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. Council member Spense Havlick and I even went to CSU and tried to walk across their artificial flume at various flow rates and depths to test our ability to walk through a flood.
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. The "100 year flood" is a statistical notion that uses historical data to attempt to indicate what areas would have a 1 percent probability of flooding in a given year. The "500 year flood" has an annual probability of 0.2 percent.
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. This was to avoid removing vast numbers of existing buildings.
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.
We had such a flood in September 2013, when a slow-moving weather pattern essentially stalled over our area, dropping heavy rain for four days straight. Some weather scientists said this was a result of "arctic blocking" — the warming in the Arctic is reducing the rate of circulation of weather patterns and increasing their intensity. Many of us have noticed the more extended spells of warm, cold, clear, or rainy weather.
The result of extended rainstorms is that the ground becomes saturated. After that, the rain falls on an essentially impervious surface and just runs off. We saw that in my neighborhood near King's Gulch, where I have lived for over 40 years. I have never seen water run more than maybe a foot deep, even in intense thunderstorms. But in September 2013 it was running over three feet deep and over six feet wide. The result was that many houses in the neighborhood were flooded as the water ran over the streets and into yards and basements. And King's Gulch has a tiny watershed compared to many.
The big flood issue that Boulder now faces is how to mitigate the risks of South Boulder Creek and the proposed CU South development. Rather than planning by looking in the rear view mirror and using regulations that are both out of date and inappropriate for the situation, the city and CU should plan for "reasonable worst case" scenarios. For example, what would happen if an extended, strong, multi-day rainstorm were to fall in the foothills upstream of Colo. 93 when the South Boulder Creek is already full with snowmelt and is overrunning the Gross Reservoir dam? (The "unreasonable worst case" is all the above, plus having Gross Dam fail!)
Given multiple days of flood level flows, the detention pond behind the proposed berm would fill up pretty quickly, and the city would be faced with making an unpleasant choice: Do we let more water spill out so that the areas downstream flood now, or do we just hope that even more intense rain later will not cause the levee to be overtopped and create a major disaster? Let's not forget the Oroville Dam scenario — a supposedly well-engineered dam was at risk of failure, because heavy rains forced the California Department of Water Resources to release a lot of water that eroded the spillway and forced the evacuation of over 188,000 people. And this dam was inspected both by the FERC and the CDWR. Apparently, being checked by a regulatory agency is no guarantee.
The solution is to use the principles, not the regulations, from 30 years ago — don't try to dam up the river, create ways for the floodwater to safely flow through while minimizing structural solutions, and keep new development well out of harm's way.