The Riddle of the Sands

Mimstry of Transport, Public Works Directorate-General of Public Works and Water Management and Water Management

National Institute for Coastal and Marine Management/R/KZ

A Tidal SystenVs Answer to a Rising Sea Level

T. Louters & F. Gerritsen Report RIKZ-94.040 October1994

Projectinformation Some years ago the National Institute for Coastal and Marine management (RIKZ) of the Rijkswaterstaat started a research program on the possible effects of an accelerated sea leve! rise, as a result of the greenhouse effect, on the geomorphology and ecology of the Wadden Sea within the framework of the Project "Impact of Sea Level Rise on Society" (ISOS), shortly called Project ISOS'WADDEN. This project is part of a national research program (NRP) on Global Airpollution and Climatfc change. The research is closely related to the coastal research program "Coastal Genesis" carried out by RIKZ, Internationally the research is connected with the Intergovernmental Panel on Chmate Change (IPCC) and with its subgroup Coastal Zone Management (CZMS). National Institute for Coastal and Marine Management/R/KZ

CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG

Louters, T

The riddie of the sands : a tidal system's answer to a rising sea level / T.Louters & F Gerritsen, ttext contributions- K. Essink ... et aL; ed.' T Louters . et al.; transl. from the Dutch] - Den Haag: Mmistry of Transport, Public Works and Water Management, Directorate-Ceneral of Public Works and Water Management, National Institute for Coastal and Marine Management (RIKZ). -111 Transl. of: Mysterie van de wadden: hoe een getijdesysteem inspeelt op de zeespiegelstijging. -1994 - Report RIKZ-94 040. - With ref. ISBN 90-369-0084-0 Subject headings: sea level, tidal systems; ecology; Wadden Sea.

Mmistry of Transport, Public Works and Water Management National Institute for Coastal and Marine Management /RIKZ Korte naerkade 1 p.o. box 20907 2500 EX The Hague The Netherlands

The Riddie of the Sands National Institute for Coastal and Marine Management/WKZ

Table of contents

1. Introduction 7

2. 'Unseen Forces': Rising Sea Level, Subsiding Sea Floor, Tide, Wind and Waves 9 2.1 introduction 9 2.2 Rise in sea level due to climate change 10 2.3 Reiative sea level rise: also influenced by sea fioor subsidence 11 2.4 Tide 13 2.5 Wind and waves 14 2.6 Sediment transport 15

3. The Wadden Sea tamed in 10,000 years 17 3.1 Evolution of the Wadden Sea at a time of a gradual rise in sea level 17 3.2 Differences between east and west 22 3.2.1 The Western Wadden Sear once a wooded peat bog 22 3.2.2 Eastern Wadden Sea, old Wadden Sea 23 3.3 Lessons from the present and the past 25

4. The Response of the Wadden Sea System to the Rising Sea Level and Human Intervention 27 4.1 Introduction 27 4.2 The system's sand and silt economy 27 4.3 The system in equilibrium 29 4.4 The system out of balance 30 4.5 Reiative rise in sea level causes sand demand 30 4.6 Reduction of the tidal basin creates sand hunger 32 4.7 Potential sources of sand: outer deltas and island coasts 32 4.8 Flats and salt marshes 35 4.8.1 Flat development 35 4.8.2 Development of salt marshes 37

How the Wadden System maintains 39

5. Looking Ahead to the Future Landscape of the Wadden 43 5.1 Introduction 43 5.2 What will be the future demand for sediment (in the coming 50to 100 years)? 43 5.3 How large is the sediment supply? 49 5.4 Is sediment demand being compensated by the supply? 50 5.5 What does the supply and demand balance mean for the tidal basins, tidal flats and salt marshes? 51 5.5.1 Expected tidal basin development 52 5.5.2 Expected flat development 52 5.5.3 Expected salt marsh development 54 5.5.4 The effect of salt marsh policy and management on the supply and demand balance 54 5.6 What does the supply and demand balance mean for the island coasts? 55

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6. Looking Ahead to the Ecology of Tomorrow 57 6.1 Introduction 57 6.2 Complex food webs 57 6.3 Ecological tolerance 62

7. ConclusJons and recommandations 63 7.1 The wadden flats in a state of flux? 63 7.2 Recommendations 63

8. Ref eren ces 65

Colophon 69

The Riddle of the Sands National Institute for Coastal and Marine Management/R/KZ

The Riddie of the Sands National Institute for Coastal and Marine Management/fi/KZ

Figure 1.1 Physically, the wadden system encompasses tidal basins with channels and flats, outer deltas and islands that are interconnected and interactive through water and sand transports along the coast and via the tidal inlets. With the alternating rise and fall of the water, large areas of the wadden (tidal flats) are submerged during flood tide and exposed during ebb tide.

flood tide ebb tïde

Figure 1.2 View of the future: factor fiction? Total inundation of the unique wadden flats is one of the greatest threats posed by the accelerated relative rise in sea leve!. This view of the future may become reality if the flats and salt marshes are no longer able to balance the rise of sea level with extra sedimentation. A large pool of salt water will be all that remains. In short, a development with disastrous consequences for flora and fauna. situation 1994: Uncovered flats and salt marshes in the Dutch Wadden Sea

5e n Helder

Situation 2100?: Uncovered flats and salt marshes in the Dutch Wadden Sea

The Riddleof the Sands National Institute for Coastal and Marine Management/R/KZ

1. Introduction

"From east and west two sheets of water had overspread the desert, each pushing out tongues of surf that met and fused. I waited on deck and watched the death-throes of the suffocating sands under the relent- less onset of the sea. The last strongholds were battered, stormed and overwhelmed; the tumult of sounds sank and steadied the sea and swept victoriously over the whole expanse." (Erskine Childers, The Riddle of the Sands, 1903)

It rarely occurs to us how amazing it really is that an area we now know as the Dutch Wadden Sea has developed in the Netherlands. The flats and salt marshes have been able to hold their own in an apparently miraculous way, while the sea level has risen many metres at different speeds in the past 7000 years. The area's thousands of years of history teaches us that, morphological- lyr the Wadden Sea is prepared for the phenomenon of a rise in sea level. The system of islands, channels, flats and salt marshes (Figure 1.1) responds dynamically to the forces of the tides, wind and waves. Flats and salt marshes are not submerged as long as nature can compensate for the rise of sea level with extra sedimentation (Figure 1.2). This additional sediment, most of which comes from erosion of the (island) coasts ( Photograph 1.1), is carried in by the sea. As a result, the islands migrate toward the mainland. At the same time, the Wadden Sea is extending landwards due to inundation of the hinterland. Consequently, the total size of the wadden area remains more or less the same. In the natural situation, in which the Wadden Sea and the islands could behave as nature dictates, the system always proved able to strike a balance between the supply of sand from eroding islands, its size and demand for sand emanating from the Wadden Sea. Even after the construction of dikes over the past 1000 years - which curbed further landward expansion of the Wadden Sea - the tidal flats have been able to adjust to the rising sea level, preserving their characteristic properties. The question of whether this will continue to be the case with the expected increase in the rate of sea-level rise is the subject of this report.

The Wadden Sea consists of a series of tidal basins with channels, shallow tidal flats and salt marshes. In this report the tidal basin is seen as one unit from a landscape point of view, forming part of the entire wadden system of islands, tidal inlets and outer deltas. The effects of an accelerated rise in sea level on the Dutch Wadden Sea wiil be described, taking other human intervention into account, such as sand and shell extraction, subsidence due to gas extraction and the policy of 'dynamic stabilization' of the coastline. This includes indications of ecological effects (Photograph 1.2). Before outlining the scope of the effects (Chapters 5 and 6), the report will describe the expected changes in driving forces (Chapter 2), the geological development of the Wadden Sea (Chapter 3) and the principles underlying the processes of change in the morphological structure of the Wadden Sea (Chapter 4).

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Photograph 1.1 Erosion of the island coasts. Morphoiogical changes of the Wadden Sea cannot be separated from morphoiogical changes of the island coasts. Thus, coastal security partly depends on developments in the Wadden Sea.

Photograph 1.2 The landscape of the Wadden Sea, with its salt marshes and flats emerging at low tide, intersected by ebb and flood channels, is of great ecologica! value. This value is affected by ecological changes to the tidal flats as a result of an accelerated rise in sea level and anthropogenic impact such as sand and shell extraction, subsiding sea floor due to gas extraction and the policy of 'dynamic maintenance' of the coastline.

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2. 'Unseen Forces': Rising Sea Level,

Subsiding Sea Floor, Tider Wind and Waves

2.1 Introduction

The unseen forces at play in the riddle of the wadden are the complex changes in the system's driving forces: the tide and the waves (Figure 2.1). The combination of these forces induces a complicated mechanism of enormous water and sand displacements, moving continuously via the tidal inlets to and from the Wadden Sea. Yet at first glance, and at small scales of time and space these forces seem hardly to affect the character of the wadden landscape as a whole (we talk in terms of days to years).

Figure 2.1

A complicated interplay between the forces of the tide and the waves creates a complex pattern of sand displacement. Large quantities of sand are transferred continuously back and forth from the coastal region of the islands and outer deltas via the tidal inlets to the tidal flats in the Wadden Sea. Since almost no sand is exchanged with the North Sea, we can say thatthe sand economy is virtuaHy closed. The displacement of sand is one part of a continuous process of striving to achieve dynamic equilibrium between the physical shape (morphology) and the continuously changingtide flows.

tide-generated current wave-generated current Wadden Se<

The net changes are very small and often difficult to measure. But because these forces work constantly and over a long time affect the wadden landscape they steer its large scale development. On long range these guiding morphological processes are influenced by climate changes, by a different storm and wind climate, but especially by a change in the rate at which the sea level rises. A subsiding sea floor also plays a role in each of these factors. We will review expected changes in the forces which steer the morphological development.

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2.2 Rise in sea level due to climate change

The sea level - or, to be precise, the mean sea level - has been rising for more than a hundred centuries. In the last Ice Age (about 10,000 years ago), when the area of the Netherlands remained free of ice, most of the North Sea was dry. As the climate became warmer, the ice-cap thawed and the sea level rose by 120 to 140 metres. This rise in sea level was very rapid at first; later, the rate gradually decreased (Figure 2.2). In general, the sea level has gradually risen in the last thousand years.

BC 6,000 4,000 2,000 0 2,000 in calendar years Figure 2.2 Bronze Iron Roman Middle Modern Mesolithic Neolithic Age Age Age Curve of the relative sea level rise Age Age 8,000 6,000 4,000 2,000 0 AA14 in the Holocene indicates the chan- c - years prior to day ge in the average sea level which is now at approximately NAP (Normal Amsterdam Level), In the beginning of the Holocene, the sea level rose rapidly, after which it slowed gradually, and in the last 2,000 years, it did not rise more -15 than 5 to 30 centimetres per Louwe Kooijmans (1976) century. -20 - Jelgersma (1979) Van de Plassche (1982)

The sea level only dropped in the Late Middle Ages (the 'little' Ice Age) (Figure 2.3). Circa 1850, the average temperature began rising again, the glaciers shrank and the sea level along the Dutch coast rose by some twenty to thirty centimetres, regaining the level of the Early Middle Ages.

(m) Figure 2.3 The average high-water curve for the southern North Sea (according 1.3 to Jensen et al., 1993) and the average global temperature (according to Barth &Titus, 1984) over 1.0 the last 1,000 years illustrate that rises and falls in global temperature correspond with rises and falls in the average high water mark (HW). 0.5 1,000 1,200 1,400 1,600 1,800 2,000 time {years)

Measurements of the mean sea level along the open Dutch coast from the past 150 years reveal a fairly constant increase of 20 cm per century (Figure 2.4). In view of the fact that at this point there is no indication of an acceleration in the rise of sea level, an increase in sea level by about 20 cm in the next century is likely.

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Figure 2.4 Average sea level in the past and the future prognosis. Development of the average sea levels of Amsterdam, Den Helder, Harlingen and Delfzijl and the average sea level of the Netherlands indicate a gradual increase in the average sea level. For the period between 1900 and 1990, the average sea level of the open Dutch coast rose at a rate of about20cm per century.

To make a prediction of the effects on the ecosystem of the wadden flats, three different rates of sea level increase have been used in the ISOS study: Current rate of increase : 20 cm per century (line A) Predicted rate of increase : 60 cm per century fline B) Worst-case scenario : 85 cm per century (line C)

«30 1B50 1870 ifflO 1910 193Q 1950 1970 1990

1700 1725 17W 1775 1300 1835 1850 1875 1900 1925 195Q

1830 1850 1B7O 1B9O 1910 1930 1950 1970 1990

urne (yearsl

Recent studies indicate, however, that the increase in the CO2 content and other 'greenhouses gases' in the atmosphere could lead to an increase in the average global temperature. Estimates of the potential rise vary widely. For the Netherlands, the sea level could, in the worst case, rise by some 85-105 cm per century. Empirical proof, however, of the projected acceleration in the rate at which sea level rises is as yet lacking. This prediction of a higher average global temperature by the increase in CO2 level (2.5° to 3° C warmer globally if the CO2 level were to doublé) is not based on measurements, but on simulations of climate models. An intensification of the so-called greenhouse effect can lead to an accelerated sea level rise. It could take a few decades before we under- stand the development of the rate of sea level rise in coastal waters with more certainty.

2.3 Relative sea level rise: also influenced by sea floor subsidence

The distance between the sea surface and floor along the Dutch coast is increasing not only as a result of the climate's becoming warmer, but also because the sea bed is subsiding. This combined action is what we call relative sea level rise (Figure 2.5).

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Figure 2.5 Relative sea level rise is the result of changes in the level of the sea floor and sea level.

initial sitjation absolute rise in sea level relative rise in sea level

A recent study carried out in connection with the Normal Amsterdam Level (NAP) proved that the level of the deep substratum of the coast changes. A natural process of sea floor subsidence has been discovered, which varies from 4 to 8 cm per century along the Dutch coast. In the eastern and south-eastern region of the Netherlands, however, the ground level has been found to be rising by 8 cm per century. In other words, the Netherlands is slowly but surely canting towards the sea. These natural shifts in the ground level have been even in nature in the past century and are related to known geological structures in the substrata. Since these upwards and downwards movements are the result of large-scale geological processes, we can draw the tentative conclusion that comparable ground shifts will occur in the century to come. In short, the floor of the Wadden Sea will probably continue to subside by 4 to 8 cm per century.

F i gure 2.6 Forecast of ultimate subsidence due to extraction from existing gas fields and prospective new gas fields (as Schiermo ïn Itoot laid down in the memorandum entitled "Impact of gas extraction on the Wadden Sea", 1993). Subsidence progresses gradually and takes on the shape of a flat dish. The largest gas field (Siochteren) is expected to result in a maximum subsidence of the Wadden Sea of 20 to 30 cm. Government permission received prior to December 1993 for extraction of gas reserves will cause subsidence of the Wadden Sea floor for another 20 to 40 years. Calculations show that the average In addition to the natural subsidence, a smaller-scale process of accelera- total subsidence resulting from gas ted subsidence is underway, initiated directly by human activity. On a extraction will be greatest in the local scale, these anthropogeomorphic changes are a result of sand, and, sediment retention areas of to a lesser degree, shell extraction. On a regional scale, ground subsi- Borndiep Channel (3 cm), Pinkegat dence is caused by gas extraction. This subsidence of the sea floor deve- (15 cm), Frisian Gat (5 cm) and lops gradually in the form of a lat saucer its maximum value occuring in LauwersGat (6 cm). the middle and from the centre its value decreasing towards the sides. The largest gas field (Siochteren) is expected to bring about a subsidence in the wadden area of 20 to 30 cm. This figure will be less for other gas fields (Figures 2.6; 2.7). In spite of the high degree of uncertainty concerning changes in the relative sea level rise to be expected, government policy does take an accelerate rise into account (Discussion report 'Coastal Protection after 1990'). For the time being, a 60-centimetre rise of the relative sea level in the century to come is being used for policy purposes as the most probable case and 85 cm per century as a worst-case scenario.

