Fundamentals of Resilient Design: Wet Floodproofing

Posted by on Apr 20, 2015

The Chesapeake Bay Foundation headquarters in Annapolis, Maryland was built close to the Bay and incorporated wet floodproofing measures, such as elevating the occupied space and using the ground level for parking. Photo: Alex Wilson

The Chesapeake Bay Foundation headquarters in Annapolis, Maryland was built close to the Bay and incorporates wet floodproofing measures, such as elevating the occupied space and using the ground level for parking. Photo: Alex Wilson

When it comes to flood risk, there are several quite different approaches for improving the resilience of buildings, and the terminology can be confusing. Here, we’ll take a look at the seemingly oxymoronic practice of wet floodproofing.

The Federal Emergency Management Agency (FEMA) defines wet floodproofing as “measures applied to a structure or its contents that prevent or provide resistance to damage from flooding while allowing floodwaters to enter the structure or area.”

There are a number of key elements to this practice that we’ll look at in turn.

Wet floodproofing strategies, including openings that allow floodwaters to enter the basement. Graphic: FEMA

Wet floodproofing strategies, including openings that allow floodwaters to enter the basement. Graphic: FEMA

Letting floodwaters flow through a building without washing it away

A key aspect of wet floodproofing is properly anchoring a structure to its foundation, but allowing floodwaters to enter a building and equalize pressures on foundation walls or lower-level walls. By contrast, if floodwaters are kept out of a building (dry floodproofing, which I’ll address in a future article), there are very significant hydrostatic forces (pressures on those walls) that have to be contended with.

With wet floodproofing, you let the water in. A common approach in coastal areas is to build on structural piers with non-structural lower-level walls or openings that break-away to allow water to flow through and under the building.

With perimeter walls, opening with break-away windows or louvers can allow floodwaters into the building (see illustration).

FEMA and the National Flood Insurance Program require wet floodproofing for buildings, especially residential buildings in flood plains in which part of the building lies below the base flood elevation (BFE)—the elevation at which flooding will occur during a 100-year storm event. Building codes often specify a design flood elevation (DFE) as the base flood elevation plus some additional mount of elevation, such as one foot.

Elevating mechanical equipment following hurricane damage in Carolina Beach, North Carolina. Photo: Dave Saville, FEMA

Elevating mechanical equipment following hurricane damage in Carolina Beach, North Carolina. Photo: Dave Saville, FEMA

Elevating mechanical equipment

Part of the wet floodproofing approach is getting stuff out of floodable parts of a building that would be damaged by flooding. FEMA calls for mechanical equipment to be elevated above the BFE, but others, including RDI, recommend going well beyond just the BFE for placement of such equipment. I like to see mechanical equipment installed at the BFE plus five feet (BFE+5) whenever possible.

The increasingly common practice of placing mechanical equipment on the roof of larger, commercial buildings and multifamily apartment buildings is an example of this strategy. That gets equipment well above the BFE (unless the building is located in an extraordinarily bad place!), though it does add other complexities, such as building fully weatherproof enclosures, and it takes up space that might otherwise be used for a solar-electric (PV) system, rainwater harvesting, or rooftop greenhouse.

The therapeutic pool at Spaulding is located on the first floor. Photo: Spaulding Rehab Hospital

The therapeutic pool at the Spaulding Rehab Hospital in Boston is located in floodable space on the first floor. The building is designed to allow flooding of the entire first floor with minimal damage.  Photo: Spaulding Rehab Hospital

Specifying materials and finishes that can get wet and dry out

An important aspect of wet floodproofing is specifying materials in floodable locations that can be soaked by flooding and dry out without lasting damage or mold growth. Standard paper-faced drywall is to be avoided in such locations, because once it’s flooded, it’s destroyed; soaked paper-faced drywall can’t be salvaged, it has to be replaced.

Other materials to be avoided in floodable spaces, such as basements, include wall-to-wall carpeting, wood and cellulostic materials, sheet materials such as wallpaper and resilient flooring that can trap moisture, and plaster finishes.

Materials that are appropriate for wet floodproofing include poured concrete floors (polished concrete makes an excellent finished floor that can be highly decorative, depending on the concrete aggregate that is exposed during the grinding and polishing process), ceramic tile, brick, and fiberglass-faced drywall (such as the Georgia Pacific DensGlass® product line).

Note that these strategies need not be limited to flood zones. Basements can flood for other reasons, such as plumbing leaks, broken water lines, and burst hoses on clothes washers or dishwashers. Such accidents can cause as much damage as a flooded river or coastal storm surge.

Spaces below the BFE that are protected using wet floodproofing measures should be limited to uses like vehicle parking and limited storage, because vehicles and stored goods can be relocated in advance of a predicted flood.

More on wet floodproofing is available from FEMA, including Technical Bulletin 7 – Wet Floodproofing Requirements (1993), Technical Bulletin 1 – Openings in Foundation Walls and Walls of Enclosures (2008) and Technical Bulletin 2 – Flood Damage-Resistant Materials Requirements (2008).

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Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. To keep up with his latest articles and musings, you can sign up for his Twitter feed. To receive e-mail notices of new blogs, sign up at the top of the page.

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