US hurricane and flood: a case study for comprehensive data

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The US is currently recovering from the impacts of two tropical cyclones, Tropical Storm Marco and Hurricane Laura, which impacted the US Gulf Coast in close succession in late August 2020. Hurricane Laura was the strongest storm to make landfall in the state of Louisiana for over a century and, although flooding was less severe than first feared, it still brought major storm surges to coastal areas of Louisiana and Texas.

The Atlantic Hurricane Season is approaching its meteorological peak, from late August through September, with the US gearing itself for more potential activity in the Gulf of Mexico. The average Atlantic Hurricane Season spans from June through November, which sees the formation, on average, of 10.1 named storms, 5.9 of which reach hurricane status. Around 2.5 of these hurricanes become major hurricanes, classified as greater than a Category 3 on the Saffir-Simpson scale.

These tropical cyclones have the potential to bring widespread flooding to the Atlantic and Gulf coasts of the United States each year, with flood risk remaining a significant threat to communities in these areas, as well as to the insurance market that serves their needs. However, FEMA maps have been the predominant representation of flood risk for the US insurance industry for decades, despite acknowledged limitations in the data. 

How might comprehensive flood data help insurers understand complex flooding from hurricanes, including an event like 2017's Hurricane Harvey, which we look at in this blog?

The need for comprehensive flood data

Flood is a complex peril and requires more granular detail than many other perils. Firstly, flood varies spatially; one house may be flooded while the neighbour is spared. This is particularly seen in urban areas with lots of small streets and infrastructure features, and high-resolution data is vital to capture the intricacies of the water movement.

Secondly, the time it takes for a rain event to pass over a region can vary significantly, from hours and minutes to days and weeks, and being able to capture these different time scales is important for a complete view of risk.

Furthermore, how extensive, and therefore how severe, a flood might be needs to be understood for insurers to effectively assess the risk. Understanding which flood type may impact a property enables a deeper understanding of how much damage may be caused. For example, coastal and storm surge flood is more typically destructive due to the added salinity in the water, whereas pluvial flooding is often cleaner and quicker to recede. Information on flood type is especially vital in the US where hurricanes like Harvey pose an additional threat – properties may be susceptible to multiple flood types simultaneously and be exposed to extreme flooding more frequently than locations away from hurricane tracks.

Hurricane Harvey and Houston: A case study

Classified as a Category 4 hurricane on the Saffir-Simpson scale, Hurricane Harvey made landfall on the east coast of Texas on August 25 2017. The storm stalled over this area for four days, peaking on September 1 2017, making it even more devastating.

Harvey is now known to be the largest and most significant rainfall event ever to impact Texas, bringing record rainfall for a tropical cyclone event with highest reported totals of 60.58 inches. An analysis of the river gauges undertaken by the USGS shows that of 74 streamflow-gauging stations in the affected area, 40 stations (54%) recorded a new highest peak flow during Hurricane Harvey (USGS, 2018).

This unprecedented amount of rainfall resulted in damage to approximately 204,000 homes, estimated to cost $125 billion, placing it as the second costliest hurricane after Hurricane Katrina (NHC, 2017).

Houston was the largest complex area affected by Hurricane Harvey, with a high population density and a concentration of insured assets. By referring to high-water marks recorded immediately after Hurricane Harvey, we can see how comprehensive data like JBA’s US Flood Map can help to understand which properties may be at risk during these types of events.

In Houston, there are 640 high-water marks recorded from Harvey. JBA’s 100-year return period flood map indicates a flood hazard at 550 of these locations (86%), whereas FEMA’s equivalent flood mapping captures 408 (63%)*. If we dig into this further, we can see that JBA’s 1,500-year return period flood map captures 616 (96%) of the high-water marks, many of which are missed from FEMA mapping due to the maps’ single return period of 100-years. The rainfall experienced during Hurricane Harvey was estimated to be a 1000-year return period event.

Figure 1: shows the distribution of High-Water Marks (black dots) in Houston overlain on JBA’s 100-year fluvial and pluvial maps.

Furthermore, when assessing hurricanes, the risk of pluvial flooding can often be overlooked compared to storm surge and fluvial flooding. This proved catastrophic during Hurricane Harvey which, as we’ve seen, saw record rainfall over a four-day period, which caused widespread pluvial flooding. This was especially destructive in Houston and other urban areas due to the prevalence of impermeable surfaces, causing increased and rapid runoff of rainfall, which quickly overwhelmed the storm drains.

Ten percent of the high-water marks recorded following Hurricane Harvey are covered solely by JBA’s pluvial flood map, while the FEMA map represents no flood hazard at these locations. This is because the FEMA map only represents fluvial and storm surge flooding, and does not account for high-intensity rainfall events.

