May 2008

Flood Mitigation

We occasionally see on TV news where buildings are inundated or submerged in flood water after heavy rain, typhoons or hurricanes. Many people believe it never happens to their own buildings. But is that really true? If you have any doubts, you should check to see if your site has been subject to such events in the past, or if it is potentially exposed.

In the U.S., you can obtain this information through Flood Insurance Rate Maps on the FEMA website ( In the other countries, similar flood maps may be available prepared by local authorities. Unfortunately, however, such information is not generally available in many developing countries, where it is most needed.

You may not be able to trust old flood maps, especially where the areas have been developed drastically since the effective date. The ability to absorb water may have been reduced significantly where the soil has been paved.

If maps are not readily available, you should try to find the following information which is typically used in the insurance industry to determine the severity of flood exposure.

  • Site elevation.
  • Elevation of the nearby body of water such as river or lake.
  • Any earthen or concrete levees protecting a river and their elevations. See article “Fixing levees isn’t easy or cheap,” USA Today, February 2, 2007 “Fixing levees isn’t easy or cheap” which identifies 122 levees in the U.S. that pose an unacceptable risk of failing in a flood.
  • Any other physical flood protection such as flood control dam, sump pump, or levee maintenance procedures.
  • Emergency planning by local authorities.

For construction projects, such information is gathered in advance for the purpose of evaluating a site.

If flooding is possible for your site, you should find out what the finished floor elevations are for important buildings or structures. You can then determine to what elevations the floodwaters are expected. Based on the potential damage to the buildings that you identify, you should consider, but not be limited to, the following permanent physical flood protection measures.

  • Properly anchor all storage tanks and piping, especially those containing flammable or combustible liquids. This also includes underground storage so that they can resist against hydrostatic forces lifting them.
  • Where possible, elevate all critical utility and production equipment above the expected flood level. For example, if the plant contains molding machines, have the electrical equipment located above the machine at the ceiling level instead of in a trench below the units.
  • All unnecessary openings located below the expected flood level should be adequately sealed.
  • All cracks in building walls and floors that are located below the expected flood level should be adequately sealed.
  • Provide valves or check valves in all underground sewer or drainage lines where their discharge outlets are located below the expected water levels during flooding. This will prevent flood waters backing up into the buildings through sewer or drainage lines.
  • Install floodwalls, flood gates, sump pumps, etc.
  • Provide the recommended maintenance on the valves and pumps to ensure that they operate properly.
  • It is also important to create emergency response plans that should include, but not limited to, the following.
  • Consider shutting down plant operations for the areas that will likely be affected by flood water. Flammable liquids or gases in particular should be shut off before flood water enters the site.
  • Relocate items susceptible to water damage such as stock, computers, etc. to a higher elevation.
  • Close valves for sewer lines to prevent back-up.
  • Relocate any outdoor equipment or storage that can have an impact on building collapse.
  • Check for any unanchored equipment, tanks or piping and anchor them, if .
  • Check for any unprotected or damaged openings on the building exterior walls, flood walls or flood gates and repair them, if possible. Protect windows and doors where necessary.
  • Have sandbags on site and place them around unprotected areas where flood water can enter.
  • Determine which emergency personnel will remain at the plant.
  • Create procedures for salvaging damaged equipment or stock and re-starting plant operations. Back-up plans should be prepared well in advance for areas that are expected to be damaged during flood events.

While a “flood” is usually defined as a large volume of water such as a rain-swollen river or lake overflowing, heavy rain events alone can cause similar damage in well developed downtown areas where no river or lake exists. Due to poor, old or inadequately designed public sewer systems, rainwater can enter basements and ground floors where important utility equipment is located. Transformers, switchgears, power control rooms, boilers or fire pumps are often seen in basement floors where real estate and occupancies are expensive. The same considerations as stated above should be given in such cases.

Risk Logic can help in determining the potential flood hazard that may exist to a building and its operations.

“Fixing levees isn’t easy or cheap,” USA Today – Friday, February 2, 2007