Spatial distribution of compound flooding events and their drivers
The U.S. experiences a considerable number of flooding events, with a total of 5,506 recorded incidents in SHELDUS along the U.S. Gulf and East coasts between 1980 and 2018 (see Fig. S1). Fig. S2 shows the total number of compound events (see Methods for how we define and identify compound events). In Fig. 1, we show the ratio of compound events versus total number of flooding events (expressed in percent) and how this varies across counties. The percentage values vary from 50–100%. We find that over 70% of total recorded flood events were compound in 83% of the counties, and no county shows less than 50% of compound events. The highest percentage of compound flooding events (ranging from 90–100%) occurred in counties along the U.S. Gulf Coast, from Louisiana to Florida’s Panhandle, and on the U.S. East Coast, between North Carolina and Delaware, and in Maine (Fig. 1). These are areas where most recorded flood events were caused by multiple flood drivers. For example, in Atlantic County (New Jersey) and in Gloucester County (Virginia) 90% and 100% of historical flood events were compound in nature, respectively. Furthermore, we find that compound event occurrence varies considerably between states, with Mississippi (MS) experiencing the highest percentage of compound flood events (89%), when averaged across all the counties, and Georgia (GA) having the lowest percentage (59.5%) (Fig. 1).
In Fig. 2, we show how often the different flood drivers were involved in creating the identified compound flooding events. We find that precipitation is most often a contributing driver, i.e., precipitation exceeded its 95th percentile threshold during 91% of all compound events (see Methods) (Fig. 2a). This highlights the importance of extreme rainfall events in creating compound flooding, including those caused by extra-tropical storms as well as hurricanes42,43. River discharge and soil moisture were also often involved in historical compound events for most counties (85% and 83% of all compound events, respectively) (Fig. 2b and 2c). The contribution of storm surge and ocean waves (described here as significant wave height) to compound flooding events varies considerably across space, with the highest percentage (ranging from 60–90%) in counties along the East coast between North Carolina and Maine (Fig. 2d and 2e).
Figure 3 shows more detailed results for the ten counties with the highest total number of recorded flooding events in SHELDUS. Doughnut charts show how often the different flood drivers contributed to compound events in these counties. For instance, in Harris County (Texas), precipitation exceeded its 95th percentile value during 119 out of 155 total compound events. This highlights the county's vulnerability to intense rainfall events, combined with other flood drivers, which, in this case, are often associated with tropical storms and hurricanes. River discharge and soil moisture also played important roles, contributing to 114 and 102 compound events, respectively, indicating the substantial influence of upstream hydrological conditions and saturated ground on the generation of compound flood events and associated impacts. Soil moisture contribution is highest for Harris County, TX, and Charleston County, SC, and markedly lower for the other counties shown. Those same two counties also have the lowest fraction of compound events where oceanographic drivers (storm surge and wave height) contributed compared to the other eight counties.
Clustering analysis of the impacts and drivers of compound flooding
Through clustering analysis (see Methods and Supplementary Fig. S3), we identify four key clusters where in each cluster, the counties exhibit, on average, similar percentages of occurrences in the different flood drivers during compound flooding events (Fig. 4). As shown above at the individual county level, precipitation, river discharge, and soil moisture (i.e., hydrological processes) have contributed to compound flooding more frequently than storm surges and waves (i.e., oceanographic processes) in many counties; those counties are part of cluster 1 and stretch along the entire Gulf and East coasts (Fig. 5). In clusters 2, 3 and 4, we find a higher occurrence of ocean waves and storm surge during compound flooding events compared to cluster 1, particularly in clusters 2 and 3 (> 16%). These clusters are comprised of coastal counties mostly between North Carolina and New Hampshire, including Chesapeake Bay, and individual counties along the U.S. Gulf coast (Fig. 5), consistent with Fig. 2. We note that while clusters 2 and 3 look similar in Fig. 4 in terms of the average values across counties, the variability can still be very different and lead to the separation of clusters. The dendrogram in Fig. S3 shows the hierarchical relationships between all counties and allows, for example, to further compare counties within our identified main clusters in terms of compound flooding drivers and how they relate to each other.
Socio-economic impact of compound flooding and their regional distribution
Flooding events can lead to property (infrastructure) and crop damage. Figure 6 shows the percentage of historical property (Fig. 6a) and crop losses (Fig. 6c), recorded in SHELDUS, associated with compound flooding events as identified in our analysis. Over 80% of historical property and crop damage has been caused by events when multiple drivers were extreme (rather than univariate) in 92% and 81% of counties, respectively. For example, Orleans Parish, Louisiana, experienced property losses of ~ US $24 billion, almost entirely attributable to compound flooding (~ 99%). Ocean County and Monmouth County in New Jersey have recorded over US$11 billion in flood damage with over 99% of this damage due to compound flooding. Those two examples from Louisiana and New Jersey also highlight the challenges when analyzing loss data that includes extreme outlier events. Hurricanes Katrina and Sandy were by far the costliest events in those counties, and both happened to be compound events where multiple flood drivers were extreme. Hence, it is not surprising that the majority of losses are linked to compound events. To account for this, we also derive the median losses from compound and non-compound events, and the ratio between the two, for all counties (Fig. 6b and 6d). A ratio larger than one indicates that compound events were more costly than non-compound events. For property losses, this is the case for 161 (out of 203) counties, and the average ratio across those counties is 26.68 (excluding the ones where no non-compound events were recorded, and a ratio cannot be calculated). This means that compound events (in terms of the median) were more than 26 times costlier than non-compound events; for crop losses that number increases to 76.02, further highlighting the damaging effects of compound flood events. Counties with ratios smaller than one indicate that non-compound events were more expensive. That is the case for 42 counties in terms of property loss and 28 counties for crop loss. Note that in some of those cases, the overall number of recorded flood loss events was small, and no compound events occurred, leading to a ratio of zero. Overall, these results show the widespread impact of compound flooding events on built infrastructure and agricultural assets along the U.S. Gulf and East coasts.