Recent Urban Floods: A simple equation

By Dr. Marshall Shepherd

Meteorology and climatology usually involve complex calculus and physics. However, as I watch breathtaking flooding in Boulder, Pensacola, Detroit, Baltimore (weather.com), and Long Island (PIX11), a rather simple equation comes to mind.

Urban Flooding = Increase in intensity of top 1% rain events + expanding urban impervious land cover + storm water management engineered for rainstorms of “last century”

I have researched and published on precipitation/urban hydrometeorological processes for over 2 decades. I am also a member of the NASA Precipitation Science team and was Deputy Project Scientist for the Global Precipitation Measurement (GPM) mission that recently launched.

As we watch roadways become rivers in Detroit and parking lots become lakes in Baltimore, it reminds me of the record 2009 Atlanta Floods that we studied in the peer-reviewed, Bulletin of the American Meteorological Society (BAMS).

That flood was a juxtaposition of extreme meteorological conditions, urbanization, and people.

I am not a big fan of linking 1 event to climate change and have written that numerous times. With flooding events, it is helpful to try to place them in a context of changing climate that will help the public with perspective.

To say this week’s Long Island, Detroit or Baltimore floods are “caused” by climate change is ill-posed and invites criticism. That is like saying because I am at archery class and hit the bullseye one time, I am going to hit it every time. Probably not likely.

BUT if somehow the bullseye is “tripled” in size, I probably have a better chance of hitting the bullseye more often.
Warming climate is likely increasing the “urban flood bullseye”, making the probability of such flooding more likely.

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. (ClimateChange.gov)

A friend, long-time Baltimore area resident and StormCenter CEO/Meteorologist Dave Jones recounts someone saying “that parking lot at BWI airport never floods.” It did yesterday, and severely.

And oh, even with a larger bullseye, I will still miss the target sometimes (e.g., there will still be cold or dry days too), but that doesn’t change the fact that I have a better chance of hitting it.

But, 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 (earthzine.org). 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 (ASCE):

“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.

Yep, our climate changes naturally. Always has. Somehow I managed to figure that out over the course of 3 degrees in atmospheric related sciences. But it now has an additional signal on top of it. And more and more impervious surface for heavy rains to fall upon.

Our Jurassic era friends didn’t know what asphalt was….