Models for flood futureproofing

Extreme storm surges and flooding in the aftermath can devastate coastal communities and ecosystems. To avoid catastrophic impacts and loss of lives, we need to be able to anticipate when this is going to happen and prepare accordingly. Models are a key tool in helping to forecast events like this, and understand how they evolve, gauge their potential impacts, and support recovery efforts. But, as a rule, these models are generally not often implemented, even where they could reasonably be deployed. One extreme surge that occurred recently in the German Baltic Sea has afforded scientists the opportunity to test the utility of these sorts of models and develop a framework for using them to manage extreme coastal flooding in the region. This has enabled Nassos Vafeidis, from the University of Kiel, to identify three phases - before, during and after the event - and suggest the specific actions that need to be undertaken in each phase…

Nassos - Our motivation was the October 2023 storm surge that hit the German Baltic Sea. It was a rather extreme event with a frequency of occurrence of one in 200 years and constituted a great opportunity for us to test our numerical models and collect data on flood characteristics, which are very rare.

Chris - How extensive was the data collection? What did you actually measure?

Nassos - Our work is mostly focused on modelling, and we use models to, for example, estimate the frequency of occurrence of these events. So we look at past events, we look at the tide gauge measurements, and we try to estimate how often these events occur. This is difficult because there are only 30, 40, 50 years of data in the best cases, in most places at least, and we're trying to predict occurrences of events that happen once in 200 years or once in 500 years. So we need to model these data. Historical information from scripts, novels or hearsay can reduce the uncertainty and improve those estimates. These estimates are essential for designing coastal protection. We also saw that extreme water levels can vary considerably spatially, and the existing networks of tide gauges do not capture this. So models allow us to produce results at much higher resolution, and this can also be used for coastal protection estimation, but also for response.

Chris - So that gives us the upfront preparation. What about the opportunities afforded when the event is evolving in real time? What can you learn? Because that presumably is a high priority—to learn from a real event as it unfolds. So what do you prescribe as the sorts of things we should be doing during the event itself?

Nassos - Absolutely. I have to point out it was great to have real-time information about how the extreme water levels would evolve in the next hour or two, and this was based on simulations. This is available, and people have access to it. But we can do more.

All this information tends to stop at the coastline, so we have no information about how the actual flooding might evolve. We can forecast that pretty well, either by looking at maps or maybe even running models in real time. We're not fully there yet because these hydrodynamic models take a few hours in the best cases to run, depending on resolution. But there are simpler models that we can use to forecast how the flood on land will evolve.

The other thing which is pretty difficult to do is collecting data during the flood. This helps us improve our models, and it's notoriously difficult because few people want to go out—just a few crazy scientists—during the flood event and start measuring water depths. But we now have instruments that are relatively cheap, such as pressure sensors, for example. We have satellites, we have drones that can give us information during the flooding, and this information can be used for emergency responses but also to improve our models—calibrate them, tweak them in a way that they can represent the events as accurately as possible.

Chris - And in the aftermath, what sorts of things should be on the priority list to measure then?

Nassos - Assessing impact is a priority. We need to respond quickly; we need to help. So rapid mapping, assessing the flood extent and depth, flood characteristics in general, and assessments of damages depending on water depth can be very quickly produced with model simulations or with measurements. And of course, all this information can also be used to go back and evaluate the work that we did in preparation for such events.

Chris - Is the purpose of this that you produce something which is bespoke for one geography? Or is what you're saying: we do this, we demonstrate with this one geography how integrating these different bits of information—the before, the during and the after—produces a model way to approach this sort of problem, and then we extrapolate that to any geography that may be affected in the future?

Nassos - This is a very interesting point. So our intention is to propose a framework for making this information more accessible, more systematic, more understandable—but also to produce actionable knowledge. So obviously, in Germany we have access to a lot of data, computational facilities which allow us to run models quickly, but there are elements of the things that we're proposing that are available everywhere, and they are becoming increasingly available. Also, the capacity to do these things is rapidly evolving, so eventually this kind of framework can be used in any other place, and it can also be populated with the elements that already exist that would be good enough to start.

Chris - How will it cater, though, for the future? Because we anticipate that with climate change, extremes of weather are going to become a lot more common, and also that general weather patterns are going to change anyway. So therefore, there's a two-dimensional change potentially coming, and any system we set up has got to be able to cater for that.

Nassos - Absolutely. I mean, this framework is flexible. So it allows, or rather requires, that after an event you go back and re-evaluate what you've done before the event. Flooding will become increasingly intense in the future just because of sea level rise—even if weather patterns do not change—and we need to prepare well for a wide range of scenarios. We also need to prepare for scenarios such as dike breaching, for example. We know that defences often fail, and this can have catastrophic effects for coastal communities. So all this preparation can take place in the context of this, and can be organised within the context of this framework and be constantly updated. So I would say, for the future, such frameworks are essential for being able to manage and to cope with such events.