The correct interpretation of the concept of a 500 or 1,000 year flood

A flood expert addresses five myths about floods and this includes the idea that a 500 year flood only happens once every 500 years:

Myth No. 3
A “100-year flood” is a historic, once-in-a-century disaster.

Describing floods in terms of “100-year,” “500-year” and “1,000-year” often makes people think the disaster was the most severe to occur in that time frame — as encapsulated by President Trump’s tweet calling Harvey a “once in 500 year flood!” He’s not alone. When researchers from the University of California at Berkeley surveyed residents in Stockton, Calif., about their perceived flood risk, they found that although 34 percent claimed familiarity with the term “100-year flood,” only 2.6 percent defined it correctly. The most common responses were some variation of “A major flood comes every 100 years — it’s a worst-case scenario’’ and ‘‘According to history, every 100 years or so, major flooding has occurred in the area and through documented history, they can predict or hypothesize on what to expect and plan accordingly and hopefully correct.”

In fact, the metric communicates the flood risk of a given area : A home in a 100-year flood plain has a 1 percent chance of flooding in a given year. In 2018, Ellicott City, Md., experienced its second 1,000-year flood in two years, and with Harvey, Houston faced its third 500-year flood in three years.

That risk constantly changes, because of factors such as the natural movement of rivers, the development of new parcels of land, and climate change’s influence on rainfall, snowmelt, storm surges and sea level. “Because of all the uncertainty, a flood that has a 1 percent annual risk of happening has a high water mark that is best described as a range, not a single maximum point,” according to FiveThirtyEight.

I am not surprised that the majority of respondents in the cited survey got this wrong because I have never heard it explained this way. Either way, the general idea still seems to hold: the major flooding/storm/disaster is relatively rare and the probability is low in a given year that the major problem will occur.

Of course, that does not mean that there is no risk or that residents couldn’t experience multiple occurrences within a short time period (though this is predicted to be rare). Low risk events seem to flummox people when they do actually happen. Furthermore, as noted above, conditions can change and the same storms can create more damage depending on development changes.

So if this commonly used way of discussing risk and occurrences of natural disasters is not effective, what would better communicate the idea to local residents and leaders? Would it be better to provide the percent risk of flooding each year?

McMansions lead to water runoff damage in Kirkwood, Missouri

The construction of new housing can lead to water issues for existing homeowners. See the ongoing case of Kirkwood, Missouri homeowners dealing with more water due to the construction of McMansions:

Next week, a new Kirkwood water runoff regulation will take effect, but longtime residents say it’s all too little too late…

Like many other longtime Kirkwood residents, Liskew said water began invading her home when new construction started near her neighborhood. Behind her home, she noticed new, large homes—often called ‘McMansions’—were being built on small lots…

That special council created a new storm water management regulation. The ordinance requires all “infill development” to capture rainfall runoff, and submit a plan to the Metropolitan St. Louis Sewer District for review and approval prior to the issuance of any building permits…

And she said it’s been an expensive, never-ending problem to worry about. In addition to several French drains, Liskew has installed a sump pump, taken out basement windows, graded a large portion of her yard and even commissioned a water runoff study to find out where the water was originating. The study found the water was coming from the neighborhood directly behind her home, and heading directly into her backyard.

See an earlier blog post on the Kirkwood situation. To some degree, the construction of any new residential units is likely to affect water runoff. Switching land from non-use or agricultural use to homes, driveways, yards, and streets will have an effect. Add to that the pejorative term McMansions used here: big homes take up more space and in the interest of keeping water away from them even more water is channeled elsewhere.

I sometimes wonder if the way water issues in suburbia work is like this: every new development attempts to push the water somewhere else and the problem simply moves onto someone else’s property. Generally, developers and municipalities do their best to move the water away from existing buildings and uses but this may not be possible depending on the topography and existing infrastructure. Cleaning up the water issues after the fact – such as in Kirkwood or the Deep Tunnel project in the Chicago region – is costly and very frustrating. But, without a commitment to avoid sprawl or widespread adoption of greener techniques, the water problems will just get pushed down the road. Flooding will continue to be a major suburban problem.

