By Thomas Mortlock (firstname.lastname@example.org)
Severe Tropical Cyclone Debbie made landfall at Airlie Beach on the Whitsunday Coast earlier on in the year, with an estimated property insurance market loss estimate over AUD $1.6 billion (PERILS, 2017). Debbie had all the ingredients for a large storm surge potential – a low and dropping pressure before landfall (down to 943 mB), high and sustained onshore wind speeds (landfalling as a Cat 4 system), a track perpendicular to the coast, and a very slow forward moving speed (7 km/hr at landfall).
Debbie also coincided with a relatively high state of tide (landfall occurring 2 hours after high water) and large waves (> 9 m), to produce a storm tide inundation, according to Risk Frontiers’ own survey estimates, of around 5 m above mean sea level. This was roughly equivalent to the height of most coastal foredunes, meaning direct coastal inundation damage to property was limited.
While the storm tide inundation could have been much higher if Debbie had made landfall two hours earlier, the storm surge itself (a combination of elevated coastal water levels due to high wind speeds and low atmospheric pressure, minus tides and waves) should have been bigger. The open question posed since March has been – why was it not?
A similar question has been asked of storms and surges in the Wadden Sea, a fringe basin in the North Sea between the Netherlands and Denmark. The article below, written by Giordano Lipari in the Netherlands, makes the point that superstorms don’t always lead to supersurges, especially in coastal areas fronted by islands. In the case of the Wadden Sea, and the article below, these are “barrier [sand] islands”, but in the case of Debbie and the Whitsunday region, the same effect may also be caused by the numerous rock and coral islands and reefs that fringe the mainland coast.
Below is an edited version of Lipari’s article. The original version can be found at bit.ly/supersurge.
The Wadden Sea (Figure 1) is a fringe basin of the North Sea delimited by a strip of barrier islands. When it comes to storms and surges, the Wadden Sea stages an intriguing three-way interaction between physiographic features (in plain language: the water container), atmospheric systems (weather), and flow patterns (water). In the Dutch part of basin, in particular, this interplay defeats the intuition that the most severe surges are caused by the most severe storms when ranked by wind speed alone.
2. Back by the beach
When raging winds raise the water against the coast, it is generally taken as ground truth that the higher the peak wind speed, the higher the peak water level. Some tide gauges in the Dutch Wadden Sea, however, showed that record-breaking surges were not caused by the most severe winds in the same control period.
The unlimited presence of water is self-evident on a shore squarely facing the ocean’s expanse. In contrast, the water volume contained in the Wadden Sea depends on the course of the waters flowing in and out across its several tidal inlets. However hard the wind pushes in the water across one tidal inlet, some water may still escape from another, leading to no noteworthy accumulation of water inside the basin. In the extreme, there is no surge if no extra water stays in and for long enough. Hence, there could be much barking in the wind, little biting in the water.
Figure 2 Hydrodynamic modelling in the Wadden Sea – the fringing barrier islands and the complex flow directions behind may be an indication of the type of situation that occurred to the lee of the Whitsunday Islands during DebbieIn a basin delimited by barrier islands [or rock islands and reefs, in the case of the Whitsunday coastline] the surges are significantly modulated by the physical geography. Only those storms causing a substantial piling-up of water behind the islands can cause severe surges, once they have managed to bring in the excess water to raise in the first place.
The arrows in the picture above, based on computer simulations, indicate qualitatively where water is going in and out at a storm’s given moment: clearly, it’s not the same everywhere, nor will it stay unchanged while the storm unfolds itself.
In sum, the Wadden Sea evidence is that high wind speeds alone are neither necessary nor sufficient to cause, or expect, record-breaking surges. Since the container drives both water storage and motion, the Wadden Sea itself determines effectively which storms result in a surge with a certain level of flood hazard with possibly counter-intuitive outcomes. The scope and degree of generality to which the cautionary tale is applicable to all/other situations is matter of orderly scientific discourse. Certainly, the severity of storm surges is pretty much a situation-specific matter, and it cannot be reduced to a single number defining the storm alone, such as the Beaufort or the Saffir-Simpson scale, except in the simplest configurations.
3. Thinking onwards and upwards
There are many ways in which the Wadden Sea insights might be helpful beyond the specifics. Societal concerns for the coastal areas are justified due to the growing concentration of the global population, to the weather anomalies and outliers expected to increase after climate change, and to the land subsidence aggravating flood-proneness.
The investigations on the Wadden Sea made it at least clear that the excess of simplification in the superstorm-supersurge anticipation could mishandle the exposure and vulnerability of certain coastal areas.
As often said — and hence attributed to Albert Einstein for good measure —, every problem statement should be as simple as possible, but not simpler. In the case of storm surge prediction – this may not be that simple.
Risk Frontiers staff and associates have significant experience in coastal process and hydrodynamic modelling, in particular, understanding the dynamics and impacts of extreme waves and water levels in Australia. In association with a consortium of eight other research institutes and government agencies, we are coordinating the analysis of an unprecedented number of coastal impact observations post-Debbie, to be published soon.