| You're on the right track, but your framing is still off wrt how the engineering design process works. Assuming that designing for a 500-yr storm has anything to do with 'predicting what a 'future 500-yr storm' (or 25-yr or 100-yr) looks like is dead wrong. Irrelevant. The 'definitions' are not left alone, they are updated as time goes on. But with historical data, and they are not extrapolated/predicted out into the future. Engineers (PEs) design by taking known criteria and then applying probabilities and factors. They do not predict criteria. It's a subtle but important distinction. A 500yr event, by definition, is actually the one year probability of a 1/500 chance event. And it's up to the designing engineer to choose and state whatever the assumptions are that go into that. But a levee designed this year will use this years current 'storm definition' just as it uses this year's building code. Not a future one. (Sometimes the storm/ event definitions seem stale because things like flood maps might only get updated every few decades.) |