improved measurement of land surface topography enabled by lidar technology. As noted above, the use of lidar data to calculate more accurate base flood elevations and floodplain boundaries reduces future flood losses and produces net benefits to the State of North Carolina. Reducing future flood losses also benefits taxpayers throughout the nation. FEMA has recently begun to support collection of lidar data along the Gulf coast, but lidar data coverage over most inland areas is still sparse.
Recommendation 2. FEMA should work toward a capability to use coupled surge-wave-structure models to calculate base flood elevations, starting with incorporating coupled two-dimensional surge and wave models into mapping practice.
A significant improvement to coastal flood mapping can be made by improving the models. Currently, base flood elevations are calculated by combining storm surge models with wave models, and using the result in models that calculate erosion and wave effects. However, modeling has greatly advanced, and it is now possible to use coupled models that account for storm surge, waves, erosion, and topographic features simultaneously.
Recommendation 3. FEMA should commission a scientific review of the hydrology and hydraulics needed to produce guidelines for flood mapping in ponded landscapes.
Methods to map landscapes in which water tends to flow from one ponded area to the next (shallow flooding) are still being developed. The primary hurdle to progress is the lack of scientific studies and models on the interactions between ponds, the volume of water temporarily stored in the depressions, and the rate at which it percolates out. Commissioning a study would not be costly and is a necessary step toward improving shallow flood mapping.
Recommendation 4. FEMA should require that every flood study be accompanied by detailed metadata identifying how each stream and coastline reach was studied and what methods were used to identify the magnitude and extent of the flood hazard and to produce the map.
One of the most important ways to quantify and communicate flood map accuracy is to document the data and methods used to study each segment of stream or coastline. FEMA’s current metadata reporting requirements do not include all the information needed to assess the quality and reliability of the data underlying the maps. For each stream or coastline mile studied, metadata should describe what input data, mapping, and modeling methods were used; the date of mapping; the contractor; and the starting and ending points.
Recommendation 5. FEMA should reference all stream and coastal studies within its Mapping Information Platform to the USGS National Hydrography Dataset.
FEMA Map Modernization has produced a large amount of geospatial data and flood hydraulic models for the nation’s streams and coastlines. The result is the most comprehensive digital description of the nation’s streams and rivers that has ever been undertaken. These data are stored in the Mapping Information Platform (MIP) on a county-by-county basis. There is no requirement that map information such as stream centerlines be consistent from one county to the next. The USGS National Hydrography Dataset is a seamless, connected map of the nation’s streams, rivers, and coastlines. Using a technique called linear referencing, it is feasible to link the FEMA stream and coastline data with the corresponding information in the National Hydrography Dataset. If this were done, FEMA flood data could become an integral part of the nation’s hydrologic information infrastructure rather than existing as a separate database.