Building for Reduced Flood Risk

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Building for Reduced Flood Risk

Siting, design, and materials choices can all mitigate risk where flooding is a danger

Posted on Sep 7 2011 by Tristan Roberts

A lot can change in two hours. At 8 a.m. Sunday, I walked the length of our half-mile driveway here in southern Vermont, checking the culverts and water bars, all fortified and cleared the day before. All good. The brook next to our driveway was raging, but staying within its banks. The Green River was doing the same across the town road.

At 10 a.m., I got a call from my neighbor that my other neighbor's house was flooding and they'd had to get out. Going back down the driveway with the hope of helping them, I found that the brook had grown to 10 times its usual width, filling the valley that this tranquil little brook usually meanders through. The Green River had done the same, covering the road and making it impossible to get anywhere. Friends who had been excited about rafting the swollen rivers canceled their plans after watching whole trees float by, and hearing boulders roll through the river.

The flooding crested before the neighbor's home was seriously damaged, but the road is badly washed out. For everyone affected by flooding in the Northeast connected to Hurricane Irene, my heart goes out to you. As I've been pulling together with my neighbors to adapt to these events, I've been wishing the best for everyone else in doing the same. As you know, it will be a long-term effort here.

Global weirding could bring more floods ...

As we think about our built environment in light of these events, we have to consider the reality of climate change. Some people prefer to refer to "global warming" as "global weirding," because our climate is a complex system, and all kinds of odd storms and weather patterns may erupt. It won't just be a linear ride of slightly higher-than-average temperatures.

As water temperatures rise in the South Atlantic, tropical storm systems will pick up more energy, resulting in higher-magnitude hurricanes on the Gulf Coast and Eastern seaboard. Elsewhere, changing precipitation patterns are expected to deliver more rainfall in intense storms that could result in river flooding.

... While development makes our landscapes less absorptive

To complicate matters, development has made our landscapes less able to absorb rainfall, says architect Don Watson, who is writing a book on "design for resilience." Watson says, "We've taken away all the absorptive capacity of our landscapes." Adapting to climate change will require making our buildings more resilient to storms and flooding. In the longer term, we need to prepare for rising sea levels and restoring the ability of our land to absorb water.

While it may be cold (and wet) comfort to owners and residents in damaged buildings, here are some tips for adapting to increased flooding, adapted from "Design for Adaptation: Living in a Climate-Changing World," an Environmental Building News article by Alex Wilson and Andrea Ward. 

Expand capacity with natural systems

Avoid building in flood zones. Flood zones are expanding — often faster than revisions to zoning regulations, meaning that simply following the law relative to the siting of buildings may not be enough. Instead of designing to 100-year floods, consider designing to 500-year floods, seeking civil engineering or surveyor assistance as needed.

Expand stormwater management capacity and rely on natural systems. More intense storms will strain the capacity of standard stormwater management infrastructure in some areas. Provide larger stormwater conveyance and detention basins, and try to rely on natural features, constructed wetlands, and other ecologically based systems to manage stormwater. "Restore the ecological services of the landscape," says Watson.

Design to survive extreme winds

Design buildings to survive extreme winds. Examples of specific measures that impart good wind resistance to a building include:

  • installing impact-resistant windows (compliant with Miami-Dade Protocols PA 201, PA 202, and PA 203) or exterior shutters;
  • installing outward-opening doors that are less likely to be pushed inward in intense wind;
  • designing walls to resist uplift using hurricane strapping and other metal fasteners that provide a continuous load path from foundation to roof;
  • anchoring walls properly to foundations or frost walls;
  • designing walls to resist shear and lateral forces using engineered wall bracing or shear panels for frame walls and proper use of re-bar for masonry walls;
  • designing roof geometries (such as hip roofs) that are less prone to wind damage than gable roofs; installing continuous roof underlayment;
  • properly installing high-strength roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. (such as 5⁄8" plywood) that will resist uplift; and
  • specifying roofing that has been tested to ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. standards for wind resistance.

Raise buildings off the ground. In flood-prone areas — even where flooding is only remotely possible — raise buildings or living spaces above ground level to minimize damage in the event of flooding. With any type of pier foundation, use great care to ensure that energy performance and airtightness are not compromised; raised floors are notoriously difficult to insulate and seal.

Specify materials and components to survive flooding

Specify materials that can survive flooding. Especially in locations where flooding or hurricane damage is likely, use materials that can get wet and then dry out with minimal damage. Such materials include preservative-treated sills and wood framing, fiberglass-faced rather than paper-faced drywall, and tile or resilient flooring rather than carpeting.

Install specialized components to protect buildings from flooding or allow flooding with minimal damage. Breakaway wall panels on pier foundations in flood-prone areas can allow floodwaters to pass under a house without destroying it. Flood vents (permanent openings in foundation walls) allow floodwaters to escape. Specialized flood barriers, including removable barriers for entrances, can keep rising floodwaters out in certain situations.

Elevate mechanical and electrical equipment. To minimize damage — and danger — from flooding, elevate mechanical equipment, electrical panels, and other equipment above a reasonably expected flood level. Even if the whole building can't be elevated to such a level, it may be cost-effective to elevate just the equipment.

Please share below your flood survival stories and thoughts on adapting to global weirding!

Tristan Roberts is Editorial Director at BuildingGreen, Inc., in Brattleboro, Vermont, which publishes information on green building solutions.


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Image Credits:

  1. Tristan Roberts
  2. Rob Prohaska
  3. Simpson Strong-Tie