Sunday, February 26, 2017

The thing about spillways



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.

8 comments:

David B. Benson said...

In the Pacific Northwest almost all of the dams have a hydropower function. So if excess precipitation is predicted more water goes through the turbines. It is most exceeding rare to spill water anymore.

Not much help for dams without this capability and even more isolated hydropower dams such as Shasta are susceptible.

Fernando Leanme said...

I take it you think the rain which ended the drought and filled water reservoirs was caused by CO2?

DrTskoul said...

I guess you must be thick....

Mal Adapted said...

FL:
"I take it you think the rain which ended the drought and filled water reservoirs was caused by CO2?"

I'll be "you".

Look, FL, everything about climate and weather is "caused" by CO2.

As for attribution of specific weather to AGW:

- CMIP5 projections show both winter mean precipitation, and the proportion contributed by atmospheric rivers, increasing substantially with AGW;

- Reliable reports demonstrate an anthropogenic component to particular AR events:

- the NAS Report Attribution of Extreme Weather Events in the Context of Climate Change says:

An array of studies continues to provide strong support for upward trends in the intensity and frequency of extreme precipitation events [citations at the link].

Argue with the NAS, not with me.

Greg said...

David B. Benson. The Oroville dam has a hydro power plant, but there was some reason they couldn't release all the water through it - either a max capacity issue, or some maintenance problem - I don't recall. Now however, the debris from the spillway has "raised channel levels to the point that dam operators can’t run Oroville Dam’s hydroelectric power plant, normally its main source of water releases."

http://www.sacbee.com/news/state/california/water-and-drought/article134918039.html

Oh, just below that in the same source it comments on the max flow capacity of the hydro plant, and the value given (14k cfs) is much less than what they had to flow over the spillway (55k cfs when the damage started, more during the height of the emergency). So that answers that.

David B. Benson said...

Thanks for the info.

Bernard J. said...

FL, your schtick is more schtwig...

Anne said...

GOOD POST