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Figure 2.7 Projecttons of the average increase in the content of the tidal basins (million m3) due to future extraction of gas prospects. If subsidence due to gas extraction is not compensated by sand sedimentation, the content of the entire Wadden Sea wtll increase by 35 million m3. This increase in content is in addition to the expansion in content by 58 million m3 as a result of the floor subsidence caused by extractions 2.4 Tide already underway at Slochteren, Ameiand-Oost, Zuidwal and Blija. In addition to changes in the mean sea level, the high- and low-water marks and the speed of ebb and flood tidal currents are also subject to change. Tidal motion is characterized by wave like behaviour, which is expressed by continuous changes in water level and tide-induced velocities along the coast and inside the Wadden Sea. The tidal wave progresses from south to north along the coast and enters the Wadden Sea through the tidal inlets, where the wave is reflected against the mainland coast. The result in the Wadden Sea is a complex interaction pattern of incoming and reflecting tidal wave components, because of which high and low water levels are varying. The tide also induces horizontal water movements: the ebbend flood currents. These are strongest in the tidal inlets and in the connnecting channels and lowest near the watersheds. The latter separate adjacent tidal basins. The shallow tidal flats in the Wadden Sea are submerged during flood tide and emerge again during ebb periods. This phenomena occurs twice a day and repeats itself every 12 h 25 min on the average. The total quantity of water which flows through the tidal inlet during flood is called flood-volume; during ebb respectively ebb-volume. Flood- and ebbvolume are also called tidal prism (Table 2.1; Figure 2.8).

Table 2.1 tidal basins average sój-face area surface area uncovered Present characteristics of the tidal tidal prism at mean high at mean low inlet systems of the Dutch Wadden in million m3 water in km2 water in km3 Sea I Marsdiep Channel 1054 712 121 II Eijerlandsche Gat 207 153 106 III VlieCat 1078 668 323 IV Borndiep Channel 478 309 1ë5 V Pinke Gat 100 65 42 VI Frisian Gat 200 130 82 VII Eijerlanderbalg Creek 70 55 28 VIII Lauwers Gat 160 145 92 IX Schild 31 :' 29 t$ X Ems-Dollard Bay 1000 520 214

In the course of time the tidal watermotion in the Wadden Sea has significantly changed due to interventions by man such as dredging- works, empolderings and damming. The two most significant changes are the damming up of Zuiderzee (1932) and Lauwerszee (1969). The general observation has been made that, along the Dutch coast, the average high water marks have risen more quickly than the low water levels. For example, measurements of the sea level at Den Helder have

The Riddle of the Sands 13 National Institute for Coastal and Marine ManagementIRÏKZ

Figure 2.8 Schematized cross section of a flood basin bordered by a dike on the landward side. The tidal prism is the total volume of water flowing into or out of a tidal basin. With small tidal basins, this volume is virtually equal to the average depth of the tida! range multiplied by the basin area.

BxL=surface of basin HW-LW= tidal range d- average depth of the flats below the high water mark

BxLxd = volume of inflowing and outflowing water

revealed a rise of 20 cm per century during the period from 1940 to 1990, while the high-water level has risen by 22 cm per century. The low-water level, on the other hand, has only risen by 12 cm per century. The significance of this for the tidal flats is that it could bring about a change in the division of the surface area between channels and tidal flats. In the last 50 years, a rise in high water levels has been measured along the coast which is an average of 5 cm per century higher than the average sea level rise for that period. It is possible that this phenomenon has something to do with the large-scale changes in tidal patterns in the North Sea. Since this development will probably continue in the future, policy takes account of an extra rise in the high water marks of 5 cm per century.

2.5 Wind and waves

The possible climate change caused by an intensification of the greenhouse effect not only has implications for the temperature (and, in turn, the sea level), but also for wind and storms and thus for the waves as important shaping forces of the Wadden Sea. The Royal Dutch Meteorological Institute (KNMI) takes into account an increase of wind speeds compared to the current climate and predicts that depression activity will increase north of the 45th parallel and decrease south of that latitude, should the atmospheric concentration of CO2 doublé. Thus far, however, no larger depression activity has yet been demonstrated in the wind regime above the Netherlands. (Figure 2.9). Even if the wind climate does not change and the intensity and frequency of storms remain the same, storm floods which exceed the critical stormsurge level will occur more often as a result of the gradual rise in sea level by 20 cm per century. In the last twenty years, an increase in wave heights has been measured. However, since decreases were also witnessed in the previous period, making projections on possible trends would not yet be justified.

TheRiddleoftheSands 14 National Institute for Coastal and Marine Management/RIKZ

Figure 2.9 180 r jü maximum gust of wiïïd ~j

Climate models predict that 1944 estimated value lu maximum hourly value 1921 depression activity in areas north of • i of wind speed 160 |- the 45th parallel will increase if the 1973 G1990 D CO in the atmosphere doubles. 1953 3 n • For the Netherlands, this means an 140 ;ïï • increase in the number of severe o a 13-1-'93 storms and of the average wind -S 120 t~' velocity. Measurements taken in E ( severe storms in the period from * j. 1910-1993 (RoyalDutch 100 \ü Meteorological Institute, KMNI,

1993) do not reveal a change in 80 L— the wind regime. The storms of 1920 1940 1960 1980 2000 today are just as severe and occur time (years) as frequently as in the past century. 2.6 Sediment transport

The sediment consists of a mixture of sand and silt. It is continuously moved back and forth along the coast and through the tidal inlets into the wadden system. The coarser material is dominantly moved near the bottom (the bottom-transport). The finer partides of sand and the partides of silt are dominantly moved by current as suspended material. In order for the flow to transport sand and silt the velocities have to exceed critical values. Part of the sediment can be deposited in the channels and on the tidal flats and salt marshes, in this way reducing depth; on the other hand erosion of the bottom can also develop, whereby sediment is picked up and transported. For the long term development of the Wadden Sea it is of utmost importance to know how big a portion of the inflow of sediment is retained in the Wadden 5ea. These net quantities are relatively small compared to the total quantities transported.

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3. The Wadden Sea tamed in 10,000 Years

3.1 Evolution of the Wadden Sea at a time of a gradual rise in sea level

In its short geological existence, the Wadden Sea has already gone through some turbulent developments. Under the influence of the rise in sea level, part of the Wadden Sea system has shifted eastward and landward and has become considerably smaller in the course of time. The island and mainland coastlines were and still are fixed by human intervention (dike construction and coastline maintenance). The history of the Wadden Sea demonstrates an ever more limited freedom of movement. History has more to teach than that, though. A gradual rise in sea level does not automatically lead to the formation of an inland sea without sand flats uncovered at low tide. Geological reconstruction of the wadden area development in the last 7,000 years has revealed that, despite a sea level rising at a gradual rate, nature managed remarkably well to maintain the geomorphological form of the tidal flats, even though the sea rose several metres (Figure 3.1).

6,900 4,900 2,500 2,000 in calerdar year Figure 3.1 10.000 8,000 6,000 4,000 14 The Jelgersma (1979) curve of refa- NAP-k; C -years prior to the present tive sea level rise expressed in (m) • calendar years and in C14 time -5-[ scale(Beetsetal.,1994). s -10 •- GO cm - 40 cm per century

''.'"" '"] more than 80 cm per century

The history of the Wadden Sea can be roughly divided into six periods.

1.100,000 to 10,000 (C14) years ago Coast farther away

At the end of the Weichselian period (some 100,000 to 10,000 (C14) years ago), the last ice age of the Pleistocene, the area of the North Sea was, for the most part, dry and the coast was far from what is now called the Netherlands. The substratum of the present-day Wadden Sea consists of moraine and alluvial deposits which were formed in the Mid and Late Pleistocene (Figure 3.2). During the last ice age, the ice-cap did not reach what is now the Netherlands, and a thick layer of sand was deposited on the irregularly shaped Pleistocene landscape. Some 15,000 years ago when the sea level was 120 to 140 metres lower than it now is, the melting of the North American ice-caps caused the sea level to start rising at a rate of many metres per century. Like the Dutch dunes, the Wadden Sea is a geologically young landscape which did not take on its current form until the warm period after the last ice age.

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Figure 3.2 Geologically speaking, the Wadden Sea area is still young. The Wadden Sea was not formed until well into the Holocene {starting about 10,000 C14 years ago). The irregulariy shaped Pleistocene landscape with deep depressions forms the substratum of what is today the Wadden Sea. Large portions of the southern North Sea were then dry.

NAP Om -3m -6 m -9m

-12 m Topsideof the Pleistocene -15 m deposits below <-15 m the current sea level 2.10,000 to 7,000 (C14) years ago (9,200 - 5,800 BC) Tidal flats in the making

To this day, little is known about the making of the wadden area. Fragmentary geological data suggests that under the influence of a rapid relative sea level rise of at least 80 cm to a few metres per century, the Dutch coast shifted inland and the seaward area was submerged (Figure 3.3a; situation 7,000 (C14) years ago). In the western region of the Netherlands, a brackish to saltwater lagoon was formed which was cut off from the predecessor of the Wadden Sea by an offshore bar (approximately at the latitude of what are now the Islands of Texel and Vlieland). This 'Wadden Sea' of old mainly consisted of estuaries, with lagoons and tidal flats, formed by the flooding by the sea of the river valleys that had formed in the Pleistocene. In an arch enclosing this sea, there was a series of islands with tidal inlets and channels between them. It is not clear at this time how far out to sea this former wadden area extended. They could therefore be termed Wadden Sea, although they did not resemble the current wadden area. The area as a whole gradually became subject to ever-increasing marine influence and was unable to keep pace with the rapid rise in sea level, causing the coast to erode and recede. Apparently, the supply of alluvial sediment to the area was too slow at that time to cause accretion of the large Wadden area. The large quantities of sediment resulting from this erosion probably contributed to the raising of the area behind the coastline.

3. 7,000 to 5,000 (C14) years ago (5,800 - 3,780 BC) Striving for equilibrium

This period progressed virtually the same as the previous one, only more slowly. The rise in sea level probably equalled some 80 to 40 cm per century. The tidal flat area still shifted landward and the Pleistocene Heights near Texel underwent severe erosion. The area started to bear more resemblance to the present-day wadden area. Due to the gradually developing barrier bars and dunes, a fairly stable coastline with tidal inlets and channels was formed in what are now the Noord-Holland and

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Figure 3.3a Figure 3.3b The Wadden Sea area approximately 7,000 (C14) years ago The Wadden Sea area approximately 5,300 (C14) years ago (i.e 5,800 BC). The sea level was several metres below the (i.e 4,000 BC). Formation of a more or less stable coastline current level. Initially it rose rapidiy (afew metres per with tidal infets and channels and, behind that, a zone with century) as a result of the melting of the ice caps. Erosion tidal flats, salt marshes and, in the higher-lying swampy caused the coast to rapidfy shift in a landwards direction and areas, peat bogs. The sea level was about 4 m below NAP large portions of the Wadden Sea area were inundated. and rose by approximately 80 to 40 cm per century.

Figure 3.3c Figure 3.3d The Wadden Sea area approximately 3,700 (C14) years ago The Wadden Sea area developed between 500-700 AD to (i.e 2,100 BC). The sea level rose by between 40 and 20 cm the intertidal area as we knew it before the Zuider Sea was per century. dammed. The sea level rise equalled some 5 to 30 cm per century.

Open water Figures 3.2; 3.3a to 3.3d are derived from a study by Salt marshes deposits (salt water or fresh water) Zagwijn (1986) of the National Geological Service and pro- Fresh water deposits Pleistocene deposits vide a reconstruction of the relief under the Dutch coastal regio n. Intertidal area Not induded

Coastal dunesand Hypothetical borders beaches Peat bogs: raisedbog s and blanket bogs

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Zuid-Holland provinces between 6,000 and 5,000 (C14) years ago (i.e. 4,900 to 3,780 B.C). Behind it, tidal flats and salt-marshes developed and the higher boggy areas became covered with a layer of peat (Figure 3.3b). It can be deduced that, under these conditions, sediment is deposited at the same rate that the sea level rises. This teaches us that \f the supply of material is Jarge enough, a rising sea level does not necessariiy mean loss of land or the formation of an inland sea. Extension is even possible, as evidenced by the next period.

4. 5,000 to approximately 3,700 (C14) years ago (3,780 - 2,100 BC) Extension of the coast

The sea level rose more slowly in this period, i.e. between 40 and 20 cm per century. In the delta area which stretches across the entire west coast of the Netherlands, enough sediment was gradually deposited to win the race against the rising sea ievel, which allowed the coast to expand in a seaward direction; first in the south and later in the north (Figure 3.3c). The sediment originally came from erosion of the receding capes (Zeeland delta area, Texel Heights) and the former outer deltas, and was transported by the North Sea and rivers. The area north of Bergen continued to erode, as did the coast of the Wadden Sea, which progressively receded, making sand available for the wadden area. Part of the elevated flats in the wadden were transformed into salt-marsh or even became dry land. The Zuider Sea was not yet a seaF but a freshwater inland lake into which rivers from the south drained.

5. 3,700 (C14) years ago until the Middle Ages Formation of the present wadden environment

Up until about 3,700 (C14) years ago, the area that is now the western Wadden Sea developed in more or less the same way as the western region of the Netherlands. After that, the area flooded and became the tidal flats. The pattern of peat bog formation and flooding dominated everywhere except at the higher-lying areas of Texel to about Harlingen. At the end of the Middle Ages, here, too, an area of shallow tidal flats developed into the intertida! flats we know from the time before the Zuider Sea was dammed (Figure 3.3d). The high-lying areas in the eastern Wadden Sea flooded some 3,000 years ago, bringing about the extension of the wadden area.

6. Post-Middle Ages until the present 'Nature under human sway'

From the Middle Ages on, the coast could no longer be said to have developed autonomously, driven purely by natural forces. Humankind steps in. Acts of intervention include dike construction, reclamation of tracts of land from the sea (empoldering) and peat-cutting, as well as the damming up of channels and parts of tidal basins. Reinforcing existing dunes to serve as dikes, constructing jetties and moles and such large-scale activities as closing off the Zuider Sea and the Lauwers Sea in the eastern part of the Wadden Sea have had a major influence on the development of the wadden system. Despite the interference, the wadden system is still far from tamed; its landscape is in a constant state of flux, as demonstrated by the shifting positions of islands, island headlands (Figure 3.4) and tidal channels (Figure 3,5).

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Figure 3.4 Dynamic shifts in island points. The development of the coastline of the Point of Ameland since the begïnning of this century illustrates the dynamic shifts in isiand points. Periods of predominant expansion or erosion alternate quickly and can cause the coastline to shift by hundreds of metres a year. The extent and direction of the coastline shift is contingent upon a complex interaction between tides and waves which deposit and remove sediment along the coast and via the tidal inlets. If less sand is deposited than is removed, the point of an island erodes quickly. If the inverse is true, quick accretion of the point occurs.

Figure 3.5 Rapid shifting of channefs and drainage area. Circa 1300, the Wantij of Schiermonnikoog was still probably at the level of the Frisian coast. The predecessor of the Zoutkamperlaag tidal basin {to the west of Schiermonnikoog) was still small and not connected to the Lauwers Sea, which still drained entirely via the Lauwers Gat. Presumably, the Wantij of Schiermonnikoog gradually shifted eastward in the period from 1350-1450, giving the Zoutkamperfaag tidal basin more and more opportunity to assume what was the Lauwers Sea's function as an outlet. The Zoutkamperlaag tidal inlet became wider and deeper while Schiermonnikoog shifted eastward. This development forces the Lauwers Gatto shift also, causingit to lose its connection with the Lauwers Sea by 1556. After the damming of the Lauwers Sea in 1969, the size of the tidal flow area was reduced.

1300

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3.2 Differences between East and West

We now view the Wadden Sea as one entity bordered by the islands to the north and the coastline of the northern mainland of the Netherlands and the IJsselmeer Dam to the south. The geological history of the region teaches us, however, that the eastern area came about in a different way than the western part, which is much younger.

3.2.1 The Western Wadden Sea, once a wooded peat bog

A major portion of the western Wadden Sea was created by a relative rise in sea level combined with human intervention. In the Early Middie Ages, a wooded peat bog was to be found to the west and south of the Texel-Harlingen line. The inhabitants dug ditches and channels in order to drain and empolder the lowlands and they cut peat for salt extraction and fuel. These human activities exposed the marshy area to flooding. Around 1,000 AD, the sea encroached upon this area and transformed it into tidal flats. The tidal basins increased in volume and the old peat in the wadden area was swept away, while the coasts of Noord-Holland, Texel, Etjerland and Vlieland (Figure 3.6) were subjected to intense coastal erosion due to the ensuing enlargement of the volume of the basin.

Figuur 3.6 The devefopment of the coastline of Vlieland Island. The North Sea coast of Vlieland has been subject to intense coastal erosion for centuries. The coastline recedes many metres per year. At the same time, the island has expanded landwards.

