Our expert answers a question on global warming

August 1st, 2013 / By: / From Our Readers

Question:

Dear Dr. Leshchinsky,

In Item 2, “Apparent cohesion,” you state: “In some zones this redundancy becomes more important due to an increase in precipitation intensity caused by global warming.”

I was wondering if you could identify the location of these “zones” and the relative or absolute increase in precipitation you are attributing to global warming in them?

John A. Kerr, P.E.

Cincinnati, Ohio

Reply
Global warming: Explaining the usage

John,

First, let me explain our use of the term “global warming.” We used it in our initial
draft as it is direct. I had someone who knows the English language well review it.
He replaced it with “climate change,” which is less controversial and perhaps politically correct. I am glad that [the editor], or perhaps my co-author, changed it back to “global warming,” which is the correct scientific term. From your email it is not clear whether you question the phenomenon of global warming or just asking about the numerical value of rainfall intensity increase associated with global warming. If you question the phenomenon of global warming, you can approach scientists who are experts in that area (I am not). If you question whether there is an increase in rainfall intensity due to global warming, the same scientists will provide you with numeric evidence and the prediction of future increase/decrease depending on the zone.

As an example, look at the report titled “Global Climate Change Impacts in the United States,” published in 2009. Please refer to the top of page 7. As you can see, the increased intensity of heavy precipitation in the U.S. during the last 50 years is significant. This information is based on science and it serves as a good predictor
of the future.

OK, answering your query. This phenomenon is real in Japan and Korea (per my co-author) and in the U.S. (see reference above). My impression is that it happens globally. While some zones may have some decrease in rainfall, others are subjected to torrential rains increasingly breaking old records. That is, while the average amount of precipitation may have changed very little or even has decreased (resulting in drought), the intensity has increased at an alarming rate. However, drainage is designed based on predicted maximum rain intensity within a given period of
time (say, a 100-year storm).

When the intensity increases, the drainage system may not function as intended, thus potentially increasing the soil’s degree of saturation. Obviously, this results in the reduction of apparent cohesion, which makes many flimsy structures, including MSE walls, function. Without redundancy this reduction will result in collapse. You may recall the numerous failures of MSE walls in Atlanta in September 2009 due to high intensity rainfalls in the period, where Georgia was formally in a drought. So the definition of “quantity” of rainfall is relative to quality of backfill (free draining) as well as to the drainage system’s actual capacity. As a geotechnical engineer, I would say that when the degree of saturation is getting close to 100% we are in trouble (not too much water is needed to cause this degree of saturation). I do not consider a situation where the water in the voids is sufficient to create hydrostatic stresses—this would be a real disaster and we do not do design for such situations.

I am sorry for the rather convoluted response. However, my response is there. From personal experience I can tell you that geotechnical engineers rarely consider the impact of global warming (well, redundancy in design may help, especially if uncertainty about rainfall intensity exists). I was involved in a forensic of a failure of a 6-mile-long MSE seawall supporting a runway. The life span of the runway was designed for 100 years. While reviewing the designs, I was surprised to see that the runway designer designed its elevation 1m (about 3ft) higher than immediately needed. The explanation was that, due to global warming, a sea rise within 100 years could be as high as 1m. Needless to say, having the runway built 1m higher than what was immediately needed added substantial cost; however, it reflects the foresight of good engineering practice. I checked with an expert in coastal engineering. He showed me clear data of a rise in sea level over the past 40-50 years (If I recall correctly, it was about 30cm as recorded along the New Jersey shore). It is regretful that only little development is done in geotechnical engineering to improve designs as related to the progressively increased intensity of rainfalls.

Dov Leshchinsky

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Click here to read the online article referenced in the letter. Send your comments to editor Ron Bygness at rwbygness@ifai.com.

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