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Understanding and Communicating Flood Risk

Expanding the National Conversation

Scott Giberson    |    Natural Hazard Risk

Each year, November 30 marks the end of the Atlantic hurricane season, and beginning on December 1, hurricane experts start to compare the season’s activity to predictions; insurers and reinsurers consider the losses and any impact to solvency plans; policymakers debate changes to state or federal insurance laws or regulations, and families and business owners in affected areas continue work to rebuild and recover.  Hurricane season provides an annual focus for the national conversation on flood risk, but the historic winter storms in recent months illustrate the need to understand flood risk beyond this season.  Understanding coastal flood risk from a hurricane is more complex than simply classifying the hurricane’s severity rating, and understanding inland flood risk is more complex than simply classifying the storm’s return period. 

Let’s consider South Carolina and Hurricane Joaquin this past fall. Joaquin did not make landfall but contributed to meteorological circumstances resulting in an immense amount of rainfall that impacted South Carolina.  Over 30 counties in the state were declared federal disaster areas due to the impact of flooding.  Part of understanding flood risk is analyzing the exposure facing those impacted by the event including property loss, risk to capital, damage to infrastructure and lost revenue. CoreLogic analysis of structure value data1 (as illustrated by ZIP code in Figure 1) for buildings in the impacted areas reveals that over $6.8 billion in structural value was exposed to flood risk.

Further, we recognize that understanding and communicating about flood risk is more than just determining whether or not a property is within a “1 percent annual chance floodplain”2, or the Special Flood Hazard Area as mapped by the Federal Emergency Management Agency (FEMA). The FEMA flood map is a critical resource for rating flood insurance, locating areas of high risk, guiding development, pursuing mitigation and identifying loans subject to the federal mandatory flood insurance requirement. The FEMA flood map can also help power analytical flood risk assessment tools that can measure the full spectrum of flood risk within the Special Flood Hazard Area and beyond.  

South Carolina Flooding Sunter County Flood Inundation

South Carolina Flooding Sunter County Flood Inundation

As illustrated in Figure 2, however, knowing whether an area is mapped as a Special Flood Hazard Area is not determinative of whether it is at any risk to flood.  This map compares the flooding that occurred in an area of southwest Sumter, South Carolina (based on preliminary inundation data released by FEMA) to the mapped Special Flood Hazard Area based on the current flood map. Importantly, the Special Flood Hazard Area represents the predicted area subject to flooding from an event of a single frequency (the 1 percent annual chance flood).  More granular flood risk assessment and analysis helps demonstrate several things: that risk exists beyond areas mapped for a single probabilistic event; that such flood maps do not show typical or necessarily expected inundation; and that actual inundation depends, to a certain extent, on unpredictable amounts of rainfall in unpredictable localities.   

In future blogs, we will take you deeper into the analysis and assessment of flood risk and how technological advances, science and data can drive greater understanding and more meaningful communication among experts, policymakers, risk managers and property owners.

1Structure value data represents estimated value of improvements on property established from CoreLogic data sourced from local government tax rolls.
2This is commonly referred to as the “100-year floodplain” although some believe that use of “100-year” in referencing this area of the floodplain provides an improper perception of the level of risk to flood in this area.

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