So what’s with all the water?

Last week, a series of heavy rainstorms resulted in newscasts that showed cars driving up to their axles on flooded streets and some motorists stranded. For the most part, these weren’t New Orleans or Mississippi category floods, just maybe a foot or less of water on paved streets. News announcers solemnly informed us that the storm drain systems had only been designed for 5 or 10 year storms, and so could not handle anything greater. A few commentators seemed to be implying that since a 100 year storm was experienced, that’s what the design standard should have been. So what’s the story? Were the designers negligent and were people flooded unnecessarily? Why weren’t the systems designed for a 100 year storm?
First off, let’s look at what a 100 year storm actually means. It’s simply a storm of a magnitude that has about a one percent chance of occurring in any given year. That works out to about once in 100 years, but that’s an average based on records and probabilities. You could get two 100 year storms in the same week. OK; so back to the original question. Why not just design for the 100 year storm and avoid all that messy flooding?
Since there is never an unlimited supply of money available for any project, pretty much everything in engineering design involves a trade-off between perfection and affordability. Structures have a certain safety factor incorporated, based on the job the structure has to do, and the consequences of failure. If a bridge fails, for instance, it can be catastrophic, but an overwhelmed street drain is more of a nuisance for most people. So a bridge will be designed to a higher standard than a storm drain, but even the bridge will not be designed to be, say a hundred times as strong as it needs to be. It isn’t affordable, and it isn’t necessary.
Since the cost of designing for bigger storms goes up rapidly, but the consequence of a failure is limited and temporary, the smaller parts of a storm drain system, such as curb inlets, will usually be designed for a five or ten year storm. Even if they are overwhelmed every few years during a major storm, the resultant flooding is seldom dangerous and will recede quickly once the storm has passed. Nothing will collapse; the water will just back up a little deeper, spread a little farther, and a remain a little longer. The bigger parts of the system, such as culverts beneath roads, will be built to a higher standard, since a failure would be far more damaging.
So that’s why even the most well-designed streets flood occasionally.

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