Combined modelling of US fluvial, pluvial and coastal flood hazard under current and future climates

Abstract

This study reports a new and significantly enhanced analysis of US flood hazard at 30 m spatial resolution. Specific improvements include updated hydrography data, new methods to determine channel depth, more rigorous flood frequency analysis, output downscaling to property tract level, and inclusion of the impact of local interventions in the flooding system. For the first time, we consider pluvial, fluvial, and coastal flood hazards within the same framework and provide projections for both current (rather than historic average) conditions and for future time periods centered on 2035 and 2050 under the RCP4.5 emissions pathway. Validation against high-quality local models and the entire catalog of FEMA 1% annual probability flood maps yielded Critical Success Index values in the range 0.69-0.82. Significant improvements over a previous pluvial/fluvial model version are shown for high-frequency events and coastal zones, along with minor improvements in areas where model performance was already good. The result is the first comprehensive and consistent national-scale analysis of flood hazard for the conterminous US for both current and future conditions. Even though we consider a stabilization emissions scenario and a near-future time horizon, we project clear patterns of changing flood hazard (3σ changes in 100 years inundated area of −3.8 to +16% at 1° scale), that are significant when considered as a proportion of the land area where human use is possible or in terms of the currently protected land area where the standard of flood defense protection may become compromised by this time. Plain Language Summary We develop a method to estimate past, present, and future flood risk for all properties in the conterminous United States whether affected by river, coastal or rainfall flooding. The analysis accounts for variability within environmental factors including changes in sea level rise, hurricane intensity and landfall locations, precipitation patterns, and river discharge. We show that even for a conservative climate change trajectory we can expect locally significant changes in the land area at risk from floods by 2050, and by this time defenses protecting 2,200 km 2 of land may be compromised. The complete dataset has been made available via a website (https://floodfactor.com/) created by the First Street Foundation in order to increase public awareness of the threat posed by flooding to safety and livelihoods. BATES ET AL.

Compound Hydrometeorological Extremes: Drivers, Mechanisms and Methods

“…The risk of compound flooding resulting from storm surge and heavy rainfall has been increasing in major coastal cities of the United States (Wahl et al, 2015). The risk of compound storm surge and heavy rainfall is projected to increase in the future (Karim and Mimura, 2008;Bevacqua et al, 2019;Bates et al, 2020;Hsiao et al, 2021). However, there are still large uncertainties in quantifying changes in the risk of compound flooding due to the insufficient skill of climate models in simulating extreme precipitation caused by storms (Zhang et al, 2019a;Roberts et al, 2020;Vannière et al, 2020).…”

Estimating River Channel Bathymetry in Large Scale Flood Inundation Models

“…Validation of these extreme inundation extents is beyond the scope of this paper, however an extensive analysis of the GFM hazard estimates using this solver is presented by Bates et al. (2021).…”

“…Flood inundation extents are expected to be sensitive to this assumption, thus, although we aim to improve the simulation of the bank full water surface profile by the GFM for a given return period, we do not reduce the uncertainty in the bank full discharge, which can be substantial (Williams, 1978). Bank full return period might be refined using data on historical flood occurrence (Hawker et al., 2020) regionalization of section observations (Castro & Jackson, 2001) or using standard of protection for engineered reaches (Bates et al., 2021; Winsemius et al., 2013).…”

Urbanizing the floodplain: Global changes of imperviousness in flood-prone areas

“…Furthermore, even areas that are protected by defenses build at a specific standard (e.g., 100 years) can become exposed in the near future due to increases of flood magnitudes for the same return period 37 . For example, a recent study 18 found that ∼2,200 km 2 of land currently defended from a 1/100-year flood will be at risk by 2050 in the United States under one of the intermediate climate change scenarios (RCP 4.5). In sum, we conclude that (i) human settlement on floodplains is increasing, with impervious surface area expansion rates accelerating; (ii) the total urban area in floodplains has nearly doubled since 1985; (iii) population size has grown on the floodplain with the highest rates in areas with 1/20 annual probability of flooding.…”

“…We argue that our analysis can act as the baseline for assessing the relationship between urbanization and flood risk, as the resulting dataset is physically-based at sufficiently high spatial resolution with global coverage. Despite their potential value, such maps can only be a component of the integrated modeling frameworks 18 required to correctly assess all aspects (socio-economic, engineering, policy etc.) of national and local flood management.…”