Utilizing Flood Insurance Claims in Coastal CONUS to Assess the Impact of Compound Flood Risks

The intricate dynamics of compound coastal flooding—from the convergence of river floods and storm surges—often lead to more severe impacts than floods caused by a single factor. The expected increase in damages from such flooding events is a combination of climate change effects on coastal storms, with the growing population and economic activities in coastal areas. In the United States, the Federal Emergency Management Agency's (FEMA) National Flood Insurance Program (NFIP) is the largest issuer of flood insurance policies, with over two million flood claim transactions available for public analysis since 1978. However, studies examining the impact of compound events on insurance claims remain sparse.

This research focuses on over 60,000 counties along the coastline of the United States, offering a comprehensive analysis of economic losses in regions affected by individual flood drivers (river or coastal), or compound (river and coastal) flooding events. To determine the predominant flooding mechanism—whether inland or coastal—for a specific location, we use a published index (D-Index, for details interested reader can check the reference here, https://doi.org/10.1016/j.jhydrol.2023.130278 ) capable of attributing flood depth to river, coastal, or combined rainfall and storm surge drivers.

Our analysis centers on the reported damages in insurance claims, comparing and contrasting claims from counties categorized as coastal, riverine, or compound. By calculating the quantile weight distance (QWD) of damages from claims in 'compound' counties versus 'single-driver' counties, we explore the relationship between storm event characteristics (rainfall and tide) and NFIP flood claims in compound events. Additionally, we examine QWD differences by comparing FEMA's high-risk flood zones (1-percent annual chance floodplain) with low and moderate-risk flood zones, where flood insurance is not mandated for homes financed through federally backed or regulated lenders.

In summary, this study provides critical insights into the complex challenges of evaluating the impacts of compound coastal flooding on insurance claims. The proposed methodology, which includes a flood type-specific mapping system and accounts for spatial variations in inundation characteristics, lays a strong foundation for a thorough and improved flood risk assessment in coastal regions of the contiguous United States (CONUS). These findings enable coastal communities to proactively manage hidden risks and strengthen their resilience against the compounded effects of environmental forces. This research offers a proactive and informed approach to mitigating the potentially catastrophic consequences of compound coastal flooding in an evolving climate and socio-economic environment.