Sunday, August 17, 2014

Recent Urban Flooding, Climate Change and Civil Engineering

In light of urban flooding in metro Detroit, Long Island and Baltimore, meteorologist Dr. Marshall Sheppard looks at the link between changing climate and civil engineering design:

 Figure 1. Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) from 1958 to 2012 for each region. There is a clear national trend toward a greater amount of precipitation being concentrated in very heavy events, particularly in the Northeast and Midwest. Image credit: NCA Overview, updated from Karl et al. 2009.

As our climate warms, scientists have warned for decades of an accelerated water cycle that will lead to more "extreme" hydroclimate (flood, drought) extremes . The recent U.S. National Climate Assessment Report, which is Congressionally mandated since the late 80s/early 90s, published these facts/graphs showing the top 1% rain events have increased in intensity everywhere in the continental U.S over the past 50 years. (, intensity is only part of the story. For the first time in history, a majority of the world lives in cities. I discuss the growth of urbanization and its implications on weather and climate in a recent Earthzine article. Urbanization requires more roads, parking lots, rooftops, and other impervious surfaces that alter the urban water cycle and optimize urban flooding potential.
Additionally, civil engineers and hydrologists use recurrence intervals/frequency (best represented as IDF curves) to design engineered systems for storm water management and drainage. A host of peer-reviewed literature affirms that many cities' storm water engineering is designed for last centuries rain storms or under an assumption of stationarity (i.e. 1950s rainstorms are just like 2014 rainstorms). Consider the abstract of this peer-reviewed paper:
"The hydrologic design standards for urban drainage systems are commonly based on the frequency of occurrence of heavy rainfall events. Observations of recent climate history indicate that the frequency of occurrence of heavy rainfall events is increasing. This increasing trend will likely continue in the future due to global warming. In this study, extending from previous analysis results for Chicago, the rainfall intensity–duration–frequency (IDF) relationships were determined to represent the climate conditions of the first and second halves of the last century. Using these IDF relationships, the impact of the observed increase in heavy rainfall events on the design and performance of urban drainage systems were quantified. This quantification demonstrated the need for updating rainfall IDF relationships to reflect changing climate conditions. In the design of new and retrofitting or replacement of old urban drainage systems, up to date IDF relationships need to be used to maintain design standards."
By the way, I didn't even get into the recent literature suggesting that Arctic amplification may be causing "wavier" jet stream patterns that could lead to more extreme events. I personally am letting that play out a bit more in the literature because I am not a big fan of "1-study" conclusions on either side of the issue. However, a recent National Research Council study has captured current literature on that issue.
The bottom line for me is that cities must strategically consider hydrometoeorology and climate in its current and future planning, storm water management design, and transportation systems.
 This is a major issue for everybody, including farmers, but it is most significant for civil engineers.  They will have to adapt their design standards to take into account more frequent occurrences of
large rainfall events.  To put things in perspective, Elwynn Taylor claims that his data showed what were considered to be 100-year storms (1% probability of occurrence in any year) is more like a 17-year storm today.  Back in my days designing storm sewer systems and detention structures, most communities required a full-flow pipe design carry the 10-year storm, with a check of hydraulic grade lines for the 25-year storm to confirm that flooding wouldn't occur.  This would indicate that some localized flooding would be expected to take place in 50-year and 100-year storms.  Likewise, many of our detention basin designs were based on controlling the outlet flow from a development during a 100-years storm in the post-development condition down to the level of a 5-year storm in the pre-development condition of the site.  If these storms are happening more frequently, and even bigger storms, which are outside of the design parameters, are also taking place more frequently, we are going to see damaging floods.

The challenge is that with the climate already changing, it is hard to figure out just how much more rain to expect, and just how frequently.  Even if we adjust the rainfall intensities to take into account the changes shown in the map above, there is a very real likelihood that intensities will continue to increase.  At the same time, these design standards impact property developers more than anyone else.  Any increases in design rainfall intensities lead to larger storm sewers and more land set aside for storm water detention, and thus higher costs for developers, and fewer and more expensive lots for development.  The developers are generally pretty influential in state and local politics, and can be counted on to lobby against such changes.  They have little interest in what happens in 15 years, they just want to get their lots sold and move on to another project.  This will guarantee that changes in design requirements will be difficult to enact.

Finally, much of this week's flooding occurred in older suburbs which were built in the days before storm water detention became standard practice.  These communities have seen significant demographic change as residents have aged and newer, more prosperous communities have sprung up further in the periphery of the metropolitan areas.  Since the communities were developed without detention facilities, it would be very expensive to construct these structures in the built environment.  With the demographic changes, it is more difficult to fund the required projects than it would have been in the past.  The more frequent flooding makes these neighborhoods less desirable to live in, which lowers property values more and makes needed infrastructure improvements even less affordable.  It is an ugly feedback loop that looks to make these areas into urban eyesores.  Whatever civil engineers decide to do with their design standards, climate change will seriously impact our communities in ways many people won't expect.  You might want to buy a life raft.

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