Towards a Mud Balance for the Trilateral Wadden Sea Area: Mud supply, transport and deposition Report Towards a Mud Balance for the Trilateral Wadden Sea Area: Mud supply, transport and deposition Report Colophon Authors Albert Oost, Ana Colina Alonso, Peter Esselink, Zheng Bing Wang, Thijs van Kessel and Bas van Maren With contributions from Theo Gerkema, Eelke Former, Julia Vroom, Andreas Wurpts, Yoeri Dijkstra, Matias Duran-Matute, Frank Kösters, Maarten Kleinhans, Bas Borsje, Hans Burchard, Emil Stanev, Peter Milbradt, Piet Hoekstra, Hein Sas and Michiel Firet Graphic design cover BW H ontwerpers Photography cover Luchtinspectie Rijkswaterstaat (Ems estuary seen from above the Dollard) ISBN 978-94-90289-57-7 Report 2021-02 This research has been scientifically supervised by the Wadden Academy and financed by the Programme Towards a Rich Wadden Sea and the Wadden Academy Published by Wadden Academy © Wadden Academy march 2021 Contact person Klaas Deen Executive Secretary T +31 (0)58 233 90 31 E [email protected] www.waddenacademie.nl The Wadden Academy is funded by the Wadden Fund. FOREWORD Muddy sediments are a vital element for the Trilateral Wadden Sea ecosystem and mud, both in the form of suspended particles in the water column as well as substrate on shoals, tidal flats and salt marshes, plays an important role in providing and developing ecosystem-services. For many centuries the import and deposition of mud has significantly contributed to the silting up and evolution of tidal basins in the Wadden Sea in response to tidal processes and long-term sea level rise. Mud accumulates in the high intertidal and supra-tidal zone and contributes to the growth of tidal flats and salt marshes. Likewise, mud dynamics and deposition plays an important role in the development of major estuaries in the Wadden Sea region. Mud has various impacts on the food web of the Wadden Sea. High concentration of fine-grained suspended matter, for example, may become a limiting factor for primary production of phytoplankton (pelagic microalgae) by increasing turbidity levels and reducing the penetration of light in the water column. High densities of microphytobenthos (benthic microalgae) are, however, generally found on the more muddy parts of the tidal flats, amongst others due to higher concentration of nutrients in the pore waters of finer sediments. Fine suspended sediment in the water column may hamper the uptake of food for suspension-feeders such as mussels and cockles, whilst deposit-feeding benthos such as mudsnails prefer muddy substrates. For already a long time, mud dynamics in the Wadden Sea is heavily affected by human activities and interventions such as land reclamation, channel deepening for navigation, maintenance dredging and more recent attempts to locally extract or demobilize mud in tidal basins and estuaries to reduce turbidity. Surprisingly though a more or less comprehensive overview of mud dynamics in the Trilateral Wadden Sea is lacking. This omission has inspired both the Wadden Academy and “Programma naar een Rijke Wadden Zee” to invite Deltares to launch a study to identify the major sources, sediment pathways and sinks of mud in the Trilateral Wadden Sea. This report provides the first mud balance for the Trilateral Wadden Sea, making use of a range of data sources such as existing literature, bathymetric charts, sediment distribution maps and measurd deposition rates. We are confident that the report is highly informative and may act as a kind of benchmark study for future work. We wish you pleasant reading! Prof. dr. Piet Hoekstra Hendrikus Venema, MSc Portfolio Geosciences and Climate Program manager Programma naar Wadden Academy een Rijke Waddenzee (PRW) Towards a Mud Balance for the Trilateral Wadden Sea Area Summary The Trilateral Wadden Sea is an extensive barrier-lagoon system composed of three main estuaries and a series of tidal basins, covering Dutch, German, and Danish territory. Although being an important nature reserve, the area also provides important economic services (through fisheries, tourism, and shipping). Because of these economic services, resulting in deepening, land reclamations and flood defences, the area is strongly influenced by human interventions – especially in the three main estuaries (the Elbe, Weser and Ems). The sedimentary processes in tidal basins are dominated by sandy and muddy sediments, with the largest mud content in the shallow tidal flats on the landward side and on the tidal divides. Most of the mud supplied to the Wadden Sea originates from the Straits of Dover, via the so- called North Sea Continental Flow. Additional but smaller contributions are from the IJsselmeer, the Ems, Weser and the Elbe Rivers, local sources and aeolian deposition. This report provides the first mud balance for the Trilateral Wadden Sea, providing detailed estimates for mud sinks, sources, and