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This is Henry Fountain for the New York Times.
At the heart of the World Trade Center redevelopment in
Lower Manhattan, is the memorial plaza, designed by
Michael Arad and Peter Walker.
The plaza covers half of the site's 16 acres and features
two fountains that sit where the twin towers stood,
surrounded by trees.
The fountains are among the largest ever created
artificial waterfalls with sides more than 170 feet long.
Water cascades over granite walls to reflecting pools 30
feet below and then falls through square voids in the
center of each pool.
Bronze parapets along the top are inscribed with the names
of the victims of the terrorist attacks.
The parapets are lighted from inside and have heating and
cooling systems, so they remain comfortable to the
touch in summer and winter.
The water flows first into calming pools
just below the parapets.
It's pumped from below, at a rate approaching 30,000
gallons an hour, through a false floor in the pools, and
then flows over a weir, or small dam.
The specially designed weir is serrated and inclined at a 45
degree angle, so the water stays in small streams that
project about four feet from the walls.
The serrations also help to prevent clogging from leaves
and other debris and can be adjusted to keep it within
1/16 of an inch of horizontal.
From the reflecting pool, the water falls over a simple
curved edge into what appears to be a
bottomless void in the center.
It then travels through large pipes into a pump room, where
it's filtered and sent back up to the calming pools.
In the winter, heaters will warm the water slightly to
prevent freezing.
The other main feature of the plaza is the forest of swamp
white oak trees, eventually more than 400 of them.
This is no simple planting job.
The plaza, itself, is a six-foot high, engineered
structure, designed to keep the trees healthy and includes
the system for collecting and reusing rainwater.
The structure sits on top of the concrete roofs of the
Memorial Museum and other underground spaces.
Short concrete walls run the length of the plaza, creating
troughs that alternately hold soil for the trees and
drainage and irrigation pipes.
Where the trees are planted, there are other walls of a
unique louvered design, meant to hold back the soil, while
also allowing the roots to grow freely.
The trees, themselves, were selected for aesthetic
features, like straight trunks and consistent shape.
They were held in a nursery in New Jersey for five years.
Their over-sized root balls carefully tended to prepare
them for the World Trade Center site.
Drains that run the length of the plaza will funnel
rainwater to underground cisterns.
During dry periods, the water can be used to
irrigate the trees.
The cistern water is pumped to large pipes running parallel
to the rows of trees.
Smaller feeder pipes let the water flow into irrigation
pipes surrounding the root ball.
These pipes are perforated and open at either end, allowing
air, as well as water, into the soil.
About six inches above the root ball are cantilevered
steel plates that prevent the soil from being compacted,
which could harm the trees.
Granite paving stones form the surface of much of the plaza,
although in some areas, grass and ground
cover will be planted.
At installation, the oak trees are about 30 feet tall, but
they are expected to more than double in height
in the coming years.