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How Water Slides Work
by Tom Harris

In the amusement park industry, the roller coaster is king. But during the hot summer months, these classic attractions get some tough competition from water slides. In the past 25 years, the world of water slides has exploded. Water parks boast slides with dozens of loops, top speeds and exhilarating, 100-foot drops. A water slide is like a wet roller coaster with no seat and no safety harness.

 In the past 25 years, water parks like Wet 'n Wild Emerald Pointe in Greensboro, NC, have sprung up all over the world.

In this edition of HowStuffWorks, we'll peek behind the scenes to find out what's involved in operating a water slide. We'll also see how the pieces of a water slide fit together and see what keeps you from flying off into the air as you whip around corners.

The Physics of Falling
At its most basic level, a water slide is a relatively tame roller coaster with no track and no car. If you've read How Roller Coasters Work, then you know that coaster cars are driven by gravity. At the beginning of the ride, the coaster car is pulled up the lift hill. As the coaster rises higher in the air, its potential energy, or energy of position, increases. Simply put, it has farther to fall. When the coaster is released at the top of the hill, gravity pulls it down the track, converting potential energy to kinetic energy, or energy of motion.

 The simplest sort of water slide is a small, curved hill that is lubricated by a stream of water.

Water slides work on exactly the same principle. But instead of a lift hill, you have a stairway. Climbing the stairs builds up a certain amount of potential energy, which turns into kinetic energy as you head down the slide. A taller slide has more potential energy to work with than a shorter slide.

On a water slide, your body, sometimes combined with a mat or raft, takes the place of the roller-coaster car. Coaster cars have wheels that roll along the track. This reduces the friction between the car and the track, so the car can keep moving. Water slides have a constant stream of water flowing from the top to the bottom. The water lubricates the slide to reduce the friction between the slide and your body.

 When you climb to the top of this towering water slide, you've built up a lot of potential energy for your trip down the slope.

Apart from total height, the main difference between particular water slides is the way they put the potential energy to work. This is determined by the shape of the slide.

The slide applies a force working against gravity. The balance of these two forces depends on the angle of the slide. When you are sliding along on a nearly level slope, gravity pulls you directly into the slide, and the slide pushes you upward. The upward force of the slide pushes nearly opposite the downward force of gravity, slowing your downward acceleration. When the slope drops sharply, gravity is still pulling you straight down, but the slanted slide is no longer pushing you straight up; it's pushing you at an angle between upward and forward. Since the slide isn't working directly against gravity, you accelerate downward more rapidly.

Speed slides and sled slides focus only on these up-and-down forces. On a speed slide, you plummet straight down a steep slope and launch into an exit flume, a long canal of water that slows you down gradually. In a sled slide (also called a toboggan slide), you glide over a series of bumps and dips. In both of these slide designs, you move forward in a straight line.

 A speed slide, like this one at Wet 'n Wild Emerald Pointe, has a dramatic drop that launches you into a long exit flume.

Serpentine slides add something new to the mix: curves. The slide snakes around on its way to the bottom, whipping you in different directions all the while. In this sort of ride, the slide structure is not only working against the force of gravity, it's working against your own inertia. When you speed toward a curve, your body naturally wants to keep going forward. If the slide were flat, you would be launched into the air at the first sharp turn. The slide has to curve up at these turns to keep you on the ride.

 Photo courtesy Splashtacular, Inc.This serpentine water slide rockets you around a series of sharp turns.

When you hit these curves, you feel a strong force acting on your body. This is the slide accelerating you -- changing your forward velocity -- so you move in a different direction. (See How Roller Coasters Work to find out more about these forces.)

For everything to work correctly in a water slide, you need a sturdy, smooth surface to glide on. In the next section, we'll look at the structural elements of a water slide.

Bits and Pieces
A small water slide, the sort you might find in somebody's backyard, has a very simple construction. It's a single piece of smooth fiberglass material, cast in the shape of a slide, supported by a metal frame.

