Introduction

In cities like Manhattan, where urban development and water bodies converge, floodproofing of structures is not just a regulatory requirement but a crucial aspect of building design. The coastal geography of New York City is considered highly vulnerable to flooding from extreme weather conditions. As we witness climate change and sea level rise, we anticipate flood risks to increase.

In a recent AIA NY presentation on flood and hurricane mitigation, a panel of experts, including building envelope consultants, a structural engineer, and an architect, shared their insights on addressing the challenges posed by extreme weather events in New York City. The discussion covered Flood Zone and Wind-Borne Debris Region designations, relevant building codes, and real-world applications, highlighting critical construction details from ongoing projects.

In this article, we will focus on Daniel Kinsley’s portion of the presentation, where he shared his expertise on flood mitigation for building envelopes. A future article will summarize Jessica Mandrick’s structural engineering perspective. GMS has completed several projects that showcase innovative floodproofing strategies to enhance building resilience and safety.

Understanding Flood Risks

Understanding how to read flood maps is crucial for assessing your flood risk. These maps delineate areas prone to flooding, categorizing regions into the 100-year floodplain for high risk, 1% chance of flooding in a given year and the 500-year floodplain for moderate risk, 0.2% chance of flooding in a given year.

Federal Emergency Management Agency’s (FEMA) Flood Insurance Rate Maps (FIRMs) classify regions into five flood zones ranging from high to moderate risk. These zones help determine the potential extent of flooding and whether flood waters will be exacerbated by wave action (indicated as VE zone). Moreover, the Base Flood Elevation (BFE) is used to indicate the expected height of floodwaters in high-risk areas. For example, a notation like AE13 on a FIRM signifies that the location is in the AE flood zone with a BFE of 13 feet. BFEs are measured using the NAVD 88 datum, which indicates feet above mean sea level. To calculate the expected flood height above the ground at your location, subtract the ground elevation from the BFE. The BFE is used in determining the appropriate Design Flood Elevation (DFE) for new construction.

A site’s DFE is established by the local building code as the minimum elevation to which a new construction on a site must be elevated to, or floodproofed up to. DFE is the sum of the BFE plus 1 to 3 feet depending on the building’s location and use.

The image below is an excerpt from a Flood Insurance Rate Map (FIRM) highlighting an area from the lower Manhattan along the Hudson River from West 24th Street to Harrison Street; an area that has seen a significant increase in new development over the past few years. This article will touch on three projects that the GMS Building Envelope Team has worked on in this stretch recently, and some of the flood mitigation solutions that were used at each projects’ ground floor.

Figure 1 FIRM for Lower Manhattan along the Hudson River from West 24th Street to Harrison Street           

Wet Floodproofing

In flooding events, there have been incidents where the hydrostatic pressure of the water pressing against foundations has caused entire homes to be dislodged from their foundations during storms. Wet-floodproofing is intended to eliminate that one directional pressure flood waters apply to a structure by allowing the water to get to both sides of a wall or foundation, therefore equally distributing the load as the water rises.

Project Example 1: Wet Floodproofing
One exemplary project of incorporating wet floodproofing is a mixed-use building in Manhattan that features residential and retail spaces. This building employs a wet floodproofing strategy with an innovative interior flood wall. Flood vents are openings to prevent interior and exterior water pressure from building up and are strategically placed within the exterior concrete curbs to allow water to enter and exit the building. This minimizes pressure on the exterior walls during flood events. The flood water passes through the perforations eliminating the hydrostatic pressure on the as water rises on both sides of the wall equally.

What is interesting here is the flood wall on the interior. The flood vents allow the water to enter the building, but the flood wall, set at the site’s DFE, prevents water from flooding the whole floor and damaging interior finishes.  The Owner made the choice to sacrifice the first 2’ of the building interior as a measure to protect everything further inboard.

Figure 2: Flood Vents

Figure 2.1: Section View

 

 

 

 

 

 

 

 

 


Dry Floodproofing

Dry-floodproofing prevents flood water from entering a building.  In some cases, a site’s DFE, may be 3, 4, 5 or more feet above street level. Typically meaning 5’ of opaque, surface.

In 2020 the New York City Department of Buildings, through their Office of Technical Certification and Research, issued Building Bulletin 2020-021 to address the increasing interest in using glazing systems as perimeter barriers against flood waters. The Building Bulletin established evaluation criteria for glazing systems used for dry-floodproofing in special flood hazard areas, or “flood-resistant glazing systems”, including a definition of “substantially impermeable” and site-specific loads based on ASCE 24.

The criteria reference the standards set forth in ANSI/FM 2510 for flood mitigation equipment and barriers.

  • ANSI – The American National Standards Institute partnered with Factory Mutual Standards, or FM Standards, FM Standards is the research and testing arm of FM Global; a major international property insurance provider.

Project Example 2 – Pre-tested (Dry Floodproofing)
This building, currently under construction, combines residential and retail functions and incorporates a pre-tested structurally glazed, flood-resistant system below DFE, which is complemented by standard glazing above. This not only ensures compliance with flood mitigation standards but also aligns with the building’s aesthetic requirements.

On the plan, the blue areas are egress and residential areas that require wet-floodproofing; flood vents and deployable barriers. The red are areas protected by dry-flood proofing.

