east-west trending sand bars prob- 1000e800 a BP (Ey, 2010). Gradually, local ring dikes were con- ably consist of eroded Pleistocene material and are a relict of this nected and raised, and by 800e700 a BP a continuous system of barrier system (Hoffmann, 2004). The sandy barrier system pro- winter dikes had been constructed along the entire West and East tected the hinterland and around 2500 years ago extensive marshes Frisian Wadden Sea coast and several parts of the North Frisian formed, consisting of thick sequences of clayey and peaty sedi- coasts (Oost, 1995; Ey, 2010). The diking was partially a response to ments with small-scale relief due to differential compaction (Meier, the increasing vulnerability to flooding due to peat excavation e 2004). Although a large part of these marshes eroded during both inside and outside the dikes e and the development of several medieval times, its remnants still dominate the morphology drainage systems (e.g. Balgzand, Lauwerszee, Dollard, Jade and of the area. Seaward of the marshes, tidal flats began to form as, for North Frisian area; Oost and Kleine Punte, 2004; Kühn, 2007; example, documented on Hallig Hooge and (Vollmer Wiersma et al., 2009). The vulnerability has resulted in some major et al., 2001). “invasions” by the sea and the (re-)opening of many embayments along the Wadden Sea (, Middelzee, part of the Lau- 3.2. Interactions of tidal inletebarrier island system over the past werszee, Dollard, and the Jade Busen; Pons and Wiggers, 1960). The millennium spatial extension of the tidal basins generated a larger tidal prism and larger sediment demand. Before 500 a BP, many larger bays 3.2.1. Coastal evolution between 1200 and 500 a BP (Fig. 3; after reached their maximum size. Partly coinciding with the onset of the Wiersma et al., 2009) Little Ice Age, successful started, from time to time The reconstruction of the West Frisian Wadden area at 1200 a BP set back by severe storm surges (Oost, 1995). By 500 a BP, large (Fig. 3) shows the situation before large-scale human interference parts of the salt marshes attached to the mainland which had within the system developed. The situation in the West Frisian area become high enough due to natural sedimentation had been is considered to be representative for especially the West and East embanked and extensive areas of land bordering the inland bays

Fig. 3. Reconstructed map of the West Frisian Wadden Sea for the situation 1200 a BP (based on Vos and Kiden, 2005; Vos and Knol, 2005). 24 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 along the West (Middelzee, Lauwerszee, Fivelboezem), East (Maade As early as 500 a BP the first large-scale attempts were made to Einbruch, Schwarzes Brack) and North (Bucht von Sielmönken) protect dune belts on the West and East Frisian barrier islands. Frisian mainland coasts had been reclaimed (Van der Spek, 1994; However, extensive livestock grazing inhibited the intended stim- Oost, 1995; Vollmer et al., 2001; Van Heteren and van der Spek, ulation of a closed vegetation cover. Serious sand drifts were 2003). The land reclamations reduced the surface area of the tidal recorded on many islands, for instance Texel, , Terschelling, basins, creating smaller tidal prisms, which in turn resulted in and Sylt (Homeier, 1964; Schoorl, 1999a,b, 2000a,b; Jessel, smaller tidal inlet systems. It was coastal squeeze on a very large 2001; Van Heteren et al., 2006). On most of the islands wind scale: while the mainland coast was prograding seaward and the erosion must have occurred especially in the inner dunes: marram barriers were retreating landward, the tidal basins became smaller. grasses benefit from drifting along the dune face. In this Settlements on most of the islands were (semi-)permanent from period, extensive sand drift dikes were built and maintained by at least 1000 a BP onwards. It has been documented that several of placing brushwood fences, which trapped and stabilized wind- the East Frisian barrier islands were plundered around 1200 a BP blown sand (Oost et al., 2004). As early as 370 a BP, the islands of (Abel, 1893; Rau, 1955). For most of the it is clear Eijerland and Texel were connected by a sand drift dike (Schoorl, that continuous inhabitation was at least present since 800 a BP, 1999b). Although such measures suggest a strong influence on based on written historical sources. From radiocarbon dating and the sedimentary development of the area, the anthropogenic pollen analysis, it appears that and Terschelling have been effects mainly slightly modified the impact of natural processes. used for agriculture in or before 1200 a BP (De Jong, 1984, 1993; Existing technology at that time was largely incapable of stopping Oost, 1995). The names of the villages and excavation on some large-scale natural developments, such as channel or churches indicate that some settlements were founded around migration. 110 0e1000 a BP (van Oosten, 1986; Edes and Glashouwer, 1990). The area in the North Frisian Wadden Sea, which was Management and the introduction of animals and plants on the especially vulnerable due to peat extraction for the production of islands initiated morphological and ecological changes. Especially salt, was struck by several catastrophic storm surges (e.g.1634,1717, dune arcs, which provided protection against flooding, were 1825 A.D.) and large areas of land were lost, largely shaping the exploited. Directives protecting the marram grass and dunes date present-day Halligen landscape (Figs. 4e6; Vollmer et al., 2001). from at least as early as 650 a BP (1354 A.D., ; Blok Only a small percentage of the inundated and eroded et al., 1899; Oost, 1995). between Rømø and Eiderstedt could subsequently be reclaimed The Halligen islands in the North Frisian back-barrier area were (Vollmer et al., 2001). at that time still much larger than at present, although sea-level The tidal waters flowing through the inlets in combination with rise, storm surges and normal tidal and wave action during medi- wave- and wind-driven currents continuously exchange enormous eval times had already dramatically changed the vulnerable peaty amounts of mainly sandy sediments between the North Sea, the landscape (Hofstede, 1991; Vollmer et al., 2001). From 1200 a BP islands, the ebb-tidal delta, the inlet and the back-barrier basin onwards, dwelling mounds were once more constructed in the (Oost, 1995). Net sedimentation occurs in the back-barrier basins at North Frisian region (Vollmer et al., 2001) as had been done before. the expense of the open North Sea coast, which is on average Between 1000 and 900 BP, stormeflood layers were deposited on retreating in a landward direction (e.g. Juist, Texel). A closer look, top of the earliest cultural layers, indicating increased marine however, reveals large differences from island to island, indicating influence. Flooding is also indicated by the fact that, at the same that on top of the long-term regional trend in coastal behavior time, the peat bogs north of the Garding-Tating system other processes are locally important. One of the clear examples is changed into a (Vollmer et al., 2001). Subsequently, the the western side of Vlieland during the 16th to 18th century. Due to North Frisian marshes were protected by dikes. These required strong into the back-barrier area the ebb-tidal drainage, which then resulted in compaction due to dewatering of delta retreated followed by the west coast of the island. Retreat of the underlying sediments and peat layers. In addition the peat the island was as much as 1.5 km in 150 years (Abogado Rios, 2009, started to oxidize resulting in further lowering of the sediment unpubl. res.). Another clear example can be found on the adjacent surfaces. barrier islands of Ameland and in the West Between 700 and 600 a BP, catastrophic storm surges inundated Frisian Wadden Sea. From reconstructions (Oost, 1995) it appears extensive areas protected by dikes. In particular, the areas con- that after 150 a BP Ameland retreated landward. In the same period sisting of thick clayey and peaty sediments that had experienced the barrier island of Schiermonnikoog, by contrast, shifted seaward, increased compaction could no longer be protected against the sea. especially after a reduction of the tidal volume of Schiermonnikoog Consequently, parts of former coastal marshes changed into tidal Inlet with a third in 1969 (Biegel and Hoekstra, 1995; Oost, 1995). flats. During the middle Ages, people produced salt by burning the Also, the eastern side showed a growth of more than 6 km, whereas peat in the coastal marshlands under a cover of clay. This peat Ameland only grew by about 1 km towards the E (Oost, 1995). extraction made the land even more vulnerable to flooding and Both developments result from the fact that the tidal inlet coastal erosion (Vollmer et al., 2001). The reconstructions around system is a sediment-sharing system. In the back-barrier area, 500 a BP show that small parts of the salt marshes on several roughly speaking, vertical accretion of the shoals keeps pace with barrier islands had already been embanked. It is thought to have sea-level rise and hence, if tidal amplitude remains constant, the commenced as early as 700 BP (Oost, 1995 ; Schoorl, 2000a). tidal volume will mainly depend on the area of the back-barrier The high North Frisian barrier islands with a Pleistocene core, tidal basin. The volume of the back-barrier tidal channels such as Amrum and Sylt were inhabited more or less uninterrupted depends directly on the tidal volume that has to be imported and since pre-historic times (Vollmer et al., 2001). exported through these channels. If this tidal volume decreases, the tidal current velocities will decrease as well and net sedimentation 3.2.2. Changes after 500 a BP (Figs. 4e6; after Wiersma et al., 2009) of sand and clays will occur in the channel until the tidal flow will From 500 a BP onwards, successful land reclamation continued establish a new equilibrium. In this morphodynamic equilibrium in the West (Lauwerszee, last remnants of the Middelzee) and East sedimentation in the channel is balanced by erosion and e apart (Harlebucht) Frisian Wadden Sea (Van Veen, 1936; Vollmer et al., from migration e the channel will maintain stable dimensions 2001). The area of embanked salt marshes gradually increased (Biegel and Hoekstra, 1995). Something similar applies to the ebb- and the size of the inland bays consequently decreased. tidal deltas, the sediment volumes of these are also related to the A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 25

