How a Lazy River Works

Lazy rivers are gaining popularity among both residential and commercial clients.

Over the past several years, they’ve become the norm, not only in waterparks, but also in resort and community pools across the country as aquatics directors look for ways to increase attendance. Now they’re popping up in backyards, thanks in part to a manufactured system reported to simplify design and installation.

But they present their own set of design challenges. Not only must a lengthy body of water move 1 to 2 miles per hour, but it must turn around curves without stalling. The special challenges have caused some aquatics designers to commission scientific research on the subject and develop proprietary design processes, calculations and spreadsheets, which they guard with their lives.

To learn about the hydraulics, PSN consulted with aquatics designers who have created lazy rivers the traditional way and with newer propulsion systems.

Heart of the System

Traditional lazy river systems are fueled by the same pumps used on pools and spas — only bigger and more of them.

Generally speaking, the horizontal end-suction centrifugal pumps can be 10 to 40 horsepower, depending on the river’s size and volume. Generally speaking, the goal is to move the river 1 to 2 miles per hour, or about 2 to 3 feet per second.

The system for motive flow, or water movement down the river, is separate from filtration. With proper calculations, the motive pumps can be placed as far away from the river as needed.

On the commercial and residential side, a propulsion system made for lazy rivers is becoming more popular. The River Flow system consists of a high-capacity, low-head, vertically mounted axial flow pump. The pumps are designed to move large volumes of water with relatively little horsepower — a 7½ hp pump can move 2,500 gallons, says Peter Davidson, president of Current Systems Inc., the Ventura, Calif.-based producer of the systems. Their design requires that the pumps be placed near the river.

Regardless of the pump type, variable frequency drives are popular here. This allows designers and operators to speed up or slow down the water in certain spots where they’ve found it appropriate. Designers also can adjust the VFDs to push more water around the corners, if needed, to move users around bends without stalling. This can especially come in handy on tight turns.

“It works out very well, and our clients are really tickled and enjoy it, because they feel like masters of their own ship,” says Ken Martin, founding principal of Orlando-based Aquatic Design & Engineering.

However, VFDs are not meant to enable sloppy calculations by those expecting to fine-tune later. Designers still must exercise precision for the most cost-effective and efficient project possible.

When designing with all this power, entrapment becomes an even bigger issue than normal. “You have all these very powerful pumps that are drawing water from the actual river channel and pushing it back there through some very high-powered nozzles,” Martin says. “So we have to be quite quite careful.”

For this reason, strict adherence to the Virginia Graeme Baker Pool and Spa Safety Act should be practiced. Martin’s company prefers to send water from the river to a collector tank before it goes back to the pump. “The pumps are never hooked directly up to the actual river itself,” he says. “[So] there’s no way they could get trapped against it.”

Motive force

Water pushes into the river through a series of jets or inlets placed throughout.

The traditional inlets are not like spa or current jets, but consist of pipe fed into the river wall or floor.

“The angle is important,” says Terry Brannon, president of The Brannon Corporation, an engineering and consulting firm based in Tyler, Texas. “Too steep an angle, and the water jet bursts through the surface like a large bubbler. Too flat an angle, and all the momentum is imparted to water at the floor and is not very efficient.”

The jets are pointed in the direction the water needs to move. Brannon then angles them 30 to 45 degrees from horizontal (for floor jets) or vertical (for wall jets). They are cut flush with the walls or floor.

They often are made with 2-inch pipe, Brannon says, and generally come in clusters of three or more at a time. On the other hand, the River Flow systems uses much larger plumbing — 10 to 12 inches, with specially made return fittings.

Many designers prefer to place the jets or inlets on the walls. For Brannon, the issue is avoiding tripping hazards. He puts the inlets on both sides of the river, pointed toward the center.

If the river forms a loop or circuit, designer David Schwartz prefers to place the jets on the outside wall to facilitate motion around curves. “That’s where the highest velocity will occur relative to the inside wall,” says the principal at Waters Edge Aquatic Design in Lenexa, Kan. “If you put it on the inside wall, it will just pile up against the outside wall. This way it’s going to curve around.”

But the walls are not as tall as the floor is wide, so they don’t have as much space. On very wide, mostly commercial rivers, it may make more sense to place the jets on the floor, in order to fit more. Brannon has seen installations where inlets were set in the floor in groups of 10 or 12.

Regardless of orientation, water should not exit the nozzles at speeds higher than 20 feet per second, for user safety. Motive flow rates generally fall between 1,000 and 1,500 gallons per minute, Schwartz says.

Davidson uses another range for nozzle speeds. He likes to see them hover around three to four times the average speed of the river. “If … the average speed of your river current is 2.5 feet per second, a good speed for the velocity at the nozzle … is [approximately] 10 fps.”

Around the bend

One of the toughest and most important challenges is getting the water to round the corners. If it moves too fast, the water can bunch up on the outer side and maybe even spill over. This reduces the water depth on the inner turn. Gravity causes rafts to slide from the higher water to the lower portion and bump into each other.

On the other hand, if the water moves too slowly, it can stall.

The tightness of the turns plays a significant role. “We’ve seen other facilities where the tubes basically get caught in an eddy, and a lifeguard has to stand in the water and push them around it, if they make the turns too tight,” Schwartz says.

Water velocity, depth and width of the river determine how tight the corners can turn. Generally speaking, the wider the stream, the wider the turn, Schwartz says. Higher speeds also require wider turns.

It helps to place wall jets in the corners, even if the rest of the jets are on the floor, Brannon says.

Nature of water

Some principles of fluid dynamics help in designing these features.

Lazy river design uses water to push water. Momentum is vital to the workings of lazy rivers: Once enough momentum has accumulated, it does some of the work in place of pumps. The process of combating inertia and reaching peak momentum can take several minutes, up to 30 in especially long lazy rivers, Schwartz says. It will take less time to bring the water back to a stand-still after the system shuts down, he adds.

Momentum makes it possible to move the water with less electrical energy. Once the preferred water velocity is reached, the pump power can be reduced up to 25 to 50%, professionals say.

This force also helps create some interesting combinations. For instance, Waters Edge recently designed a lazy river that passes through a wave pool from one side to the other, at a municipal waterpark called Long Branch Lagoon, in Dodge City, Kansas. The rafts enter the wave pool on one side and float across to an opening on the other. It’s done without jets in the wave pool itself. Instead, the momentum continues from the rest of the river — even when the waves are on.

Other things contribute to this ability. To start, water moves differently in a wave pool than a lazy river. “The waves don’t push outward, they pulse up and down,” Schwartz says. “The lazy river is pushed horizontally, so the water flow doesn’t interfere too much with each other.”

Additionally, water is not compressible like a gas. This helps it effectively maintain its volume, shape and direction as it passes through the other body of water, with relatively little dissipation. “If you overcome enough inertia and gain enough momentum, you’re moving the water through the other water,” Schwartz says. “Instead of thinking of it as a liquid, think of it as a very flexible solid, so it pushes through the wave pool.”

In this case, the waves in the pool were mild, measuring only about 12 to 18 inches from trough to crest. But the waves would skew the riders slightly. To guarantee that riders make it through the opening back into the river, designers calculated how much the waves would push riders out, and offset the river opening accordingly.

In residential settings, Davidson has designed rivers to do the same thing in regular pools. The flow moves across the pool so riders can re-enter the river on the other side.

It takes some pretty sophisticated calculations to figure this out, and should be done by qualified engineers. Designers also must be careful about where to do this. For instance, rivers should never be interrupted near a turn, whether to cross another pool, or provide an entry and exit area for rafts. If the designer does, the river will lose momentum, Brannon says.