Hurricane Ida became the ninth named storm of the 2021 hurricane season, tied as the fifth strongest in history, and may yet be the costliest storm to hit the mainland U.S (Aon 2021).
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Rapid intensification
Following its initial classification as Tropical Storm Ida on 26 August, the storm rapidly intensified over the following 3 days, intensifying from a category 1 storm to a category 4 within 24-hours. Ida reached its peak as a Category 4 hurricane just before making landfall near Port Fourchon, Louisiana at 16:55 UTC on Sunday 29 August, with maximum sustained winds of 150mph (241 kph) (NHC, 2021, NBC News, 2021b). This ties with Hurricane Laura in August 2020 as the strongest storm to ever hit Louisiana, and ties as the fifth strongest to impact mainland U.S.
Meteorologists suggest that the path of the storm assisted with this rapid intensification, with Ida being fuelled by warm water on the surface and below the surface in the gulf (NYTimes, 2021a).
The remnants of tropical cyclones that travel inland over central and eastern USA (Figure 1) are sometimes known as “second acts” and can often go on to cause damage and heavy rainfall hundreds of miles beyond the coastal area where the storm initially made landfall (The Washington Post, 2021a). While moving through the Northeast, the remains of Hurricane Ida collided with jet streams in the mid-latitudes (Bloomberg, 2021). These jet stream disturbances added additional energy and provided a source of invigorated uplift to tropical remnants (The Washington Post, 2021a).
Rainfall events in the Northeast because of these second acts have been increasingly common since the mid-1900s, with the heaviest rainfall events increasing by 55% in the Northeast between 1958 to 2016 (CBS News, 2021). This is a result of warmer waters along the Northeast coast, with warmer air having a greater capacity to hold more moisture (CBS News, 2021).
After making landfall, by midnight on Sunday Ida had weakened to a Category 1 hurricane with sustained windspeeds of 95mph (153kph) (NBC News, 2021).
Figure 1: Track of Hurricane Ida and rainfall accumulation between 27 August – 3 September 2021. The rainfall accumulation data are derived from satellite data (Source: NASA Precipitation Measurement Mission).
Louisiana
Prior to making landfall, Hurricane Ida was forecast to produce a storm surge of up to 16ft (4.8m) along the coast of southeast Louisiana (BBC, 2021).
While coastal areas in Louisiana such as Grande Isle, Shell Beach, Lafitte, Barataria and Port Fourchon experienced a storm surge, the surges seen were lower than forecast and ranged from 3-8ft (1-2.5m) (NASA Earth Observatory, 2021).
However, these strong winds and storm surges still resulted in back up of the Mississippi River due to flows being reversed. The river level recorded at the USGS gauge in Bell Chasse rose around 7ft (2m) and the flow reversed to around 0.2m/s in the opposite direction (CNN, 2021). In comparison, storm surge during Hurricane Katrina measured up to 24-28ft (7-8.5m).
A total of 28 deaths were reported in Louisiana as of 13 September (AP News, 2021a). One million people in the state, including much of the city of New Orleans, lost power for 10 days (BBC, 2021), with some of the later deaths in New Orleans linked to power shortage, heat and carbon monoxide poisoning from the use of backup generators (NYTimes, 2021c). The number of electricity pylons brought down in Louisiana and Mississippi was greater than Hurricane Katrina, Zeta and Delta combined (NYTimes, 2021c).
After Hurricane Katrina, a 192-mile flood defence system was constructed, with levees, walls, storm gates and pumps installed to protect New Orleans against floods (NYTimes, 2021d). Following Hurricane Ida, no part of the defence system was overtopped or breached (NYTimes, 2021d) with no flooding reported in New Orleans, Jefferson or St. Bernard parishes (AP News, 2021b).
Flooding did occur in other areas of Louisiana. 60 miles (96.5km) away in Larose, storm surge from Ida overtopped a modest levee maintained by the Lafourche Parish government, and flooding was reported to have entered houses (NYTimes, 2021e). Similarly, flooding occurred in LaPlace, a western suburb of New Orleans, where work on a levee project recently began but is not due to finish until 2024 (AP News, 2021b).
This flooding was coupled with damage, debris and loss of power across the state due to high windspeeds, which will take some time to recover from.
The Second Act – Floods in Northeast and New York City
The remnants of the storm brought heavy rainfall and flash flooding across the East Coast, most notably in New York and New Jersey, with states of emergency declared in these areas on the evening of Wednesday 1 September.
Over 40 deaths have been reported as a result of flooding across the Northeast (Guardian, 2021), including Pennsylvania and Connecticut. This includes 25 fatalities in New Jersey, 16 in New York, four in Pennsylvania and one in Connecticut state (NYTimes, 2021g).
Of the 13 reported deaths in New York City, many of these sadly occurred due to residents being unable to escape from flood water in basement apartments. Some streets along Brooklyn were flooded to around 0.3-0.4m as rainwater rushed to basements of properties (NYTimes, 2021h).
Figure 2: Some locations identified as flooded around New York City (Source: Media reports, Twitter; Map by JBA Risk Management).
Why were floods in New York City so severe?
High rainfall
The National Weather Service New York recorded a total of 3.15 inches (80mm) of rain in Central Park in just one hour between 20:51 – 21:51 on 1 September (National Weather Service New York, 2021a). This is the highest one-hour total ever recorded at this location.
Based on the Climate Summary for 1 September, the Central Park gauge station recorded 7.13 inches of rainfall for a 24-hour period (National Weather Service Weather Forecast Office, 2021). This is the fifth largest daily rainfall recorded over 150 years. Based on an extreme value analysis for the daily rainfall, the estimated return period of the rainfall event in New York City is approximately 100 years (Figure 3).
