The estuaries in the SW Netherlands, a series of distributaries of the rivers Rhine, Meuse and Scheldt known as the Dutch Delta, have been engineered to a large extent as part of the Delta Project. The Voordelta, a coalescing system of the ebb-tidal deltas of these estuaries, extends c.10 km offshore and covers c.90 km of the coast. The complete or partial damming of the estuaries had an enormous impact on the ebb-tidal deltas. The strong reduction of the cross-shore directed tidal flow triggered a series of morphological changes that continue until today. This paper aims to give a concise overview of half a century of morphological changes and a sediment budget, both for the individual ebb-tidal deltas and the Voordelta as a whole, based on the analysis of a unique series of frequent bathymetric surveys. The well-monitored changes in the Voordelta, showing the differences in responses of the ebb-tidal deltas, provide clear insight into the underlying processes. Despite anthropogenic dominance, knowledge based on natural inlets can still explain the observed developments. Complete damming of the three northern estuaries Brielse Maas, Haringvliet and Grevelingen resulted in a regime shift, from mixed-energy to wave-dominated, and sediments are transported in landward and downdrift direction. This results in large morphodynamic changes – sediments are redistributed from the delta front landward – but small net volume changes – a 0.1–0.2 × 106 m3 a−1 increase in volume over the period 1965–2010 – since the dams block sediment transport into the estuaries. Sediment volume losses of 106 m3 a−1 are observed on the ebb-tidal delta of the partially closed Eastern Scheldt and still open Western Scheldt estuary. As a result of a reduction of the estuarine tide in the mouth of the Eastern Scheldt, the north–south-running North Sea tidal wave has gained impact on its ebb-tidal delta, which causes morphological adjustments and erosion of the Banjaard shoal area. Moreover, the Eastern Scheldt ebb-tidal delta delivers sediment to its neighbours. The stable ebb-tidal delta configuration in the Western Scheldt, despite major dredging activities, illustrates that these large inlet systems are robust and resilient to significant anthropogenic change, as long as the balance between the dominant hydrodynamic processes (tides and waves) does not alter significantly.

This paper describes the background of five palaeogeographical maps between the Marsdiep and the Weser River, and discusses the natural and anthropogenic processes driving the coastal changes during the last part of the Holocene. Before 2500 BC, during the first half of the Holocene, tidal basins were formed in the lower lying Pleistocene valley system as a result of the Holocene sea-level rise. The tidal basins were filled during the second half of the Holocene and on the deposits from the Pleistocene in the hinterland large coastal peat bogs developed. These peat bogs were vulnerable and sensitive to marine ingressions when the peat surface subsided due to drainage, compaction and erosion. During the Subatlantic (450 BC to present), the different ingression systems in the coastal area between Marsdiep and Weser had their own histories in timing and evolution. The ingressions were naturally caused by lateral migration of coastal barrier and tidal-inlet systems or by changes in the natural drainage system in the hinterland. From the Late Iron Age onwards, humans started to be the major cause of ingressions. By reclaiming and cultivating the seaward margins of coastal peat bogs, these areas subsided significantly and were flooded by high storm surges. When coastal areas were embanked during the historical period, the situation for the lower lying peat lands became more dramatic. When the sea dikes breached, the peat land was flooded, leading to casualties and huge material damages and loss of land. Drowning of the peat lands of the Jade and Dollard in the 14th and 15th centuries are examples of such catastrophes.