Canadian Underwriter

Tampa/St. Petersburg most vulnerable to storm surge flooding damage, with loss potential of US$175 billion: Karen Clark

August 27, 2015   by Canadian Underwriter

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The largest losses from storm surge along the Gulf and East coastal locations in the United States are concentrated in relatively few places, but flooding from a 100-year hurricane could spur losses exceeding US$100 billion in three cities, notes a new study from Karen Clark & Company (KCC).

The study report, Most Vulnerable US Cities to Storm Surge Flooding, identifies Tampa/St. Petersburg as the metropolitan area most vulnerable to flooding damage with a loss potential of US$175 billion. The vulnerability “is due to unique coastline features, local bathymetry and the low coastal elevations,” notes the report issued last week, 10 years to the month following the devastation wreaked by Hurricane Katrina. [click image below to enlarge]

New Orleans and New York would likely have losses exceeding US$100 billion, the report says

The two other cities that would also likely have losses exceeding US$100 billion are New Orleans and New York.

The report examines and ranks the most vulnerable cities on the U.S. Gulf and East Coasts based on the estimated property damage and losses likely to be experienced in specific events. Using KCC’s RiskInsight high-resolution coastal flooding model and detailed databases of property exposures, “the storm surge impacts were estimated for over 300 events at 10-mile spaced landfall points, and cities were then ranked by the estimated property damage, including building, contents and time element losses.”

The U.S. cities most vulnerable to storm surge flooding by losses to residential, commercial and industrial properties from a 100-year hurricane – rounded to the nearest US$5 billion – are as follows:

• Tampa/St. Petersburg – estimated loss of US$175 billion; 50% of the population lives on ground elevations less than 10 feet; and the 100-year hurricane for Tampa is a strong Category 4 storm with peak winds of 150 miles/hour (mph).

• New Orleans – estimated loss of US$135 billion; because of such low elevation and marshy terrain, storm surges can travel tens of miles inland before attenuating; and the 100-year hurricane for the Gulf region is a strong Category 5 hurricane.

• New York City – estimated loss of US$100 billion; geographic areas likely to experience high surges are Lower Manhattan, Staten Island and the south shore of Long Island; the estimated loss includes damages in new Jersey and along the New England coastline; and the 100-year hurricane is a Category 3 storm with peak winds of 120 mph.

• Miami – estimated loss of US$80 billion; although from a physical perspective, storm surge hazard is relatively low, vulnerability stems from the sheer magnitude of property values near the coast along with low coast elevations; and the 100-year hurricane is a strong Category 5 storm with peak winds of 165 mph.

• Fort Myers – estimated loss of US$70 billion; most of the population of the city and surrounding towns is below 10 feet elevation; and the 100-year hurricane is a Category 5 storm with peak winds of 160 mph.

• Galveston-Houston – estimated loss of US$55 billion; winds positioned at the mouth of the Galveston Bay will cause the high waters to be channelled into the bay, thereby accentuating the water heights; and the 100-year hurricane for Texas is a strong Category 5 hurricane.

• Sarasota – estimated loss of US$50 billion; and the 100-year hurricane is a Category 5 storm.

• Charleston – estimated loss of US$45 billion. [click image below to enlarge]

The dark purple areas in the RiskInsight storm surge footprint indicate water levels of 10 feet or more; the green areas indicate elevations of less than 10 feet

For most hurricanes, notes the report, “wind is the primary driver of property damage and economic disruption. But for certain storms, like Katrina, the storm surge flooding can cause tremendous devastation and economic loss – in some cases, dwarfing the wind damage.”

Pointing out that most of the flood damage potential is not currently insured, KCC contends “private flood insurance presents a significant opportunity for insurers that have the right tools for understanding the risk.”

The report makes clear that every coastal location is subject to storm surge flooding from a 100-year hurricane, but the largest losses are concentrated in relatively few places along the coast. Four cities expected to experience among the largest potential losses in a 100-year hurricane are located in Florida, with the state’s west coast being more vulnerable than its east coast.

The 100-year hurricane – which KCC reports should be thought of as an infrequent, but not unexpected occurrence – is the type of event for which there is a 1% probability of occurrence in any given year. “It’s important to note that the 100-year wind event may not produce the 100-year flood event because weaker storms that are larger in size can produce equivalent or even more storm surge than stronger, smaller storms,” the report states.

After Charleston, “the next largest losses are produced by storms impacting several smaller metropolitan areas along the East coast. For example, as was seen in Superstorm Sandy, even a weak storm making landfall along the Mid-Atlantic coast can cover a large area and produce large storm surge losses,” states the report.

“The size of the storm and the track angle are very important influences on storm surge. In general, larger storms create more coastal flooding, primarily because longer stretches of coastline are impacted,” KCC explains. “Storms travelling inland perpendicular to the coast will also create higher peak surge.”