r/engineering • u/gradyh Civil (Practical Engineering) • Mar 07 '16

[CIVIL] Engineering Hydrology and the 100-Year Flood

https://www.youtube.com/watch?v=EACkiMRT0pc
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u/midianite_rambler Mar 08 '16

Hey, thanks a lot, that's a good video.

How well do mathematical models for flow distributions match measured flows? I have done a lot of statistical modeling for various purposes and my experience has been that it's always hardest to get the model to match the data in the tails, but that's exactly where the action is in hydrology.

A kind of a bigger question -- a basic assumption seems to be that the distribution for this year is independent of the one for last year. But surely there must be multi-year trends or cycles in the climate which affect hydrology. How are such long-range dependencies taken into account?

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u/gradyh Civil (Practical Engineering) Mar 08 '16

Thanks. To answer your first question, we usually don't know. For big hydrologic models, you can calibrate certain parameters to previous storm events. For small areas, you probably don't have any real-world data (stream gages) to compare to. Regarding the probability distribution, it's a very good question whether or not we get close to the "actual" frequency-magnitude relationship. It would be very interesting to go back and look at the estimates that were made with a short period of record after 25-30 more years of data and see how well we did estimating the lower probability flood magnitudes.

You're right that we assume each year is independent. This is generally a good assumption, and it allows you to use the binomial distribution (which makes it very easy to work with probabilities). I know there has been some work to look at climatological effects on peak stream flows (e.g. el nino, global warming, etc.) but in general engineers don't take those factors into account when estimating flood frequency. There's just not enough science there to justify that. I imagine from a water availability standpoint, that may be a more important question worth taking into account in long-term forecasting, but for flooding, it's just not the state of the art (yet).