Home » Online Exclusives » Know the Flood Risk Before You Build

Know the Flood Risk Before You Build


How to build to protect from flood waters

Contractors need to know the flood risks before building.

The historic rainfall that devastated parts of Houston earlier this year exposed the nation’s vulnerability to flooding that can destroy homes and businesses in an instant. In addition to death and injury, flooding can cause extensive infrastructure and property damage, social instability and economic disruption. According to the National Flood Insurance Program (NFIP), the average commercial flood claim was nearly $89,000 between the years 2010 and 2014 with nearly 25 percent of businesses closing permanently after a flood.

Extreme weather, sea level rise and other effects of climate change have put many communities at high risk of flooding. Along the coast, cities such as Seattle, Baltimore, New York, Boston, Tampa and New Orleans could face $60 billion a year in losses annually from flooding by mid-century, according to ClimateCentral.org. “Nuisance” flooding, minor coastal flooding that 50 years ago would have been caused by a strong hurricane, can be caused today by a high tide resulting in flooded roads, homes and businesses.

In the Midwest, a new study from Washington University in St. Louis indicates that federal agencies may have underestimated 100-year flood levels by as much as five feet. FEMA, the Army Corps of Engineers and other agencies use 100-year flood levels to predict potential flooding, designate flood zones, define flood insurance maps, and design levees and other structures to withstand floods. Any miscalculation can have serious implications for business development along major rivers. For example, the projected high-water mark for a 100-year flood event on the Mississippi River in St. Louis is 51.5 feet, 21 feet above flood stage, according to the study.

Urban development and man-made river control systems like levees, dikes and dams have exacerbated the flood risk in these areas. Robert Criss, PhD, professor at the university and author of the study, said the high-water marks on river systems in the Midwest are rising about an inch a year, 10 times greater than annual sea level rise due to climate change.

Given the frequency and severity of flood events in recent years, the U.S. government made changes to the federal flood plain maps that now place thousands of properties inside flood zones, requiring their owners to obtain flood insurance for the first time. The new flood maps show higher waters and wider areas at risk to potential flooding. Greater demand for flood insurance plus a jump in insurance claims due to recent floods have driven insurance rates up by a factor of 10 in some areas.

Contractors who plan to build in flood-prone communities must take prudent steps to assess the flood risk and then take action to avoid or mitigate the threat.

Many U.S. cities are at risk of flood

Many U.S. cities are at risk of flood

Before starting a building project, contractors should first contact local building department officials to understand the building code and floodplain management requirements for project sites.

To ensure project sites are reasonably safe from flooding, the NFIP requires that structures are designed to prevent flotation, collapse and lateral movement during a flood; flood resistant materials are used; the building is engineered and constructed to minimize flood damage; and HVAC/plumbing equipment is designed or located to prevent water entry.

Updated federal flood maps and changes to the NFIP have forced communities to alter building codes and comply with the new floodplain regulations to qualify for federal disaster assistance and flood insurance. While regulation of floodplain land use is the responsibility of state and local government, state and local ordinances generally follow NFIP requirements plus those set by the state or community.

Regulation can be achieved through designated flood ways, encroachment lines, zoning ordinances, subdivision regulations, special use permits, flood plain ordinances and building codes. Typically communities adopt or enforce flood plain management ordinances and laws to define regulatory floodplains and control development. The NFIP makes affordable, federally backed flood insurance available once minimum NFIP regulations are met.

Contractors should also review these other important sources of information:

  • Flood Insurance Rate Maps which define flood boundaries for FEMA flood zones and base flood (100-year flood) elevations, and map the results of recent flood insurance studies
  • Flood Boundary Maps
  • Flood Insurance Studies, which show mean water levels and wave elevations along the shoreline
  • FEMA Technical Bulletins, such as Floodproofing Non Residential Buildings (FEMA P-936) and NFIP Technical Bulletin 3-93
  • State and local land use regulations, which govern land use, can be more restrictive than FEMA and the NFIP, and may set higher standards based on local conditions in the interest of safety.

Flood risk is defined as the potential losses associated with a flood in terms of costs and consequences, expected probability and frequency of event, and exposure to floods. Residual risk is the level of risk not off-set by hazard-resistant design or insurance, such as the continued cost of rebuilding.

Generally, contractors can follow this rule of thumb to assess the potential consequences of a flood event. First, determine the construction/replacement cost per square foot as applied to floodable levels. Then, add the cost of business disruption that includes a realistic period for resumption of operations after a flood. The time required to move back in and resume business is typically longer than predicted.

According to the NFIP, flooding is the #1 natural disaster in the U.S. However, most property owners protect against burglary and fire even though a flood event is more likely. For projects in flood zones, contractors should consider the likelihood of a flood event over a given time period, the likelihood of frequent/less severe events like nuisance flooding or flash floods, and less frequent/more severe events such as hurricanes and 100-year floods.

A common misconception is that a “100-year flood” is a flood event that happens once every 100 years, which may give contractors a false sense of security about the flood risk for a project site. In fact, a 100-year flood statistically has a 1 in 100 probability of occurrence – a one percent chance — in any given year. Using the same statistical process, a 50-year flood has a 1 in 50 probability of occurrence, a two percent chance, in any given year. A 10-year flood has a 1 in 10 probability of occurrence, a 10 percent chance, and so on. Therefore, it is possible for a 100-year flood to happen two years in a row, and a 10-year flood event to happen 10 times in a single year.

The 100-year flood is the national standard used by the NFIP and all federal agencies for the purposes of requiring the purchase of flood insurance and regulating new development. At a minimum, commercial contractors should build to standards that prepare for flooding scenarios that may seem unlikely, but actually have a statistical probability of occurrence.

Risk can be reduced or managed physically through flood mitigation and financially through insurance. Risk reduction is a combination of physical measures such as building codes, design and construction, best-in-class solutions, safety factors and insurance measures. Financial exposure to residual risk – that which can’t be prevented or avoided – can be reduced by purchasing insurance, but should not be a substitute for a properly designed and constructed building.

Structures built to specifications for a 500-year flood event will have a much lower risk of being flooded over 30 years versus structures built to the base flood elevation. Use of freeboard is another way to mitigate risk by providing additional protection for floods exceeding the base flood elevation and typically results in lower insurance premiums. NFIP insurance and private insurance can reduce exposure to any remaining risk. Even after these measures, some risk to flooding will remain.

To avoid or manage flood risk, commercial contractors and their clients have few options: move the building outside of the flood zone, elevate the structure above base flood elevation, build earthen barriers like berms, dikes and walls, or floodproof. According to the Army Corps of Engineers, dry floodproofing is the best choice for urban structures and is recognized by the NFIP as an acceptable option for commercial businesses.

Dry floodproofing requires that all large openings have flood barriers, flood panels or flood doors installed, areas below water level must resist infiltration, all small openings must be sealed, buoyancy effects should be considered and a method for pumping out leakage must be provided. FEMA and the NFIP allow dry floodproofing for commercial buildings because relocation or elevation would be a significant business hardship. Furthermore, most commercial structures can’t be elevated, and employees can deploy floodproofing solutions and practice fire-drill style floodproofing deployment plans.

Commercial contractors should take proactive measures to minimize flood risk before beginning any project, and particularly in flood-prone areas. The frequency of flood events and the high cost of floods on businesses and communities across the U.S. will continue to drive demand for flood protection and other measures to reduce the probability of catastrophic loss due to flooding.

Tom Osborne is the owner and president of Flood Panel, LLC where he oversees design, development and operations for the manufacture of flood mitigation products for commercial buildings in flood zones nationwide.