Sediment budget and morphological development of the Dutch Wadden Sea: impact of accelerated sea-level rise and subsidence until 2100

Abstract

The Wadden Sea is a unique coastal wetland containing an uninterrupted stretch of tidal flats that span a distance of nearly 500km along the North Sea coast from the Netherlands to Denmark. The development of this system is under pressure of climate change and especially the associated acceleration in sea-level rise (SLR). Sustainable management of the system to ensure safety against flooding of the hinterland, to protect the environmental value and to optimise the economic activities in the area requires predictions of the future morphological development.The Dutch Wadden Sea has been accreting by importing sediment from the ebb-tidal deltas and the North Sea coasts of the barrier islands. The average accretion rate since 1926 has been higher than that of the local relative SLR. The large sediment imports are predominantly caused by the damming of the Zuiderzee and Lauwerszee rather than due to response to this rise in sea level. The intertidal flats in all tidal basins increased in height to compensate for SLR.The barrier islands, the ebb-tidal deltas and the tidal basins that comprise tidal channels and flats together form a sediment-sharing system. The residual sediment transport between a tidal basin and its ebb-tidal delta through the tidal inlet is influenced by different processes and mechanisms. In the Dutch Wadden Sea, residual flow, tidal asymmetry and dispersion are dominant. The interaction between tidal channels and tidal flats is governed by both tides and waves. The height of the tidal flats is the result of the balance between sand supply by the tide and resuspension by waves.At present, long-term modelling for evaluating the effects of accelerated SLR mainly relies on aggregated models. These models are used to evaluate the maximum rates of sediment import into the tidal basins in the Dutch Wadden Sea. These maximum rates are compared to the combined scenarios of SLR and extraction-induced subsidence, in order to explore the future state of the Dutch Wadden Sea.For the near future, up to 2030, the effect of accelerated SLR will be limited and hardly noticeable. Over the long term, by the year 2100, the effect depends on the SLR scenarios. According to the low-end scenario, there will be hardly any effect due to SLR until 2100, whereas according to the high-end scenario the effect will be noticeable already in 2050.

Adaptation to uncertain sea-level rise; how uncertainty in Antarctic mass-loss impacts the coastal adaptation strategy of the Netherlands

“…We estimated required sand nourishment volumes based on the area of the marine coastal zone (∼4000 km 2 ) and the rates of SLR. Impacts of SLR on the intertidal surface area of the Wadden Sea are estimated using the ASMITA model [69], which simulates sediment exchange between the North Sea coastal zone and the Wadden Sea. For each tidal inlet and related basin, critical SLR rates are calculated at which the maximum imported sediment volume is no longer sufficient to match SLR.…”

“…Part of the supplied sand will be transported to the Wadden Sea, an UNESCO heritage site and the largest system of intertidal sand and mud flats in the world. However, the sediment transport capacity into the Wadden Sea is limited [69]. Hence, high rates of SLR will eventually lead to drowning of the system by a reduction in flat surface area and/or height [69].…”

“…However, the sediment transport capacity into the Wadden Sea is limited [69]. Hence, high rates of SLR will eventually lead to drowning of the system by a reduction in flat surface area and/or height [69]. For the Dutch Wadden Sea, the critical rate at which the intertidal flats will start to 'drown' is between 6 and 10 mm yr −1 [88].…”

“…However, current observations of global [45,89] and local SLR rates [84] are still much lower, and SLR would need to accelerate considerably to reach these critical rates before 2040. On the other hand, land subsidence could amplify local SLR rates, and subsidence rates (here due to gas and salt extraction [69]) will thus also influence at what moment in time the critical rates will be exceeded.…”

The development of the tidal basins in the Dutch Wadden Sea until 2100: the impact of accelerated sea-level rise and subsidence on their sediment budget – a synthesis

“…This information will be combined to provide the best possible projection of future developments in this area. This synthesis summarizes the information presented in the papers by Vermeersen et al (2018), Fokker et al (2018) and Wang et al (2018) in this special issue of the Netherlands Journal of Geosciences and combines projections of local sea level, subsidence and sediment transport into an assessment of the future state of the Dutch part of the Wadden Sea. We will restrict ourselves to projections of the future morphological development using the SLR scenarios presented by Vermeersen et al (2018) in combination with the prognoses for subsidence caused by gas and salt extraction for the individual fields in the Wadden Sea by Fokker et al (2018).…”