The Riddle of the Sands

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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

The Riddle of the Sands

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).

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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

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National Institute for Coastal and Marine Management/WKZ

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

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National Institute for Coastal and Marine Management/R/KZ 6. 6.1 6.2 6.3 7. 7.1 7.2 8.

Looking Ahead to the Ecology of Tomorrow Introduction

Complex food webs Ecological tolerance

ConclusJons and recommandations The wadden flats in a state of flux? Recommendations

Ref eren ces Colophon 57 57 57 62 63 63 63 65 69

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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 wad-den 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 2 1 0 0 ? : Uncovered flats and salt marshes in the Dutch Wadden Sea

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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 addi-tional 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, shal-low 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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National Institute for Coastal and Marine Management/fl//CZ

2. 'Unseen Forces': Rising Sea Level,

Subsiding Sea Floor, Tide

r

Wind and Waves

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.

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).

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 morpho-logical development.

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National Institute for Coastal and Marine Management/RIKZ

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.

Figure 2.2

Curve of the relative sea level rise in the Holocene indicates the chan-ge in the averachan-ge 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 than 5 to 30 centimetres per century. BC 6,000 Mesolithic 8,000 4,000 Neolithic 6,000 2,000 Bronze Age 4,000 Iron Age 0 Roman Age 2,000 Middle Age 2,000 Modern Age 0 AA c14 in calendar years

- years prior to day

-15 20 -Louwe Kooijmans (1976) Jelgersma (1979) Van de Plassche (1982) Figure 2.3

The average high-water curve for the southern North Sea (according to Jensen et al., 1993) and the average global temperature (accor-ding to Barth &Titus, 1984) over 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).

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) 1.3 1.0 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)

« 3 0 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

F i gure 2.6

Forecast of ultimate subsidence due to extraction from existing gas fields and prospective new gas fields (as 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 extrac-tion of gas reserves will cause subsidence of the Wadden Sea floor for another 20 to 40 years. Calculations show that the average total subsidence resulting from gas extraction will be greatest in the sediment retention areas of Borndiep Channel (3 cm), Pinkegat (15 cm), Frisian Gat (5 cm) and LauwersGat (6 cm).

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.

Schiermo ïn Itoot

In addition to the natural subsidence, a smaller-scale process of accelera-ted subsidence is underway, initiaaccelera-ted directly by human activity. On a local scale, these anthropogeomorphic changes are a result of sand, and, to a lesser degree, shell extraction. On a regional scale, ground subsi-dence is caused by gas extraction. This subsisubsi-dence of the sea floor deve-lops gradually in the form of a lat saucer its maximum value occuring in the middle and from the centre its value decreasing towards the sides. The largest gas field (Siochteren) is expected to bring about a subsiden-ce 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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National Institute for Coastal and Marine Management/R/K2

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 already underway at Slochteren, Ameiand-Oost, Zuidwal and Blija.

2.4 Tide

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

Present characteristics of the tidal inlet systems of the Dutch Wadden Sea tidal basins I Marsdiep Channel II Eijerlandsche Gat III VlieCat IV Borndiep Channel V Pinke Gat VI Frisian Gat

VII Eijerlanderbalg Creek VIII Lauwers Gat IX Schild X Ems-Dollard Bay average tidal prism in million m3 1054 207 1078 478 100 200 70 160 31 1000 sój-face area at mean high water in km2 712 153 668 309 65 130 55 145 :' 29 520

surface area uncovered at mean low water in km3 121 106 323 1ë5 42 82 28 92 t$ 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

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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 green-house 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.

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Figure 2.9

Climate models predict that depression activity in areas north of the 45th parallel will increase if the

CO3 in the atmosphere doubles.

For the Netherlands, this means an increase in the number of severe storms and of the average wind velocity. Measurements taken in severe storms in the period from 1910-1993 (RoyalDutch Meteorological Institute, KMNI, 1993) do not reveal a change in the wind regime. The storms of today are just as severe and occur as frequently as in the past century. 180 r 160 | -140 ;ïï -S 120 t~' E (

* j .