Deep Tunnel as the wrong solution to water issues (plus alternative uses)

Henry Grabar poses an interesting question: what if the Deep Tunnel project, one of the largest civil engineering feats in the world, does not solve flooding and stormwater issues in the Chicago region?

What if Chicago took a wrong turn in 1972 when, in the spirit of civic grandee Daniel Burnham (“Make no little plans”), it opted to build the world’s largest sewers instead of making all possible efforts to keep rainwater out of them? Scott Bernstein, the founder of the Center for Neighborhood Technology, says that the Deep Tunnel imposed a massive opportunity cost because the city and the district did little else to adapt. The MWRD spent billions on what engineers call “gray infrastructure” (pipes, tanks, pumps) and virtually nothing on “green infrastructure”: rain barrels, detention ponds, green roofs, porous pavements, and other adaptations that would have kept water out of the system…

The project proceeded regardless. Even with downsized reservoirs and a longer time frame, Chicago’s ambition captured the attention of civil engineers around the world. Today, most U.S. cities whose combined sewer overflows are governed by consent decrees with the EPA are working on Chicago-style digs. St. Louis, which has the fourth-largest sewer system in the country, is under a consent decree to commit $4.7 billion to ending its overflows through deep tunnels…

While engineers’ penchant for megaprojects endures, some American cities are preaching deterrence. If Chicago built a bathtub, Philadelphia is trying to transform itself into a sponge with park space, street trees, and permeable pavement. The city is spending $2.4 billion to implement the nation’s largest green infrastructure plan, an experiment that positions it as the anti-Chicago. The city thinks keeping water out of the system will save billions of dollars compared to a rejected tunnel proposal—and that green initiatives will produce positive externalities, like improving air quality and creating verdant streets.

In Chicago, meanwhile, the MWRD has committed to creating just 10 million gallons of green infrastructure capacity under its EPA consent decree. Compare that to neighboring Milwaukee, a deep-tunnel city that now believes its green infrastructure will, by 2035, surpass the capacity of the tunnels and hold up to 740 million gallons of rain where it falls.

Hindsight may always be a little tricky in these cases as we have the advantage now of being able to see the Deep Tunnel project in action. Does it actually accomplish its goals? Was all the spent money worth it? At the same time, a project of this magnitude should have generated plenty of discussion and at least a few alternative options.

If Deep Tunnel does not work as intended or does not solve all of the flooding and stormwater problems, I wonder if it could be used in other ways. I’m thinking of other major infrastructure projects that have been reversed or reused, like urban highways that are torn out (like in Boston or San Francisco) or former railroad lines turned parks or recreation areas (think the High Line). Some other options for the Deep Tunnel:

  1. Underground roads. With Elon Musk’s Boring Company working on underground roads plus Chicago’s legacy of Lower Wacker Drive, perhaps traffic could be rerouted deep underground.
  2. Underground freight movement. Given Chicago’s railroad bottleneck, this could be an interesting solution.
  3. An underground park and recreation area. It would certainly be unique. Think a combination of spelunking, rock climbing, and exploration.
  4. A military installation and testing area.

Teaching kids about Chicago’s Deep Tunnel project

Kids should know about one of the largest civil engineering projects in the world: the Deep Tunnel project in and around Chicago.

DeepTunnelNotebaertNatureMuseum

This is from the Riverworks exhibit at the Peggy Notebaert Nature Museum in Chicago. While some of the pieces of the exhibit failed to work the day we visited, I think I could see the purpose of the The Deep Tunnel exhibit: the floodwaters would be diverted away from the city.

The concept may appear simple and explainable to kids but the execution in real life is not. The exhibit suggests the flooding the past is now alleviated by Deep Tunnel. Yet, the problems are likely to go on in a region that continues to expand and change. Remediating water and flooding issues is a very difficult task compared to altering development at the beginning.

