Sizing the Delta Tunnels

Typical Annual Water Supply and Demand for the California Aqueduct.

Typical Annual Water Supply and Demand for the California Aqueduct.

In yesterday’s Los Angeles Times, George Skelton seems incredulous that the planned Delta Tunnels must be so large.  But Mr. Skelton is a journalist, not an engineer.  And he didn’t do the math.  So let’s do it for him.  See his article at:

http://www.latimes.com/local/politics/la-me-cap-water-20150709-column.html

We’ll use one of the MOST Five formulas, Q=Av, to find the flow rate (Q) that two, 40-foot diameter tunnels will enable.  For the velocity, we’ll use 6 feet per second.  The result of this calculation is a flow rate of 30,000 acre-feet per day, or 0.03 million acre-feet (MAF) per day.  That’s a lot — it could fill Pyramid Lake in a week!  But is this too much, Mr. Skelton?

The Delta Tunnels would be the missing segment of the California Aqueduct, which is intended to supply about 4 MAF per year.  Using another of our MOST Five formulas, Time = Volume / Flow Rate, we find that the Delta Tunnels could transmit the entire 4 MAF in just 133 days.  So maybe Mr. Skelton is correct?

Well, if customers would take water at a constant rate, and if nature supplied us with water at a constant rate, we could plan on a constant transfer of water for 365 days each year.  However, that is just not the case — and especially so in California.

Our graphic today is an attempt to show the typical supply and demand profiles for the California Aqueduct over the course of a calendar year.  And here our problem is quite evident: there’s little supply when we need it most, and WAY too much water during the Spring flood period.  If we could capture the Spring floods alone, we could supply all the water needed for a year by the California Aqueduct!  This is why the Delta Tunnels are sized as they are.

(We also need a place to store that water south of the Delta, but that’s a blog for another day.)

Capturing Spring flood waters improves water quality for California Aqueduct customers, and reduces the risks of flooding in the Sacramento Delta.  It also means that the natural directions of water flow through the Delta are preserved, thus improving the environmental conditions there.  And the cost of these benefits is using water that would only flow into the Pacific Ocean, possibly wreaking havoc along the way.  That is a huge win for all parties involved.

But the price is bigger Delta Tunnels than Mr. Skelton would like to see.  MOST is fine paying that price, because we did the math.

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