wadden 1500

Dunes 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).

tidal volume in the basin: the sand volume decreases with growth and dune development reflecting sand supply from the ebb decreasing tidal volume. If the tidal volume decreases, part of the deltas and shoreface. The islands of , , Bal- sand eroded from the ebb-tidal delta is transported into the trum, , Juist, Schiermonnikoog and Ameland started to channels and into the back-barrier area. The relationships between grow rapidly eastward and sometimes were eroded at their west- sand volume of the ebb-tidal delta and volume of the tidal channels ward side. Dunes became larger on many places on the islands though are not linearly correlated with tidal volume. This may along the entire Frisian barrier chain. In the North Frisian Wadden result in a mismatch between the amount of sediment that area, Amrum grew and the Havsand and Juvre Sand in the inlet becomes available due a reduction in size of the ebb-tidal delta in between Sylt and Rømø merged with Rømø. Fanø extended response to a reduction in tidal volume (or tidal prism) and the northwards with the formation of new dune ridges, because ebb- amount of sediment required for the morphological adaptation of tidal deltas were turned southward by the littoral drift and the channels within the tidal basin, depending on the configuration subsequently moved landward by the waves. The Skallingen of the basin and the infill of the back-barrier channels. If this results barrier-spit eroded and retreated after the 1960s (Ehlers, 1988). in a surplus of sediment, this sand will be transported along the downdrift barrier island coast, most frequently in the form of 3.3. Behavior of an individual barrier island on a time scale of shoals. After longshore and cross- dispersion, this surplus of decades to a few centuries sediment will contribute to the seaward progradation or stabili- zation of the shoreline. Schiermonnikoog Inlet has a long history of In recent morpho-ecological studies of the West and East Frisian shrinking tidal basins and decreasing tidal volumes since about 600 barrier islands, the concept of the so-called prototype island is aBP(Oost, 1995) due to land reclamation projects in the tidal basin developed and applied (Fig. 7, Löffler et al., 2011). The prototype (mainly Lauwerszee embayment) resulting in net progradation of island illustrates and integrates most of the common morpholog- the barrier island in contrast to Ameland, where hardly any recla- ical aspects of the natural islands of the West and East Frisian part mation occurred in the back-barrier basin of Ameland Inlet. of the system. It demonstrates both the temporal and spatial In the past 150 years, a series of notable morphological changes coherence between the large-scale morphological units, which occurred (Ehlers, 1988). These changes primarily concern island form the island under natural conditions. The large-scale 26 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38

wadden 1850

Dunes 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: 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). morphological units are characterized by developments on a time island will demonstrate accretion (e.g. Schiermonnikoog) or scale of decades to centuries, and a spatial scale of tens of square erosion (e.g. Norderney), respectively. Within this long-term kilometers. For that reason, this morphological concept also development there commonly is a shorter cycle of sedimentation provides a robust conceptual framework for the ecology of the due to the merger of shoals (Joustra, 1971; Oost, 1995; Israel and islands, because each large-scale morphological unit is character- Dunsbergen, 1999) and (channel-driven) erosion associated with ized by a range of abiotic conditions relevant to both flora and the autonomous dynamics of the ebb-tidal delta. With increasing fauna. Such abiotic conditions are expressed in terms of e for size of the tidal volume and hence the size of the ebb-tidal delta, example e the exposure to waves and currents, wind and salt spray, the duration of the cycle will increase, from decennia for tidal the average bed level, the amount of relief, the role of surface water volumes of a few 100 $ 106 m3 to more than a century for tidal and groundwater, the availability of nutrients, and the composition volumes of the order of several 1000 $ 106 m3 (Oost, 1995; Schoorl, of the sediment or substrate. The prototype island (Fig. 7) is char- 1999a,b; Oost et al., 2004). acterized by five large-scale morphological units, which mutually overlap and influence each other (Hoekstra et al., 2009; Löffler 2) Dune arcs et al., 2011). This unit comprises large dune complexes that form an arc 1) Island heads with curved and partly parallel dune ridges, encircling former beach plains with tidal marshes along the central island coast. These encompass the updrift coast of the island adjacent to the The exact nature of the rows of dunes is still under debate, but tidal inlet. This is the area where the sandy shoals of the ebb-tidal dune belts at the accretion side of the island consist of primary delta merge with the island and eolian sands are mobilized on the dunes (Van Heteren et al., 2006), whereas those at the erosive beach plains from where they are blown into the dune belts (see: sides of the islands consist of parabolic dunes, formed by the FitzGerald et al., 1984, 1994; Cheung et al., 2007). In the long-term, reworking of primary dunes. The large dune complexes have processes of accretion and erosion alternate with each other been stabilized by natural vegetation development. The dune (Oertel, 1977). Depending on the long-term sand surpluses or arcs provide coastal protection to major human settlements on deficits in the adjacent ebb-tidal delta the downdrift coast of the many of the islands. A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 27

wadden 2000

Dunes 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).