It is interesting to think how else this engineering feat could be presented to children. I could imagine a scaled model that kids could walk through to help give them a sense of the size of the sewers needed as well as the size of some of the water reservoirs. Deep Tunnel is not intended for minor amounts of water; this is supposed to help protect millions of people on a fairly regular basis. Communicating the sheer size could fascinate kids. Or, perhaps some sort of computer game where kids play the role of an engineer or expert as they make choices about where to divert water. Come to think of it, where is this version of Simcity or Roller Coaster Tycoon – “Infrastructure Builder” or “Sewer Wars” or something catchier.

Invasion of McMansions in Kirkwood, Missouri

Teardown McMansions have infiltrated an older neighborhood in a well-off St. Louis suburb:

Residents said not only are smaller historic homes getting wiped out in the process, but the large houses are causing problems for some of their next-door neighbors…

The one next door to her on Cleveland Avenue was erected last year and is nearly twice the size of the original home. It’s a four-bedroom home on the market for more than $800,000.

She said it’s created a real problem for her. The new home’s rain runoff has turned her driveway into a lake…

The city says the builders have followed all the community’s guidelines:

“…The new house on this site sits closer to the neighbor’s driveway, which may explain the confusion. Yes, the new home was built per permit specifications. The City requires the contractor to have the top of the foundation surveyed prior to beginning framing. The floor system is then verified to determine that the finished floor height is as allowed.

A follow-up story from several days later says the new McMansions are affecting more houses:

Since the homes were built around 2015, Reed said her mother’s basement has constantly been flooded and her backyard has turned into a swamp…

The ITeam recently discovered a Kirkwood ordinance that said new developments cannot cause water run-off problems for surrounding properties.

But attorney Paul G. Henry said getting the city to enforce it could be difficult…

We repeatedly asked Kirkwood officials about why they don’t appear to be enforcing their own ordinance but they declined to answer. Instead, they recommended that we file an information request.

Such issues could put a suburb in a sticky situation: should it protect the properties of elderly citizens who have lived in the community for a long time or allow new property owners to construct homes to their liking? Whose property rights prevail? There is probably some middle ground here where the teardowns can be regulated in such a way to provide a little protection to neighbors (whether this involves water issues or residents are concerned about the changing character of their neighborhoods) but these regulations could take some time to discuss and enact.

Replace Houston McMansions and sprawl with what?

At least a few of the homes flooded in Houston are McMansions. For example, see this video of rescue efforts in one neighborhood where water is past the first floor of McMansions.

Once the waters recede, what will happen to these McMansions? Critics of such homes argue that they are often poorly built. Are they worth restoring and rebuilding or will homeowners pursue other options? What will communities approve and will they promote other options beyond big single-family homes?

Rarely do suburbs and big cities have opportunities to rethink past development decisions on a large scale. Houston is known for sprawl and a number of commentators (including me) have already suggested that such an approach often does not work well with flooding and water issues. Yet, overturning decades of sprawling suburban development is a difficult task and is likely even harder when residents just want to get back into their homes.

 

“People care about flooding…they don’t care about stormwater management”

An article discussing the difficulties of avoiding flooding in a sprawling city like Houston includes this summary of a key problem:

One problem is that people care about flooding, because it’s dramatic and catastrophic. They don’t care about stormwater management, which is where the real issue lies. Even if it takes weeks or months, after Harvey subsides, public interest will decay too. Debo notes that traffic policy is an easier urban planning problem for ordinary folk, because it happens every day.

It is difficult to get people interested in infrastructure that does not effect them daily or they do not see it. Yet, flooding is a regular issue in many cities and suburban areas and it can be very hard to remedy once development has already occurred. Indeed, it is difficult imagine abandoning full cities or major developments:

The hardest part of managing urban flooding is reconciling it with Americans’ insistence that they can and should be able to live, work, and play anywhere. Waterborne transit was a key driver of urban development, and it’s inevitable that cities have grown where flooding is prevalent. But there are some regions that just shouldn’t become cities.

Given the regularity of flooding in developed areas, it is interesting to consider that there are not more solutions available in the short-term. Portable and massive levees? Water gates that can be quickly installed? Superfast pumps that can remove water?