The Canals of… Cedar Rapids?

While May 2008 went down in history as a prolific month for severe weather and tornadoes, June 2008 will be remembered for its flooding. Though it was relatively dry and warm over much of Oklahoma during June, our neighbors to the northeast in Iowa, Illinois, and Missouri dealt with flooding that evoked memories of 1993, a year when floods struck up and down the Mississippi valley, and drawing parallels with the tragic aftermath of Hurricane Katrina. So major was the flooding that even the candidates for president took notice; Barack Obama took time off from campaigning to fill sandbags in Illinois. This month, we’ll take a brief look at the extent and effects of these floods, then dive into the meteorological (and hydrological) causes behind them.

Many cities were hard hit by the floods. Pictured above is downtown Cedar Rapids, IA, covered in floodwaters. The flooding in Cedar Rapids was among the worst ever recorded in the city, submerging more than a thousand city blocks and shutting down much of the downtown business district. Iowa’s capital city, Des Moines, had to be partially evacuated on June 13, when floodwaters breached a levee.

As floodwaters from Des Moines and Cedar Rapids flowed south and east, they eventually emptied into the Mississippi river, leading to more floods and necessitating an evacuation of the small river town of Oquawka, IL, on June 14. Concerted sandbagging efforts helped hold back the waters in other river towns like Burlington, IA, and Quincy, IL. Record crests were reported along the Mississippi river; the river level measured at Burlington was expected to exceed the 1993 record by nearly a foot, and the river crested 10 feet above flood stage in Hannibal, MO.

Flooding was not confined to the Mississippi valley, either. Heavy rains (some areas receiving over 10 inches in a single day) in Indiana inundated nearly the entire the town of Paragon with floodwater; 29 of the state’s 92 counties were declared federal disaster areas. In a rather bizarre turn of events, persistent heavy rains caused the failure of a dam in Wisconsin, causing Lake Delton to completely drain, washing away a highway and several homes. In all, 13 deaths have been reported as a result of the past month’s flooding, and untold thousands of homes have been lost.

As in 1993, some of the worst flooding resulted after levees broke, releasing the waters of the swollen Mississippi into surrounding fields and towns. While levees are crucial in protecting towns near rivers from flooding, they can ironically exacerbate the situation, as the levees confine the river to a narrower channel, resulting in a higher crest. In fact, several cases were observed both during 1993 and during this June where a levee broke on one side of the river and water levels were actually observed to drop by several inches on the opposite side of the river, as water was allowed to flow into previously dry land. Between June 15 and 18, levee failures resulted in the flooding of several Mississippi river towns, including Gulfport and Meyer, IL.

But what has been the cause of all these floods and levee failures? If you followed your instincts and guessed ‘too much rain’, give yourself a gold star. There’s much more to it than just that, though. More specifically, the floods were so widespread and severe because of too much rain in too little time over a very wide area. The chart below, produced by NOAA, depicts observed precipitation over the Upper Midwest during the first half of June. Totals above 8” cover most of southern Wisconsin and nearly half of Iowa, and bullseyes of up to 16 inches can be seen, most notably in central Indiana.

Geography also played an important role; nearly all the heavy rainfall that we can see from this chart lies within the Mississippi River Basin—that is, given time, it will flow into streams which flow into rivers which eventually merge with the Mississippi. This helps explain why flooding was so devastating along the Mississippi river in particular. Finally, let’s take one more step back and look at the prevailing synoptic pattern. During much of early June, the Upper Midwest was caught under a relatively static pattern, with southwesterly flow aloft, a persistent stationary front at the surface, and multiple low pressure systems that moved through the area, causing heavy rainfall, and ultimately resulting in the pattern we saw above.



Story is © Nate Snook, 2008