Built in the U.S.A.: Inches to Protect a City from Floods

In the heart of the Heartland, Des Moines, Iowa, officials spent $3 million to raise a century-old bridge by 54 inches.

Like a vaulter straining to clear a high bar, the newly raised Red Bridge—which reopened in late May 2017—gives the downtown area an extra 6 inches of breathing room above floods like the ones that washed through the city in 1993 and 2008.

Des Moines city officials spent $3 million to raise the Red Bridge 54 inches.Those 6 inches are critical in meeting insurability standards set by the U.S. Federal Emergency Management Agency (FEMA).

Public and private sector partners have invested millions of dollars in recent years to develop riverfront amenities in the Iowa state capital. Improvements include converting the Red Bridge, a former Union Pacific Railroad structure that spans the Des Moines River, to pedestrian use. Failing to meet FEMA flood standards could have led to those assets being torn out and replaced.

The decision to raise the Red Bridge was “between getting accredited and not getting accredited” by FEMA, says Pam Cooksey, P.E., Des Moines City Engineer.

The 45-day Red Bridge lift began in the fall of 2016 with two end spans lifted by cranes off their seats and set on shoring alongside the bridge. Four other spans were raised 14 inches using hydraulic jacks. Blocking was installed, the jacks were repositioned, and the process was repeated until the bridge rose the required 54 inches (4.5 feet). Piers also were raised to support the new bridge height with new concrete.

(Watch a time-lapse video of the bridge being raised in late 2016.)

Raising the Red Bridge is part of a broader 10-year effort to replace and raise three vehicle bridges, install pump stations, raise levees, and better protect the city’s wastewater treatment facility from inundation during floods.

“You’ve Got to Be Kidding”

To be sure, at least some of the $110 million infrastructure program is aimed at replacing structurally deficient bridges. But underlying the capital program is the realization by federal, state, and local officials that floods with a 1% probability of occurring—so-called “100-year floods”—are more likely.

(Contrary to the popular press, a 100-year flood does not mean that a flood in 2008 won’t be duplicated until 2108. Instead, it means that each year has a 1% chance of such a flood. That means a 100-year flood can occur two years in a row. As a result, a homeowner with a typical 30-year mortgage has better than 1 in 4 chance of experiencing a “100-year” flood over those 30 years. “That’s a different ballgame,” says one source interviewed for this article.)

Indeed, a 2011 U.S. Army Corps of Engineers study that updated flood frequency models used on the Des Moines River found that the probabilities associated with 50, 100, and 500-year floods were “significantly higher than indicated in the past,” says Jason Smith, flow risk management program risk manager for the Rock Island District office. That meant elevations associated with levees and bridges were “not meeting FEMA standards for accreditation.”

(Click to enlarge.) Stream drainage features across the Midwest. Des Moines lies in the cream-colored area on the left. Credit: USACEThe Corps's finding took Des Moines officials by surprise.

“You have got to be kidding; what’s going on?” was the reaction in her office, Cooksey recalls. “We were not pleased with the results,” she recalls. The Corps study revealed that the city’s levee system would fall short--literally--of reliably protecting the downtown from a 1% probability flood.

What’s more, flood control reservoirs both north and south of the city do little to ease river flows during a 1%-probability flood event such as in 1993 and 2008. The Corps of Engineers-owned Saylorville Lake 10 miles north of the city is "ineffective” in a 100-year flood, Cooksey says. “Water just flows over the emergency spillway.”

Greater Flood Risk, but Why?

Multiple factors could be at play in the increased flood risk. For one thing, Iowa’s landscape is one of the most altered in North America. The state’s agricultural industry has groomed large swaths of farmland across the state to boost crop yields and improve efficiency. To that end, roughly 40% of the state’s farmland is underlain with drainage tiles. These tiles help to move water off of fields and into streams and rivers.

But most of the landscape changes are 100 years old or older. The 2011 Corps of Engineers study says, “The major land use conversion to agricultural land use occurred prior to the 20th century.” The Corps’ expectation is that “the influence of land use change on annual maximum flows is minimal.”

A second possibility is that it simply is rainier. Evidence suggests that idea may hold water.

The Corps study says that notion was raised as far back as 1970 by officials tasked with developing guidelines for flood frequency analysis. The estimated frequency of flood event volumes has “increased significantly” in recent years, the 2011 Corps study says. “The increased frequency is clearly due to the much wetter period that has occurred since 1993.”

Witold Krajewski, Iowa Flood Center. That is backed up in part by researchers at Iowa State University’s Iowa Flood Center, which was formed in response to the 2008 floods.

“Independent studies by scientists at University of Iowa show there are increasing trends of precipitation in the Upper Midwest” including Iowa, says Witold Krajewski, Flood Center director.

Weaknesses

Even so, a relative lack of long-term data exists about stream flows and rainfall variability. The U.S. Geological Survey maintains stream gauging stations across Iowa as well as the U.S. Krajewski says there are no more than 150 such gauges across Iowa, and that the number nationwide is around 4,000, down from a peak of around 8,000.

Standard flood prediction techniques, he says, call for recording the annual peak flow at a stream gauge and then fitting that single point to a distribution model. Extrapolations then are made and from those predictions are made about regional flood risk. Typically, missing are factors that include land use and topography.

“There are weaknesses everywhere you look” Krajewski says.

Despite the drawbacks, the Iowa Flood Center used the standard approach in completing a six-year effort to update flood inundation maps for the entire state of Iowa that FEMA and others use to assess risk. Krajewski says the Center decided to develop the maps using standard procedures because they wanted the maps to be approved.

“It’s a tough situation,” he says. “We are not quite ready to challenge in a quantitative way the status quo.”

Raising Levees

That leaves communities such as Des Moines making long-term infrastructure investment decisions based on models that are at best inexact. Compounding things is what is proving to be a time-consuming process to raise levees protecting the city.

In September 2016, the Iowa Watersheds Project ended with the completion of over 150 built structures including ponds, terraces, wetlands, water and sediment control basins, and on-road structures. The Iowa Flood Center received $4.5M from the U.S. Department of Housing and Urban Development for the five-year project aimed at mitigating flood risk in select Iowa watersheds.That’s because the Corps of Engineers decided that it was not in the federal interest to raise the levees anywhere from a few inches to a few feet. The Corps determined the cost would be greater than the benefit, says Jason Smith.

As a result, it falls to the city itself to ask for and then pay for any modification, which it says are necessary. But to do that, the city has to file what’s known as a “408 package” of applications and supporting documentation. That package is based on a federal statute that required Corps approval before an entity such as Des Moines can modify a levee.

The city filed its paperwork in mid-May. It could take as long as two years for the review process to be completed, Pam Cooksey says.

The 408 process can lead to some awkward logistics. For example, one levee that needs to be raised is owned by the Corps but is linked by hydrology to the city. That leaves the city engineer’s office having to apply for and then wait for federal approval to spend city money to pay for improvements to a Corps-owned asset.

(Editor's note: Investment in critical infrastructure is gaining increased attention at the federal, state, and local levels around the U.S. Engineering360 presents an ongoing series of articles, "Built in the U.S.A.," that look at projects, approaches, technologies, and engineering challenges that lie at the heart of this push to modernize existing infrastructure and build for future needs.)