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The 2013-14 European winter storm season has been pretty active so far. Early in the season, Windstorm Christian raced across northern Europe, followed by Xaver in early December, and then storms Dirk, Erich, Felix and Anne hit the U.K., Ireland, and northwest France over the Christmas and New Year period.

To date the season has been a great demonstration of how northern Europe is a common target for winter storms. However, this week sees the 5th anniversary of Windstorm Klaus, reminding us that storms can also impact southern Europe, affecting regions not acclimatized to extreme winds and causing severe damage.

What happened when Klaus hit and what have we learned from it?

Can such a storm occur again in the near future and more importantly, can we predict it, or at least estimate how bad it could be?

Windstorm Klaus sprung to life on January 23, 2009 in the central Atlantic, directly in line with southern France. The climate backdrop to this storm was pretty uncharacteristic. The large-scale Icelandic low-pressure system and the Azores high-pressure system were farther south than usual. Also, the North Atlantic Oscillation (NAO) was entering a negative phase.

A positive phase of the NAO creates favorable conditions for strong storms to pass over northern Europe, as Lothar and Anatol did in 1999. But a neutral or negative phase of the NAO can lead to storms that affect southern Europe and this is exactly what happened with Windstorm Klaus.

By midnight on January 24, as Klaus approached land, it had a central pressure of 963 hPa, comparable to Windstorm Lothar. Winds reached severe gale force in the southwest of France, peaking with gusts above 140 km/h at coastal locations near Bordeaux, accompanied by violent seas with wave heights of several meters. Local infrastructure was severely disrupted by fallen trees and electricity pylons.

Over 1.7 million households were without power immediately after the storm and over 60% of maritime pines in the Forêt des Landes were destroyed. Once the damage had been appraised, Klaus was estimated to have caused insured losses of €2.5billion (US$3.4 billion).

Shortly after the event, RMS scientists Dr. Navin Peiris and Dr. Christos Mitas conducted a reconnaissance survey, which helped to enhance our understanding of building vulnerability in this region. They observed frequent non-structural wind damage, such as the uplifting of roof tiles and collapsed chimneys, but also direct wind damages from tree fall, due to the high density of trees in close proximity to properties.

Damage EUWSK laus

Source: RMS 2009 reconnaissance

Closer examination of the roof damage revealed little evidence of proper fixation, particularly along roof edges, leaving them more vulnerable to wind damage. Another observation was the use of canal-type tiles, which are prone to uplift from the build up of air pressure, caused by strong winds. Also, damage was more frequent in residential properties, compared to commercial or industrial buildings that are generally engineered in line with building codes.

This survey, combined with an assessment of claims data, provided us with an enhanced understanding of regional vulnerability differences. For example, we observed a significantly lower fragility of buildings in the Perpignan area compared to the southwest of France.

modeled and observed EUWS

Ratio of the modeled and observed losses by postcode using non-regionalized vulnerability functions. Variation supports need for distinct vulnerability regions.

This information is vital for us to continually develop and inform our models, in order to represent the risk accurately. Due to the inherent uncertainty in the climatic phenomena driving windstorms, it is not possible to forecast exactly when the next strong storm will hit southern Europe. Catastrophe models provide a range of possible events, which can help the insurance industry prepare for the next big event.

The RMS Europe Windstorm Model contains storms comparable to Klaus, including some that impart larger wind intensities and damages. The below image compares two examples of stochastic storms with the actual Klaus wind footprint to illustrate storms that could potentially cause insured losses similar to or higher than Klaus.

Klaus And Stoch

Currently we are in a close to neutral phase of the NAO, so does that mean a Klaus type storm could occur this winter? No one can answer that question for certain, but a model at least enables us to explore the possible worst-case scenarios and be prepared.

