Look at the image above. This is a scene of flooding in Cornwall, England. Notice the three foot tall blue planks across the entrances of the shops? Those are flood barriers and is one form of dry floodproofing we will be discussing today!
What is Dry Floodproofing?
Dry floodproofing is the adaptation method to keep water out of the building. Obviously, this is preferred by most property owners. It requires making the structure water tight as possible, and sealing any spaces below flood risk level.
Forms of Dry Floodproofing
There are two sectors or dry floodproofing: temporary and permanent. It is fairly obvious from the names what the are. Before deploying any dry floodproofing method, research and make sure that it is the most effective for the building and the type of flooding that is experienced. Flood Proofing Tests: Tests of Materials and Systems for Flood Proofing Structures (U.S. Army Corps of Engineers 1998) and the Floodproof Construction: Working for Coastal Communities (Southeast Region Research Initiative, 2011) are good starting points for this research.
Temporary dry floodproofing is short term and does not promise a high level of protection. The most common form of temporary dry floodproofing is using sandbags. This is often a last-minute measure that people use in order to decrease their flood risk during an event. For example, in Florida before a hurricane counties and cities may allow residents to purchase or pick up sandbags in order to have an extra layer of protection for their property. Membranes are another method deployed in which the building is wrapped in plastic sheeting or other waterproof material. Ideally, the membrane would be higher than the flood level. However, this method has a lower success rate due to potential damage from debris puncturing or ripping the membrane, or solar radiation causing the plastic to deteriorate. Temporary methods require more intervention and preparation ahead of time.
Permanent dry floodproofing offers a higher level of protection than its temporary counterpart. When looking at openings, such as doors and windows, these are imperative to seal as they offer the least resistance to water. If possible, filling these openings to match the solid wall is recommended, however not always feasible. Shifting the openings vertically above the anticipated flood level and filling in below that level is also an option, however, not always feasible. There has been experimentation in using submarine glass and window systems, which shows promising results as it is able to withstand water pressure. Once this method is installed, however, the windows will become inoperable in order to ensure watertight qualities.
One of the more common interventions is the use of flood shields, as seen in the previous images. These methods are often fitted to the building itself and deployed ahead of a potential flood event. Below are FEMA examples of different flood shields. Like everything discussed previously, what flood shield is deployed depends on the characteristics of the opening, flooding, and building. A flood shield is only as good as the wall it is attached to. Passive flood shields require less human intervention, as the floodwaters trigger a ballast system and push the flood shield closed. Hinged or sliding requires intervention to deploy and is very visible. Also, if the opening is large, like a garage door, then steel or reinforced aluminum plate is recommended. However, smaller openings will commonly use aluminum. Everything depends on the situation and architecture, always consult a professional before installing a system in order to ensure the best results.
Advantages and Disadvantages
We have talked about what dry floodproofing is, and some of its methods. But the fact of the matter is, just like every adaptation method, it is only effective in certain situations. This is even more so when being applied to a historic building, such as those in Nantucket. The potential of irreversible damage is high, not only from the water but from the materials deployed. Filling in windows, especially in a historic context, is not recommended in any form. Shifting openings vertically is less harmful, but not really recommended. Installing visible flood shields is not ideal due to the permanent damage the bolts may impose on the historic materials. The visibility of the shields, especially the sliding and hinged shields, is also not ideal as they disrupt the visual continuity of the historic environment. The National Park Service released Guidelines on Flood Adaptation for Rehabilitating Historic Buildings in 2019. The Recommended and Not Recommended sections are in the photo gallery below. This outlines the National Park Service’s stance on employing dry floodproofing measures on historic buildings. Also below are general advantages and disadvantages for employing dry floodproofing measures on any building.
- Effective for the flood depth it was designed for
- For non-residential buildings, it fulfills requirements for reducing flood insurance premiums
- Does not require additional land beyond the building footprint
- The structure is kept in place and may avoid too significant changes in appearance
- May be fundable under FEMA mitigation grant program
- Not technically feasible for flood depths more than 3 feet, or a structure with a basement due to hydrostatic pressure and bouyancy
- Cannot bring a residential building into compliance with floodplain and insurance standards
- Require human intervention and consistent maintenance
- Can fail or be exceeded by large floods, offering no protection than a regular opening
- In case of failure, the building should not be occupied during flooding
- Subject to leaking, therefore flood pumps or wetproofing methods may be necessary
- Does nothing to minimize damage from high flood velocity, wave action, erosion, or debris impact
National Park Serice Recommended and Not Recommended
We cannot stress enough the need for research before employing any adaptation method on a building. Deploying the incorrect method can cause more damage than the flood itself. Dry floodproofing methods are effective when used in the correct situations and are properly maintained to ensure effective deployment. Temporary methods are best for very infrequent flooding, whereas the more permanent methods are best for slightly more frequent flooding. But regular flooding, high-velocity flooding, or high depth flooding is not ideal for either temporary or permanent flooding. Historic buildings require even more care due to their already protected nature and sensitive materials. Ensure you are doing the best for your building as possible: research, reserach, research!
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