Back almost 30 years ago, when I was on the City Council, we engaged in a very detailed study of the major drainages that flow through Boulder, and the likely damage that could result from floods. Our objective was to come up with appropriate risk mitigation standards… Out of this study came Boulder’s regulatory standard using the 100-year and 500-year flood maps and also the high hazard areas, which were based on such flows.
Setting the rules based on these standards was a compromise. The council did end up requiring some buildings that were at very serious risk to be torn down. But buildings in areas at somewhat lower risk were left in place, even though they never should have been built in the first place.
The current regulations need some serious updating. First, they are not appropriate for areas where development can be avoided; they were created for already-developed areas, and so compromise the level of protection. Second, the frequency/intensity forecasts are really just educated guesses because the historic events are so infrequent, so they form a weak basis for doing quantitative risk assessment. Third, and critically important, the climate is changing, so we can expect more and more intense flood events.