The world’s coasts and deltas are progressively threatened by climate change and human activities. The degree at which coastlines can adapt to these changes strongly depends on the sediment availability. The availability of muddy sediments is however poorly known. This study aims at developing a mud budget for the world’s largest system of uninterrupted tidal flats: the Wadden Sea. The resulting mud budget is nearly closed: ~ 12 million ton/year enters the system on its western end, ~ 1.5 million ton/year is added by local rivers, while ~ 12 million ton annually deposits or is extracted by anthropogenic activities. A mud deficit already exists in the downdrift areas, which will only become more pronounced with increased sea level rise rates. Mud is thus a finite resource similar to sand, and should be treated as such in sediment management strategies. Resolving future challenges will therefore require a cross-border perspective on sediment management.@en

Effective adaptation to sea-level rise is critical for future flood protection. Nature-based solutions including salt marshes have been proposed to naturally enhance coastal infrastructure. A gently sloping grass-covered dike (i.e. Wide Green Dike) can be strengthened with clay accumulating locally in the salt marsh. This study explores the feasibility of extracting salt-marsh sediment for dike reinforcement as a climate adaptation strategy in several sea-level rise scenarios, using the Wide Green Dike in the Dutch part of the Ems-Dollard estuary as a case study. A 0-D sedimentation model was combined with a wave propagation model, and probabilistic models for wave impact and wave overtopping. This model system was used to determine the area of borrow pits required to supply clay for adequate dikes under different sea-level rise scenarios. For medium to high sea-level rise scenarios (>102 cm by 2100) thickening of the clay layer on the dike is required to compensate for the larger waves resulting from insufficient marsh accretion. The model results indicate that for our case study roughly 9.4 ha of borrow pit is sufficient to supply clay for 1 km of dike reinforcement until 2100. The simulated borrow pits are refilled within 22 simulation years on average, and infilling is projected to accelerate with sea-level rise and pit depth. This study highlights the potential of salt marshes as an asset for adapting flood defences in the future.

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