Learn more about First Street Foundation's average annual loss (AAL) data and methodology.
First Street’s AAL work and methodology are documented in this document titled The Cost of Climate. More specific information on the AAL work can be found below.
Please note, that First Street's AAL data is only available through Risk Factor. For more information on what's included in each of the Risk Factor plans please review this article.
The AAL values are the products of the damage functions from the HAZUS framework, which return % of structure damage. The AAL is the percent of the structure value that is impacted based on the full risk profile (a depth times likelihood integration). You can multiply it by the property value of a given structure to get a proxy for the $ value of damage from the flood risk profile in any given year. For the presentation on Flood Factor, and our own internal research, we have computed our own Property Value (AVM) indicator and then multiplied that property value (AVM) by the county assessor office's "improvement %" which allows us to better understand the value of the structure (relative to the value tied to the land). We are unable to share the improvement % or AVM due to contractual restrictions, but for context, the improvement % averages about 0.70 (70%) across the country, and a high-resolution property value indicator (such as census block group or tract averages) could stand in for property specific values. In application, AALs can be produced by taking the following approach to estimate damage values per property:
AAL dollar value = (AAL * AVM) * 0.70
Inputs into HAZUS framework for calculating AALs
- Land use type (residential property types),
- Number of units,
- FEMA flood zone and BFE,
- First Flood Elevation (in reference to BFE),
- Number of stories
- Year built (pre-firm/post-firm)