Pluvial flooding: Protecting your property from a growing risk

Building and preparing for a 100-year flood has traditionally been regarded as sufficient protection against these seemingly rare weather events. However, taking into consideration climate change, extreme weather trends and overburdened stormwater infrastructures across the U.S., it may be wiser for property owners to protect themselves from flooding outside of 500-year floods. If that sounds far-fetched, unfortunately, it’s becoming the reality.

The increase in severity and frequency of pluvial flooding — a term that includes both flash floods and surface water runoff — continues to plague communities and businesses across the U.S. Even more daunting is Zurich Claims data that shows more than 50% of flood claims are occurring in areas designated outside the 500-year flood plain by the Federal Emergency Management Agency (FEMA). These higher-elevation areas, traditionally considered lower risk for flooding, are increasingly threatened by pluvial floods. And so, subsequently, is the property built upon them.

What is pluvial flooding?

Pluvial flooding is a term that will gain notoriety as long as urbanization and the severe weather patterns fostered by climate change continue to collide. It happens when extreme precipitation, such as microbursts, drops several inches of water in a short period of time, overwhelming the ability for the affected area to efficiently remove the water. Melting snow can also contribute to these events. (This is in contrast to fluvial flooding, caused by rivers and other bodies of water overflowing their banks.)

In recent years, total amount of precipitation has resulted from these intense single-day events, according to the Environmental Protection Agency (EPA). A recent EPA report, citing records that date to 1901, found that global precipitation has increased at an average rate of 0.04 inches per decade, while precipitation in the contiguous 48 states has increased at a rate of 0.20 inches per decade.¹

Different types of pluvial flooding

Two types of pluvial flooding can challenge communities and stormwater systems:

Surface water floods

Surface water flooding occurs when rainfall is not absorbed by the ground as it travels to a body of water (e.g., a river, creek, or drainage basin). When this happens, water may collect in low-lying areas, which then can affect below-grade spaces with or without a drainage system present. With nowhere to go, the water can collect on streets and into surrounding structures. Because this surface water typically rises gradually and remains shallow, people fortunately have time to move to safety. Although surface water flooding creates no immediate threat to lives, it may still cause significant economic damage to property.

Flash floods

Flash floods are characterized by an intense, high-velocity flow of water triggered by torrential rain that falls within a short amount of time, impacting the immediate vicinity and nearby elevated terrain. A historic flood event occurred in July 2023 in Vermont, where 4 to 9 inches of rain fell in a 48-hour period.² A deadly example of flash flooding occurred in the Northeast after Hurricane Ida in 2021, when 6 to 10 inches of rain fell within a few hours, leading to devastating flooding and over 40 deaths. For context, consider that in impacted cities such as New York City and Newark, N.J., the historical average for rainfall for the entire month of September is just 4 inches.³

Flash floods can also occur when there’s a sudden release of water from an upstream levee or a dam. Flash floods are extremely dangerous and destructive, not only because of the water’s force, but also from hurtling debris swept up in the flow. These “debris flows” can strike without warning, destroying everything in their path.

How severe is the risk from surface flooding?

Surface flooding can lead to catastrophic damage. This was observed in the flooding that resulted from 2017’s Hurricane Harvey in Houston, which dropped 27 trillion gallons of water in Texas and caused about $125 billion in damage.⁴ More recently, some of the heaviest rain on record fell in the San Diego, California, area in January 2024, causing millions of dollars in damage to homes and businesses.⁵

The catastrophic potential of surface flooding challenges the assumption that large flooding risks only occur near a significant body of water. Surface water runoff is playing a growing role in flooding events and needs to be better accounted for in flood risk.

Surface water runoff management is critical

Urbanization is a significant cause of surface water flooding because less permeable surfaces such as concrete and asphalt are replacing the natural flood protection provided by water-absorbing grasses, plants, trees, and wetlands. Urban flooding results in $9 billion in damages every year, notes the American Society of Civil Engineers (ASCE).⁶

Stormwater infrastructure is struggling to keep pace with the growing demands of extreme weather and the proliferation of urban development. Not only are many of these systems aging, but they were also built to accommodate rainfall averages and models that are far more modest than today’s reality.

Preventing surface water flooding

New building projects, including buildouts on existing property, can alter the flow of stormwater as well as the ability for existing infrastructure to handle the surface water. This also includes the need to address the potential degradation in surrounding-ground grading, which can cause unintended consequences to water flow or even water ponding in other areas of your property or a nearby neighbor’s.