3) Washover complexes However, almost all of these wide channels/openings at the end of the dune arcs have disappeared in past centuries due to the A (former) washover complex is normally found at the end of development of sand drift dikes. a dune arc with an alternation of foredunes, remnants of dunes, and local depressions. Shallow creeks form a connection between the 4) Island tails North Sea and the Wadden Sea during storm surges. The associated supratidal plains form low depressions, separated from the beach Downdrift tails of the islands are limited in width up to a few by a beach ridge or high and accommodate a pioneer vege- kilometers. Dunes alternating with (former) washover fans and tation, tidal salt marshes and low dunes (Hoekstra et al., 2009). channels, 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. In addition, 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 salt marshes may also occur. If dynamics are somewhat reduced due to dune 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 Sea coast of the islands form the natural link with other morpho- ecological units (Guillèn and Hoekstra, 1996; Hoekstra et al., Fig. 7. The prototype West and East Frisian barrier island is characterized by the presence of five large-scale morpho-ecological units: (1) island head; (2) dune arc; (3) 1999; Houwman, 2000). Often these beaches are characterized by washover complex; (4) island tail; (5) Beach and shoreface (after Löffler et al., 2011). a considerable width and a rather low gradient (1:100) (compare 28 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38

Wijnberg, 1995; Ruessink, 1998, 2010; Ruessink et al., 1998, 2011, in 9000 press). 8000

3.4. Major human measures and coastal interventions in past 7000 centuries 6000

3.4.1. Influence of man in the past 150 years (Figs. 5 and 6) 5000 After 150 a BP, wide-spread land reclamation occurred. Over 4000 the period 500 a BP to the present the tidal flat area decreased by Number up to 58% of which a large part occurred in the past century 3000 (Delafontaine et al., 2000). Parts of the West and East Frisian 2000 Wadden Sea were also used for harbor development (e.g. Ems, 1000 Weser and Elbe estuary and Jade ). Next to that, , canalization and deepening of the larger estuaries (Ems, Weser, 0 Elbe) significantly influenced the hydrography, hydrodynamic 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 processes and morphological development of these waters (see for Year instance: Winterwerp, 2009). Another important intervention in the West Frisian Wadden Sea Fig. 8. Population development of the barrier island Norderney (data: historical & internet sources). The boom of the population occurred after 1797 when the island was the closure of the Zuiderzee in 1932 A.D. It led to a sediment became a bathing resort. demand of 500e1000 $ 106 m3, which is stilled by gradually importing sediment from the North Sea side of the barrier system, especially from the ebb-tidal deltas (Sha, 1990a; Elias, response, new coastal defense structures were developed, which 2006; Elias et al., 2012, acc.). Since that time the westernmost with the exception of nourishments, reduced the mobility of Wadden Sea in front of the Zuiderzee has been adapting to the new sediment and the morphological development of the islands, such hydrodynamic conditions, and over the period 1927e1997 nearly as (Table 1): 400 $ 106 m3 of sediment have accumulated in the back-barrier, especially in the abandoned tidal channels (Berger et al., 1987). A Dikes (at least since 650 a BP on Texel; Oost et al., 2004, but large part of it was derived from the North Sea coastal zone, causing perhaps even 1000 a BP on some of the islands (Oost, 1995); strong retreat of the ebb-tidal delta of Texel Inlet. Together with Protection (since at least 650 a BP on Rottumeroog; Oost, 1995) that, retreat occurred of the adjacent coasts of the barrier island of and planting of marram grasses (since at least 350 a BP on south-Texel (800 m in 50 years) and Northern , where Norderney); a large ebb-delta channel formed near the coast (Oost et al., 2004; Sand drift fences and formation of sand drift dikes (since at Elias, 2006). From the developments, it can be judged that the least 370 a BP on Texel); effects of the will continue to influence the area for (Sub)littoral stone defense works (since at least 300 a BP in the centuries (Elias, 2006; Elias et al., 2012, acc.). The process is ex- Inlet); pected to lead to serious ebb-delta erosion (Elias et al., 2012, acc.). Groynes (since about 200 a BP); Retreat of the ebb-tidal deltas may on the long run also exert Revetments and seawalls (since 150 a BP on Norderney; influence on the barrier coasts since shelter decreases and sand Thorenz, 2011); may become less easily available (Hofstede, 1999a,b). Harbor moles (since at least 150 a BP) and massive discharge Smaller enclosures such as part of the embayment of Schier- sluices; monnikoog Inlet in 1969 A.D. have resulted in an infill of the main Sand nourishments (since 60 a BP Norderney island in 1950/51; back-barrier channel connected to it and erosion of the ebb-tidal Kunz, 1990). delta, all tuned to the decrease of tidal prism from 300 to 200 $ 106 m3. A substantial part of the sediment became available Substantial were the hard protection measures taken at the to the downdrift island of Schiermonnikoog. The adjustment northwestern and western heads of the islands of Ameland, Bor- processes took at least about 4 decades (Biegel and Hoekstra, kum, Nordeney, , Spiekeroog and Wangerooge (Oost, 1995; 1995). Also in the North Frisian Wadden Sea hydrodynamical Thorenz, 2011). The constructions have halted the process of tidal and morphological effects were produced by the construction of inlet migration (e.g. Norderney, Nummedal and Penland, 1981). In the dams connecting Rømø (1949 A.D.) and Sylt (1927 A.D.) to the some cases, for instance Ameland and Texel inlets, this caused mainland, and the construction of the Eider storm surge barrier in extreme deepening (Oost, 1995; Schoorl, 1999b). In addition to 1973 (Reise et al., 1996). In the case of Rømø the location of the groynes and revetments, seawalls were erected to protect the dam did not fully coincide with the position of the tidal watershed barrier coast in places vulnerable to erosion (cf. Elko and Davis, (or drainage divide). Subsequently, the tidal prism of the areas 2006). The earliest were built of basalt; later, asphalt mounted North and South of the dam was redistributed; a process which with stones was used in order to increase the roughness of the was followed by an adjustment in local channel morphology. surface and decelerate the waves. All these measures also influ- Furthermore, subsidence of some tidal inlet systems and a subse- enced sediment transport along and perpendicular to the island quent increase of sediment demand resulted from sand mining coasts. In the same period some barrier islands have been given up within the tidal systems (above 13 m MSL) and due to the for maintenance by man. For instance, in 1991 it was decided by exploitation of gas and oil. Volumes of subsidence are of the order the Dutch Ministry of Transport, Public Works and Water of 0.1e10 $ 106 m3. Management that the uninhabited barrier island Rottumeroog On the barrier islands, the increase in tourism during this would no longer be managed and to let nature take its course. Due period resulted in the development of seaside resorts along the to considerable protest of several stakeholders it lasted until 2001 Wadden Sea coasts and a growth of the island population (Fig. 8). before the advice was really implemented and it was decided to Permanent structures were built in highly dynamic locations and stop all management of the dunes and let natural forces of wind were soon threatened by coastal retreat (Ehlers, 1988). In and water take over. A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 29

Table 1 Some 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 after Thorenz, 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 24 Vlieland ee 1854 13 18 Ameland 1960e1969 e 0.6 1874 2.8 6.5 ee1869 21 35 Juist 1913 e 1.4 ee1914 e 7 Norderney 1857 2.0 4.7 0.5 e 1846a 12 32 Baltrum 1873 e 1.7 1.8 0.2 1873 14 14 Spiekeroog 1874 1.6 1.9 0.8 e 1873 12 12 Wangerooge 1874 3.1 5.7 ee1818b 14 23 Total 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.

