The impact of future sea-level rise on the tides

Abstract

Tides (along with mean sea-level and surges) are a key component in coastal extreme water levels. This investigation begins by assessing the effect of future sea-level rise (SLR) on the tides of the northwest European Continental Shelf. Tides here are dominated by semidiurnal constituents; therefore the focus is on changes in the M2 constituent and the spring and neap tides. The validated operational Dutch Continental Shelf Model is run for the present day sea-level as well as uniform 2 and 10m SLR scenarios. M2 tidal amplitude responds to SLR in a spatially non-uniform manner, with substantial amplitude increases and decreases in both scenarios. The North Sea M2 tidal response is not proportional to SLR between 2 and 10m. In the 2m SLR scenario the M2 constituent is particularly responsive in the resonant areas. Changes in the spring tide are generally larger (-49cm St. Malo to +35cm Cuxhaven) than those in the M2, neap or shallow water tides. With SLR the depth, wave speed and wave length are increased causing changes in near resonant areas. In expansive shallow areas SLR also causes reduced energy dissipation by bottom friction. These mechanisms result in the migration of tidal amphidromes and complex patterns of non-proportional change in the tide with SLR. These substantial alterations to the tides are contrary to some previous studies.
These results motivate a subsequent investigation into the effect of future SLR on the global tides. We use a fully global forward tidal model, OTISmpi, to simulate the response of the four primary tidal constituents (M2, S2, K1, O1) as well as mean high water (MHW) and maximum range to various SLR scenarios. Attention is paid to changes at the 136 largest coastal cities (populations >1 million), where changes would have the greatest significance. A refined model setup is shown to have good skill at representing the present day tides. Uniform SLR scenarios 0.5-10m with fixed coastlines show the tidal amplitudes in shelf seas globally to respond strongly (increases and decreases) and non-proportionally to SLR. The changes in K1 and O1 tides are confined to Asian shelves. With 0.5m, 1m and 2m SLR MHW changes exceed ±10% of the SLR at 13, 13 and 10 of the 136 cities, respectively. Uniform SLR scenarios including coastal recession show a stronger and increasingly negative MHW response. The regularly opposing signs of change between the fixed and recession cases are explained through the opposing effect of the perturbations on the natural period of oscillation of the basin. These results suggest it may be possible to influence the sign of the tidal amplitude change through coastal management strategies. Non-uniform SLR, due to ice melt, causes the largest difference from the uniform SLR tidal response at high latitudes, in the near field (diminished response) and far field (amplified response) of the mass loss.
Changes in the tide will influence: coastal flooding, renewable and nuclear power generation, water reliant industry, sediment transport, dredging, shipping, tidal mixing fronts and intertidal habitats.

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