It doesn’t always take superstorms to get supersurges

A lay exposition on why extreme surges may follow from not-so-extreme storms in the presence of barrier islands

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1. Big can fail, little can hit

Satellite view of the Wadden Sea

The Wadden Sea is a fringe basin of the North Sea delimited by a stripe of barrier islands. The picture shows the extent of the basin from the Netherlands in the south to Denmark north. (The whitey sandy shoals, characteristically exposed only at low tide, are rendered for effect.) 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 (ranked by wind speed alone).

Complex ball outcomes in limited playing fields Drawing an example from basic experiences with ball games, everybody knows that the harder you hit a ball, the more energy it will gain in the direction you hit it. Trivial only in an unlimited playing field, though. If 6 seconds worth of mundane basketball do not lead you astray from lofty environmental hydrodynamics, click on the picture for a Youtube clip (has audio) showing things not going the linear way, because hard (ring, an opponent’s head?) and soft (net) constraints conjure for complex outcomes.

Brute force is neither necessary nor sufficient to make a hit — as in ‘remarkable result’ — out of a hit — as in ‘strong action’. Note here how linguistic usages often cloud useful distinctions and reveal hidden assumptions at the same time. Actions (storms) and results (surges) are logically separate. Yet we can easily slip into short-cutting their mutual connection, to the point of merging distinctions in a single word, like in hit above. We naturally tend to assume that one action only leads to one result, the more so the more powerful the action is, in a sort of too-big-to-fail bias. Until your action-hit doesn’t result-hit, of course: until, for example, your expectations do not predict much of what you observe or not at the right moment.

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. For example, that of 1 November 2006. In a previous employment of mine I was tasked to look critically into the specifics of the storm-and-surge pairing in the Wadden Sea — more in [3]. There, two unchecked anticipations had to be challenged.

On the one hand, the water motion downwind of the barrier islands is constrained by the space made available by shorelines and bed depths (the physiographic features). Water follows its own slope from high to low levels. Thereby, as soon as the slope of the water levels permits, the water moves alongshore from one side of the basin to the other and across the wind. This is in contrast with the canonical behaviour of the surge from an open sea, where the water charges the land head-on in the wind direction.

On the other hand, 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 one other inlet, leading to no noteworthy accumulation 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.

Storm surge of 1 Nov 2006. Hindcast from Lipari and Adema (2010)

In a basin delimited by barrier islands 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. And — here comes the last element of the triad — storms themselves are evolving atmospheric patterns, with concomitant changes of speed and direction at any one place, and differences between places far apart. The wind above the tidal inlets will drive in more water while it blows in the direction of the tidal channel (in the guise of pushing an open door). And that alignment will change while the wind veers and backs, here and there, according to the moment of the storm. 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.

Notably, the analysis also showed that, discounting the presence of tide, the same surge height at one reference location had been generated by two storms with remarkably different developments. In particular, one peaked at 22 m/s (force 9, strong gale) and the other at 27 m/s (force 10, storm), whereas we would have expected that force-10 conditions had raised the waters more than force 9. Conjecturably, for every conceivable superstorm, there might be another lesser storm causing the same surge, until the barrier islands are overwashed, at least… The ‘match’ between the storm development and the basin geography leading to a severe surge is not even unique.

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. The sources

This section is probably for those interested in the nitty-gritty of the studies, else the closing section [4] is already waiting for you. Since recently, the two reports highlighting the mechanisms discussed above are freely available on-line. The information owner is Rijkswaterstaat, the Dutch governmental agency for the main water systems, namely the department Water, Verkeer en Leefomgeving (water, traffic and environment). You can access and circulate the reports from the links

  • http://bit.ly/WaddenSeaSurgeGeneration — This is Simulation studies for storm winds, flow fields and wave climate in the Wadden Sea (Lipari et al, 2008). The analysis of extreme-surge records, water-level and wind records, as well as hindcasts show the criticality of wind direction for disentangling the relationships between storms, surges and sub-basins. The study also assesses the hazard posed by fictional storms deprived of the natural wind speed and direction, confirming that these factors must not be overseen.
  • http://bit.ly/WaddenSeaPrototypeStorms — This is Viability study of a prototype windstorm for the Wadden Sea surges (Lipari and van Vledder, 2009), an attempt to parametrize the surge-generating potential of a storm and identify whether the spatial variability of the storm can be discounted by referring to one measurement point. Numerical experiments aimed to infer approximated storm patterns (‘eyecasts’) that could work out as a template for a larger class of surge-generating storms, alas unsuccessfully.
  • http://bit.ly/WaddenSeaSurges — This is the conference poster The generation of severe surges in the Dutch Wadden Sea (Lipari and Adema, 2010), summarizing the two reports above, possibly serving as aperitif before diving into the investigations.