Also, dune areas developed quite differently due to human southern spit of Skallingen and the ebb-tidal delta of the Skallingen interventions (Nordstrom et al., 2007). Whereas natural dunes are Inlet (Grådyb). The processes can partly be attributed to the low and irregular in form (e.g. Spiekeroog) the artificial dunes or dredging of the navigation channel and the associated dumping of sand drift dikes are high and often strongly interconnected (e.g. the sand at the southern side of the inlet, which is thereby lost for Schiermonnikoog). Partially they were formed to protect the the sediment circulation around the southern part of Skallingen inhabited areas and partially to close off the open parts of the coast. and on the ebb-tidal delta. Once formed these were often repeatedly re-enforced and grew to 5 m or more above mean sea level, especially when the first dune 2) The effects of nourishments on the morphology of the dunes at row could be kept, which became quite normal once nourishments the beach became important to maintain the coastline. The dunes were also It has been shown that on the West Frisian Barrier islands an used as a reservoir for drinking water. In the Netherlands, the amount of sediment is blown into the dunes from the beach, production has been restricted, but on the East Frisian Islands, the which is in volume 44% of the volume of the nourishments. production is often more than the annual precipitation leading to Depending on the management, several reactions are possible a gradual lowering of ground water tables. Furthermore, dunes are (Arens, 1999; Arens et al., 2007, 2008, 2010): used on many places as recreational areas. Infrastructure, holiday A) no dynamics in the dune face area, only minor migration of houses, hotels and campings have been constructed, often near the the dune foot. No sediment is transported into the dunes shore outside of safe zones, leading to extra demands for safety behind it and no primary dunes do develop. from flooding. B) Dynamics are mainly restricted to the zone in front of the In recent decades, there has been a growing tendency to use dune face and the top of the dune face. Embryonic dunes sand nourishment rather than hard structures to actively protect develop, which may develop to new dunes. The dunes the coast. A good example is Sylt where at Hornum, concrete behind the dune face do not receive sands and are hence tetrapod barriers were placed in 1960e1967 in cross- (jetty) and not rejuvenated. longshore direction (dune foot revetment; Sistermans and C) Dynamics are slight to strong, and sand transported by the Nieuwenhuis, 2002). Also, seawalls were built in Westerland. The wind can reach the area behind the dune face. Due to the measures proved to be counterproductive in the long run since they dynamics embryonic dunes can form which may develop contributed to disrupt longshore sediment transport, thus gener- into new dunes and older dunes are rejuvenated by the ating lee-erosion south of the jetty. The seawall at Westerland import of calcareous sands. suffered from severe damage during storm surges as a result of D) Dynamics are very strong. Depending on the beach foreshore lowering in front of the structure. Thereupon authorities dynamics embryonic dunes may form or not. Due to (arti- started in the early 1970s with beach nourishments and flexible ficial) openings in the dunes sand can reach the older solutions such as revetments. These measures did not dunes, which are thus developing in a natural way. dispel the need for hard protection but improved their efficiency 3) The effects of strange sand (Stuyfzand et al., 2010) and life expectancy (Sistermans and Nieuwenhuis, 2002; http:// www.ubcwheel.eu/index.php/gpdb/article/1733). An advantage of The sand has to be dredged somewhere , consequently nourishments is that the beaches can maintain their dynamic introducing sand that in terms of hydraulic or geochemical behavior states, keeping natural processes of erosion and sedimentation may be different from the local sands, thereby affecting the habitats. more intact. For many of the North Frisian barrier islands the Studies revealed that indeed the sand differs in grainsize and availability of nourishment sand is limited. geochemical characteristics and thus may be of influence. Effects of nourishments are (Holzhauer, 2011; Oost, 2012)are: 4) Other effects (Holzhauer, 2011) 1) The influence of dredging of sand In recent studies it could not be demonstrated that nourish- Depending on the depth on which sand for nourishment is ments on the foreshore did influence significantly the current dredged this may influence the development of the coastal zone. In velocities, bottom roughness, sediment characteristics (also on the the West Frisian area sand is dredged on water depths of 20 m and beach), infauna (also on the beach) and epifauna. Sediment nour- more, thus trying to avoid effects on the coastal development. This ishments did also not influence the sanderling, which is depending is different per country. This is illustrated by the erosion of the on the infauna of the beach. 30 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38

