Why We Don’t Design Our Cities To Withstand 1000-Year Floods

Why We Don’t Design Our Cities To Withstand 1000-Year Floods

The floods in the US state of Colarado are being described as “biblical”, and that word seems to fit for once. Boulder, for example, usually gets around 380mm of precipitation annually. This year, that amount has fallen in the 10 days since September 9 alone. On September 12, they received 229mm in one day.

As floodwaters covering an area the size of Delaware turned suburban neighbourhoods into massive lakes and rerouted creeks through canyon roads, estimates sailed cleanly past the qualifications for a 100-year flood, then surpassed 500-year flood levels. Now scientists think the disaster could be classified as a 1000-year flood event.

Satellite imagery by NASA shows Colorado’s Front Range before and after the floods. Dark blue and black are floodwaters. (The images have been artificially coloured.)

When I went to the University of Colorado, I used to see signs posted along the Boulder Creek path: “Climb to Safety In Case of a Flash Flood.” It seemed incongruous in a city that saw 315 days of sunshine, where the only people I ever saw climbing were hanging from canyon walls on purpose. But Boulder has long been recognised as a ticking time bomb. In 1975, the prominent geographer Gilbert White named a Boulder flood as one of the country’s top natural disasters waiting to happen and outlined a detailed plan to reduce the potential devastation. There’s a memorial to White in Boulder that shows flood levels through out the years, as well as the predicted level for 100-year, 500-year and 1000-year floods.

That’s the thing: Everyone knew exactly what was going to happen eventually. So why haven’t the 15 Colorado counties affected by the floods made the changes that could have stopped the water from ripping apart these cities and claiming eight lives?

As early as 1923, the Olmstead Brothers (the sons of Central Park designer Frederick Law Olmstead) recommended keeping a large area of open space around Boulder Creek, via Women Environmental Artists Directory

“Nothing that you can build is ever going to prevent all flooding,” says Dr Josephine Axt, the chief of planning for the Los Angeles District of the US Army Corps of Engineers, which manages levees, dams and other urban river projects across the country. “If you get a big enough storm it doesn’t matter what dam or channel you have, it can be overcome.” The Corps of Engineers works is doing so by evaluating its projects on a cost-risk basis, says Kerry Casey, a senior hydraulic engineer. “We’re trying to find the best project we can build that absorbs the risk requirement at the best cost.”

That level of risk is what the Corps of Engineers means when they (and insurance companies) say “100-year flood,” but that terminology is actually falling out of favour, says Axt. “That implies it’s only going to happen very 100 years, which is not the case. We can have three years in a row that have 100-year floods,” she says. “Now we look at ‘annual chance exceedance,’ which gives the sense that it’s a probability.” So the chance (or Annual Exceedance Probability, AEP) of a 100-year flood happening in any given year is one per cent. A 1000-year flood? .1 per cent.

The Los Angeles River near downtown Los Angeles

Another evolution has been the strategy behind those Corps of Engineers projects. “20 years ago, we called it flood control, then flood reduction,” says Axt. “Now it’s flood risk management.” Where once the Corps of Engineers hoped to harness rivers, now they’re allowing the water to flow more naturally, flooding (within reason) without endangering lives or property.

The best known (and most cinematic) of these “controlled” rivers is in Los Angeles, where most of the 51-mile river was channelised into concrete banks after several catastrophic floods in the early part of the 20th century. But the LA River also best illustrates the Corps of Engineers’ changing values. Their new proposal released last week recommends that 11 miles of the river be restored to its natural state, a big step towards what many groups hope will lead to revitalising the entire river.

The proposal for the Taylor Yards section of the Los Angeles River, from the LA River Revitalization Master Plan

There are several portions of the 82km river that already feature a “soft-bottom”, or naturalised gravel riverbed, and the city has even opened up sections of the river for kayaking the past two summers. But instead of the angled cement walls that quickly flush floodwaters out to the Pacific Ocean, the flow will be slowed by diverting it into side channels, wetlands and underground cisterns. Infrastructurally, the unimaginative walls will be replaced with “creativity with concrete”, materials that restore the ecosystem and prevent the water from washing the vegetation away, says Casey: turf-reinforced mats, erosion-control blankets, concrete structures with radial spokes like giant jacks buried in the banks. “The water is still coming fast, but we need to prevent erosion.”

The Missouri River flooding near Jefferson City, Missouri on July 30, 1993; photos via Missouri Department of Transportation

When I was 16 and growing up in St Louis, the Missouri and Mississippi Rivers experienced a 300-year flood event. It was the highest water levels ever recorded and some areas experienced flooding for six months. The 1993 flood caused the most widespread damage in US history, flooding over 77,700 square kilometres, inundating 75 entire cities (several of which were relocated) and destroying over 100,000 buildings.

One of those buildings was my parents’ lawn care business in the Chesterfield Valley, just outside St Louis. Like many of the residents of the Valley, who were attracted to the area by affordable and available real estate, they knew the risk of building in a floodplain, but it didn’t feel imminent. Their building was 2km from the water, with a 100-year levee that separated them from the river. Their store was only five years old when the river poured into the valley, drowning the building in three metres of water. When the waters receded, there were jets from a nearby airport stranded on their property.

In collaboration with the Corps of Engineers, the business owners in the Valley worked to build a private levee to 500-year levels to protect their community. It bought them some time. But there are signs that levees only cause more problems by channelling more water into a narrower space. One levee built to protect a city in 1970 might be the one that aids the destruction of a suburb in 2010. In 2011, engineers dramatically blew up a levee on the Mississippi to save several small towns. Between the construction of levees and encroaching development, rivers often don’t have anywhere to go.

Images collected by a drone of the flooded and inaccessible St. Vrain and Left Hand Creeks, in Boulder County, image by Falcon UAV

In the short term, engineers also need to learn about how their structures behave before they’re overwhelmed in order to design better solutions. In Colorado, the owner of a company that sells unmanned aerial vehicles (UAVs) got a lot of attention this week when he tried to use a drone to help assess damage to roads and bridges. Apparently he was ordered to stop by the Boulder County sheriff department — though the track record for drones in floods is proven.

Since 2009, drones have been used successfully in flooding events on the Red River in Grand Forks, North Dakota to survey weaknesses in the current infrastructure and make hydrological predictions. (The rest of the time, these particular drones patrol the US/Canada border for drug trafficking and other immigration-related issues.) The drones make sense: Satellite imagery is limited in detail, and helicopters sometimes can’t fly in the inclement weather that usually accompanies flash flooding. If drones could be used to locate people stranded by flooding, or warn residents ahead of time, they might be able to help save lives too.

Still, one big issue remains: We love building our cities too close to dangerous rivers. And that’s unlikely to change soon. “Is the government going to tell people they can’t build their cabin next to the stream?” says Axt. “They don’t want to be told that.” A 2009 executive order aimed to toughen restrictions on floodplain development and discourage the addition of levees and dams that give hazardous areas a false sense of security.

Especially after the exceptionally devastating coastal flooding events of Katrina and Sandy, cities might finally be paying attention, looking at environmental elements like marshlands that can provide both flood protection and open space opportunities. But maybe there’s bigger cultural problem with American cities that needs to be addressed first. “We had a meeting with Dutch engineers and when they asked what we designed for — we said 100 years,” says Casey. “They design for a 1000-year event.”

[Live Science, Discover, Climate Central; New York Times; FEMA]

Top picture: Roadsidepictures