Detailed on the right in Figure 4, are two systems separated by a steel tube support. The blue arrow indicates the direction of the flood loading. The steel is set 1’ above the design flood elevation. In Dry-floodproofing applications,

Below the steel is a structurally glazed flood-resistant glazing, and above is the typical captured glazing system. The architect could choose to extend the flood-resistant glazing (FRG) to the full height of the glass opening, but that would be costly, so instead the systems are separated and designed to look similar.

 

Figure 4: Details of flood-resistant glazing

At top left in Figure 4, the first thing you may notice is the thick glazing assembly. Per the ANSI/FM standard, laminated glazing is required within flood-resistant glazing systems; glass is allowed to crack during a flood event but must not shatter.

The interior lite two 3/8” lites laminated with a SGP interlayer is the workhorse here providing the flood and debris resistance. The exterior lite provides the aesthetic. It the low-e coating, and it matches the outer lite of the glazing used above DFE and throughout the rest of the building.

The framing system is a typical aluminum storefront system. The manufacturer has previously subjected their system to independent lab testing. The OTCR Bulletin states that if the proposed system is previously tested for loading equal to or higher than what is expected at your site, project specific testing can be waived.

Project Example 3 – Custom System (Dry Floodproofing)
Now, what if a pre-tested system doesn’t work for your project, either due to the loading the site anticipated, or purely for aesthetics.

This former warehouse building was renovated and expanded and is now a commercial office project with ground floor lobbies and retail spaces. Per the building code, alterations to Pre-FIRM buildings that increase their non-compliance with flood mitigation codes, are required to become compliant. This building was originally built in 1934; well before FIRMs were first established in New York City in 1983.

Custom flood-resistant glazing was used to meet mitigation requirements, shown in yellow in Figure 5.

Figure 5: First floor plan with FRG highlighted.

For this project, DFE is about 5′ above grade in some locations. Some of those locations were designated to be Retail spaces and the Architect and Owner did not want 6′ of opaque surface before vision glazing started. Prospective retail tenants would prefer pedestrians could look into their space from this sidewalk. This meant lowering the sill of the existing openings below DFE. A custom FRG system was designed for this project.

The approach here was similar to the previous example. We were to provide an FRG system up to DFE +1, and a typical captured storefront system above and make them look cohesive. You can see in the bottom of Figure 6, the portion of the opening that would become FRG.

Figure 6

The system is structurally glazed with steel mullions. The steel is more robust than stock aluminum extrusions and can handle the heavy loads calculated for the site.

There are aluminum extrusion beauty caps adhered to the glazing to match the aesthetic of the standard storefront elsewhere. Unlike in the pre-tested system, there were more options for customization. It just meant it had to be tested.

 

Testing

The OTCR Bulletin requires lab flood load testing if the proposed FRG system is not previously factory tested and approved according to the ANSFM2510 standard; all FRG systems must pass the laboratory testing protocol under OTCRDOB BB 2022-021 OTCR. For this project, a mock-up of the custom system was installed at a testing laboratory.

In Figure 7, we are looking at the interior face of the system. The exterior face is within the steel tub.

Figure 7

A protocol of two tests was required for this specimen. A hydrostatic bathtub test and a debris impact test:

Hydrostatic Load Test
The tub is filled with water and left for 24 hours. The rate of leakage through the system is recorded periodically. The allowable leakage is less than a tenth of a gallon per hour per linear foot. There is an allowable amount as nothing is deemed impenetrable.

Debris Impact Test (if Hydrostatic Load Test passed)
This is to determine how the system will hold up when rising flood waters enable large debris to strike the building. In this case, the lab simulates a telephone poll striking the specimen. You can see in the center picture, the lab uses a piece of wood fixed on a steel hammer. The hammer is mounted on a lift and it is swung against the glass. The glass may break, and it did, but it cannot shatter.

Figure 8: Hydrostatic Bathtub Test and Debris Impact Test (left to right)

This is then followed by another 24 hour hydrostatic load test to see how the system holds up following impact.

Implementation

When testing was completed, these units were fabricated and installed on site.

Figure 9 Exterior: You can see the location of DFE relative to grade and the glazing extending below it.
Figure 9 Interior: During a flood event, the flood-resistant glazing system transfers hydrostatic load to the columns and surrounding concrete floodwalls. In the image, you can see that at a portion of the building, there is a double height space extending below grade. We added buttresses to reinforce the columns and flood wall.

Figure 9

Completion

Here are some photos of the finished conditions.

Figure 10

Figure 11

Discussion

These projects illustrate the effectiveness of both wet and dry floodproofing strategies. While each comes with its own set of challenges, such as compliance with local codes and integration into existing architectural designs, the tailored solutions by GMS demonstrate flexibility and innovation in urban flood mitigation. The success of these projects extends beyond individual buildings, influencing industry standards and encouraging other architects and developers to adopt similar strategies. Regulatory bodies also benefit from these case studies, which help refine and update floodproofing requirements to reflect the capabilities of modern engineering and architectural solutions. As urban landscapes continue to evolve, the insights gained from these projects will undoubtedly shape future approaches to floodproofing in architecture.

References and Further Reading

For more detailed technical specifications and building codes relevant to floodproofing, readers are encouraged to consult the New York City Building Department’s guidelines and the American National Standards Institute’s standards on flood mitigation equipment.

Department of Buildings – NYC Housing Recovery
Federal Flood Risk Management Standard | FEMA.gov
Guidance for FEMA’s Risk Mapping, Assessment and Planning | FEMA.gov
American National Standards Institute – ANSI Home