3.4.2. The impact on morpho-ecological units of an individual on abandoned again. Due to this, large areas of beach plain and marshes have changed into . Human interventions in the last two centuries have seriously affected the morphology and ecology of the islands. Major effects are: 4) The island tails i) the large-scale erosion, movement and deposition of sediment around islands in relation to morphological adjustment due to On the the presence of the sand drift dikes closure works and large constructions; ii) the lack of mobility of prevented the transport of water, sediment and nutrients to the sediment being fixed by constructions or “trapped” in, for example, interior or even Wadden coast of the islands. The sand drift dikes sand drift dikes and iii) the more recent artificial supply of sand in the were also intensively maintained by planting marram grass and framework of nourishment projects to protect the coast. All measures placing sand fences further limiting the eolian transport across the have in common that they are related to “sediment management” island. This lack of natural dynamics led to fast development and within the system and that they have significantly altered the coastal succession of the vegetation. After it was decided that the sand drift dynamics of, in particular, the West Frisian barrier islands. We will dikes on the uninhabited island tails would no longer be main- now discuss the way in which these developments are visible in the tained new openings formed in the sand drift dikes on some landscape of the morpho-ecological units of an island. locations but this has not seriously improved the situation. The contrast to the East Frisian barrier islands is huge now. Large sand 1) Island heads drift dikes are absent there and we see a much more natural environment with highly dynamic ecosystems. Depending on the sand surpluses (e.g. Schiermonnikoog) or deficits (e.g. Norderney) in the ebb-tidal delta the downdrift coast 5) Beaches and shorefaces of the island adjacent to the tidal inlet may receive sediment or erode. As erosion is being hindered by stonework and nourish- Beaches and upper and lower shorefaces are often nourished or ments to protect the nearby inhabited dune arcs the net effect is kept with stoneworks. In this way the natural retreat of the barrier often an (updrift) elongation of the islands. Also, the island heads islands thereby slowly cannibalizing themselves over the past become either bulwarks or at least semi-stable areas where dune thousands of years has come to a halt. The erosive coasts have erosion is largely prevented. One has to keep in mind that dune become accretional static coasts. This implies less reworking of erosion is often a source of sediment for new dune building sediment from the dune face: sand, which is eroded from the dune processes. So by interrupting the natural cycle of erosion and face or the beach, is replenished with sand dredged from deeper deposition the impact of eolian processes is reduced. waters. This sand has commonly not the same sedimentological properties as the original beach sands (compare Guillèn and 2) Dune arcs Hoekstra, 1996), often contains a lot of shell fragments and, as a result, is often less suitable for eolian transport. The large dune complexes that form an arc have been stabilized by coastal protection works, planting of marram grasses and 4. Barrier island management: discussion forestation. It resulted in a reduction of eolian transport in the dunes. The formation of high sand drift dikes at the dune face on 4.1. Introduction many of the Frisian barrier islands resulted in further reduction of eolian transport into the so-called grey dunes behind them. The In this discussion we will answer the three major questions that characteristic vegetation of the grey dunes of grasses and lichens we addressed in the introduction of this paper. depends on poor nutrient conditions and a slight sand import. The reduced dynamics led to the formation of shrubbery. This is further 4.2. What can we learn in terms of ICZM from the natural enhanced by the increase of atmospheric nitrogen deposition and development and behavior of the Wadden Sea barrier island precipitation and the decrease of grazing (Van Dobben and Slim, system? 2011, acc.). On many parts the characteristic open landscape of the island is being replaced by a climax vegetation and a process of An overarching theme in barrier island management is the role rejuvenation is lacking (Löffler et al., 2011). Present nature of coastal (morpho)dynamics in determining safety (or the oppo- conservation strategies, such as the removal of top soils and site: creating risks), in providing the boundary conditions for the shrubberies do not address the causes of the problem e the lack of dynamics of ecosystems and in delivering ecosystem services, such dynamic processes e but simply are dealing with the symptoms as for the environment and the tourism industry. Coastal zone and, consequently will not be sustainable. On the long run it will be managers of the Wadden Sea barrier island system have to be impossible to provide the envisaged nature restoration, since the aware of the following morphological conditions and sediment measures taken are short-lived, often have a very local character budget considerations: and also depend on seed banks that are becoming poorer in species (Löffler et al., 2011). 1) Presence of outcrops and inherited Pleistocene topography and paleomorphology 3) Washover complexes Up to this day the Pleistocene initial topography is a dominant The wide openings at the end of the dune arcs have all been factor in the development of the north-south trending North Frisian closed off by sand drift dikes at the North Sea side and mostly by Wadden area, which is characterized by a steeper Pleistocene relief dikes at the Wadden side, except for the one on Spiekeroog. On the and lower availability of loose sediments. On a smaller scale deeply West Frisian Islands the storm surge disaster of 1953 had as incised river valleys, which formed during the Pleistocene low- a consequence that many sand drift dikes were built over the full stand, determine to a large extent the present-day positions of length of the islands to link the individual dune systems and to several major estuaries such as Elbe, Weser/Jade and Ems (Streif, protect the coast. This strategy was part of a major master plan to 2004). Furthermore Pleistocene outcrops determine locally the close off the entire West Frisian Wadden Sea; a plan that was later formation of the coastal zone such as the southern spit of Sylt, and A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 31 near Skallingen (Blåvands Huk). The rigid clays deposited or over- may have consequences for the demand of sediment when sea- ridden by ice during the Ice Ages such as boulder clays are known to level rise accelerates. As long as sedimentation can keep up with be resistant to erosion. It has been argued that the boulder clay sea-level rise within the back-barrier basin, the sediment demand outcrops on and in front of the southwest part of Texel Island from the North Sea coast (and thus erosion) will increase linearly actually form the reason why Texel forms the knick point between with the rate of sea-level rise. However, if sedimentation cannot the N-S trending Holland coast and the W-E trending Frisian barrier keep up with sea-level rise, which is expected to occur at lower coast (Schoorl, 1999a). The resistance against erosion also explains rates of sea-level rise for larger basins than for smaller (Oost et al., the seaward position the barrier islands of Amrum and Sylt (Fig. 1). 1998), wave and tidal energy within the basin increase leading to The influence of the rigid clays on channels, which are incised into enlargement of the tidal channels and even more difficult condi- the Pleistocene sediments, and their possible influence on island tions for sedimentation on the shoals (Flemming and Barholomä, dynamics may also be of importance. An example is a pipeline 1997; Van Ledden, 2003; CPSL, 2010). A tidal basin might then which was entrenched through a Pleistocene clay ridge in the start to function as a source of sediment instead of a sink. Inlet in the East Frisian Wadden Sea and which was Depending on the original dimensions of the tidal system the subsequently covered with sand (Grann, 1997). Much to the changing demand for sediment in the back-barrier area may thus surprise of the engineers the sand was quickly eroded, leaving have different implications for the management of the North Sea a hanging pipeline and leading to a reorientation of the main coasts of the barrier islands. channel via the pipeline trench. The boulder clay which had orig- inally been present was more resistant to erosion than the under- 4) Climate change lying sand and had hindered the reorientation of the inlet channel. Currently the dredging of a deeper channel thalweg is considered As discussed, the striking coincidence of so many positive through the boulder clay W of Borkum, which may have its effects shoreline changes after 150 BP makes it very likely that some on the barrier island: special attention is given to it by the state common processes were responsible. Since the period after 150 authorities. a BP marks the end of the Little Ice Age (A.D. w1450e1850), However, the larger part of the islands coastal morphology is the a change in circulation following this period has been suggested result of repeated reworking of sediments and thus totally gov- as a driving mechanism for the observed changes (Ehlers, 1988). erned by the present-day natural forces and interactions between Partially it will, of course, also be the resultant of human induced the various morphological units. In such areas morphodynamics poldering of mainland areas resulting in a reduction of tidal volume are determined by Holocene sands and clays and not so much the in inlets and surpluses of sand, which became available to the Pleistocene relief. In that case the following points are of islands. Furthermore, the decrease of dynamics on inhabited importance: islands and the furthering of the development of vegetation will have enhanced local dune development, especially since AD 1960. 2) Holocene line-up of barrier islands and ebb-tidal deltas However, given the fact that also uninhabited islands show a different development from the period before 150 BP, the During its long Holocene evolution, since 5000e6000 yrs BP the hypothesis of the Little Ice Age aftermath might well be correct. In islands and ebb-tidal deltas along the North Sea side have been that case the adaptation will not continue indefinitely: wide-spread lined up during their Holocene coastward migration, forming coastal might give way to wide-spread erosion once a more or less uninterrupted barrier chain along the Frisian coasts. again just as in the past (e.g. Lindhorst, 2007). Coastal managers Momentarily there are management plans to push the coast locally then will be facing larger net erosion in the future. In this light it seaward, or to keep the coast in its present position. Other parts of might be wise to store as much sand as possible on the islands by islands or entire islands or ebb-tidal deltas are allowed to continue allowing natural processes. to migrate towards the mainland as they have done during the large part of the Holocene. As yet, it is not clear what would be the effects 5) Reactions of the sediment-sharing system of allowing such disturbances in a chain of lined-out barrier islands. If retreat of parts of the chain becomes strong it will most likely An inlet system is, as discussed in chapter 3, primarily a water- result in increasing erosion of the promontories and disturbances and sediment-sharing system with a tidal inlet, the ebb-tidal delta, in the coast-parallel sand transports. adjacent barrier islands and the tidal basin with channels, shoals, tidal flats and salt marshes. Barrier island development is thus 3) Balance between sea-level rise and sedimentation directly linked to tidal inlet system development. A natural change or an intervention by man may have repercussions for other parts The observation that around 5000 a BP sediment accumulation of the system. The initial increase and subsequent decrease of the rates locally exceeded the rate of sea-level rise, whereas in other tidal volumes of inlet systems due to sedimentation and diking places areas drowned, indicates that the balance between sea-level must have influenced the development of the North Sea coasts of rise and sedimentation is rather subtle (Flemming, 2002). It implies the barrier islands strongly, by initiating coastal erosion when that the availability of sediment from the North Sea side and the sediment was needed and sedimentation when surplus sediments possibility to settle in the back-barrier area both play an important became available. This is illustrated by the differences in coastal role. The availability is partly determined by the variation in tidal development of the barrier islands of Ameland and Schiermonni- volume and wave climate (Van Goor et al., 2003; Grunnet et al., koog. It should be realized that thus caused accretionary or 2005), and for the other part by changes in the North Sea facing erosional tendencies on the islands will eventually come to an end, cross-shore coastal profile. The potential for sediment to settle when the sediment-sharing system is moving towards equilibrium depends on the hydrodynamic conditions in the back-barrier basin. in the long run (decades to centuries). During the sedimentary infill and shrinking of the back-barrier Sometimes, however, a closure triggers a strong increase in basin towards the present day, the energy gradient between the sediment demand in the back-barrier basins and thus results in mainland coast and the inlet became steeper, thus making it more coastal erosion of e in the first onset e the ebb-tidal deltas, as is the difficult for especially finer-grained sediments to deposit and case with Texel inlet. The effects are often ignored, as management accumulate (Flemming and Nyandwi, 1994). If the idea is correct, it is often focused on the immobilization of the island and the 32 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 protection of the inhabitants, infrastructure, harbors, polders, and a fast succession (Löffler et al., 2011) resulting in a climax vegeta- preservation of the dunes and tidal marshes (i.e. everything above tion and the loss of the characteristic open landscape. It was rec- MSL). Nourishing of sand or building groynes and revetments may ommended in earlier studies (Williams et al., 2009; Feagin et al., counteract erosion of the island caused by the sediment demand in 2010), that a more sustainable strategy of barrier island manage- the back-barrier area, but may not prevent the erosion and retreat ment and nature development should be based on a natural degree of the ebb-tidal delta (Elias et al., 2012, acc.). This, in turn, may lead of substrate mobility. To that end the focus for managing barrier to less shelter and sediment supply to the islands and hence to islands should be on maintaining the integrity of ecosystem greater difficulties in maintaining them (FitzGerald et al., 1994; services, their functioning and natural process development. In Abogado Rios, 2009). Examples are the Texel and Skallingen inlets. 2005 the United Nations Millenium Ecosystem Assessment (MEA) divided the ecosystem services in four categories: production, such 6) Dynamics on the barrier island between the larger morpho- as the production of food and water; regulating, such as the control ecological units of climate and disease; cultural, such as spiritual and recreational benefits and supporting, such as nutrient cycles and crop pollination The larger morpho-ecological units on a barrier island are in (DeFries et al., 2005). Traditionally, coastal management mainly close interaction with each other. In this the beach and foreshore focused on strengthening especially one or more of the production- play an important role as conveyor belt of sediment between the , regulating- or cultural services -, paying little attention to the other units in a coast-parallel direction. Next to that natural supporting services (e.g. Morton, 2008). As a result, many examples dynamics will allow for coast-perpendicular wind and waterflow of traditional coastal management projects show a negative impact driven transports of sediments, thus allowing the island to move in on supporting services, undermining sustainability of the system. a back-barrier direction or to accrete vertically. This is also the case on most of the Wadden barrier islands.