Most water park slides have a similar structure, but on a much larger scale. Obviously, it's not feasible to use a single piece of fiberglass for a giant, curving slide. Water park slides are formed from dozens of fiberglass segments fastened together with heavy-duty bolts. Typically, the individual segments fit together like sections of a toy race track.

Each segment has one end with a raised lip and one end with a sunken step. When you fit two segments together, the lip of segment A rests on the step of segment B. This ensures that the segments hold together, with a smooth seam between them. Ideally, the slide feels like a single unit to the rider. Slides typically use completely enclosed tubes for the sharpest turns, to make sure everyone stays in.

These segments rest on a framework of steel girders. The girders may be positioned directly below the slide, or they may sit adjacent to the slide, supporting it with sturdy cantilevers.

 Photo courtesy Splashtacular, Inc.This water slide is built around several steel columns, which hold up the fiberglass segments with strong cantilever supports.

Water parks generally buy new slides from an outside manufacturer. The manufacturer designs the slide and builds all of the individual pieces. The water park hires a local contractor to take these pieces and put the whole thing together according to the manufacturer's directions. It's just like building a toy race track or model train, but on a massive scale.

 In this serpentine water slide, the sharpest curves are completely enclosed, so riders won't go flying off into space.

The actual slide structure is only half of the ride, of course. In order to zip down the slide, you need a constant stream of water. In the next section, we'll look at the pumping system that generates this stream.

Slip Slidin' Away
As we've seen, a water slide needs a constant stream of water to reduce friction between you and the fiberglass surface. To maintain this stream, the water park has to get a supply of water to the top of the slide. Most water slides do this with a pump, housed in a building near the base of the slide. In the standard design, the pump motor turns a drive shaft, which is attached to a propeller. The spinning propeller drives water forward, in the same way an airplane propeller moves air particles.

The pump draws water from a collection sump, typically the pool at the base of the slide, and pushes it up through a narrow pipe to the top of the slide. In this way, the water running down the slide is constantly recycled. In some parks, the water is cycled through several connected pools before it is pumped back up to the top of a slide.

 One of the pump rooms at Wet 'n Wild Emerald Pointe. In these rooms, pumps draw water in from a collecting pool, pass it through a filtering and chlorinating system and pump it out to slides and other rides in the park.

In a typical set-up, the water line has a check valve, also called a one-way valve, positioned between the pump and the top of the slide. Water can only flow upward through this valve. This makes things easier for the water park facilitators. When they shut off the pump at night, all of the water from the check valve to the top of the slide sits in the pipe. When they turn the pump on again in the morning, they don't have to wait for the pipes to refill; the water starts flowing immediately.

To keep everything sanitary, the water in the collection pool is also pumped through a strainer and a filter system. The typical filter is a large container filled with sand, which sits on top of a layer of gravel. Water is pumped from the top of the container to the bottom, through the sand and gravel layers. The sharp edges of the fine sand particles trap the bits of dirt in the passing water.

 Filters and water pumps at Wet 'n Wild Emerald Pointe

At night, the park managers reverse the flow of water through the filter. As water moves up through the sand, it dislodges the bits of dirt, cleaning the filter. This backwash is pumped out to the sewer line. In a typical collecting pool, all of the water is passed through the filter several times a day. Any swimming pool is constantly losing water -- through filtering, evaporation and people carrying water away in their swim suits. To keep the pools filled, the park has to pump in more water, either from a well or the city line.

 Photo courtesy Splashtacular, Inc. This novel water slide, manufactured by Splashtacular, Inc., begins and ends inside the building.

Water slides continue to advance at a breakneck pace. One of the most interesting advancements on the horizon is the so-called "water coaster." In the past, water slides have been gravity-driven rides; the water doesn't do much more than help you along on your descent. But some newer designs actually use water to push you uphill. In these rides, the pump system drives high-pressure water to several points along the slide. When the slide dips, the water jets propel you up the next hill. With this element, designers can make slides that carry you in a complete circle, like a roller coaster. It's really amazing what you can do with only water, plastic, fiberglass and gravity.