An ever improving link with the North Sea caused the Flevo lake to be gradually transformed during the Middie Ages into a basin with brackish water influenced by tides and later even into a saltwater basin now better-known as the Zuider Sea. Until long after the Middie Ages, the inhabitants attempted to reverse the process of land loss in the western Wadden Sea and Zuider Sea by constructing dikes - with varying degrees of success. It was not until the seventeenth century that man won the battle against the sea and, from then on, humankind managed to empolder larger and larger tracts of land. In every respect, land reclamation gained the most ground in the nineteenth and twentieth centuries, reaching its apex with the polders created after the closing off of the Zuider Sea during the first half of this century. To this day, the dikes, including the IJsselmeer Dam, have had a major influence on the sediment economy and hydrodynamics of the western Wadden Sea and the latter is still influenced by the altered tidal pattern created by closing off parts of the basin.

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3.2.2 Eastern Wadden Sea, old Wadden Sea

The area to the east of the Vliestroom Channel and to the north of the Texel-Harlingen line developed into an intertidal area earlier than the western portion of the Wadden Sea due to the lower elevation of the Pleistocene surface in the eastern Wadden Sea, causing this area to be affected by the sea sooner. In the Holocene epoch, large quantities of sediment were swept away from the North Sea coast and the foreshores of the islands, causing the shape of the coast to change radically. Influenced by the rise in sea ievel, the Wadden Islands and the iniets have been moving landward for the past 5,000 years. Study of remainders of ancient outer deltas and filled-up channels of former tidal iniets has revealed that about 5,000 to 6,000 (C14) years ago, the position of the coastline of Ameland and Schiermonnikoog islands used to be farther north by 11 and 15 kilometres, respectively. Consistent with geological findings, sources from the Roman and Early Middle Ages reveal that intertidal flats and barrier islands existed at least as long ago as the beginning of the Common Era (Quote).

There have been tidal flats in the wadden area since time immemorial, as illustrated by this quote by Pliny from 47 AD

... but so also are the races of people called the Greater and the Lesser Chauci, whom we have seen in the north. There twice in each period of a day and a night the ocean with its vast tide sweeps across in a flood over a measureless expanse, covering up Nature's age-long controversy and the region disputed as belonging whether to the land or to the sea. There this miserable race occupy elevated patches of ground or platforms built up by hand above the level of the highest tide experienced, living in huts erected'on the sites so chosen, and resembling sailors in ships when the water covers the surrounding land, but shtpwrecked people when the tide has retired, and round their huts they catch the fish escaping with the receding tide. It does not fall to them to keep herds and live on miik like the neighbouring tribes, nor even to have to fight with wild animals, as all woodland growth is banished far away. They twme ropes of sedge and rushes from the marshes for the purpose of setting nets to catch the fish, and they scoop up mud in their hands and dry it by the wind more than by sunshine, and with earth (turves) as fuel warm their food and so their own bodies, f rozen by the north wind. Their only drink is supplied by storing rain-water in tanks in the forecourts of their homes (Quote: Pliny, 47 AD Hist. Nat. XVI, 1.).

Anthropogenic influences on the development of the Wadden Sea gradually intensified. The first inhabitants of the northern coast settled on the higher-lying salt-marsh banks and levees. The construction of terpen (mounds used for refuge and as high ground upon whtch to build) and dikes gained in importance as time passed. At first, the dikes were built to protect the land from flooding and only later for the purpose of empoldering the land. In the period from 1000-1100 AD, the Middle Sea was empoldered and, from 1300 on, the Lauwers Sea was gradually empoldered as well. In the Dollard estuary, empoldering began after 1520. The heightened empoldering activity reduced the size of the sediment retention area. In response to this, the iniets and channels became shallower. At the same time, the island coasts eroded and the tidal iniets shifted slowly but surely to the east and closer to the coast. In the second half of the Middle Ages, the pace at which the Wadden Islands migrated towards the coast slowed, since dune vegetation considerably reduced the formation of new wash-overs (dike breaches due to storm flooding) and dune erosion.

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Figure 3.7 Migration and disappearance of the wadden Islands. In the past the wreck-masters had to constantly relocate their quarters on Rottumeroog to keep their feet dry. Rottumeroog has been 'ambling' eastward for centuries. The total surface area of Rottumeroog remains almost the same (twelve to thirteen hectares), but the island is becoming lower. In the long run, 40 to 100 years from now, the island is expected to be LOWWatermak swallowed up by the river Ems. High-Watermark41921-1930 5 1930-1956 6 aften 956 d at low tide

Cartographic data from 1980

Figure 3.8 Empoldering and saft marsh development (after Dijkema, 1987). The Wadden Sea comprises sizable areas of relatively high-lying intertidal salt marshes with vegetation, calfed 'kwelders'. These salt marshes are a modest remainder of what once was an expansive landscape of brackish marshlands and salt-marshes, peat bogs and lakes that, up until about a thousand years ago, were situated along the border zone between Pleistocene and marine deposïts. After this period, the Dutch and Frisians began enclosing the inhabited areas with dikes. The sea, however, soon broke through in many places, causing new salt marshes to be created where dikes burst and along the outside of the diked-in villages as sand and silt sediment was deposited (e.g. Lauwers Sea, Doüard Bay, Middle Sea). Step by step, these dike bursts were repaired. It was not until after 1600 that the inhabitants managed to hold back the sea once and for all and, in the interplay between the development of salt marshes and dike construction, fewer and fewer salt marshes remained. The size of the mainland salt marshes decreased, and thus the opportunities for creating polders. Coastal farmers found themselves gradually impelled to actively promote the accretion of salt marshes by digging ditches and building fascine dams. Initially, there was little to show for their efforts. After 1935, the government initiated large-scale land reclamation works. The majority of the current mainland salt marshes in the northern area of the coast of Friesland and Groningen is the resuit of these activities. In the western part of the Wadden Sea, the mainland mud-flats were barely significant. The mud-flats of the islands, on the other hand, expanded in the 18th century to an impressive size of 88.5 km thanks to the shelter afforded by the 'man-made' drift dunes of Koegras (1610) and Eijerland (1629). When these areas were completely enclosed by dikes in 1817 (Koegras) and 1835 (Eijerland), the area of salt marsh decreased. In 1969, the portion of the Lauwers Sea comprising saft marshes and tidal flat was empoldered. The salt marshes of today are made up of small parts of the Balgzand Shoaf, the salt marshes and summer polders of the Frisian mainland, the northern coast of Groningen and the area along the edges of Dollard Bay.

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An island which did shift position at a considerable pace is Schiermonnikoog; in the period from 1300 to 1850, the island is estimated to have moved 3.5 km on the western side and 7.5 km on the eastern side, in other words, some 1 km per century. Not all of the islands shifted so quickly. In spite of major changes caused by island migration (Figure 3.7), extensive empoldering (Figure 3.8) and the disappearance of the islands of Bosch and Heffesant (as a result of the All Saints' Day Flood of 1570), the shape of the eastern portion of the wadden system remained, morphologically speaking, almost the same.

Figure 3.9 Coastline development over the last 100-140 years.

3.3 Lessons from the present and the past

The history of the wadden system teaches us that a great part of the sediment made available by the constantly receding coast (Figure 3.9) contributes to the elevation of the Wadden Sea. The wadden system, consisting of the coast of the islands, outer deltas, tidal inlets and tidal basins, has managed to adapt in the past to the rising sea level by moving landwards (Figure 3.10). Although channels, flats and island coastlines and headlands can undergo highly dynamic changes locally, the basic morphological character of the wadden system as a whole has barely changed in the past centuries. Humans have fixed the position of part of the island coast and most of the mainland coast over the years, limiting the wadden system's freedom of movement Fixing larger portions of the island coast can disturb the balance between the sand supply of the eroding island coast and sedimentation in the wadden area. If this state of imbalance should mean that the sea level rises more quickly than the rate of sedimentation in the wadden area, the tidal basins will not receive sufficient sediment in order to sustain the intertidal areas. Were this to be the case, the flats and saltmarshes would be swallowed by the sea.

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Figure 3.10 island Lessons from the past. saltmanh arthSea The history of the Wadden Sea Wadden Sea teaches us that as long as the sand supply of the islands and the sedi- 3,000 BC mentation of the wadden area are in balance, the natural rise in sea level results in the a slow landward shifting and raising of the Wadden Sea and Islands. Over the course of time, not only has humankind seen to it that the mainland coast of the 1;,000 BC wadden system is fixed, but also that the coastline of the islands

remains the same. The current islam! coast protection policy is geared towards 'dynamic stabitization' of the islands position of 1990. This is in contrast to the natural response of the isfands to migrate towards 1,000 AD the mainland. The result of the this stabilization is that the natural adjustment by the wadden system to processes such as sea level rise and floor subsidence can occur only within the current boundaries of the Wadden Sea and that the After ,1990;

equilibrium between the sand sea level rise supply of the islands and sedi- sedimentation mentation in the tidal flats might erosion dynamic stabiliiation: be dramatically upset. supplemental sand or seaward coastal protection dike

The Riddle of the Sands 26 National Institute for Coastal and Marine Managemertt/R/KZ

4. The Response of the Wadden Sea System to the Rising Sea Level and Human Intervention

4.1 Introduction

Changes in the physical forces of tide and waves pjay a crucial role in the morphological development of the wadden. They constantiy shape the landscape of the Wadden Sea, causing flats to submerge and re-emerge elsewhere, channels to change their course and salt marshes to form and then be washed away. Yet, when viewed across a short span of time (we are speaking in terms of years), the influence of these physical forces have on the geomorphologicaf picture of the Wadden Sea as a whole seems negligible at first glance. This is because the net changes are very slight and often very difficult to measure. In geological terms, tidal basins are short-lived, existing for a period varying from a few hundred to a few thousand years. A characteristic example is the former tidal basin of Alkmaar-Bergen which was open and active from 6,000 to 3,000 BC and which silted up soon thereafter (circa 1,250 BC). A rise in sea level and storm floods that are accompanied by (dike) breaches can, however, bring about a rejuvenation of tidal basins that have filled up with sand or silt. The question as to whether the Wadden Sea will keep its present character or whether it will fill up with sand in the future requires insight into the physical mechanisms that underlie the development of the wadden system. The tidai systems of the entire globe display certain simiiarities in terms of morphology and tide. Current knowledge of the dynamic processes that determine the landscape of the tidal flats is insufficient to allow us to devise reliable mathematical models with which to make forecasts having a very high degree of accuracy in terms of time (e.g. years) and space (e.g. metres to kilometres). Information about the geological development of the Wadden Sea and empirical knowledge of the relationships between tidal characteristics and large-scale morphological landforms such as flood basins, inlets and outer deltas therefore formed the underpinnings of a forecast of the morphological effects of a rising sea level. To discover the empiricaj relationships specific to the Wadden Sea, the extensive sounding data and information on flows and waves in the Dutch wadden area were used.

4.2 The system's sand and silt economy

Physically, the wadden system forms an ensemble of islands, inlets, outer deltas, and a series of adjacent tidal basins with channels, flats and salt marshes, connected to and interacting with one other by the longshore transport of sediment. Some sand exchange also occurs between adjacent tidal basins and between the wadden system and the depths of the North Sea (deeper than the -20 m line), but the quantities exchanged are small and basically negligible in the time frame of this discussion. Viewed from a geologicaf time scale, the sand exchange with the deep shelf could play a role. In addition, sand is transported along the coast, resuiting in a northern-moving sand transport and a portion ending up in or being removed from the Wadden Sea. The difference between the amount of sediment imported and exported along the coast to and from the system is slight enough to consider its influence

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Figure 4.1 Sand sharing system The sediment budgets in the tidal inlet, tidal basin, the outer delta and neighbouring island coasts are all interlinked and balanced with one another. The system's sediment equilibrium is virtually closed. If sand is removed from one part of a tidal basin, the system will attempt to regain equilibrium by adding sand to the deepened area.

outer delta Den Helder

on the various tidal basins negligible. To all intents and purposes, the wadden system can be said to have a closed sand economy. If a part of a tidal basin in such a closed system as this becomes deeper, the system reestablishes equilibrium by importing sediment from another part of the systems. This type of system is also referred to as a sand-sharing system (Figure 4.1). Of the sediment that settles in the Wadden Sea, some 70-80% consists of sand and the remainder is silt. The closer we get to the salt marshes and watersheds (wantijen), the higher the silt content. These spatial differences in the sediment composition are the result of the fact that transport of coarse material requires a faster current speed than does fine material. Coarser sand is found in the inlets and on the bottom of connecting channels where faster current speeds are measured. Fine-grained sand and silt can be moved mainly further back into the basin, on flats and in the salt marshes, where the current slows. A distinction has been drawn between sand and silt, because their composition and behaviour differ considerably.

Figure 4.2 Estimated average annual quantity of sand transported through the tidal inlets to the Wadden Sea at flood tide (in million m3 peryear).

The silt carried in suspension to the Wadden Sea comes from the North Sea. It originates from the rivers (e.g. the Rhine), the English Channel, the Flemish Banks, discharges into the sea of dredgings from the Rotterdam port (Loswal Noord) and the bed of the North Sea. Every time the tide goes in and out, large quantities of sediment (sand and silt) flow to and from the Wadden Sea. The net quantities that remain behind in the Wadden Sea are generally small (10-30%) in comparison to the total sediment load during flood and ebb tide (Figures 4.2; 4.3).

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Frisian gat Figure 4.3 spring tide The annual quantity of sand and 4 April 1991 silt that are transported in and out silt and sand transport of the tidal inlets is much larger than the net quantity transported. This figure illustrates the sand and siit movements during spring tide at one measurement site in the inlet of the Frisian Gat.

4.3 The system in equilibrium

There is a correlation between the dynamic behaviour of the Wadden Sea and the net sediment balance. The net sediment balance is the quantity of sediment that permanently settles or erodes within a speci- fied period of time. In other words, if - for whatever reason - the import of sediment to the Wadden Sea exceeds the export of sediment, the tidal basins become shallower and, inversely, if sediment export exceeds import, the wadden area deepens. In a situation of equilibrium, the average quantity of sand transported over a prolonged period to the Wadden Sea equals the quantity that exits the Wadden Sea. We call this static equilibrium. In the event the sea level rises, the system can establish equilibrium if the speed at which it rises keeps equal pace with the rate of sand suppletion. We call this dynamic equilibrium. The rise in sea level is a process that does not always occur at the same rate. Fluctuations in the average trend can occur, with the sea level rising at a rate that is faster or slower than the average value. This equilibrium is consequently also dynamic. The landscape of the wadden area is in a continual process of change due to meandering channels and tidal flats going through alternating periods of becoming deeper and shallower. The form of the tidal curve is instrumental in the generation of a net sediment transport (or residual transport) to the Wadden Sea. A high- speed flood tide that lasts a short time, for instance, transports more sand than a slower, long ebb tide. The tidal properties described here - such as a short intense flood tide or a quick turn of the tide at low water - are created by the deformation of the tidal wave under the action of friction with the bed in the channels and on the flats.

The relocation of sediment during flood and ebb tide are part of the system's constant endeavour to achieve dynamic equilibrium between its form (morphology) and the ever-changing conditions of the tidal currents and waves. This balance is influenced at different levels of time

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and space. At the g!obal and regional Ieveis, this baiance is controlled by the large-scale effects of climate change, which bring about changes in the sea level, tides and waves. At the regional and local Ieveis, the landscape of the wadden area is influenced by various acts of human intervention, such as reduction of the size of the tidal basin by closing it off, empoldering and iand reciamation works, and the extraction of gas, sand and shells. The combination of these effects influences the current conditions and the amount of sediment that shifts and, in turn, the system's net sediment baiance.

4.4 The system out of baiance

The history of the development of the wadden system has taught us that the morphology of the Wadden See adapted to the rising sea level by adding sand from the longshore transport, on the one hand, and from other parts of the system, most notably the coasts of the eroding islands and the otiter deltas, on the other. The net sediment baiance of the wadden system shows the response to disruptions of the dynamic equilibrium. Relatively sudden changes in the morphological structure of the tidal area, such as the closing off of the Zuider Sea (1932) and the Lauwers Sea (1969) - cause abrupt changes in the hydrodynamics (tidal movements) and sedimentation economy of the system. Gradual processes like the formation of salt marshes and an accelerated sea level rise also influence the baiance of the system as a whole.

Since the Middie Ages, the characten'stic landscape of the wadden area has not or scarcely changed, despite its reduction in size. Soundings reveal that the net quantity of sand that has entered the Wadden Sea since the thirties is larger than that which has left it. Sedimentation has been able to keep pace with the current rise in sea level. The wadden area is still in a process of adapting to past acts of human intervention and have not yet regained a new state of equiiibrium. Can the developments observed be explained? How will the responses unfold and which other responses can be expected if the sea level rises more quickly or in the event of anthropogeomorphic disruptions?