4.3. What has been the impact of human measures and 4.4. Is our present-day ICZM strategy a successful, integral and interventions on barrier island development in the last decades and sustainable approach that is able to cope with future aspects of centuries? global change or do we have to adopt our coastal policies?

Human measures and interventions have strongly influenced There is a clear connection between the various spatial scales the development of the barrier islands, in terms of sediment considered which are strongly coupled to the temporal scales over deliverance to the island (see above), safety from flooding and which the characteristic developments take place. This applies as natural development of the island itself. Originally safety was found well to the regional development during the Holocene of the barrier on natural relief of the dunes and dwelling hills. After that closed island chain, as to the interconnectedness of the barrier island with duneedike rings were maintained, of which the dune vegetation the adjacent tidal inlet systems, as to the local large-scale was somewhat protected. Gaps between the dune arcs were left morphological units, which together compose the barrier island. open allowing eolian transport and inundation transport so that But it also applies to relations between the various spatial scales: parts of the island could accrete in a natural way. larger scales to a large extent set the stage where smaller scale From the 17th century onwards sand drift dikes were formed at developments can take place. This interconnectedness implies that the dune face on the West Frisian islands and parts of the East and the management of a barrier island cannot be seen in isolation but . Also, marram grasses and trees shrubs has to be considered in terms of interactions on various spatial and stabilized the dunes. Before that vegetation density was mostly temporal scales. Until now it is not taken strongly into consideration lower and there was ample possibility for sediment transport by in the ICZM approach that morphodynamics of a barrier island is wind or water on many locations, resulting in a more dynamic also connected to the developments which take place on larger landscape. Due to plantation the vegetation trapped the trans- spatio-temporal scales and the relative strong influence by man on ported sand more effectively and erosion was avoided better. these in the past two centuries. This may lead to surprises like for Where needed the coast was protected by stoneworks, forming instance accelerated coastal erosion due to changes in the net a bulwark at the western side of many islands. The fixation of inlets sediment supply to the coast, or fast retreat of an ebb-tidal delta, and sandy coasts limits the possibility of inlet migration and leading to changes in wave attack on the barrier island. We advocate longshore barrier island dynamics (compare Elko and Davis, 2006). to take into account the bigger picture of relevant developments on Due to all the anthropogenic attempts to stabilize the islands, also larger spatio-temporal scales and to make this a consciously sand transport from the shoreface to the beach and onto the islands considered part of barrier island management. The list given in the has been reduced. above discussion (4.2) indicates that exact outcomes of such The later beach and shoreface nourishments allowed for sand developments may differ from island to island. In general, a hierar- transport along and perpendicular to the island coasts and into the chical order of influences should be considered (Fig. 9): back-barrier and to the ebb-tidal deltas. Thus the integrity of the sediment-sharing system was to some extent restored. However, 1) Is the Pleistocene paleomorphology important to the island? the approach was mainly derived from the rather reactive coastal protection policies aiming at restoration of erosion and maintaining If the answer is yes it should be taken into account in terms of the sand drift dikes. Only locally eolian (Terschelling beach pole less free morphological development than in the situation of only 10e15) or inundation transport (Spiekeroog Leegde) of sediments Holocene sediments (predominantly sand and some gravel, shells onto the islands is allowed. Sufficient sediment import is, however, and clay) influencing the development. In the latter case the on the long run essential everywhere outside of the dike-dune ring integrity of the barrier-ebb-delta chain should be considered. if the island is to accrete vertically and to keep up with rising sea Although the effects of forming seaward bulwarks while at the levels. As such allowing sediment import onto the island has to be same time allowing other parts of the chain to retreat coastward are part of a robust and sustainable strategy to guarantee safety in the yet not fully understood it is clear from experience that this may centuries to come. well lead to unwanted accelerations in local erosion. It thus may The massive stabilization also reduced the opportunities for become more cost-efficient (also in terms of ecosystem services) to pioneer vegetation. Dune belts and tidal marshes have experienced nourish those other retreating parts of the barrier chain, for A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 33

