Q&A: Siphoning Hoover Dam

Question: They say Hoover Dam could potentially reach deadpool level in the near future, shutting off flow to the lower Colorado River. To keep the river flowing, could they use a siphon to draw water from Lake Mead at dead pool level and below? — DF, Toronto, Canada.

Answer: Excellent question. Siphons are pretty amazing devices. Unfortunately, there’s a height limit above which siphons can’t work, and it’s set by the atmospheric pressure. At normal sea-level barometric pressure (14.7 psi) that height limit is 32 feet. As you can see in the graphic, the lift over Hoover would need to be 337 feet. The lake is currently at 1067 feet (as of this writing) and that’s a mere 170 feet above deadpool.
 
Not being able to tap into that remaining 815 billion gallons would have devastating affects on ecosystems downstream — not the least of which are the millions of people who rely on the Colorado River for water. There are only a few significant tributaries and springs to feed the river downstream of the dam.
 
The graphic shows what happens if we try your siphon solution. We put the inlet as deep as possible in case water levels further decrease. We run the pipe over the top of the dam to feed the river downstream. A pump would be needed to get the siphon started. We fire up the pump, water is pushed into the intake pipe and reaches the top of the dam, at which point it flows toward the outlet pipe under its own momentum with the help of gravity. When the water hits the downward sloping pipe it accelerates rapidly eventually reaching the outlet and emptying into the river, as shown.
 
However, at the moment the pump is shut off, the siphon “breaks” by cavitation. While the pump is running, the pressure from the impeller keeps the water flowing. With the pump off, the weight of the water in the vertical intake pipe could no longer by supported by atmospheric pressure. That limit is reached at only 32 feet of lift, and we need 10X that much lift to get over the dam.
 

The next graphic shows why cavitation happens when you try to siphon water more than 32 feet. Water in the intake pipe reaches the 32 feet limit and can be pushed no higher by atmospheric pressure because 32 feet of water (regardless of pipe diameter) exerts a downward pressure of 14.7 psi.

Water already past that point is pulled by gravity toward the outlet. This reduces the pressure at the top to below the vapor pressure of water (0.2 psi at 25°C). That causes it to boil, releasing dissolved air and creating a cavitation break. Water in the outlet pipe drains into the river breaking the seal, and the water in the intake pipe drains back down into Lake Mead.

If you want a more technical, mathematical explanation for the 32 feet limit, you’ll need to consult Bernoulli’s Principle. It relates the parameters of pressure, density, gravity, elevation, and speed in a moving fluid.

When the dam was built back in the 1930s there were tunnels at the perfect elevation to drain below deadpool. They were needed to divert the river’s flow while the dam was being constructed. Alas, the engineers never foresaw deadpool actually happening, so the tunnels were capped with steel plates and filled with concrete. Modern boring technology could reopen them, but once done there would be no more electrical power production unless new turbines were installed in those tunnels. Even then, power production would be a fraction of what the dam was designed to produce.

Operable gates could be installed on the reopened tunnels, just in case the river recovers and Lake Mead starts to fill again. That’s a viable option, but it would be a huge and costly project. However, there’s an easier solution that involves no drilling.

Use your original idea of a pipe over the dam, but keep the pump running. It could be powered by a solar and/or wind farm. And it would be cheaper than re-boring a tunnel and installing operable gates. Then, if the river does recover, they can reroute the power running the pump back into the grid — until the next deadpool is reached. But that pump would need to be a monster.

The Colorado River’s flow rate is currently around 45,000 cfs so it would require that amount of pumping to maintain the flow. By comparison, in the Central Arizona Project (CAP) system, the six huge (66,000 hp) pumps that lift Colorado River water 800 feet over the mountains move around 3000 cfs, and require a power supply of 285 MW. Those same pumps used at Hoover Dam, where the lift is only 337 feet, would deliver around 7000 cfs, so the river would flow, but at only 15% of its current flow rate.

You can see the conundrum faced by water managers. As always, solutions will involve trade-offs. In this case it’s between the need for water, how much energy you’re willing to spend, and how much disruption to the ecosystem you’re willing to tolerate.

Next Week in Sky Lights ⇒ White Hydrogen

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