Moreover, the problems can literally have a ripple effect, impacting local or even downstream communities where the drainage system connects enroute to the ultimate outlet.

Why pluvial flooding is such a difficult risk to anticipate

Flood maps from FEMA, though not the only mapping reference to predict the risk of a property to flooding, are probably the most familiar guide for planners, builders, and homeowners alike. Unfortunately, these maps have limitations:

• FEMA maps typically account for fluvial flood hazards, but usually do not account for pluvial impact nor the combined impact of fluvial and pluvial flooding.
• The maps do not account for all streams and watersheds in the U.S. The Association of State Floodplain Managers estimates that only about a third of the nation’s rivers and streams, and 46% of shoreline, are represented.⁷
• Although updates are scheduled every five years, many maps have been found to be older. This is especially problematic given the speed with which urbanization has continued alongside significant weather events of recent years.  For example, there are properties in New York City that experienced flooding during Superstorm Sandy (2012) but are still not required to purchase flood insurance because FEMA and the city recognize maps from 2007 that understate the actual flood risk around the five boroughs.⁸
• FEMA only maps U.S. locations.

How businesses can better manage the risk from storm waters

Given the changing tide of flood risks, property owners need to adopt a more nuanced understanding of pluvial risks. For instance, different areas of a property may pose different flood risks. The flood risk at the entrance of a property can be dramatically different from, say, the back of the site with acres of asphalt parking lot, or areas adjacent to different types of grading.

We’re not suggesting rejecting FEMA maps. When it is up to date, FEMA mapping considers only accredited or provisionally accredited levees, includes levee condition, and provides onsite assessment of river channels and overpasses. These are all beneficial when evaluating fluvial flooding, which in turn should be considered along with pluvial risks for an optimal understanding of overall flood risk.

To take a more comprehensive view of flood hazards, Zurich has incorporated new technology to help our customers identify and understand the growing challenges of pluvial flooding. These tools can help identify pluvial risks at various location levels of any given property.

Our Zurich Resilience Solutions Risk Engineering team makes multi-faceted assessments with a risk-based, rather than a hazard-focused, approach. Though the initial hazard level will be determined from a hazard map, we will then take into account a variety of hazard characteristics not included in the maps. In addition, we incorporate additional, relevant information for a particular site, including but not limited to event duration and local terrain conditions, such as urbanization that reduces soil absorption, or potential contaminants, such as underground storage tanks, etc. The critical exposures at a location (e.g., buildings, contents and/or storage) that could be threatened by the hazard are also considered as part of the assessment.

By combining information from different data sources, Zurich can help customers better identify both fluvial and pluvial risks for a more complete overall understanding of their flooding risk.

For more insights to help businesses prepare for, respond to and recover from floods, visit Zurich’s Flood Resource Hub.

To learn more about pluvial risks and how Zurich can help you get a more complete picture of your property’s flood risk, email Zurich Resilience Solutions-Risk Engineering.

Visit our website to explore our wide range of tailored, holistic solutions to help customers of all sizes protect their entire portfolio of business.

Joffre Mishall is Head of Property, U.S. National Accounts, for Zurich North America. Before taking on his current role, he served as Zurich’s Midwest Regional Property Leader, Property Portfolio Manager, Property Domestic Portfolio Manager and Property Claims Large Loss General Adjuster Manager.

References

1. “Climate Change Indicators: U.S. and Global Precipitation.” Environmental Protection Agency. Updated 7 July 2022.
2. The Great Vermont Flood of 10-11 July 2023: Preliminary Meteorological Summary (weather.gov).
3. Hershner, Monica and Nigel Chiwaya. “Map: Here's How Much Rain Ida Dumped on the Northeast.” NBC News. 2 September 2021.
4. “Hurricane Harvey.” The Texas A&M University System. Accessed October 2021.
5. California declares emergency after San Diego 'thousand-year storm' - Los Angeles Times (latimes.com).
6. “Stormwater: 2021 Report Card for America’s Infrastructure.” American Society of Civil Engineers. Accessed October 2021.
7. “Flood Mapping for the Nation.” Association of State Floodplain Managers (ASFPM) Foundation. January 2020.
8. “10 Years After Superstorm Sandy: The Paradox of More Flooding and Less Insurance | Moody's RMS" Moody’s October 27, 2022