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Definition

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Dry floodproofing is a method of flood preparation that involves building designs and material choices that do not allow for the entry of floodwaters into the structure. This resilience measure should be designed to account for the height of building flood elevations, buoyancy and hydrostatic pressures of flood waters, and wave action.

Dry floodproofing differs from wet floodproofing in that wet floodproofing measures allow the entry of water (see figure at right for further comparison).

Dry floodproofing measures:

  • Sealing building walls: Since most wall materials will leak, it is generally necessary to apply waterproof sealants to the building walls, all structural joints (such as where walls meet foundations), and openings for utility lines. Cement- and asphalt-based coatings are often the most effective sealant. However, if applied to outside walls, these coatings can drastically change the appearance and may be susceptible to puncturing. Addition of a new masonry veneer over the coating can provide additional protection and aesthetic appeal. Clear coatings, such as epoxies and polyurethanes, can be applied to exterior walls without changing the appearance, but tend to be less effective than cement- and asphalt-based materials.
  • Permanently seal openings: It may be possible to permanently seal the lower portions of some window or door openings. For example, all or part of a low window can be replaced with brick or glass block.
  • Flood shields for openings: Windows or doors that extend below the flood protection level require temporary installation of watertight shields over the openings. The material used and installation methods depend on the width of the opening, flood depth, and other considerations. A plan must be developed and implemented to ensure that flood shields are properly installed prior to a flood.
  • Interior drainage: A good interior drainage system must be provided to collect water that leaks through the sealant or sheeting and around the shields. This system typically requires a sump pump and an emergency power source, such as a portable generator, to enable operation during a power outage.
  • Other: It may also be necessary to strengthen the walls, anchor the building, install backflow valves in sanitary and storm sewer lines, elevate utility systems, anchor fuel tanks, or implement other measures.


Issues Addressed


Dry floodproofing addresses the need to accommodate increased flood risk, and protect the interior of buildings from flood damage. New York City has experienced flash floods, storm surges, storm tides, river floodings, and inland flooding events caused by extreme weather such hurricanes, tornadoes, nor'easters and high precipitation events.

New York City’s more than 600 miles of coastline means a large volume of the built environment exists within 10 feet of average sea level. Global sea level rise is a slow, yet steady, process, which incrementally increases the chances for various types of flooding that impact coastal areas. The New York City Panel on Climate Change has stated that “as sea level rises, coastal flooding associated with storms will very likely increase in intensity, frequency, and duration.” Increased precipitation is another projected factor in future climate change models. Volume of annual precipitation may increase by as much as 10% by the 2080’s. These combinations of risk factors are resulting in the adjustment of Federal flood hazard mapping to show increased flood risk assessments that continue to rise throughout the next century.

Special Initiative for Rebuilding and Resiliency (SIRR)

The SIRR makes the following recommendation regarding Dry Floodproofing:
  • Dry floodproof all commercial buildings below the flood line


Considerations


Dry floodproofing is appropriate for masonry buildings that are slabs-on-grade and those buildings that do not have basements. Wood frames can only use this technique if veneered in brick, masonry, or stone. Above 3 feet, an engineer should do structural calculations. New buildings in the flood plain must utilize other flood mitigation techniques, but older buildings may use dry floodproofing to bring themselves in line with newer regulations.

An important thing to consider is that many dry floodproofing techniques need to be manually put into place prior to a flood by an able-bodied person.


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