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Many (re)insurers may be taken aback by the level of claims arising from floods in the French Riviera on October 3, 2015. The reason? A large proportion of the affected homes and businesses they insure in the area are nowhere near a river or floodplain, so many models failed to identify the possibility of their inundation by rainfall and flash floods. Effective flood modeling must begin with precipitation (rain/snowfall), since river-gauge-based modeling of inland flood risk lacks the ability to cope with extreme peaks of precipitation intensity. Further, a credible flood model must incorporate risk factors as well as the hazard: the nature of the ground, such as its saturation level due to antecedent conditions, and the extent of flood defenses. Failing to provide such critical factor can cause risk to be dramatically miscalculated. A not so sunny Côte d’Azur This was clearly apparent to the RMS event reconnaissance team who visited the affected areas of southern France immediately after the floods. “High-water marks for fluvial flooding from the rivers Brague and Riou de l’Argentiere were at levels over two meters, but flash floodwaters reached heights in excess of one meter in areas well away from the rivers and their floodplains,” reported the team. This caused significant damage to many more ground-floor properties than would have been expected, including structural damage to foundations and scouring caused by fast-floating debris. Damage to vehicles parked in underground carparks was extensive, as many filled with rainwater. Vehicles struck by more than 0.5 meters of water were written off, all as a result of an event that was not modeled by many insurers. The Nice floods show clearly how European flood modeling must be taken to a new level. It is essential that modelers capture the entire temporal precipitation process that leads to floods. Antecedent conditions—primarily the capacity of the soil to absorb water must be considered, since a little additional rainfall may trigger saturation, causing “saturation excess overland flow” (or runoff). This in turn can lead to losses such as those assessed by our event reconnaissance team in Nice. Our modeling team believes that to achieve this new level of understanding, models must be based on continuous hydrological simulations, with a fine time-step discretization; the models must simulate the intensity of rainfall over time and place, at a high level of granularity. We’ve been able to see that models that are not based on continuous precipitation modeling could miss up to 50% of losses that would occur off flood plains, leading to serious underestimation of technical pricing for primary and reinsurance contracts. What’s in a model? When building a flood model, starting from precipitation is fundamental to the reproduction, and therefore the modeling, of realistic spatial correlation patterns between river basins, cities, and other areas of concentrated risks, which are driven by positive relationships between precipitation fields. Such modeling of rainfall may also identify the potential for damage from fluvial events. But credible defenses must also be included in the model. The small, poorly defended river Brague burst its banks due to rainfall, demolishing small structures in the town of Biot. Only a rainfall-based model that considers established defenses can capture this type of damage. Simulated precipitation forms the foundation of RMS inland flood models, which enables representation of both fluvial and pluvial flood risk. Since flood losses are often driven by events outside major river flood plains, such an approach, coupled with an advanced defense model, is the only way to garner a satisfactory view of risk. Visits by our event reconnaissance teams further allow RMS to integrate the latest flood data into models, for example as point validation for hazard and vulnerability. Sluggish growth in European insurance markets presents a challenge for many (re)insurers. Broad underwriting of flood risk presents an opportunity, but demands appropriate modeling solutions. RMS flood products provide just that, by ensuring that the potential for significant loss is well understood, and managed appropriately.…

Laurent Marescot
Laurent Marescot
Senior Director, Model Product Strategy, RMS

Based in Zurich, Laurent initially joined RMS in 2008 as part of the Zurich account management team, servicing the European (re)insurance and ILS market. He then moved to the model product management group, leading the technical product management team for European climatic perils, such as windstorm, severe convective storm and flood. Since 2014, he has joined the model product strategy group for Europe model product line.

Prior to RMS, Laurent worked 3 years at the Swiss Federal Institute of Technology Zurich (ETHZ) as a Research Associate and Lecturer, managing multidisciplinary natural hazard research projects. Laurent still lectures regularly on geophysics and catastrophe modeling at universities, and gives seminars and invited talks in international meetings. He is a Lecturer and Scientific Collaborator at the University of Fribourg (Switzerland). Laurent co-authored numerous industry publications, reviewed scientific articles and proceeding papers. He holds an MSc in Geology from the University of Lausanne and a PhD in Geophysics from the University of Lausanne and the University of Nantes.

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