4.5 Relative rise in sea level causes sand demand

If in the future the greenhouse effect should cause the sea level to rise more rapidiy, dynamic equilibrium can only be restored if sedimentation likewise increases. In other words, if more sediment is withdrawn from the sediment flow and deposited in the tidal basins. This hinges on the precondition that the entire wadden area will have to become somewhat deeper relative to the rising sea level. This slight depth increase will cause a s\ight reduction in the average current speeds in the channels and over the flats (Figure 4.4). As there is a correlation between sediment transport and current velocity, the transport capacity wil! drop much more than the current velocity slows. The f lood stream carrying a sediment load can then depositthe sand in the tidal basin. The ebb stream does not have enough force to remove the total quantity of sand brought in. Thus, over a long period, the quantity of sand that flows into the basin is on average larger than the quantity that flows out. This phenomenon is based on the sand retention mechanism of a deepened basin. The current situation is such that the quantity of sediment brought in by the flood tide already exceeds the quantity returned to the sea by ebb transport, but a sea level rising at an accelerated pace and a relatively deeper tidal basin would cause the difference between the import and export of sediment by flood and ebb current to become

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Figure 4.4 The effect of a reiative rise in sea level causes the average depth of the tidal basin to increase and, accordingly, the current to sandtransportcapacity decelerate. This results in an exponential decrease of the sand transport and in sedimentation in the Wadden Sea. reiative sea level rise

larger than it is under the current circumstances. The property of a deepened tidal basin to retain large quantities of sand is termed 'sand hunger or sand demand of the Wadden Sea'.

When the rate at which the sea level rises accelerates, the tidal basin becomes somewhat deeper and grows hungry for sand. Structurally and constantly, the basin is, as it were, thrown slightly off balance. In the beginning of this process, the sand retention capacity of the deepened basin gradually increases. The more the sea level rises, the greater the increase in the sand retention capacity, until dynamic equilibrium is regained. The system's response to the rise is delayed and the average basin level thereby becomes slightly lower in relation to the sea level (Figure 4.5). The development of the system into a new state of dynamic equilibrium is contingent upon the degree of rise in sea level and the supply of sediment. The total quantity of sand required yearly to restore dynamic equilibrium is directly proportional to the sea level rise.

dynamic adjustment dynamic Figure 4.5 equilibrium period equilibrium If the sea level rises at an accelerated rate, the tidal basin deepens slightly over time in relation to the rising sea level. As the sea level rises, the tidal basin's ability to trap sand gradually increases until dynamic equilibrium is restored.

speed of rise in sea level (crr per century)

time in years

If the supply of sediment is not sufficient to allow the tidal area to keep pace with the sea level rise, dynamic equilibrium cannot be regained. In that case, the wadden area will gradually lag behind the rise in the sea level, eventually bringing aboutthe area's inundation.

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If the basin or parts of it become deeper due to subsidence resulting from gas, sand and shell extraction, the effect is the same as a relative rise in sea level: a demand for sand is created, but then at a more localized level. Here, too, the total amount of sand required to restore the dynamic equilibrium is directly proportionate to the depth increase.

A relative rise in the sea level can also influence the tide. Tide simulations of the current situation of the wadden area with a fictitious water level increase of 1 metre reveal that the volume of water flowing in and out of the inlets and the tide ranges in the basin can be enlarged by dynamic effects. In general, these dynamic effects are much less significant in the small basins than in the larger tidal basins. A sudden 1-metre rise of the water level is, however, not realistic, because the morphology of the wadden area adapts to a great extent by means of sedimentation. Accordingly, these effects have been quantitatively disregarded.

4.6 Reduction of the tidal basin creates sand hunger

In physical terms, it can be reasoned that, in a state of dynamic equilibrium, there is a correlation between the dimensions of an tidal inlet and the size of the tidal phsm (see page 39). The larger the tidal prism, the greater the tidal inlet. A similar correlation applies to the dimensions of channels lying closer shorevvard. Part/al damming up of a tida! basin, resulting in a reduction of the tidal prism, leads, in turn, to sand hunger and to sedimentation in the basin. Empolderingthe edges of the wadden area engenders an analogous process,

4.7 Potential sou rees of sand: outer deltas and island coasts

Potential sources that can satisfy the tidal basins' sand hunger are the outer deitas, the island coasts, and coasts of the Noord-Holland main- iand. In a state of dynamic equilibrium, the sand volume of an outer delta is linked to the tidal prism (see page 39). When sand hunger is caused by a disruption resulting in a decrease in the volume of inflowing and outflowing water, the outer delta will tend to decrease in size and serve as a source of sand to adjust to the change. When the volume of water remains constant, this will not be the case. In that case, sand is drawn from the island coasts.

Ctosing off of Lauwers Sea: outer delta as sand source

When Lauwers Sea was closed off, the tidal prism and basin volume were abruptly reduced. This damming caused a dramatic disruption of the dynamic equilibrium of this tidal system. The equilibrium reiationship between the basin volume and the tidal prism makes it evident that, after damming, the basin volume of the Frisian inlet is not in equilibrium with the tidal prism, i.e. the basin is too big in relation to the t/dal prism (see page 40). This creates sand hunger in the tidal basin. Because the basin has become too big, the current speeds in the tidal basin dropped drastically after damming. In this case the flood current carrying sediment is given the opportunity to deposit sediment in the tidal basin. The ebb current is not strong enough to carry out the total quantity of sediment carried in. The result is net sedimentation in the basin (Figure 4.6).

To satisfy the basin's subsequent sand hunger, sand is supplied by the outer delta of the Frisian inlet sytem, which shrinks. Erosion of the outer delta will continue until the dynamic equilibrium between the size of the outer delta and the tidai prism has been restored.

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volume changes cumulative in millions of cubic metres Figure 4.6 Gradual damming of the Frisian Gat in 1969 caused a reduction in the volume of water flowing in and out through the tidal inlet. The system is restoring its equilibrium through erosion and reduction of the size of the outer delta and sedimentation in the tidal basin.

-30

1970 1975 1980 ,. 1985 1990 time in years The tidal basin and outer delta of the Frisian Gat are still in the process of adjustment, although at a much slower rate than in the years immediately following the closing of the Lauwers Sea in 1969.

Closing off of the Zuider Sea: the outer delta as a sand source to serve as a temporary buffer against coastal erosion

History teaches us that since the Middle Agesr the dynamic equilibrium between the dimensions and hydrodynamic conditions of the current system of the Marsdiep tidal basin and Zuider Sea was upset by the erosion of enormous peat tracts (see Chapter 3). As a result, a stretched- out tidal basin was created, connected with the North Sea by the Marsdiep and Vlie inlets to the north and causing the shallow Zuider Sea to the south to function as filling basin. Over time, this tidal system endeavoured to restore its equilibrium by means of sedimentation. The current speeds in the Zuider Sea were slow, limiting sediment transport and sedimentation to fine-grained material. Sand was transported in and around the inlets where the currents were faster. We do not know for certain whether the tidal basins had already completed the adjustment process and regained equilibrium before the closing of the Zuider Sea (1932). It is, however, plausible that the inlets and connecting channels had basically reached a state of equilibrium. The closing of the Zuider Sea (1932) has had a major effect on the tidal movements to the north of the IJsselmeer Dam. Although the size of the tidal basin has decreased substantially since then, the dynamic effects of tide and the remaining basin has increased the tidal range and tidal prisms of the Marsdiep basin and, to a lesser extent, the Vlie. This act of human intervention abruptly disturbed the process of reaching a dynamic equilibrium, especially in the Marsdiep tidal basin. A clear sedimentation trend can be observed in the Marsdiep basin as it adapts to the closing off of the Zuider Sea. Large quantities of sediment have been deposited in this basin thus far, particularly in the closed off tidal channels near the IJsselmeer Dam. In spite of the eniarged tidal phsm in the inlet, the channels further back into the basin are too wide for the tida! currents there. This situation generates sand demand. The outer delta of the Marsdiep iniet system, the coast of Texel Island, and the mainland coast of Noord-Holland are potential suppliers of sand. In view of the fact that the tidal phsm of the Marsdiep basin increased after the Zuider Sea was closed off, it can be expected that the size of the outer delta will expand. Observations reveal, however, that the volume of the outer delta has actually decreased.

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A possible explanation is that the basin's huge demand for sand causes the basin to draw sand from the outer delta first, instead of from the coast. Over a long period of time, this process will expectedly reverse and the outer delta will begin expanding, at the expense of the neighbouring coasts.

Empoldermg and land redamation projects: outer deltas as sand sources

Over the years, peopie have empoldered large tracts of the peripheral zones of the wadden area which, due to silt accumulation, were above the high water level, and built fascine dams to promote land redamation. The sediment found in these areas consists mainly of silt and fine sand, materials which settle under calm conditions. The result of this accretion, be it anthropogenic or natural, is that the size of the tidal basin, and thus usually the tidal prism, is reduced fittle by little. Compared to damming, this process progresses naturally and fairly gradually. The effect of the prism's continually becoming smaller is that the dynamic equilibrium is upset over and over again. In this case, too, a reduction in the size of the basin creates sand hunger, which is satisfied by resorting to the outer deltas for sand.

Relative rise in sea level: island coasts as sand source

Sand hunger caused by a relative rise in sea ievel cannot be sated permanently by the outer delta. As mentioned above, the relative rise in sea level has no effect to speak of on the volume of the inflowing and outflowing water. Consequently, the outer delta will ultimately maintain the same size, If the outer delta were to supply sand at all, it would be replenished in the course of time. The only constant sources of sand to feed the tidal basin in this case are the island coasts (Figure 4.7) and the littoral drift. This is not to say, however, that the outer delta is fixed: the volume of the outer delta is the volume of sand that will shift in relation to the contiguous coastal profile. The contribution from the littiral drift is small. The extraction of minerals (sand and shell extraction, subsidence due to gas extraction) triggers a similar mechanism as does a rise in sea level: the tidal basins become hungry for sand and the island coasts erode and recede. The full magnitude of coastal erosion can be forecast with a fair degree of accuracy, but exactly where, when and how much erosion will take piace at any given spot cannot be predicted without conducting a specific study of the area in question.

TheRiddleoftheSands 34 National Institute for Coastal and Marine Management/R/KZ

Figure 4.7 A schematic coast profile showing that a relative nse in sea leve! causes coastal erosion.

roiume of sand reteased as coast receöes effect of rising sea leuel

effect of retragradation

volwme of sand required to keep up witfi sea level rise

effect of rising sea leve

effect of retrogradaöon and sea level rise

4.8 Flats and salt marshes

In the tidal basins, the channels, but more particularly the flats and salt marshes, characterize the wadden landscape.

4.8.1 Flat development

The development of flats has been highly contingent upon the exchange of sediment between the channels and flats. During high tide, the current in the channels is fastest just before or at the same time as the flats are flooded (Figure 4.8). In general, the sediment concentration is highest when tidal water flows out over the flats before high water, and sediment is carried to the flats. The current over the flats slows down considerably, causing sedimentation, most notably in the period when the tide is turning. Sandy ridges are built up along the edges of the channels which can be a few decimetres higher than the flats behind them. More sediment is carried in during flood tide than is carried away during ebb tide. The ebb current flowing over the tidal flats is concentrated in the 'prielen1, small troughs that are dry at low water.

Measurements conducted since 1930 show that the flats in the Wadden Sea grow by 2-3 mm a year, or 20 to 30 cm each century (Figure 4.9). This increase corresponds with the natural relative rise in sea level. On average, the increase in flat elevation has equalled the average rise in sea level in recent years.

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Figure 4.8 Process measurements at tidaf flats indicate that sand is deposited more easily on shallow tidal flats. The sand carried by flood tide to the shallow tidal flats is more than the sand removed by ebb tide.

9 10 11 12 13 14 15 16 17 18 19 20

Figure 4.9 The flat system in the Wadden Sea displays a wide range of rates of elevation increase. Measurements have shown that the average increase rate is about 21 cm per century. This growth is the same as the present rise in sea leve).

-1.0 2.0 (m) = average vertical growth rate

vertical growth rate in m per 100 years

fall in flat elevation rise in flat elevation

measurements sedimentation

The Riddle of the Sands 36 National tnstitute for Coastal and Marine Management/R/KZ

The flats build up primarily in calm weather. Storms in the Wadden Sea can cause waves that are 0.5-1 m highr which affect the transport of sediment near the bottom in the shallow areas of the Wadden Sea, since the wave movement on the surface causes the sand on the bottom to move to and fro. This moves sand up from the bottom by turbulent eddies, whereby the sand concentration in the water rises, allowing greater amounts to be transported and making it more difficult for the sediment to be deposited onto the tidal flats, while facilitating its removal. During periods of stormy weather, the flats flatten out, during calm weather, though, the flats are built up rather than flattened.

When the sea level rises at an accelerated rate, the average depth of the wadden area increases somewhat. As the current travels across the flats, the speed decreases, as does the sand-transporting capacity, to a relatively greater extent than in the channels, causing a relatively higher amount of sedimentation on the flats. When the sea level rises in the Wadden Sea, sand will thus first be deposited on the flats (Figure 4.10).

schematized effect of 10-cm relative rise in sea level Figure4.10 The infiuence of the relative rise in percentage 20 sea level on the movement of tidai channets water and the sand transport is 15 relatively more intense on the flats than in the channels, because flats 10 are the preferred deposition site for 5 sediment. 0

-5

-10

-15

-20 | depth increase n current velodty | sand transport

4.8.2 Development of salt marshes

Salt marshes can develop in (hydrodynamically) calm, sheltered areas of the Wadden Sea. The calm environment and an adequate supply of sediment create the conditions for a relatively high level of deposition of fine sand and silt. When these areas are uncovered long enough due to a steady rise in elevation, the first salt marsh plants can establish themselves. The silt in the deposition provides the nutrients required. The first pioneer vegetation establishes itself as soon as the elevation (and consequent shelter from wave impact) allows the germination of the seeds. The salt marsh vegetation slows the current down still further, consolidating sedimentation and thereby the accretion of the wadden flats. Continued raising, further maturation of the soil and the succession of the vegetation produce well-developed differentiated salt marshes.

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Measurements from the last thirty years (1960-1992) have shown that the average annual siltation of fine material in the salt marsh zone of Friesland is 18 mm, and that of Groningen 13 mm (Figure 4.11). On the flats side of the islands, the average annual siltation is lower, about 3-8 mm. It seems that the salt marsh system has the capacity to compensate for a whole range of speeds at which the sea level rises by raising elevation. Salt marshes trap silt effectively, especially in calm weather. During periods of stormy weather, the waves have a greater erosive effect on the salt marshes, particularly in the pioneer zone. Wave movement hinders the sedimentation of the silt that is carried into the salt marshes.

siltation rate of wadden salt marshes in mm per year Figure 4.11 Sitation rate of Dutch salt 18 marshes (after Oost e.a., 1993) 13-20

10 ---

7 3-8 6

fel l I i 1 l Frisian salt Groningen salt other mainland islands and low mainland pioneer marshes mars hes 5 alt mars hes zone

The Riddle of the Sands 38 National Institute for Coastaf and Marine Manage me nt/RIKZ

How the Wadden System maintains In the following paragraphs, we will briefly look at a number of characteristic empirical relationships We rarely stop to consider the fact that the large sea between the tidal prism, on the one hand, and large- inlets, or 'gats', of the Wadden Sea have managed to scale geographicaf units such as outer delta, tidal inlet survive throughout the centuries. Apparently, the and tidal basin, on the other. sediment carried in and out by the tidal currents is in balance. This is not the complete story, however, because the beds of the tidal inlets have gradually Relationship between cross-section and tidal prism increased in height due to the rise in the sea level. In other words, a tidal inlet is not in a fixed state of A typicai unit of measure for an inlet is the narrowest balance. The equilibrium is dynamic: fluctuations in cross-section at the opening. A clear connection has the supply of sand or current speed, such as during been established between the size of this cross- spring and neap tide, can temporarily cause sand to section and the tidal prism. Although not universally be added or removed, making the inlet deeper or equivalent, this correlation is probably the same for shallower. The profile within a tidal inlet can change, areas that form a morphological unit. Empirical study too, for instance, due to the shifting of channels. has shown that the cross-section of an inlet increases Despite these morphological changes, the general almost linearly with the tidal prism. If we limit our- character of the system has remained constant. selves to the large inlets, a linear connection can safely be assumed. Figure A indicates the relationship between the Characteristic equilibrium relationships average tidal prism and the cross-section al area at NAP (Normal Amsterdam Level, or average sea When in a state of morphological balance, relation- level). The same type of relationship exists for cross- ships exist between the area's morphometric and sectional areas of channels in the basin and the tidal hydrodynamic values present themselves. One of the prism at the site of the cross-section. key mechanisms behind morphological changes in a tidal area is the tidal prism. It should not come as a surprise that in the past various countries have con- Relationship between volume of sand in the outer ducted studies to determine the relationship between delta and the tidal prism the morphological balance of the tidal inlets and the tidal prism. Tidal currents transport large quantities of sand through the tidal inlets. Because the ebb fiow spreads These relationships enable us to forecast morphologi- out beyond the inlet, sand is deposited outside the cal adjustment if changes occur in the Wadden Sea inlet and a system of channels and banks is created: system, whether caused by human intervention or the outer delta. This outer delta is a 'storehouse' for a natural processes. The correlations are not universal, large quantity of sediment. We now see that there is and are influenced by site-specific circumstances, a connection between the volume of the sand in the such as waves, the size of sand grains, the volume of outer delta (above the profile of the coastline) and sand transported through the inlet. When making the tidal prism. The deltas of the Wadden Sea appear general forecasts, these factors can be disregarded to match the general picture of coasts with a and the relationship can also be assumed to apply to reasonably strong wave action (Figure B). an accelerated rise in sea level.

