The Federal Emergency Management Agency (FEMA) does not classify approximately 6 million homes and other properties in the United States, which are estimated to be at risk, as high-risk flood zones. In total, about 14.6 million real estate properties with some level of risk lie outside the official zones. Some of the maps used to define these zones were created about twenty years ago, so they reflect neither current development nor changes in the hydrology of the areas.
FEMA maps are based primarily on historical data and show the probability that water will reach a given area. They do not forecast future scenarios that account for climate change, nor do they determine what the consequences will be for a specific home. How severely an individual structure is damaged depends on the depth of the water, the age and condition of the building, as well as insurance coverage and repair costs, which determine how recovery after a flood unfolds. Even on a single street, two homes can have substantially different risk, and this circumstance makes general maps an incomplete tool for residents.
This is stated in a study conducted by scientists from Rice University together with colleagues from Arizona State University and the University of Colorado Boulder. The researchers assessed nearly one million homes in Houston for their vulnerability to flooding, and the results were published in the journal Applied Geography. Earlier similar work in the city showed that more than half of the 100,000 buildings prone to flooding lie outside official risk zones. Houston is one of the most vulnerable large cities in the country precisely because of the combination of low-lying terrain, intensive development, and frequent heavy rainstorms that regularly test the city's drainage systems.
For the assessment, the authors considered four factors: the depth of possible water, the condition of the building, mobility, and the financial capacity of residents. The flood depth was taken from a national model for parcels in Harris County. The age and condition of buildings were determined from county property appraisal records. Mobility was assessed by traffic intensity nearby, within a radius of half a mile and three miles, since this determines whether people can evacuate in time or access assistance after roads are flooded. The financial component reflects how well a household can withstand losses and cover repairs without outside support.
The most significant detail of the study concerns the scale of the differences. The spread of risk estimates within individual neighborhoods turned out to be greater than the spread between neighborhoods. That is, the average figure for a neighborhood can conceal the most risky individual homes. This means that relying only on general zones or on averaged data by block misses some households that most need attention, and that protection decisions should be made at the level of specific addresses.
For homeowners, the researchers advise checking the flood history of the parcel, the age of the home, and the availability of insurance coverage. It is worth knowing that most standard homeowners insurance policies do not cover flood damage, so a separate policy is important, and the absence of such protection can leave a family alone with recovery costs. Communities, in turn, can use address-level risk assessments together with local records to more precisely direct limited public funds toward protecting specific buildings. According to the researchers, similar data exist in many parts of the United States, not only in Houston, so the approach can be scaled to other cities with similar risks.