US flood defences:
fail to prepare or
prepare to fail?

Flood management in the US is under scrutiny following the catastrophic aftermath of recent flood and hurricane events, in which heavy rainfall along with under-performance of flood defences led to higher than expected flooding of property. Combined with low insurance penetration for flood, these events have served to significantly raise public concerns regarding the country’s ability to cope with flooding.

Many US flood defences are levees/floodwalls, channelisation and backwater areas. According to United States Army Corps of Engineering’s (USACE) figures, over 14 million Americans live behind the protection of levees (USACE, 2018a). It is estimated there are around 100,000 miles of levees in the US, although the actual figure is unknown (USACE, 2018b).

Some of these defences are highly effective. For example, the Mississippi River and Tributaries project has prevented an estimated USD $478 billion in flood damages between 1928 and 2012 (Camillo, 2012). However, it would be a mistake to think that all those living behind flood protection schemes are not at risk to flood. Performance of flood defences in the US must be critically assessed in light of recent events. The economic and insured losses for hurricanes and flooding in 2017 in the US were almost USD $130 billion and USD $48 billion respectively, with approximately USD $1.6 billion in insured damage for the 2018 Atlantic hurricane season so far (Aon, 2018).

Levees: For better or for worse?

A recent study found that the levee systems in the lower Mississippi basin are exacerbating flooding in that region rather than providing the intended protection (Munoz et al., 2018). This study reported that the magnitude of 100-year flood has increased by 20% over the past five centuries, with around 75% of that increase attributable to river engineering (Munoz et al., 2018). Straightened channels, spillways and enlarged levee systems all contribute to speeding up the propagation of a flood wave and increase peak discharge for a given flood, producing more damage than providing protection.

Breaking point?

As soon as defences are put in place, they will begin to degrade over time, with the deterioration affecting the level of protection they offer.

According to the Federal Emergency Management Agency (FEMA), approximately 85% of levees built by the USACE are locally owned and maintained, meaning they are not maintained by any Federal agency but maintained by the state in which the defence is located (FEMA, 2018; USACE, 2018b). This leads to a lack of consistency and national-level knowledge of defence condition, which can create safety and financial risks (USA Today, 2016).

Defences don’t necessarily have to physically fail – they may just be insufficient for the type or direction of flooding experienced during an event. An example of this is Lumberton canals, North Carolina. It is believed that the canals were less efficient at diverting flood water caused by Hurricane Florence due to debris still being present in the channels, which was left after Hurricane Matthew in 2016 (Washington Post, 2018). Unfortunately, it was still present despite the mayor reaching out to the USACE for help (Washington Post, 2018).

During Florence, reports suggest levees in Lumberton did eventually fail (HuffPost, 2018) but that the defences were also flanked by water flowing through a railway underpass (Washington Post, 2018). Residents had to resort to constructing a makeshift barrier in the underpass at the last minute due to the railway company’s refusal to allow the plans to go ahead sooner (Time, 2018). This makeshift barrier in the underpass eventually failed and thousands of homes were affected. This highlights the complexities of who oversees the management of flood defences during an event when it lacks a centralised body. Although flooding is not a common occurrence in this area, with the devastation of hurricanes Matthew and Florence still fresh in local minds, it may not seem so infrequent. 

The damage of Hurricane Katrina

A prominent example of the dangers of lack of maintenance and human interference with drainage basins can also be seen in the levee failures in New Orleans when Hurricane Katrina hit in 2005. This was the third time in the last century that a major failure of levee defences due to flood had occurred in New Orleans (Discover Historic Travel, 2012). Storm surge caused overtopping and failure of numerous defences.


The flooding caused by Katrina and these failures cost over 1800 people their lives as well as USD $165 billion in damages across numerous states (USACE, 2018; National Oceanic and Atmospheric Administration (NOAA), 2018a). The 1.7 million claims made to private insurance companies amounted to USD $41.1 billion, with a further USD $16.1 billion in payments from the National Flood Insurance Program and an additional USD $3 billion to offshore losses (Insurance Information Institute, 2010).

For the states which had been impacted, the effects of Katrina were exacerbated by Hurricane Rita which hit 31 days later, creating another USD $15 billion in damages due to the already compromised defences. More recently, Hurricane Harvey caused an estimated USD $127.5 billion in damage across multiple states, most of which was caused by widespread flooding due to exceptionally heavy rainfall, particularly devastating Houston, Texas (NOAA, 2018b). Discussions of building raised defences in Houston were spurred on by Hurricane Ike in 2008, but plans never went ahead (Vox, 2017). Although some of Houston’s highways are designed to drain floodwater, much of the urban sprawl has been built with little consideration to flood zones.

How can we help you manage flood exposure in the US?

The failure or overtopping of defences is a very real possibility and one that shouldn’t be overlooked. Whilst defences may protect against a smaller event, once defence systems are overwhelmed, the results can be more catastrophic than would otherwise have been the case.

At JBA, we provide a Defended Areas dataset to enable insurers and reinsurers to account for this possibility. Created using the best available third party sources, including FEMA’s National Flood Hazard Layer and USACE’s National Levee Database, the dataset highlights areas which are inundated in our maps but may in fact be defended. This gives our clients the best of both worlds; they can see what would happen if defences were assumed to fail in our flood maps, as well as what might be protected by defences, allowing more informed business decisions based on individual risk preferences.

If you would like more information on our US datasets, including our new 5m US flood maps, or would like to inquire about our hurricane flood footprints, get in touch.


Munoz et al., 'Climatic Control of Mississippi River flood hazard amplified by river engineering', 2018, accessed via

Aon, 'US Economic Losses', 2018, accessed via

FEMA, 'What is a levee?', 2018, accessed via

Washington Post, 2018, accessed via

HuffPost, 2018, accessed via

Time, 2018, accessed via

Discover Historic Travel, 'An abbreviated history of New Orleans levees and flood control projects', 2012, accessed via

USACE A, 'Levee System in the US', accessed via

USACE B, 'Strong Levee Safety Programs in All States', accessed via:

Insurance Information Institute, 'Hurricane Katrina fact file', 2010, accessed via

Camillo, 'Divine Providence: The 2011 Flood in the Mississippi River and Tributaries Project', 2012, accessed via

NOAA A, 'Billion-Dollar Weather and Climate Disasters: Table of Events', accessed via

NOAA B, 'Costliest U.S Tropical Cylones table updated', accessed via

USA Today, 2016, accessed via

Vox, 2017, accessed via

News &

News GRiP expands into flood intelligence with JBA Risk Management tie-up

South African Spatial Technology and Data Specialist GRiP partners with JBA to deliver advanced flood risk intelligence across Africa.

Learn more
News JBA Risk Management teams up with Oxford University for infrastructure study

JBA and Oxford University join forces to research the risks of climate extremes on infrastructure networks worldwide today and in the future.

Continue reading
News International Women’s Day 2024

JBA women talk about their achievements, support for other women in the workplace, and ideas to #InspireInclusion for International Women's Day 2024.

Continue reading
News Mind the physical-risk due diligence gap

A failure to use good quality data and sophisticated climate change intelligence to understand the impact of flood on physical assets could be putting investors' portfolios at risk.

Continue reading