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Mapping the Zone: Improving Flood Map Accuracy
insurance rates because the hazards are represented more accurately.
Recommendation. FEMA should begin mappingE zones to better serve insurance and floodplainmanagement needs.
MAPPING FLOOD RISK
Risk is defined as the product of the probability of an event and the consequences of its occurrence (Einstein, 1988). For there to be a risk, there must be a hazard consisting of an initiator event, a receptor, and a pathway linking the two. For example, in the event of heavy rainfall (the initiator), floodwater may propagate across the floodplain (the pathway) and inundate housing (the receptor) that may suffer material damage (the consequence). If the consequences of an event can be mitigated by some intervening measure (e.g., presence of a levee, floodwall, or other structure), the probability that the intervening measure will function as designed must be factored into the risk equation.
Hazard and risk maps are essential tools for helping the public understand the challenge it faces by living in a flood hazard area. They can also help communicate the inundation risks associated with global warming and sea level rise. However, although much has been written on risk communication in general, little formal research has been done in the United States on effective ways to use maps to communicate flood risk to those in the floodplain. What studies exist indicate continued problems of low market penetration (Dixon et al., 2006) and communication associated with FEMA’s flood hazard maps (e.g., the annual chance terminology is still not commonly used by government officials, the media, or the public; Galloway et al., 2006) and the potential benefits of risk mapping (IPET, 2008).
A hazard map shows the location and probability of a hazard. FEMA’s paper and digital Flood Insurance Rate Maps (FIRMs) are hazard maps because they show floodplain boundaries that indicate different flooding probabilities (i.e., 1 percent and 0.2 percent annual chance floods). A risk map not only shows the hazard probability, but also includes the probability that protection systems (e.g., levees, dams) will operate properly and the consequences of failure of the system for a given event. While DFIRMs will be needed for the National Flood Insurance Program for some time, the communication of flood risk to better inform the public and support an effective mitigation program will require FEMA to shift its risk mapping communication focus to a higher and more technical level. Geographic Information System (GIS) and database technologies and the widespread availability of the Internet offer opportunities to leverage the Map Modernization investment to effectively communicate risk through improved maps and websites. Tools such as Hazards U.S. (HAZUS) provide communities, private companies, and others with an understanding of GIS the opportunity to learn more about the risks they face.
Hazard and Risk Maps
Considerable effort is underway in the United States and abroad to take advantage of new mapping capabilities to portray up-to-date information that floodplain occupants need and will use. Maps can integrate information about the flood hazard with information about the economic, social, or environmental consequences of flooding. In 2008, the European Commission published an atlas of flood maps that provides examples of the best mapping techniques used in 19 European countries, the United States, and Japan.4 The atlas contains examples of maps designed to support risk communication, land use planning, emergency notification and response, insurance rating, and historical analysis. The maps reflect involvement at the national, regional, and local levels and public-private partnerships.
The Czech government, in cooperation with Swiss Re, an international insurance company, and MMC, a European GIS company, has developed the Flood Risk Assessment Tool, an interactive system that identifies up to six different risk zones within the floodplain. The tool is similar to FloodSmart prepared under FEMA’s Map Modernization Program, but has a higher level of discrimination. Users are able to enter a database and extract information about an area of interest. Figure 7.1 illustrates a map with four risk zones.
The German state of Rheinland-Pfalz has developed maps for the Mosel River Basin that portray