How the Water Will Fall
Because waterfalls in nature come in different sizes, Eliasson wanted The New York City Waterfalls to be different sizes, too. The waterfall under the Brooklyn Bridge will be a full 80 feet wide, but to fit under the bridge it could only rise 90 feet. The remainder of the falls were scaled to pair well with the surrounding buildings. The Governors Island waterfall will be the same volume as the one at the Brooklyn Bridge, but stretched to 120 feet tall and 60 feet wide. The Brooklyn Heights Promenade waterfall will also be 120 feet tall, and the Pier 35 waterfall ten feet shorter. Both will be 30 feet wide.
1. THE SCAFFOLDING
Each waterfall is made of aluminum structural scaffolding of a kind often used in hoist towers for construction elevators. Each scaffolding structure sits in a concrete base, which is placed on a three-inch layer of sand and stone dust (trucked in from a quarry in Elmont) sandwiched between blankets of sheet plastic. This layer is called a “bond-breaker,” because it protects the land below from being marred by the concrete foundation. When the waterfalls finish running, in October, the site will be left untouched.
2. THE PUMP
Black PVC intake pipes, eighteen inches in diameter, will draw up to 7,000 gallons of water per minute to each pump, which sit on land. From there, the water will shoot to the top of the falls through a ten-inch riser pipe. According to hydraulics designer Robert Benazzi, sewage pumps would not be powerful enough, so the team chose Thompson pumps, which are usually used for draining construction sites. When the waterfalls are turned on each morning, it will take fifteen seconds for the pumps to rev up to 300 horsepower, their maximum capacity, which is roughly the strength of a 2005 Land Rover. They will run all day. To reduce its carbon footprint, the Public Art Fund has purchased renewable-energy credits.
3. THE FROTH
Benazzi says that the first renderings he saw depicted water falling in a smooth, glassy curtain. But when he met Eliasson, the artist said he actually wanted something that looked more natural and frothy. To aerate the water at the top of the fall, Benazzi started to design a weir, or platform, of sheet metal studded with small teethlike barriers. “The more pieces of metal the water hits as it pours down,” he explains, “the more it churns, the more of a natural waterfall effect you get.” The final design incorporates two weirs: one that creates an articulated stream, and another that creates ruffles on top. After water rises in the trough at the top of the falls, some of it will be directed into the three-inch-wide channels of a laminar-flow weir. The rest of the water in the trough will keep rising to crest over the upper, turbulent-flow weir. This water will make its choppy, agitated way past dozens of miniature studs, tumbling and churning until it reunites with the water from the laminar-flow weir, and all of it, finally, goes flying off the edge.
4. THE INTAKE-FILTER POOLS
Eliasson’s waterfalls were designed to avoid disrupting the East River ecosystem as much as possible for a 100-foot stream of pounding water. (The weight of the water falling at the Brooklyn Bridge waterfall will be about ten tons per second, spread over 80 feet.) To break the water’s fall, the team designed solid-bottomed submersible intake filter pools, twelve feet deep. According to Michael Tumulty, one of the waterfalls’ environmental consultants, the pools will be surrounded by a mesh whose openings are less than the size of the head of a pin, so as to avoid sucking in fish, or even fish larvae. Equipped with bumpers that rise about a foot above water level, they will also help secure the plunge area from daredevil swimmers or kayakers.