By Ted Cooke, CAP General Manager
4/10/2019

In Arizona, we say “water flows uphill,” thanks to the remarkable engineering of the Central Arizona Project aqueduct system.

Canal at SRP Interconnect

The 336-mile system took 20 years to build and cost $4 billion, and is an engineering marvel that has contributed dramatically to our quality of life and the sustainability of the state’s water supply and economy.

Despite the fact that CAP supplies water to the majority of Arizona’s population, many people don’t know much about this amazing infrastructure. So, when I have the opportunity, I enjoy explaining these details -- how it works, its features, the specific equipment it includes and even some items it doesn’t include, and why.

The CAP system moves water from the Colorado River near Lake Havasu to the terminus just south of Tucson. For much of its journey through the system, CAP water is moved by gravity. However, the terminus is higher in elevation than the river. Consequently, when it is first diverted from the river, and then several times more along the way, CAP water must lifted vertically, after which it is allowed to flow by gravity through the contours of the Arizona desert until it needs another “lift” – totaling nearly 3000 vertical feet from diversion off the river to the end of the system south of Tucson.

The CAP aqueduct system was precisely designed to descend at a slope of approximately five inches per mile (0.008%), which is just enough “tilt” for the weight of the water to overcome the friction of the sides and bottom of the aqueduct. Now that is some incredible engineering, especially given this was accomplished in the 1970s and 1980s, without the benefit of the vast technological advances of the past 40 years to assist in the design, construction and operations of a highly efficient system.

CAP Elevation

There are 14 pumping plants along the aqueduct, plus Waddell Pump Generating Plant that not only pumps water into Lake Pleasant, but also releases water from the reservoir, generating electricity in the process. The largest pumps in the system are the six at the Mark Wilmer Pumping Plan on the Colorado River that weigh in at 66,000 horsepower each. One of these pumps moves enough water to fill an Olympic sized swimming pool in 30 seconds!  

In addition to the pumping plants, this incredible system boasts turnouts that divert water into customers’ systems, check structures that help control the flow of water in the CAP canal and Lake Pleasant, our largest storage reservoir that is also a recreational destination.

So what doesn’t the CAP system include? Things that do not contribute directly to or may interfere with the cost effective and reliable delivery of CAP water. Things that may slow down or interrupt the flow of water or interfere with our efficient operation and maintenance of the system.

I am sometimes asked, “Why not cover the canal with solar panels to generate energy and reduce evaporation?” Or, ”Have you thought about installing low head turbines in the canal to recover energy from the flowing water?” Neither of these features are included because there are real technical and financial challenges to making these projects viable. 

CAP was designed to be a very efficient user of electricity, which is the largest single expense involved in moving CAP water. It is a perfectly balanced system, using just enough power to lift the water to the next elevation, and then using the kinetic energy of the flowing water and the potential energy of the weight of the water itself to overcome friction and transport it to the next pumping plant. Instream devices used to harvest even small amounts of energy from the flowing water in the canal, such as small turbines to generate electricity, would effectively reduce the capacity of the system, limiting CAP’s ability to fulfill its mission to deliver Colorado River water.  The only way to overcome this loss of energy would be to input more energy into the pumped water.  However, since no device is 100% efficient, the amount of energy generated by a turbine would be less than the energy added to the pumped water due to friction and other losses.  In addition, there is no cost-effective way to move the generated power to the electrical grid or other point of use.

Solar panels would not have the same capacity limiting effect but introduce other challenges.  A solar project that would cover the canal would carry the additional costs of the structures needed to suspend solar panels over the canal, which is quite wide – 80 feet for much of the canal. There would need to be additional electrical transmission facilities to move the generated power onto the electrical grid. It would also be much more difficult to maintain the canal, since the suspended solar panels would interfere with access to the canal itself, and the mounting infrastructure for the panels would take up precious space on the O&M roads on either side of the canal that are used for the movement of heavy maintenance equipment with no room to spare. Compared to solar projects that could be (and are) constructed on abundant vacant land located near transmission systems, a CAP-over-the-aqueduct solar project is a very expensive and impractical alternative.

So, what about the opportunity to reduce water losses? It turns out that the CAP aqueduct is lined making it a low loss system.  On average, the canal loses about 1 to 2 percent of the volume diverted from the Colorado River.  It is unclear how effective covering the canal would be in terms of reducing the evaporative loss, but even if it was 100% efficient, the modest water savings would yield only a very small change in the actual CAP water supply.  While water is certainly of tremendous value in Arizona, the modest water savings considered in context of the enormous cost involved in covering the canal and building transmission lines does not pencil out. 

So, as you go about your normal daily activities, you may rest assured that CAP is hard at work delivering a water supply that many Arizona communities and industries rely on…that is a key to our economy and our way of life…supported by a 336-mile system that is a true engineering marvel.

 


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