Projecting Changes in Expected Annual Damages From Riverine Flooding in the United States

Abstract

Inland flood risk in the United States is most often conveyed through maps of 1% annual exceedance probability (AEP) or “100‐year” floodplains. However, monetary damages from flooding arise from a full distribution of events, including floods both larger and smaller than the 1% AEP event. Furthermore, floodplains are not static, since both the frequency and magnitude of flooding are likely to change in a warming climate. We explored the implications of a changing frequency and magnitude of flooding across a wide spectrum of flood events, using a sample of 376 watersheds in the United States where floodplains from multiple recurrence intervals have been mapped. Using an inventory of assets within these mapped floodplains, we first calculated expected annual damages (EADs) from flooding in each watershed under baseline climate conditions. We find that the EAD is typically a factor of 5–7 higher than the expected damages from 100‐year events alone and that much of these damages are attributable to floods smaller than the 1% AEP event. The EAD from flooding typically increases by 25–50% under a 1 °C warming scenario and in most regions more than double under a 3 °C warming scenario. Further increases in EAD are not as pronounced beyond 3 °C warming, suggesting that most of the projected increases in flood damages will have already occurred, for most regions of the country, by that time. Adaptations that protect against today's 100‐year flood will have increasing benefits in a warmer climate by also protecting against more frequent, smaller events.

Climate change, riverine flood risk and adaptation for the conterminous United States

“…Additional details on the methods used for extracting future return intervals can be found in Wobus et al (2017) and Wobus et al (2019) .…”

“…Until recently, the data required to prioritize adaptation investments at local scales have not been available at a sufficiently granular scale to inform these decisions. In addition, riverine flood maps from FEMA are typically available for only one or two recurrence intervals (typically the “100-year” and “500-year” flood events), which does not allow calculation of the expected annual damages (EAD) from floods of all magnitudes ( Ward et al, 2011 ; Wobus et al, 2019 ). Finally, most publicly available flood risk products do not sufficiently account for the impact of climate change on the future magnitude and frequency of floods.…”

Combined Modeling of US Fluvial, Pluvial, and Coastal Flood Hazard Under Current and Future Climates

“…Lastly, climate change may be poorly handled in large‐scale hazard and risk analyses, and in many studies (e.g., Wing et al., 2018) it is not even considered at all. Many large‐scale assessments of future fluvial flood hazard (Alfieri et al., 2016; Hirabayashi et al., 2013; Wobus et al., 2019) are driven with the output of Global Circulation Models that may poorly represent extreme rainfall (Gregersen et al., 2013; Kendon et al., 2014) and which still do not fully resolve tropical cyclones (Emanuel, 2013). This is especially problematic for countries like the US where tropical cyclones play a central role in driving flood losses (Smith et al., 2010) and where hurricane‐induced coastal flooding is likely to increase in the future (Marsooli et al., 2019).…”

Assessing Property Level Economic Impacts of Climate in the US, New Insights and Evidence from a Comprehensive Flood Risk Assessment Tool

“…On average, direct flood losses have risen from USD 4 billion annually in the 1980's to roughly USD 17 billion annually from 2010 to 2018 [2]. Increasing development of flood-prone areas is a key driver of rising [3][4][5] and climate change is expected to exacerbate losses even further [6][7][8][9]. In both the international and U.S. cases, the current and future expectations around flood risk and damages make the need for readily available impact assessment tools that can be created at scale and the use of publicly available data and transparent methods imperative.…”

“…In recent studies, researchers have examined AALs on a broader scale. Wobus et al [6] examine how riverine flood damages across 376 US watersheds are expected to change in response to rising global temperatures. They find that expected annual damages are 5 to 7 times higher than damages expected from the 1% annual chance event and that a significant share of the losses attributable to a more comprehensive evaluation of flood risk.…”