Figure A. redi iwre rr Various scientists all over world have VlieGat,.-" proved that in the case of dynamic Marsdiep t equilibrium, the cross sectional area 3orndiep increases with the tidal prism in the Frisian Gatafi ei damming narrowest part of the inlet. This means [5969 .--"•' Frtsian jat before damm Fn ian Oal(i98 (19691 rlar>dBche Ga that if the tidal prism diminishes for Ö|£ «. Lsuwc any reason whatsoever, the cross- • Gat section of the inlet will diminish • Elje tanderb * PirfeGat proportionally. Creek

Stfulo

0 50 100 200 500 1,000 2, tidal ptism In mlltion cubic metres

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Figure B. A number of scientists have established a relationship of equilibrium between the volume of sand in the outer delta and the tidal prism. This relationship implies that the volume of the outer delta is subject to a slight exponential increase with the tidal prism, meaning that if, for any reason whatsoever, the tidal prism diminishes, the size of the outer delta will diminish as welt. Research has also shown that the relationship is partly affected by the wave climate. When wave tnovement is strong, the volume of the outer deltas is smaller than when wave movement is

weak. 10 20 50 100 200 500 1000 2000 5000 10000 tidal prism in millions of cubic metres

Relationship between the volume of the tidal basin the basin and the tidal prism, as determined by and the tidal prism measurements taken in the Wadden Sea.

Tidal currents shift sediment, filling and emptying tidal basins. The formation of channels and flats Adaptive behaviour of tidal basins creates a state approximating equilibrium. There is a certain connection between the size of the channels Morphological adjustments are not immediate; and how much the basin can hold, expressed as a restoring dynamic equilibrium requires time. relationship between the volume of the basin and the It has been observed that the rate of adjustment is tidal prism (Figure C). usually proportional to the magnitude of the disruption. Some changes are abrupt, for instance, those caused by partial damming of a tidal basin, Relationship between intertidal area and tidal prism while other proceed more slowly, such as the rising sea level. Adjustment to these types of changes When the sea level rises, a portion of the sediment follows an exponentiaf curve over time, with a certain carried in by the flood tide is deposited on the flats. lag effect (Figure E). The relationship between channels and flats is thus expected to relate to the inflow of sediment and, in turn, to the size of the tidal prism. The larger the tidal inlets (and tidal prism), the smaller the intertidal areas in proportion to the total surface of the basin. Figure D shows the relationships between the proportion of intertidal area and total surface of

contenl of tidal basin below NAP in millions af cubic metrc Figure C

•• • Marsdiep Channd jS' aftei-1932 The dimension of the flood basin is • • Epw^DoHsrd determined by the tidal prism. 2000 Vlie Gat The greater the tidal prism, the greater 1,000 after1932

the content of the basin becomes. '. ^-'» ! Borndiep 500 • Oiannel • For the Dutch mud-flats, the content of damming (1969] *-- "^y dammln^ (i%9) the basin is more or less identical to the 200 Ei* lardsche Cal^- Lauwef5 Cal tidal prism. This relationship explains why if the content of the flood basin is 10° .A-Pin reduced by a dam, for example, the 50 • effen or Iismming öjerianderbalg 4 tidal prism will diminish proportionally. Creed --' ' 1 ^"'''Schild

100 200 500 tidal prism in millions of cubic metres

The Riddle of the Sands 40 National Institute for Coastal and Marine Management/R/KZ

intertidal atea in proportion to Bie total surface of the basin Figure D Schild Large basins have relatively more chan- -4

nels than small ones. In other words, rbslg * ~~- Creek -ne large basins have a greater drainage 1 Pinke Gat I . "•-->, » ' Bomdfep capacity than small ones. Since the lijcrlarnjsche Gat! ' - •*.; Charme! 0,6 surface area of the flats between low

and high water forms the difference >1 14 VlieGat A between the total basin area and the ° ^-" channel area beneath low water, it is precisely the smaller basins that have QJ2 1 relatively large numbers of intertidal MaTidiep Channd after dainn ing Zuiderzee areas in comparison to the larger *

basins. 100 200 500 Hdal prisrn in millions of cubic metfes

The large-scale empirical sediment balance model: The ebb flow loses the power to carry all the MORRES imported material back out and sand is consequently deposited in the basin. Based on a ümited number of characteristic empirical and internal relationships, the large-scale empirical When the rise in sea Ievel accelerates, the sand- sediment balance model cailed MORRES retention capacity of the basin will initially be less (MORphological RESponse model) was developed. It than is actually required in order to keep up with the is used to calculate sudden (e.g. damming) and rise. As time passes, however, the basin's ability to gradual (e.g. sea ievel rise) morphological changes. trap sand will improve until sedimentation in the Given the fact that the relationships are not universal, basin is in step with the sea Ievel rise. Thus, the the model yields general outcomes. The model average depth of the Wadden Sea will keep pace distinguishes three large-scale geographical units: with the accelerated rise in the long run, but with a outer delta, gat and tidal basin, each of which is 'lag' in the adjustment of the depth, which is greater given a morphometric value. than in the existing situation. MORRES calculations Water movement is characterized by the tidal prism do not reveal how the increased average depth is and range. The tidal range is a constant in the distributed across the flats and channels. calculations. Consequently, if the sea Ievel rises more quickly, the Sediment exchange between the North Sea and the flats could deepen somewhat, but closer research tidal basin is based on the sand-trapping concept and into the mechanism of sand exchange between the the hypothesis that sand can be deposited in the channels and flats shows that there is little change in basin when the sea leve! rises more quickly as long as the elevation of sand flats in relation to high water it becomes deeper relatively. The depth increase and that the flats are where sand tends to settle. causes a slight deceleration of the currents in the Provided there is enough sediment, we can assume channels and over the flats in the basin. Since sand that the tidal basin will deepen on average, especially transport is proportional to the current speed (a third due to the deepening or widening of the channels. power is usually used), the transport capacity will A deepened tidal basin's tendency to accumulate drop much more when the current speed slows. large quantities of sand is what we call 'sand hunger'

Figure E Adaptive behaviour of tidal basins when faced with abrupt changes (I) and with changes varytng over time {II).

'X^j^^i&i^^ft^^S^^

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or 'sand demand' in the basin. The system's progress from gas, sand and shell extraction gives rise to sand towards restoring its dynamic equilibrium is hunger, but it is more localized. In this case, the total contingent on the extent of the sea level rise and the quantity of sand required to restore dynamic sand supply. In the model, the sediment supply is equilibrium is directly proportionate to the depth defined as the average annual quantity of sand that increase. ends up in the inlet by means of longshore transport and which is transported by flood tide through the As mentioned above, the MORRES model operates inlet into the basin. Only a small portion of that on the assumption that the tidal range is constant. amount remains behind in the basin, contributing to Slight increases in tidal ranges have actually been the elevation of the Wadden Sea. observed and demonstrated by tidal calculations. The The total quantity of sand ultimately required per implication is firstly that more sediment will be year in order to restore dynamic equilibrium is carried to the Waddon Sea and secondly that it is directly proportionate to the rate at which the sea possible that the relationship between the surface level rises. areas of the flats and the basin will decline. This is mainly of relevance to the larger basins of Marsdiep, Just like a relative sea level rise, the depth increase of Vlieand Ems Dollard. the basin or parts of it due to subsidence resulting

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5. Looking Ahead to the Future Landscape of the Wadden

5.1 Introduction

In looking ahead to the wadden landscape of the future, we must examine the forecast natural developments and anthropogeomorphological effects on the sediment balance of the Wadden Sea. Such large-scale natural developments as a rise in the sea level and a climate change (frequency of storms, temperature) are difficult to influence regionally. Acts of human intervention, on the other hand, can usually be controlled. How can we guide human actions in such a way as to prevent harm to the wadden system? To do so, we must first know how various measures and effects influence the system's sediment balance and what the repercussions are for the future structure of the wadden landscape.

5.2 What will be the future demand for sediment in the coming 50 to 100 years?

The expected demand for sediment in the Wadden Sea as a key compo- nent of the sediment balance resulting from natural developments and human intervention is discussed below (see Table 5.1; Figure 5.1)

Table 5.1 Wadden Sea Sediment demand (106m3 per year) Amounts of sediment of the Dutch Effects 1990 2040 2090 Wadden Sea required annually (in million m3 per year) in the coming 1) Sea level rise 4-5 4-5 4-5 50 to 100 years to compensate sea 20 cm per century level rise and acts of 2) Extraction of sand and shells 8-9 (in period 6(D 6(D anthropogeomorphological effects. 1960-1987} 3) Extraction of natural gas 0.3 1-2 0 4) Past engineering works 2-3 0.7 0.3 2 5) Natural accretion in the salt 0-9 0-9(2) 0-9< > marshes and peripheral zones

Total sediment requinement 14-24 12-21 11-20

A)Extra sediment required 4-5 6-7 at sea level rise of 60 cm per century B) Extra sediment required at sea level rise of 85 cm per century 6-7 9-10

1 dependmg on policy on sand and shell extraction 2 depending on nature conservation policy of the peripheral zones and salt marshes

TheRiddleof theSands 43 National Institute for Coastal and Marine Management//ï//CZ

Figures 5.1: An estimate of the total demand for sediment in the Wadden Sea in 2040 The demand for sand in the Wadden Sea depends on the increase in the rise of the sea leve), mineral extraction (gas, sand and shells) and reduction of the basin size due to damming, empoidering and land redamation projects. The potential sand suppiy is the average annual bad that flows through the tidal inlets into the tidal basins at tide. As long as the potential sand suppiy far outstrips the demand, it is likely that the wadden Sea will be able to compensate by means of sedimentation. Throughout the Dutch Wadden Sea, with the exception of the Marsdiep Channel and Ems-Dollard Bay, the sand suppiy seems to be adequate to meet the demand.

sand suppiy sediment demand in 2040 in milllions m3 in millions m3

scenarios of sea level rise scenario 20 cm/century scenario 60 cm/century scenario 85 cm/century

Wadden Sea 8.75 11.05

0.75

Marsdiep Channel 1.12 1.81 2.17

1.41 0.7 1.41 0.7

Eijerlandse Gat

0.27 0.78 1.06

0.2

Vlie Gat

2.07 2.53 1.22

0.05 1.89 0.45

Borndiep Channel 0.46 0.89 1.13 4.4 0.05 0.55 0.1 0.55

sand suppiy towards effect due to effect due to accretion effect due to expected effect due to past effect due the tidal basins sea level rise in the salt marshes and sand and shell extraction engineering works to gas peripheral ïones extraction

The Riddle of the Sands 44 National Institute for Coastal and Marine Management/fi/KZ

sand supply sediment demand in 2040 in millions m3 in millions m3

scenarios of a sea level rise scenario 20 cm/century scenario 60 cm/century scenario 85 cm/century

Pinke Gat 0.09 0.3 0.41 0.13 0.1 0.1 0.13 0.13

Frisian Gat 0.81 0.21 0.39 0.1 0.1 0.39 0.05 0.05 0.1 0.1 0.39

Eijerlanderbalg Creek 0.06 0.21 0.29

0.08 0.08 0.08

0.58 0.78

0.44 0.44

Schild 0.05 0.18 0.25 1.6

0.07 0.07 0.07

Ems-Dollard Bay 3.44 3.44 3.44 10.3

TheRiddleoftheSands 45 National [nstitute for Coastal and Marine Management/RIKZ

The atrrent rise in the sea level requires 4 to 5 million m3 of sediment annually, an accelerated rise would require 6 to 10 million m3 extra

The present rise in the sea level of 20 cm per century causes a sediment hunger in the Wadden Sea (including the Ems Dollard Bay) of 4-5 million m3 annually. Some 75% of this sediment demand is answered by the large tidal basins of the Marsdiep, Vlie and Ems Dollard. A rise of 60 cm per century would require an extra annual quantity of sediment of 6-7 million m3 after hundred years. In the worst-case scenario of a rise of 85 cm per century, the extra demand for sediment on the coast would require an extra 9-10 million m3 annually after hundred years.

The damming of the Zuider Sea and the Lauwers Sea will require 2 to 3 million m3 of sediment annually for decades to come

Past damming works wil! influence the future sand economy in the Wadden Sea for many years to come. The system has still not adjusted to the damming of the Zuider Sea (1932) and the Lauwers Sea (1969). The Marsdiep basin is still in the process of adapting to the damming of the Zuider Sea (1932). Since then, some 200 million m3 of sediment has been deposited in this basin. Making a correction for the sand extracted in the same period, the calculation of the annual sediment demand created by the damming equals about 3 million m3. A large proportion of this sediment ended up along the Frisian coast and in the closed tidal channels along the IJsselmeer Dam. Calculations based on sounding data reveal that the area of the Texel inlet has completed up to 80% of the process of achieving a new state of equilibrium, but that the back of the basins adjust much more slowly, requiring another 2 or 3 centuries. It goes without saying that if sand is extracted from the basin, the process of adjustment will be slower, because extraction will increase the basin's sand hunger. The magnitude of the tidai volume and the dimensions of the tidal basin created after the damming display many similarities with the Vlie, which is now more or less in a state of dynamic equilibrium. In light of this, we may ask ourselves whether in the long term (e.g. centuries from now) the Marsdiep will develop the same geomorphological characteristics as the Viie now has. We consider this prospect a possiblility but not within the time frame considered (50-100 years). This scenario is less likely at a high rate of sea level rise (85 cm per century and more) when a shallow basin near the Frisian coast could remain open. However if it would develop, it would require an estimated total of another 700-900 million m3 sediment to restore the dynamic equilibrium of the Marsdiep tidal basin. The sediment demand created by the damming is approximated at an annual 1-2 million m3 in the next few decades, after which the annual demand for sediment will gradually decrease. The damming of the Lauwers Sea in 1969 threw the Frisian tidal inlet sytem out of (dynamic) equilibrium. An estimated total of 50 million m3 of sediment is required to recover this equilibrium in the tidal basin. In the period 1970-1987, some 30 million m3 of this total was deposited in the basin, which corresponds with an annual sand demand of 1 to 2 million m3. At a present net annual sediment inflow of a little under 1 million m3, the system is expected to take 30 years to reach a new dynamic equilibrium with a total remaining demand of about 20 to 30 million m3 of sediment, an amount that will be supplied primarily by the outer delta.

The total annual demand for sediment caused by the damming of Zuiderzee and Lauwerszee is expected to be between 2 and 3 million m3 in the coming decades.

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Sand and shell extraction in the Wadden Sea may require 6 million m3 a year

In the period 1960 to 1990, some 8-9 million m3 of sand was extracted annually from the wadden area as a whole, approximately 4,5 million m3 of which originated from the Ems Dollard basin. The remainder came from the rest of the Wadden area. The government is currently conducting a more cautious policy with regard to sand extraction (as laid down in the policy memorandum "Wadden Sea: Part 3 of the Cabinet Position on Key Planning Decisions"). Deepening due to sand extraction in the Wadden Sea has the same effect as a rise in sea level: in the long run, it has a negative impact on the island coasts (Photograph 5.1). The policy is intended in the long term to limit the sand extraction to maintenance dredging in navigation channels.

Photograph 5.1 Sand extraction from the Wadden Sea wiil have a substantial effect on the sediment economy in the Wadden Sea.

Nevertheless, the expected, future annual volume of sand extraction of 6 million m3 will have a substantial effect on the sand economy in the Wadden Sea. Most of the sand (about 4,5 million m3 yearly.) is extracted from the Ems-Dollard estuary, and particularly in that section of the main navigation channel of the Ems estuary administered by Germany. The volume of shell extraction is far more modest than that of the sand extraction. The existing cautious government policy on this industry is to be continued, and allows for a maximum volume of about 0,14 million m3 ayear.

Subsidence caused by gas extraction: annual sediment demand of 1 to 2 million m3.

