Data Acquisition

Flood damages are compiled by National Weather Service Weather Forecast Offices across the United States and its Territories. Each office can obtain data from emergency managers, the U.S. Geological Survey, the U.S. Army Corps of Engineers, power utility companies and newspaper articles.   In the event that a direct number cannot be obtained from these sources, the Forecast Office uses a guideline of storm damage estimates to produce their own estimate. These estimates are complied nationally and quality controlled to produce the figures you are viewing.

The NWS continually gathers data from each Weather Forecast Office, and employs a rigorous process to quality control the damage estimates. By March of each year, the annual figures dating back to 1903 are adjusted to inflation using the Construction Cost Index from the McGraw Hill Construction Engineering News-Record (http://enr.construction.com/economics/ - subscription required). Finally, the annual figures are delivered to the U.S. Army Corps of Engineers, who uses these figures to prepare a report to United States Congress.

Explanation of Table

The data represented are for water years, starting in October and ending in September.  For example, Water Year 2009 starts on October 1, 2008, and ends on September 30, 2009. Older years generally contain less accurate information than more recent years.

Each year, the entire set of data (all previous years) are adjusted to inflation using the latest Construction Cost Index (CCI) numbers provided by the “Engineering News-Record”. For example, in 1990 the raw damage amount was $1,636,431,000. To adjust this number to inflation, we take the latest year’s CCI (for water year 2010, that number was 8802), and divide it by the CCI of the year in question (4732 in 1990) to come up with an adjustment factor of 1.86010 for 1990. We then multiply the raw number from that year, by the adjustment factor, to arrive at the adjusted number displayed above.$1,636,431 x (8802/4732) = $3.04 Billion.

Cautions on the Accuracy of these Data

Flood damage estimates are reported in many different ways, and are subject to a wide variety of errors. Estimates come from federal, state, or county level government officials. If these estimations cannot be made, the reporting official from the NWS must make an approximation of the damages, a method which is prone to a high degree of subjectivity and inaccuracy.  Damages are often underreported, and many times the information never makes it to the NWS Forecast Office responsible for reporting these figures. As stated above, the National Weather Service’s primary mission is providing weather information and services to save life and property, not on post-event reporting.

One of the most critical discrepancies of these data occurs with storm surge related flooding caused by tropical cyclones. Coastal flooding caused by storm surge is not counted in the figures presented here. The record season of 2005, with Hurricanes Katrina and Rita, were undoubtedly enormous flooding events. However, the damages and deaths associated with Hurricane Katrina were largely due to storm surge, and not fresh water flooding (associated to rainfall). Therefore, the annual figure of $43B for water year 2005, although much higher than any other year, does not account for most of the flooding produced by Katrina.