Why the CAP canal shape matters — a tale of the trapezoid

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CAP Canal Shape

Remove the water from a section of the CAP canal and you’d see what looks like a foundation for an amphitheater.

If you were to stand on the canal bottom, gaze from the narrow base to the gradually widening sides, and imagine a water line on the surface, you’d probably notice the four-sided shape of a trapezoid.

This canal shape, as well as narrowing width and shallowing depth  as it tapers along the entire 336-mile system, is not a coincidence.

It was all part of CAP’s original design to reliably deliver Colorado River water to central and southern Arizona in an efficient manner.

This design was forged long before the first CAP water deliveries in 1985.

Why a trapezoid?

The short answer: CAP’s specific trapezoidal shape is a balance of hydraulic efficiency and constructability. Take a look.

CAP Canal Shape Infographic

Water in the canal is flowing by gravity and generating friction where it is in contact with the canal lining on its bottom and sides. This friction slows the water down, so large pumps must lift the water up at various intervals along the CAP system to continue allowing it to flow by gravity through the canal.

Start wide, end narrow — deliver reliably

The original CAP design concepts were driven by economics as much as pure hydraulics.

One example: the first portion of the canal which stretches 25 miles from Mark Wilmer Pumping Plant at Lake Havasu to Bouse Hills Pumping Plant is also wider and deeper – a neck-wrenching 80 feet wide at the bottom and 200 feet wide at the top.

In a sense, this section serves as an “in-line storage reservoir,” allowing CAP to pump water from the river into this short-term storage area during off-peak hours when energy is cheapest. Meanwhile, flow can remain constant throughout the rest of the system. This gives flexible daily “shaping” of power usage as well.

The canal bottom narrows as it progresses from the beginning of the system through the western and southern portions of the aqueduct. Over the course of the 336 mile-long system, the bottom width starts at 80 feet and whittles down to 12 feet.

The CAP: an engineering marvel

Geeky Engineering Fact: In canal design, variations of canal bottom width, longitudal slope, side-slope and canal depth are used to produce an appropriate canal shape for a desired flow and topography.

A system designed primarily for economical efficiencies, CAP water flows reliably from Lake Havasu to Tucson, an engineering wonder that supplies water to more than 80 percent of the state’s population.

Call it — if you so desire — trapezoidal technology.