Hardly No Pleistocene restrictions to paleomorphology development on important? smaller scales Yes

Periodic sand No Inlet system not supply to barrier in dynamic Restrictions island equilibrium? development on smaller scales Yes e.g. Sylt, Human influenceNo Amrum & Texel; No on morpho- Natural Elbe, Weser & ecological units Ems Estuary dynamics strong? prevail, resilient Barrier island has Yes Soft where possible, hard sand where really needed surpluses or deficits for Reduced natural decades to dynamics and decay centuries, of natural values, e.g. e.g Texel Borkum, Ameland

Fig. 9. Scheme to evaluate ICZM-policies for the West and East Frisian Barrier Islands. instance in the form of ebb-delta nourishments in channels which available to allow natural processes to develop and to create robust are threatening to erode a barrier island. ecosystems. The same applies to the factor time: the system will need time to tune and adjust to changing external conditions (in 2) Is the inlet system not in dynamic equilibrium? the order of several decades up to centuries). Furthermore, it should be realized that developments may not always occur on the If the answer is yes the net sand deliverance to an island might exact same place as original (before human intervention). There- be either showing a long-term (decennia to centuries) surplus or fore, instead of focusing on nature conservation a paradigm shift is deficit, which is temporary. Such insights may influence the coastal needed in barrier island management towards stimulating the nourishment and development schemes of barrier islands. Nour- development of nature dynamics. Stopping management of Rot- ishments will be necessary even when the inlet is in equilibrium if tummeroog, allowing natural forces to take over, leading to a strong the coast is to be maintained on its position. morpho-ecological development of the area is an example of this ‘working with nature’. To our opinion the best solution is to allow 3) Is human influence on morpho-ecological units strong? the geo-biological processes to take their natural course as much as possible. Only where this is not feasible other management prac- Often the answer will be yes as the supporting ecosystem tices should be applied. As a rule: soft and with respect for mor- services of many units are restricted due to all coastal protection phodynamical integrity where possible, hard where really needed.Itis measures and adverse effects of nourishments, especially with our expectation that this e to a certain extent e will also regulate respect to dune development. Moreover, the increasing tourism on the booming use by tourism of the barrier islands. Examples are the the islands is based on use of production, regulating and cultural development of the National Park Schiermonnikoog and the barrier ecosystem services, taking a gradual increase of demands for island of Spiekeroog, where large parts of the islands are naturally granted. This situation is not sustainable on the long run. We will developing. But even there morphodynamics is often unnecessarily expand on this below. reduced due to earlier restrictive measures. In the past decade the National Park Service (NPS) As stated above, many barrier islands originally consisted of five called for natural processes to be maintained and for human- major morpho-ecological units, which mutually overlap and altered systems to be restored on barrier islands to allow natural influence each other. Each of these units is characterized by its own processes to function and natural to develop and evolve. dynamics, morphology and ecology. The interconnectedness and For it is stated that: “Storms and associated processes such the morphological development of the various units are to a large as waves, , currents and relative sea-level change are critical extent governed by the (possibility of) transport of fresh and salt elements for the formation and evolution of [ ] barrier islands, sand water, wind, sediments, nutrients and salts. All of these factors or dunes, back-barrier sand flats and lagoons and vegetated wetlands. processes are vital to nature development. Many barrier islands, or Processes such as wave run-up, and barrier breaching, parts of them, have changed and are used so strongly by man that which occur during elevated storm surges, are all necessary processes restoration of natural conditions is near to impossible. Neverthe- in enabling the efficient transfer of sediments, nutrients and marine less, in many places there are alternatives for the present-day water from the Atlantic Ocean across the barriers.” (Williams and management, which envisage a more integrated approach (Löffler Foley, 2007). In order to restore nature sufficient space should be et al., 2011). Restoring interconnection between the various units 34 A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 and allowing natural developments to once more take their course present. If washover complexes are restored then: i) pioneer will be beneficial for barrier island safety, maintenance and nature species will re-occur; and ii) additional sand is deposited on the development. All will benefit by the resulting increase of resilience, barrier islands from the North Sea side, resulting in a higher resil- which is reached by increasing the ability of (part of) the islands to ience of these natural plains with respect to sea-level rise (Van exchange nutrients, waters and sediments by natural forces. The Dongeren and Van Ormondt, 2007) and iii) the washover development of such new approaches is nowadays without risk to complexes will act as a gateway for eolian transport across the safety, since today it is technically feasible to take rapid, corrective island, further stimulating aggradation. measures such as sand nourishments, stopping of eolian processes, closing openings etc. 4) Island tails The following alternative barrier island management options are available (see also Löffler et al., 2011): Often the island tails are among the more natural parts of the West and East Frisian barrier islands and need little maintenance 1) Island heads or restoration. Locally on the West Frisian Islands sand drift dikes are present which are relatively easy to reactivate, for instance by At many island heads where sandy ebb-delta shoals merge with opening up these dunes, so that: i) small flooding openings form the island, erosion and sedimentation alternate over many decades (Ten Haaf and Buijs, 2008); ii) eolian sand drift may increase the often with an overarching longer term deficit or surplus of sedi- topographical level in the area; and iii) tidal marshes may form. ments and sometimes the possibility of Pleistocene paleorelief Also, a more natural development of the island tail leads to more influences (see above discussion). Taking this into account, reac- natural and ecologically diverse dune system (De Jong et al., tivation of dunes, perhaps in combination with removing the top 2011). soils of the valleys, is thought to have the following effects: i) That such an approach may also form a partial answer to rejuvenation of lime-rich soil; ii) increase in the dynamics of the accelerated relative sea-level rise is illustrated by the developments area; iii) increase in the ability for the more inland area to build up on Skallingen in the North Frisian Wadden Sea. At the Skallingen vertically with sea-level rise (increasing resilience); and iv) peninsula coastal protection was abandoned some 20 years ago. improving the conditions for the establishment of dry pioneer plant This has allowed the dune ridges to be breached during severe species of the xeroseries and for the wetter hygroseries. Where net storm surges leading to the deposition of large washover fans. It has erosion dominates, larger sand nourishments and new dune been shown that this process results in local gain of sediment and formation may provide the desired level of safety (see Arens and washover fans, which subsequently built up to new dune ridges Mulder, 2008). perpendicular to the coast (Christiansen et al., 2004; Nielsen and Nielsen, 2006). 