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Mapping the Zone: Improving Flood Map Accuracy
accuracy. The accumulation of information from flood events has intermediate and long-term benefits. Post-flood inspections yield information needed to improve models and update the maps. For example, inundation maps of the June 2006 floods in New York are being used to update Flood Insurance Rate Maps created in 1985. Knowledge about how the built environment responds to floods and coastal surges leads to improved building design and safer siting and thus to reduced future damage, social losses, and the need for federal disaster assistance. Similarly, experience responding to floods leads to more robust plans for emergency services and thus minimizes future loss of life and property. The information gained also contributes to society’s underlying knowledge base across multiple disciplines.
FEMA BENEFIT-COST ANALYSES
In 1997, FEMA analyzed the incremental costs and benefits of modernizing its Flood Hazard Mapping Program (FEMA, 1997). The analysis considered all costs, including costs for flood data updates, map maintenance, new mapping, conversion to new standards, and customer service. It also calculated three benefits that could be quantified with reliable data:
Reduced damage to new residential properties,
Reduced damage to new non-residential structures, and
Reduced costs of map reviews.
The first two were calculated by determining the annual damage that would be prevented by designing new construction using more accurate flood data and subtracting the increased construction costs for complying with NFIP requirements (up to 5 percent). The third was based on estimates of the time saved by using improved maps and digital products for mortgage and permit applications and flood insurance policy ratings. The study found incremental benefits of $1.75 billion and incremental costs of $848 million over a 50-year period, for a benefit-cost ratio of 2.1.
In 2000, FEMA repeated the analysis, modifying the projected number of new structures in SFHAs and factoring in survey responses on flood map inventory needs from all mapped communities (the original analysis considered only 10 percent of mapped communities; FEMA, 2000). The updated analysis yielded incremental benefits of $1.33 billion and incremental costs of $799 million, for a benefit-cost ratio of 1.7. The analysis also estimated how the new construction benefit would change over time. The benefits to new construction are greatest in areas that are unstudied or studied through approximate methods because no flood elevation data are available to site new buildings. As more flood elevation data become available through map modernization, the benefits for new construction decline. FEMA estimated that factoring in this declining benefit decreases the benefit-cost ratio to 1.5.
FEMA’s Office of Inspector General audited its cost estimate for the Map Modernization Program in 2000 (OIG, 2000). It found that FEMA’s methodology was sound and no major costs were overlooked, but that the estimate could be significantly in error because costs were not always verified or drawn from reliable sources, some assumptions (e.g., cost of flood studies) have a major effect on cost, and cost savings from partnerships and technological innovation (e.g., use of lidar) were not considered. FEMA agreed with most of the findings and outlined steps for improving future cost estimates in the report’s appendix. The revised costs have not yet been incorporated in a benefit-cost analysis.
NORTH CAROLINA CASE STUDY
Many benefits and costs are too varied to assess generically—case studies are required to understand them at the local level, where implementation occurs. The North Carolina Floodplain Mapping Program (NCFMP) determined the costs and three benefits of more accurate maps in three different physiographic regions in North Carolina and also examined the costs and benefits of different flood study methods for the entire state (NCFMP, 2008). The communities chosen represent the typical level of development within three physiographic regions: Pasquotank County in the coastal region, Mecklenburg County in the piedmont region, and the city of Asheville in Buncombe County within the mountain region (see Chapter 1, “Case Studies”). Geospatial data necessary to complete the assessment (e.g., parcel boundaries attributed with zoning, building value, and construction date; digital flood hazard information) were available for each of