Girding Against Return of the Windy City in Manhattan
By DAVID W. DUNLAP
Published: March 25, 2004
AUSTIN J. TOBIN PLAZA was the official name of the vast open space at the World Trade Center. But most New Yorkers knew it as the Barren Windswept Plaza.
There is a chance they will recognize its successor, too, just by the gusts.
"Typically, pedestrian-level wind conditions will be in the comfortable range," the draft environmental impact statement said about the planned World Trade Center redevelopment. "However, during some limited time periods, uncomfortable conditions may occur and activities like sitting, standing and walking may be impeded."
"A few hours per year, conditions severe enough to limit activities, produce difficult walking conditions and, at times, pose potential safety problems may occur," the statement continued. "Occasionally, access to some areas of the site may be limited."
Absent landscaping, wind screens and perhaps even the redesign of buildings, the impact statement concluded, wind levels for pedestrians "would be comparable to pre-Sept. 11 conditions."
And those could be fierce, said Verizon, which has a central office and switching center at 140 West Street, just north of the trade center site. "In prior years," Verizon said in its critique of the impact statement, "on certain days wind conditions at 140 West Street were so severe that pedestrians needed to hold onto ropes to allow travel."
No amount of nostalgia for the twin towers makes that an inviting prospect.
"It's outrageous," said Petra Todorovich, associate planner at the Regional Plan Association, which cited the wind analysis in its critique of the impact statement. "You want to create a public space that would be welcoming to pedestrians."
There is no reason why that cannot be done, said Nicholas Isyumov, a research consultant to the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario in London, Ontario, which has studied wind conditions around the trade center site since 1965, three years before the twin towers began to rise.
"The last World Trade Center was one of the windiest spots in New York," Dr. Isyumov said in a telephone interview. "Technology has advanced so that there is no excuse for creating outdoor spaces that are dangerous. Or, for that matter, unpleasant."
The plaza's windy reputation was established in its first decade. In 1977, the owners of a building across Church Street filed a lawsuit claiming that winds generated by the towers, reaching 60 miles per hour, had caused thousands of dollars of damage. By the early 90's, the problem was well enough recognized to have been addressed in a daring structural proposal by Davis, Brody & Associates (now Davis Brody Bond), as part of an overall renovation for the Port Authority of New York and New Jersey.
"Once you took care of the wind, you could program the plaza 12 months a year," said the architect Steven M. Davis. Studies were conducted at the University of Western Ontario, involving engineers at Leslie E. Robertson Associates and Ove Arup & Partners.
The proposed solution was a 330-by-330-foot open cable net, suspended from both towers, that would have covered the plaza 100 feet above ground, like a permeable ceiling. Though open to the elements, it would have been dense enough in cross-section to deflect southwesterly winds blowing over the trade center hotel. "It was like skipping a stone on water," high over pedestrians' heads, explained the architect J. Max Bond Jr.
As Davis, Brody was completing the ambitious renovation plan in 1995, the Port Authority had begun to explore selling or leasing the World Trade Center to a private developer. The canopy was ultimately unbuilt.
NO ONE is proposing a canopy over the future open spaces at the trade center site, but this month the City Planning Commission urged that the tower design guidelines incorporate setbacks deep enough to serve as wind breaks.
Tall buildings can act as sails that catch the wind and direct it downward, an effect more dependent on relative height than absolute height. In other words, the street-level wind produced by a 70-story tower among 20-story neighbors will typically be greater than that produced by a 70-story tower among 50-story neighbors.
Because wind also spills around corners, the space between buildings can affect its force. Perhaps the windiest spot at the old trade center, Dr. Isyumov said, was the channel between the north tower and the Custom House at 6 World Trade Center. That suggests that one of the trickiest places in the new complex may be the channel between the skyscraping Freedom Tower and the low-rise performing arts center.
Kevin M. Rampe, the president of the Lower Manhattan Development Corporation, said yesterday that the impact statement offered the "worst-case scenario."
"What it points to is the importance of considering the impact of the wind," he said. "Certainly this is something we've been mindful of, and we want to make sure we don't repeat the mistakes of the past."
The corporation will undertake wind-tunnel studies with a view to lessening pedestrian-level winds. It has already incorporated building setbacks in its proposed design guidelines. Other steps have also been taken. A key revision in the memorial design called for a far more lushly landscaped plaza. "One of the best forms of wind protection are trees," Dr. Isyumov said.
Though he has not formally analyzed the Freedom Tower project, Dr. Isyumov said the fact that its structural mass will diminish as it rises would almost certainly be an advantage in reducing street-level wind. "You still catch the wind," he said, "but you don't catch as much of it."