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What happens when you plan for the future using assumptions from the present?

Well, for one Swiss water engineer, you get a whole lot of sh*t.  Or as he likes to call it, “sediment.”  Let me explain:

David is a colleague of mine and an PhD candidate at the ETH, one of Switzerland’s most prestigious technical schools. Over dinner one night he told me the following story:

A couple decades ago, Zürich water engineers needed to upgrade their sewage capacity. Basically, they needed a bigger pipe. So a team of engineers examined water usage data and extrapolated some ideas from it.

Since water usage had been climbing for the last few decades due to population growth, they thought it logical to assume it would continue to rise in step with population growth. The pipe they designed, built and installed took into consideration this increase in water usage and would provide more than enough capacity to last at least thirty years.

Great job, fair enough.

The problem, however, is this:  Given recent trends and technological innovations, water usage in Zürich has actually decreased in the last decade despite increasing population growth. The sewage pipe actually has far too much unused capacity.

So what? you say.  So here’s what:  When a sewage pipe such as this does not have enough water flowing through it, sediment (read:  sh*t) tends to build up in the pipe, causing an increase in bacteria which threatens the safety of the water supply. This dramatically increases the cost of treating the sewage.

(Note: I knew absolutely none of this prior to my dinner with Dave.)

Long term planning is all fine and well, except the entire process involves predicting a future scenario you have no ability to predict. How many transit lines have been built using this exact same argument? Just because you predict x number of people will ride your transit line doesn’t mean that many actually will.

Better to make your infrastructure plans fluid so that they may contract or expand as needed.

Otherwise you may wind up with nothing more than a sediment-clogged pipe.