Sea-level rise impact on European shelf tide
Résumé
Sea-level rise (SLR) not only affects total water levels, but it can also modify tidal dynamics. Hence, several studies investigated the SLR effect on the Western European continental shelf tide (mainly the M2 component) for various coastal defence schemes. The present study further investigates this issue, using a modelling-based approach to analyse the effect of uniform SLR scenarios from -0.25 m to +10 m on high/low tide water level and the tidal components.
Assuming coastal defences along present’ days shorelines, the patterns of increase / decrease of high tide level (annual maximum water level) are stationary in most of the area (70%). They increase in most of the domain (especially Northern Irish Sea, Southern North Sea, German Bight) and decrease mainly in the western English Channel. These changes are varying almost linearly with the SLR as long as SLR remains below +2 m above present sea-level. They can account for +/-15% of regional SLR. These high tide level changes patterns slightly differ from M2 pattern changes (e.g. along the French Atlantic coast, high tide level increases whereas M2 amplitude decreases). The analysis of the 12 largest tidal components highlights the need to take into account at least the M2, S2, N2, M4 and MS4 when investigating SLR effects on the tide. The linear behaviour of tide changes with the SLR is highly sensitive to the coastal defence strategy (i.e. let flood or not), with high tide levels varying much less linearly with SLR when further flooding is allowed inland, in particular in the German Bight. However, several areas appear weakly sensitive to this choice: therefore, an increase of ~6% SLR (resp. decrease of ~15% SLR) can be considered as a predictable effect of tide/SLR interactions in the Northern Irish Sea (resp. in the Western English Channel). Finally, the effect of a plausible non-uniform SLR scenario is simulated: high water level is very weakly sensitive to the (non-)uniformity of SLR, local rates of increase/decrease (relative to local SLR) being similar to the ones obtained from uniform SLR scenarios. The SLR effect on the tide appears like an additional uncertainty to deal with in flood hazard studies focussing on next decades/centuries.