A couple months ago, I wrote about the seemingly oxymoronic practice of wet floodproofing—protecting buildings from flooding by allowing the floodwater to enter the building by using materials that aren’t damaged by water and that don’t support mold growth. In this article, we’ll take a look at the very different approach of dry floodproofing.
The Federal Emergency Management Agency (FEMA) defines dry floodproofing as “a combination of measures that results in a structure, including the attendant utilities and equipment, being watertight with all elements substantially impermeable to the entrance of floodwater and with structural components having the capacity to resist flood loads.”
In other words, dry floodproofing is about keeping floodwater out and keeping the building dry.
There are a number of key elements to this practice that we’ll look at in turn.
Where dry floodproofing can be used
While implementing various measures to keep floodwater out of a building may seem like a no-brainer, there are actually some very significant limitations and risks. If building foundations and above-ground walls are to be made into water-tight flood barriers, there may potentially be very significant forces pushing on those walls.
These forces, referred to as hydrostatic pressure, come from the water pushing against the walls. Those forces can be huge—which is why the glass in a large aquarium has to be very thick. (The 23-feet-deep, 6.3 million-gallon ocean tank at the Georgia Aquarium has an acrylic viewing wall that’s 23 feet tall, 61 feet long, and two feet thick—needed to withstand the tremendous pressure experienced at the bottom of the tank.)
The same principle applies—though not at such an extreme—with dry floodproofing. The walls of a dry-floodproofed building need to be carefully engineered and built. For the most part, this is a solution for non-residential buildings only. This is not a practice that is recommended for single-family homes and most other residential buildings, though it may be feasible for mixed-use buildings.
To function effectively as a barrier to floodwater, walls must be impervious up to at least the design flood elevation—and I prefer that the design flood elevation be as much as five feet above the base flood elevation (BFE). With the hydrostatic pressure that results from just a few feet of floodwater on one side of a wall, even small cracks become significant leakage sites. Impervious wall systems used in dry floodproofing are most commonly poured concrete.
Reinforced, poured concrete offers high strength, good moisture blockage, and an ability to get wet without damage. Concrete masonry unit (CMU) construction is less satisfactory, due to cracking that is common between CMUs. Wood-frame construction is not recommended, due to high moisture seepage rates and damage (including mold growth) from wetting.
Concrete formulations, admixtures, and surface treatments can help to make the concrete less permeable than standard concrete for situations in which frequent wettings will occur.
Flood barriers for doors and windows
Specialized flood barriers are available to protect penetrations from floodwater entry. At entry doors, specialized tracks can be added to the door frame into which flood panels can be installed in advance of an anticipated flood. Similar barriers are available for windows. Such flood barriers should have multiple closure points and gaskets to provide watertight seals. Presray is a leading manufacturer of such barriers.
Most of these are active dry floodproofing interventions, however. They require time to deploy in advance of a flood. This can be a limitation to dry floodproofing—the need to actively engage in advance of a flood. Very clear and specific management protocols must be established for such buildings.
With existing buildings, in situations in which openings in walls aren’t serving much function (such as windows in a basement), it may make sense to fill in the opening and associated window wells, thus eliminating that point of floodwater entry.
Interior drainage, sump pumps, and backflow preventers
Spaces that are dry floodproofed, such as basements, should have either drainage to daylight (that will remain daylight during a flood) or to a sump with an automated sump pump. If there is a critical-load circuit served by a generator or battery storage system, the sump pump(s) should be on that circuit so that they will operate during a power outage.
Backflow preventers on sewer lines and/or floor drains are also very important in dry floodproofed buildings to prevent backflowing into a basement. This is especially important when combined sanitary/storm sewers are being used.
Dry floodproofing mechanical systems and other core services within a building
In a building that is not suitable for full dry floodproofing—for structural reasons most commonly—it still may be possible to dry floodproof basement mechanical systems elevator cores, electrical facilities, and other key services. Reinforced CMU or concrete walls can be built to cordon off such spaces, with specialized, watertight doors leading to those protected areas.
In some cases, those floodproofing barriers may need to extend only a few feet up from a slab floor, though the greater the height of full protection, the more resilient the system. A sump pump or other drainage is still important is such spaces to handle leakage or seepage.
When watertight doors are used to seal off a mechanical room, provision may be needed for emergency egress up into the building, in case someone is trapped. Such access may also be important to access mechanical and electrical systems that are cut off from the normal stairway.
Dry floodproofing, like wet floodproofing, is a practice that can be used in some applications to enhance the resilience of a building. Dry floodproofing does not make sense in every situation, however, and obtaining professional design support is absolutely crucial with this approach.
More on dry floodproofing is available from FEMA, including Guidance Document P-936, Floodproofing Non-Residential Buildings (July 2103).
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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.