It is just a few feet high. But it could profoundly affect a visitor's experience by creating the need for a wall running more than 200 feet along the Greenwich Street side of the memorial, marking an abrupt grade change between plaza level and sidewalk.
Between this wall and the museum complex at Fulton Street and a stairway at Liberty Street, there would be only two small areas where one could walk straight into the memorial from Greenwich Street, the approach most visitors will probably take.
To the public, the tough part of designing the memorial may seem to have passed last year with the choice of a concept, the twin voids of "Reflecting Absence," and the selection of architects: Michael Arad, Peter Walker & Partners and Davis Brody Bond.
But new challenges are unfolding, largely out of sight, as planners struggle to reconcile an architectural vision with the physical realities of a complex site. At stake in these negotiations are elements as important as the whole east side of the memorial, which may depend on what may seem like arcane questions of roadway layout.
The bump — really, more of a hump — would occur in Greenwich Street at Dey Street. Here, the surface would be two feet higher than at Fulton Street to the north and six feet higher than at Liberty Street to the south.
The Port Authority of New York and New Jersey, which owns the site and will build the streets, said that height would permit the installation of as many utility lines as are needed within the roadbed, directly above the subway tunnel for the Nos. 1 and 9 trains, with sufficient covering over the tunnel to satisfy Metropolitan Transportation Authority standards. The slope would allow water drainage and prevent puddling in the street.
However, it would also mean that Greenwich Street would rise almost five feet higher than the memorial plaza. There is not room enough to accommodate a stairway at this location because the footprint of the south tower — an immovable element in the equation — is too close to the sidewalk.
And the plaza cannot be canted upward to meet Greenwich Street because the voids will have walls of cascading water within, which must be kept absolutely level. Any slope in the plaza would be apparent around the perimeter of the voids.
If Greenwich Street must rise that high, the Lower Manhattan Development Corporation has concluded that a wall will be needed along much of the adjacent part of the memorial. Not only would that wall block physical access but it would also diminish the designers' goal of direct sight lines into the plaza from the surrounding sidewalks.
As an alternative, the corporation has proposed lowering Greenwich Street by four feet. That would permit a level approach to the memorial at Dey Street. (Because of the changing topography around the site, stairways will still be needed to reach the plaza level from the sidewalks at the southeast, southwest and northwest corners.)