100 \ü 1921 •

jü maximum gust of wiïïd ~j

1944 estimated value lu maximum hourly value

i of wind speed G1990 1973 D 1953 n • • o a 13-1-'93 80 L— 1920 1940 1960 1980 2000 time (years) 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).

Figure 3.1

The Jelgersma (1979) curve of refa-tive sea level rise expressed in calendar years and in C14 time scale(Beetsetal.,1994). 10.000 6,900 4,900 8,000 6,000 2,500 4,000 NAP-k; (m) • - 5 - [

s

- 1 0 •-2,000 in calerdar year

C14-years prior to the present

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 cal-led 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 gradual-ly developing barrier bars and dunes, a fairgradual-ly stable coastline with tidal inlets and channels was formed in what are now the Noord-Holland and

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Figure 3.3a

The Wadden Sea area approximately 7,000 (C14) years ago (i.e 5,800 BC). The sea level was several metres below the current level. Initially it rose rapidiy (afew metres per century) as a result of the melting of the ice caps. Erosion caused the coast to rapidfy shift in a landwards direction and large portions of the Wadden Sea area were inundated.

Figure 3.3b

The Wadden Sea area approximately 5,300 (C14) years ago (i.e 4,000 BC). Formation of a more or less stable coastline with tidal infets and channels and, behind that, a zone with tidal flats, salt marshes and, in the higher-lying swampy areas, peat bogs. The sea level was about 4 m below NAP and rose by approximately 80 to 40 cm per century.

Figure 3.3c

The Wadden Sea area approximately 3,700 (C14) years ago (i.e 2,100 BC). The sea level rose by between 40 and 20 cm per century.

Figure 3.3d

The Wadden Sea area developed between 500-700 AD to the intertidal area as we knew it before the Zuider Sea was dammed. The sea level rise equalled some 5 to 30 cm per century.

Open water

(salt water or fresh water)

Fresh water deposits

Intertidal area

Coastal dunesand beaches

Peat bogs: raised bogs and blanket bogs

Salt marshes deposits

Pleistocene deposits

Not induded

Hypothetical borders

Figures 3.2; 3.3a to 3.3d are derived from a study by Zagwijn (1986) of the National Geological Service and pro-vide a reconstruction of the relief under the Dutch coastal regio n.

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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 exis-ting dunes to serve as dikes, construcexis-ting 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 wad-den system is still far from tamed; its landscape is in a constant state of flux, as demonstrated by the shifting positions of islands, island head-lands (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.

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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 rece-ding tide. It does not fall to them to keep herds and live on miik like the neighbou-ring 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

swallowed up by the river Ems. LHiO Wgh-Watermark41921-1930Watermak 5 1930-1956 6 a f t e n 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

Lessons from the past. The history of the Wadden Sea teaches us that as long as the sand supply of the islands and the sedi-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 wadden system is fixed, but also that the coastline of the islands remains the same. The current 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 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 equilibrium between the sand supply of the islands and sedi-mentation in the tidal flats might be dramatically upset.

island

saltmanh

arthSea Wadden Sea

3,000 BC

1;,000 BC

islam!

1,000 AD

After ,1990;

sea level rise sedimentation erosion

dynamic stabiliiation: supplemental sand or seaward coastal protection dike

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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 accompa-nied 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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Figure 4.3

The annual quantity of sand and silt that are transported in and out 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.

Frisian gat spring tide 4 April 1991 silt and sand transport

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 move-ments) 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 develop-ments 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 some-what 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 sedi-ment 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 decelerate. This results in an exponential decrease of the sand transport and in sedimentation in the Wadden Sea.

sandtransportcapacity

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.

Figure 4.5

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. dynamic equilibrium adjustment period dynamic equilibrium 00 60 40 20

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 simula-tions 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 sedi-ment is given the opportunity to deposit sedisedi-ment in the tidal basin. The ebb current is not strong enough to carry out the total quantity of sedi-ment carried in. The result is net sedisedi-mentation 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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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.

volume changes cumulative in millions of cubic metres

-30

1970 1975 1980 ,. 1985

time in years

1990

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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National Institute for Coastal and Marine Management/R/K<?

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 dam-ming, 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.

The Riddle of the Sands