Seismic studies under the Wadden Sea have shown up geological structures which may contain gas, theso called 'prospects'. Extraction from these potential gas fields will result in regional subsidence of the Wadden Sea. If we assume that the fields will ultimately be fully exhausted, the estimates for total sediment demand to compensate for the subsidence caused by extraction from these new gas fields range from 30 million m3 to 63 million m3, with an expected value of 43 m3. This process of subsidence will be at its peak during the initial decades of extraction. As the process progresses, the degree of subsidence will

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Figuur 5.2 Subsidence due to gas extraction induces sand hunger in the basin. The subsidence will be at its peak during the initial decades of extraction and subsequently tail -° 50 50 % off. The subsidence of the seabed will eventually be compensated for by sedimentation. Assuming a period of 40 years of extraction, about 70% of the hunger for new sand will be stilled by the end of 100 %

that period. The remaining 30% time ir years will be compensated for in the 60 years thereafter. Effects of gas morohological adjustment extraction on the morphology will be feit for a long time.

gradually decrease. The total useful life of this type of gas field is 20-40 years. The estimated extra demand for sediment of 45 million m3 will result in an adjustment period which will affect the coast for 100 years. Assuming a period of 40 years during which gas can be extracted effectively, about 70% of the hunger for new saediment will have been stilled by the end of that period (Figure 5.2). The annual demand for sediment on the coast of the wadden will, therefore, increase for the first 40 years by about 1 million m3. The remaining 30% of the total demand for sediment will be compensated for in the 60 years thereafter.

Subsidence caused by the extraction of gas from existing fields will create an additional demand for sediment of about 58 m3 (including Ems-Dollard Bay), which will also be met by sedimentation in about 100 years time. This is expressed in an annual demand for sediment for the coming decades of about 1 million m3. This places an additional burden on the Dutch Wadden Sea coast, and particularly on the eastern sections.

Salt marshes and silt accretion of the peripheral zones of the flats require 0 to 9 miilion m3 annually

The presence of vegetation in the salt marshes and the silt deposition conditions strongly determine the amount of silt that accumulates in the peripheral zones of the Wadden Sea and in the salt marsh works. In the past, the rate of silt accumulation and accretion was greatly influenced by human intervention. The history of empoldering of the wadden area from the year 1000 until 1970 provides a good impression of the rate of accretion. As it turns out, the pace of accretion has varied widely in the last 700 years, from 0,26 km2 (period 1000-1200) to 3,13 km2 (period 1800-1900). The average annual accretion at the current rate of increase in sea level and of silt deposition roughly corresponds with 0,07% of the surface of the basin on the eastern side of the tidal flats area and 0,035% on the western side. This means that the wadden area annually decreases in size by 1,5 km2. After 1970, we see that the peripheral zones of the wadden area more or less stabilizes. It reasons that the accretion of the peripheral zones has tapered off in the last decades. Whether this trend of stabilization will continue or whether it is simply a natural fluctuation is unknown. The forecast therefore takes account of an accretion ranging from 0 km2 to an average accretion of 1,5 km2 extending over many years. The resulting demand for sediment in the Wadden Sea as a

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whole, calculated according to the current rate of increase in the sea level, amounts to 0-9 million m3 per year. But a mere portion of that is deposited in the peripheral zones; the majority is required for the basin's adjustment. The sediment requirement may be of the same magnitude as the extra sand required in the case the sea level rises at an accelerated pace of 60 to 85 cm per century.

Whether the future rate of accretion and the consequent sand hunger in the wadden basins will change if the sea level rise accelerates is stiil uncertain and depends partly on nature conservation policy and management of the salt marsh development.

5.3 How large is the sediment supply?

The supply of sediment to satisfy the sand hunger in the Wadden Sea is determined by three factors: (i) the nature and size of the sediment sources, (ii) the possibilities of transporting the sediment from its source to the Wadden Sea and (iii) the difference between the incoming and outgoing sediment transport. The following sections look at each of these factors in more detail.

(i) Islands and outer deltas offer enough sand, the North Sea enough silt

The supply of sand stored in the islands and the outer deltas seems 'infinitely' large in comparison with the sand demand in the Wadden Sea. The sand does not exactly offer any resistance to relocation, as evidenced by the ever-eroding shoreline. But not all of the sand in this coastal zone is an available source of sand for the Wadden Sea. Every open sea inlet has an outer delta, whose sand volume is in direct relation to the tidal prism. In other words, the outer delta always requires a certain amount of sand to maintain equilibrium, The sediment supply is also partly determined by the Dutch coastal policy. As long as the coastal management authorities plan to compensate erosion with sand suppletion, and do not decide to proceed to large-scale fixing of the islands by means of solid coastal protection works, the islands will continue to function as a nearby source of sand for the Wadden Sea. It is important that the sand used for these suppletions come from the depths of the North Sea and not from the wadden system itself in order to augment the supply of sand to the system. Adding sand to the shallow coastal zones wil! eventually lead to a steepening of the coastline, which could influence the supply of sand to the wadden area, although this cannot be said with any certainty. Coastal suppletions drawn from the Wadden Sea will not increase the supply, simply being a relocation of sand within the system.

The North Sea is a vast source of silt, and is constantly supplemented by silt inflow from the Channel, as well as from rivers (Rhine!) and specific dump sites for silt (such as Loswal Noord near the mouth of the New Waterway), As long as river management or the policy on the dumping of dredgings does not change to such an extent that the supply of silt to the North Sea alters dramaticaüy, the North Sea will continue as an inexhaustible source of silt for the Wadden Sea.

(ii) Tide and waves are high-capacity sediment carriers

The sea, which powers its way into the inlets during flood tide and recedes with hardly less force on the ebb, can carry huge amounts of sand and silt with it.

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Based on measurements of the sand transported in the Vlie inlet, estimates have been made of the annual inflow of sand at high tide in the other inlets. The estimate for the entire Wadden Sea expressed in total inflow during flood tide is: an annual volume of 40 to 60 million m3 of sand (see figure 4.2). The total inflow of silt into the Wadden Sea during flood tide has been estimated at 50-100 million m3 every year. The current speed in the channels and wave action on the coast and on the outer delta have a major influence on the transport of sand; To a much lesser degree the transport of sand is affected by its supply. Silt, on the other hand, reacts differently: even if the current speed remains the same, the quantities of silt transported can vary. This distinction is important for residual or net transports through an inlet. There is a direct connection between the resulting transport of sand and the way in which the current speed develops during a tidal cycle; this is not necessarily the case for silt. The measurements do, however, show that the tide's capacity to carry sediment is much larger than the total sediment hunger of the Wadden Sea.

(iii) Measurements provide an indication ofrates of sediment deposition

Based on the analysis of sounding data accumulated in the six western tidal basins since 1925, the annual sedimentation in the entire Dutch Wadden Sea has been estimated at 14-24 million m3. At an average bed composition of 70%-80% sand and the rest silt, this means that, under the current deposition conditions, about 10-20 million m3 of sand is deposited in the Dutch Wadden Sea every year.

5.4 Is sediment demand being compensated by the supply?

The future sand suppiy may be a restrictive factor

It seems that for the entire Dutch Wadden Sea, the current net sedimentation of 14-24 million m3 is adequate to meet the need for sediment caused by mineral extraction, shrinkage of the basin due to damming, empoldering and the construction of land reclamation works and the current rise in sea level (see table 5.1). In other words, the Wadden Sea can compensate for this demand by means of sedimentation, although at the expense of the island coasts. If, however, we consider the demand for sediment in individual basins, we see that the demand for sediment in the tidal basins of the Marsdiep and the Ems-Dollard, at a rise in sea level of 60 cm per century or more, comes very close (50% or more) to the total flood transport to these basins. Full compensation by means of sedimentation is uniikely in the Marsdiep tidal basin and Ems-Dollard estuary, which may result in the inundation of the intertidal areas after a long period of time (centuries). It may be possible that silt will compensate for a portion of the sand deficit. Silt sedimentation, however, occurs oniy in those areas where the current speeds are slow.

It goes without saying, therefore, that intervention in the Marsdiep basin and Ems-Dollard estuary, which affect the demand for sand or the long-term sand supply, can have serious long-term consequences forthe development of both the tidal basin, the islands' coastline and the coast of Noord-Holland province. Such a disruption, in which the total demand for sand in the basin closely approximates the total supply (50% or more), does not occur in the other tidal basins of the Wadden Sea. This prompts us to expect that sedimentation will ensure that, during the next century, these tidal basins wil! be able to keep up with developments in the chosen scenarios for an accelerated rise in sea level.

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Sand suppletion as coastal protection is beneficial to the wadden

Sand suppletion is the government's main instrument by which to protect the coast from erosion. An additional amount of sand is applied to the coastal profile. In order to combat structural coastal erosion, an annual average of 1-2 million m3 of sand was spread on the beaches of the Wadden Sea islands and along the northem part (Kop) of Noord- Holland between 1979 and 1993. This sand suppletion compensates for the erosion of the coasts of the islands in a more or less natural way and calls a halt to the islands' natural tendency to recede. At the same time, suppletion forms a sand supply to satisfy the Wadden Sea's sand hunger created by partial damming and an (accelerated) rise in sea level. Existing policy should take a rise in the cost of coastal maintenance into account for the iong term, since the loss of sand along the island coasts will increase more rapidly due to the considerable intensification of the Wadden Sea's sand hunger caused by an accelerated rise in sea level, sand and gas extraction. The current cost price of sand suppletion averages 10 Dutch guilders per cubic metre.

Researchers suspect that attempts to protect the islands by stabilizing dunes and beaches can eventually stimulate the loss of sediment. Constant withdrawal of sand from the foreshore of the island coasts could lead to a steepening of the foreshore. During storms, this couid result in increased erosion of dunes and beach.

By way of an experiment, additional sand was applied to the underwater shore of the island of Terschelling. The consequent decrease of the offshore beachslope and the extra volume of sand in the zone near to the coast may, in time, cause beaches and dunes to erode more slowiy and, at the same time, this sand suppletion can serve as a supply to satisfy the Wadden Sea's sand hunger. Although this wil! require higher sand input than when and is deposited on the beach, the costs per m3 are lower. Research will have to show whether this alternative to beach suppletion is effective in the long term.

Seaward coastal protection restricts sand flow to the tidal flats

Where coastal erosion is so severe that sand suppletion may not be the most efficiënt solution to preserving the coastline (the Texel Headland, the western point of Ameland and the eastern point of Vlieland), it may be an idea to shift the coastline seawards by building training wal Is or sea groynes. This will probably reduce the withdrawal of sand needed to satisfy the system's sand hunger from the coastal profile due to the action of waves and tide. If so, it will upset the dynamic equilibrium between sand supplied by the island coasts and the sand demand of the Wadden Sea. It is also questionable whether under those condition, the Wadden Sea will receive enough sediment to satisfy the sand hunger caused by damming and the (accelerated) rise in sea level.

5.5 What does the supply and demand balance meart for the tidai basins, tidal flats and salt marshes?

The tida! flats and salt marshes are important ecological and landscape elements of the Wadden Sea. It is, therefore, important to ascertain whether these intertidal areas can cope with an accelerated rise in sea level. Whether these flats and salt marshes can compensate for this rise by elevation again depends on the exchange of sediment between the channels and intertidal area. If the sediment supply to the flats should fail to keep up with the degree of rise in sea level for any reason

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whatsoever, the flats and salt marshes will eventually become inundated. The turning point between continuation or inundation of flats and salt marshes is contingent upon the sediment balance.

Photograph 5.2 Gas extraction near the isfand coast of Ameland

5.5.1 Expected tidal bas in development The tidal basin becomes somewhat deeper on average

We have seen that when the sea level rises (at an accelerated rate), the Wadden Sea attempts to restore dynamic equilibrium by becoming somewhat deeper in relation to the sea level, enabling the basin to trap more sediment. If the sea level rises at an accelerated pace of 60 to 85 cm per century, the tidal basin will gradually deepen over a fifty-year period by an average of 1 -2 dm (in relation to the raised sea level), and by 2-3 dm over a period of a hundred years.

5.5.2 Expected flat development Flats are the preferred areas for sedimentation

Process measurements on and near the shallow flats in the Wadden Sea, Oosterschelde tidal basin and Westerschelde estuary indicate that the elevation of flats can adjust "fairly quickly" to changes and the average high water level. In other words, when the high water level rises, we see that, under the existing conditions of exchange and sediment supply, shallow flats are the preferred places for sediment to settle. Empihcal study of the elevation changes of the Wadden Sea flats in the period 1925-1987 reveal that the flats keep pace with the current rate of sea level rise of an average 20 cm per century. The maximum measured speed of rise by flats in certain parts of the Wadden Sea amount to 80 to 130 cm per century. Viewed over a period of many years, we see that the Wadden Sea flats are predominately becoming elevated. It follows that, given the current supply of sand and conditions of sediment exchange between the channels and flats, the flats uncovered at low tide will be able to keep up with a sea level rise of 20 cm per century by increasing in elevation during the century to come. However, as the Wadden Sea coastline will have to supply the sand, this will go hand in hand with its advancing erosion. The flats remain the preferred locales for sedimentation when the sea level rise is accelerated. The average depth of the flats uncovered at low tide will increase somewhat in relation to the average sea level.

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When there is sufficient sediment coming into the tidal basins, this gradual depth increase of the flats in the time period considered is slight (approx. 1 dm) for the sea level scenarios selected and it is mainly the edges of the flats that deepen, involving scarceiy any loss in surface area of the flats. The overall surface area of the Wadden Sea flats, with the possible exception of the Marsdiep and Ems-Dollard tidal basins, will not change significantly in the next fifty to a hundred years, provided there is no change in the basin's sediment supply. If, however, the sea level rises at a rate of 60 cm or more per century, the situation of the Marsdiep Channel and Ems-Dollard Bay may become such that the sediment demand exceeds the net inflow of sediment, retarding the flats' elevation process. The intertidal zone would no longer rise at the same rate as the sea level and eventually (centuries from now) the flats would become inundated. If the sea level rises at 85 cm per century and the net sediment inflow is halved, the effect on the Marsdiep of tb/s deceJerated flat elevation is calculated to be relatively slight after a 100-year period (approx. 1 dm). Further diminishment of the sediment supply, for whatever reason, consolidates this process of reduction of the intertidal areas. This type of situation could arise in the Ems-Dollard tidal basin, because its sediment demand is large in relation to the inflow of sediment given oirrent mineral extraction, but is unlikely in the other tidal basins, since their sediment demand is lower than the import of sediment.

Besides the influence exerted by an accelerated rise in sea level, the Marsdiep basin is still in the process of adjusting to the damming of the Zuider Sea. The intertidal area of the Marsdiep basin now covers 121 km2. Presumably, once the Marsdiep has regained equilibrium, its flats will eventually (e.g. many centuries from now) have the same percentage of flat area as the Vlie has now (324 km2). Not much is certain about the rate of development of tidal area in the Marsdiep Channel, all the more so because the deep peripheral zones created by damming will first have to be raised. Measurements taken over the last sixty years indicate that an average of 0,3-0,5 km2 of intertidal area will develop in this basin every year. Based on these findings, the expectation is that the Intertidal area of the Marsdiep Channel will increase by several dozen km2, provided the rise in sea level and supply of sediment remain constant for the next hundred years.

It remains to be seen whether the flats can keep up this growth in elevation in an increasingly rough wave climate at an accelerated rise in sea level. This requires more knowledge of the process and research. As we saw in the previous chapter, waves are created by the power of wind and storm. On average, as the force and number of storms increase, the larger waves will occur more frequently in the Wadden Sea basin. More storms, therefore, mean more intensive erosion of the flats. The accelerated relative rise in sea level will contribute to this intensification, since it will lay the flats bare to the erosive action of the waves. If a more severe storm climate should take over, we must take account of the fact that the flats will not be able to keep up fully with the rise in sea level. In the long term, the intertidal area may remain submerged as a result. However, given the fact that the process will span a number of centuries, we expect that the loss in the next 100 years will be minimal.

Whether an accelerated rise in sea level will bring about significant changes in the profile of the sand flats is uncertaïn, but we do not anti- cipate any major changes. Nor is it known whether the rising sea level wili cause the larger flats to fragment while maintaining the same, or virtually the same, total surface.

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What we do know is that the faster the sea level rises, the finer the con- sistency of the material deposited. If, however, the sand hunger in the basin and the sediment supply of the eroding Wadden Sea coastline are in balance, the composition will barely change, if at all.