These high-water marks demonstrate the need for comprehensive flood data. If insurers had been able to assess flood risk in Houston using a fully comprehensive flood map, representing a full range of return periods and flood types, more homeowners might have been able to claim on insurance policies rather than relying on the limited Disaster Relief Assistance pay-out from the federal government.

Closing the protection gap

Events like Hurricane Harvey demonstrate the vast protection gap that exists in the US. Flood insurance penetration has long been dangerously low, exposing a huge proportion of the US population to substantial economic hardship in the wake of flood events. Many homeowners in the US do not choose to buy flood insurance, and their decision is heavily influenced by whether their home is located in a FEMA 100-year flood zone (as shown in Figure 2). Even within these high-risk zones, flood insurance is only mandatory in some mortgage applications.

Figure 2: The differences between JBA and FEMA flood maps. The red hatching is the extent of FEMA’s maps and covers the areas in which a homeowner would require insurance if purchasing a mortgage. The JBA maps, covering fluvial and pluvial flood (blue and purple), show a much larger area at risk.

Furthermore, flood is often viewed as too risky to write by insurers due to the complexity of the peril and limitations in historical industry data such as FEMA’s mapping, thus reducing the flood coverage options available for home and business owners.

Hurricane Harvey sadly exposed this fact, with just 15% of homes in Harris County, Texas, insured against flood damage in August 2016, and just 28% of homes insured for flood in areas deemed “high-risk” (Quartz, 2017). The aftermath of Harvey cost the NFIP an average $103,000 per homeowner in payments to those who had purchased flood insurance from them, while the households who had not purchased flood insurance had to depend on Disaster Relief Assistance, which paid out an average $6,000 per household.

This doesn’t need to be the case. With more reliable information on flood risk, insurers can write flood with more confidence, thus helping to reduce the protection gap and reduce federal emergency money pay-outs following severe flood events.

New data for assessing flood risk

JBA’s new high-resolution US Flood Map can help insurers better understand flood risk across the country, including the risk posed by another event like Hurricane Harvey. 

The maps cover the three major flood types of fluvial, pluvial and storm surge, which can be viewed individually or in combination for a more comprehensive view of the risk compared to typical industry data.

This is furthered by bespoke methods specifically designed to capture hurricane scenarios like Hurricane Harvey. JBA modelling experts researched and designed hydraulic models to incorporate the impact of tropical cyclones and non-tropical cyclones on flooding and, for the Gulf and Atlantic coastal areas, used Applied Research Associates Inc’s hurricane storm surge levels to generate a representative flood map that can provide a greater insight into these hurricane-prone areas. Being able to accurately model the complex flooding caused by the combination of flood types enables a deeper insight into flood risk and potential loss drivers. 

The maps are modelled at 5m resolution nationwide, providing a more realistic view of flooding around properties and urban infrastructure for greater distinction of risk and a consistent approach to flood assessment across the contiguous US, which had previously been impossible using standard industry data. The maps cover a range of return periods, from 20 years up to 1,500 years, to capture more extreme events that might be missing from other datasets, and flood depths are also provided.

This granular, comprehensive, nationwide data can help insurers to more confidently understand and write flood risk across the US. Find out more using the form above or email the team.

* Analysis of the high-water marks is based on a buffered 10m zone around the points.

References

Blake, E and Zelinsky, D. 2018. National Hurricane Center Tropical Cyclone Report: Hurricane Harvey. National Hurriance Center. Available at: https://www.nhc.noaa.gov/data/tcr/AL092017_Harvey.pdf

Timmons, H, 2017, Why 85% of Houston homeowners have no flood insurance, August 29 2017,
https://qz.com/1063985/hurricane-harvey-why-85-of-homeowners-in-houston-dont-have-federal-flood-insurance/, 27th August 2020

Watson, K.M., Harwell, G.R., Wallace, D.S., Welborn, T.L., Stengel, V.G., and McDowell, J.S., 2018, Characterization of peak streamflows and flood inundation of selected areas in southeastern Texas and southwestern Louisiana from the August and September 2017 flood resulting from Hurricane Harvey: U.S. Geological Survey Scientific Investigations Report 2018–5070. Available at: https://pubs.usgs.gov/sir/2018/5070/sir20185070.pdf

Wattles, J, 2017, Hurricane Harvey: 70% of home damage costs aren't covered by insurance, September 1, https://money.cnn.com/2017/09/01/news/hurricane-harvey-cost-damage-homes-flood/index.html, 27th August 2020