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A hurricane’s overlooked weapons of destruction

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Throughout the 2021 North Atlantic hurricane season, expect the media to focus on one number: category strength. Since the 1970s, hurricanes have been measured on the five-point Saffir-Simpson scale, which categorizes hurricane strength by wind speed.

“But it is not a reliable predictor of damage,” says Iwan Stalder, Head of Accumulation Management at Zurich Insurance Group. “The scale excludes central pressure driving storm surge, rainfall and resulting inland flooding, the physical size of a storm, and other important factors.”

Stalder points to the 2018 hurricane season as an example of the limitations of the Saffir-Simpson scale. In October 2018, Hurricane Michael was measured as a strong Category 4 when it made landfall in Florida and entered Georgia as the first Category 3 on record. Its strong winds and the rising seas it whipped up – known as storm surge – obliterated part of the coast.

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Yet a month earlier, relative to the affected exposure, Hurricane Florence caused comparable damage when it hit the coasts of North and South Carolina, but it was given a lower Category 1 classification.

“The reason we see discrepancies between the category of a hurricane and the damage it causes is that wind speed is just one destructive element in a hurricane’s arsenal and the Saffir-Simpson scale does not capture this complexity,” explains Stalder.

“Category 4 and 5 hurricanes are terrifyingly destructive. But large, wet, slow tropical storms and Category 1 hurricanes like Florence that result in torrential rainfall and flooding can be just as destructive and deadly.”

Four key factors

Mathias Graf, Zurich’s Head of Catastrophe Research & Development, explains that about 80 percent of the insurance industry’s losses incurred by Hurricane Michael were due to wind damage. Whereas wind was only responsible for around 20 percent of the damage caused by Hurricane Florence.

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There were four key factors, explains Graf, that caused Hurricane Florence to cause so much damage despite its Category 1 status:

  • Size: Florence extended 80 miles (130 km) from the eye. This huge size meant it damaged a wide area of land and impacted a larger population of people than Michael.
  • Surge: Florence’s size meant it impacted a larger expanse of coastline with the resulting storm surge causing more destruction than the wind.
  • Pace: Florence slowed to just 2-3 mph (3-5 km/h) after making landfall allowing it to unload more rain causing greater inland flooding.
  • Duration: Florence’s slow movement meant structures were battered and weakened for many days, even before the hurricane made landfall.

Other factors can also come into play, Graf explains, as happened when Superstorm Sandy hit the U.S. coast in 2012. Although not strong enough to be classified a hurricane by the National Hurricane Centre (NHC), Sandy was one of the most destructive storms ever due to unprecedented storm surge. This was due to a rare constellation of factors that included pressure, high tide and even the phase of the moon.

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“Understanding the impact of a hurricane requires complex models,” Graf says. “We use inputs from at least seven companies and complement these with our own Zurich View of catastrophe modeling. These models are regularly updated as scientific research and insights derived from every new hurricane bring new findings.”

How to respond

But what should you do if you are a business or individual wanting to understand how best to plan for a hurricane?

“Don’t just rely on the media’s reporting of the category strength to base your preparation or evacuation plans,” says Clark Cochran, Senior Risk Engineering Consultant and regional specialist for Zurich Climate Change Resilience Services. “We advise customers to actively seek out information from weather channels and other sources about the potential area of impact, duration of the storm, and rainfall totals.”

Equipped with this knowledge, Cochran says a business can make informed decisions on how to prepare and respond to a hurricane. Using the key factors from Hurricane Florence as an example, questions that may need to be addressed include the following:

  • Size: If the hurricane’s impact will be far-reaching, do you have enough materials and supplies on site for preparations before landfall if local resources become scarce? What about recovery and repairs after the event? Do you have any servicing agreements in place with emergency remediation contractors to quickly begin cleanup and restorations?
  • Surge: Can storm surge or tidal flood waters potentially reach your property? What exposed contents could be raised from basement or ground level areas to higher elevations? Are there critical documents or electronics that need to be relocated away from the storm path?
  • Pace: Are the site’s water drainage systems (e.g. storm drains, culverts, and stormwater basins) in good condition and cleared of obstructions so that surface runoff can discharge properly? How about building roof drains, gutters, balcony scuppers, etc.?
  • Duration: Is there any building damage that might be a “weak link” to a prolonged windstorm event such as detached roof flashing, deteriorating exterior walls, or leaking windows and doors? What measures can be taken to better protect your property against flying debris and wind driven rain? Do emergency generators on site have enough fuel if the storm lasts longer than expected?

“Careful planning and preparation are crucial for specific and effective responses to questions such as these,” explains Cochran. “Through the development of a Windstorm Emergency Response Plan, regional hazards can be identified, and strategic damage mitigation steps established for the preparation, response, and recovery phases of a hurricane. A windstorm preparedness plan is a living document and should be reviewed and practiced before each hurricane season to ensure the plan is still up to date and actionable.”

Hurricanes are complex natural hazards that require more than just a wind speed categorization to enable you to plan for their impact. Be informed and be prepared.

Related Links:

Keep an Eye on Natural Hazards (PDF)

Learn more about Zurich Climate Change Resilience Services

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