This rainfall was exacerbated by the rainfall experienced as a result of Tropical Storm Henri just one week earlier. On 21 August, Henri dumped 1.94 inches of rain within an hour (22:00-23:00) in Central Park, which broke the record for rainfall. This was followed by 4.45 inches of rain (National Weather Service New York, 2021b) within a day on 21 August.
The compound impacts from rainfall from these two storms may have put extra pressure on the drainage system, and cause soil and drainage to be saturated before Hurricane Ida affected the Northeast region. In New York City, the sewer and drainage system were designed to manage just 1.75 inches of rain in an hour (The Washington Post, 2021a).
Figure 3: Extreme Value Analysis based on highest daily rainfall recorded between 1869 – 2020. During Hurricane Ida, New York Cityreceived 7.13 inches of rain within 24 hours which is estimated to be a 100-year event based on historical rainfall record. (Historical rainfall data source: https://www.weather.gov/media/okx/Climate/CentralPark/DailyHighestPrecip.pdf)
Urban and suburban sprawl
Billions of dollars were spent constructing coastal flood defences around New York City following Hurricane Sandy, which caused storm surges and massive floods along coastal areas, the bay and rivers in New York City. However, these coastal defences were insufficient to protect the city from pluvial floods (surface water floods) caused by intense rainfall (Technology Review, 2021). The latter caused floods in subway stations, parks, roads and properties throughout New York.
Pluvial floods are common in cities and urbanised areas as a high percentage of land cover is made up of roofs, buildings, and roads made of asphalt, which leads to higher runoff above ground. This, coupled with a lack of high capacity drainage system, means water can build up on the surface leading to floods (NYTimes, 2021i).
Similarly, high property prices and pressures for housing leads to more people living in basement flats which are susceptible to floods. The impacts of this were also seen in London in July 2021 which was also covered by our Event Response team.
Insured and economic losses
Based on recent estimates from Goldman Sachs, losses to the insurance and reinsurance industry from Hurricane Ida could reach between USD $30 and 40 billion (Artemis, 2021). However, with the ongoing pandemic and higher demand for materials and labour after a catastrophe event, the final cost may be higher (Reuters, 2021) and it is likely to be one the costliest hurricanes to impact the mainland U.S. based on total economic losses (Artemis, 2021).
In comparison, Hurricane Katrina in 2005 resulted in USD $87 billion worth of claims after adjusting for inflation (Reuters, 2021). Superstorm Sandy resulted in total economic losses greater than USD $60 billion in October 2012 (Reuters, 2021). In 2020, Hurricane Laura resulted in insured losses of approximately USD $10 billion (Reuters, 2021).
Future Outlook & Climate Change
As reported in the Fourth National Climate Assessment report, the U.S. Northeast and Midwest are likely to see heavy rainfall events greater than the 99th percentile of daily values increase by 40% in 2070-2099 when compared to 1986-2015. These increases are likely related to thunderstorm clusters and its associated rainfall (Fourth National Climate Assessment, 2021). From 1958 to 2016, observed annual precipitation in the heaviest 1% of rainfall increased by 55% in the Northeast (Fourth National Climate Assessment, 2021).
Following on from a report published in May 2021, the New York City Panel on Climate Change (NPCC) has suggested that global warming will cause New York City to be significantly wetter and rainfall may increase by 25% as we approach 2100 (NYC Mayor’s Office of Resiliency, 2021). This would put extra stress on the city’s drainage system, which has been upgraded over the years and now comprises a traditional parallel drainage-sewer system, nature based Bluebelts and green infrastructure meant to capture pluvial runoff (NYC Office of the Mayor, 2021).
However, with effects of climate change looming in the near future, more work may be required to protect New York City and other vulnerable areas from floods and intense rainfall over the next few decades.
That being said, trends in extreme events like floods are harder to determine as multiple factors could have an impact on flood extent, for example land-use type, water management and flood defence strategies (Fourth National Climate Assessment, 2021).
With many recently published articles and journals and significant hurricane activity in recent years, there are many ongoing discussions about the impact of climate change on hurricane intensity. While there is no direct indication that global warming will lead to more frequent hurricanes, increase in sea surface temperatures could increase the formation of major hurricanes (NBC News, 2021). Based on a published journal, the probability that a storm will develop into a Category 3 or higher hurricane increased by 8% per decade (NBC News, 2021). Furthermore, a warmer climate has been linked to wetter hurricanes - for one degree of warming, the atmosphere is able to hold around 7% more water vapour (NYTimes, 2021b).
It’s vital that organisations act now in response to flood risk. We offer flood mapping and probabilistic flood modelling worldwide, providing flood risk insights at any location globally. This includes the highest resolution flood maps available for mainland US at national scale, to help re/insurers, financial organisations, and the International Development sector to better understand and manage flood risk.
To find out more about our flood data and how it can help you, get in touch with the team.
This report is covered by JBA’s website terms – please read them here.
References
APNews, 2021a. 2 more deaths from heat attributed to Ida in Louisiana. [online] Available at: https://apnews.com/article/hurricane-ida-louisiana-storms-hurricanes-baton-rouge-595999dd47a9240b018ac0a672315ec8
APNews, 2021b. New Orleans levees pass Ida’s test while some suburbs flood. [online] Available at: https://apnews.com/article/environment-and-nature-new-orleans-floods-suburbs-f2a033ef3eca9b98d55351a0736657d9
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BBC, 2021a. Hurricane Ida: One million people in Louisiana without power. [online] Available at: https://www.bbc.com/news/world-us-canada-58378788
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