4. Thinking onwards and upwards

This knowledge was gained during the preparative studies for the WTI (Wettelijk Toetsinstrumentarium) 2011 commissioned by Deltares on behalf of Rijkswaterstaat. The studies confirmed that, in the Wadden Sea playing field, the surge-generating storms must be sought for carefully and considered in their entirety as fully-unsteady, space-varying processes. Indeed, ‘supersurges’ do not follow from ‘superstorms’ everywhere. These insights led to a reconsideration of the regulatory hydraulic boundary conditions for the coastal safety assessment in the Netherlands. Beside the summaries above, see the full bibliographic information at the bottom.

There are many ways in which the Wadden Sea insights might be helpful beyond the specifics. Great 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.

In a physics-based perspective, climate change means a change of the way in which weather changes, not forgetting that weather has been variable also in a stable climate. Any storm pattern, whether for its speed or its direction or a particular combinations thereof, underlines how some back-barrier basins might be more vulnerable to flooding, others more resilient, or some others ambivalent in their response. 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. Intriguingly, while the barrier islands act as a storm filter, for knowing in advance for which storms they do so and to which degree, we’d have to look at all storms.

As often said — and hence attributed to Albert Einstein for good measure —, every problem statement should be as simple as possible, but not simpler. That implies that in our enquires it is perfectly fine, and even economically foresighted, to start off from as complex (or complicated) statements as need be, lest we help harm’s way, or waste ecosystem services offered by the present arrangements, or both.

As a closing note, these Wadden Sea investigations have been created some time ago as consultancy assignments, hence within limited production times. Please feel free to share back updates, corrections and amplifications. If you believe that this knowledge is still topical to advance understanding, clarify targets, device solutions and/or enrich skills, do not hesitate to get in touch with me. My current project Watermotion | Waterbeweging makes it easy to contribute my expertise for any further development.

Thank you for your reading.


References

Please feel free to download, use and circulate them:

  • Lipari G., Vledder G.Ph. van, Adema J., Cleveringa J., Koop O., Haghgoo A. Simulation studies for storm winds, flow fields and wave climate in the Wadden Sea. Alkyon Hydraulic Consultancy & Research, Report A2108, November 2008. Prepared for Deltares within the WTI-2011 programme of the Dutch Ministry of Transport, Public Works and Management. http://bit.ly/WaddenSeaSurgeGeneration (PDF, 58 MB)
  • Lipari G., Vledder G.Ph. van. Viability study of a prototype windstorm for the Wadden Sea surges. Alkyon Hydraulic Consultancy & Research, Report A2239, May 2009. Prepared for Deltares within the WTI-2011 programme of the Dutch Ministry of Transport, Public Works and Management. http://bit.ly/WaddenSeaPrototypeStorms (PDF, 4 MB)
  • Groeneweg, J. Lipari, G. Towards wave modeling under extreme conditions in a tidal basin. WISE (Waves in Shallow Environments) Meeting 2009, Ensenada (MX), 27-30 April
  • Lipari G., Adema J. The generation of severe surges in the Dutch Wadden Sea. Insights from data, hindcasting and numerical experiments. Storm Surges Congress 2010, Hamburg (DE), 13-17 September http://bit.ly/WaddenSeaSurges (PDF, 1.5 MB)
  • Lipari G. Not just in the wind: the Dutch Wadden Sea and its very own storm surges. The Netherlands Centre for Coastal Research, 2015. Book of abstracts of the NCK Days, Camperduin (NL) 18-20 March. http://bit.ly/WaddenSeaSurgesNCK2015 (PDF, 4 MB)

This document

This exposition first appeared on 5 September 2017 in LinkedIn Pulse, where is still accessible from bit.ly/supersurge (registration may be required for viewing). To bookmark and share this one article, freely accessible from Word Press, use bit.ly/supersurges instead.

Image credits

The images are hyperlinked to their sources and associated copyright indications. Please click on them for complete information and a fuller enjoyment.


last edited September 2017 · authored by Giordano Lipari

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