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