Increasing flooding risk affects human safety and health, property, infrastructure, economies, and ecology in many basins across the U.S.

This finding is from chapter 3 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.

Description of evidence base: The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 2: Our Changing Climate, Ch. 21: Northwest, Ch. 19: Great Plains, Ch. 18: Midwest, Ch. 16: Northeast, and over 500 technical inputs on a wide range of topics that were received as part of the Federal Register Notice solicitation for public input. Observed Trends: Very heavy precipitation events have intensified in recent decades in most U.S. regions, and this trend is projected to continue (Ch. 2: Our Changing Climate). Increasing heavy precipitation is an important contributing factor for floods, but flood magnitude changes also depend on specific watershed conditions (including soil moisture, impervious area, and other human-caused alterations). There is, however, some correspondence among regions with significant changes in annual precipitation (Ch. 2: Our Changing Climate), soil moisture (Figures 3.2 and 3.3), and annual flood magnitudes (Figure 3.5).a7f8dbf5-3ec8-4ee1-8740-014006b72bfd Flooding and seawater intrusion from sea level rise and increasing storm surge threaten New York, Boston, Philadelphia, Virginia Beach, Wilmington, Charleston, Miami, Tampa, Naples, Mobile, Houston, New Orleans, and many other coastal cities (Chapter 25: Coasts). Projected Trends: Projections of future flood-frequency changes result from detailed hydrologic227f0b59-69f2-47ab-8359-29e4fc383e11 ffaab9ac-cd7b-47d2-a7df-174dbc479bbb 37288764-91af-4c74-a5f2-fdf7f088c918 and hydraulic models of rivers that simulate responses to projected precipitation and temperature changes from climate models. Consequences: Floods already affect human health and safety and result in substantial economic, ecological, and infrastructure damages. Many cities are located along coasts and, in some of these cities (including New York, Boston, Miami, Savannah, and New Orleans), sea level rise is expected to exacerbate coastal flooding issues by backing up flood flows and impeding flood-management responses (see Ch. 16: Northeast and Ch. 25: Coasts).d0797088-3f92-4cfc-be8d-15027a28378e Projected changes in flood frequency and severity can bring new challenges in flood risk management. For urban areas in particular, flooding impacts critical infrastructure in ways that are difficult to foresee and can result in interconnected and cascading failures (for example, failure of electrical generating lines can cause pump failure, additional flooding, and failure of evacuation services). Increasing likelihood of flooding also brings with it human health risks associated with failure of critical infrastructure (Ch. 11: Urban)2a41e99d-d13e-4b17-b69b-933e4ce5c692 da7c07ae-84b8-49f6-8911-0e035b9b16cb f77ef996-fbed-4d64-887a-ee9525aa84a1 f8d67434-9bff-4c8d-847e-492b7bf8ee2c from waterborne disease that can persist well beyond the occurrence of very heavy precipitation (Ch. 9: Human Health),dba82efa-be61-4edd-af85-ee5e3ed07139 from water outages associated with infrastructure failures that cause decreased sanitary conditions,c0f2e34e-88a2-4e63-88ba-408e2efe5162 and from ecosystem changes that can affect airborne diseases (Ch. 8: Ecosystems).05a0fdc2-15ec-44a8-9962-d857c295e281

New information and remaining uncertainties: Large uncertainties still exist as to how well climate models can represent and project future precipitation extremes. However, precipitation extremes are expected to intensify as the atmosphere warms, and many floods result from larger portions of catchment areas receiving rain as snowlines recede upward. As rain runs off more quickly than snowfall, this results in increased flood potential; furthermore occasional rain-on-snow events exacerbates this effect. This trend is broadly expected to increase in frequency under general warming trends, particularly in mountainous catchments.73760c11-7b97-4876-a24f-8fb54b01bca9 fb52dd18-06f2-4774-9262-b00501ff730d 9417de56-6893-4102-86b7-ad7da4e68b8e 59b1ce0d-27ed-4224-835a-666a4006d9e8 7dc6aae5-efaa-489a-807e-8b92e7bcd546

Assessment of confidence based on evidence: Future changes in flood frequencies and intensities will depend on a complex combination of local to regional climatic influences and on the details of complex surface-hydrologic conditions in each catchment (for example, topography, land cover, and upstream managements). Consequently, flood frequency changes may be neither simple nor regionally homogeneous, and basin by basin projections may need to be developed. Nonetheless, early results now appearing in the literature have most often projected intensifications of flood regimes, in large part as responses to projections of more intense storms and more rainfall runoff from previously snowbound catchments and settings. Therefore confidence is judged to be medium that increasing flooding risk affects human safety and health, property, infrastructure, economies, and ecology in many basins across the U.S.

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• Water Resources Sector Technical Input Report in Support of the U.S. Global Change Research Program, National Climate Assessment - 2013 (50d47cc1)
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