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Analysis: Development plus downpours ramped up creek flooding

This article first appeared in the St. Louis Beacon: September 17, 2008 - St. Louis is on pace to exceed the rainfall received in any year since 1871, when accurate record keeping began. This heavy rain and unwise development have caused serious flooding in practically all of our regional watersheds, large and small.

While floods on our large rivers receive plenty of attention, floods on our small creeks can be equally devastating and provide interesting comparisons to regional floods.

Flooding on major rivers is a familiar scenario. Widespread, heavy rains fall for weeks, and the water runs off the land. The rivers swell, attaining levels higher than they should because they are constricted by levees and wing dikes. A flood wave forms and moves downstream, challenging levee after levee, town after town, typically over a period of days to months.

The National Guard is called in to help, sandbags are heroically stacked, and the news stories roll. Politician eagerly grab microphones and use yet another opportunity to shamelessly direct fortunes in ways that aggravate the problem. After it's all over, in our region at least, people and developments return to the floodplains with redoubled vigor.

Flash flooding on smaller creeks is fundamentally the same, yet there are some remarkable differences that have been aggravated by impervious construction. Too much rain is delivered and the creeks swell, but the response is profound, fast and recurrent.

While the record flood peak on the Mississippi River conveyed about 6 times more water than normal flow, the peak flows on these creeks can be a hundred to several thousand times normal. The waters rise to peak levels within a few hours of rainfall delivery. A day later, little remains of the event save damaged property, frazzled nerves and the debris and mud.

The heavy rainfall last Sunday provided our most recent example. The remnants of Hurricane Ike went right over St. Louis, delivering 4 to 5 inches of rain in as many hours. Many creeks rose three feet or more an hour.

The tiny Black Creek watershed delivered several times the average flow of the Meramec River, until the gaging station was knocked offscale. Deer Creek in Ladue rose from 1 to 18 feet before the rain stopped, and at Maplewood it topped 21 feet only an hour later; yet water at these sites had respectively receded below 5 feet by early afternoon and by evening.

Many businesses along Hanley Industrial Court, along Litzsinger Road and elsewhere flooded to several feet. My son, a Horton Watkins High School student, tells me that basement classrooms were flooded and that a construction trailer situated on the parking lot was toppled. I saw many local residents mopping up and drying furniture a day later. All this happened in a few hours, yet the Mississippi River is steadily rising, a few inches an hour, three days later.

Robert Criss is a professor in the department of Earth and Planetary Sciences at Washington University. He is the coauthor of the 2003 book, "At the Confluence: Rivers, Floods, and Water Quality in the St. Louis Region." 

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