Regional Wind Vulnerability in Europe AIRCurrents 04.2011 Edited Editor s note: European winter storms cause significant damage. Their expected annual insured losses far surpass those of any other peril in the region. To manage the risk, companies need catastrophe models that can take into account not only the specific characteristics of the hazard, but also the different vulnerabilities in Europe s different regions. by Robert Zalisk Generating reliable estimates of damage and loss for winter storms in Europe is a challenge. Both the hazard itself known more precisely as extratropical cyclones (ETCs) and the vulnerabilities of the building stock across Europe have complexities and variabilities that do not readily conform to traditional modeling practices. Conventional techniques do not adequately capture the complex three-dimensional wind fields of extratropical cyclones. Similarly, the geography, climate, and wind patterns that characterize the various regions of Europe contribute to idiosyncratic local building practices and hence to different vulnerabilities to ETCs. Winter storm Kyrill (2007), for example, crossed the United Kingdom and traveled all the way to Poland and the Baltic states. Figure 1 below shows the tracks of several of the major ETCs of the recent past: Anatol, Lothar, and Martin (all in December of 1999), Erwin (2005), Kyrill (2007), Klaus (2009), and Xynthia (2010). Extratropical Cyclones Differ from Tropical Cyclones in Ways That Matter The characteristics of extratropical cyclones differ from those of their tropical counterparts in ways that are significant for estimating potential insured losses. The wind speeds of ETCs are low relative to those of hurricanes, rising only to roughly 120 to 175 kilometers per hour, the equivalent of Category 1 or Category 2 strength on the Saffir-Simpson Hurricane Wind Scale. Additionally, however, ETCs can extend far inland often with wind fields that actually intensify rather than diminish. Consequently, European extratropical cyclones have very large damage footprints. Figure 1. Tracks of Anatol, Lothar, Martin, Erwin, Kyrill, Klaus, and Xynthia, 1999-2010, illustrate the deep inland penetration of ETCs. (Source: AIR)
Relatively Small Individual Claims Can Quickly Add Up In light of their low to moderate wind speeds, ETCs generally produce relatively minor damage 1 (mostly nonstructural, such as to a building s roof covering, and/or cladding) but, importantly, over very large areas. Major ETC events can impact millions of residential, commercial, and agricultural structures. vulnerability of buildings across the continent. Each country has developed construction practices and, more recently, building codes that respond to its particular historical experience. Thus, buildings in regions that frequently experience severe winter storms tend to be less vulnerable to them because they have been subject over time to more robust local construction practices and more stringent building codes. In France, for example, for a given wind speed, buildings in Normandy which lies open to the sea and is highly exposed to winter storms are, in general, less vulnerable than buildings in south-central France, which historically have been relatively sheltered from ETCs. Similarly, the building stock in the northern areas of the United Kingdom, particularly Scotland, consistently resists damage from wind events better than the building stock in other regions of the UK. As in France, the most vulnerable buildings in the UK are in the southeast, where extreme winds are relatively uncommon but tend to cause severe damage when they do occur. Figure 2. Typical damage patterns in residential (top-left), small commercial (topright), small industrial (bottom-left), and agricultural structures (bottom-right) from European winter storms. (Source: AIR) Even though individual claims in such cases may average only between one and two thousand Euros, because of the high insurance penetration throughout most of Europe, total losses from such events can quickly rise to billions of Euros. Winter storm Anatol (1999), for example, initiated more than half a million claims across Denmark, Sweden, Germany, and the Baltic states claims that came to insured losses of more than 2 billion (in 1999 Euros). 2 In the case of ETC Kyrill a few years later, insured losses came to more than 6 billion (in 2007 Euros). 3 Just how these losses add up and where depends on regional differences in building vulnerability. Regional Differences in Vulnerability Europe experiences considerable diversity of storm climate. From the UK to the Baltic nations, storms strike one region more or less often than they strike another, and with higher or lower intensity. Over decades indeed, centuries such local weather patterns have produced differences in the These differences in vulnerabilities are reflected and in some measure have been systematized in the collection of structural design codes (Eurocodes) that began to be developed and adopted in Europe in the 1990s. Published by the European Committee for Standardization, the codes are designed to ensure that buildings meet certain safety requirements. Under the Eurocode directive, design wind loads are based on the wind speed gusts of a storm that could occur in a region once in fifty years that is, structures must be designed to be able to withstand the highest wind speeds such an event would generate. Each country has developed a basic design wind speed map based on its own local wind speed history. The map outlines the various areas or zones throughout the country that have had a similar historical storm experience. The Eurocode uses standardized design wind loads and corresponding design requirements across different countries. Figure 3 below shows the geographical contours of the different design wind gust speeds consolidated for all of Europe. 2
