US: highlights from "the first national flood risk assessment"

BACKGROUND

A new national report from the First Street Foundation provides a comprehensive national analysis of the state of flood risk in the continental U.S. The findings are the result of the First Street Foundation’s new Flood Model, a high precision, climate adjusted model that assesses flood risk at the individual property level today and into the future. These results are being made publicly available for the first time through a new tool, Flood Factor™, a free online source of high-quality probabilistic flood risk information.

The model represents the culmination of years of research and development made possible by building upon existing knowledge and peer reviewed scientific applications regularly used in the identification of flood risk. This effort was undertaken with the goal of making flood risk transparent, easy to understand, informative, and available to all. The model was produced in partnership with researchers and hydrologists from Fathom, Rhodium Group, and leading researchers from the country’s top academic institutions.

Significantly, the model allows for an understanding of risk from any type of flooding event, including fluvial (riverine), pluvial (rainfall), storm surge, and tidal sources, and addresses the reality that these sources have been, and continue to be, impacted in different ways by changing environmental factors. First Street Foundation’s modeling process has integrated those factors directly into the final risk statistics. In doing so, the model evaluates flooding from multiple risk sources (fluvial, pluvial, surge, and tidal) while also integrating current and future environmental considerations, all at the property level.

The full report consists of a high-level methodological overview, national summary and state-by-state analysis of the lower 48 United States and D.C., with a focus on providing insight into new findings around flood risk, adaptation, and how changing environmental factors impact future flood risk. The risk identified by the First Street Foundation Flood Model highlights significant variations within and across regions, states, and cities in the U.S. Most relevant for this report is the uneven risk identified across and within these localities, and the regular deviations in identified risk when examining properties located in FEMA’s Special Flood Hazard Areas (SFHA).

NATIONAL OVERVIEW

Key Finding:Many more American homes and businesses are at risk of severe flooding than previously understood.

At the national level, the First Street Foundation Flood Model identifies around 1.7 times the number of properties as having substantial risk* compared to the FEMA 1-in-100 SFHA designation. This equates to a total of 14.6 million properties across the country at substantial risk, of which 5.9 million properties and property owners are currently unaware of or underestimating the risk they face because they are not identified as being within the SFHA zone.

Washington D.C. (438%), Utah (419%), Wyoming (325%), Montana (311%), and Idaho (290%) show the greatest difference between the First Street Foundation Flood Model estimates and FEMA SFHA designation, due mainly to First Street’s nationwide coverage while FEMA’s mapping in some of these locations is not yet complete.

There are locations where First Street estimates risk is less than that designated by the FEMA SFHA, and while there are differences in this deviation county-by-county and city-by-city, at a state-wide level Arizona, New Jersey, and Louisiana are the only states that show a lower count of properties currently with substantial risk in the First Street model in comparison to the FEMA SFHA. However, when adjusting for future environmental changes, in Arizona, additional properties fall into that risk categorization. In Louisiana, after adjusting for sea level rise that approaches or exceeds protective levee heights, the deviation shifts as the First Street methods uncover an additional 332,700 properties with substantial risk by the year 2050, in turn showing 248,800 more properties with substantial risk than FEMA defines currently. Similarly in New Jersey, adjusting for environmental changes shifts the First Street estimate from 8,100 fewer properties currently at substantial risk than FEMA, to identify73,600 more properties at substantial risk in 2050 than current FEMA estimates.

While the aforementioned states show the biggest deviation between First Street and FEMA in terms of the number of properties facing significant risk, the First Street Foundation Flood Model also calculates the number of properties facing any risk** of flooding. When looking at this broader level of risk, which is beyond the FEMA SFHA definition, the data identifies 23.5 million properties in the U.S. as at-risk over the next 30 years. Of these properties, 3.6 million were categorized as facing almost certain risk, with a 99% chance of flooding at least once over the next 30 years.

At a more granular level, the results shed light on the unevenness in which changing environmental factors will impact regions of the country differently, and prove the need to incorporate more localized data at a property level in order to fully understand flood risk. Viewing risk at a summarized city, county or state level looks very different than the property-level data Flood Factor will deliver. A property’s Flood Factor is an indicator of its comprehensive flood risk, ranging from 1–10. Properties with higher Flood Factors are either more likely to flood, more likely to experience high floods, or both.

USES & IMPLICATIONS: NOW AND INTO THE FUTURE

The availability of the First Street national property-level data enables a wide range of adaptation and policy efforts, including making it possible for individuals, as well as industry and government leaders to:

• Understand the risks associated with their property and take active steps to mitigate them.
• The real estate, mortgage, insurance, and investment communities to have a consistent property-level dataset to judge the severity and value of the risk associated with the properties in their portfolio.
• Federal, state, and local governments to have a new tool for informed policymaking to guide public investment towards adaptations to reduce the risk and build resilience to flooding.

Additionally, First Street has created the First Street Foundation Flood Lab, a collection of academic and industry researchers who will drill into our data to derive the information necessary to further our understanding of flood risk, its consequences, and possible solutions.

These experts represent a wide swath of disciplines, including finance, economics, public policy, risk management, hydrology and engineering who will examine the implications of flood risk data on the mortgage industry, coastal communities, government policy, the National Flood Insurance Program, housing market, low-income and disadvantaged communities, and other related topics. Enabled by data sharing agreements among the data providers and participants, the insights generated by the Flood Lab researchers will enable the data to be applied more rapidly and to greatest effect.

* Substantial risk is calculated as inundation 1 cm or more to the building in the 100 return period (1% annual risk).

** Any risk is calculated as inundation of 1 cm or more to the building in the 500 return period (0.2% annual risk). See methodology for full model details.