Keystone Science School has a great partnership with Denver Water, Aurora Water and Colorado River District. They are a strong supporter of the H2O Outdoors program which is a free overnight program for any high school student living in Colorado. Along with supporting the H2O Outdoors program our partners have provided several trainings for our staff. In a recent training the instructors asked Matt Bond, Youth Education Program Manager for Denver Water, would it be possible to scuba dive and survive a swim through the Roberts Tunnel, a 23-mile trans-mountain diversion which travels 23 miles connecting the Dillon Reservoir to the South Platte River? There’s are a ton of variables and the sheer pressure of water and rock would be the main concern. Matts answer is below and please DO NOT ATTEMPT!
First, let’s take care of the physics and math. When full, the high-water line at Dillon sits at 9,017 feet above sea level. The inlet for the tunnel sits at 8,846 feet, which is 170 feet below the high-water line. A quick Google search said that 165 feet is about the deepest anyone would say is the max for recreational diving due to the issues like oxygen toxicity and nitrogen narcosis. For every foot of water depth, you gain 0.43 pounds per square inch (psi). That means when the reservoir is full, the water going into the tunnel 170 feet below is already at 73 psi, which is pretty close to the 80 psi-maximum plumbers like to see for residential water pressure.
Along the tunnel’s 23.3-mile journey, the water drops an additional 180 feet between the inlet and when it daylights again. To figure the pressure at the end of the tunnel, you have to know the depth of the water above. When full, there is 350 feet of elevation difference between the surface of Dillon and the bottom of the tunnel. That depth about the deepest even technical/professional divers go, and they need decompression stops and special gas mixtures. Do the math and that means the pressure at the outlet of the tunnel is about 150 psi. Yesterday, the elevation of the reservoir was 9,009 feet, which drops the pressure to 147 psi. I like to look at it this way. The average-sized woman weighs a tad less than 147 pounds (and apparently is 5’4” tall). A square inch is about the size of a quarter. If you dove all the way to the outlet of Roberts Tunnel, it would feel like you had glued quarters all over your body, and then invited an average-sized woman to stand on each one. Not very comfortable, I’d say. And if some silly daredevil opted to try to make the trip, it would take quite a big air tank. At the current flow rate of 107 cubic feet per minute, it would take more than 19 hours to get from the reservoir to the end of the tunnel.