5.5.3 Expected salt marsh development Salt marshes compensate for the rise in sea leve!

The rate of siltation and the vegetation are interdependent and together determine whether the salt marsh is able to compensate for the rise in sea level by increasing elevation. The rise in sea levei could cause the elevation of the salt marshes to fall and they may shrink as a result. It is, however, uniikely that the area of the mainland salt marshes will develop pioneer vegetation or revert to bare mud-flats. This is partly due to the fact that the capacity of the salt marshes to trap sediment increases the faster the sea level rises. Current siltation in the low and middle- elevation mainland salt marshes averages 130 to 180 cm per century. A future average rise in high water of 20 cm per century presents no problem for the mainland salt marshes, provided the supply of silt and the sedimentation conditions do not alter. Island salt marshes are formed by a relatively minor deposition of silt on a high sand bank or expanse of beach. From the limited siltation data available on island marshes, it is possible to deduce that siltation runs at 30 to 80 cm per century. Whether we can take this siltation data on the island salt marshes as a critical value for their capacity to compensate for a possible accelerated rise in sea level or whether this depends on management, is unclear. If the storm climate increases, we can expect the erosive action of the waves on the salt marshes, particularly in the pioneer zone, to increase in turn. It is still uncertain whether siltation in these areas will be able to keep step with the trend of accelerated rise in sea level under such extreme conditions, and this also partly depends on management strategies.

5.5.4 The effect of salt marsh policy and management on the supply and demand balance

The policy on and management of salt marshes in the Wadden Sea (as laid down in the Primary Decision on Spatial Planning in the Wadden Sea) is designed to preserve the existing area of salt marshes and expand the area of mainland salt marshes in the region west of Holwerd. The Cabinet would like to see this preservation and expansion take place as naturaliy as possible. So, exactly what effect does policy on and management of salt marshes and the peripheral zones have on the supply and demand balance of the sediment of the Wadden Sea?

Stimulating the formation of salt marshes and land accretion on the peripheral zones increases sediment hunger

The stimulation of siltation of the peripheral zones of the Wadden Sea and of the sedimentation areas of the salt marsh works causes the Wadden Sea to shrink and become land more quickly, thereby reducing lts tidal prism and, in turn, increasing its sediment hunger. Fascine dams have proved to have an enormous effect on siltation in the first four years after they are built. During that period, the level of siltation can be as much as 10-20 cm. After the first four years, the process proceeds at a more natural rate. Another effective management measure is to dig ditches in the salt marshes to enhance drainage and allow the salt marshes to increase in size. By stimulating siltation, the salt marshes will be able to cope with an accelerated rise in sea level and perhaps even

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expand, but this will be at the expense of the island coasts and, more notably, the outer deltas. Civen the close relationships between the coastal policy and nature policy, an integral approach to the system would be advisable.

Photograph 5.3 Tidal flats are the preferred areas for sedimentation

"Disempolderment" may reduce the sand hunger and allow the salt marsh environment to recover

Expanding the mainland salt marshes by means of disempolderment leads to an increase in the tidal prism, and, in turn, reduces the demand for sand in the tidal basin, which again reduces coastal erosion. In other words, disempolderment brings about a reduction of the burden placed on outer deltas and expands the area of salt marshes. Where possible, and ecologically sound, it may be advisable to disempolder the summer polders to allow the originally extensive salt marsh environment to recover. Expansion of the area of salt marshes by disempolderment would be worth considering and studying as a management measure, both in terms of coastal policy and nature.

5.6 What does the supply and demand balance mean for the island coasts? Increased erosion of the isiand coasts and outer deltas

A rise in the demand for sediment in the Wadden Sea due to subsidence caused by gas, sand and shell extraction implies additional erosion of the island coasts over time. This extra erosion is supplementary to the demand for sand caused by the rise in sea level. If the basin is partially dammed, which cuts the tidal prism quite abruptly, the outer deltas form the primary source of sediment. Assuming that the composition of the bed of the Wadden Sea will hardly change in the future, we can say that 70 to 80% of the total demand for sediment of 14 - 24 million m3 will have to be compensated for by erosion of the island coasts and outer deltas. For a rough estimate of the degree of retrogradation of the 150-km island coastline and bordering coast of Noord-Holland required to meet the demand for sediment in the basin, we have assumed that the profile

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of the receding coastline remains constant. If the coastal profile is able to recede freely, we can determine the retrogradation of the coast at a given degree of sand hunger.

The present rise in sea level will cause steepening and retrogradation of the coastline by 1 metre per year

Measurement of the development of the low-water line along the coast of the Netherlands over the past 120 years show the average recession of the low-water line along the islands and to the north of Umuiden to be about 0,5-6 m. A small part of this (1-2 dm) can be attributed to the direct effect on the coastal profile of the rise in sea level. For the most part, coastal erosion is congruent with the hunger for sediment in the Wadden Sea. There are, of course, regional variations. In some places, the coast is eroding, while elsewhere it is expanding, such as the island of Schiermonnikoog. However, the general picture indicates that the Wadden Sea islands and part of the neighbouring coastal zone of Noord-Hofland are eroding. Under the 'mfluence of the sediment demand in the Wadden Sea, the retrogradation at the current rise in sea level of 20 cm per century averages 1 m a year. The retrogradation of the island coasts expected in the future at a rise in sea level of 20 cm per century translates into 1 m a year. The consequence of dynamic preservation of the islands' basic coastline is that the foreshore will become steeper in the long run,

An accelerated rise in sea level causes extra retrogradation of1to3m a year

In order to be able to satisfy the basin's extra demand for sand caused by the accelerated rise in sea level, the Wadden Sea coast will have to recede more rapidly. The additional average annual retrogradation at a rise in sea level of 60 cm per century has been estimated at about 1-2 m a year. At the worst-case scenario rise in sea level of 85 cm per century, retrogradation of the coastal profile will average an additional 2-3 m a year. This rate of retrogradation of the coastal profile is in line with results as laid down in the discussion report 'Coastal Defence after 1990'.

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6. Looking Ahead to the Ecology of Tomorrow

6.1 Introduction

Morphological research has provided no indication of radical future change in the potential ecological functions of the Wadden Sea, since it is expected that the size of the flat area will barely decrease in the next 50-100 years. This is also true of the area of salt marshland. It is highly likely that this slight morphological change will have a minimal effect on future life in the Wadden Sea. As noted above, the tidal basins of the Marsdiep and Ems Dollard may not have access to a sufficiently large sediment supply to regain a new state of equilibrium at an accelerated rise in sea level of 85 cm per century. Over a period of 50-100 years, the effects remain slight, but viewed over a much longer time (centuries), the deficit in the sediment balance will cause the flats to sink.

In the long term, minor effects and gradual change could bring about major ecological change, however. After all, an accelerated relative rise in sea level always results in net sedimentation in the Wadden Sea, where the material is deposited as a result of diminished transport capacity. As the relative rise in sea level gains speed, this process will become more pronounced. Sedimentation of sand and silt in the basins and particularly on the flats will doubtless affect the turbidity, the permeability to light, the supply of nutrients and, consequently, the functioning of the ecosystem. What effect this relatively slow change in abiotic factors will have on the ecosystem and how quickly this change will be absorbed into the ecosystem is still not entirely clear.

Furthermore, it is possible that the change in climate will have a more significant ecological impact, causing a change in the temperature regime, for example. However, the possible effects this may have are outside the scope of this research. Apart from the relative rise in sea leve! and climatological change, human activity also plays an important role in the ecosystem, albeit rather difficult to distinguish from natural processes. Many recent ecological changes in the Wadden Sea can be traced back to human activity, as in the case of the IJsselmeer Dam, which put an end to anchovy fishing, and the disappearance of the European oyster from the Wadden Sea as a result of overfish'ing.

6.2 Complex food webs

Our knowledge of the reaction of the ecological system in the Wadden Sea is based on what we know of the complex food webs that are highly dependent on a variety of abiotic factors (Figure 6.1). Changes or shifts in the controlling abiotic factors provide indications of the way in which life on the mud-flats would react in the event of an accelerated rise in sea level. The following sections look briefly at a number of abiotic preconditions affected by the accelerated rise in sea level that are important to primary production and to mud-flat life in subsystems like the salt marshes, tidal flats and the sublittoral.

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Figure 6.1 Complex food webs. Changes in abiotic factors brought about by a rise in the sea level can influence the complex ecosystem of the Wadden Sea.

Primary production forms an uncertain factor in the future

The ecosystem in the Wadden Sea is governed to a large extent by processes that interconnect via material flows, with 'eat to be eaten' as a key aspect. An important link in the basis of the food webs in the Wadden Sea is the primary production of phytoplankton and microphyto-benthos, both of which determine the amount of food available for higher organisms. At an accelerated rise in sea level, the conditions for sedimentation become more favourable. The extraction of sediment from the water reduces the turbidity and increases the permeability to light This may bring about an increase in primary production. Whether this will actually occur is uncertain, because present policy focuses on cutting the discharge of waste into the sea. This will probably lead to a drop in the amount of nutrients available (phosphate and nitrate, hence eutrophication) in the nextfew decades.

Figure 6.2 frequëncy of Odaf floodirig (numbera pee year) If we move from the mud-flats to 2.5 _ the dike. we can distinguish a number of zones: mud-flat, pioneer zone, low salt marsh zone, middie and high salt marsh zones. To a large extent, the elevation determines the duration of sea water inundation, the frequency of tidal inundation and the drainage of the land (after Dijkema, 1987). \ NAP J duration of tidal ffooding ijfc (hours pertide)

Life in the salt marshes keeps up with the rise in sea level

Salt marshes form a complex ecosystem in which both physical and biological factors play a role. Generally speaking, we can distinguish a number of zones if we move from the mud-flats to the dike: mud-flats, the pioneer zone, the low salt marsh zone, the middie salt marsh and the high salt marsh (Figure 6.2). To a large extent, the elevation determines the duration of salt water inundation, the frequency of tidal inundation and the drainage of the land. The relatively sparsely vegetated pioneer zone is a fairly dynamic area that is flooded almost every day.

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During the initial phase of establishment of salt marsh plants, soil aeration and stability are the key factors, while soil salinity and biological factors, such as competition between the plants, become more important as the elevation increases due to siltation. The low salt marsh, which is flooded hundreds of times a year, begins at an elevation of 10-20 cm above Mean High Water. The middle salt marsh zone starts at about 35 cm above Mean High Water. The high salt marshes, which form the highest regions, are only flooded a few times a year. Moving from the pioneer zone to the high salt marshes, the duration of inundation decreases, as does the tolerance of the vegetation to inundation in sea water. Since salt marshes can keep up with the rise in sea level thanks to sedimentation, the frequency of sea water inundation will hardly change at all.

The suitability of the salt marshes as nesting grounds for birds varies from breed to breed. Some types of bird prefer short vegetation and others prefer ta.IL These properties of salt marsh vegetation are far more affected by grazing livestock and mowing than by the effects of rises in sea level. The single effect of rises in sea level on the significance of salt marshes for nesting birds is consequently very slight.

Life on the tidal mud-flats adapts

The tidal mud-flats are an important link in the Wadden Sea's ecosystem. Within the area of the flats, the elevation in relation to the average sea leve! (the duration of inundation per tide) and the type of sediment partly determine the amount of mud-dwelling fauna. The number of mud-dwellers that can Üve on the tidal flats is mainly determined by the total surface area of those flats. The same applies to the amount of food available to foraging birds (while dry) and young fish (while submerged).

In the Wadden Sea, the largest numbers of mud-dwellers and the broadest assortment of species per unit of surface area occur on the flats with an average elevation of between about -7 and +2 dm below or above Normal Amsterdam Level and an average sediment type with a silt content of 8-23%. The high and sheltered flats along the edges of the Wadden Sea, most of which with sediment rich in silt, are important as a breeding ground for mud-dwellers such as the lugworm, the 'nun' (a bivalve mollusc, Macoma balthica) and the sea centipede (Photograph 6.1). These flats can be found in the areas of the former land reclamation works and the zones sheltered against westerly winds just under the islands of the

Photograph 6.1 Along the eclges of the Wadden Sea, most of which with sediment rich in silt, are important as a breeding ground for mud-dwellers such as the lugworm, the 'nun' (a bivalve mollusc, Macoma balthica) and the sea centipede. Many of the birds procure therefore their food along these edges of the Wadden Sea.

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Wadden Sea, and on the mainland coast of Noord-Holland. The flats that are subject to strong dynamic forces, such as those in the outer deltas of the gats, are home to only a few types of worm that have adjusted to that particular environment.

Many of the birds that live on the Wadden Sea procure their food walking or wading on the flats that stand above water at low tide (Figure 6.3). One period of low water is usually insufficient for a bird to find enough food for the day. To compensate for this lack of time, the birds also look for food in the second period of low water. Any fall in the elevation of the mud-flats will reduce the foraging period available to them. Under the present tidal conditions in the Wadden Sea, our rough estimate would be that if the elevation falls by about 1 cm, the foraging period that is lost as a result would be about 3 minutes or, in other words, a maximum of 1 % of the foraging time. The consequences of a reduction in this period of a few per cent would be negligible, if it were not for the fact that the time birds have to eat their fill does not usually permit any flexibility. It appears in practice that there are considerable variations in the supply of food on the individual flats. Roughly speaking, however, the congestion among birds foraging for food is very much correlated to the total biomass of mud-dwellers. This means that a drop in the supply of mud-dwellers would have immediate repercussions on the number of birds using a mud-flat.

Figure 6.3 During a tidal cycle from ebb to flood, it is very busy on the mud- flats. Various kinds of btrds come to eat, Mealtimes and fare are not the same for every type of bird, however (after Swennen, 1982).

Blaïi -heaited gut

Research has shown that at a low level of sedimentation of the mud- flats, the total biomass exceeds that on eroding flats. This is because young shellfish and other food animals have better chances of establishing themselves. On the edges of flats, where the sediment is regularly churned up, the supply of food is generally more limited than on the flats themselves. On the smaller flats with a relatively large edge surface area, the average food supply per surface area unit is less than on the larger mud-flats. A change brought about by a rise in sea level which caused the large flats to fragment, although the total surface area remained constant, would, therefore, lead to a reduction in the food supply. If the sea level rises, the mud-flats will grow as well. The zoning of mud- dwellers will probably be able to adjust fairly well. However, the total number of mud-dwellers will fall if the surface area of the flats diminishes. A complicating factor is a change that could occur in the elevation profile of the flats. If the flats became steeper, some birds would have less time to forage. How the profile of the mud-flats will change as a result of the rising sea level is as yet unknown.

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Photograph 6.2 Worms and shellfish play an important role in the fixation of sand, silt and organic matter. These filter feeders have therefore a positive effect on the stabilization of the tidal flats.

Worms and shellfish play an important role in the fixation of sand, silt and organic matter. These mud-dwellers are able to fix considerable amounts of material with their faeces and pseudofaeces, which can be swirled up later by the waves. Together with other filter feeders, these animals are responsible for part of the sediment retention capacity of the flats, and can stabilize the mud-flats and sandbanks (Photograph 6.2). The microphyto-benthos and bacteria also play a part in increasing the stabilizing effect by secreting slime and cementing the sediment. Research into mussel beds in the Wadden Sea has shown that mussel seed that falls onto the remnants of old beds forms relatively stable beds that are resistant to storms. In years with good crops, mussel beds also grow on sandy substrates. Although these beds are often far more susceptible to storms, a small part of the beds develop into structured old mussel beds with mussels in various age groups. At an accelerated rise in sea level, the filter feeders have a positive effect on the stabilization of the mud-flats.

Photograph 6.3 The high sandy banks perform an essentiai function as a resting place and rearing area for seals and their pups.

For seals, the high sandy banks, directly adjacent to channels, perform an essentiai function as a resting place and rearing area for their pups (Photograph 6.3). The channels supply them with food, such as shrimps and flatfish. If the relative rise in sea level increases, we expect these high-lying sandbanks to endure.

Sublittoral

The sublittoral, the shallow parts of the Wadden Sea that are always submerged, harbours about the same wealth of mud-dwellers as the tidal flats, making it a significant feeding ground for diving ducks such

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as the eider duck and the scaup. The total surface area of the shallow sublittoral detemnines the numbers of mud-dwellers in the area (down to 5 metres beneath mean sea level). In general, the current in this part of the sea is fa i rly slack compared to the deep channels, so that the sediment contains a certain amount of silt. The deeper channels, on the other hand, are much poorer, because the high current speeds make it almost impossible for mud-dwellers to establish themselves. Since the rise in sea level is accompanied by sedimentation, the shallow sublittoral will continue to exist.

For the Wadden Sea birds that feed on the creatures that live on the bed of the sublittoral (many types of fish and crustaceans), the deeper the water, the more energy the diving birds have to expend to reach their food. It is a known fact that an increase in depth of decimetres and metres has a demonstrable negative effect on their food intake. At an expected slight change in the depth of the sublittoral, this negative effect is minor and hardly measurable.

