Introduction The Wadden Sea is the world’s largest uninterrupted system of tidal flats and barrier islands. Over a period of more than 7000 years a wide variety of barrier islands, channels, sand and mud flats, gullies and salt marshes formed under a temperate climate, rising sea level, and, especially during the last century, human interventions. The geological evolution of the Dutch coast showed that the Holocene Wadden Sea could keep up with relative sea- level rise. Relative sea-level rise caused expansion of the basin, which increased the volume of sediment accommodation space and generated a net landward sediment transport. Erosion of the adjacent shorelines contributed significantly to the sediment supply, leading to landward retreat of the entire barrier-inlet- basin system with conservation of its basic characteristics (see e.g. Van Straaten, 1975; Flemming & Davis, 1994). In recent times, increasing population density, extensive coastal development and shoreline protection structures such as dykes, seawalls, revetments, groins and jetties have increasingly impacted or constrained the natural dynamics of many coastal systems including the Wadden Sea. sea-level rise is likely to accelerate in the future due to global warming, (e.g., Meehl et al., 2007). Field observations suggest that some systems remain stable as sediment import, tidal-flat and salt marsh accretion can keep pace with certain rates of relative sea-level rise (Nichols, 1989; Van der Spek & Beets, 1992; Cahoon et al., 2000; Morris et al., 2002; Bartholdy et al. 2007; Madsen et al., 2007), while other systems degrade and finally drown (Kennish, 2001; Van Wijnen & Bakker, 2001). It is however uncertain whether, or how, anthropogenic pressure might impede the ability of natural systems, such as the Wadden Sea, to respond to changing conditions in the future, especially in the 293 Netherlands Journal of Geosciences — Geologie en Mijnbouw | 91 – 3 | 2012 Netherlands Journal of Geosciences — Geologie en Mijnbouw | 91 – 3 | 293 - 310 | 2012 Morphodynamic development and sediment budget of the Dutch Wadden Sea over the last century E.P.L. Elias 1,* , A.J.F. van der Spek 1,2,3 , Z.B. Wang 1,2 & J. de Ronde 1 1 Deltares, P.O. Box 177, 2600 MH Delft, the Netherlands. 2 Faculty of Civil Engineering and Geosciences, Delft University of Technology, PO Box 5048, 2600GA Delft, the Netherlands. 3 UNESCO-IHE, Institute for Water Education, PO Box 3051, 2601 DA Delft, the Netherlands. * Corresponding author. Email: edwin.elias@deltares.nl. Manuscript received: May 2011, accepted: May 2012 Abstract The availability of nearly 100 years of bathymetric measurements allows the analysis of the morphodynamic evolution of the Dutch Wadden Sea under rising sea level and increasing human constraint. The historically observed roll-over mechanisms of landward barrier and coastline retreat cannot be sustained naturally due to numerous erosion control measures that have fixed the tidal basin and barrier dimensions. Nevertheless, the large continuous sedimentation in the tidal basins (nearly 600 million m 3 ), the retained inlets and the similar channel-shoal characteristics of the basins during the observation period indicate that the Wadden Sea is resilient to anthropogenic influence, and can import sediment volumes even larger than those needed to compensate the present rate of sea-level rise. The largest sedimentation occurs in the Western Wadden Sea, where the influence of human intervention is dominant. The large infilling rates in closed-off channels, and along the basin shoreline, rather than a gradual increase in channel flat heights, render it likely that this sedimentation is primarily a response to the closure of the Zuiderzee and not an adaptation to sea-level rise. Most of the sediments were supplied by the ebb-tidal deltas. It is, however, unlikely that the sediment volume needed to reach a new equilibrium morphology in the Western Wadden Sea can be delivered by the remaining ebb-tidal deltas alone. Keywords: Wadden Sea, morphodynamics, tidal basins, ebb-tidal deltas, impact large-scale engineering works