For the building design community, the impact of Hurricane Sandy didn’t just stop with the clean-up. It has influenced the way we design buildings in New York City today and for the future. Regardless of the phase of a project, after Sandy, the design team had to reconsider their design to accommodate flood mitigation measures. One such project was a high-end (or luxury) residential building project at 150 Charles Street in the West Village neighborhood of Manhattan.
The 15-story residential building, designed by CookFox Architects with the Witkoff Group as the developer, is 240,000 square feet and includes over 90 apartments, extensive green terraces, a swimming pool, gym, and parking garage. WSP provided MEP (mechanical, electrical, and plumbing) engineering services as well as fire protection engineering. Excavation was completed and construction documents were at 90 percent complete when the project site was partially flooded by the hurricane. An adjacent building exterior wall collapsed into the project site and the area was without power for one week.
After Hurricane Sandy, the developer had its team of architects and engineers rethink the building design. A list of items was proposed to be incorporated into the project design to make the building resilient in the event of future storms. All resiliency suggestions were implemented. However, the efforts delayed the project by six weeks and added as much as $3 million to its cost.
Resilience Measures
The first major concern was the loss of power to the area. The installation of two natural gas emergency generators that would accommodate basic building operations was incorporated into the design. Finding a location for the generators was a challenge in itself. As the project was already 90 percent designed, the team was tasked with finding an additional 760 square feet of available roof space. Through several iterations and brainstorming sessions, it was decided to install the two generators above the elevators. In addition to the roof space, riser space for emergency generator conduits was required throughout the project. Both obstacles were overcome by the design team through collaboration.
In the event of a blackout, the two natural gas generators would run building services such as the fire alarm system, elevators, sewage ejector pumps, domestic water pumps, water heaters, and emergency egress lighting. Also, each apartment would be equipped with at least one electrical outlet connected to the emergency generator. WSP designed for a large current draw in the individual apartment outlets, assuming that a power strip will be plugged into the single emergency outlet for multiple electrical devices. A 20 amp breaker was added per apartment, capable of handling 1.92 kW, whereas a normal convenience receptacle would draw 0.18 kW.
Other resiliency tactics implemented were for flood control. The subcellar level has a recessed swimming pool, with a bottom elevation 46 feet below the 100 year floodplain. Hurricane Sandy’s storm surge raised the water level an additional 9.4 feet above the predicted astronomical tide levels for Battery Park City (approximately 2 miles south west of the project site). The following measures were taken to keep the building dry in the event of another superstorm:
- Floodgates have been designed to encircle the building at the perimeter of the project site. Posts installed at regular intervals with designed locking flood panels can be assembled in a matter of hours. These panels extend 5 feet above the lowest point of the adjacent sidewalk to mitigate the effects of future flooding. The flood panels and posts are stored in the basement of the building. Permanent footings and anchor plates were installed at the sidewalk, into which these posts would be placed if a flood event is predicted. Structural consideration had to be given to the location and the design of the post support system.
- A flood mat was designed by the structural engineer to resist external groundwater pressure up to that of a flood level. Due to the depth of the subcellar pool, water pressure at 50 feet below the surface of the flood surge can be large.
- Backflow prevention valves were added to the sanitary and stormwater outlet of the facility which is approximately 20 feet below the 100 year floodplain. The backwater valves were designed to withstand the pressure, keeping the flood water out of the sanitary piping system and from backing into floor drains and plumbing fixtures.
The sanitary distribution system was redesigned for the project as well. Originally, the cellar and subcellar sanitary system was piped to a sump pump located in the subcellar level. It was designed for a 100 year flood event. To address water levels experienced during Hurricane Sandy, the ground floor plumbing fixtures and floor drains were included in piping to the sump pump. The size of the sump pumps was increased from two pumps with a 500 GPM capacity to three pumps with a 700 GPM capacity. This increased the capacity 40 percent and created more redundancy.
- Specific rooms identified as critical spaces - electrical switch gear room, boiler rooms, gas meter and water meter rooms, telecommunications room - were designed with cast-in-place concrete walls for flood mitigation. In addition, the doors in the flood control walls were designed with anchors for demountable flood panels.
