Expert Report

Mapping the Zone: Improving Flood Map Accuracy (2009)

Topics:

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Flooding is the leading cause of natural disaster in the United States. High-quality, digital mapping is essential to communicating flood hazards to those at risk, setting appropriate insurance rates, and regulating development in flood-prone areas. As the Federal Emergency Management Agency (FEMA) nears the end of its Map Modernization Program, the agency, along with the National Oceanic and Atmospheric Administration, asked the National Research Council to convene a committee to examine the factors that affect the quality and accuracy of flood maps, assess the costs and benefits of map improvement efforts, and recommend ways to improve flood mapping, communication, and management of flood-related data. The resulting report concludes that even the most expensive aspect of making more accurate maps -- collecting high-accuracy, high resolution topographic data -- yields more benefits than costs, and that, to complete the goals of its Map Modernization Program, FEMA should continue to invest in updating and improving its flood maps.

Key Messages

  • Coastal flood maps can be improved significantly through use of coupled two-dimensional storm surge and wave models and improved process models, which would yield more accurate base flood elevations.
  • FEMA's transition to digital flood mapping during the Map Modernization Program creates opportunities for significant improvements in the communication of flood hazards and flood risks through maps and web-based products.
  • Flood maps with base flood elevations yield greater net benefits than flood maps without.
  • The most appropriate flood study method to be used for a particular map depends on the accuracy of the topographic data and the overall flood risk, including flood probability, defined vulnerabilities, and consequences.
  • Topographic data are the most important factor in determining water surface elevations, base flood elevation, and the extent of flooding and, thus, the accuracy of flood maps in riverine areas.