In about 1930, about 150 km2 of eelgrass (Zostera marina) disappeared as a result of an infectious disease. There is currently about 2 km2 of eelgrass left in the Dutch Wadden Sea. The eelgrass never recovered from the disease. The different hydraulic conditions in the western part of the Wadden Sea caused by the construction of the IJsselmeer Dam, combined with severe water turgidity and the effects of drag-net fishing, probably contribute to preventing its recovery. As we have said, the turgidity may diminish at an accelerated rise in sea level, thus improving the conditions for the deveiopment of eelgrass in the future.

6.3 Ecological tolerance

!t is hardly possible to indicate a close relationship of abiotic factors in which Üfe is possible for any of the organisms in the Wadden Sea. In other words, the level of ecological tolerance of life in the Wadden Sea is high. The influence of the tides (salt content, current speed) and the morphological dynamics (conversion of sediment, variation in types of sediment, variation in duration of inundation of the flats) cause the occurrence of a whole range of variation in a 24-hour period. Life in the Wadden Sea has adapted to this variation. Partly and primarily as a result, the ecological effects on the flora and fauna induced by an accelerated rise in sea level are mostly slight. Only if the most unfavourable scenario were to become reality, and the supply of sediment were to diminish, would more and more of the Wadden Sea become permanently submerged. This would mean that in the long term, the habitat could change so much that some sorts of flora and fauna wili no longer feel at home.

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7. Conclusions and recommandations

7.1 The wadden flats in a state of flux?

As we have seen in the previous chapters, we can expect the Wadden Sea to be able to keep up with a relative rise in sea tevel of 20 to 60 cm per century in the coming centuries. Exceptions are the Marsdiep tidai basin and Ems Dollard estuary. Generally speaking, flats and salt marshes will not become permanently inundated, because they can trap sediment effectively. Their sand hunger is satisfied by sand that comes mainly from the island coasts. The composition of the sediment will probably remain unchanged. Only in very extreme situations, at a rise in sea level of 85 cm or more and a sharp increase in wind speeds, will the morphology of the Wadden Sea change to such an extent that the flats disappear forever, but such a situation would take many centuries to evolve. Even subsidence of the seabed as a result of gas extraction will be localized and temporary, and will have hardly any consequences for the morphology of the Wadden Sea as a whole. Reducing sand extraction from the Wadden Sea is advisable. Present sand extraction and extraction in the near future, which will mainly take place in the Marsdiep Channel and Ems-Dollard Bay, still places a substantial load on the sediment economy of these tidai basins. We can, therefore, conclude that a relative rise in sea level will probably affect the morphologicai structure of the Wadden Sea at local level, but that the characteristic dynamic system of channeis and tidai flats will continue to exist in the next hundred years.

Given the high degree of ecological tolerance, the ecological effects on the level of the tidai basins are expected to be limited, localized and temporary. An accelerated rise in the sea level could cause the specific location of habitat zones to shift, but the character of the Wadden Sea ecosystem in itself wil! remain unchanged. Only if the worst-case scenario were to become reality and if the supply of sediment were to be reduced would an ever-larger portion of the wadden flats be submerged over time. The result would be a long-term habitat change and some species would no longer feel at home.

7.2 Recommendations

In light of the fact that the sediment hunger in the Wadden Sea increases as the sea level rises more rapidly, this study has led for the island coast and the unique wadden area to the following general recommendations: (-) Further reduction of sand extraction in the Wadden Sea in general and the Marsdiep and Ems Dollard tidai basins in particular.

(-) Cautious policy on the stimulation of siltation in the peripheral zones and salt marshes. These measures increase the sediment hunger in the tidai basins of the Wadden Sea and tax the sand supplies of particularly the outer deltas. The outerdelta wil! tend to decrease in size.

(-) Expanding the salt marsh area by disempoldering could reduce the demand for sediment of the Wadden Sea and merits further study.

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{-) The Wadden Sea's sediment hunger is satisfied by sand that comes mainly from the island coasts. It is preferable to protect the coast by means of suppletion with sand from outside the wadden system instead of constructing solid coastal works. Seaward coasta! protection restricts the sediment flow to the tida! basins.

(-) We do not yet have sufficient knowledge of the system's processes, especially the exchange of sand and siit between the channels and flats. Further study is required so as to fine-tune forecasts of the behaviour of channels and flats in response to an accelerated sea level rise and to better visualize how the various components of the area will develop. This would also enable us to present a more detailed picture of the ecological implications of changes.

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8. References

The following literature was gratefully used to Dijkema K.S., P. Bouwsema & J. van den Bergs 1989. wrïte this report: Possibilities for the Wadden Sea marshes to survive future sea-fevel rise. In; C.H. Ovesen (ed.), Saltmarsh management in Barth M.C, & J.G, Titus. 1984, Greenhouse effect and sea the Waddensea Region, Proceedings of the Working level rise: a challenge for this generation. Van Norstrand Conference, Römö, Denmark, 10-13 October 1989; Reinhold, New York; 325 pp 125-147 pp

Bartholdy J.A. & M. Pejrup. 1990. Sedimentology of the Dijkema K.S , J.H. Bossinade, P. Bouwsema & R.J. de Glopper. Danish Wadden Sea The Common future of the Wadden Sea. 1990. Salt marshes in the Netherlands Wadden Sea: Rising Expert report. WWF, Husum. high tide levels and accretion enhancement. In: J.J, Beukema, W.J. Wolff & J.J.w M. Brouns (eds), Expected effects of Beets DJ , L. van der Valk & M. i F Stive 1992. Hofocene climatic change on marine coastaf ecosystems Kluwer evolution of the coast of Holland Marine Geology, 103, 423- Academie Publishers, Dordrecht; 173-188 pp. 443 pp Dijkema K S., J van den Bergs, J.H, Bossinade, P. Bouwsema, Beets DJ,A J.F van der Spek & L van der Valk. 1994 R.J. de Glopper & J W.Th.M. van Meegen. 1988. Effecten van Holocene ontwikkeling van de Nederlandse kust. Rijk rijzendammen op de opslibbfng en op de omvang van de Geologische Dienst, RGD rapport 40.016; 46 pp. vegetatiezones in de Friese en Groninger landaanwinnings- werken. Rijkswaterstaat, Directie Groningen, NotaGRAN Beukema J J 1976. Biomass and species richness of the 1988-2010. macrabenthic animals living on the tidal flats of the Dutch Wadden Sea. Netherlands Journal of Sea Research, 10, 236- Eisma D & W.J. Wolff. 1980. The development of the wes- 261 pp. tern most part of the Wadden Sea in historical time In: l< S Dijkema, H.E. Reineck & W.J. Wolff (eds.), Geomorphology of Burgt, van de J.H, 1936. Over de duinen en stuifduinen op de the Wadden Sea Area, Report 1 of the Wadden Sea Working Waddeneilanden, Texel, Vlieland en Terschelling. Group; 95-103 pp. Rijkswaterstaat, Nota 28 februari 1936. Eisma D, 1981. Supply and deposition of suspended matter in Dankers N. & J.J. Beukema. 1981. Distributionaf pattems of the North Sea. Special publication of the International macrozoobenthic species in relation to some environmentai association of sedimentologists, 5; 415-428 pp. factors. In N Dankers, H Kohl & W.J Wolff (eds.), Invertebrates of the Wadden Sea, Balkema, Rotterdam, 69- Gerritsen F. 1956. Enige opmerkingen over de ontwikkeling 103 pp. van de Dollart. Rijkswaterstaat, Nota CSD 55-17.

Dankers N., K 5. Dijkema, P.J.H. Rerjnders &CJ. Smit. 1990. Giesen W.BJ.T. 1990. Wasting disease and present eelgrass De Waddenzee in de toekomst - waarom en hoe te bereiken? condition. Rapport van het Laboratorium voor Aquattsche Rijksinstituut voor Natuurbeheer, RIN-rapport 90/19; 5-112 pp. Ecologie, Katholieke Universiteit Nijmegen; 138 pp.

Dekker R. 1989. The macrozoobenthos of the subtidal wes- Giesen W.B.J.T., M.M. van Katwijk & C. den Hartog. 1990a. tern Dutch Wadden Sea. In Biomass and species richness. Temperature, salinity, insolation and wasting disease of Netherlands Journal of Sea Research, 23, 57-68 pp. eelgrass (Zostera manna L.) in the Dutch Wadden Sea in the 1930s. Netherlands Journal of Sea Research, 25; 395-404 pp. Dijkema K.S 1987. Changes of salt-march area in the Netherlands Wadden Sea after 1600. In: A.H.L Huiskes, Giesen W.B.J.T., M.M. van Katwijk & C den Hartog. 1990b. G.W.P.M. Blom & J. Rozema (eds ): Vegetaton between land Eelgrass condition and turbidity in the Dutch Wadden Sea. and sea, Junk, Dordrecht; 42-49 pp. Aquat. Botany, 37; 71-85 pp.

Gnede J W. & W. Roeleveld. 1982. De geologische en paleogeografische ontwikkeling van het noordelijk zeeklei- gebied. KNAG-Geografisch Tijdschrift, XVI, 5; 439-454.

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Biegel E.J. 1993. Morphological changes due to sea level rise Gerritsen F. 1990. Morphological stability of inlets and tidal in tidal basins in the Dutch Wadden Sea versus concepts channels in the Western Waddensea. Proceedings of the 7th morphological response model MORRES. Institute for Marine international Wadden Sea symposium, Ameland 22-26 and Atmospheric Research Utrecht (IMAU), Oct.1990. Report 93.14; 58 pp. Gerritsen F. 1993. Review of three papers regardingthe effect Biegel E.J. & P. Hoekstra. 1992. Morphologicaf response char- of increased sea level rise on the Wadden Sea. University of acteristics of a tidal basin, Frisian Inlet, the Netherlands Hawan, Department of Ocean Engineering, Summary report, (prep.). In: Tidal Clastics 1992, Wilhemshafen, Volume 151. july 1993.

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Gerritsen F,, T Loutere. 1994. Marsdiep volgen of verdrinken Lambeek J J.P. 1991. Biotic-abiotic relations in the benthic Werkdocument RfKZ\OS-94.157X, systems of the Dutch Waddensea. Rijksuniversiteit Utrecht, Geografisch Instituut, Vakgroep Fysische Geografie, Report Glim G.W., G. Kool, M F. Lieshout & M de Boer. 1986 Erosie GEOPRO 1991.017 (IRO); Rijkswaterstaat, Dienst en sedimentatie in de buitendelta van het Zeegat van het Vlie Getijdewateren, Notitie GWAO 91.10091 1933-1982. Rijkswaterstaat, Directie Noord-Holland, Deelonderzoek nr.7, Rapportage ANWX 86.H210 Ministerie van Verkeer en Waterstaat 1993. Invloed van gaswinning op de Waddenzee Rijkswaterstaat, Dienst Glim C.W., G Kool, M.F. Lieshout & M de Boer. 1987. Erosie Getijdewateren, Directie Groningen, Directie Friesfand, en sedimentatie in de binnendelta van het Zeegat van Texel Directie Noord-Holland; 33 pp. 1932-1982 Rijkswaterstaat, Directie Noord-Holland, Deelonderzoek nr.1, Rapportage ANWX 87.H201. Mulder J P M 1992. Effects of an increased sea level rise on geomorphology and ecological functioning of the Wadden

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Glim GW.N.de Graaff, G. Kool, M.F. Lieshout & M de Boer. Nijkamp H. & C. Slegtenhorst. 1992. Some representatie 1988. Erosie en sedimentatie in de binnendelta van het Zeegat species of the Wadden Sea and their dependence on the van het Vlie 1933-1983. Rijkswaterstaat, Directe Noord- (physical) environment. Report ISOS*2, November 1992. Holland, Deelonderzoek nr3, Rapportage ANWX 88 H204 Project Group Wade 1992 Effects of an increased sea levei Glim G.W , G Kool, M.F. Lieshout & M de Boer 1989. Erosie rise on geomorphology and ecological functioning of the en sedimentatie in de binnendelta van het Eijerlandse Gat Wadden Sea. 1934-1982. Rijkswaterstaat, Directie Noord-Holland, Rijkswaterstaat, Dienst Getijdewateren, Progress report Project Deelonderzoek nr.2, Rapportage ANWX 89.H202. Group WADE, GWAO-92.197X, November 1992.

Glim G.W , G. Kool, M.F. Lieshout & M de Boer. 1990. Erosie Rakhorst H D , G Kool & M.F. Lieshout. 1992. Erosie en en sedimentatie in de buitendelta van het Eijerlandse Gat en sedimentatie in de buitendelta van het Zeegat van Ameland aangrenzende kuststroken 1926-1983. Rijkswaterstaat, en aangrenzende kuststroken 1926-1989 Rijkswaterstaat, Directie Noord-Holland, Deelonderzoeken nr.6 en nr 10, Directie Noord-Holland, Deelonderzoek nr. 12, Rapportage Rapportage ANWX 90.H204. ANV-92 201

Hoozemans F.J.M. 1990 'Effecten zeespiegelstijging op mor- Spek, van der A.J.F. & DJ. Beets. 1992. Mid-Holocene fologische ontwikkeling van de Waddenzee'- Werkplan ISOS-2, evolution of a tidal basin in the western Netherlands1 a model for future changes in the northern Netherlands under Kleef, van A W 1991. Inventarisatie meetgegevens conditions of accelerated sea level rise. Sedimentary Geology, Waddenzee Rijksuniversiteit Utrecht, Geografisch Instituut, Proc. SEPM/IGCP 274 Res Conf. Quart. Coast. Evol., Vakgroep Fysische Geografie, Report GEOPRO 1991.04 Talahassee, Flonda, May 1991. (IRO), Rijkswaterstaat, Dienst Getijdewateren, Notitie AOFM- 91 10.010

Kleef, van A.W. 1991 Empirical relationships for tida! inlets, basins and deltas. Rijksuniversiteit Utrecht, Geografisch Instituut, Vakgroep Fysische Geografie, Report GEOPRO 1991 019 (IRO).

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The Riddle of the Sands 68 National Institute for Coastal and Marine Management/fi//CZ

Colophon

Text contributions: dr. K. Essink (RIKZ), Prof dr ir F. Gerritsen (University of Hawaii), drs W Groenewoud (Meetkundige Dienst), dr. J.P.M Mulder (RIKZ), drs. A.P. Oost (University of Utrecht), dipl-ing G K Lorenz (Meetkundige Dienst}, drs T. Louters (RIKZ), ir. J.C. de Ronde (RIKZ)

Authors: drs. T. Louters (Projectmanager (SOS*WADDEN), Prof.dr ir. F Gerritsen

Editors: drs, T. Louters, Prof.dr.ir. F Gerritsen, dr. J.P.M. Mulder, drs. R. Hisgen (Direct Dutch b.v.)

Lay out: M.A Naber, B. Immers, W. Storm (Section Visual lay out, RIKZ)

Acknowledgement: Many have contributed to this report, which is a reflection of studies commissioned by RIKZ within the framework of ISOS*Wadden. The authors want to express their sincere appreciation to all who have contributed to this study. In pacticufar they acknowledge contributtons from the following individuals- RIKZ, Rijkswaterstaat; drs. R Misdorp, drs. F.H.LM. Steyaert, ir. T van Heuvel, P.C. Beukenkamp, A W. Hokke, Mw. A.M. Walburg and Directorate Noord-Hofland, Rijkswaterstaat; ir H.D Rakhorst, M. de Boer, G.W. Glim, G. Kool, M.F. Lieshout, G. Rookhuyzen, D.L Ulm. Special thanks go to those individuals who cooperated with RIKZ in the project in special tasks. University of Utrecht (RUU); dr. P.G.E.F. Augustmus, drs. E J Biegel, drs. A.W, van Kleef (presently Directorate Zeetand), drs. J.J.P. Lambeek (presently Delft Hydraulics), Delft Hydraulics, ir. W.D. Eysink, drs FJ M. Hoozemans, AIDEnvironment (SEA Division); drs. H. Nijkamp, drs. C. Slegtenhorst, National Geological Service (RGD); dr.D.J. Beets, drs A.J.F, van der Spek, dr. R.T.E. Schüttenhelm, Technical University of Delft (TUD), ir A van Dongeren, University of Miami; Prof dr ir, J van de Kreeke. Finally smcere appreciation is due to the National Research Program (NRP) represented by; dr.ir. T. Schneider (programleader}, drs. S. Zwerver (secretary Global Airpoflution and Climatic Change) and Prof.dr. W.J. Wolff ((IBN-DLO)- coördinator Wadden-theme).

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