Oct. 19 was such a crazy day for many people due to the storms. Mother Nature decided to not play nice and surprised everyone by unloading a lot of water from the sky in a short period of time. Manholes and side sewer vents overflowed, intersections flooded, and unfortunately some people's homes were damaged in the process.
The good thing is that in most instances City departments came together to help those areas in need. This came in the form of pumping out water, cleaning debris, or sanitizing homes and businesses so people weren't displaced. Still, there was a lot of damage done.
What happened? Well, San Francisco's combined sewer system is designed to carry around 0.8 inches/hour of flow. This is based on historical weather models... and most of the time it's enough. For example, during the Oct 13 storm or any of the rainstorms over the past two years (albeit there weren't enough) you didn't hear reports of whole neighborhoods flooding.
Clogged catch basins and intersections yes, but not rivers of water and manhole geysers.
On Monday, some of our gauges showed that nature dropped a whopping 0.71 inches of rain in 20 MINUTES in many areas. Even in areas previously prone to flooding (where we subsequently upgraded our system), the storm intensity thrown our way was too much, too fast. Of course, the upgrades helped, and some areas could have fared far worse, but that's understandably little consolation to those who had their homes, businesses, and lives affected.
Rather than blab on about the system... I'll direct you to a a very informative web post written by San Franciscan Jeff Diehl (WARNING: some expletives are used in the page).
More to come about our combined sewer system in coming weeks. We'll be reevaluating where our system goes from here as we move forward with plans to rebuild it -- and we'll be incorporating a lot of different strategies, like Low Impact Design (which is mentioned in Jeff's post).
JEFF DIEHL'S WEB POSTING
Were you affected on Monday? Post your story in the comments section.