transport using a combination of existing literature, bathymetric charts, sediment distribution maps, observed deposition rates, and dredging information. The total mud supply to the Trilateral Wadden Sea is estimated at 12.1 to 16.5 million ton/yr. Mud is mainly deposited on the upper tidal flats connected to the mainland of the Trilateral Wadden Sea, in the tidal marshes, and in the sheltered embayments, providing a sink of sediments. An additional sediment sink is sediment extraction, whereby sediment is dredged and placed on land. The total amount of mud deposition and extraction is estimated at 10.8 to 11.3 million ton/yr. This implies that currently, the mud sources are larger than the mud sinks, but not much larger. Sand-mud mixtures are either sand-dominated or mud-dominated, resulting in spatial segregation of sand and mud. Especially the inland embayments such as the Dollard, the Leybucht and the Jade Bay are characterised by high mud contents. This segregation is an important characteristic of the system, especially when evaluating the response of the system to sea-level rise and local anthropogenic disturbances. At present, the (mud-dominated) upper tidal flats and salt marshes are accreting at a faster rate than sea-level rise, while some subtidal sections are eroding, especially in the German Wadden Sea. It remains unclear to what extent this redistribution is the result of the observed increase in tidal amplitude or mean sea-level, and therefore related to local human interventions or global sea-level rise. The most important human interventions influencing mud dynamics are land reclamations and channel deepening. Historic land reclamations resulted in a major loss of sediment sinks where fine sediments naturally accumulate. This loss in sediment accommodation space resulted in more sediment remaining in suspension and therefore higher sediment concentration and more eastward transport of sediment. Both land reclamations and channel deepening (for navigation purposes) lead to amplification and asymmetry of the tides, promoting landward transport of sediments in the estuaries. iii Towards a Mud Balance for the Trilateral Wadden Sea Area How ong-term sea-level ri ll impa eposition o mud i presen ifficul to redict. short-term basin will probabl keep a wi sea-level rise ulting n increased edim deposition This ould mpl rogressively mor mud uld e deposit in west Wadd Sea. total moun o ud transported thin system limit and efo mud ailability m becom limit in East Wadden Sea. high sea-level ri th system m partl dro ch may elerat resul o d-mud interaction nstance, disappearan o dy sheltering e mudfla may o erosion f iously rading udfla resulting reasing ud deposition ove tim oweve the prediction imited y ou present-da understanding h system sponse over h full xten o h tri-lateral Wadd iv Towards a Mud Balance for the Trilateral Wadden Sea Area Table of contents 1 INTRODUCTION 1 1.1 Background 1 1.2 Research questions 1 1.3 Study area 3 1.4 Report outline 4 2 A GENERAL DESCRIPTION OF THE STUDY AREAS 5 2.1 The Dutch Wadden Sea area 5 2.2 The Ems estuary 11 2.3 The Lower Saxony Wadden Sea 13 2.4 The Weser estuary 17 2.5 The Elbe estuary 20 2.6 The Schleswig Holstein Wadden Sea area 22 2.7 The Danish Wadden Sea 30 3 A BRIEF OVERVIEW OF MAIN PROCESSES 35 3.1 Determining factors and timescales 35 3.2 Gross and net fluxes 36 3.3 Residence time of mud 37 4 SUSPENDED SEDIMENT CONCENTRATION 39 4.1 Spatial variations 39 4.2 Temporal variations 43 5 MUD AVAILABILITY IN THE SEDIMENT BED 49 5.1 Dutch Wadden Sea 49 5.2 Ems-Dollard estuary 50 5.3 Lower Saxony Wadden Sea 50 5.4 Weser Estuary 52 5.5 Elbe Estuary 52 5.6 Schleswig Holstein Wadden Sea 53 5.7 Danish Wadden Sea 53 5.8 Sand-mud segregation 54 5.9 Sand and mud in the Wadden Sea: two different worlds? 54 6 TOWARDS A MUD BUDGET 57 v Towards a Mud Balance for the Trilateral Wadden Sea Area 6.1 Mud sedimentation in the basins 57 6.2 Mud sedimentation in saltmarshes 65 6.3 Dredging activities 71 6.4 Internal reworking 75 6.5 The North Sea 77 6.6 Synthesis: a mud balance for the Wadden Sea and North Sea 88 6.7 Discussion: uncertainties 90 7 HUMAN INTERVENTIONS 93 7.1 Introduction 93 7.2 Poldering and closure works 93 7.3 Deepening and port construction 94 7.4 Landfills and sediment extraction 98 7.5 Managed realignment and depoldering 101 7.6 Foreland marsh development 103 8 SEA-LEVEL RISE 105 8.1 Past mud sedimentation 105 8.2 Present-day mud sedimentation: effect of tidal amplitude and storms 107 8.3 Future mud sedimentation 109 9 MODELLING PRACTICES: FOCUSING ON MUD 113 9.1 Process-based models 113 9.2 Aggregated models 116 9.3 Idealised models 118 10 DISCUSSION AND CONCLUSIONS 119 10.1 Discussion of the research questions 119 10.2 Testing the hypothesis 125 11 REFERENCES 129 12 WADDEN SEA RESEARCHERS AND PRACTITIONERS 153 vi 1 INTRODUCTION 1.1 BACKGROUND Recent stimate (Colina onso al.