Combined Modeling 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.

U.S. flood risk in the Anthropocene

“…The physical ood data used to calculate risk in this paper were published in Bates et al, 23 itself an evolution of the rst spatially continuous U.S. ood model presented by Wing et al 20 and the globalscale modelling methods of Sampson et al 33 In this section, the main methods and model validation studies are outlined. For more information, the reader is referred to Bates et al 23 The ood inundation model, at its core, solves the local inertial formulation of the shallow water equations in 2D (based on LISFLOOD-FP) 33,34 over a regularly spaced 1 arc second (~20-30 m in the US) grid. This formulation has been shown to produce indistinguishable answers to the full solution of the shallow water equations for typical ood inundation problems (i.e., subcritical ows), given typical input data errors.…”

“…Channels are thus parameterised under the assumption that they can convey a certain return period discharge (generally the 2-year ow, rising to 5-year in arid regions), with their bed elevations thus estimated using an inverted gradually varied ow solver (which solves for water height rather than discharge). 38 Return period discharges for channel bed estimation, and indeed for the extreme ows to simulate ooding are computed using a regional ood frequency analysis (RFFA) based on the methods of Smith et al 39 and further extended in Bates et al 23 This involved pooling almost 7000 USGS river gauges into proximal and hydrologically similar groups in order to compute an index ow for every cell on the ow accumulation array (with upstream area >50 km 2 ). Since ow records are generally too short to understand extreme ow behaviour, the RFFA substitutes time for space by again pooling hydrologically similar river gauges to derive growth curves.…”

“…The recent work of Bates et al 23 addresses the limitations of existing ood models in the US, fusing the accuracy of local studies with the spatial continuity of large-scale models. They present a hydrodynamic ood model at 30 m spatial resolution accounting for all major ood drivers and built with a welldocumented ood protection database.…”

“…25 The impact of these projected changes to hurricane behaviour on coastal surges are most keenly felt in Virginia, the Carolinas, and the west coast of Florida, while the contribution of sea level rise to future coastal oods dominates the remaining stretches of the Atlantic and Gulf coasts. 23 Intensifying rainfall, both hurricane and non-hurricane, is also expected to drive up risk in inland counties of Florida and the Northeast. Climate risk hotspots are further found in some already-risky western counties in California, Oregon, and Washington.…”

InundatEd-v1.0: a height above nearest drainage (HAND)-based flood risk modeling system using a discrete global grid system

“…Figure 1 shows the input DEM with elevation values given in meters, and the dams and gauging stations used in this study. The resolution of the DEM and LULC data is 30 m × 30 m. The vertical accuracy of the DEM is 0.34 m ± 6.22 m, i.e., 10 m at the 90 % confidence level (Beaulieu and Clavet, 2009). The vertical datum used is the Canadian Geodetic Vertical Datum of 2013 (CGVD2013).…”