three case study areas by the North Carolina Floodplain Mapping Program showed that the use of detailed studies and limited detailed studies that generate base flood elevations results in net benefits to the state. In contrast, the use of approximate study methods, which do not yield base flood elevations, results in net costs. This is significant because detailed and limited detailed studies in North Carolina rely on lidar data, and even though lidar surveys are expensive, the costs to map the three study areas are outweighed by the benefits of more accurate maps.
Finding 4. 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.
The North Carolina benefit-cost analysis showed that a combination of different study methods produces the greatest economic benefits to the state as a whole. The best study method depends on the characteristics of the area being mapped, such as the present and future potential of flooding, the potential for population growth, the availability of land for development, and the likely economic value of structures to be built. The quality of the topographic data is also important. Where accurate topographic data are available, an accurate base flood elevation can be calculated, a more accurate map can be produced, and thus better decisions can be made about appropriate use of the floodplain.
Finding 5. 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.
FEMA is moving from simply portraying flood hazard and flood insurance rate zones on maps to communicating and assessing risk, an ambitious goal that leverages the digital flood-related information and maps produced during the Map Modernization Program as well as FEMA tools for estimating flood damage and loss (i.e., Hazards U.S. Multi-Hazards software). To communicate risk, the maps and products must show not only where flood hazard areas are located, but also the likely consequences of flooding (e.g., damage to houses, coastal erosion). Inundation and risk maps beginning to be produced by U.S. federal and state government agencies and by other countries have attributes that merit FEMA’s attention.
Maps that show only floodplain boundaries have the disadvantage of implying that every building in a designated flood zone may flood and that every building outside the zone is safe. Providing floodplain residents with the elevation of structures relative to the expected height of a number of floods offers a better way to define graduated risk (from low risk to high risk). Where the necessary data are available (e.g., structure elevation, base flood elevations, flood protection structure performance), a geographic information system could be used to personalize flood risk to individual addresses.
The body of the report contains focused recommendations on how to improve specific aspects of FEMA’s flood data, models, and mapping. The following overarching recommendations address Tasks 4 through 6 and are based on the analysis of information presented throughout the report.
Recommendation 1. FEMA should increase collaboration with federal (e.g., USGS, NOAA, U.S. Army Corps of Engineers), state, and local government agencies to acquire high-resolution, high-accuracy topographic and bathymetric data throughout the nation.
Riverine mapping methods are well established, although improvements could be made in calibrating rainfall-runoff models, updating regression equations (many of which are more than 10 years old) more frequently, and increasing the use of two-dimensional models developed by the research community. The greatest improvement, however, would come from use of high-accuracy, high-resolution topographic data. Improved measurements of channel, lake, estuarine, and near coastal bathymetry would augment the