EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Re-assessing extreme sea level events through interplay of tides and storm surges

Stephen Outten1, Tobias Wolf1, Fabio Mangini1, Linling Chen2, and Jan Even Nilsen3
Stephen Outten et al.
  • 1Nansen Environmental and Remote Sensing Center, Bjerknes Centre for Climate Research
  • 2Department of Earth Science, University of Bergen
  • 3Institute of Marine Research

Flooding events pose an ever increasing threat in a warming world. Safety standards for buildings and infrastructure are often based on past observations of local sea level, as measured by tide gauges and remote sensing systems. However, sea level at a given location is not an isolated property and is determined by a combination of factors. For extreme sea level events, there are two factors that of particular importance: the astronomical tide, and storm surges. In this work, we analysed measurements from 21 stations in the Norwegian tide gauge network, disentangling the factors contributing to the previously observed extreme events.

By separating the observed sea level into a tidal component and a storm surge component, we found that in many cases the observed extreme sea level events were caused by an extreme storm surge coinciding with only a moderate tide, or an extreme tide coinciding with only a moderate storm surge. This raises the possibility of a ‘super-flooding’ event, where an extreme storm surge may occur with an extreme tide. Even in the short records examined in this study (less than 40 years), the combination of the highest observed tide with the highest observed storm surge would greatly exceed in the 1000-year return level event at many locations. This is often used as a national standard for critical infrastructure.  

We further complement the work by analysing the storm tracks close to Norway. By relating the storm surges with the individual storms giving rise to them, we found that many storm surges during extreme sea level events were related to cyclones of only moderate intensity. Combined with the previous findings, this work suggests the need to assess extreme sea level return values for future construction and infrastructure planning as the result of a multi-variable system. This is in contrast to basing such assessments on the single variable of observed sea level as it is done today.

How to cite: Outten, S., Wolf, T., Mangini, F., Chen, L., and Nilsen, J. E.: Re-assessing extreme sea level events through interplay of tides and storm surges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8000,, 2020


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  • CC1: Comment on EGU2020-8000, Joanne Williams, 06 May 2020

    Thanks Stephen, it's always a relief to see skew surge used for this task. It's not surprising that you find large skew surges when there's not the extreme total water levels - the advantage of using that metric is that you pick up on the events that didn't happen to cause flooding so extend your database. I like your simple plotting of the highest event there could have been with the right combination of circumstances. And then plus sea-level rise ...

    You could also plot the extreme lows with the same data set - which can be interesting to harbour managers.

    In several sites in the UK, Netherlands & US east coast I had to include seasonality in order to get statistical independence of tide and skew surge but I didn't look at Norway. It'd be good to follow up on this, have you got any more detail/code/data written up?

    • AC1: Reply to CC1, Stephen Outten, 06 May 2020

      Thanks for the comment.
      We hadn't considered the idea of extreme lows, but that is definitely worth looking into.
      We do have data and codes, but we're not making them available just yet as we are still in the process of publishing the work. 
      If you Like, I'd be happy to email you as soon as our publication is in final review and send you details.
      best regards,

      • CC2: Reply to AC1, Joanne Williams, 06 May 2020

        Great, I'll look out for it. Cheers, Jo