Study: Flood Risk Underestimated – Rising Due to Climate Change

One of the key findings of a recent National Academy of Science panel on abrupt climate change, is that its important not just to look at potential sudden tipping points in climate effects, but also to consider that a steady, incremental creep in climate changes can cause human systems and infrastructure, built for the conditions of 50 or 100 years ago, to fail suddenly in the face of the new normal.

Washington University:

As floodwaters surge along major rivers in the midwestern United States, a new study from Washington University in St. Louis suggests federal agencies are underestimating historic 100-year flood levels on these rivers by as much as five feet, a miscalculation that has serious implications for future flood risks, flood insurance and business development in an expanding floodplain.

“This analysis shows that average high-water marks on these river systems are rising about an inch per year — that’s a rate ten times greater than the annual rise in sea levels now occurring due to climate change,” said Robert Criss, PhD, professor of geology in the Department of Earth and Planetary Sciences in Arts & Sciences and author of the study.

Published this month in an advance online issue of the Journal of Earth Science, the findings are important, Criss said, because many of the nation’s flood-control river levee systems are not engineered to withstand floods that rise much higher than the projected 100-year flood level.

Any flood that rises even a few inches over the top of a 100-year levee has the potential to cause a catastrophic breach of the flood control system, he warns.

Based on complicated equations currently used by key federal agencies, such as the Federal Emergency Management Agency (FEMA) and the U.S. Army Corps of Engineers (USACE), the official 100-year flood level is a key national index of potential flood severity. Levees are commonly designed to withstand floods at “100-year” levels and “100-year” flood zones are delineated on detailed flood insurance maps produced by FEMA.

Criss, a hydrogeologist who has studied water flows on major rivers for decades, has long argued that man-made river control systems, such as levees, locks, dams and navigation-enhancing dikes, have gradually increased the odds of catastrophic flooding by tightly constricting river channels and preventing floodwaters from flowing naturally into surrounding wetlands and floodplains.

Global warming and the resulting increase in extreme weather cycles has only added to the flooding risk in recent years, he said.

In his study, Criss argues that the statistical formulas now used to set federally-recognized official levels for 100-year flood events are grossly inaccurate because they assume conditions are the same as they were many decades ago, when the rivers were relatively untamed and global weather patterns were more consistent.

In a 2008 study, he showed that flooding patterns along the Mississippi River near Hannibal, Missouri, were already in an extreme range — far beyond what would be expected using the official federal flood risk calculations.

Since the publication of that study, floods at Hannibal have exceeded the “10-year” flood levels in 2009, 2010 and 2011; in both 2013 and 2014, the area experienced stages that were officially designated as “50-year” floods, he said.

“Such outcomes are far too unlikely to be attributed to a nearly continuous succession of statistical flukes, and instead must be attributed to faulty calculation of flood risk,” Criss said. “Many factors such as climate change and in-channel structures are causing flood levels to rise, so realistic estimation of future flood levels must take these changes into account.”

A perfect example is shown in this video from August 1014, when the Detroit area was hit with enormous rainfall that overwhelmed sewers, storm drains, and even freeways that were constructed to meet the conditions of 50 years ago, and have performed well enough, ’til now.

Washington Post:

Detroit Local 4 Meteorologist Paul Gross was astounded by the flooding event:

I have lived my entire life and worked my entire career here, and I have never seen as widespread a flooding event. … I also remember some individual intense thunderstorms that flooded ONE freeway. But I don’t ever remember EVERY freeway being flooded out.