What’s the difference between Risk Rating 2.0 Premium Estimate Maps, traditional FEMA flood insurance rate maps (FIRMs), and other flood risk maps?

Risk Rating 2.0 Premium Estimate Maps

The geographic flood insurance rate maps presented on this website are based on the NFIP Risk Rating 2.0 premium calculation methodology. This is the methodology now used by FEMA for all NFIP insurance policies issued as of April 2022. Risk Rating 2.0 is composed of premium base rates and numerous multipliers all of which are calibrated to reflect actual historical claims. The multipliers are a function of geographic variables such as distance to and elevation relative to flood source, though there is no underlying physical model of flooding. Instead, there is simply a listing of the key geographic variables associated with flooding, and a calibration of their importance in adjustments to the lookup tables that relate physical geographic variables to risk multiplier values.

Traditional FEMA Flood Insurance Rate Maps (FIRMs)

By contrast, traditional FEMA flood insurance rate maps (FIRMs) are based on numerically computed physical models of flooding. A flood frequency analysis is performed to determine flows associated with various recurrence interval events (e.g., the 100-year or 1% annual chance exceedance event). The dimensions of the river, and any culverts or bridges at road crossings are surveyed in the field and entered into the geometric component of standard modeling software (typically HEC-RAS). The topography of the surrounding terrain is added using best available data. The equations coded in the software reflect known empirical/physical relationships between flow, velocity, cross-sectional area, and flow resistance in the channel and floodplain in typical channel cross-sections and at culverts and bridges. The result includes flood profiles (water surface elevations versus distance downstream) and flood inundation maps. Flood profiles include the 10-year, 50-year, 100-year, and 500-year events, while the inundation maps show just the 100-year and 500-year inundation area. These traditional floodplain mapping projects are intense efforts involving numerous hours of data entry, model evaluation, post-processing, and detailed quality checks.

National Flood Risk Map

A third flood risk product is the national physically-based flood risk model and mapping, e.g., the Fathom model used in Flood Factor. The Flood Factor model uses regional flow frequency equations to estimate flood event flow rates throughout the country. It also uses a combined 1D/2D riverine flood modeling approach and includes a pluvial flooding component to consider the potential impacts of intense rainfall combined with poor drainage. And it considers storm surge along the shoreline using historical buoy data. While packaged as a “probabilistic model”, it uses a deterministic/physical modeling approach to relate quantity of precipitation and flow to water distribution over the land surface.

Advantages, Limitations, and Use Cases

Each of the above three approaches has its advantages, limitations, and use cases.

The traditional FEMA FIS FIRM maps, with their deterministic modeling are resource intensive and time consuming to develop. But its strength is its very attention to detail and modeling of local specifics (bridges and culverts constricting flow and increasing upstream flood levels; dams and weirs creating long backwaters). Importantly, these maps are still used in floodplain regulation.

The national flood risk model leverages publicly available national data sets and applies deterministic models on a national scale. Their uniformity of approach helps eliminate likely inconsistencies in the patchwork of FEMA FIRM maps and allow for regional and national comparison of risks. And they add pluvial risk. But without use of local survey data for riverine bridges, culverts, and weirs, and without modeling storm sewers for pluvial urban risk, they likely have numerous local flaws. Caution is appropriate in using nationally based maps to make local flood risk decisions, without efforts to quality assure and tweak the model locally.

Risk Rating 2.0 Premium Estimate Maps are also national in scope with maps produced here on the county/parish scale. Risk Rating 2.0 calculates insurance premium values calibrated to match the distribution of actual flood insurance claims on a national basis. While having physical elements, Risk Rating 2.0 is a true probabilistic or statistical model reflecting probable risk without replicating physical flood conditions. Likely this method also has local mismatches in estimates relative to actual risk and even relative to the physical process of flooding. But despite these potential inconsistencies, Risk Rating 2.0 geographic rate maps do in fact reflect actual NFIP premium rates and thus the impact on wallets and pocketbooks.