hahaha Yea, that is exactly right. Design input is based on a rainfall return period that is pulled out of the air. Intensity is based on averages that change periodically. Head losses and coefficients of runoff are a best guess. Time of concentration is also a best guess and is very likely to be incorrect. So given all this airy fairy design input we work everything else out to the nearest fraction of a mm. Go figure. But...if it all goes horribly wrong, and flooding and damage occurs somewhere, the lawyers come looking for someone to blame. If we can prove that we complied with the all the local rules and standards we are off the hook. If not, well....life could get interesting.

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hahaha Yea, that is exactly right.

Design input is based on a rainfall return period that is pulled out of the air. Intensity is based on averages that change periodically. Head losses and coefficients of runoff are a best guess. Time of concentration is also a best guess and is very likely to be incorrect.

So given all this airy fairy design input we work everything else out to the nearest fraction of a mm. Go figure.

But...if it all goes horribly wrong, and flooding and damage occurs somewhere, the lawyers come looking for someone to blame.

If we can prove that we complied with the all the local rules and standards we are off the hook. If not, well....life could get interesting.