Intertidal flat development in the Frisian Inlet under accelerated sea-level rise

Abstract

The Frisian Inlet in the Dutch Wadden Sea consists of two subbasins: the dynamic Pinkegat and the more stable Zoutkamperlaag. External drivers, including the Lauwerszee closure (1969), gas extraction (since 1986), and storm events, have shaped tidal dynamics and intertidal flat development. While historical sediment budgets have sustained these flats, rising sea levels may outpace sediment input, threatening the Wadden Sea’s role in the East Atlantic flyway. This shows the need to assess how tidal systems such as the Frisian Inlet adapt to changing conditions. However, the direct impact of accelerated sea-level rise is difficult to isolate within natural variability. To address this, the Lauwerszee closure and gas extraction are proxies, having caused long-term shifts in tidal dynamics and intertidal area development that mimic the expected consequences of rising sea levels. This study analyzes the morphological evolution of the Frisian Inlet using hypsometric curves, cumulative surface area changes, and tidal flat slope gradients. Interpolation techniques, variability analysis, and climate assessment further quantify the dynamics in both subbasins. Additionally, sedimentation rate under different sea-level rise and subsidence scenarios is evaluated alongside the potential ecological implications of tidal flat slope evolution. The central question in this research is: How does the Frisian Inlet respond to accelerated sea-level rise in terms of spatio-temporal intertidal flat development? Following the 1969 closure of the Lauwerszee, wave-driven erosion intensified in Pinkegat, while Zoutkamperlaag remained more stable. Post closure, wind redistributed sediment in Zoutkamperlaag, but reduced tidal inundation limited supply to higher flats. As a long-term response, both basins experienced mid intertidal zone decline due to infilled channels and increased wave action, though Zoutkamperlaag’s resistant layers provided greater protection. Gas extraction since 1986 had no impact, but storms (2010–2023) caused erosion in Pinkegat and relative stability in Zoutkamperlaag due to contrasting dynamics. Vaklodingen’s low temporal resolution limits the detection of these short-term trends. Slope differences affect sedimentation and ecology: Zoutkamperlaag promotes sedimentation but limits adaptability, while Pinkegat retains less sediment but enhances foraging. Based on sea-level rise and subsidence scenarios, sediment supply may be insufficient to maintain current topography, as Zoutkamperlaag would require a sedimentation rate of 0.69–0.82 Mm³/yr and Pinkegat would require 0.33–0.37 Mm³/yr.