Yes, that's a waterfall behind the house. Anderson dam spillway in full force now. #CaliforniaStorms @CBSSF pic.twitter.com/B54AWVHbsM— Len Ramirez (@lenramirez) February 20, 2017
San Jose evacuated 14,000 residents this week in the worst flooding in decades. I used to be on the board of the water district responsible for flood control in Silicon Valley. Somewhat restrained finger-pointing between the water district and the city has started over what went wrong, and we'll eventually get the facts.
The thing I wanted to talk about was spillways - what people may not realize when they hear that water is flowing from a dam's spillway is that means, with some exceptions, that the dam no longer serves a flood protection function. A spillway could be considered a directed-overtopping of the dam, an intentionally-cut notch in the top of the dam so that when the dam is just about to overtop, the water exits down the spillway rather than cascading randomly and destructively anywhere on the dam's face.
If water's flowing in the spillway in this kind of directed-overtopping, then the dam has used up all its storage function. Any increase in flow upstream of the dam results in increased flow below the dam. Some extremely large dams have multiple spillways, but the principle of lost flexibility in holding back floods still applies.
Coyote Creek which flooded San Jose could pass for a river by western United States standards. It has two dams along the main stem of the creek, one of them the biggest in the county. Both were spilling immediately prior to the flood, so both could do nothing when the heaviest rains came. It doesn't leave you defenseless, but it makes things a lot harder.
The obvious thing is to not build in a floodplain, but that's easier said than done. Another obvious thing is to not monkey around with climate change.