Changes in the North Sea extreme waves

The current approach to obtain hydraulic boundary conditions for the Dutch water defences involves the transformation of offshore wave conditions to nearshore. The transformation consists of defining uniform wind fields with extreme wind velocities and associated offshore wave conditions, and using these to run the wave model SWAN (Simulation of Waves in Nearshore Areas, Ris et al., 1999) in stationary mode, computing the corresponding nearshore extreme waves. There are two aspects in this approach that may have a negative effect on the quality of nearshore extreme value estimates. First, the stationary assumption may not hold, either the modelled storm never occurs or occurs with a different return period. Secondly, in the current approach no attention is paid to effects of climate variability on wave extremes.

In this study, we use SWAN in the non-stationary mode to produce a timeseries of nearshore long-term wave heights and analyse it using both stationary and a nonstationary extreme value models. Our final goal is to answer questions such as: What are the differences between the extreme value estimates obtained using a non-stationary wave modelling approach and those obtained with the currently used stationary approach? Has the North Sea extreme wave climate changed in the last decades, and how is it expected to change in the future? In terms of future changes in wave climate, this is essentially an illustrative study indicating how one can obtain preliminary estimates of possible effects of climate change in the definition of hydraulic boundary conditions for the Dutch coast.

Wave hindcasts from 1958 to 2001 are computed. The hindcast timeseries of significant wave height (Hs) at MP1, a meassuring station off the Dutch coast, is analysed using a stationary extreme value approach. The 100-yr return value estimate obtained from such analysis is then compared with the corresponding estimate obtained by running SWAN in stationary mode with the estimated 100-yr uniform wind field and the corresponding wave boundary conditions. In order to look for trends or other systematic temporal variations of Hs in the last decades at MP1, the hindcast timeseries is also analyzed using a non-stationary extreme value approach.