Why does the US need
another flood map?

US flood insurance is at a critical juncture. With the reauthorisation of the National Flood Insurance Program (NFIP) extended again to September 2020, there’s no doubt that changes could be afoot. Quite what those changes could be is uncertain, but decisions about the NFIP will have to be made eventually. However, no matter what the changes are, insurers will want to be armed with as much high-quality information about flood risk as possible to enable them to capitalise on the new landscape.

The Federal Emergency Management Agency (FEMA) already makes its flood maps available to everyone. So, all’s good, right?

Well, not really. A brief internet search will indicate that FEMA’s maps have deficiencies, a fact highlighted particularly in the wake of catastrophes like those of the 2017 and 2018 hurricane seasons. These deficiencies are why insurers are keen to consider other sources of information.

JBA’s new US 5m Flood Map has been developed to address the limitations of other flood data available in the market.

Can flood maps go out of date?

Development of FEMA’s national maps began in the 1970s, with hydrology and engineering specialists making use of science and tools that were considered cutting-edge at the time. Although these maps represented the latest technology, large parts of the country haven’t had their maps reviewed or updated for years, sometimes decades. Technology changes rapidly, as does the natural environment when humans turn up with bulldozers to shape it to their needs.

Florida is a shining example of economic development. With an impressive boom in the early 2000s, creating a need for more housing and bigger roads, the extent of landscaping and engineering was spectacular. Many areas are now unrecognisable compared to how they were 20 years earlier. It’s reasonable to suggest that flood maps created more than 10 years ago are now inappropriate for use in hazard and risk assessments. It’s not that the FEMA maps are bad; they’re just badly out of date. The impacts of this are most strongly felt in areas that have undergone significant economic development.

How much difference do environmental changes make to a flood map?

We did some modelling to investigate, using Palm Shores as a case study.

Figure 1: aerial imagery from 1994 vs Aerial imagery from 2016. The 2016 image shows a significant amount of new housing across the area, accompanied by landscaping to create artificial ponds and waterways.

We didn’t have access to digital elevation data from 1994, so we used something more recent for our modelling. All model inputs were the same between the two models, apart from digital elevation data:

  • Model 1 used National Elevation Data (NED) which, until 2015, was the most up-to-date, high quality elevation data available from United States Geological Survey (USGS).
  • Model 2 used the more recent 3DEP elevation data available from USGS, partially made up of lidar.

Both elevation datasets were formatted to 5m resolution.

Figure 2: flood map based on NED (model 1) vs flood map based on 3DEP (model 2).

If we zoom in closer, to the Sherwood Park area, we can really see how the choice of flood map has a big impact on the level of hazard associated with individual houses.

Figure 3: flood map based on NED vs flood map based on 3DEP.

The comparison highlights the importance of using the most recent available elevation data. As an insurer, you would view the flood risk very differently depending on which of the above flood maps you use.

The map based on the older elevation data has a blocky appearance and is likely to overestimate flood hazard for some properties whilst completely missing flood hazard for others. The map created using the new 3DEP elevation data much better reflects the location of waterbodies, low-lying areas and flow paths along streets and between buildings, which is important for accurately modelling the behaviour of flood water.

What else makes a good flood map?

Having acquired the most appropriate elevation data, we’re now ready to tackle the complex phenomena of flooding.

It’s important to understand all flood types that can affect an area. Different flood types are best modelled using different approaches to reflect their different underlying causes, but we can’t model them in isolation of each other, particularly in hurricane-prone areas. The impact of flooding from rivers and rainfall occurring simultaneously with a storm surge is known to have been the cause of many flood claims from hurricanes Harvey and Irma in 2017.

At JBA, we've ensured that this compound flooding phenomenon is captured in our mapping process. We’ve worked with experts Applied Research Associates, using their wind-driven extreme sea levels to map storm surge as a separate hazard layer, as well as using the surge conditions to adjust the coastal boundaries for our inland flood mapping.

We’ve also considered flooding caused by intense rainfall overwhelming drains. It’s clear that flooding can happen miles from any river or coast, yet many maps don’t represent this type of flooding at all. Some map providers do try to capture it, but don’t model at a resolution high enough to adequately represent flow paths along small streets and between buildings.

JBA’s 20 years’ experience of flood mapping at a national scale has taught us the importance of using the best available data to properly address flooding from heavy rainfall. Figure 4 clearly illustrates how a 30m map struggles to represent the flow of water down narrow features such as walkways, resulting in a blocky appearance, whereas a 5m map is able to capture the movement of water in complex urban areas in a much more realistic way.

Figure 4: 30m flood map vs JBA 5m flood map. High-resolution mapping is needed to fully understand flood hazard in heavily urbanized regions such as Downtown Miami.

How can JBA help manage US flood risk?

We’re known as The Flood People®. Understanding and modelling flooding is very much what we do and we’ve been busy researching the complexities of this in the US. We’ve developed novel methods to ensure that our flood maps not only make use of the most up-to-date input data, but also account for the other details often neglected.

JBA’s US 5m Flood Map enables a comprehensive understanding of fluvial (river), pluvial (rainfall) and storm surge flood hazard. We’re also creating US Flood Pricing Data, which translates the flood maps into an easy to use dataset for calculating the annualised cost of flooding at any location.

For more information on managing your flood risk in the US, please contact our team.

Background info
FEMA's faulty flood maps (Bloomberg): https://www.bloomberg.com/graphics/2017-fema-faulty-flood-maps/

Outdated + backward-looking maps (NRDC): https://www.nrdc.org/experts/joel-scata/femas-outdated-and-backward-looking-flood-maps

Outdated science (Scientific America): https://www.scientificamerican.com/article/national-flood-insurance-is-underwater-because-of-outdated-science/

Increasing risk of compound flooding from storm surge and rainfall for major US cities (Wahl et al): https://www.nature.com/articles/nclimate2736

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