An urban promenade for the 21st century, the High Line’s linear roof garden offers sophisticated drainage and planting solutions.
By Frank Edgerton Martin
The reborn High Line is one of the first places in New York City that really has a 21st century feel. The irony is that the High Line’s new LED lighting, sleek benches, and emerging milieu of star architect hotels and condo towers are all built on 19th and early 20th century infrastructure.
Designed by James Corner, principal at Field Operations, with architects Diller Scofidio + Renfro, the High Line will ultimately extend 1.5 miles through Manhattan’s Meatpacking and Chelsea Districts. Much of the design’s brilliance is that it calls attention to the time layers of the surrounding city landscape and not its own. There are places, such as near 14th Street, where undulations in the walking deck surface encourage a slower pace to look at the views over the Hudson River and into the Meatpacking District. At 10th Avenue, the High Line angles over the street with a framed view of the Statue of Liberty and Ellis Island.
The railroad engineers who built this line in the 1920s to get freight trains off the streets certainly didn’t plan for this aesthetic outlook for growing trees and gardens on what is essentially a linear roof deck. Almost a century later, the High Line’s renewers deserve credit for preserving it without adding competing statements of their own and for using the retrained design palette of Piet Oudolf’s plantings that keep the deck on the High Line as a “thin” foreground for looking at the city from a forgotten aspect.
Yet, the very thinness of this old rail line presents significant technical challenges for drainage and the creation of a soil profile deep enough to support plants and trees. Annette Wilkus, ASLA, principal of SiteWorks LLC, often consults design firms such as Field Operations on technical issues and site-specific solutions. The High Line presented stormwater challenges, including its elevation from the street and its linear form. The solution required a tailored combination of products.
A green roof system was installed throughout all the planting beds of the High Line. Approximately 25mm thick, this system has an egg-carton-shaped type of base, filled with gravel and covered with a layer of geotextile filter fabric. Soil is placed over the system. Set at the planter plank interface in combination with perforated metal is another geotextile layer that holds the soil in place while also preventing rodent infestation. (This nonwoven material is a standard drainage geotextile that meets or exceeds commonly specified DOT and commercial drainage protection requirements.)
Wilkus explains, “Drains were located throughout this green roof system wrapped with filter fabrics.” Set at surface level, each of the drains is wrapped in filter fabric, then surrounded by 152mm of gravel. Because the narrow gauge of the project allowed little room for water retention, water runoff leads directly into the city stormsewer system. Yet this water, having already percolated through a rich array of flowers and grasses, is cleaner than water drained from surrounding hard surface streets and roofs.
Without the drainage and filtering options made possible by geosynthetic materials and emerging green roof systems, daring 21st century reuse projects such as the High Line would be much harder to build and sustain. Clearly this linear promenade’s elevated views, cool breezes, and soft plantings are meeting some hunger felt by New Yorkers—especially those who live downtown with few big parks of their own. Since opening in June 2009, the High Line has received its millionth visitor and is now a major tourist attraction.