2) Dune arcs 5) Beach and shoreface The dune arcs offer safety to man (Roelvink et al., 2009, in press). If there are many rows of dunes between the beach and the One of the characteristics of a natural barrier island is the inhabited area, this offers locally possibilities for eolian sand seasonal and decadal alternation of sedimentation and erosion transport. The transport can easily be stopped at a sufficient phases of the beach. The present-day state of technology is such distance from the inhabited areas. In this way: i) sedimentation in that the beach’s position can be safeguarded. This can be done by the (inland) dunes is stimulated again, allowing vertical aggrada- means of hard works, which obstruct or tion; ii) pioneer species have more possibilities to survive; and iii) prevent natural beach morphodynamics and sediment dynamics in larger dune massifs may come into existence, providing larger fresh general. However, given the strong interconnectedness of the water buffers and supplying the dune valleys with groundwater islands with the back-barrier and ebb-tidal deltas, and the strong rich in dissolved carbonates. Any sediment losses thus caused on connection between the beach and the rest of the island, the better the beach have to be compensated by nourishments if the beach solution seems to be nourishments (Hoekstra et al., 1994, 1999; see position is to be maintained. As illustrated by Landry (2007) and also Williams and Foley, 2007). Thus far nourishments have been Costanza and Farley (2007), it is possible to optimize nourishment carried out to primarily maintain the coastline. However, the schemes by taking the value of houses, recreation and nature into present-day state of technology make it possible to steer nourish- account. In this way a coupling between dune arcs and beaches ments in terms of location, frequency, volumes and form, so that would be restored. Also, dune arcs may be allowed to form again not only safety is maintained but also nature is furthered and where they were once present, either by actively assisting in the partially restored (see also Van der Wal, 1999). Large volumes of first steps of the process (e.g. West Vlieland) or by bringing back the beach or ebb-delta nourishments may, for instance, lead to the washover complexes and removing sand drift dikes (see below). (temporal) formation of new dunes and habitats, but hinder the New dune arcs such as the one currently forming in the eastern part development of older more landinward dunes (Arens, pers. com.). of Spiekeroog which are not enclosed by dikes on the Wadden Sea Such approaches are especially interesting on the NW parts of the side also provide ample conditions for tidal marsh development island coasts where through natural developments strong sedi- with gradients from high to low, salt to fresh, sandy to clayey. mentation and erosion patterns occur on a decadal scale. It should be realized however, that under natural conditions sand supply at 3) Washover complexes many other coastal locations is periodic but very limited. Nour- ishing large amounts of sand may severely disturb the more natural Many of the islands once had one or more washover complexes, processes of sand transport and coastal development in such which were mostly closed by sand drift dikes. A good surviving places. example is the so-called Spiekeroog Leegde. Restoration of such For the West and East Frisian islands a simple scheme can be washover complexes is only possible if it does not form a threat to deduced from the various scales when addressing coastal zone human safety. Restoration will often consist of the removal of management (Fig. 9). If questions are answered with a “yes” than a sufficient length of sand drift dike and removing part of the top the influence should be considered in barrier island management. soils, preferably in places where the washovers were originally The outcome however is always similar: on the long run barrier A.P. Oost et al. / Ocean & Coastal Management 68 (2012) 18e38 35 islands and their nature may become more resilient when natural where really needed. In this way large parts the islands can accrete processes are furthered as much as possible. to cope with sea-level rise, safety will be maximized and pioneer conditions will not disappear on the islands. 5. Conclusions Ethical statement The morphological development of the West and East Frisian barrier islands is the outcome of a hierarchy of morphological The work described in or manuscript has not been published developments on several spatio-temporal scales. In this the larger previously. It is not under consideration for publication elsewhere. scale settings to a large extent determine which smaller scale Its publication is approved by all authors and tacitly or explicitly by developments can take place. The natural development and the responsible authorities where the work was carried out, and behavior of the Wadden Sea barrier island system depends on: that, if accepted, it will not be published elsewhere including electronically in the same form, in English or in any other language, The presence of outcrops and inherited Pleistocene topography without the written consent of the copyright-holder. and paleomorphology, which may dictate to some extent the development of the islands, with promontories of rigid Pleis- Acknowledgements tocene deposits such as on Sylt and Texel as the most striking examples. This paper is partly based on a previous document or position The Holocene alignment of the chain of barrier islands and ebb- paper (in Dutch) initiated by the Wadden Academy (part of the tidal deltas, which is currently under pressure as man tries to Royal Dutch Academy of Arts and Sciences) to discuss relevant keep some (important parts) of the islands in position whereas geoscientific issues of the Wadden Sea (see Speelman et al., 2009). other parts are allowed to erode. The position paper addresses three major subjects: the geological The balance between sea-level rise and sedimentation, which development and structure of the region (including the presence may become disturbed in the future when sea-level rise is too and development of mineral/natural resources), the Holocene fast for the back-barrier basin to keep up with. coastal evolution and “present-day” . The Other adaptations to climate change such as changes in wind position paper was almost entirely based on data from the Dutch directions, which may have led to sediment surpluses for the part of the Wadden system. Our present paper focusing on barrier islands during the past 150 years. islands is based on an extended and strongly edited version of the Reactions of the sediment-sharing inlet system on the position paper on the Holocene evolution updated with our study increases and decreases in tidal volume due to changes in the on the Holocene evolution of the trilateral Wadden Sea on request back-barrier areal extent leading to surpluses or deficits of of the Common Wadden Sea Secretariat (CWSS; see Wiersma et al., sediments on the barrier islands which form part of the sedi- 2009) and our insights on barrier island management (see Löfller ment-sharing system. et al., 2011). Dynamics on the barrier island between the larger morpho- ecological units, which in their natural form do allow References for both coast-parallel and coast-perpendicular sediment transports. Aagaard, T., Orford, J., Murray, A., 2007. Environmental controls on coastal dune formation; Skallingen Spit, Denmark. Geomorphology 83, 29e47. Abel, O., 1893. Kaiser Karls Leben von Einhard. In: Die Geschichtschreiber der The impact of human measures and interventions on barrier deutschen Vorzeit, 13, 9th Cent., third ed., 1st vol. Verlag der Deutschen island development in the last decades to centuries has led to Buchandlung, p. 62. disturbances in the sediment-sharing inlet systems and hence to Abogado Rios, M.T., 2009. Spatial and temporal analysis of the shoreline variations surpluses or deficits of sediment becoming available to many of the and morphological development of the Barrier Island Vlieland, the Netherlands. Internship Report Msc. System Earth Modeling, Faculty of Geosciences, Utrecht barrier islands. Furthermore some parts of the barrier chain have University, The Netherlands. been allowed to retreat landward whereas other parts are kept in Arens, S.M., 1999. Evaluatie Dynamisch Zeereepbeheer. Vergelijking situatie 1988 en their position, which may lead to local increases in erosion. On the 1997. Arens Bureau voor - en Duinonderzoek in opdracht van Rijkswa- terstaat, Dienst Weg e en Waterbouwkunde, RAP99.01. barrier islands itself human measures have led to an increase in Arens, S.M., Mulder, J.P.M., 2008. Dynamisch kustbeheer goed voor veiligheid en safety from flooding and at the same time a decrease in natural natuur, land þ water. Magazine voor civiele- en milieutechniek 48 (9), 33e35. dynamics and hence supporting ecosystem services. Natural values Arens, S.M., Löffler, M.A.M., Nuijen, E.M., 2007. Evaluatie Dynamisch Kustbeheer Friese Waddeneilanden. 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