Posted on July 29 2015
Image: Flickr
Reservoirs are an ingenious solution to sustaining life in places where it doesn’t rain throughout the year, and there aren’t a lot of natural lakes. Store your water when you can get it, and save it for a, well, non-rainy day. Reservoirs are tapped by means of what is called an intake structure, which is basically a big pipe tower that lets in water at some depth above the bottom so you can access deep water. The water then flows by gravity to a conduit that carries it to a treatment plant. Lake Mead was built in the 1930s and designed to meet the needs of much of the Southwest. Water allocation promises were made to various states and Mexico based on the rainfall conditions existing at that time. Unfortunately, those were abnormally wet times. As we boldly wander into a drier world, Lake Mead (and Lake Powell upstream) are lowering to levels not seen since they first filled. At 1,220 feet in elevation, Lake Mead is full, and it has fallen to about 140 feet below this level. As of this month, it stands at 1,076 feet. If the water level was to get down to 1,000 feet, the lowest intake port, the second of two, will be exposed. Still a lot of water in the lake, but no easy way to get it where it needs to go. Waterworks at this scale are all done by gravity – it takes too much power to pump massive quantities of water up into conduits if the levels fall too low. Clearly, the long and enduring drought of the Southwest is approaching a significant crisis point.
Enter the hydraulic engineers. Never ones to let a little hard work get in the way of grand plans, (I mean, these guys built Hoover Dam in the first place) they figured out a way to get that deeper water. The project was informally called The Third Straw. All it took was drilling a 24-foot diameter tunnel through three miles of rock underneath the bottom of Lake Mead. Piece of cake. It did, however, take three years to eat it. The hydraulics of this scheme are, of course, complex, but they went more or less as follows.
As a preliminary step, workers blasted a 60-foot deep pit in the rock bottom of the lake and poured a concrete pad underwater. On this pad in the pit was placed a 1,200 ton pre-cast intake structure with a vertical riser tube. Concrete was then poured around that. Then the real fun began. After drilling a 600-foot deep vertical shaft onshore, a starter tunnel was hewn out to make way for the next step. That involved a humongous drilling machine made in Germany which was lowered to, and reassembled at the bottom of the hole. This behemoth then began grinding away, horizontally, towards the intake structure. For three miles it trudged on, in a wide curving arc, for 3 years. But not without problems. The starter tunnel leaked so much they had to realign the route. The drill cut across a fault line, which caused leaking and a shutdown of operations. One worker was killed in an accident. Various other leaks slowed progress and raised the cost. In late 2014, however, the drill met the intake structure wall, less than two inches off target. Once it drilled into that, it made the connection for water to eventually flow. Of course they first have to back the drilling machine to where they started, take it apart and lift the pieces out again. Then the whole system is allowed to flood, which pops out a huge plug in the intake structure. But hey, child’s play, right?
This enormous project is really all about saving Las Vegas. With the Third Straw at 860 feet, much more water is available for removal and transport to Sin City. However, if water levels drop to 900 feet, it’s not possible to release any more water through Hoover Dam to flow down to Arizona, California and Mexico. Colorado received some strong snow and rain storms this past winter and spring, but not so much in the western part of the state that feeds the Colorado River. Lake Powell, upstream of Lake Mead, may rise about 7 feet whereas normal years bring 35-foot rises. The Third Straw may save Las Vegas, but the downstream states are still facing the possibility of losing a major water source.
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