Fragmented Patterns Seen in the Recent History of U.S. Floods

A new study by scientists from the U.S. Geological Survey and the Vienna University of Technology in Austria examined the recent history of floods in the United States for the time period 1940-2013. The scientists found some regional trends, but no widespread national pattern of flood change.

Using a novel approach based on data from a carefully selected set of 345 U.S. streamgages, the investigation set out to resolve the apparent contradiction between findings by climate scientists of increases in the frequency and magnitude of heavy precipitation in most parts of the country and other studies that have failed to identify statistically significant increases in floods. The science team considered the years 1940-1969 to be a base period for comparison with flood properties in more recent years.

“An important prerequisite for effective flood risk management is to have an accurate assessment of how flooding is changing over time,” said Stacey Archfield, USGS scientist and lead investigator of the team. “Of course, changes in climate as well as land- and water-use management are each potential sources of change in flooding frequency or magnitude. But the relative influence of these factors across broad areas has been difficult to discern.”

To reduce the direct influence of human factors on the data, the USGS streamgages selected for the study were geographically representative but were located in watersheds that are relatively free of the blurring effects of water storage reservoirs, high concentrations of urban land, storm surges, or other coastal flooding.

The team’s conclusion was that most of the assessed regions have not experienced significant change in key flood characteristics over the 1940-2013 period with some notable exceptions.

The northern Great Plains and Upper Mississippi Valley show statistically significant, but small, decreases in the frequency of floods, but large increases in the peak magnitude, volume, and duration of floods. For example, some portions of that area experienced more than a doubling of the average volume and average duration of flood events relative to the base period.

Large parts of New England exhibit the opposite behavior, showing large, significant increases in the frequency of flood events and significant (but small) decreases in the average peak magnitude, average volume, and average duration of flood events. In some parts of New England, the average frequency of floods increased from 2 floods per year to an average of 4 or 5 events per year relative to the base period.

Moving beyond simply testing for the significance of trends in each region, the research team also evaluated the likelihood that the number of regions with significant trends could have occurred by chance. In considering the frequency, average volume, and average duration of flood events, the number of regions with significant trends in either direction was found to be greater than the number of significant trends that would have been occurred by chance. This finding confirms that the number of regions with observed significant trends in these flood properties was, in fact, substantive.

In addition, the number of regions with increasing trends in the peak magnitude was significantly greater than the number with negative trends. The authors note, however, that there were far more regions with no significant trend in any of the flood characteristics than the number of regions with either positive or negative trends.

The analyses show that generalizations about the nature of flood trends are not possible at this time. The analyses further suggest that a nuanced approach is required to address flood change for the purposes of management of flood hazard, rather than uniform approaches applied to a regional or national domain.

The authors of the study stressed the importance of continued streamflow monitoring and continued re-analysis of flood records, using a wide range of possible measures of changing flood behavior as only one part of an overall strategy needed to increase the ability to forecast the trajectory of flood conditions. This ability to forecast flood behavior is needed to guide natural resource and natural hazard planning and management over the coming decades.

The study “Fragmented patterns of flood change across the United States” was recently published in the journal Geophysical Research Letters.

Source: U.S. Geological Survey

Flooded areas of Baton Rouge, Louisiana, are shown Aug. 15, 2016. U.S. Coast Guard photo by Petty Officer 1st Class Melissa Leake
Flooded areas of Baton Rouge, Louisiana, are shown Aug. 15, 2016. U.S. Coast Guard photo by Petty Officer 1st Class Melissa Leake