It rained for months in Mississippi before the levee at Mounds Landing broke on 21 April 1927, triggering a series of events that left roughly 70,000 square kilometers of land across three states underwater, hundreds of people dead, and several hundred thousand more homeless. The 1927 flood on the Lower Mississippi River was one of the most destructive in U.S. history. The cost of the flood came out to $1 billion in 1927 dollars—a sum equal to a third of the federal government’s annual budget that year. The magnitude and speed of the river’s rise at Mounds Landing before the levee gave way are still unparalleled.
Despite the widespread and lasting consequences of the flood, the events that led up to it are not well understood. There are no records of the flood peak discharge measurements from the U.S. Geological Survey, the U.S. Army Corps of Engineers, the Mississippi River Commission, or the state of Arkansas. And estimates of the rainfall that triggered the flooding are equally uncertain, ranging from storms over a period of just a few weeks to many months.
Such gaps in knowledge can lead to gaps in preparation. The National Weather Service in the United States uses observations of past storms to imagine hypothetical floods—the worst-case flooding scenarios for the Lower Mississippi—that then influence the design of flood control systems in the region. But the storm events that led to the 1927 flood are not among them. Here Smith and Baeck examine the rainfall events preceding the 1927 flood to determine how the distribution of precipitation over time and across the states that make up the Lower Mississippi River basin led to record flooding.
The researchers used downscaling simulations—numerical manipulations of data to visualize regional conditions in higher detail—from the Weather Research and Forecasting Model. To set the boundary conditions for their analysis, the team applied 20th Century Reanalysis fields, an expanded data set that includes climatic measures for the entire past century.
The team supplemented the simulations with rain gauge data from the National Centers for Environmental Information (formerly National Climatic Data Center) and analyzed measurements from sparse USGS stream gauging stations and the Army Corps of Engineers to get a sense of the magnitude of the flood peak. The authors noted that the peak discharge of the Mississippi River around Arkansas City—nearly 70,000 cubic meters per second—is the largest in USGS records of floods.
The team found that the 1927 flood could be traced to a series of major storms that battered the river basin throughout March and April of that year. Systems of thunderstorms, some of which produced tornadoes, were important agents of extreme rainfall and flooding. Daily rain gauge observations over that period reveal heavy rainfall in the lower region of the Mississippi River basin, with the highest accumulation in Mississippi occurring near Mounds Landing. Both rain gauges and their simulations singled out Arkansas at the epicenter of heavy rainfall before the flooding.
All the major flooding events in the Lower Mississippi for more than a century have been linked to exceptionally large water vapor transport from the Gulf of Mexico, and the researchers found that 1927 was no exception. The unusual, westward position of the North Atlantic Subtropical High—a migrating high-pressure area in the Atlantic Ocean—brought extreme amounts of warm, moist air up from the Gulf of Mexico, setting up the central United States for a spring of heavy rains.
The study shows that downscaling simulations that use 20th Century Reanalysis fields can be helpful in elucidating the extreme precipitation patterns that can result in catastrophic flooding. (Water Resources Research, doi:10.1002/2015WR017927, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), Reimagining a fatal flood, Eos, 97, doi:10.1029/2016EO047909. Published on 17 March 2016.