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Stopping local flash floods requires cooperative approach, say experts

This article first appeared in the St. Louis Beacon, June 23, 2009 - A week after the St. Louis area was victimized by severe flash flooding, some experts are emphasizing that a community-wide approach that includes buyouts and smarter development will be needed to fix the area's flooding problems.

The floods that struck St. Louis last Monday and Tuesday left many people, especially in oft-victimized University City, frustrated and angry with the lack of progress made in stopping the floods. But the Metropolitan St. Louis Sewer District (MSD) says there needs to be a multi-jurisdictional conversation and effort to stop the floods from damaging homes and businesses.

WHAT'S CAUSING THE FLOODS?

The recent flash floods have happened, experts say, because parts of St. Louis County, including many of the areas affected by floods lately, lie in floodplains, which are flat, chronically flood-prone areas near waterways. "A floodplain, by definition, is going to flood," said Lance LeComb, spokesman for MSD. "It's not a matter of if; it's a matter of when."

Development in and near floodplains is one major cause because it destroys grassy areas that absorb water, replacing them with surfaces that are impervious to water, such as buildings, parking lots, sidewalks and roads, to name a few. Floodplains are typically attractive places to develop because they are flat and provide a nearby source of water.

When rain hits impervious surfaces, the water has nowhere to go, except into nearby rivers and creeks, which can overflow if they receive water too quickly and put nearby areas under water.

Bob Criss, an expert on hydrogeology at Washington University, said too much development has occurred in recent decades, increasing the amount of impervious surfaces significantly. That's magnifying the amount of runoff so much that even small amounts of rain have resulted in severe flash flooding, and it's caused areas to flood that didn't flood as recently as 20 years ago.

This development has happened even as St. Louis County's population has declined by 2.4 percent since 2000 -- from 1.01 million to an estimated 991,830 in 2008, the latest figures available.

But Criss said development needs to slow down or there will be serious consequences, including even worse flooding.

Criss said last Monday and Tuesday's rainfall, totaling about 3.4 inches, was "trivial," yet it caused flooding that he might have expected from several inches more rain.

Still, the amount of rain isn't the only factor; the rate of rainfall often dwarfs the amount in importance, according to Ben Miller, a meteorologist at the National Weather Service in Weldon Spring.

"If you have 3 inches over a 24-hour period that falls at the same slow, steady rate, rivers are going to go up, but it's not going to be a big deal," Miller said. "But if you get that 3 inches in less than an hour, that's going to cause problems."

Last Monday and Tuesday, the highest hourly rainfall total was only about an inch, but experts say the presence of so much impervious surface is producing the same effect.

Criss also suggested that MSD's method of piping away the runoff water that the agency's storm drains absorb could also worsen the flooding issue. The pipes deliver that water into creeks and streams at a very rapid pace when it rains. "That makes the flood peaks much higher," Criss said.

But LeComb said the runoff water its drains take up would otherwise end up flowing into waterways anyway, making Criss' claim "a moot point."

SOLUTIONS TO FLOODING

MSD is powerless to do anything since it is not a flood management agency, LeComb said. It doesn't own the local rivers and creeks, which it says are private property. And it doesn't have statutory or financial power, LeComb said, to do any projects to mitigate the flooding.

Instead, some experts say municipalities need to start working together, and residents need to push their municipal governments to do buyouts and change how they do development.

LeComb said property buyouts and wiser development may be the only viable long-solutions to the problem.

"This needs to be addressed at the municipal level," LeComb said. "Until people put that demand on their municipal governments, things are not going to change."

The Army Corps of Engineers tried to devise a project to prevent the long-troublesome River Des Peres from flooding, but found that no such project would work. Instead, the Corps has recommended buyouts of homes and buildings in some areas around the river prone to frequent flooding, including Wilson Avenue.

Residents of University City have made some progress. Earlier this month, the city approved $1 million for buying up 26 homes along Wilson, to match $4 million in state Emergency Management Agency funds.

LeComb said the Army Corps has recommended buyouts of as many as 150 homes; that could push the cost into the several millions of dollars.

MSD has implemented a stormwater fee that charges building owners based on how much impervious surface they have. That charge goes down if owners eliminate some of their impervious surface; MSD hopes the system will encourage people to do just that.

Criss agreed that replacing impervious surfaces with permeable ones like grass will help, but he said MSD's stormwater fee is unfair because it penalizes old and new structures equally. "It's not the old structures that are causing the problems," Criss said.

Criss also suggested "green" architectural and landscaping techniques, such as systems that catch runoff water and store it for reuse, including cisterns, and putting in grassy areas to reduce the need for storm drains.

LeComb encourages homeowners and renters to buy a flood insurance policy, either through a private agency or through the Federal Emergency Management Agency.

Puneet Kollipara, an intern at the Beacon, is a student at Washington University. 

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