Figure 3. Building design wind gust speeds at a height of 10 meters (derived from EN 1991-1-4 [ABI, 2003]). Countries in green denote those included in the AIR Extratropical Cyclone Model for Europe. Europe also exhibits variations in vulnerability that are attributable to influences other than weather histories. These can result from differences in insurance coverage, different claims adjustment practices, how rigorously building codes are enforced, lower or higher levels of building stock maintenance (economically poorer regions having less well-maintained buildings) and even different rates of fraud 4. Modeling Regional Vulnerability Thus, Europe s diversity presents something of a challenge to the risk manager and to the modeler. Two buildings described in claims as being virtually identical each a twostory wood-frame residential structure, for example would exhibit different vulnerabilities if one were located in the south of France and the other in Norway. The AIR Extratropical Cyclone Model for Europe addresses the issue through the use of regional-specific damage functions that capture the effects of regional construction practices on building vulnerability. AIR engineers base these functions on the Eurocode design wind speeds; while the design wind speed maps outline regions of local weather activity, they thus also indicate areas of different local construction practice. Figure 4 below illustrates how the AIR model identifies regional variations in vulnerability in Europe. Figure 4. Regional vulnerabilities implemented in the AIR model. Buildings in regions that frequently experience higher winds tend to be less vulnerable. (Source: AIR) The illustration places Europe s highest vulnerabilities in areas such as south-central France, where building codes use lower design wind speeds than in areas like Scotland, where very high wind speeds are common and where, over time, buildings were built to withstand them. In the south of France, homes tend to be of lighter masonry and stucco, with low-to-medium pitched roofs covered with tile. Homes in Scotland are of heavier masonry, with medium-to-high pitched roofs that feature better connections between their roof coverings and their roof framing. Other regional differences in vulnerability are captured implicitly. The physical, or engineering-based, models normally used to determine the resistance of structural and non-structural building components have inherent uncertainties at the low to moderate wind speeds of ETCs. These limits make establishing a direct relationship between wind loads and building resistance by engineering analysis problematic. Thus, both post-disaster survey information and extensive claims data analyses are also needed to develop reliable damage functions. When correlated with locations, survey 3
findings together with loss experience data reflect the specific array of conditions and practices that are common to a particular place (such as how building codes are enforced and how buildings are maintained). Thus to the extent that this information is used to develop the damage functions for a region, the functions implicitly capture all such local factors that may affect vulnerability. To develop damage functions for its European ETC model, AIR has used post-disaster survey findings from all major European winter storms starting with Anatol in 1999, as well as more than three billion Euros of claims provided by nearly twenty different insurance companies. The claims cover the major countries of Europe and storms that date back to ETC Daria in 1990. Closing Comments Version 13 of AIR s Extratropical Cyclone Model for Europe, which will be released later this year, adds six countries in central Europe and the Baltic region to the 12 existing modeled countries of Western Europe thereby covering virtually all of Europe that is impacted by ETCs. Supported lines of business include residential, commercial, industrial, agricultural, greenhouse, forestry, and automobile. Europe is a large and diverse region, one with considerable diversity in storm climate and seasonal storm intensity. To provide companies with the ability to assess the risk from winter storms in all their manifestations, the AIR model recognizes and accounts for differences in building codes and construction practices, as well as other influences that result in different vulnerabilities from region to region across the European continent. 4
References 1 It should be noted that within ETCs, surface winds can sometimes be enhanced by substructures, such as sting jets and gravity waves. In such cases, wind speeds and damage can be severe. Such substructures and their effects, however, tend to be highly localized. 2 Munich Re Group (2002). 3 Swiss Re (2008). 4 See the AIR Current Anatomy of a Damage Function: Dispelling the Myths. About AIR Worldwide AIR Worldwide (AIR) is the scientific leader and most respected provider of risk modeling software and consulting services. AIR founded the catastrophe modeling industry in 1987 and today models the risk from natural catastrophes and terrorism in more than 50 countries. More than 400 insurance, reinsurance, financial, corporate and government clients rely on AIR software and services for catastrophe risk management, insurance-linked securities, site-specific seismic engineering analysis, and property replacement cost valuation. AIR is a member of the ISO family of companies and is headquartered in Boston with additional offices in North America, Europe and Asia. For more information, please visit www. air-worldwide.com. 2010 AIR Worldwide. All rights reserved. 5