Research ArticleENVIRONMENTAL STUDIES

Managed retreat through voluntary buyouts of flood-prone properties

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Science Advances  09 Oct 2019:
Vol. 5, no. 10, eaax8995
DOI: 10.1126/sciadv.aax8995
  • Fig. 1 FEMA-funded voluntary property buyouts to manage flood risk.

    FEMA supports buyouts of flood-prone properties through several grant programs. Under FEMA regulations, the property acquisitions are voluntary. Property owners, generally homeowners, must agree to sell their properties, and eminent domain or condemnation powers cannot be used. Grant applications to FEMA are submitted via states, territories, or federally recognized tribes (“grant recipient” in figure). The buyouts are administered by subgrantees, which are most often local governments (i.e., city or county governments). Subgrantees can also include state agencies, federally recognized tribes, tribal agencies, and private nonprofits. After a buyout, the land is maintained as open space. Icons, modified in figure, are from thenounproject.com.

  • Fig. 2 Flood risk and voluntary property buyouts across the United States.

    (A) Exposure to riverine and rainfall-driven flood hazards. The fraction of each county’s area that is 1% annual chance floodplain is shown. (B) Flood-related property damage. For each county, cumulative damage over 1989–2016 is depicted (as million US$2016). (C) Buyouts of flood-prone properties. The number of FEMA-funded voluntary property buyouts in each county is shown for program years 1989–2017 (as cumulative no. of bought-out properties). Major river systems are illustrated. Insets, not to scale, show available data for Alaska, Hawaii (no bought-out properties), and Puerto Rico.

  • Fig. 3 Trends in FEMA-funded buyouts of flood-prone properties over program years 1989–2017.

    (A) Number of bought-out properties and buyout projects, by decade. (B) Number of bought-out properties in U.S. states and territories, by decade of buyout-project program years. (C) Number of bought-out properties under each relevant FEMA grant program and federal disaster declaration incident type. Relevant FEMA grant programs: Hazard Mitigation Grant Program (HMGP), Flood Mitigation Assistance Grant Program (FMA), the Pre-Disaster Mitigation Grant Program (PDM), the Repetitive Flood Claims Grant Program (RFC), and the Severe Repetitive Loss Grant Program (SRL). (D) Structure and residency types of bought-out properties, where specified.

  • Fig. 4 Flood-related exposure in counties in which voluntary property buyouts have versus have not occurred.

    Flood-related exposure is shown by (A) number of flood-related disaster declarations including individual assistance in the county over 1989–2017, (B) fraction of county area that is 1% annual chance floodplain, and (C) cumulative flood-related property damage over 1989–2016 (as million US$2016). Within each panel, the density plot and mean are orange for counties in which voluntary property buyouts have been administered by any subgrantees over program years 1989–2017, whereas they are blue for counties in which buyouts have not occurred. ***P ≤ 0.001 for Welch’s unequal variances t test for differences in means. Data are included for all geographic regions available for each flood-related measure: (A to C) continental United States; (A and C) Alaska and Hawaii; (A) Puerto Rico. The number of counties relevant to each panel is therefore as follows: (A) 3220 counties (2072 in blue, without buyouts and 1148 in orange, with buyouts), (B) 3108 counties (1993 blue and 1115 orange), and (C) 3142 counties (2019 blue and 1123 orange).

  • Fig. 5 Socioeconomics and demographics of communities and residents participating in buyout programs, evaluating counties in which local governments have administered buyouts of flood-prone properties.

    In these 1087 counties, city or county governments have served as FEMA subgrantees over program years 1989–2017. These counties with local government–administered buyouts are compared to the 2019 counties in which no buyouts have occurred. (A and B) Density plots and means for counties with (orange) and without (blue) local government–administered buyouts for indicators averaged over 2012–2016: (A) median household income in county, divided by median household income in state, and (B) county population density, divided by state population density. (C and D) Within counties in which local governments have administered buyouts, density plots and means for ZCTAs in which property buyouts did (orange) and did not (blue) occur (16,718 ZCTAs in total: 2807 with buyouts and 13,911 without buyouts). In these panels, relative income and population density indicators compare ZCTAs to their corresponding counties. ***P ≤ 0.001 for Welch’s unequal variances t test for differences in means. Each panel includes data for the continental United States, Alaska, and Hawaii. Note that the x axis in some panels is truncated; data included in means and t tests, but not visualized graphically, are as follows: (A) 2 counties, (C) 65 ZCTAs, and (D) 179 ZCTAs.

  • Fig. 6 Variable importance in random forest model classifying counties with and without buyouts.

    Variables included encompass all factor categories assessed throughout this study: county-level exposure to flood-related hazards and county-level and state-level socioeconomic and demographic indicators. Results are shown for 100 model iterations, each based on 1000 decision trees. Mean decrease in Gini index indicates the importance of the variable in classifying counties as buyout or non-buyout counties. The 3108 counties included are those for which all assessed variables are available: 1115 with buyouts and 1993 without buyouts.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax8995/DC1

    Supplementary Material

    Fig. S1. Flood-related property damage in the continental United States, Alaska, and Hawaii.

    Fig. S2. Spatial patterns of flood-related federal disaster declarations over 1989–2017.

    Fig. S3. Spatial and temporal trends in flood-related property damage over 1989–2016.

    Fig. S4. FEMA-funded buyouts of flood-prone properties over program years 1989–2017, by grant program.

    Fig. S5. The frequency of buyout projects (no. of projects) of different sizes (no. of bought-out properties) for overall program years 1989–2017 and for specific decades 1989–1998, 1999–2008, and 2009–2017.

    Fig. S6. Flood-related exposure in counties in which voluntary property buyouts have and have not occurred.

    Fig. S7. Socioeconomics and demographics of communities participating in buyout programs, evaluating counties in which local governments have administered buyouts of flood-prone properties.

    Fig. S8. Socioeconomics and demographics of residents participating in buyout programs.

    Fig. S9. Population and population density within counties with local government–administered buyouts.

    Fig. S10. The duration of FEMA HMGP projects with property buyouts over program years 1989–2017.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Material
    • Fig. S1. Flood-related property damage in the continental United States, Alaska, and Hawaii.
    • Fig. S2. Spatial patterns of flood-related federal disaster declarations over 1989–2017.
    • Fig. S3. Spatial and temporal trends in flood-related property damage over 1989–2016.
    • Fig. S4. FEMA-funded buyouts of flood-prone properties over program years 1989–2017, by grant program.
    • Fig. S5. The frequency of buyout projects (no. of projects) of different sizes (no. of bought-out properties) for overall program years 1989–2017 and for specific decades 1989–1998, 1999–2008, and 2009–2017.
    • Fig. S6. Flood-related exposure in counties in which voluntary property buyouts have and have not occurred.
    • Fig. S7. Socioeconomics and demographics of communities participating in buyout programs, evaluating counties in which local governments have administered buyouts of flood-prone properties.
    • Fig. S8. Socioeconomics and demographics of residents participating in buyout programs.
    • Fig. S9. Population and population density within counties with local government–administered buyouts.
    • Fig. S10. The duration of FEMA HMGP projects with property buyouts over program years 1989–2017.

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