The impact of sea level rise on tides, waves and tidal sand ridges in the North Sea.
MetadataShow full item record
Tidal sand ridges are large-scale bedforms that occur on continental shelves with strong tidal currents and a sandy bottom. The length of these features is in the order of several kilometres, the height in the order of tens of meters and they evolve on centennial time scales. The key objective of this study is to quantify the impact of sea level rise (SLR) on the long-term evolution of tidal sand ridges on a realistic shelf. This also requires the assessment of the changes in tidal and wind waves with a rising sea level. For this, an existing shelf model (Delft3D-SWAN) is used, and applied to model the evolution of tidal sand ridges on the Belgian continental shelf. These ridges are subject to change in mean sea level and related changes in tidal and wave forcing with time. First, the changes in tides and waves as a result of SLR are quanti?ed through nesting of the shelf model into a larger-scale North Sea model (called DCSM-ZUNO). Second, starting from an alongshore uniform and sloping bottom, the shelf model is run in absence of SLR, until mature tidal sand ridges develop on the shelf. Finally, these ridges were used as a starting point for assessing morphodynamic changes due to SLR or differences in tidal and wind wave forcing. Different SLR scenarios were explored, using rate of SLR over the last century (2 mm/yr), the current rate of SLR (3:3 mm/yr) and different SLR rates derived from IPCC projections (3,3 mm/yr, 4,5 mm/yr, 5,4 mm/yr and 11 mm/yr). Model results show that on the Belgian shelf a larger water depth, resulting from SLR, causes the tidal wave to travel faster and to have a larger amplitude, while the tidal current amplitudes decrease. The signi?cant wave height and the peak wave period increase with a rising sea level, while the wave orbital velocity slightly decreases. The weakening of tidal currents and wave orbital velocities results in a decrease of sediment stirring at the bed. When imposing present-day conditions for waves and tides, the tidal sand ridges are able to keep pace with the rising sea level, even for the most extreme SLR scenario (11 mm/yr). When the effects of SLR on waves is imposed on the boundaries, the off-shore migration of the tidal sand ridges is reduced. However, the ridges are still able to keep pace with the rising sea level. Finally, when the effects of SLR on both the waves and tides are accounted for, the tidal sand ridges can keep pace with SLR, up to a SLR rate of 11 mm/yr, where the growth rate is lower than the rate of SLR.